Radio Frequency (RF) Interference Analysis and Optimization
By Farhana Jahan ID: 061-19-342 Md. Rafiqul Islam ID: 061-19-370 Md. Mohibul Hasan ID: 061-19-373

A thesis report presented in partial fulfillment of requirements for the degree of Bachelor of Science in Electronics and Telecommunication Engineering

Supervised by Mohammed Humayun Manager (Network Department) ADVANCED DATA NETWORKS SYSTEM LIMITED Red Crescent Concord Tower (19th floor) 17, Mohakhali Commercial Area, Dhaka-1212

Department of Electronics and Telecommunication Engineering

DAFFODIL INTERNATIONAL UNIVERSITY
October 2009

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APPROVAL PAGE
This thesis titled „Radio Frequency (RF) Interference Analysis and Optimization‟, Submitted by Md. Rafiqul Islam, Md. Mohibul Hasan and Farhana Jahan to the Department of Electronics and Telecommunication Engineering, Daffodil International University, has been accepted as satisfactory for the partial fulfillment of the requirement for the degree of Bachelor of Science in Electronics and Telecommunication Engineering and approved as to its style and contents. The presentation was held on 19th October 2009.

Board of Examiners
Mr. Golam Mowla Choudhury Professor and Head Department of Electronics and Telecommunication Engineering Daffodil International University ---------------------(Chairman) Dr. M. Lutfar Rahman Dean & Professor Faculty of Science and Information Technology Daffodil International University ---------------------(Member) A K M Fazlul Haque Assistant Professor Department of Electronics and Telecommunication Engineering Daffodil International University ---------------------(Internal) Dr. Subrata Kumer Aditya Professor Department of Applied Physics, Electronics and Communication Engineering University of Dhaka ---------------------(External) ii

ABSTRACT

Wireless Local Area Network (WLAN) applications are new, fast growing telecommunication protocols operating mainly in a free Industry-Scientific-Medicine (ISM) frequency bands. There is an increasing demand for Wireless Local Area Network (WLAN) systems to meet the emerging data communication challenges with respect to reliability, performance and cost. The choice of free frequency bands make it very attractive from commercial point of view. The wired networks have some problems which can not be overcome; these can be achieved by using WLAN.

This thesis report presents various WLAN performance analysis of different standards from IEEE 802.11 family & the interference issues involved in operating WALN, configuring network and simulation for establishing a WLAN connection at place including some theoretical and practical ideas about the radio frequency properties, modulation techniques, antennas and interference. The performance specifications include parameters such as channel reuse, frequency, bandwidth, and all this experience can be helpful to work in any kind of telecommunication sector using wireless communications.

The aim of this work is to study the possibilities that the Wireless Local Area Networks (WLAN's) offer to indoor localization and to implement an application that will help to improve performance and interference free wireless network.

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DECLARATION
We hereby declare that the work presented in this thesis report titled “RADIO FREQUENCY (RF) INTERFERENCE ANALYSIS AND OPTIMIZATION” is done by us under the supervision of Mohammed Humayun, Manager (Network Department) ADVANCED DATA NETWORKS SYSTEM LIMITED, in partial fulfillment of the requirements for the degree of Bachelor of Science in Electronics and Telecommunication Engineering. We also declare that this thesis is our original work. As far as our knowledge goes, neither this report nor any part thereof has been submitted else where for the award of any degree or diploma.

Students: Farhana Jahan ID: 061-19-342 Department of Electronics and Telecommunication Engineering ---------------(Signature) Md. Rafiqul Islam ID: 061-19-370 Department of Electronics and Telecommunication Engineering ---------------(Signature) Md. Mohibul Hasan ID: 061-19-373 Department of Electronics and Telecommunication Engineering ---------------(Signature) COUNTERSIGNED Supervisor: Mohammed Humayun Manager (Network Department) ADVANCED DATA NETWORKS SYSTEM LIMITED ---------------(Signature)

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ACKOWLEDGMENTS

First of all, we would like to express our gratefulness to Almighty Allah – the most merciful, the most beneficent to give us the capability to complete the thesis successfully.

Then we would like to express our sincerest gratitude to our supervisor Mohammed Humayun who has inspired, encouraged and co-operated us in every possible ways to make this thesis a success. His helpful suggestions and supply of materials regarding this thesis are also gratefully acknowledged.

We would also like to express our gratefulness to Mr. Ikhtiar Uddin Ahmed (Sohel), Wireless (RF) Engineer, Network Operation Department, BANGLA LION LTD. who helped us in different aspects to start this thesis work. He also guided us during the thesis and writing the report of this work in various ways.

We also thank our family members for giving us the mental and financial supports in completing this project.

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Table of content

CHAPTER 1: INTRODUCTION
1.1 Aims and objectives 1.2 Background 1.2.1 Cellular Concept 1.2.2 Fundamental of RF communication and WLAN network 1.3 WLAN principle and RF properties 1.3.1 Amplitude 1.3.2 Frequency 1.3.3 Wavelength 1.3.4 Phase
1.4 WLAN standards 1.4.1 IEEE 802.11 1.4.2 802.11a 1.4.3 802.11b 1.4.4 802.11g 1.4.5 802.11n 1 1 2 2 3 3 4 5 6 7 7 7 7 8 8 8 8 9 10 11 12

1.5 Applications of WLAN 1.5.1 Wi-Fi Network 1.5.2 Wi-Fi Hotspot 1.5.2 Wireless Sensor Networks 1.5.3 Voice over Wireless LAN (VoWLAN)
1.6 Organization of this report

CHAPTER-2: RADIO FREQUENCY AND INTERFERENCE
2.1 Radio frequency 2.2 Radio frequency behavior
13 13

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2.2.1 Gain 2.2.2 Loss 2.2.3 Reflection 2.2.4 Refraction 2.2.5 Diffraction 2.2.6 Scattering 2.3 Radio frequency spectrum 2.4 Licensed Band 2.4.1 Federal Communications Commission: 2.4.2 ISM and UNII Bands 2.4.2 900 MHz ISM Band 2.4.3 2.4GHz ISM Band 2.4.4 5.8GHz ISM Band 2.5 Unlicensed 2.5.1 Unlicensed National Information Infrastructure (UNII) Bands 2.5.2 Lower Band 2.5.3 Middle Band 2.5.4 Upper Band 2.6 Licensed vs. Unlicensed 2.7 Radio frequency interference 2.7.1 Radio frequency interference 2.7.2 Causes 2.8 Types of Radio frequency interference 2.8.1 Adjacent channel interference 2.8.2 Co-channel interference 2.8.2.1 Adverse weather conditions 2.8.2.2 Poor frequency planning 2.8.2.3 Overly-crowded radio spectrum 2.9 Factors of RFI (Radio Frequency Interference) 2.9.1 Physical objects 2.9.2 Radio frequency interference

13 14 15 16 16 18 19 21 21 22 23 23 23 24 24 24 24 25 25 26 27 28 30 30 31 31 31 31 32 32 32

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2.9.3 Electrical interference 2.9.4 Environmental factors

33 33

CHAPTER-3: RADIO FREQUENCY INTERFERENCE ANALYSIS
3.1 Background 3.2 Sources of Interference
3.3 Analysis of Radio Frequency Interference: 3.3.1 D-Link Access Point 3.3.2 NetGear Access Point 3.3.3 Analytical software 3.4 Co-channel Interference Analysis 3.4.1 Configuration setup 3.4.2 Access Point Configuration 3.4.3 RF analysis for co-channel interference with NetStumbler 3.4.4 Spectrum analysis 3.5 Adjacent Channel Interference 3.5.1 Access Point Configuration 3.5.2 Spectrum analysis 3.5.3 RF analysis for Adjacent channel interference with NetStumbler 3.5.4 Spectrum for this configuration 3.6 Spurious Emissions 3.7 Out-of-Band Emitters 3.7.1 Desensitization 3.7.2 Intermodulation 3.7.3 Intermodulation Signals from Non-linear Power Amplifiers 3.7.4 Intermodulation from Non-linear External Elements 3.8 In-Band Emitters 3.8.1 Harmonic and Parasitic Outputs 3.8.2 Overlap of Antenna Patterns 35 36 36 36 37 39 40 40 41 43 43 44 44 47 47 49 50 50 50 50 51 52 52 53 53

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3.8.3 ISM System Antenna Alignment Problems 3.8.4 Back lobes and Side lobes 3.8.5 Reflections and Fading 3.8.6 Cellular Antenna Overlap

53 54 54 54

CHAPTER-4: Optimization
4.1 RFI Optimization 4.2 Cell Design 4.3 Frequency assignment 4.4 Power control 4.5 Co-channel interference optimization 4.6 Adjacent Channel Interference optimization 4.7 Methods of Dealing with Interference- The Physical Layer 4.7.1 Modulation and the C/I Ratio 4.7.2 Antennas 4.7.3 TDD Synchronization 4.8 Methods of Dealing with Interference- The MAC Layer 4.8.1 Frame/Slot size 4.8.2 Automatic Retransmission Request 4.8.3 Centralized Transmission Control 4.8.4 PMP Applications – Cellular Deployments 4.8.5 PMP Applications - Spot Deployments 4.8.6 PTP/Backhaul Applications 55 56 56 57 59 60 61 61 63 63 64 64 65 66 67 67 69

CHAPTER-5: CONCLUSION REFERENCES APPENDIX

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72

74

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LIST OF TABLES CHAPTER - 2
Table 2.1: Different frequency wavelengths Table 2.2 Wireless Obstacles Found Indoors 20 34

CHAPTER - 3
Table 3.1: Frequency spectrum for Co-channel Interference. Table 3.2: Frequency Spectrum at Adjacent Channel Interference Table 3.3: Interference analysis for the adjacent channel (overlapped). 44 47 49

CHAPTER - 4
Table 4.1: Frequency Spectrum for Co-channel Interference Table 4.2: Non-overlapping frequency spectrum for optimization Table 4.3: Interference Area of Adjacent Channel Table 4.4: Channel selection for interference optimization 59 60 61 61

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LIST OF FIGURES CHAPTER - 1
Figure 1.1 Amplitude Figure 1.2 Frequency Figure 1.3 Wavelength Figure 1.4 Phase Figure 1.5: Wi-Fi Network Figure 1.5: Wireless Sensor Network Figure 1.6: Voice over Wireless LAN Network 4 5 6 6 9 10 11

CHAPTER - 2
Figure 2.1: Power gain Figure 2.2: Power loss Figure 2.3: Reflection Figure 2.4: Refraction Figure 2.5: Diffraction Figure 2.6: Scattering Figure 2.7: Different radio frequency spectrum Figure 2.8: Different band of RF spectrum Figure 2.9: Variety of possible interferers 14 15 15 16 17 18 19 25 29

CHAPTER - 3
Figure 3.1: Access Point (D-Link)
Figure 3.2: Primary configuration of D-Link access point. Figure 3.3: Access Point (NetGear) Figure 3.4: Primary configuration of NetGear access point Figure 3.5: Signal level analysis with NetStumbler software

36 37 38 38 39

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Figure 3.6: Channel setup for co-channel interference Figure 3.7: Access Point (D-Link) configuration Figure 3.8: Access Point (NetGear) configuration Figure 3.9: Co-channel interference analysis with NetStumbler Figure 3.10: configuration for Adjacent Channel interference analysis Figure 3.11: configuration setup for adjacent channel analysis (DIU) Figure 3.12: signal to noise ratio for Adjacent Channel Interference Figure 3.13: Channel changing for Adjacent Channel Interference Figure 3.14: Signal to noise ratio for Adjacent Channel Interference. Figure 3.15: Back lobes and side lobes

40 41 42 43 45 46 48 48 49 54

CHAPTER-4
Figure 4.1: Cell design or Access Point placement Figure 4.2: 2.4GHz frequency spectrum Figure 4.3: Frequency planning in a cellular deployment Figure 4.4: TDD MAC Frame Figure 4.5: Poor coverage in a dense deployment Figure 4.6: Canopy coverage in a dense deployment Figure 4.7: Canopy AP site with backhaul

56 57 64 65 68 68 70

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CHAPTER- 01 INTRODUCTION
1.1 Aims and objectives When evaluating wireless technology, the possibility of radio frequency interference disturbing a wireless network link sometimes decreases the overall network performance. This paper provides background on interference, design implementation of a WLAN, describes the analysis and measuring radio frequency interference, and some implementation to overcome interference in the WLAN network.

