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Lte Initial Access

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LTE Initial Access
Like all mobile communication systems, in LTE a terminal must perform certain steps before it can receive or transmit data. These steps can be categorized in cell search and cell selection, derivation of system information, and random access. The complete procedure is known as LTE Initial Access and is shown in the Figure below. After the initial access procedure, the terminal is able to receive and transmit its user data.


Initial synchronization
Successful execution of the cell search and selection procedure as well as acquiring initial system information is essential for the UE before taking further steps to communicate with the network. For this reason, it is important to take a closer look at this fundamental physical layer procedure. This section focuses on the cell-search scheme defined for LTE and the next chapter describes reception of the essential system information.
As in 3G (WCDMA), LTE uses a hierarchical cell-search procedure in which an LTE radio cell is identified by a cell identity, which is comparable to the scrambling code that is used to separate base stations and cells in WCDMA. To avoid the need for expensive and complicated network and cell planning, 504 physical layer cell identities of is sufficiently large. With a hierarchical cell search scheme, these identities are divided into 168 unique cell layer identity groups in the physical layer, in which each group consists of three physical layer identities. To remember this hierarchical principle, consider the example of first names and surnames. According to statistics, the most common English surname is “Smith”, which corresponds to physical layer cell identity group 0. The second most common surname is “Johnson”, which represents the physical layer cell identity group 1. This example can be extended to the last group, which would be “Rose”. The most common male first names are “James”, “John”, or
“Robert” and female names are “Mary”, “Patricia”, and “Linda”. Each first name represents one of the three physical layer identities.
This information is now transmitted using two different signals, generated by Layer 1.
The two signals, carrying the physical layer identity and the physical layer cell identity group, are the primary and the secondary synchronization signals respectively. This means that the complete cell search procedure consists of two steps to identify the cells’ identity as shown Graphically in the Figure below:


Primary Synchronization Signal (PSS)
The UE first looks for the primary synchronization signal (PSS) which is transmitted in the last OFDM symbol of the first time slot of the first subframe (subframe 0) in a radio frame. This enables the UE to acquire the slot boundary independently from the chosen cyclic prefix selected for this cell. Based on the downlink frame structure (Type
1, FDD). The primary synchronization signal is transmitted twice per radio frame, so it is repeated in subframe 5 (in time slot 11). This enables the
UE to get time synchronized on a 5 ms basis, which was selected to simplify the required inter-frequency and inter-RAT measurements. LTE must accommodate handover to and from other radio access technologies, such as GSM/GPRS/EDGE,

Secondary Synchronization Signal (SSS)
After the mobile has found the 5 ms timing, the second step is to obtain the radio frame timing and the cells’ group identity. This information can be found from the SSS. In the xtime domain, the SSS is transmitted in the symbol before the PSS . The SSS also has 5 ms periodicity, which means it is transmitted in the first and sixth subframes (subframes 0 and 5) as shown in the Figure below. Like the PSS, the SSS is transmitted on 62 of the 72 reserved subcarriers around the DC subcarrier.


LTE Cell selection and reselection criteria
The previous section described how initial cell selection will work and the difference between LTE FDD and TD-LTE. However, only when specific criteria are fulfilled is the UE allowed to camp on that cell. These criteria for cell selection as well as cell reselection for LTE are specified in [3]. It is further illustrated by a description of the two procedures: In the initial cell selection procedure, as described in the previous sections, no knowledge about RF channels carrying an E-UTRA signal is available at the UE. In that case the UE scans the supported E-UTRA frequency bands to find a suitable cell. Only the cell with the strongest signal per carrier will be selected by the UE. The second procedure relies on information about carrier frequencies and optionally cell parameters received and stored from previously-detected cells. If no suitable cell is found using the stored information the UE starts with the initial cell selection procedure. S is the criterion defined to decide if the cell is still suitable . This criterion is fulfilled when the cell selection receive level is [pic] is computed based on the Equation below:

[pic]is the measured receive level value for this cell, i.e. the Reference Signal Received Power (RSRP). This measured value is the linear average over the power of the resource elements that carry the cell specific reference signals over the considered measurement bandwidth. Consequently, it depends on the configured signal bandwidth. In the case of receiver diversity configured for the UE, the reported value will be equivalent to the linear average of the power values of all diversity branches. [pic] is the minimum required receive level in this cell, given in dBm. This value is signaled as QrxLevmin by higher layers as part of the System Information Block Type 1 (SIB Type 1). QrxLevmin is calculated based on the value provided within the information element (-70 and -22) multiplied with factor 2 in dBm.

[pic] is an offset to Qrxlevmin that is only taken into account as a result of a periodic search for a higher priority PLMN while camped normally in a Visitor PLMN (VPLMN). This offset is based on the information element provided within the SIB Type 1, taking integer values between (1…8) also multiplied by a factor of 2 in dB. This gives a wider range by keeping the number of bit transmitting this information. The offset is defined to avoid “ping-pong” between different PLMNs. If it is not available then Qrxlevminoffset is assumed to be 0 dB.

[pic]is a maximum function as shown in Equation 5. Whatever parameter is higher, PEMAX- PUMAX or 0, is the value used for PCompensation. PEMAX [dBm] is the maximum power a UE is allowed to use in this cell, whereas PUMAX [dBm] is the maximum transmit power of an UE according to the power class the UE belongs too. At the moment only one power class is defined for LTE, which corresponds to Power Class 3 in WCDMA that specifies +23 dBm. PEMAX is defined by higher layers and corresponds to the parameter P-MAX defined in [2]. Based on this relationship, PEMAX can take values between -30 to +33 dBm. Only when PEMAX > +23 dBm PCompensation is it considered when calculating Srxlev. The P-MAX information element (IE) is part of SIB Type 1 as well as in the "RadioResourceConfigCommon" IE, which is part of the SIB Type 2.

As explained above, all parameters except for Qrxlevmeas are provided via system information. In a real network a UE will receive several cells perhaps from different network operators. The UE only knows after reading the SIB Type 1 if this cell belongs to its operator’s network (PLMN Identity). First the UE will look for the strongest cell per carrier, then for the PLMN identity by decoding the SIB Type 1 to decide if this PLMN is a suitable identity. Afterwards it will compute the S criterion and decide for a suitable cell or not.
The Figure above shows one possible scenario in a real network. Assume that the UE belongs to network operator 1. There are two other carriers also operating an LTE network but of course at different frequencies. The terminal receives all base stations but at different power levels. Based on the above definition the UE will select the strong cell for each carrier . Using this the UE will start with network operator 3 and figure out after decoding the SIB Type 1 that the PLMN saved on the USIM does not match to the transmitted one. From this information it will stop with its attempt and proceed to the next strongest signal, which is operator 2 . Now the PLMN does not correspond so the UE will continue with signal 3 (green) – and the PLMN will match. The UE continues to use the information in SIB Type 1 and Type 2 to compute the cell selection criteria. In this example, the parameters transferred and belonging to eNB1 do not fulfill S > 0 where the UE will move along with demodulating and decoding the information provided by eNB2. S > 0 is fulfilled and the UE starts camping on this cell.

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