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.


Friday, 30 December 2016

Reasons for RRC Connections Reestablishment

As per 3GPP spec, 36.331 (RRC Protocol Spec), RRC Connection Reestablishment initiates after these conditions occur :
1. Upon detection of RLF (Radio Link Failure)
2. After Handover failure
3. Integrity check failure indication received from lower layers.
4. RRC connection Reconfiguration Failure.
5. Mobility from E-UTRA failure.

Thursday, 29 December 2016

SSS in LTE (secondary synchronization signals)

SSS stands for secondary synchronization signals.

SSS comes in the symbol just before PSS comes. If you don't know, how did we detect PSS, then goto this the previous post of PSS and SSS in LTE (Primary and secondary synchronization signals)

SSS is basically an m-sequence or Maximum length sequence.

Now, What is m-sequence?
M-sequence is a pseudo random binary sequence. These sequence can be generated just by cycling through every possible state of a shift register of length resulting in a sequence of length . Here, Three m-sequences, each of length 31, are used to generate the secondary synchronization signals.

SSS Generation

Two binary sequences, each of length 31, are used to generate the SSS. Sequences s0(m0) and s1(m1) are different cyclic shifts of an m-sequence, ˜s. The indices m0 and m1 are derived from the cell-identity group, NID(2) and determine the cyclic shift. The values can be read from table 6.11.2.1-1 in "Physical Channels and Modulation." 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA)."
The two sequences are scrambled with a binary scrambling code (c0(n), c1(n)), which depends on NID(2).
The second SSS sequence used in each radio frame is scrambled with a binary scrambling code (z1(m0)z1(m1)) corresponding to the cyclic shift value of the first sequence transmitted in the radio frame.

Binary Sequence Generation

s0(m0) and s1(m1) are given by the following equations.
s(m0)0=˜s((n+m0)mod31)
s(m1)0=˜s((n+m1)mod31)
˜s is generated from the primitive polynomial x5+x2+1 over the finite field GF(2).
c0(n) and c1(n are given by the following equations.
c0(n)=˜c((n+N(2)ID)mod31)
c1(n)=˜c((n+N(2)ID+3)mod31)
˜c is generated from the primitive polynomial x5+x3+1 over the finite field GF(2).
z1(m0) and z1(m1) are given by the following equations.
z(m0)1=˜z((n+(m0mod8))mod31)
z(m1)1=˜z((n+(m1mod8))mod31)
˜z is generated from the primitive polynomial x5+x4+x2+x+1 over the finite field GF(2).

Mapping of the SSS

The scrambled sequences are interleaved to alternate the sequence transmitted in the first and second SSS transmission in each radio frame. This allows the receiver to determine the frame timing from observing only one of the two sequences; if the first SSS signal observed is in subframe 0 or subframe 5, synchronization can be achieved when the SSS signal is observed in subframe 0 or subframe 5 of the next frame.
The SSS is transmitted in the same subframe as the PSS but one OFDM symbol earlier. The SSS is mapped to the same subcarriers (middle 72 subcarriers) as the PSS.
The SSS is constructed using different scrambling sequences when mapped to even and odd resource elements.
  • Even resource elements:
    • Subframe 0: d(2n)=s(m0)0(n)c0(n)
    • Subframe 5: d(2n)=s(m1)1(n)c0(n)
  • Odd resource elements:
    • Subframe 0: d(2n+1)=s(m1)1(n)c1(n)z(m0)1(n)
    • Subframe 5: d(2n+1)=s(m0)0(n)c1(n)z(m1)1(n)

d(n) is mapped from lowest subcarrier to highest subcarrier.

References:
[1]mathworks.com
[2] 3GPP TS 36.211. "Physical Channels and Modulation." 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA). URL: http://www.3gpp.org.