1.2 Background
The ability to communicate with people on the move has involved remarkably since Guglielmo Marconi first demonstrated radio‘s ability to provide continuous contact with ships sailing the English Channel. That was in 1890, and since then new wireless communications methods and services have been enthusiastically adopted by people throughout the world. Particularly during the past ten years the mobile radio communication industry has grown by orders of magnitude, fueled by digital and RF circuit fabrication improvements, new large scale circuit integration, and other miniaturization technologies which make portable radio equipment smaller, cheaper, and more reliable. Digital switching techniques have facilitated the large scale deployment of affordable, easy to use radio communication networks. These trends will continue at an even greater pace during the next decade. Communications that were formerly provided by wires are now provided by radio (wireless) means. Thus wireless communication, which uncouples the telephone from its wires to the local telephone exchange, has exploded.

In these prospects, the better microwave link design, higher bandwidth, greater coverage area and radio frequency overlook is needed. Here we tried to discuss the background of designing a radio frequency network which can be called as WLAN or Wireless Local Area Network.
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1.2.1 Cellular Concept
The design object of early mobile radio systems was to achieve a large coverage area by using a single, high power transmitter with an antenna mounted on a tall tower. While this approach achieved very good coverage, it also meant that it also possible to reuse those frequencies throughout the systems, since any attempt to achieve frequency reuse would result in interference. The cellular concept is the major breakthrough in solving the problem of spectral congestion and user capacity. It offered very hard capacity in a limited spectrum allocation without any major technological changes. The cellular concept is a system level idea which calls for replacing a single power transmitters with any low power transmitters, each providing coverage to only the service area. Each base station is allocated a portion of the total number of channels available to the entire systems. And near by base stations are assigned different groups of channels so that all the available channels are assigned to a relatively small number of neighboring base stations. Neighboring base stations are assigned different groups of channels so that the interference between the base stations is minimized.

1.2.2 Fundamental of RF communication and WLAN network
In radio channel/frequency communication radio waves are used as a transmission medium. It is time dependent electromagnetic fields produce waves that radiate from the source to the environment. The radio wave based radio communication system is vulnerable to the environmental factors like mountains, hills reflectors. The radio signal depends on the distance from the base station, the wavelength and the communication environment. The main problems of radio communication which affects the RF signals are: Reflection Refraction Diffraction Scattering Absorption and Interference
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RF signals are high frequency alternating current (AC) signals composed of electromagnetic energy. RF signals are generated as electrical energy by the transmitting radio, passed along a copper wire to the antenna, and then radiated into the air by the antenna. The antenna converts the wired signal to a wireless signal, and vice versa. In a wired LAN, the electrical signal travels predictably, it propagates within the wire, with little interference. By contrast, the RF signals used in a WLAN, an administrator must have a solid foundation in the properties of RF radiation and theory. A wireless LAN or WLAN is a wireless local area network that enables client computers and the server to communicate with one another without direct cable connections. By using electromagnetic waves or Radio waves, WLANs transmit and receive data over the air, and thus minimize the need for wired connections. There are several different technologies by which these WLANs can be implemented depending on the requirements of the users. Furthermore, several WLAN standards have been developed in order to ensure interoperability between products from different vendors.

1.3 WLAN principle and RF properties
All radio frequencies have the properties of amplitude, frequency, wavelength, phase, and polarity.

1.3.1 Amplitude
Amplitude is the objective measurement of the degree of change (positive or negative) in atmospheric pressure (the compression and rarefaction of air molecules) caused by sound waves. Sounds with greater amplitude will produce greater changes in atmospheric pressure from high pressure to low pressure. Amplitude is almost always a comparative measurement, since at the lowest-amplitude end (silence), some air molecules are always in motion and at the highest end, the amount of compression and rarefaction though finite, is extreme. In electronic circuits, amplitude may be increased by expanding the degree of change in an oscillating electrical current. A woodwind player may increase the amplitude of their sound by providing greater force in the air column i.e. blowing harder.
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Figure 1.1 Amplitude

The change in pressure caused by a passing sound wave, we can measure the change in RF energy caused by a passing RF wave. This change is known as the amplitude of the signal. The amplitude of an RF signal is analogues to a voltage level in an electrical signal. Higher amplitude signals are more likely to show high signal strength when they are received by an 802.11 station. Amplitude is the vertical distance, or height, between crests. For the same wavelength and frequency, different amplitude can exist. It represents the quantity of energy injected in the signal. This value is usually regulated as it can affect the receivers. Amplitude is the most basic quality of an RF signal. Other qualities of the signal, such as frequency, wavelength, and phase are all based on variation of the signal‘s amplitude over time. One quality, polarity is independent of the signal‘s amplitude.

1.3.2 Frequency
The time between one peak in the signals amplitude and next peak is constant from peak to peak. The number of times per second that the signal amplitude peaks is the signals amplitude peaks is frequency of the signal. The frequency determines how often a signal is seen. Frequency is measured in Hertz. 1 cycle per second is 1 Hertz. Low frequency travel farther in the air than higher frequencies.

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Figure 1.2 Frequency For cyclical processes, such as rotation, oscillations, or waves, frequency is defined as a number of cycles, letter ν (nu). In SI units, the unit of frequency is hertz (Hz), named after the German physicist Heinrich Hertz. For example, 1 Hz means that an event repeats once per second, 2 Hz is twice per second, and so on. This unit was originally called a cycle per second (cps), which is still sometimes used. Heart rate and musical tempo are measured in beats per minute (BPM). Frequency of rotation is often expressed as a number of revolutions per minute (rpm). BPM and rpm values must be divided by 60 to obtain the corresponding value in Hz: thus, 60 BPM translates into 1 Hz. The period is usually denoted as T, and is the reciprocal of the frequency f: or periods, per unit time. In physics and engineering disciplines, such

as optics, acoustics, and radio, frequency is usually denoted by a Latin letter f or by a Greek

The SI unit for period is the second.

1.3.3 Wavelength
The distance between crests of a wave. The wavelength determines the nature of the various forms of radiant energy that comprise the electromagnetic spectrum. For electromagnetic waves, the wavelength in meters is computed by the speed of light divided by frequency (300,000,000/Hz). For sound waves, the wavelength is determined by 335/Hz.

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Figure 1.3 Wavelength The wavelength is the distance between crests. The higher the frequency, the shorter the wavelength.

1.3.4 Phase
The phase of an oscillation or wave is the fraction of a complete cycle corresponding to an offset in the displacement from a specified reference point at time t = 0. Phase is a frequency domain or Fourier transform domain concept, and as such, can be readily understood in terms of simple harmonic motion. The same concept applies to wave motion, viewed either at a point in space over an interval of time or across an interval of space at a moment in time. Simple harmonic motion is a displacement that varies cyclically, as depicted to the right. It is described by the formula:

where A is the amplitude of oscillation, f is the frequency, t is the elapsed time, and θ is the phase of the oscillation. The phase determines or is determined by the initial displacement at time t = 0. A motion with frequency f has period T ═ 1⁄f

Figure 1.4 Phase
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1.4 WLAN standards
1.4.1 IEEE 802.11 When wireless networks initially started appearing in the market, there was no unique standard to ensure interoperability between different products. In order to combat this, the IEEE 802.11 committee was formed in 1990. The purpose of the IEEE 802.11 standard (also known as the WiFi standard) was to define specifications that allow wireless LAN devices from different vendors to operate together. Frequency Hopping Spread Spectrum (FHSS) operating in the 2.4 GHz band at 1 and 2 Mbps Direct Sequence Spread Spectrum (DSSS) operating in the 2.4 GHz band at 1 and 2 Mbps Infrared operating at 1 and 2 Mbps Further amendments were made to the protocol later on. These include the 802.11a, 802.11b and 802.11g amendments.

1.4.2 802.11a
The 802.11a amendment to the original protocol was ratified in 1999. It uses a technology called Orthogonal Frequency Division Multiplexing (OFDM) rather than Spread Spectrum and operates in the 5 GHz band. Using this technique, data rates of up to 54 Mbps are possible. Unfortunately it is not compatible with 802.11b devices unless the device implements both standards. The advantage of 802.11a is that instead of operating in the crowded 2.4 GHz band (which is also used by cordless telephones, microwave ovens, Bluetooth devices and other devices), it operates in the 5 GHz band which is relatively free of interference. However, the higher frequency has less penetrating power thus reducing the indoor range from 50m to 30m.

1.4.3 802.11b
The 802.11b amendment was ratified in 1999 and is an extension of the original standard, improving the DSSS technology. It uses a technique called Complementary Code Keying (CCK) which increases the data rate to 11 Mbps. Because it is similar to the original, it was easy for
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manufacturers to adapt to 802.11b and due to the higher data rate it became the most widespread technology for wireless LANs.

1.4.4 802.11g
In 2003 a new amendment called 802.11g was ratified. Like 802.11b, it operates in the 2.4 GHz band using DSSS but allows data rates of up to 54 Mbps. This is done by using Orthogonal Frequency Division Multiplexing (OFDM) similar to that used by 802.11a. An advantage is that 802.11g is compatible with 802.11b hardware. Vendors were quick to incorporate the new standard in their products and currently most devices in the market can operate using 802.11a, b and g. As mentioned before, the drawback of using 802.11b and 802.11g is that they operate in the crowded 2.4 GHz band and thus are susceptible to interference.

1.4.5 802.11n
In 2004, the IEEE announced plans for a new amendment to the 802.11 standard: 802.11n. This amendment will incorporate Multiple-Input Multiple-Output (MIMO) technology into 802.11 to allow a proposed data rate of 540 Mbps. The amendment is expected to appear some time after July 2007

1.5 Applications of WLAN
There are too many applications in WLAN. They are Wi-Fi network, Wi-Fi Hotspots, Sensor network, VoLAN etc. Some applications are discuss below.

1.5.1 Wi-Fi Network
Wireless Fidelity (Wi-Fi) is an inexpensive, short range, line-of-sight, broadband wireless technology that uses the same unregulated radio frequencies as microwave ovens and cordless phones. It is essentially a wireless local area network that can be deployed so as to serve a single business such as a coffee shop, or deployed citywide in what is referred to as a mesh network. By changing the economics of high speed Internet access, Wi-Fi has the potential to dramatically increase the number of people and businesses who have access to e-commerce, distance education, e- government, telemedicine and other electronic services. A Wi-Fi network has four basic elements:
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Access Points (radio transmitters on poles) Wired or wireless connection to a base station A Wireless ISP (WISP) Wired or wireless connection from base station to the WISP‘s server.

Figure 1.5: Wi-Fi Network

1.5.2 Wi-Fi Hotspot
A single Wi-Fi coverage area, like a single building or a park usually covering an area no larger than a football field. There are about 22,000 hot spots in the United States today and their number is forecast to grow to 40,000 by 2007. Coverage provided by hotspots is isolated and sporadic. In Bangladesh, there created a Wi-Fi hotspot in the Dhaka University area. And several areas are under processing for the Wi-Fi network. A zone is unified by service, not geography. It is an aggregation of cooperating hotspots sharing a single management system. A single login allows an end-user to access the network anywhere in the geographic area covered by the zone. A zone may cover a large area like a mall or convention centre, but the area covered need not be contiguous.

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1.5.2 Wireless Sensor Networks
Smart environments represent the next evolutionary development step in building, utilities, industrial, home, shipboard, and transportation systems automation. Like any sentient organism, the smart environment relies first and foremost on sensory data from the real world. Sensory data comes from multiple sensors of different modalities in distributed locations. The challenges in the hierarchy of: detecting the relevant quantities, monitoring and collecting the data, assessing and evaluating the information, formulating meaningful user displays, and performing decision-making and alarm functions are enormous. The information needed by smart environments is provided by Distributed Wireless Sensor Networks, which are responsible for sensing as well as for the first stages of the processing hierarchy. The figure shows the complexity of wireless sensor networks, which generally consist of a data acquisition network and a data distribution network, monitored and controlled by a management centre. The plethora of available technologies makes even the selection of components difficult, let alone the design of a consistent, reliable, robust overall system.

