#SSB : Synchronization Signal Block
Ajay Kumar Singh
Technical Lead-SIT |Experienced Telecom Testing Professional with Expertise in 2G | 3G | LTE | 5G NR-SA/NSA | Protocol Testing | UE logs Analysis| RAN/ORAN Testing | RF Planning and Optimization
A Synchronization Signal Block (SSB) is a block of 20RBs (240 REs) in frequency domain and 4 OFDM symbols in time domain which carries PSS, SSS, PBCH and DMRS of PBCH. It is also referred as SS/PBCH Block.
It is broadcasted by gNB in DL with the periodicity of (5/10/20/40/80/160) in ms and specified by ssb-periodicity in SIB1 message. If this field is absent, UE assumes a periodicity of 5ms. For Initial cell selection UE assumes a periodicity of 20ms.
As an SSB uses 240 REs and 4 OFDM symbols in frequency domain and time domain respectively, hence the total number of REs in SSB are 240x4 = 960 REs.
Out of 960 REs,
Out of 576 REs of PBCH and its DMRS
The DMRS is distributed across the Symbol 1, 2 & 3 and their location depends upon PCI.
As per below formula, the Subcarriers used by DMRS of PBCH is given as,
Where, v = PCI mod 4
There are 1008 unique physical-layer cell identities (PCI) which is given by,
Let us take an example,
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Consider PSS =0 and SSS =1
Then, PCI = 3x1 + 0 = 3
Now, v = (PCI mod 4) = (3 mod 4) =3
Then, DMRS position (REs used by DMRS) will be,
Contents of SSB:
#Synchronization Signals: PSS and SSS are DL Synchronization Signals used by UE for DL Synchronization.
With the help of PSS and SSS UE obtain DL synchronization and calculates the PCI of the serving cell. UE use the PCI to distinguish the Cells.
#PBCH: Physical Broadcast Channel
PBCH is downlink Channel used by gNB to broadcast MIB (Master Information Block) and other L1 information which is mentioned below.
#DMRS:Demodulation Reference Signal.
It is used by a UE for channel estimation for demodulation of associated physical channel i.e., channel estimation for demodulation of PBCH in this case.