Primary and secondary synchronization signals (PSS & SSS) in LTE


Cell synchronization is the very first step when UE wants to camp on any cell. From this, UE acquires physical cell id (PCI), time slot and frame synchronization, which will enable UE to read system information blocks from a particular network.

UE will tune it radio turn by turning to different frequency channels depending upon which bands it is 
supporting. Assuming that it is currently tuned to a specific band / channel, UE first finds the primary 
synchronization signal (PSS) which is located in the last OFDM symbol of first time slot of the first subframe (subframe 0) of radio frame as shown in figure (green squares). This enables UE to be synchronized on subframe level. The PSS is repeated in subframe 5 which means UE is synchronized on 5ms basis since each subframe is 1ms. From PSS, UE is also able to obtain physical layer identity (0 to 2).

In the next step UE finds the secondary synchronization signal (SSS). SSS symbols are also located in the same subframe of PSS but in the symbol before PSS as shown in the figure(yellow squares). From SSS, UE is able to obtain physical layer cell identity group number (0 to 167).

Using physical layer identity and cell identity group number, UE knows the PCI for this cell now. In LTE 504 physical layer cell identities (PCI) are allowed and are divided into unique 168 cell layer identity groups where each group consist of three physical layer identity. As mentioned earlier, UE detects physical layer identity from PSS and physical layer cell identity group from SSS. Assuming physical layer identity = 1 and cell identity group=2 then the PCI for given cell is

PCI = 3*(Physical layer cell identity group)+ physical layer identity = 3*2+1 = 7

Once UE knows the PCI for a given cell, it also knows the location of cell Reference signals as shown in figure (red and black squares). Reference signals are used in channel estimation, cell selection / reselection and handover procedures. 



LTE Resource Grid (Source: http://paul.wad.homepage.dk/LTE/lte_resource_grid.html)

After cell synchronization procedure, UE will proceed to read Master information and other System information blocks
Please see below sections
Master information block (MIB)
System information block 1 (SIB1)
System information block 2 (SIB2)


47 comments:

  1. AnonymousFebruary 24, 2013 at 7:06 PM

    Excellent job here. Thanks very much. I finally have come to understand this complex procedure intuitively.

    ReplyDelete
  2. UnknownMarch 12, 2013 at 9:23 PM

    "From PSS, UE is able to obtain physical layer cell identity group number (0 to 167)."

    I think you mean "From SSS".

    ReplyDelete
    Replies
    1. Adnan BasirMarch 12, 2013 at 10:36 PM

      Hi thanks for pointing out this mistake.

      Delete
    2. UnknownApril 25, 2017 at 5:20 AM

      Hi ,
      Can you please explain on which channel PSS & SSS transmitted???

      Delete
  3. fatouApril 11, 2013 at 3:31 AM

    Hi, please can you help me
    I don't understand how can we know from the PCI the RS location?
    thank you

    ReplyDelete
    Replies
    1. Adnan BasirApril 11, 2013 at 4:33 AM

      Hi,
      The location of RS is fixed in time domain but changing in frequency domain. There are only six different locations or shifts possible for RS allocations. In other words for 84 PCIs there is one fixed location (84*6=504). So once the UE knows PCI, it also knows which one of the six possible shift/location is used for RS allocation
      Use this resource grid generator http://paul.wad.homepage.dk/LTE/lte_resource_grid.html and you will understand what i am saying. As you increase the PCI value from 0 and above, the RS location is repeating in cycle after 6 shifts

      Delete
    2. fatouApril 12, 2013 at 2:26 AM

      ok!!! Thank you very much! we get it!

      Delete
    3. Anjalika KumarJune 1, 2015 at 2:38 AM

      Hi can you please explain RS allocation is affected by MIMO or diversity scenarios

      Delete
  4. zinebApril 12, 2013 at 7:36 AM

    Hi,

    I am using VSA for a LTE signal demodulation. knowing the PCI,I have already set the RS frequency shift, but still the demodulation is not correct.
    Is there any other parameters to set apart from the center frequency, the bandwith and the RS location...?
    If any ideas, please let me know

    Thanks,

    ReplyDelete
    Replies
    1. Adnan BasirApril 14, 2013 at 11:00 PM

      Hi Zineb,
      I dont have much experience on VSA tools, sorry for that

      Probably you can ask question here
      http://www.home.agilent.com/owc_discussions/forum.jspa?forumID=87

      Br,
      Adnan

      Delete
  5. zinebApril 15, 2013 at 8:44 AM

    ok Adnan
    thank you

    ReplyDelete
  6. Tomasz StraszydłoApril 29, 2013 at 3:20 AM

    Hi,

    I wonder how the UE keep the synchronization after achieving initial synchronization. I mean, whether it constantly monitor the PSS and SSS or it keep synchronization in another way. What is the difference in IDLE and CONNECTED mode in this regard?