Figure 1.5: Wireless Sensor Network
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1.5.3 Voice over Wireless LAN (VoWLAN)
Voice over IP (VoIP) is maturing and achieving increasing corporate acceptance as device costs decrease and VoIP phones improve. Employing VoIP over wireless LAN (VoWLAN), freeing the telephone from the cord, is an obvious next step. It is expected that the enterprise VoWLAN market will initially be driven by specific corporate needs, such as warehouse and retail sales tracking and control, ubiquitous mobile telephony in medical campuses and hospitals, and mobile security applications. VoWLAN phones are already being offered to enterprises by leading vendors, while integration of WLAN and cellular technology (dual mode handsets) is also being pursued by numerous vendors and carriers in several countries. WLAN provides an excellent opportunity to enable VoIP, since it combines the cost effectiveness of VoIP solutions with cordless mobility. However, VoWLAN deployments have special needs in order to be effective. VoWLAN requires a very strong uplink to reduce latency and jitter. It requires age and seamless mobility complete cover that allow strong security, without interrupting service with constant handoffs. In addition, increased capacity is needed to provide a sufficient number of simultaneous voice calls. Users must always be close to an AP to cope with power constraints of VoWLAN devices, allowing them to use lower transmission powers, and to transmit at the highest data rate

Figure 1.6: Voice over Wireless LAN Network
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.1.6 Organization of this report  This thesis report has five chapters in total.  The first chapter describes a brief idea about Fundamental of RF communication, WLAN network, standard, RF principles and applications.  The second chapter contains theory about the radio frequency Interferences and its different types  In third chapter, we described the analysis of radio frequency interferences.  In Fourth chapter here is some techniques of radio frequency interference optimization.  The fifth and final chapter is the concluding chapter where few suggestions of our work are provided along with the concluding remarks.

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CHAPTER-2 RADIO FREQUENCY AND INTERFERENCE

2.1 Radio frequency
Radio frequencies are high frequency alternating current (AC) signals that are passed along a copper conductor and then radiated into the air via an antenna. An antenna converts/transforms a wired signal to a wireless signal and vice versa. When the high frequency AC signal is radiated into the air, it forms radio waves. These radio waves propagate (move) away from the source (the antenna) in a straight line in all directions at once.

2.2Radio frequency behavior
RF is sometimes referred to as "smoke and mirrors" because RF seems to act erratically and inconsistently under given circumstances. Things as small as a connector not being tight enough or a slight impedance mismatch on the line can cause erratic behavior and undesirable results. The following sections describe these types of behaviors and what can happen to radio waves as they are transmitted

2.2.1 Gain
Gain, illustrated in Figure 2.1, is the term used to describe an increase in an RF Signal‘s amplitude [2]. Gain is usually an active process; meaning that an external Power source, such as an RF amplifier, is used to amplify the signal or a high-gain antenna is used to focus the beam width of a signal to increase its signal amplitude.

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Figure 2.1: Power gain

However, passive processes can also cause gain. For example, reflected RF signals combine with the main signal to increase the main signal's strength. Increasing the RF signal's strength may have a positive or a negative result. Typically, more power is better, but there are cases, such as when a transmitter is radiating power very close to legal power output limit, where added power would be a serious problem.

2.2.2 Loss
Loss describes a decrease in signal strength (Figure 2.2). Many things can cause RF signal loss, both while the signal is still in the cable as a high frequency AC electrical signal and when the signal is propagated as radio waves through the air by the antenna. Resistance of cables and connectors causes loss due to the converting of the AC signal to heat. Impedance mismatches in the cables and connectors can cause power to be reflected back toward the source, which can cause signal degradation. Objects directly in the propagated wave's transmission path can absorb, reflect, or destroy RF signals. Loss can be intentionally injected into a circuit with an RF attenuator. RF attenuators are accurate resistors that convert high frequency AC to heat in order to reduce signal amplitude at that point in the circuit.

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Figure 2.2: Power loss Being able to measure and compensate for loss in an RF connection or circuit is important because radios have a receive sensitivity threshold. A sensitivity threshold defined as the point at which a radio can clearly distinguish a signal from background noise. Since a receiver‘s sensitivity is finite, the transmitting station must transmit signal with enough amplitude to be recognizable at the receiver. If losses occur between the transmitter and receiver, the problem must be corrected either by removing the objects causing loss or by increasing the transmission power.

2.2.3 Reflection
Reflection, (as illustrated in Figure 2.3) occurs when a propagating electromagnetic wave impinges upon an object that has very large dimensions when compared to the wavelength of the propagating wave [3]. Reflections occur from the surface of the earth, buildings, walls, and many other obstacles. If the surface is smooth, the reflected signal may remain intact, though there is some loss due to absorption and scattering of the signal.

Figure 2.3: Reflection

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RF signal reflection can cause serious problems for wireless LANs. This reflecting main signal from many objects in the area of the transmission is referred to as multipath. Multipath can have severe adverse affects on a wireless LAN, such as degrading or canceling the main signal and causing holes or gaps in the RF coverage area. Surfaces such as lakes, metal roofs, metal blinds, metal doors, and others can cause severe reflection, and hence, multipath. Reflection of this magnitude is never desirable and typically requires special functionality (antenna diversity) within the wireless LAN hardware to compensate for it.

2.2.4 Refraction
Refraction describes the bending of a radio wave as it passes through a medium of different density. As an RF wave passes into a denser medium (like a pool of cold air lying in a valley) the wave will be bent such that its direction changes. When passing through such a medium, some of the wave will be reflected away from the intended signal path, and some will be bent through the medium in another direction, as illustrated in Figure 2.4.

Figure 2.4: Refraction Refraction can become a problem for long distance RF links. As atmospheric conditions change, the RF waves may change direction, diverting the signal away from the intended.

2.2.5 Diffraction
Diffraction occurs when the radio path between the transmitter and receiver is obstructed by a surface that has sharp irregularities or an otherwise rough surface. At high frequencies,
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diffraction, like reflection, depends on the geometry of the obstructing object and the amplitude, phase, and polarization of the incident wave at the point of diffraction. Diffraction is commonly confused with and improperly used interchangeably with refraction. Care should be taken not to confuse these terms. Diffraction describes a wave bending around an obstacle (Figure 2.5), whereas refraction describes a wave bending through a medium. Taking the rock in the pond example from above, now consider a small twig sticking up through the surface of the water near where the rock. As the ripples hit the stick, they would be blocked to a small degree, but to a larger degree, the ripples would bend around the twig. This illustration shows how diffraction acts with obstacles in its path, depending on the makeup of the obstacle. If Object was large or jagged enough, the wave might not bend, but rather might be blocked.

Figure 2.5: Diffraction Diffraction is the slowing of the wave front at the point where the wave front strikes an obstacle, while the rest of the wave front maintains the same speed of propagation. Diffraction is the effect of waves turning, or bending, around the obstacle. As another example, consider a machine blowing a steady stream of smoke. The smoke would flow straight until an obstacle entered its path. Introducing a large wooden block into the smoke stream would cause the smoke to curl around the corners of the block causing a noticeable degradation in the smoke's velocity at that point and a significant change in direction.

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2.2.6 Scattering
Scattering occurs when the medium through which the wave travels consists of objects with dimensions that are small compared to the wavelength of the signal, and the number of obstacles per unit volume is large. Scattered waves are produced by rough surfaces, small objects, or by other irregularities in the signal path, as can be seen in Figure 2.6.

Figure 2.6: Scattering

Some outdoor examples of objects that can cause scattering in a mobile communications system include foliage, street signs, and lampposts. Scattering can take place in two primary ways. First, scattering can occur when a wave strikes an uneven surface and is reflected in many directions simultaneously. Scattering of this type yields many small amplitude reflections and destroys the main RF signal. Dissipation of an RF signal may occur when an RF wave is reflected off sand, rocks, or other jagged surfaces. When scattered in this manner, RF signal degradation can be significant to the point of intermittently disrupting communications or causing complete signal loss. Second, scattering can occur as a signal wave travels through particles in the medium such as heavy dust content. In this case, rather than being reflected off an uneven surface, the RF waves are individually reflected on a very small scale off tiny particles.

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2.3 Radio frequency spectrum
The term Radio Frequency (RF) refers to the electromagnetic field that is generated when an alternating current is input to an antenna. This field, also called an RF field or radio wave, can be used for wireless broadcasting and communications over a significant portion of the electromagnetic radiation spectrum -- from about 9 kilohertz (kHz) to thousands of gigahertz (GHz). This portion is referred to as the RF Spectrum. As the frequency is increased beyond the

Figure 2.7: Different radio frequency spectrum RF spectrum, electromagnetic energy takes the form of infrared (IR), visible light, ultraviolet (UV), X rays, and gamma rays.
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Many types of wireless devices make use of RF fields -- radio, television, cordless and cellular telephones, satellite communication systems, and many measuring and instrumentation systems used in manufacturing. Some wireless devices, such as remote control boxes and cordless mice, operate at IR or visible light frequencies. The RF spectrum is divided into several ranges, or bands. Each of these bands, other than the lowest frequency segment, represents an increase of frequency corresponding to an order of magnitude (power of ten). The chart at the top of the page depcits the eight bands in the RF spectrum, showing frequency and bandwidth ranges. Internationally, the RF spectrum is allocated by the International Telecommunication Union (ITU) to various classes of service according to different regions of the world. Within the United States and its possessions, the RF spectrum is further allocated to non-Government and Government users. The Federal Communications Commission (FCC), acting under the authority of Congress, is responsible for the allocation and assignment of frequencies to non-Government users.

Table 2.1: Different frequency wavelengths
Designation Very Low Frequency Low Frequency Medium Frequency High Frequency Very High Frequency Ultra High Frequency Super High Frequency Extremely High Frequency Abbreviation VLF LF MF HF VHF UHF SHF EHF Frequencies 9 kHz - 30 kHz 30 kHz - 300 kHz 300 kHz - 3 MHz 3 MHz - 30 MHz 30 MHz - 300 MHz 300 MHz - 3 GHz 3 GHz - 30 GHz 30 GHz - 300 GHz Free-space Wavelengths 33 km - 10 km 10 km - 1 km 1 km - 100 m 100 m - 10 m 10 m - 1 m 1 m - 100 mm 100 mm - 10 mm 10 m - 1 mm

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2.4 Licensed Band
Most computer-related hardware and technologies are based on some standards, and wireless LANs are no exception. There are organizations that define and support the standards that allow hardware from different manufacturers to function together seamlessly. In this chapter we will discuss the FCC‘s role in defining and enforcing the laws governing wireless communication and the IEEE‘s role in creating standards that allow wireless devices to work together. We will also cover the different frequency bands on which wireless LANs operate, and examine the 802.11 family of standards. We will discuss some of the major organizations in the wireless LAN marketplace as well as the roles they fill in the industry. Finally, we will cover some of the emerging technologies and standards and discuss their impact on the wireless LAN industry. By understanding the laws and the standards that govern and guide wireless LAN technology, you will be able to ensure that any wireless system you implement will be interoperable and comply with the law. Furthermore, familiarity with these statutes and standards, as well as the organizations that create them, will greatly enhance your ability to research and find the latest information about wireless LANs.

