3GPP LTE standard TS36.211 specifies the down link synchronization signals (DL SSs) in section 6.11. The specification defines the DL SSs in two types: primary synchronization signal (PSS) and secondary synchronization signal (SSS). DL SSs are needed to acquire frequency and time synchronization to a cell and determine the physical layer identity of the cell.
The structure and characteristics of the PSS and SSS are as follows:
1. PSS and SSS are transmitted in the central 6 RBs for all possible system bandwidth (6 - 110 RBs): PSS and SSS are each mapped to the central 62 subcarriers around the unused D.C. subcarrier.
2. DL SSs are transmitted twice per 10 ms radio frame (the two PSSs within a radio frame are identical in a given cell). For the FDD case, the PSS is located in the last symbol of the first slot of subframe of 0 and 5 and the SSS is located just prior to the PSS. For the TDD case, the PSS is located in the third symbol of subframe 1 and 6 and the SSS is located three symbols ahead of the PSS.
3. Three different PSS sequences are used for the cell identity in the group.
4. Three (root = 25, 29, 34) Zadoff-Chu sequences of length 63 (ZC(31) is not used for the D.C. subcarrier) are used for PSS.
5. Length 62 sequences of SSS are an interleaved concatenation of two length 31 BPSK-modulated binary codes (SSC1 and SSC2). The concatenated sequence (m-sequence) is scrambled with a scrambling sequence given by the PSS. The possible combination of SSC1 and SSC2 are 168, representing the physical layer cell identity group.
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