UEs on LTE perform Cell Search to achieve the following objectives
Physical Layer Cell ID: The UE can find a suitable cell meeting the radio thresholds and discover its Physical Layer Cell ID.
Normal/Extended Cyclic Prefix: Find out if c the cell is using a Normal or a Extended Cyclic Prefix.
TDD/FDD Cell: Find out if it is a TDD/FDD Cell.
Time Synchronization: To align the UE FFT window with that of the eNB. In LTE DL, OFDMA involves IIFT at eNB and FFT at UE. If the FFT window is not aligned with the NW, it will lead to ISI (Inter Symbol Interference) when UE tries to decode the DL.
Frequency Synchronization: The Phase and frequency of the Local Oscillator as well as the Sampling frequency between the sender and receiver may deviate after a certain time. This again leads to ICI(Inter Channel Interference) and ISI. Frequency synchronization provides the UE with Carrier Frequency Offset(CFO) and Sampling Frequency Offset(SFO) to correct the Local Oscillator frequency and the Sampling Frequency so that it is in perfect synch with the NW.
The UE has to do the Cell Search at different trigger points, two of them are:
Initial Synchronization(UE switched ON): When the UE for the first time tries to find a suitable cell to camp on to. The UE, after successfully camping on the cell goes on to Random Access and CN registration.
New Cell Search(during HO or cell re-selection): When the UE is already connected to the NW, but has to search for a new better cell as the current cell cannot support it. It continues searching for a cell until a cell satisfying the radio thresholds and PLMN selection criteria are met. The UE might go on to do a Tracking Area Update if the cell selected has a TAI not in its list of TAIs.
Cell search is based on two synchronization signals, the Primary Synchronization Signal(PSS) and Secondary Synchronization Signal(SSS). These two combined enable the UE to time and frequency synchronize and obtain the necessary information to read the Physical Broadcast Channel(PBCH). The UE only has to scan the center 62 sub carriers(930 KHz) of the entire Bandwidth as the Synchronization Signals are limited to that portion independent of the bandwidth. The cell search is optimized in order to meet the stringent requirements for the UE to be able to get synchronized and find a new cell in given time.
Primary Synchronization Signal(PSS): This signal carries a Zadoff-Chu binary sequence of size 62. The PSS occupies the center 62 subcarriers independent of the transmission bandwidth. It is transmitted in the last OFDM symbol of the 0th and 10th slot of each radio frame. Therefore it occurs every 5 ms. Modulation is BPSK with one binary digit of the sequence mapped to one resource element.
Secondary Synchronization Signal(SSS): This signal carries interleaved concatenation of 2 length 31 sequences. The concatenated sequence is scrambled with a scrambling sequence given by the sequence used in primary synchronization signal. It occupies the symbol just before the PSS i.e. the second last OFDM symbol of the 0th and 10th slot of each radio frame. Modulation is BPSK with one binary digit of the sequence mapped to one resource element.
Cell Search Procedure:
The UE starts with detecting the PSS and the 62 bit sequence that it carries. After this the UE is slot aligned to the NW transmission. The code used in the PSS is one of the three codes derived using 3 root indices. Therefore the 62 bit sequence indicates the value of NID(2).
Since the SSS is place in the symbol preceding PSS, the UE can detect SSS coherently along with PSS, or separately depending upon implementation. The precise position of SSS will depend upon the length of the Cyclic Prefix and the frame structure. Therefore the UE is aware of the CP and FDD/TDD used in the cell after successfully detecting the PSS and SSS.
The scrambling sequence used to de-scramble the SSS is derived from the 62 bit code in PSS. The SSS sequence is formed by two cyclic shifts of a single 31 bit M sequence. The cyclic shift indices depend upon the Physical Layer Cell ID group (0 ... 167) [ table Table 6.11.2.1-1 in 36.211] . Therefore after decoding the SSS sequence, the Physical Layer Cell ID group number NID(1) is known to the UE.
Physical Layer Cell ID is thus given by:
The order of the above two codes is alternated in the two transmissions of SSS in a radio frame, making the two transmissions of SSS different from each other. Therefore the UE becomes aware of the 10ms Radio frame boundary after detecting 2 consecutive SSS.