2.4.1 Federal Communications Commission:
The Federal Communications Commission (FCC) is an independent United States government agency, directly responsible to Congress. The FCC was established by the Communications Act of 1934 and is charged with regulating interstate and international communications by radio, television, wire, satellite, and cable. The FCC's jurisdiction covers not only the 50 states and the District of Columbia, but also all U.S. possessions such as Puerto Rico, Guam, and The Virgin Islands. The FCC makes the laws within which wireless LAN devices must operate. The FCC mandates where on the radio frequency spectrum wireless LANs can operate and at what power, using which transmission technologies, and how and where various pieces of wireless LAN hardware may be used.
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2.4.2 ISM and UNII Bands
The FCC establishes rules limiting which frequencies wireless LANs can use and the output power on each of those frequency bands. The FCC has specified that wireless LANs can use the Industrial, Scientific, and Medical (ISM) bands, which are license free. The ISM bands are located starting at 902 MHz, 2.4 GHz, and 5.8 GHz and vary in width from about 26 MHz to 150 MHz In addition to the ISM bands, the FCC specifies three Unlicensed National Information Infrastructure (UNII) bands. Each one of these UNII bands is in the 5 GHz range and is 100 MHz wide. When implementing any wireless system on a license-free band, there is no requirement to petition the FCC for bandwidth and power needs. Limits on the power of transmission exist, but there is no procedure for receiving permission to transmit at such power. Furthermore, there are no licensing requirements and, thus, no cost associated with licensing. The license-free nature of the ISM and UNII bands is very important because it allows entities like small businesses and households to implement wireless systems and fosters the growth of the wireless LAN market. Such freedom from licensing carries with it a major disadvantage to license-free band users. The same license-free band you use (or intend to use) is also license-free to others. Suppose you install a wireless LAN segment on your home network. If your neighbor also installs a wireless LAN segment in his home, his system may interfere with yours, and vise versa. Furthermore, if he uses a higher-power system, his wireless LAN may disable yours by ―whiting out‖ your wireless traffic. The two competing systems don‘t necessarily have to be on the same channel, or even be the same spread spectrum technology. There are three license-free ISM bands the FCC has specified that wireless LANs may use. They are the 900 MHz, 2.4 GHz, and 5.8 GHz bands.

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2.4.2 900 MHz ISM Band
The 900 MHz ISM band is defined as the range of frequencies from 902 MHz to 928 MHz. This band may be additionally (and correctly) defined as 915 MHz ± 13 MHz. Though the 900 MHz ISM band was once used by wireless LANs, it has been largely abandoned in favor of the higher frequency bands, which have wider bandwidths and allow more throughput. Some of the

wireless devices that still use the 900 MHz band are wireless home phones and wireless camera systems. Organizations that use 900 MHz wireless LANs find out the hard way that obsolete equipment is expensive to replace should any piece of their hardware malfunctions. A single 900 MHz radio card may cost as much as $800 and might only be able to transmit at speeds up to 1 Mbps. In comparison, an 802.11b compliant wireless card will support speeds up to 11 Mbps and sell for roughly $100. Finding support or replacements for these older 900 MHz units is almost impossible.

2.4.3 2.4GHz ISM Band
This band is used by all 802.11, 802.11b, and 802.11g-compliant devices. The 2.4 GHz ISM band is bound by 2.4000 GHz and 2.5000 GHz (2.4500 GHz ± 50 MHz), as defined by the FCC. Of the 100 MHz between 2.4000 and 2.5000 GHz, only the frequencies 2.4000 – 2.4835 GHz are actually used by wireless LAN devices. The principal reason for this limitation is that the FCC has specified power output only for this range of frequencies within the 2.4 GHz ISM band.

2.4.4 5.8GHz ISM Band
This band is also frequently called the 5 GHz ISM Band. The 5.8 GHz ISM is bound by 5.725 GHz and 5.875 GHz, which yields a 150 MHz bandwidth. This band of frequencies is not specified for use by wireless LAN devices, so it tends to present some confusion. The 5.8 GHz ISM band overlaps part of another license-free band, the Upper UNII band, causing the 5.8 GHz ISM band to be confused with the 5 GHz Upper UNII band, which is used with wireless LANs.
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2.5 Unlicensed 2.5.1 Unlicensed National Information Infrastructure (UNII) Bands
The 5 GHz UNII bands are made up of three separate 100 MHz-wide bands, which are used by 802.11a-compliant devices. The three bands are known as the lower, middle, and upper bands. Within each of these three bands, there are four non-overlapping DSSS channels, each separated by 5 MHz. The FCC mandates that the lower band be used indoors, the middle band be used indoors or outdoors, and the upper band be allocated for outdoor use. Since access points are mostly mounted indoors, the 5 GHz UNII bands would allow for 8 non-overlapping access points indoors using both the lower and middle UNII bands.

2.5.2 Lower Band
The lower band is bound by 5.15GHz and 5.25GHz and is specified by the FCC to have a maximum output power of 50mW. When implementing 802.11a compliant devices, the IEEE has specified 40mW (80%) as the maximum output power for 802.11a-compliant radios, reserving the lower band for indoor operation only.

2.5.3 Middle Band
The middle UNII band is bound by 5.25 GHz and 5.35 GHz and is specified at 250mW of output power by the FCC. The power output specified by IEEE for the middle UNII band is 200mW. This power limit allows operation of devices either indoors or outdoors and is commonly used for short outdoor hops between closely spaced buildings. In the case of a home installation, such a configuration might include an RF link between the house and the garage, or the house and a neighbor‘s house. Due to reasonable power output and flexible indoor/outdoor use restrictions, products manufactured to work in the middle UNII band could enjoy wide acceptance in the future.

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2.5.4 Upper Band
The upper UNII band is reserved for outdoor links and is limited by the FCC to 1 Watt (1000mW) of output power. This band occupies the range of frequencies between 5.725 GHz and 5.825 GHz, and is often confused with the 5.8 GHz ISM band. The IEEE specifies the maximum output power for this band as 800mW, which is plenty of power for almost any outdoor implementation, except for large campuses or long-distance RF links.

2.6 Licensed vs. Unlicensed
In most worldwide applications of the electromagnetic spectrum, channel frequencies are assigned under a legal license. This licensing process stringently controls the installation practices and the system performance specifications. Transmit powers have maximum limits, and the pointing of the system antennas is carefully regulated. These procedures ensure that most of the world‘s licensed systems have a reasonable chance of operating independently and securely, without having to worry about other legal signals interfering. In recent years, unlicensed communications system designs have been allocated to certain

Figure 2.8: different band of RF spectrum
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narrow frequency spectrum bands previously reserved for a variety of industrial, scientific and medical (ISM) applications. ISM bands are authorized under FCC Rules, and can be used for unlicensed communications applications. FCC Part 15.247 covers the 2.4 – 2.5 GHz band. For the bands at 5.7 – 5.8 GHz, operating under FCC Part 15.401, higher powers are permitted using directional antennae which increase the chances of unlicensed communication systems interfering with other systems. Unlicensed systems do not require FCC approval for their purchase and installation. Instead, the system equipment manufacturer must certify that the operating equipment meets ―Type‖ approval, which insures that proper use is made of the frequency spectrum and the emitted power envelope.

2.7 Radio frequency interference Background
Since the earliest days of radio communications, the negative effects of interference from both intentional and unintentional transmissions have been felt and the need to manage the radio frequency spectrum became apparent. In 1933 a meeting of the International Electro technical Commission (IEC) in Paris recommended the International Special Committee on Radio Interference (CISPR) be set up to deal with the emerging problem of EMI. CISPR subsequently produced technical publications covering measurement and test techniques and recommended emission and immunity limits. These have evolved over the decades and form the basis of much of the world's EMC regulations today. In 1979, legal limits were imposed on electromagnetic emissions from all digital equipment by the FCC in the USA in response to the increased number of digital systems that were interfering with wired and radio communications. Test methods and limits were based on CISPR publications, although similar limits were already enforced in parts of Europe.

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In the mid 1980s the European Union member states adopted a number of ‗New approach‘ Directives with the intention of standardizing technical requirements for products so that they do not become a barrier to trade within the EC. One of these was the EMC Directive (89/336/EC) and it applies to all equipment placed on the market or taken into service. Its scope cover all apparatus ―liable to cause electromagnetic disturbance or the performance of which is liable to be affected by such disturbance‖ This was the first time there was a legal requirement on immunity as well as emissions on apparatus. And although there may be additional costs involved for some products to give them a known level of immunity, it increases their perceived quality as they are able to co-exist with apparatus in the active EM environment of modern times and with fewer problems. Many countries now have similar requirements for products to meet some level of EMC regulation.

2.7.1 Radio frequency interference
Radio frequency (RF) is an electromagnetic wave that oscillates between the audio and infrared in the electromagnetic spectrum. Many natural RF signals exist in nature and indeed travel throughout the universe. But typical radio frequency interference (RFI) is defined as any ―unwanted‖ signal received by a device that prevents clear or best ―wanted‖ signal reception. This means that RFI can affect any signal receiving system such as televisions, computers, audio and security systems, and even automatic garage door openers. In the wireless world, interference, by definition, is a situation where unwanted radio signals operate in the same frequency channels or bands - i.e. they mutually "interfere," disrupt or add to the overall noise level in the intended transmission. Interference can be divided into two forms based on whether it comes from your own network(s) or from an outside source. If the

interfering RF signals emanate from a network under your control, whether it is on the same cell tower or several miles away, it is termed "self-interference." If the opposing signals come from a network, device, or other source that is not under your control, it is termed "outside interference." Thus, the definition of what type of interference is being combated is not based on technology,
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but ownership. In licensed bands, it is self-interference alone that must be taken into account; however given a more or less known operating environment (the radio spectrum will only have signals transmitting that are under control by a single entity) proper product design and network deployment can reduce these interferers to a level where they do not impact network performance. Self-interference is not a phenomenon that is confined to licensed band operations; license-exempt bands must address the same issues. The techniques and design elements of a given product that serve to reduce and tame self-interference in licensed band operations can be applied directly to the license-exempt systems. Taking advantage of the experience gained in cellular network product design, the Motorola Canopy BWA system embodies many of these same features and deployment guidelines in delivering reliable, self- interference-free operation.

2.7.2 Causes
The FCC handles a wide range of signal interference complaints, RFI is but one of them. And RFI is hard to track down. This is because any device that generates or uses radio frequency as part of its operation can also cause radio frequency interference. The interference is broadcast through the air as radio waves or conducted through power lines. Naturally, communication equipment can cause interference, as can microwave ovens, computers, street lamps, aquariums, lighting systems and other electrical devices and equipment. So, the interference may not necessarily originate with your neon signs. In fact, it may not even be your business causing the problem. One of our clients once thought a sign we made for him was interfering with his photographic and film development equipment but the interference changed in pitch. As it turned out, a construction company working down the street was using an industrial saw that generated noise that traveled through power lines into the photo shop. The only solution was to isolate which construction equipment caused the interference then schedule certain off-hours for its use. Most modern home electronic equipment and devices contain design features and circuitry to filter harmful radio signals and any subsequent electrical interaction. But often-inexpensive equipment houses inadequate filtering circuitry. This causes most complaints and the subsequent solution to land squarely back in the complainer‘s lap since the FCC states that home electronic
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equipment ―must accept any interference received.‖ And local authorities have no power to assist; they are preempted by federal law to act even if the transmitter is operating illegally, unlike complaints of loud music or use of noisy machinery. Hence, all RFI charges must be filed with the FCC. But here‘s an inside tip, sometimes you can get home electronic equipment manufacturer‘s to provide RFI filters at no charge. It‘s worth a shot if you are the recipient of unwanted signal noise. But in the case of our complaining HAM radio operator, his allegation took precedence in the eyes of the FCC. There‘s another kind of unwanted noise called sixty-cycle interference. It‘s more common than broadcast interference and is one type of signal noise that neon is notorious for producing. Usually, it‘s overheard on telephones and gets louder the closer the phone gets to the problem neon. The handset or handset cord picks up the interference because it acts like an antenna. The best way to rid this trouble is to house all conventional transformers and as much wiring as possible in well-grounded metal housings. Adding extra insulation to high-tension wiring prior to encasing in grounded metal will go a long way in preventing arcing. Arcing too produces RFI as well as the better-known neon failure or the least liked neon caused fire.

