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)
Please see below sections
Master information block (MIB)
System information block 1 (SIB1)
System information block 2 (SIB2)
Excellent job here. Thanks very much. I finally have come to understand this complex procedure intuitively.
ReplyDelete"From PSS, UE is able to obtain physical layer cell identity group number (0 to 167)."
ReplyDeleteI think you mean "From SSS".
Hi thanks for pointing out this mistake.
DeleteHi ,
DeleteCan you please explain on which channel PSS & SSS transmitted???
PSS and SSS are signals transmitted in PDSCH.
DeleteHi, please can you help me
ReplyDeleteI don't understand how can we know from the PCI the RS location?
thank you
Hi,
DeleteThe 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
ok!!! Thank you very much! we get it!
DeleteHi can you please explain RS allocation is affected by MIMO or diversity scenarios
DeleteHi,
ReplyDeleteI 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,
Hi Zineb,
DeleteI 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
ok Adnan
ReplyDeletethank you
Hi,
ReplyDeleteI 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
Hi,
DeleteMy 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
Thank you for your response,
DeleteDo you know the 3GPP reference, where it is described?
Hello!
ReplyDeleteSorry for offtopic:
Сan be packaged in one RB for the user data and signaling information?
Hi.. Yes user data (DRB)and signalling information (SRB) can be sent in one resource block ..
ReplyDeleteIts mentioned here that PSS is repeated in sub-frame 5.
ReplyDeleteBut, I have read that PSS is sent in sub-frame 0 and sub-frame 9.
Can any1 please explain.
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 ?
DeleteHi Adnan,
ReplyDeleteIn 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
Hi Pavan,
DeleteThis 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
This comment has been removed by the author.
ReplyDeleteJoin www.TelecomTube.com
ReplyDeleteHi Adnan,
ReplyDeleteThanks 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
hi,
ReplyDeletei 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???
Adnan hi,
ReplyDeleteLong 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..!
Hi Utku,
DeleteGood 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
adjacent or neighbor cells *
Deletemight 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.
ReplyDeleteAnd also which one is 0 and 10th slot, 7th symbol as there is no information of all those things.
by either a preferred frequency list or by scanning a 100 KHs until it gets a correlation hit with the PSS
DeleteHi why SSS comes before PSS any specific reason
ReplyDeleteJoin www.telecomtube.com
ReplyDeleteThis comment has been removed by the author.
ReplyDeleteHi Adnan,
ReplyDeleteSince 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?
How does UE know to such slot and symbol it has to camp on to perform synchronization, thx!
DeleteThe 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,
DeleteHi Adnan,
ReplyDeleteWhy PSS is positioned after SSS if PSS needs to be decoded first?
UE get frequency from PSS, yes or no and How?
ReplyDeleteHi adnan,
ReplyDeleteUE get frequency from PSS yes or no and How?
Thanks
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).
DeleteKnowing 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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PSS & SSS are transmitted through which channel???
ReplyDeleteThis is great information about Primary and secondary synchronization signals. I really love it. Thanks for Sharing with us.
ReplyDeleteRegard - Blog Tutorial Supporter
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
ReplyDeleteThank you. Very clear..for long time i did not get the clarity. now got it
ReplyDeleteComplexity has been simplified in this article ...Thank You very much.
ReplyDeleteCould 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?
ReplyDeleteCould you please let me know the way to calculate the Sync Signals (PSS&SSS) Power for a 40W, LTE 2100 MHz Network.
ReplyDeleteThis stays as one of the best and attractive post I have read yet. Great sharing this post.
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Why sss is different in one subframe and pss is same?
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