5G NR: Secondary Synchronization Signal (SSS)


Cell search is the procedure for a UE to acquire time and frequency synchronization with a cell and to detect Physical layer Cell ID (PCI) of the cell.
During cell search operations which are carried out when a UE is powered ON, mobility in connected mode, idle mode mobility (e.g. reselections), inter-RAT mobility to NR system etc., the UE uses NR synchronization signals and PBCH to derive the necessary information required to access the cell.
Similar to LTE, two types of synchronization signals are defined for NR; Primary Synchronization Signal (PSS) and the Secondary Synchronization Signal (SSS). The Synchronization Signal/PBCH block (SSB) consists of PSS, SSS and Physical Broadcast Channel (PBCH).
Synchronization signals can also be used by the UE for RSRP and RSRQ measurements.
For more details about SSB, visit 5G NR: Synchronization Signal/PBCH block (SSB), for details about PSS, visit: 5GNR: Primary Synchronization Signal (PSS) and for PBCH/MIB, visit 5G NR:PBCH and Master Information Block (MIB).

Physical-layer Cell Identity (PCI)
-   There are 1008 unique PCIs defined in 5G NR, double of that in LTE (504).
-   1008 NR PCIs are divided into 336 unique PCI groups and each group consisting of three different identities.
-   PCI of a cell can be calculated using;
NIDCell = 3 * NID(1) + NID(2) where NID(1) {0,1, … ,335} and NID(2) {0,1,2}
-   The UE derives PCI group number NID(1) from SSS and physical-layer identity NID(2) from PSS.
Secondary Synchronization Signal (SSS) 
-   The time-frequency structure of SSB is illustrated in the figure below. It can be seen from that the SSS is located in the third OFDM symbol within the SSB and span over 127 subcarriers.

















-   UE knows the timing of SSS after detecting PSS.
-   SSS is transmitted periodically at SSB periodicity.
-   In frequency domain, SSS occupies same length as PSS so that, the same frequency filter can be shared between PSS and SSS detectors.
-   As the total number of PCIs are 1008, there are 3 PSS sequences and 336 SSS (=1008/3) sequences associated to 1-of-3 PSS sequence defined.
-   SSS is generated by using BPSK modulated Gold sequence of length 127. Gold sequences can generate much larger number of mutually low-correlated sequences than its sequence length. This is very useful since the number of SSS sequences (336) are too large as compared to the sequence length (127).
-   The Gold sequence is generated by multiplying two BPSK modulated m-sequences with cyclic shifts determined by PCI. The details are as shown in the figure below.

-   The SSS sequence of length 127 is mapped on to 127 subcarriers. The mapping is in the increasing order from subcarrier number 56 through 182.
-   Once UE decodes PSS, it knows one out of three identities for NID(2) and after decoding SSS, PCI group number NID(1) is known to the UE. Using NID(1) and NID(2), the PCI can be calculated as NIDCell = 3 * NID(1) + NID(2)
 Reference: 3GPP TS 38.211 and 38.213