Figure 2.9: Variety of possible interferers
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Interference results from a variety of sources, usually other transmitters in the area, both licensed and unlicensed. Whether licensed or unlicensed, sources of interference cause the same results— impaired system performance. The only difference is that there are more potential uncontrolled sources of interference in the unlicensed bands. Figure 2.9 diagrams a variety of possible interferers. The ―Affected System,‖ a microwave link for an Ethernet data bridge in the 2.4 – 2.5 GHz ISM band, is shown in the center. When signals from other systems (shown as Figures 2.9 [a], [b], [c], [d], [e] and [f]) reduce the affected system‘s carrier/interference ratio (C/I) below its specification margins, the radio‘s data processing fails. [a] is a high-power broadcast signal. [b] is a leaking microwave oven in the ISM band. [c] is a similar system whose transmitter signal overflies its own receiver. [d], [e], and [f] have signals that enter by reflection, side-lobes or back-lobes.

2.8 Types of Radio frequency interference
The two major type of system generated cellular interference are co-channel interferences and adjacent channel interferences. Even though interfering signals are often generated within the cellular system, they are difficult to control in practice (due to propagation effects). Even more to control interference due to out of band users, which arises without warning due to front end overload of subscriber equipment or intermittent inter modulation products. In practice the transmitters from competing cellular carriers are often a significant source of out of band interference, since competitors often locate their base stations in close proximity to one another to provide comparable coverage to customers.

2.8.1 Adjacent channel interference
Adjacent-channel interference or ACI is interference caused by extraneous power from

a signal in an adjacent channel. ACI may be caused by inadequate filtering, such as incomplete filtering of unwanted modulation products in frequency modulation (FM) systems,

improper tuning, or poor frequency control, in either the reference channel or the interfering channel, or both. ACI is distinguished from crosstalk. Broadcast regulators frequently manage the broadcast spectrum in order to minimize adjacentchannel interference. For example, in a WLAN network with more then one AP in a single
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region cannot be configured with adjacent frequencies — that is, if one AP is configured with channel 1 then the other AP should not use channel 2. Similar restrictions formerly applied to third-adjacent frequencies as well.

2.8.2 Co-channel interference
Co-channel interference or CCI is crosstalk from two different radio transmitters using the same frequency. There can be several causes of co-channel radio interference; three examples are listed here.

2.8.2.1 Adverse weather conditions:
During periods of abnormally high pressure weather, VHF signals which would normally exit through the atmosphere can instead be reflected by the troposphere. This troposphere ducting will cause the signal to travel much further than intended; often causing interference to local transmitters in the areas affected by the increased range of the distant transmitter.

2.8.2.2 Poor frequency planning:
Poor planning of frequencies by broadcasters can cause CCI, although this is rare. A very localized example is Listowel in the south-west of Ireland. The RTÉNL UHF television transmitter systems in Listowel and Knockmoyle (near Tralee) are on the same frequencies but with opposite polarization. However in some outskirts of Listowel town, both transmitters can be picked up causing heavy CCI. This problem forces residents in these areas to use alternative transmitters to receive RTÉ programming.

2.8.2.3 Overly-crowded radio spectrum:
In many populated areas, there just isn't much room in the radio spectrum. Stations will be jampacked in, sometimes to the point that one can hear loud and clear two, three, or more stations on the same frequency, at once. In the USA, the FCC propagation models used to space stations on the same frequency are not always accurate in prediction of signals and interference. An example of this situation is in some parts of Fayetteville, Arkansas the local 99.5 FM KAKS is displaced
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by KXBL99.5 FM in Tulsa, particularly on the west side of significant hills. Another example would be of Cleveland's WKKY 104.7 having interference from Toledo's WIOT 104.7 FM on the Ontario shore of Lake Erie, as well as Woodstock's CIHR (on rare occasions), which is also on 104.7 FM, due to the signals travelling very far across Lake Erie. Co-channel interference may be controlled by various radio resource management schemes.

2.9 Factors of RFI (Radio Frequency Interference)
Because wireless signals travel through the atmosphere, they are susceptible to different types of interference than standard wired networks. Interference weakens wireless signals and therefore is an important consideration when working with wireless networking. Wireless interference is an important consideration when you‘re planning a wireless network. Interference is unfortunately inevitable, but the trick is to minimize the levels of interference. Wireless LAN communications typically are based on radio frequency signals that require a clear and unobstructed transmission path. The following are some factors that cause interference:

2.9.1 Physical objects:
Trees, masonry, buildings, and other physical structures are some of the most common sources of interference. The density of the materials used in a building‘s construction determines the number of walls the RF signal can pass through and still maintain adequate coverage. Concrete and steel walls are particularly difficult for a signal to pass through. These structures will weaken or at times completely prevent wireless signals.

2.9.2 Radio frequency interference:
Wireless technologies such as 802.11b/g use an RF range of 2.4GHz, and so do many other devices, such as cordless phones, microwaves, and so on. Devices that share the same channel can cause noise and weaken the signals.

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2.9.3 Electrical interference:
Electrical interference comes from devices such as computers, refrigerators, fans, lighting fixtures, or any other motorized devices. The impact that electrical interference has on the signal depends on the proximity of the electrical device to the wireless access point. Advances in wireless technologies and in electrical devices have reduced the impact that these types of devices have on wireless transmissions.

2.9.4 Environmental factors:
Weather conditions can have a huge impact on wireless signal integrity. Lightning, for example, can cause electrical interference, and fog can weaken signals as they pass through. Many wireless implementations are found in the office or at home. Even when outside interference such as weather is not a problem, every office has plenty of wireless obstacles. Table 2.2 highlights a few examples to be aware of when implementing a wireless network indoors.

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Table 2.2 Wireless Obstacles Found Indoors

Obstruction

Obstacle Severity

Sample Use

Wood/wood paneling

Low

Inside a wall or hollow door Inside walls Couches or office partitions Windows Windows High-volume traffic areas that have considerable pedestrian traffic Walls Outer wall construction Mirror or reflective glass Metal office partitions, doors, metal office furniture Aquariums, rain, fountains

Drywall Furniture

Low Low

Clear glass Tinted glass People

Low Medium Medium

Ceramic tile Concrete blocks

Medium Medium/high

Mirrors

High

Metals

High

Water

High

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CHAPTER-3 RADIO FREQUENCY INTERFERENCE ANALYSIS
3.1 Background
Signal interference in wireless networks negatively affects transmission coverage and mobile capacity, limiting the overall network performance. Unavoidable signal interference is becoming more prevalent in the wireless community with the increasing number of active transmitters on the RF spectrum. The spectrum is shared among different systems and services such as mobile communications, mobile radios, paging, wireless local area networks, and digital video broadcasting. In addition to the licensed systems, the spectrum is also occupied by unlicensed transmitters, reflections, and fading. The composition of all these signals is making a very complex environment which must be routinely monitored in order to maximize service performance. The effect of unwanted energy due to one or a combination of emissions, radiations, or inductions upon the reception in a radio communications system, manifested by any performance degradation, misinterpretation, or loss of information which could be extracted in the absence of such unwanted energy. More simply – ―unwanted reception that disrupts communications‖ is refers to the interference. Unless some interference is accepted, no wireless communications could occur; the spectrum would have little or no value; total avoidance of interference is impractical and counterproductive. It should be noted that whether the interfering signal is in-band or out-of-band, the signal is almost certainly coming through the antenna, down the cable, and into the affected receiver. Therefore, a spectrum analyzer connected to the operating system antenna will serve as a substitute measuring receiver which will display and help identify unwanted signals. Remember that the system‘s band pre-selection filters are inside its receiver, so many out-ofband signals are naturally present at its antenna input connector. Interference generally only affects receiver performance. Although it is possible that a source of interference can be physically close to a transmitter, the characteristics of the transmitted signal will not be affected. Thus, the first step in recognizing if interference has corrupted a receiver is to learn the characteristics of the signal that the affected system is intended to receive.
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3.2 Sources of Interference:
The sources of interference can be described by some parameters as listed bellow Co-channel Interference Adjacent Channel Interference Out of Band Emissions Spurious Emissions Intermodulation

3.3 Analysis of Radio Frequency Interference:
In this analytical part we design a network by using two Access Point (AP), where in the working part we considered channel assigning, spectrum of 2.4GHz, frequencies of the channels, co-channel and adjacent channel etc. In this design we use the D-Link AP and NetGear AP.

3.3.1 D-Link Access Point:
The figure shows the practical picture of the Access Point. First we have to input the power and connect it with a pc by RJ-45 cable. Then we use the user name and password in the Internet explorer window as describe in the use manual.

Figure 3.1: Access Point (D-Link)
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The next figure describes the configuration processes. The mode of the device is in Access Point mode. SSID is changed into DIU. SSID broadcasting enabled, frequency channel fixed into channel 11 which is 2.462GHz. And primarily we use the open system authentication system.

Figure 3.2: Primary configuration of D-Link access point.

3.3.2 NetGear Access Point:
The figure shows the practical picture of the Access Point. First we have to input the power and connect it with a pc by RJ-45 cable. Then we use the user name and password in the Internet explorer window as describe in the use manual.

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Figure 3.3: Access Point (NetGear) The next figure describes the configuration processes. The mode of the device is in Access Point mode which is 802.11g and 802.11b. SSID is changed into DIU-1. SSID broadcasting enabled, frequency channel fixed into channel 1 which is 2.412GHz. And primarily we use the open system authentication system.

Figure 3.4: primary configuration of NetGear access point
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3.3.3 Analytical software:
We use NetStumbler software in our analysis. NetStumbler is a utility that works under Windows and is meant specifically for 802.11b wireless networks. Under the 802.11 standard, most wireless hardware vendors are compatible. However, security is the most commonly overlooked element. NetStumbler has released its software in an effort to increase awareness of the inherent problems with wireless communication security, or lack thereof. The objective is to see that vendors concentrate on security while maintaining the functionality necessary in wireless products. The single Access Point (SSID DIU-1) gives the signal strength better which has no interference due to same frequency channel. The figure is showing the signal strength level around -45dBm.

Figure 3.5: signal level analysis with NetStumbler software.
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3.4 Co-channel Interference Analysis:
Co-channel interference is crosstalk from two different radio transmitters using the same frequency spectrum. In our paper we configure the access points with the SSID DIU and DIU-1 respectively and took the RF channel status from software named NetSumbler.

3.4.1 Configuration setup:
For co-channel interference analysis, the configuration setup is done by some steps. Two Access Points are given the SSID DIU and DIU-1. The NetStumbler analysis software is used here. Frequency channel assigned to the same frequency channel 11.

Figure 3.6: Channel setup for co-channel interference.
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3.4.2 Access Point Configuration:
We assign SSID DIU in the D-Link Access Point. DIU configuration setup is given below:

Figure 3.7: Access Point (D-Link) configuration

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Another SSID DIU-1 is assigned in the NetGear Access Point. DIU-1 configuration is given below:

Figure 3.8: Access Point (NetGear) configuration.

For co-channel interference, we assign the frequency channels in channel 11 which is 2.462GHz. both covered a certain ESS.

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3.4.3 RF analysis for co-channel interference with NetStumbler:
The figure shows the signal to noise ratio for co- channel interference.

Figure 3.9: Co-channel interference analysis with NetStumbler.

3.4.4 Spectrum analysis:
The 2.4GHz frequency channel spectrum is given below. The channel11 is fixed for co-channel intereference for DIU and DIU-1.

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Table 3.1: Frequency spectrum for Co-channel Interference.

Interference area is denoted by red color. By using the same frequency channel in both access points the interference is affecting the total channel. The range is 2.452GHz to 2.472GHz. Here used channel is 2.462GHz. The same channel overlapped each other.

3.5 Adjacent Channel Interference
For Adjacent channel interference analysis, the configuration setup is done by some steps. Two Access Points are given the SSID DIU and DIU-1. The NetStumbler analysis software is used here.

3.5.1 Access Point Configuration:
The frequency channel of access point DIU-1 is changed into channel 6 which is 2.437GHz.

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Figure 3.10: configuration for Adjacent Channel interference analysis. Other access point DIU is set into previous configuration setup. This is given below:

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Figure 3.11: configuration setup for adjacent channel analysis (DIU)

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3.5.2 Spectrum analysis:
The 2.4GHz frequency channel spectrum is given below. The channel11 is fixed for co-channel interference for DIU and DIU-1.

