Saturday, 31 December 2016

Why does PSS & SSS synchronization need?

Before answering the exact question, let's go back little at once UE is switched on.
When UE is switched on, it feels there are numerous signal around itself with different frequencies. So, it is now confused, with which signal he has to go with? Which is its right signal? So it starts scanning the radio signals which it supports. UE supports some specific bands. Bands contain a range of frequencies like band 7.

Band: 7Uplink Freq: 2500 - 2570 MHzDownlink Freq: 2620 - 2690MHz
So it scans the frequencies which is supported by UE, and read PSS. To know more about how to read PSS and SSS, go to PSS signals and SSS signals.

So, Between UE is turned on and UE starts receiving and sending data it needs to synchronize with it's supported frequencies. This is done by PSS and SSS.

Now, Where is PSS transmitted?
PSS is transmitted in the last symbol of 0th and 10th slot in a frame. It gets repeated in each 5 msec time interval.

The green bar in above pic is the PSS.

What does PSS conveys?
PSS conveys the cell id ( N1) in the group. The range of N1 would be 0 to 2. So total 3 id. PSS helps in synchronizing subframe and slot.

So, Where is the SSS transmitted?
SSS is transmitted in the penultimate symbol of 0th and 10th slot in a frame i.e just before PSS. And same as PSS it gets repeated in each 5 msec time interval.

The yellow bar in above pic is the SSS.

What does SSS conveys?
SSS conveys the cell group id (N2). The range of N2 would be 0 to 167. So total 168 groups. SSS helps in synchronizing frame.

Using cell id and cell group id, we calculate cell identity as N2*3 + N1. So the range of cell id would be 0 to 503. So total 504 cell ids.

Lets, calculate a cell id. If N1 is 1 and N2 is 50, so the cell id would be,

Cell id = 50*3 + 1 = 151

Note, this cell identity is NOT the physical global cell identity (PCI). PCI is always unique across the world and transmitted in SIB1.

So, Let's conclude this post, PSS and SS helps in synchronizing Slot and Frame respectively. As well as helps in finding the Cell id in that region. Till now we just synced with the signal transmitted over the air.