HET452/HET718 Wireless Communications

Multiple Access Techniques for Wireless Communications
Lecture eight

Outline of Lecture
• The purpose of this lecture is to describe the main multiple access techniques for wireless
– Frequency Division Multiple Access – Time Division Multiple Access – Code Division Multiple Access

• (Packet Radio Multiple Access)

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10-2

Multiple Access Techniques
• Multiple access schemes allow many mobile users to share simultaneously a finite amount of radio spectrum • Frequency Division:
– Allocate part of the spectrum to each user

• Time Division
– Allocate a time slot within the spectrum to each user

• Code Division (Spread Spectrum)
– Allocate the whole spectrum to each user, but apply a spreading code so that other users’ communication will appear as (low level) noise
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10-3

Multiple Access Schemes
Frequency Division: FDMA code time frequency Time Division: TDMA code time frequency
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10-4

Code Division: CDMA code/power time frequency

Duplexing
• Enable a user to talk and listen at the same time
– Full Duplex communication

• Frequency Division Duplexing (FDD)
– Two distinct bands for each user
• Forward band provides traffic from the base station to the user • Reverse band provides traffic from the user to the base station

• Time Division Duplexing (TDD)
– Users access the channel in assigned time slots – Usually limited to short range systems (cordless phones)
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10-5

Narrowband and Wideband
• Narrowband and wideband
– Defined in relation to coherence bandwidth
• Narrowband the bandwidth of the channel similar to CB • Wideband bandwidth of channel much greater than CB

– Coherence bandwidth inversely related to rms time spread
• Range of bandwidth over which channel has a flat response

• Narrowband more susceptible to multipath and frequency selective fading than wideband

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10-6

Frequency Division Multiple Access
Allocate part of the spectrum to each user

Frequency Division: FDMA code time frequency
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10-7

Frequency Division Multiple Access
• • • • FDMA assigns individual channels to individual users Each user allocated a unique frequency band FDMA usually implemented in narrowband Symbol time of a narrowband signal is large compared with the average delay spread
– Very little intersymbol interference in FDMA – Little need for equalization

• Needs tight filtering to minimize adjacent channel interference
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10-8

Time Division Multiple Access
• Divide the radio spectrum into time slots • One user is allowed to transmit or receive within the slot • Often superimposed on an FDMA scheme (GSM)
Time Division: TDMA code time frequency
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10-9

Time Division Multiple Access
• TDMA assigns a single channel to multiple users • Channel is divided into time slots • Time slots are allocated to each user within a frame
– Transmission occurs in ‘buffer and burst’ mode

• A frame contains a preamble, information and tail bits
Preamble Information Message Slot 1 Slot 2 Slot 3 Trail bits Sync. bits Information Data Trail bits Slot N Guard bits
10-10

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Time Division Multiple Access
• Symbol time is smaller than FDMA • Will usually require equalization to eliminate intersymbol interference • Buffering of voice samples causes voice delay

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10-11

Problem on GSM Delay Spread
• The GSM bit rate is 270.8333 kbps per channel. What is the GSM symbol time? • What RMS delay spread is tolerable for GSM systems? • In which of the following environments will GSM performance be acceptable without equalization?
– – – – – Indoor Open area Suburban Urban Hilly 0.01 - 0.05 µs < 0.2 µs < 1.0 µs 1 - 3 µs 3 - 10 µs

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10-12

Spread Spectrum Multiple Access

Power/code

frequency CDMA = Code Division Multiple Access
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10-13

History of spread spectrum
• Patented by actress Hedwig (Hedy) Lamarr
– (Did Harry Potter’s owl use spread spectrum??)

• Rapidly hop from one carrier frequency to another
– Lamarr’s used a pianola (mechanical piano, controlled by holes punched in a roll of paper)

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10-14

Principle of spread spectrum
Spreading (before transmission)
Noise spectrum

depreading (after reception)

Original signal

freq

Spread signal Narrowband noise

freq

freq

freq Original signal

Fading Faculty of Information and Communication Technologies

freq

freq
10-15

Principle of spread spectrum
• Take a given transmitter power • spread that power over a large part of spectrum (large frequency range) • this mixes a weak (but wide) signal with the (strong?) background noise • find a way to recover the weak signal from the noise

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10-16

Matched filters for orthogonal signals
Filter (expected signal) × × × × × FDMA TDMA ≈0 ≈0 ≈1 ≈0 ≈0

Sprd Spec

? ? ? ? ?
10-17

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Matched filters for spread spectrum
• Signals from different users should be “orthogonal” – Correlation is 0 ∫ s (T ) s (T ) dt = 0 • FDMA: different carrier frequencies are orthogonal • TDMA: different time slots are orthogonal • Spread spectrum uses signals which are spread in time and spread in frequency but are still orthogonal to each other • Noise orthogonal to the spreading function is also removed at the receiver
T 0 1 2

