What are synchronization signals? Why do we need them? How are they and where are they transmitted in LTE? The below article should explain the same. Basically, as the name suggests the synchronization signals are needed for a UE which is trying to enter the network to get synchronized to the eNodeB or even for a UE to maintain its already gained synchronization. There are two synchronization signal in LTE downlink, Primary synchronization signal (PSS) and Secondary synchronization signal (SSS), below you find more details about these signals,
Primary Synchronization Signal (PSS)
PSS is a zadoff-Chu sequence of length 62, whose root index is chosen based on the NID2 value, which is got from the physical cell ID. There can be three different NID2 values (0, 1, 2), hence there are 3 different root indexes (25, 29, 34) corresponding to the NID2 values. The length of the PSS is 72 subcarriers or 6 resource blocks, out of 72 only 62 subcarriers are having valid PSS data, remaining 10 subcarriers (5 on each side) are zero padded.
PSS Resource Mapping
The PSS is always mapped to the central 72 subcarriers, this is to assist the UE to decode the PSS irrespective of knowing the system bandwidth. The central 72 subcarriers may not always align with the resource block boundary, it can always exist in half RBs also. For Eg: In case of 5 MHz the central 6 RBs do not exactly align with the center of the bandwidth, hence the PSS mapping is done as, first 6 subcarriers in second half of RB9, next 60 subcarriers to RB10 to RB14 and remaining 6 subcarriers in first half of RB15. The PSS is always mapped in last symbol of first slot in subframe 0 and 5, when it is a FDD system and in 3 symbol of first slot in subframe 1 and 6, when it is a TDD system
Since PSS is a Zadoff-Chu sequence, when plotted as a constellation diagram we should see a circle
Secondary Synchronization Signal (SSS)
The SSS is a combination of 2 31 length binary sequence, where these binary sequences are function of NID1, there can be 168 different NID1 values, hence there are 168 different binary sequence corresponding to these NID1. Also these sequences differ between subframe 0 and subframe 5, infact this the way UE gets the subframe number within the radio frame. These binary sequences are also scrambled with a scrambling sequence which is function of NID2, hence creating a coupling between PSS & SSS.
The SSS is also mapped similar to PSS in the frequency domain, occupying the central 72 subcarrier, with 62 valid SSS subcarriers. But SSS is mapped to the last but one symbol of first slot in subframe 0 and 5 for FDD and last symbol of second slot of subframe 0 & 5 in a TDD system. Since SSS is a binary sequence, when plotted we should see two dots on the x-axis.
The symbol location of PSS/SSS in time domain is different between a FDD and TDD system as this helps the UE to identify, if this is a FDD or a TDD system.
Since the location of PSS/SSS is always fixed in frequency domain, the UE can easily do a correlation at the expected band to get the PSS/SSS, from which the UE can aquire many parameters such as the physical cell ID (From NID1 & NID2), duplexing mode( from the location of PSS/SSS in time domain), subframe number( from the SSS sequence), slot boundary as well.
For a new UE, the PSS/SSS helps to get synchronized to the eNodeB and for a idle UE within the service of eNodeB, the PSS/SSS helps to maintain the synchronization. Hence these synchronization signals play a very important role in LTE.