LTE: Detection of PSS and SSS

Question

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 ?

http://cdn.rohde-schwarz.com/dl_downloads/dl_application/application_notes/1ma150/1MA150_0E_cell_searchselection_in_LTE.pdf

Answer 1

As we know, LTE offers bandwidth flexibility. UE does not know the bandwidth of the system when it is performing the cell search procedure. So, independent of the network's bandwidth, the PSS and SSS is always located in the center 6 RBs and the UE always uses 64 FFT during this procedure. Only after reading the Master Info Block, the UE realizes the downlink cell bandwidth and starts using the corresponding FFT.