    Thank you in advance

    ReplyDelete
    Replies
    1. Adnan BasirMay 2, 2013 at 9:21 PM

      Hi,
      My understanding is as below
      * PSS and SSS are for initial synchronization only (idle to connected ) or every time RACH is required
      * During call, reference signals are used for synchronization
      * In idle mode, UE monitors PA-RNTI on every paging cycle

      Delete
    2. Tomasz StraszydłoMay 15, 2013 at 2:13 PM

      Thank you for your response,
      Do you know the 3GPP reference, where it is described?

      Delete
  7. WeterokASPSeptember 17, 2013 at 10:54 AM

    Hello!
    Sorry for offtopic:
    Сan be packaged in one RB for the user data and signaling information?

    ReplyDelete
  8. Adnan BasirSeptember 18, 2013 at 12:17 PM

    Hi.. Yes user data (DRB)and signalling information (SRB) can be sent in one resource block ..

    ReplyDelete
  9. Anil BhattSeptember 25, 2013 at 1:43 PM

    Its mentioned here that PSS is repeated in sub-frame 5.
    But, I have read that PSS is sent in sub-frame 0 and sub-frame 9.
    Can any1 please explain.

    ReplyDelete
    Replies
    1. Adnan BasirOctober 6, 2013 at 10:18 PM

      primary synchronization signal is transmitted twice per radio frame, so it is repeated in subframe 5 to provide 5m synchronization.. what is your source of information ?

      Delete
  10. PavanSeptember 26, 2013 at 10:21 AM

    Hi Adnan,

    In your previous posts you explained:-
    In other words for 84 PCIs there is one fixed location (84*6=504).

    Does this 84 number is derived from 12 (sub carriers) * 7 Symbols = 84 [Resource grid] from 1.4 Mhz BW ?
    or is this Resource grid block [12 *7] is constant for all BW's in LTE ?

    Please explain and correct me if y understanding is wrong

    ReplyDelete
    Replies
    1. Adnan BasirOctober 6, 2013 at 11:18 PM

      Hi Pavan,
      This number 84 is not derived from 12*7 .. you are getting it wrong.. what i meant is that if we change PCI value by one then there is one shift in RS in frequency domain. but after every 6 shifts all RS comes to same location. so if 504/6 = 84...there fore we can say that 84 PCIs share same RS location

      Delete
  11. Shahid SNovember 30, 2013 at 12:40 AM

    This comment has been removed by the author.

    ReplyDelete
  12. Shahid SNovember 30, 2013 at 12:42 AM

    Join www.TelecomTube.com

    ReplyDelete
  13. Nidal ZarifehDecember 2, 2013 at 9:55 AM

    Hi Adnan,

    Thanks for the amazing blog. I need a detailed description (on bit or byte level) of SIB1. Please give me some direct reference.
    Best regards,
    Nidal

    ReplyDelete
  14. KamikazeDecember 9, 2013 at 10:53 PM

    hi,
    i have a query about the part just below and above pss/sss which is in black.
    1) it says it is unused by a specific antenna port or undefined for all antenna port , what does this mean exaclty??

    2) also if one were to map PRS (positioning reference signal), will it be mapped in this black spots ??? i mean it is exactly not pss/sss, and in specs it says that PRS will not be mapped to the resource elements reserved for PSS/SSS and PBCH???

    ReplyDelete
  15. Utku ÖztürkJanuary 29, 2014 at 4:07 PM

    Adnan hi,
    Long time no see after Helsinki (=
    I have a question about PCI planning. Is it technically possible to assign different SSS for the cells on the same enodeb (at same earfcn)...? Thus the PCIs will be non-consecutive..!

    ReplyDelete
    Replies
    1. Adnan BasirJanuary 30, 2014 at 10:42 AM

      Hi Utku,
      Good to hear from you... EARFCN basically defines the bandwidth allocated to a given cell.. while PCI is used to identify a cell and is used in synchronization process.. so it is possible for two adjacent cells to have same EARFCN but PCI has to be different of-course else there will be synchronization issue. If the PCI is different then SSS is also different for these two cell...