Table 3.2: Frequency Spectrum for the Adjacent Channel Interference

The table shows the network is assigned by adjacent channel. Channel 6 and channel 11 are assigned to the access points. Although the frequency channels are not same, the higher order harmonics of the channels causes interference which is shown in the NetStumbler analysis Software.

3.5.3 RF analysis for Adjacent channel interference with NetStumbler:
After setup the all procedure the signal level is analyzed by NetStumbler software. The interference is greater than the co-channel interference. The figure shows the signal to noise ratio for Adjacent Channel interference:

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Figure 3.12: signal to noise ratio for Adjacent Channel Interference. Now if we assign a channel to the channel 10 then the interference increases. The configuration given below:

Figure 3.13: channel changing for Adjacent Channel Interference.
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3.5.4 Spectrum for this configuration:
The frequency channel set to the channel 6 and channel 10. The 2.447GHz frequency is overlapped. The spectrum is given below: Table 3.3: Interference analysis for the adjacent channel (overlapped).

The RED area shows the interference area which is at the channel 8 or 2.447GHz frequency. So the SNR increased than the adjacent channel without overlapped. The signal strength for these types of interference taken by NetStumbler software is given below:

Figure 3.14: signal to noise ratio for Adjacent Channel Interference.

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From the figure we can see the power level variation is from -45dBm to -65dBm which is caused by the overlapped adjacent channel interference.

3.6 Spurious Emissions:
Emissions on a frequency or frequencies which are outside the necessary bandwidth and the level of which may be reduced without affecting the corresponding transmission of information. Spurious emissions include harmonic emissions, parasitic emissions,

Intermodulation products and frequency conversion products, but exclude out-of-band emissions.

3.7 Out-of-Band Emitters
The following paragraphs describe some typical sources of interfering signals and their effects. Even though the affected system is designed to reject signals outside its assigned band, there are effects caused by out-of- band emitters which can impact the in-band performance.

3.7.1 Desensitization
When a high-powered transmitter, such as a UHF TV broadcast station, is nearby, the affected receiver can be driven into RF overload even though its signal is well out-of-band. See Figure 1[a]. This happens when the affected receiver‘s pre-selection filter is not adequate. The high power signal leaking into the affected receiver will drive the operating point of the front-end amplifier up through its dynamic range characteristic. This destroys the normally-required linear amplification process, introducing inter modulation distortion and serious data errors.

3.7.2 Intermodulation:
The production in a nonlinear element of a system and of frequencies corresponding to the sum and difference frequencies of the fundamentals and harmonics thereof that are transmitted through the element. Intermodulation can be defined by some points: Intermodulation arises as a result of transmitter harmonics or the mixing of two or more signals.

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The mixing product from two or more signals can be including fundamental frequencies or harmonic frequencies. Harmonic fundamentals – 2 *F1, 3*F1, etc Mixing of two or more frequencies –IM2: F1 ±F2 – IM3: 2 * F1 ± F2; F1 ± 2 * F2, F1 + F2 + F3, F1 – F2 + F3, F1 + F2 – F3, F1 – F2 –F3 –IM5, IM7, etc Not a clearly defined bandwidth, may have ―fuzzy‖ RF envelope Bandwidth may shift as the fundamental transmitters deviate May be intermittent as one or more of the contributing transmitters turns on and off May cover up entire protected band if IM product is of higher order or protected band is narrow. The low order mixing products may fall close to the signal of interest this leads to difficulty in dealing with the interfering signal. The IM product could be generated by combinations of internal sources transmitted, internal sources received, and external sources.

3.7.3 Intermodulation Signals from Non-linear Power Amplifiers
Modern wireless systems receive, transmit, and process hundreds of channels of voice or data at a

common base station. Most of those channels a reprocessed via common antennas and broadband power amplifiers at the final stage for cost effectiveness. The multiple channel signals are combined in front of the final power stages and then amplified together. The linearity specifications of those final power stage amplifiers are very tight since a non-linear char- act
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eristic can cause cross-frequency signal products to be produced and emitted. Those cross-

frequency signals might cause interference within their own system‘s operating band or cross over into other systems.

3.7.4 Intermodulation from Non-linear External Elements
This interference mechanism is called the ―rusty fence‖ syndrome. If two high-powered transmitted signals, f and f, impinge on some random rusty 1 2 element such as a steel fence, a rusty metal roof, or even corroded coaxial cable elements, an electric effect sometimes takes place. The corrosion junction acts like a rectifying diode and mixes all the transmit signal shitting it. These results in a whole list of new signals, called intermodulation products, which are re- transmitted. These signals are mathematical combinations of the original transmitted signals, such as (f – f ), 1 2 (2f – f ), (3f +2f ), etc. 1 2 1 2 While this effect is typically a random problem, there are certain transmitter frequency assignments which and f signals have exact cause the mixed products to fall right on top of other assigned bands. When the f 1 2 frequency spacing equal to the affected receiver‘s input, it tries to accept these re-transmitted intermodulation products as its own inband data.

3.8 In-Band Emitters
Non-licensed wireless Local Area Networks (LANs) using frequency-hopping (FHSS) or directsequence-spread- spectrum (DSSS) techniques spread the useful data modulation over a wider band. They operate in the ISM band which is also the home frequency of the typical microwave oven. Microwave ovens operate at the 2.4 GHz resonance frequency of the water molecule. Although the spread spectrum modulation schemes defend against interference from the oven leakage, the location and power level of the leakage may overcome the interference resistance.

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3.8.1 Harmonic and Parasitic Outputs
If broadband output power amplifiers are driven far into saturation, signals begin to get compressed resulting in more than the intermodulation effects. The signal clipping produces harmonics of the

broadband transmit signal and these spurious signals coming out of the antenna might interfere in other receiver pass bands. Parasitic signals are caused when a power amplifier deteriorates into a random oscillation mode. Ham radio transmitters deliver high power and can occasionally degrade to the point of emitting unintended spurious outputs.

3.8.2 Overlap of Antenna Patterns
Antenna pattern overlaps between communication systems frequently occur. There are a variety of interference causes attributed to antenna pattern conflicts.

3.8.3 ISM System Antenna Alignment Problems
In non-licensed systems, users simply install their newly purchased equipment without any licensing needed. This commonly occurs in business parks where companies install data links

between office buildings. It is easy to foresee how one system signal becomes an interferer in an affected receiver. The signal pattern where the intended transmit signal over flies its own receiver and comes right into the aligned antenna pattern of the affected receiver. While modulation designs are supposed to offer some rejection of interference due to different frequency- hopping parameters or different DSSS code patterns, it is possible that the interfering signal levels at the Affected receiver might still overwhelm the rejection tolerance of the modulation scheme. It should be noted that even if the antenna pattern lobes of the affected system are relatively narrow (high gain), there is still considerable sensitivity to signals that are as much as 20 to 30 degrees off bore sight.

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3.8.4 Back lobes and Side lobes
There are side lobe and back lobe characteristics in every antenna. This means that interfering signals might cause problems if they enter one of the side lobes or the back lobe of the affected system. Typical side lobe and back lobe sensitivity is only 15 – 30 dB down from the main lobe.

Figure 3.15: Back lobes and side lobes

3.8.5 Reflections and Fading
The affected system often operates in signal environments which affect its system signals. Heavy rainfall attenuates microwave frequencies. Buildings, hills, and other natural obstructions bend or cause multiple paths to form between transmitter and receiver. These multiple paths, or multi paths, lead to destructive signal cancellations and cause random fades in signal strength. Other buildings might reflect interference into the side of the affected antenna‘s main lobe. Low flying airplanes can cause a moving reflection which might degrade data randomly.

3.8.6 Cellular Antenna Overlap
Cellular systems, with their theoretical hexagonal base station cell pattern spacing, take advantage of frequency band re-use by assigning the same frequencies to cells that are spaced just one cell distance away. As such, any given cell antenna that happens to be misadjusted for tilt can easily over fly the adjacent cell and impinge on an affected receiver two cells over where the signal frequency assignments are the same. 54

CHAPTER-4 OPTIMIZATION

4.1 RFI Optimization:
Wireless Local Area Network (WLAN) is currently among the most important technologies for wireless broadband access. The IEEE 802.11 technology is attractive for its maturity and low equipment costs. The overall performance of a specific WLAN installation is largely determined by the network layout and the radio channels used. Optimizing these design parameters can greatly improve throughput and system performance. A thorough analysis of the performance of both 802.11b and 802.11g products in common network deployments can help alleviate the confusion and help readers make an informed choice. The analysis will use published information, published data and the system performance parameters readily available from the Federal Communications Commission (FCC), International Telecommunications Union (ITU) and the Institute for Electrical and Electronic Engineers (IEEE).

In the previous chapter we have shown and analyzed several types of interference. This chapter describes the optimization processes. We have to select the frequency channel carefully to reduce the adjacent channel and co-channel interference. There are 14 channels in the 2.4GHz frequency spectrum which are referred to the licensed free bands. As it is licensed free, many other devices use the same frequency band which causes interference.

At first glance the task of a network planner is to build up an infrastructure which offers sufficient coverage to fulfill the given requirements concerning the capacity demands. Due to the low number of available non-overlapping frequency channels (especially with standards using the 2.4 GHz band), the problem of co-channel interference has a major impact on the performance of the network and should therefore also be considered. This results in the need to also include the carrier assignment in the planning process depending on the standard used for the network installation.
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For optimization, we have to go through some steps. The steps of optimizing the Radio Frequency Interference in a WLAN are described in this chapter.

4.2 Cell Design
The designing of an indoor WLAN is done by two access points (D-Link and NetGear). So one Extended Service Set (ESS) is created in the area. Access Point placement or location selection is important part to get the better performances. In our university campus, we use to cover the 2 nd floor with Wi-Fi network. Access point distance, frequency channel assignment, SNR, signal strength, coverage the whole floor with reliable bandwidth etc. are the important factors to design a WLAN in home or office.

Figure 4.1: Cell design or Access Point placement.

4.3 Frequency assignment
The 2.4GHz licensed has the 13 frequency channels. For designing issue, we have to assign the frequency channel carefully which should be non overlapping channel.

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Figure 4.2: 2.4GHz frequency spectrum.

IEEE 802.11g uses spectrum around 2.4GHz that is divided into 13 channels with center frequencies 5MHz apart. (Channel availability varies across geographical regions due to different spectrum regulations.) In addition to the center frequencies, the standard specifies a power envelop by which the signal must drop by at least 30dB below peak energy at ± 11MHz and by at least 50dB at ± 22MHz from the center frequency. Channels at least 24MHz apart are often considered to be non-overlapping. This yields at most three non-overlapping channels, i.e., channels 1, 6, and 11. Therefore, channel assignment for IEEE 802.11g Wireless LANs is usually understood as a frequency assignment problem with three available frequencies. However, this view is simplified. First, a powerful transmitter operating on channel 1 can effectively interfere to those operating on channel 6 or even channel 11. Second, three channels are insufficient for WLANs with high stations density. Third, in some countries, e.g., Spain and France, the total number of available channels does not exceed four and all the channels mutually overlap. Moreover, the network planner may want to avoid using some channels at a certain AP to limit interference coming from neighboring WLANs. To resolve these issues, a realistic channel assignment model for WLANs planning must be flexible in choosing a set of available channels and must be able to consider solutions with overlapping channels.

4.4 Power control
Transmitter power control, however, can also be used to control co-channel interference, i.e. interference from other users using the same channel (code or time slot). For this purpose, the optimum transmitter power configuration is found by solving an Eigen value problem. The basic
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models as well as the concepts of achievable C/I, up/down-link equivalence and C/I- balancing are introduced. Both the interference limited (noise-less) cases as well as models including thermal noise are treated. Results show that substantial improvements in system capacity can be achieved, particularly in conjunction with Dynamic Channel Allocation.