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10-18

Frequency Hopping Spread Spectrum
• Frequency used for channel changes throughout the duration of the communication
– The communication ‘hops’ from one frequency to another

• Receiver and Transmitter agree on how the hopping will occur
– Pseudo Noise Sequences

• Slow and fast frequency hopping • Slotted and unslotted frequency hopping • Orthogonal and Random frequency hopping
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10-19

FHMA and Frequency Selective Fading
Frequency

Time
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10-20

DS-CDMA
• DS (Direct Sequence) • CDMA (Code Division Multiple Access) • sender and receiver share a known code for that channel
– “spreading code”

• Any two long random binary sequences are “almost” orthogonal
– Don’t need as much co-ordination between users
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10-21

Code Division Multiple Access
• Use signals which have transmission bandwidth several orders of magnitude greater than the minimum required RF bandwidth • A spreading code converts a narrowband signal to a wideband signal • Spread Spectrum provides resiliance to multipath fading

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10-22

DS-CDMA transmission
Original signal (digital bit stream) (low speed)

wideband modulator transmitted RF signal Note: wide spectral bandwidth because of high speed spreading code
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Spreading code (pseudo random binary sequence) (high speed)

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DS-CDMA spreading example
1 original +1 signal -1 spreading +1 code -1 transmitted+1 bit stream -1 1 signal bit time spreading code inverted here 1 0 0 1 time

time

time

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10-24

DS-CDMA decoder received RF signal spreading code timing control averaging over bit time

threshold detection decoded bit stream

correlator output

decides 1s and 0s

Note: increased noise just increases bit error rate
10-25

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Spread Signal: Signal recovery
• Composite signal contains messages sent by many users
– Messages occupy the same (wideband) frequency

• Shared frequency (no FDMA), no time slots (no TDMA) • “Correlate” received signal with spreading code
– large +ve correlation = data 1, large -ve correlation = data 0 – small correlation = “noise”

• (Correlation is a comparison of 2 signals over time
– take many samples (pair of signals sampled at same time) T – multiply each sample pair ∫0 s1 (T )s2 (T ) dt = 0 – sum (average) the multiplied samples)
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10-26

Orthogonal codes: Walsh Functions
• Need to separate the messages based on their coding
– To separate individual messages, coding must be orthogonal
• MobileBasestation: random codes to minimise coordination • BasestationMobile: use truly orthogonal codes

• Walsh functions: Commonly used orthogonal codes • Mirror the properties of orthogonal trigonometric functions
– In the same way that trigonometric functions can be summed, transmitted and then recovered, so can Walsh Functions

• Walsh Functions are the rows of Hadamard matrices of successive orders
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10-27

Hadamard Matrices and Walsh functions
A Hadamard matrix is a square n × n matrix whose rows have elements that are either +1 or –1, and each row has an arrangement of +1's and –1's so that each row is orthogonal to each other row. They are defined recursively: H1 = [1]
H 2k H 2 k −1 = H 2 k −1 H 2 k −1  − H 2 k −1  

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10-28

The first few Hadamard Matrices
1 1 H2 =  1 − 1  
1 1 1 1 − 1 1 H4 =  1 1 − 1 1 − 1 − 1 

1 1  1 1  − 1  H 8 = 1 − 1 1 1 1   1 1 

1

1

1

1

1

1

−1

1 −1 1 −1 1 1 −1 −1 1 1 −1 −1 −1 1 1 −1 −1 1 1 1 −1 −1 −1

−1 1 −1 −1 1 −1 1 −1 −1 −1 −1 1 −1 −1 1 −1 1 1

1 − 1  − 1  1 − 1 1  1  − 1

etc….. •the rows of these matrices are the Walsh functions
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10-29

Sending a bit using DS-SS and Walsh Codes
1. 2. 3. 4. 5. 6. All users use the full spectrum to transmit a signal Receiver and Transmitter both know what Walsh code is used Each bit is spread across the full bandwidth using the shared orthogonal sequence Bit sequences from multiple users are summed together to form a composite signal Each receiver takes the composite signal and multiplies it by the orthogonal sequence The resulting message is averaged over the message bit time to give the original bit
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10-30

Sending a bit using DS-SS and Walsh Functions
1. Encode bits +1, +1 and –1 using the following Walsh codes
+1, -1, +1, -1, +1, -1, +1, -1 +1, +1, -1, -1, +1, +1, -1, -1 +1, -1, -1, +1, -1, +1, +1, -1

2. 3.

