LTE PSS AND SSS

[plannerguy]

Dear experts


While going through the initial cell search mechanism in lte(fdd mode) understood that in a 10msec radio frame we have two PSS and two SSS . why is required to have two sync channels? also SSS apears first then PSS. why is it designed like that.

Rgds
Plannerguy

[giegala]

Hi,

PSS allows quick correlation because there are only 3 sequences for PSS. Because of that, it just gives timing(or synch) for slot. Hence it is not possible to figure out where frame starts. SSS is used for that because two different SSS sequences will be transmitted at 2 different occasions per frame.

Besides that difference in time between SSS and PSS will give:
- whether cell is using Extended or Normal CP
- whether cell is FDD or TDD

[plannerguy]

Hi

Thanks for quick reply.

But my query is why we have two PSS configured in a radio frame is single not sufficient? also since PSS is decoded first , why SSS is placed before PSS.

Rgds
Plannerguy

[giegala]

Hi

this little of guessing from my side, but:
- if you are sending 2 PSS, detection of a cell is 2 times quicker - very important for incoming inter-freq & inter-rat mobility (like HO)
- if you'll look on the first occurrence of PSS in a Frame, there is MBCH right after it, which is 4 symbols long. Probably putting MBCH in front of PSS is more challenging than putting SSS.

on other hand because all the receivers operate in digital domain, there should be no difference in what comes first - PSS or SSS.