Synchronization Signal (Primary and Secondary)

A UE that has just found an LTE carrier knows nothing about the cell on it. It does not know the physical cell ID, and it does not know where a radio frame starts. The PSS and the SSS give the UE both. This page is a short summary, and the pages linked at the end carry the details.

How do PSS and SSS make up the physical cell ID ?

Let's start with the numbers, because the rest of the design follows from them. LTE has 504 physical cell IDs, and neither synchronization signal alone can carry all of them. So the cell ID is split in two parts, and each signal carries one part.

i) Each cell transmit one of 168 unique sequences on its secondary sync channel (each bit sequence is 62 bit sequence)

ii) The Primary Sync channel carries one of three unique sequence (62 element Zadd-off chu sequence)

iii) So total number of Sync code combination become 168 x 3 = 504 and each of these combination become a physical cell ID of a cell.

36.211 v19.3.0 clause 6.11 writes the split as N_ID^cell = 3N_ID⁽¹⁾ + N_ID⁽²⁾. The group number N_ID⁽¹⁾ runs from 0 to 167 and comes from the SSS. The identity inside the group, N_ID⁽²⁾, runs from 0 to 2 and comes from the PSS.

The two signals are built differently. The PSS is a frequency-domain Zadoff-Chu sequence, and N_ID⁽²⁾ selects its root index from Table 6.11.1.1-1: 25, 29 or 34. The values of this sequence are complex, not bits. The SSS is an interleaved concatenation of two length-31 binary sequences. 36.211 scrambles this concatenated sequence with a scrambling sequence given by the PSS. So the UE must detect the PSS before it can decode the SSS.

The PCI has two parts : N_ID^cell = 3N_ID⁽¹⁾ + N_ID⁽²⁾, with 168 groups of three identities.
PSS carries the identity inside the group : one of three Zadoff-Chu roots, 25, 29 or 34.
SSS carries the group, and it depends on PSS : the SSS is scrambled with a sequence given by the PSS, so PSS detection comes first.

Where are PSS and SSS in the frame ?

The UE has to find the two signals before it can use them, so their position in the frame matters as much as their content. The position also differs between FDD and TDD, which is the first thing to check when a receiver finds nothing.

For the location of Sync signals in downlink frame structure, see Downlink Frame Structure page.

In frame structure type 1, FDD, 36.211 maps the PSS to the last OFDM symbol of slots 0 and 10. The SSS sits in the OFDM symbol just before it. In frame structure type 2, TDD, the PSS moves to the third OFDM symbol of subframes 1 and 6. The SSS stays in subframes 0 and 5, in the last symbol of slots 1 and 11. In both cases, each signal appears twice per radio frame, 5 ms apart.

In frequency, both signals use the 62 subcarriers around the DC subcarrier, with 5 reserved subcarriers on each side. That makes 72 subcarriers, or 6 resource blocks, whatever the channel bandwidth. So the UE can search for them before it knows the bandwidth of the cell.

The two PSS instances in a radio frame are identical, so the PSS alone gives only a 5 ms timing. The SSS removes this ambiguity. In 36.211, the combination of its two length-31 sequences differs between subframes, so the UE learns whether it sees subframe 0 or subframe 5. With that, the UE knows where the 10 ms radio frame starts.

PSS moves between FDD and TDD : the last symbol of slots 0 and 10 in FDD, and the third symbol of subframes 1 and 6 in TDD.
Both signals sit in the centre 6 resource blocks : the UE can find them without knowing the channel bandwidth.
SSS gives the frame timing : its sequence combination tells subframe 0 from subframe 5.

Where to read more

Each item below goes one level deeper than this summary. The Matlab pages use the LTE Toolbox, and the physical layer pages cover PSS and SSS one at a time.

If you are interested in how these synchronization signal is physically generated and how it looks like in constellation, refer to following pages.

Reference

TS 36.211 v19.3.0 - E-UTRA physical channels and modulation, clause 6.11 synchronization signals