Controlling the transmitter power level has been a frequently used tool in many cellular communication systems as well as wireless local area network. In most modern system, both base stations (here access point) and mobiles (client device) have the capability of real time (dynamic) adjustment of their transmitter powers. There are several reasons why this tool may be effective in order to enhance the performance (the capacity) of a wireless system: Enhanced adjacent channel protection: In radio systems with receivers with limited "dynamic range" it is necessary to combat "near-far" problems, where two signals on separate "channels" but with a large difference in signal level may interfere. Typical applications are the DS-CDMA systems that have been proposed lately. These systems suffer from what could be called "adjacent code" interference. The aim of the power control scheme in these systems is to maintain the received powers from all mobiles within a cell at a constant level and thus compressing the dynamic range of received signals.
Co-channel Interference Management: This type of interference is caused by spectrum reuse and will thus be present also in systems with perfectly orthogonal signals (for instance "ideal" TDMA, FDMA). By proper power adjustment, the detrimental effects of co-channel interference can be reduced. This allows for a more "dense" reuse of resources and thus higher capacities.

The last item, the possibility to manage co-channel interference by means of dynamic power control. The objective of these schemes is to choose the transmitter powers in each base-mobile link such that a sufficient transmission quality (signal-to-interference ratio) is maintained in all communication links. It is now well known that the two of the objectives listed above, in general, lead to different power control schemes. In particular, achieving a good adjacent channel protection by maintaining a constant received power will result in little or no reduction in the cochannel interference.
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We are now trying to describe some processes of the co-channel interference and adjacent channel interference optimization techniques according to our interference analysis.

4.5 Co-channel interference optimization
The contributions of our work are the following. First, we present analytical model that can be used for the two-step sequential planning of WLANs. For the first step, we took the analytical data from the network by choosing proper locations for a given number of APs (here two APs are used). For the channel assignment step, we use the unlicensed frequency band which is 2.4GHz and select frequency channel from 13 channels. We faced two interference problems in the WLAN. We measured the signal strength and interference by software. And in this chapter suggest some techniques to minimize co-channel and adjacent channel interference.

For channel overlapping co-channel interference occurred. The interference area is large; the total channel faces the interference. Interference area is from 2.452GHz to 2.472GHz. Table 4.1 shows the co-channel interference area in red color.

Table 4.1: Frequency Spectrum for Co-channel Interference.

So channel overlapping is an important part of any WLAN network performance. We have to choose the frequency channel carefully to minimize the interference. To reduce the interference area, we have to select a non-overlapping channel. In our practical implementation, we have chosen two non-overlapping channels. One is channel 6 (2.4737GHz) and the other one is channel 11 (2.462GHz). The table 4.2 shows the non-overlapping frequency channel spectrum. There is no interference area and from analysis we showed that the interference is reduced.

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Table 4.2: Non-overlapping frequency spectrum for optimization

4.6 Adjacent Channel Interference optimization
Adjacent Channel interference or ACI is interference caused by extraneous power from a signal in an adjacent channel. ACI may be caused by inadequate filtering, such as incomplete filtering of unwanted modulation products in frequency modulation (FM) systems,

improper tuning, or poor frequency control, in either the reference channel or the interfering channel, or both. ACI is distinguished from crosstalk. Broadcast regulators frequently manage the broadcast spectrum in order to minimize adjacentchannel interference. For example, in a WLAN network with more than one AP in a single region cannot be configured with adjacent frequencies — that is, if one AP is configured with channel 1 then the other AP should not use channel 2. Similar restrictions formerly applied to third-adjacent frequencies as well. Non-overlapping but adjacent channels create interference by the higher order harmonics of the signals. It is illustrated in table 4.2.

If we select frequency channel 6 and channel 10, than there is a overlap in between 2.447GHz. Table 4.3 shows the spectrum of these kinds of adjacent channel interference. The red area shows the interference area.

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Table 4.3: Interference Area of Adjacent Channel.

To optimize these types of problems, we have to select the frequency channel carefully. If we select channel 1 and channel 11, then there will be no chance for interference because the channels are far from each other. The higher order harmonics cannot interfere and there is a huge band gap. Table 4.4 shows the channel selecting for optimizing the adjacent channel interference. Table 4.4: Channel selection for interference optimization.

If we have to enlarge our network, we can easily do it by using these 2.4GHz channels. But we can reuse frequency channels assembled by frequency reuse factor.

4.7 Methods of Dealing with Interference- The Physical Layer

4.7.1 Modulation and the C/I Ratio
At the most fundamental level, an interfering RF source disrupts the digital transmission by making it too difficult for the receiving station to "decode" the signal. How much noise or interference a digital RF transmission can tolerate depends on the modulation used. Fundamentally, modulation is the method whereby zeros and ones are communicated by varying one of three aspects of a radio signal. The three portions of an RF signal that can be changed or modulated are phase, frequency, and amplitude. Shifting the properties of any of these
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parameters can be used to communicate different "states." These states, in turn, are translated to zeros and ones for binary communications. For example, with frequency modulation, if the sine wave is at frequency one, it is decoded as a zero. If the sine wave is shifted slightly to frequency two, this is decoded as a one. This type of modulation is referred to as Binary FSK (BFSK), or Frequency Shift Keying. In this example, a system must only be able to tell the difference between one of two states or phases. More complex modulations, such as 16QAM(Quadrature Amplitude Modulation), attempt to differentiate among 16 different possible states of an incoming signal. The advantage to 16QAM is that it conveys more information per bandwidth or more bits/Hz. The disadvantage lies in the fact that, in order to distinguish among the 16 different states, the signal must be very clean and very strong relative to background noise or, even more important, interference. The ability of a receiving station to decode an incoming signal at the most basic physical layer is dependent on a factor called the "carrier to interference ratio," or C/I. This fancy-sounding term means exactly what it says: how strong the desired signal (the carrier) is relative to the unwanted signals (the interference). C/I ratios are based primarily on the modulation used, with more complex

modulations requiring higher C/I numbers than more robust modulations, such as BFSK. The Canopy product employs BFSK for modulation. With this modulation the C/I ratio

necessary to operate properly with an error rate of 1x 10-4 bits per second is only 3dB; i.e. the wanted signal need be only 3dB higher in power than the unwanted interferers. A system operating with 16 QAM at these levels would require a C/I ratio of roughly 12 to 14dB. Putting this into perspective, with every 3dB of additional signal strength, the power of a signal is doubled. This means that the Canopy system, with its C/I ratio of 3dB, can tolerate an interfering signal that is many times more powerful than a 16QAM system and still operate at the specified error rate. Whether the interference is from another cell site on the network or another network completely, the Canopy system employing BFSK modulation will tolerate substantially higher levels of interference before the communication stream becomes impacted. All other PHY layer techniques are designed to improve this most fundamental measurement of network robustness and operational effectiveness by sustaining the necessary C/I level.

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4.7.2 Antennas
When a BWA signal is followed from end to end, it leaves the radio and travels first through a transmitting antenna, over the air to a receiving antenna, and into the radio. The antenna, an important component in the RF chain, can also have an impact on how well the network tolerates interference, both internal and external. Antenna performance is specified in a variety of ways, but for the purpose of this discussion, the one that is most important is the front- to- back ratio. The front-to-back ratio of an AP antenna indicates how much of an incoming signal will be absorbed coming into the front of the antenna as compared to how much of a signal arriving at the back of the antenna is absorbed. When deploying networks in a cellular topology, the performance of the antenna in rejecting unwanted signals from behind is an important feature. The Canopy system, with its integrated antennas at the AP, has a front-to-back ratio of 20dB. Coupled with the excellent C/I ratio, this means a Canopy AP receiving a signal at threshold (the weakest signal it can still detect) can be hit with an interfering signal from behind, either internal or external, on the order of -60dBm and still support connections at an acceptable error rate.

4.7.3 TDD Synchronization
BWA networks that use Time Division Duplex for separating upstream and downstream communications are ideally suited for asymmetric traffic, such as data. The ability to adjust the amount of bandwidth dedicated for upstream and downstream communications without changing hardware is a powerful feature. TDD systems operate by transmitting downstream (from the AP to the SM) for a period of time 1ms for example. Following a short guard time, the SMs then transmit on the same frequency in the upstream. For a cell site with more than one radio operating in TDD mode, it is important that all the sectors of the cell transmit and receive at precisely the same time. Otherwise, if sector 1 is transmitting when sector 2 is receiving, sector 2's incoming transmission can be interfered with even if they are on different frequency channels because the sector 1 signal is so close it is strong enough to "flood" or overwhelm the electronics in sector 2. When deploying a TDD system in a cellular topology, it is desirable to be able to use the same frequency in each cell site even though those cell sites are possibly several miles away.
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This means sector 1 from AP A may interfere with sector 1 of AP B. The frequency planning used for the Canopy product is displayed in Figure 2, showing how these signals can interfere. In this case, inter-cellular synchronization is required, making sure that all the sectors in all the cell sites are properly timed and synchronized in terms of downstream and upstream communications. Delivering tight synchronization across potentially hundreds of square miles can be a challenge. With the Canopy system, designed for large scale, dense network deployments, TDD synchronization is a critical requirement. This has been solved with the use of a GPS signal. These precise satellite signals are used for timing and, ultimately, transmit/receive synchronization, thus tying all sectors in a Canopy network to the same "clock."

Figure 4.3: Frequency planning in a cellular deployment.

4.8 Methods of Dealing with Interference- The MAC Layer
Up until this point the discussion has centered on various physical layer techniques for addressing interference. Often overlooked, but as important in combating unwanted signals, is the design approach taken in the MAC layer.

4.8.1 Frame/Slot size
A typical MAC frame for a TDD system such as Canopy is shown below. As can be seen, the upstream and downstream portions of the frame are divided into slots, each slot carrying what
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can be termed a "radio data packet," or RDP. The original data, an IP packet datagram, for example, is segmented into packets that fit into a RDP. Despite all the best deployment design and use of the extremely robust Canopy system, there will be instances where interference will overcome these measures and corrupt a MAC frame or a portion of a MAC frame. When this happens, the corrupted data must be sent again. If the

MAC frame is designed for large RDPs on the order of several hundred bytes, the entire slot must be re-transmitted even if only a small amount of this packet is damaged. The impact on network throughput as a result can be large, with a few bytes in error causing hundreds of bytes to be re-sent. Canopy solves this problem by using RDPs sized at 64 bytes. With this smaller RDP size, the re-transmission can be contained to only those bytes that were damaged, thus avoiding the re-send of large chunks of valid data.

Figure 4.4: TDD MAC Frame

The 64-byte slot could have been made even smaller, but as RDP size decreases, the slot header which is fixed becomes a more significant portion of the packet data, hence increasing the MAC layer overhead. In addition the 64-byte slot is ideally sized for handling the TCP acknowledgements sent for most IP packets.

4.8.2 Automatic Retransmission Request
As discussed earlier, small amounts of interference can have large impacts on end-to-end network performance. This is tied to the way TCP/IP networks were designed to operate in the
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wired world. This protocol was designed to operate over wire, where interference was assumed to be negligible. The protocol design calls for a positive acknowledgement sent from the receiving station to the sending station for every IP packet sent out. If the sending station does not receive the TCPACK in a certain amount of time, it is assumed that the cause was congestion of the network - not an error resulting from transmission impediments. When encountering congestion, TCP responds by dramatically slowing down the transmission and then increasing transmission speed slowly. In a BWA network, a lost or corrupted packet is not due to congestion but interference. But the TCP protocol has no knowledge or ability to account for higher error rates, and responds by slowing down the end- to-end data rates. This is the phenomenon that can multiply a small amount of RF interference into significant network degradation. In the Canopy networks, this is not a factor. Recognizing the dilemma of

combining TCP/IP with wireless networks and the attendant error rates, the Canopy system solves the problem with a feature called Automatic Retransmission request or ARQ. ARQ actually inspects the RDPs that come into the receiving SM and looks for errors. If an error is detected, the SM (or AP) will send a request to the sending entity to re-send the RDP. All of this is accomplished two layers below TCP in the protocol stack. The net effect is that as far as TCP is concerned, it never receives a packet of data with an error as a result of the wireless portion of the network. This prevents TCP from invoking the slow start algorithm, keeping the end-to-end data rates at the peak or just slightly below peak operational rates.

