Exploring 5G — Synchronization signals
There are 2 types of synchronization signals PSS(primary synchroniztion signal) and SSS(Secondary synchronization signal) in 5G. we will first explore their location in resouce grid first and then eventually go into the details of each one .
To understand the mapping of PSS and SSS we need to explore a new term called SS/PBCH block .
Time-frequency structure of an SS/PBCH block
- A SS/PBCH block carries PSS,SSS,PBCH and related DM-RS.
- A SS/PBCH block consists of 4 OFDM symbols in time domain and 240 subcarriers in frequency domain.
- Mapping of all channels in time and frequency domain is determined using following table. in this table v = PHY_CELL_ID mod 4.
- Lets visualize the SS/PBCH block using above table.
- SS/PBCH block will be transmitted in half frame i.e. 5 ms (5 subframes) of a radio frame.
- Location of first symbol of SS/PBCH block is dependent on subcarrier spacing and is calculated as follows.
Lets take few examples to understand the location of SS/PBCH blocks .
Example 1 : subcarrier spacing = 30 KHz , carrier frequency > 3 GHz (Case A)
from the above table yellow highlighted values will be used .
n = 0,1
symbol index = {2,8}+14*n
first take 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}
total number of index (L)= 4 and symbol index set = {2,8,16,22}
From Above diagram you can see there are 4 SS/PBCH blocks in half frame with starting indexes 2,8,16,22 for case A.
Example 2: subcarrier spacing = 30 KHz , 3 GHz<carrier frequency < = 6GHz (Case B)
here n = 0,1 ; symbol index = {4,8,16,20}+28*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}
total number of index(L) = 8 and symbol index set = {4,8,16,20,32,36,44,48}
- Similarly you can calculate L value and each index for each subcarrier spacing. here L is # beams.
- A parameter(ssb-PositionsInBurst) for time domain location of SS-blocks from RRC is also sent to UE in SIB-1 . And one more parameter ssb-PeriodicityServingCell is sent in SIB-1 to know periodicity of SSB.
SIB1 ::= SEQUENCE {
frequencyOffsetSSB ENUMERATED {khz-5, khz5} OPTIONAL, -- Need R-- Time domain positions of the transmitted SS-blocks in an SS-Burst-Set (see 38.213, section 4.1)ssb-PositionsInBurst SEQUENCE {
inOneGroup BIT STRING (SIZE (8)),
groupPresence BIT STRING (SIZE (8)) OPTIONAL -- Cond above6GHzOnly},
-- The SSB periodicity in msec for the rate matching purpose (see 38.211, section [7.4.3.1])ssb-PeriodicityServingCell ENUMERATED {ms5, ms10, ms20, ms40, ms80, ms160, spare1, spare2}
- For initial cell selection, a UE may assume that half frames with SS/PBCH blocks occur with a periodicity of 2 frames.
There are few more terms introduced in 5G related to synchronization , lets explore them in detail.
SS Burst
SS burst is a set of SS blocks(SS/PBCH block) in a half subframe . we have seen few examples where in a half subframe there are multiple SS/PBCH blocks present .
In CASE B example there are 8 SS/PBCH blocks (starting indexes 4,8,16,20,32,36,44,48) present in half frame . this set is called SS burst. similarly there will be different number of SS blocks in one SS burst for different subcarrier spacing.
Lets draw case B again to visualize SS burst.
Each SS blocks (in this example total 8) are for different beam. SIB-1 parameter ssb-PositionsInBurst tells about time domain position of SS block in the SS Burst.
ssb-SubcarrierOffset from MIB tells about frequency location of SS block.
In summary gNB will send a list of MIB and SIB-1 corresponding to each beam and for each SS block (corresponds to each beam ), time and frequency location will be send in MIB and SIB-1.