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what is the use of PSS and SSS in LTE?

+3 votes
what is the use of PSS and SSS in LTE?
posted Sep 3, 2013 by Vikram Singh

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These are downlink synchronization signals which are used by the UE to obtain the cell identity and frame timing.

1 Answer

+2 votes

To get lte service, Ue has to establish radio connection(AS connection) with eNB and NAS connection with MME. To send a NAS message towards MME. UE makes radio connection with eNB, which is known as RRC connection setup procedure. For all these things, Ue need to acquire system information from eNB, eNB transmits system information in the form of MIB and SIBs. MIB is transmitted over PBCH channel and SIBs are transmitted over PDSCH channel. So UE first read MIB and get some information from MIB so that It can read SIBs. To read MIB message, UE perfroms cell synchronization (Primary and Secondary secondary synchronization).

Actually there are total 504 identifiers, that that can be used to identify a cell at physical level. Total 504 identifiers are divided in 168 groups and each group has 3 values(0,1,2)

PSS: Primary synchronization tells about one of three values(0,1,2)
SSS: Secondary synchronization tells about one of the groups ( 1 to 168)

Cell physical layer indentity is calculated by = 3 * (SSS value) + (PSS value).

PSS also known as slot level synchronization and SSS known as frame level synchronization. After doing this UE is synchronized in downlink with eNB. Now UE can reads MIB message from PBCH channel and SIBs from PDSCH channel.

answer Sep 3, 2013 by Vimal Kumar Mishra
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I understand that according to the standard a 64 FFT is used by the UE for cell search for all Bandwidth configurations(see section 2.2.1 page 7 in the below document). Suppose we are transmitting at 20 MHz which means we do a 2048 FFT on the transmitter side for the whole resource grid, including the 6th and 7th symbols corresponding to PSS and SSS in Subframe 0 and 5. In this case how is it possible for UE to do cell search using 64-FFT. Although PSS and SSS use only the central 62 subcarriers, shouldn't the UE use the same FFT length used in the transmitter side ? Also, how is the DL resource grid processed by the UE ?Does the UE extract Symbols 6,7 of Subframes 0,5 and process it separately with 64-FFT(after low pass filtering) while using 2048 FFT for the rest of the symbols ?