      Example

      Lets assume we have eNodeB with two cells. Following configuration is possible

      Cell1 Earfcn: 2600
      Cell 1 PCI : 10

      Cell2 Earfcn: 2600
      Cell2 PCI: 11

      PCI can be any value as long as they are not same for adjacent cells

      Delete
    2. Adnan BasirJanuary 30, 2014 at 10:44 AM

      adjacent or neighbor cells *

      Delete
  16. Smruti Ranjan SahooMarch 8, 2014 at 9:51 PM

    might be this is a very silly question can you please explain how UE will know that this is the bandwidth where i will look for PSS and SSS , since till that point MIB has not been broadcast.
    And also which one is 0 and 10th slot, 7th symbol as there is no information of all those things.

    ReplyDelete
    Replies
    1. Chris PadleyFebruary 17, 2015 at 10:42 AM

      by either a preferred frequency list or by scanning a 100 KHs until it gets a correlation hit with the PSS

      Delete
  17. Dhiraj GulatiMarch 26, 2014 at 10:39 PM

    Hi why SSS comes before PSS any specific reason

    ReplyDelete
  18. Mark JohnsonJuly 24, 2014 at 3:15 AM

    Join www.telecomtube.com

    ReplyDelete
  19. Mark JohnsonJuly 24, 2014 at 3:16 AM

    This comment has been removed by the author.

    ReplyDelete
  20. S.K. DakshOctober 2, 2014 at 9:42 AM

    Hi Adnan,

    Since SSS uses only one OFDM symbol(#5 in # 0Subframe & #5 in #5 Subframe) using QPSK. So, it can only represent a 2bit information. But there could be 168 possible Pseudo Random sequences of SSS.
    Then how UE gets the information of one of the 168 possible combinations of SSS?

    ReplyDelete
    Replies
    1. DARVISH DARVISHIJanuary 15, 2015 at 2:27 PM

      How does UE know to such slot and symbol it has to camp on to perform synchronization, thx!

      Delete
    2. Chris PadleyFebruary 17, 2015 at 10:50 AM

      The UE initially has no timing info by decoding successfully the PSS it then achieves slot and symbol sync, probably why PSS is at the end of the slot in FDD. By working out which of the 168 sequences in use on SSS the channel is decoded,

      Delete
  21. SunnyFebruary 4, 2015 at 9:02 AM

    Hi Adnan,
    Why PSS is positioned after SSS if PSS needs to be decoded first?

    ReplyDelete
  22. alhaddadFebruary 15, 2015 at 9:05 AM

    UE get frequency from PSS, yes or no and How?

    ReplyDelete
  23. alhaddadFebruary 15, 2015 at 9:11 AM

    Hi adnan,
    UE get frequency from PSS yes or no and How?
    Thanks

    ReplyDelete
    Replies
    1. Chris PadleyFebruary 17, 2015 at 10:44 AM

      No UE must retune and step at 100 KHz until it gets a correlation hit with the PSS, (or goes to last known freq.... typical 3pp UE behavior).
      Knowing the EARFCN is only useful in FDD for the UE to calculate the UL freq.....it already knows the Dl freq.....

      Delete
  24. Assault CubeHackOctober 26, 2015 at 3:47 PM

    Bonjour à tous ! sur mon site vous trouverez des codes Free Wifi privé et qui ne ce désactive jamais ou du moin pour longtemps !

    ︻̷̿┻̿═━一 identifiant free wifi gratuit

    ReplyDelete
  25. UnknownNovember 27, 2015 at 11:53 PM

    PSS & SSS are transmitted through which channel???

    ReplyDelete
  26. Blog SupporterFebruary 11, 2016 at 10:09 AM

    This is great information about Primary and secondary synchronization signals. I really love it. Thanks for Sharing with us.

    Regard - Blog Tutorial Supporter

    ReplyDelete
  27. UnknownJune 20, 2017 at 10:16 PM

    I have a question regarding the PSS and SSS. IF PSS and SSS are transmitted on all the 4 antenna ports, then how will UE decode the PSS and SSS.indentify which antenna RS is present where in 4x4 MIMO

    ReplyDelete
  28. Biplab MallickJuly 5, 2017 at 5:36 AM

    Thank you. Very clear..for long time i did not get the clarity. now got it

    ReplyDelete
  29. Sai RamNovember 27, 2017 at 10:58 PM

    Complexity has been simplified in this article ...Thank You very much.

    ReplyDelete
  30. korada harshaDecember 29, 2017 at 7:23 AM

    Could be a basic question. How could the PSS and SSS could be indentified without knowing the bandwidth information. As MIB is decoded after frame synchronization and cell identification?

    ReplyDelete
  31. Venkata SubramanianMarch 12, 2018 at 10:56 PM

    Could you please let me know the way to calculate the Sync Signals (PSS&SSS) Power for a 40W, LTE 2100 MHz Network.

    ReplyDelete

Subscribe to: Post Comments (Atom)