4.8.3 Centralized Transmission Control
Some BWAMAC protocols, such as that used in the IEEE 802.11 standard, operate in what is referred to as a distributed control manner. This means that each SM has the ability to send a packet at its own discretion. Typically in this scenario the SM will "listen," and if it does not hear any transmissions, it will assume the channel is clear and send its data. The problem arises if the sending SM cannot hear other SMs. In this instance, two or more SMs may send a packet at the same time, corrupting both and causing a retransmission. Interference is also a culprit in blocking SMs from hearing each other with the same effect. Canopy solves this problem by implementing a demand contention access scheme where the AP controls all
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transmissions in the sector, both upstream and downstream. An SM will only send its data when allowed. If an SM's request to send data is interfered with, it will wait and try again, but at no point will it ever transmit into the uplink data channel until it is permitted by the AP.

4.8.4 PMP Applications – Cellular Deployments
Cellular deployments are chosen when a service provider decides that they want to deliver ubiquitous BWA service to a large area. The benefit of a cellular approach lies in the fact that all of the area will be covered uniformly. The downside lies in the increase in up-front planning and design required. For systems with less robust modulation schemes, the C/I requirements often drive complex frequency plans. This is because before a given frequency channel can be used again at a second cell site, it has to be far enough away to satisfy the C/I requirement. [As noted in sidebar below, every time the distance is doubled, the signal fades by 6dB.] Thus, to ensure the "interfering signal" from cell site A does have not an impact the next time that channel is used, its signal level strength must be reduced. This is accomplished by ensuring it is several cell radii away (actual distance also depends on antennas and other factors such as line of sight) from any of the other sites using that channel. The net effect for a cellular deployment with a system using higher- order modulations typically means more channels are required in order to satisfy the C/I ratio. Thus the ability to deploy efficiently in a cellular format depends on many factors, such as the C/I ratio, antenna performance, and TDD design and synchronization. All of these factors, as well as ease of design for the network, are clearly understood by the Canopy design team, and have been addressed in the Canopy product.

4.8.5 PMP Applications - Spot Deployments
Many BWA deployments start out being installed in what is referred to as a "spot deployment" model. This topology refers to a single cell site, or possibly several, that are not geographically contiguous but are chosen to serve specific areas of need. This is contrasted with a cellular deployment approach in which the goal is to provide BWA coverage across an entire region and hence the cells or AP sites are deployed such that there are no LOS gaps in coverage.
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Figure 4.5: Poor coverage in a dense deployment. When a BWA system is deployed in a spot method, frequency coordination and planning are not usually an issue, and each cell site is installed with what is best for that area of coverage alone as a deployment guide. For spot deployments, a problem arises when an operator starts adding many AP central locations to an area. As is explained in the cellular deployment model above, self-interference from neighboring AP‘s or SMs on the same frequency channel rapidly becomes a problem. Because little if any frequency planning was performed as the sites were rolled out, the problem with self-interference can become very serious very quickly.

Figure 4.6: Canopy coverage in a dense deployment.
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The result is that operators will be severely constrained in where and how close they can install additional AP sites. As is shown in Figure 4a, this will result in large coverage gaps. The Green shaded areas can be served by the AP sites shown. Red represents areas that can't be served due to interference between AP sites. Large gaps in coverage not only prevent a service provider from serving potentially lucrative areas of interest, but also leave the public at large with a poor perception of the service. It only takes a few potential customers to hear, "I'm sorry, we cannot serve your business" to get the word out that the service is "not real." The Canopy system's design minimizes these impacts, allowing the service provider to grow the network far beyond less-robust BWA systems. With Canopy, an operator utilizing a spot deployment methodology will be able to significantly increase the number of AP sites without the coverage gaps of other BWA networks. Fundamentally, more AP sites means greater coverage, which means more

customers per AP. The net effect is simple -- more revenue per square mile. Figure 4b shows a dense deployment using the Canopy system. Note that vast increase in coverage area (shown in green).

4.8.6 PTP/Backhaul Applications
While most of the talk in BWA surrounds PMP networks and products, a large percentage of these systems will be located in areas where the infrastructure is not well developed. The location of the AP site is chosen based on where potential customers are located, where a good high tower or building can be used, etc. What does not factor into this decision is the question of where is there a good connection to the Internet or a private network. Most times the AP will be located such that there is no significant copper or fiber connection into the WAN. It almost seems ludicrous that BWA networks can be hindered by the lack of wire-line infrastructure to connect the data aggregated at the AP to the WAN. Nonetheless, it is a reality that for BWA systems to be effective, they need to have a PTP solution in the arsenal to provide the AP-toWAN connection when no wire-line options exist. Figure 5 depicts the Canopy solution. The issue with BWA PTP links lies in the fact that they carry all the traffic for a AP site, or data from possibly hundreds of customers. The net effect is that this link becomes both mission critical and a single point of failure - a dangerous combination.
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Figure 4.7: Canopy AP site with backhaul.

For the same reasons a service provider may have chosen license exempt equipment for their BWA deployment, they will likely need a license-exempt solution for the backhaul or PTP link. This places a burden of reliability and robustness on the PTP connection. All of the features described previously that are implemented in Canopy to "harden" it against interference in PMP applications are applicable to the PTP application. While the likelihood of interference in PTP links is substantially less (due to two narrowly focused antennas, as discussed above) the impact is substantially greater. When the Canopy solution is used in a PTP application, the system employs a slightly different MAC, reducing overhead and delivering more use- able or sellable traffic on the link. The Canopy PTP solution is not limited to just backhauling traffic from Canopy cell sites, although they have been designed to work together interference free. With such a robust

approach to wireless communications, the Canopy PTP system has the reliability necessary for connecting cellular telephony as well. Indeed, any time a high capacity dedicated connection that has increased reliability requirements occurs, the Canopy PTP solution can serve the need. Schools, government, enterprise campuses separated by a road - all are candidates for this approach. No longer will leased lines be needed to sup- port mission-critical applications between two points. Not only are PTP wireless links more economical, they are also more scalable, allowing for users to deploy the amount of bandwidth needed.
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CONCLUSION

In this paper a new approach is described to optimize WLANs in regard of coverage, interference and required availability. It may be necessary for the network planner to vary planning parameters and check the corresponding results. However, due to the short computation time, this is not a major drawback. In this report, first we have theoretically studied the technical details about WLAN, RF properties, antenna devices, and network architectures. After that we have done our practical experiment in our lab with Access Points namely D-Link, NetGear, and laptops with Wi-Fi device and using these devices we created two different ESS of WLAN network. After establishing the network connection we measured the signal strength using NetStumbler software at different places by moving our laptops and also measured the performance of WLAN which is very cost effective and reliable comparing with wired technologies for indoor applications. Our main issue was to deal with different types of interference in the WLAN network and to improve the performance. The causes of different interference like Adjacent channel interference, Co-channel interference and many other causes we described in the 2nd and 3rd chapter. To know the interference we have to know the properties or characteristics of radio frequencies. In the analytical part, we analyzed the signal strength of WLAN using Net-Stumbler software and tried to measure different types of interference. We showed the area of interference caused by the co-channel and adjacent channel. In optimization part, we described how to optimize and get better performance by implementing the access point placement, frequency channel assignment, selecting non-overlapping channel etc. We have seen that dynamic transmitter power control is an efficient tool for managing cochannel interference and thereby improving the capacity of cellular systems. Clearly any optimum combined DCA/power control scheme should maintain C/I-balance on all channels.

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REFERENCES
[1] White paper, ―Controlling Interference: The Motorola ® Canopy™ Approach‖, August 2002 ―Spectrum Monitoring and Interference Analysis‖ –by James West – Summitek Instruments (IEEE Denver Section TPS Workshop, April-2005)

[2] AEIN, J.M., "Power balancing in systems employing frequency reuse", COMSAT Tech Rev, Vol 3, No 2, Fall 1973.

[3] FUJII, T., SAKAMOTO, M., "Reduction of Cochannel Interference in Cellular Systems by Intra-zone Channel Reassigment and Adaptive Transmitter Power Control", Proc IEEE Veh Tech Conf, VTC-88, 1988, p 668-672

[4] ZANDER, J., "Performance of Optimum Transmitter Power Control in Cellular Radio Systems", IEEE Trans Veh Tech, vol 41, no 1, February 1992, p 57-62.

[5] M. Unbehaun, M. Kamenetsky, ―The evolution of wireless LANs and PANs - On the deployment of picocellular wireless infrastructure‖, IEEE Wireless Communications, Volume: 10, Issue: 6, pp. 70 – 80, Dec. 2003

[6] C. Carciofi, A. Cortina, C. Passerini, and S. Salvietti, ―Fast Field Prediction Techniques for Indoor Communication Systems,‖ 2nd European Personal and Mobile Communications Conference (EPMCC), Bonn, pp. 37 - 42, Nov. 1997

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[7] Cheng, C.-M., Hsiao, P.-H., Kung, H. T., and Vlah, D.: Adjacent Channel Interference in Dual-radio 802.11 Nodes and Its Impact on Multi-hop Networking, IEEE GLOBECOM 2006, San Francisco, CA, November 2006

[8] IEEE 802.11 Working Group, ―Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications‖, IEEE 802.11 standard, including 802.11a,b,g and h amendments, Sep 1999-2005

[9] A. Mishra, et. al. , ―Partially Overlapped Channels Not Considered Harmful‖, SIGMetrics/Performance‘06, June 26–30, 2006, Saint Malo, France [10] ―Certified Wireless Network Administrator Official Study Guide‖, Dreamtech Press, 3rd edition pp # [11] Wi-Fi Security by - Stewart S. Miller McGraw-Hill networking pp # 9, 20-24. 66-71 [12] Wireless communication by – Theodore S. Rappaport pp # [13] Mobile cellular telecommunication by – Willium C. Y. Lee pp# [14] User manual ―Netgear‖ [15] User manual ―D-Link‖

Web site addresses: [1] www.awe-communications.com [2] www.netgear.com [3] en.wikipedia.org [4] www.cisco.com [5] www.currentdirections.com [6] www.IEEE.org [7] www.FCC.com [8] www.cwnp.com [9] www.google.com/scholar
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LIST OF ABBREVIATIONS

____________________________________________________
AC ACK ACI AP ARQ BFSK BPM BWA CCI CCK C/I CISPR CPS DIU DSSS EHF EMI EMC ESS FCC FHSS : Alternating Current : Acknowledgement : Adjacent-channel interference : Access Point : Automatic Retransmission request : Binary Frequency Shift Keying : Beats per Minute : Broadband Wireless Access : Co-channel interference : Complementary Code Keying : Carrier to interference ratio : International Special Committee on Radio Interference : Cycle per Second : Daffodil International University : Direct-Sequence-Spread- Spectrum : Extremely High Frequency : Electromagnetic Interference : Electromagnetic Control : Extended Service Set : Federal Communications Commission : Frequency Hopping Spread Spectrum
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FSK FM GPS HF IEC IEEE IM IR ISM ITU LAN LF MAC MF MIMO OFDM PMP PTP QAM RDP RF RFI RPM SHF

: Frequency Shift Keying : Frequency modulation : Global Positioning System : High Frequency : International Electro technical Commission : Institute of Electrical and Electronics Engineers : Intermodulation : Infrared : Industrial, Scientific, and Medical : International Telecommunication Union : Local Area Network : Low Frequency : Media Access Control : Middle Frequency : Multiple-Input Multiple-Output : Orthogonal Frequency Division Multiplexing : Point-to-Multipoint : Point-to-Point : Quadrature amplitude modulation : Radio Data Packet : Radio Frequency : Radio Frequency Interference : Revolutions per Minute : Super High Frequency
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SNR SSID SP TCP TDD UHF UNII UV VHF VLF VoIP VoWLAN WLAN Wi-Fi WISP

: Signal to Noise Ratio : Service Set Identifiers : Switch Processor : Transmission Control Protocol : Time Division Duplexing : Ultra High Frequency : Unlicensed National Information Infrastructure : Ultra Violet : Very High Frequency : Very Low Frequency : Voice over Internet Protocol : Voice over Wireless Local Area Network : Wireless Local Area Network : Wireless Fidelity : Wireless Internet Subscriber Protocol

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