Primary and Secondary Synchronization Signals

Both primary and secondary synchronization signals are designed to detect all type of UEs.  The synchronization signals always occupy the 62 sub-carrier of the channel, which make the cell search procedure same regardless of channel bandwidth. Although 72 subcarriers (6 RB) are available, only 62 sub-carriers are used so that the UE can perform the cell search procedure.

The primary synchronization signal subcarriers are modulated using a frequency domain Zadoff-Chu Sequence. Each subcarrier has the same power level with its phase determined by the root index number in sequence generator as defined in 36.211 .

There are 504 unique physical-layer cell identities. The physical-layer cell identities are grouped into 168 unique physical-layer cell-identity groups, each group containing three unique identities. The grouping is such that each physical-layer cell identity is part of one and only one physical-layer cell-identity group. A physical-layer cell identity

N(cell, ID) = 3N(1, ID) + N(2, ID) 

is thus uniquely defined by a number N(1,ID) in the range of 0 to 167, representing the physical-layer cell-identity group, and a number N(2,ID) in the range of 0 to 2, representing the physical-layer identity within the physical-layer cell-identity group Three different cell id is used for the primary synchronization signal i.e N(2, ID). The root index corresponds to the cell identity N(2, ID).

The secondary signal is used to identify cell-identity groups. The number and position of subcarrier are same as for the primary synchronization signal: that is the central 62 sub carriers. The sequence generation function utilizes an interleaved concatenation of two length 31 binary sequences as defined in 36.211.

The secondary synchronization signal gives a cell-identity group number from 168 possible cell identities N (1, ID). The mapping of the sequence to resource elements depends on the frame structure.

For FDD,  frame structure type 1, the primary synchronization signal shall be mapped to the last OFDM symbol in slots 0 and 10. Let NRBDL = 50 (10MHz and NRBSC = 12) for Normal Cyclic prefix NDLsyml = 7 Resource elements  (k,l) in the OFDM symbols used for transmission of the primary synchronization signal. k will range from 269 to 330 subcarriers and l shall be equal to 6 (even slot) Resource elements  (k,l) in the OFDM symbols used for transmission of the secondary synchronization signal. k will range from 269 to 330 subcarriers and l shall be equal to 5 (even slot).