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)


42 comments:

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

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

    I think you mean "From SSS".

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    Replies
    1. Hi thanks for pointing out this mistake.

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

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

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    Replies
    1. 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

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

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

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  4. 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,

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    Replies
    1. 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

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  5. 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

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    Replies
    1. 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

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

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

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

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  8. 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.

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    Replies
    1. 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 ?

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  9. 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

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    Replies
    1. 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

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  10. This comment has been removed by the author.

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  11. 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

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  12. 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???

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  13. 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..!

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    Replies
    1. 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

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  14. 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.

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

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  15. Hi why SSS comes before PSS any specific reason

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  17. 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?

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

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    2. 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,

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

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

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

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    Replies
    1. 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.....

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    ReplyDelete
  22. PSS & SSS are transmitted through which channel???

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

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    ReplyDelete