Physical Layer Cell ID: The UE can find a suitable cell meeting the radio thresholds and discover its Physical Layer Cell ID.
Normal/Extended Cyclic Prefix: Find out if c the cell is using a Normal or a Extended Cyclic Prefix.
TDD/FDD Cell: Find out if it is a TDD/FDD Cell.
Time Synchronization: To align the UE FFT window with that of the eNB. In LTE DL, OFDMA involves IIFT at eNB and FFT at UE. If the FFT window is not aligned with the NW, it will lead to ISI (Inter Symbol Interference) when UE tries to decode the DL.
Frequency Synchronization: The Phase and frequency of the Local Oscillator as well as the Sampling frequency between the sender and receiver may deviate after a certain time. This again leads to ICI(Inter Channel Interference) and ISI. Frequency synchronization provides the UE with Carrier Frequency Offset(CFO) and Sampling Frequency Offset(SFO) to correct the Local Oscillator frequency and the Sampling Frequency so that it is in perfect synch with the NW.
The UE has to do the Cell Search at different trigger points, two of them are:
Initial Synchronization(UE switched ON): When the UE for the first time tries to find a suitable cell to camp on to. The UE, after successfully camping on the cell goes on to Random Access and CN registration.
New Cell Search(during HO or cell re-selection): When the UE is already connected to the NW, but has to search for a new better cell as the current cell cannot support it. It continues searching for a cell until a cell satisfying the radio thresholds and PLMN selection criteria are met. The UE might go on to do a Tracking Area Update if the cell selected has a TAI not in its list of TAIs.
Cell search is based on two synchronization signals, the Primary Synchronization Signal(PSS) and Secondary Synchronization Signal(SSS). These two combined enable the UE to time and frequency synchronize and obtain the necessary information to read the Physical Broadcast Channel(PBCH). The UE only has to scan the center 62 sub carriers(930 KHz) of the entire Bandwidth as the Synchronization Signals are limited to that portion independent of the bandwidth. The cell search is optimized in order to meet the stringent requirements for the UE to be able to get synchronized and find a new cell in given time.
Primary Synchronization Signal(PSS): This signal carries a Zadoff-Chu binary sequence of size 62. The PSS occupies the center 62 subcarriers independent of the transmission bandwidth. It is transmitted in the last OFDM symbol of the 0th and 10th slot of each radio frame. Therefore it occurs every 5 ms. Modulation is BPSK with one binary digit of the sequence mapped to one resource element.
Secondary Synchronization Signal(SSS): This signal carries interleaved concatenation of 2 length 31 sequences. The concatenated sequence is scrambled with a scrambling sequence given by the sequence used in primary synchronization signal. It occupies the symbol just before the PSS i.e. the second last OFDM symbol of the 0th and 10th slot of each radio frame. Modulation is BPSK with one binary digit of the sequence mapped to one resource element.
Cell Search Procedure:
The UE starts with detecting the PSS and the 62 bit sequence that it carries. After this the UE is slot aligned to the NW transmission. The code used in the PSS is one of the three codes derived using 3 root indices. Therefore the 62 bit sequence indicates the value of NID(2).
Since the SSS is place in the symbol preceding PSS, the UE can detect SSS coherently along with PSS, or separately depending upon implementation. The precise position of SSS will depend upon the length of the Cyclic Prefix and the frame structure. Therefore the UE is aware of the CP and FDD/TDD used in the cell after successfully detecting the PSS and SSS.
The scrambling sequence used to de-scramble the SSS is derived from the 62 bit code in PSS. The SSS sequence is formed by two cyclic shifts of a single 31 bit M sequence. The cyclic shift indices depend upon the Physical Layer Cell ID group (0 ... 167) [ table Table 6.11.2.1-1 in 36.211] . Therefore after decoding the SSS sequence, the Physical Layer Cell ID group number NID(1) is known to the UE.
Physical Layer Cell ID is thus given by:
Physical Layer Cell ID = 3*NID(1) + NID(2)
The order of the above two codes is alternated in the two transmissions of SSS in a radio frame, making the two transmissions of SSS different from each other. Therefore the UE becomes aware of the 10ms Radio frame boundary after detecting 2 consecutive SSS.
Thanks for detailed effort.
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