5G NR: SS/PBCH Block – Synchronization

A SS/PBCH block comprises of –

• Primary Synchronization Signal (PSS)
• Secondary Synchronization Signal (SSS)
• PBCH
• PBCH DM-RS

SS/PBCH block size –

• Time Domain – 4 OFDM symbols
• Frequency domain – 240 contiguous subcarriers (20 PRBs)

Antenna port p = 4000 is used for transmission of SS/PBCH Block.

2 Types of SS/PBCH Block –

• Type A –  for Sub 6 GHz frequency
• Type B –  for mmWave

Primary Synchronization Signal (PSS) /Secondary Synchronization Signal (SSS)

• Helps UE to get a frame synchronized.
• Made of 127 value sequences and each sequence mapped to 1 symbol.
• It helps to determine Physical cell Id.

Physical Broadcast Channel (PBCH)

• Carries Master Information Block (MIB).
• PBCH payload contains the following information –
  • System Frame Number (SFN) LSB bits.
  • SS/PBCH Block Index (for mmWave)
  • Subpart to determine SS/PBCH Block location in frequency (Sub 6Ghz)
• Length – 3 symbols.

PBCH – Demodulation Reference Signal (PBCH-DMRS)

• PBCH-DMRS helps with the decoding of PBCH.
• Every 4th RE in PBCH is PBCH-DMRS (25% of PBCH)

Time-frequency structure of an SS/PBCH block

Frequency Domain

In the above table

§  v = PHY_CELL_ID mod 4

§  and  represent the frequency and time indices respectively, within one SS/PBCH block.

Time Domain –

Few examples to understand the time domain location of SS/PBCH blocks –

Example 1 –

§  SCS = 15 Khz

§  Frequency = Sub 3Ghz

§  n = 0, 1

§  Symbol Index = {2, 8} + 14*n

For n = 0, symbol index = {2,8} + 14*0 = {2,8}
For n = 1, symbol index = {2,8} + 14*1 = {2+14*1 ,8+14*1} = {16,22}

§  Number of possible SS/PBCH Block = 4

§  SS/PBCH Block start symbol = {2,8,16,22}

Example 2 –

§  SCS = 30 Khz

§  Frequency = 3Ghz < Freq < 6Ghz

§  n = 0, 1

§  Symbol Index = {2, 8} + 14*n

 For n =0, symbol index = {4+28*0,8+28*0,16+28*0,20+28*0} = {4,8,16,20}

 For n =1, symbol index = {4+28*1,8+28*1,16+28*1,20+28*1} = {32,36,44,48}

§  Number of possible SS/PBCH Block = 8

§  SS/PBCH Block start symbol = {4,8,16,20, 32, 36, 44, 48}

http://spectracells.com/2019/11/09/5g-nr-ss-pbch-block-part-1-synchronization/