Unlike other physical channel and signal, the characteristics of SSS (LTE) and NSSS(LTE-NB) differs widely. As compared below, SSS is based on m-sequence and NSSS is based on Zadoff chu sequence.

Followings are the topics to be covered in this page.

• How does NSSS carry the cell ID ?
• How is NSSS mapped to resource elements ?
• Reference

How does NSSS carry the cell ID ?

NSSS has to tell the UE two things that NPSS cannot: the physical cell ID, and where the UE is in time. The picture below puts the 36.211 clause 10.2.7.2 formula for NSSS above the clause 6.11.1 formula for the LTE SSS, and it points out the NSSS terms one by one.

NSSS combines a Zadoff-Chu sequence of length 131, a binary scrambling sequence b_q and a phase rotation θ_f. LTE SSS interleaves two m-sequences instead.

Start with the length. n runs from 0 to 131, so NSSS has 132 elements. That is 12 subcarriers times 11 OFDM symbols, which is the whole NSSS subframe. The Zadoff-Chu part uses n' = n mod 131, so the sequence of length 131 wraps around for its last element.

The cell ID is split across two parts of the formula. The root index u = N_ID^Ncell mod 126 + 3 takes 126 values. The index q = ⌊N_ID^Ncell / 126⌋ picks one of the four binary sequences in 36.211 Table 10.2.7.2.1-1. So 126 roots times 4 sequences give 504 combinations, one for each physical cell ID from 0 to 503.

The last term carries timing. θ_f = 33/132 (n_f/2) mod 4 changes with the frame number n_f. NSSS appears in every second radio frame, so n_f/2 counts NSSS occasions, and mod 4 gives four different rotations. The UE can therefore tell which of four consecutive NSSS occasions it received, and that marks an 80 ms boundary.

• 132 elements fill one subframe : 12 subcarriers times 11 OFDM symbols.
• u and q together give the cell ID : 126 roots times 4 binary sequences cover all 504 physical cell IDs.
• θ_f gives the 80 ms timing : four phase rotations over four NSSS occasions, 20 ms apart.
• LTE SSS works differently : it interleaves two m-sequences and changes between subframes 0 to 4 and 5 to 9.

How is NSSS mapped to resource elements ?

Another big difference between SSS and NSSS is Resource Element mapping and transmission pattern. As illustrated below, NSSS fills out 11 OFDM symbols from the end of a subframe (subframe 9). It is transmitted at subframe 9 of every two radio frames.

NSSS fills all 12 subcarriers of the last 11 symbols in subframe 9 of the even radio frames. LTE CRS resource elements puncture it.

• Every second radio frame : the frame row at the top shows NSSS in subframe 9 of the first and third frames only.
• All 12 subcarriers : NSSS uses subcarrier 11 as well, unlike NPSS, which leaves it unused.
• The last 11 symbols : 36.211 Table 10.2.7.2.2-1 gives 11 NSSS symbols for normal cyclic prefix, so symbols 3 to 13 carry NSSS.
• LTE CRS takes priority : the red resource elements inside the green block are LTE CRS.

36.211 v19.3.0 clause 10.2.7.2.2 adds three rules to the picture. First, where a resource element overlaps an LTE CRS, the NSSS element for it is not sent but is still counted in the mapping. Second, the UE shall not assume that NSSS uses the same antenna port as any downlink reference signal, or the same port as NSSS in another subframe.

Third, higher layers can tell the UE that different precoders are applied to NSSS over a number of consecutive NSSS occasions. The clause also covers TDD. For frame structure type 2, NSSS moves from subframe 9 to subframe 0.

Reference

[1] 3GPP TS 36.211 v19.3.0 - clause 6.11.1 for the LTE SSS, clause 10.2.7.2 for NSSS sequence generation and mapping