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LTE: Why PSS and SSS are placed side by side ?

+1 vote

Why these two are placed side by side within "first time slot" of 0th and 5th sub frame of a radio frame ?
Is it due to any mathematical calculation or something else ?

posted Mar 25, 2014 by Vimal Kumar Mishra

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1 Answer

+4 votes
Best answer

There are two cell search procedures in LTE: one for initial synchronization and another for detecting
neighbor cells in preparation for handover. In both cases, the UE uses two special signals broadcast on each cell:
Primary Synchronization Sequence (PSS) and Secondary Synchronization Sequence (SSS). The detection of these signals allows the UE to complete time and frequency synchronization and to acquire useful system parameters such as cell identity, cyclic prefix length, and access mode (FDD/TDD). At this stage, the UE can also decode the Physical Broadcast Control Channel (PBCH) and obtain important system information.

Synchronization signals are transmitted twice per 10 ms radio frame. The PSS is located in the last OFDM symbol of the first and 11th slot of each radio frame which allows the UE to acquire the slot boundary timing independent of the type of cyclic prefix length. The PSS signal is the same for any given cell in every subframe in which it is transmitted (the PSS uses a sequence known as Zadoff-Chu).

The location of the SSS immediately precedes the PSS – in the before to last symbol of the first and 11th
slot of each radio frame. The UE would be able to determine the CP length by checking the absolute position of the SSS. The UE would also be able to determine the position of the 10 ms frame boundary as the SSS signal alternates in a specific manner between two transmissions (the SSS uses a sequence known as M-sequences).

In the frequency domain, the PSS and SSS occupy the central six resource blocks, irrespective of the system channel bandwidth, which allows the UE to synchronize to the network without a priori knowledge of the allocated bandwidth. The synchronization sequences use 62 sub
-carriers in total, with 31 sub-carriers mapped on each side of the DC sub-carrier which is not used. This leaves 5 sub-carriers at each extremity of the 6 central RBs unused

The above is a copy&paste from a google search.

I'm a Mobile Packet Core Eng. not Radio, but my understanding is

  1. The division into two signals is aimed to reduce the complexity of the cell search process;
  2. The SSS is used to establish the length of Cyclic Prefix (CP), the PSS is just right after it for easines

For more go to Section 6.11 of 36.211

answer Mar 25, 2014 by Bart Barton
Nice explanation Bart...
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As LTE PCI ranges from 0 to 504, why PCI has been divided into PSS and SSS ?

+2 votes

I know PSS and SSS will occupy last and penultimate symbol simultaneously in slot#0 and slot#10. How can we ensure they are occupying that positions only? And what is the reason behind those particular positions?

+5 votes

I understand that according to the standard a 64 FFT is used by the UE for cell search for all Bandwidth configurations(see section 2.2.1 page 7 in the below document). Suppose we are transmitting at 20 MHz which means we do a 2048 FFT on the transmitter side for the whole resource grid, including the 6th and 7th symbols corresponding to PSS and SSS in Subframe 0 and 5. In this case how is it possible for UE to do cell search using 64-FFT. Although PSS and SSS use only the central 62 subcarriers, shouldn't the UE use the same FFT length used in the transmitter side ? Also, how is the DL resource grid processed by the UE ?Does the UE extract Symbols 6,7 of Subframes 0,5 and process it separately with 64-FFT(after low pass filtering) while using 2048 FFT for the rest of the symbols ?

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