Understanding PSS,SSS(SSBlock) in NR and LTE

Hi Folks,

In this article I will try to explain about what is SS/PBCH block and how this is differ from LTE. But before we deep dive into this topic this is must(or good) to know what is this PSS and SSS.

PSS and SSS signal help UE to decode the Physical Cell ID. knowing physical Cell ID is very must and important for UE before it can initiate further call/data process with network.
PSS — Primary Synchronization Signal is present in central 72sub-carrier. mapped to 72 active sub carriers(6 resource blocks), centered around the DC subcarrier in slot 0 (Subframe 0) and slot 10 (Subframe 5) in FDD and slot 2 (Subframe 2) and slot 12 (Subframe 6) in TDD.
SSS — Secondary Synchronization Signal, Mapped to 72 active sub carriers(6 resource blocks), centered around the DC subcarrier in slot 0 (Subframe 0) and slot 10 (Subframe 5) in FDD, just one symbol before the PSS.
Below image will clear some of you doubt:
PSS and SSS is present in those two above mentioned position in every radio frame.

above example is with 1.4Mhz channel b/w and hence 6PRB.
If you notice here PSS(green) is present in only central 72 subcarrier and present in last OFDM symbol , slot 0 of subframe 1 and slot 0 of subframe 5(FDD). and SSS present one symbol before the PSS.
These PSS and SSS are generated using Zad-off-Chu sequence .
After decoding the PSS UE knows Physical layer cell identity and after decoding SSS UE knows Physical layer cell identity group.

PSS — contains value 0,1,2.
SSS — contains value 0,1…167
Physical Cell Id — 3*SSS+PSS = For eg: 3*10+2 = 32
So in LTE maximum number of Physical cell value could be 504.
Cell Id = 3*167+2 = 503. So 0….503 = 504

So 32 is the Cell(primary cell)selected by UE to read MIBs/SIBs and do other process.

Now UE knows 32 is the Cell Id for him. Now it will try to decode MIBs/SIBs which are broadcasted in PBCH and PDSCH. Remember MIB flows from BCCH->BCH->PBCH and SIB flows from BCCH->DLSCH->PDSCH.
and if you notice the above grid, PBCH(Blue) is present in subframe 0. So once UE decode PBCH it will get to know the MIB contents(24bits in LTE)

So combination of PSS+SSS and PBCH is called SSblock in LTE.

Similarly in NR we have SSBlock(SSS,PSS and PBCH). PSS and SSS helps UE to get Frame, Slot, Symbol synchronization apart from getting Physical Cell Id info. In NR maximum physical cell could be double of LTE i.e 1008
Physical Cell Id = 3*SSS+PSS
where SSS can range from {0…335} and PSS{0,1,2}
Physical Cell Id = 3*335+2 = 1007 i.e 0….1007 = 1008 physical cells

Below diagram shows where exactly in time domain and frequency domain SSB block is present in resource grid. first see the diagram carefully and Iwill explain each part one-by-one

So from above picture it is very much evident that PSS and SSS location in time domain is 1 OFDM symbol and PBCH location spread across 3 OFDM symbol in which it occupies all the 240SC(20RB*12SC) in OFDM symbol 1 and 3 and occupies 48SC(4RB*12SC) in OFDM symbol 2. So can you tell me how many OFDM symbol PBCH takes in resource grid ?
576RE = 240+240+48+48.
Now we already know PSS will be available between SC 56, 57.. 182 similarly SSS available between 56,57..182. This information can be gathered from below table. This table tells frequency domain location for PSS,SSS and PBCH etc.

Now to understand where does PSS,SSS present exactly in SSblock, we need to understand SS Burst. This SS Burst is confined to 5ms window so this is present either in first half of frame or second half of frame. Each SS Burst will contains SS Blocks, and to know how many SS Blocks are present in SS Burst , it depends on carrier frequency. See below table:

So if carrier frequency is ≤3Ghz then ,4 SSblock will present in SSBurst.

Now will understand SSB Location in time domain. let’s see below table. i will explain in resource grid what this table says:

Let’s consider the first row where SCS is 15Khz. OFDM starting symbol will be{2,8}+14n. If Freq ≤ 3Ghz, SSB will be start from symbol 2,8,16,22. and same i have explained in the resource grid how it will look, and frequency domain allocation for PSS,SSS and PBCH i have explained above.

Q: How UE knows which SSB is active in a Burst ?
A: gNB tells this to UE in SIB1 message. ServingCellConfigCommonSIB (given below)ssb-PositionInBurst

That’s all folks. This is my initial understanding after reading specs and other 5G links. Please let me know if something is wrong or incorrect i will try to check and update that. Thanks for understanding.