Receiver and Transmitter both agree to use the above Walsh codes Encode each bit by multiplying it by its Walsh function giving
+1, -1, +1, -1, +1, -1 +1, -1 +1, +1, -1, -1, +1, +1, -1, -1 -1, +1, +1, -1, +1, -1, -1, +1

4.

Composite signal is the sum of the three Walsh encoded bits
+1, +1, +1, -3, +3, -1 -1, -1 Faculty of Information and Communication Technologies
10-31

Sending a bit using DS-SS and Walsh Functions (cont)
5. Each receiver receives the composite signal and extracts their bit by multiplying the composite signal by their Walsh code
+1, +1, +1, -3, +3, -1 -1, -1 Bit 1: Bit 2: Bit 3: 1 -1 1 3 3 1 -1 1 1 1 -1 3 3 -1 1 1 1 -1 -1 -3 -3 -1 -1 1 1 -1 1 3 3 1 -1 1 = 8 1 1 -1 3 3 -1 1 1 = 8 1 -1 -1 -3 -3 -1 -1 1 = - 8

6.

Each receiver takes the average value of their bit over the bit time
Bit 1: Bit 2: Bit 3:

Averaging over the bit time, the recovered bits are +1, +1, -1 (!!)
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10-32

Problem using Walsh Functions
• What is the aggregate signal when the following Walsh Functions are used to send the bits +1 and -1?
+1 +1 +1 +1 +1 +1 -1 -1

• Take the aggregate signal and show how the Walsh Functions can be used to extract the transmitted bits

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10-33

Some advantages of Spread Spectrum
• Much more resistant to multipath fading than narrowband
– Takes advantage of multiple paths

• As number of users increases, performance decreases in a smooth manner
– Can tolerate large number of users – Causes increase in perceived noise

• No need for complex planning of cellular frequencies
– Allocate entire spectrum to each cell – CDMA systems have a cluster size N = 1
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10-34

Summary
• Multiple Access Schemes
– Frequency Division Multiplexing – Time Division Multiplexing – Code Division Multiplexing
• Frequency Hopping • Direct Sequence

– (Random Access Schemes)

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10-35

CDMA cellular mobile
• RF channel 1.25 MHz wide
– same RF channel used in all cells – no frequency reuse pattern required

• Co-channel interference increases background noise
– lowers the signal/noise ratio – increased bit error rate – not audible crosstalk between user channels – 80MHz apart

• Base Station and mobile use different frequencies

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10-36

Random Access Schemes
• • • • Used for access to shared channels Typically signalling for call setup and tear down Similar concepts to Ethernet CSMA/CD Basic problem is dealing with collisions when two or more users attempt to access the same channel • Two classes of schemes
– ALOHA schemes – Carrier Sense Multiple Access schemes

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10-37

ALOHA Schemes
• Pure ALOHA
– Mobile station transmits information packet when upper layers deliver it – Base station checks parity of transmitted message – If parity ok then acknowledgment sent to mobile station – If not ok then collision has occurred and no acknowledgment is sent – Mobile station waits slightly more than the round trip time for the acknowledgement – If no acknowledgment Mobile stations waits a random amount of time before retransmitting
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10-38

Slotted ALOHA
• Transmissions only occur on slot boundaries • Base station sends a beacon signal for timing information • Twice throughput of standard ALOHA

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10-39

Carrier Sense Multiple Access
• Mobile station monitors the channel before sending • When detects that the channel is unused will send the message
– Persistent CSMA

• Will monitor transmitted message to see if anyone else has started transmission • If so then will back off an arbitrary amount of time

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10-40

p-Persistent and Collision Detection
• Persistent and p-persistent CSMA
– Persistent CSMA will transmit as soon as the channel is detected as being idle – p-persistent CSMA will transmit with a probability of p

• CSMA with Collision Detection
– Same as Ethernet – CSMA/CD will transmit as soon as the channel is detected as being idle – But will stop transmission if collision is detected – Will wait a random amount of time before attempting retransmission
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10-41

‘Hidden Terminal’ Problem
• Not always easy to detect collisions with wireless
– Two mobile stations trying to communicate with the base station – Both mobile stations in range of base station but out of range of each other – CSMA not effective when two mobile stations attempt to transmit concurrently

• Some protocols to deal with problem, but… • ALOHA popular for cellular wireless
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10-42

Comparison of schemes
Throughput for short roundtrip times 0.18 0.37 0.53 0.82 0.94 Throughput for long roundtrip times 0.18 0.37 0.16 0.14 0.13
10-43

ALOHA Slotted ALOHA Persistent CSMA Non-persistent CSMA CSMA/CD

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