#SSB : Synchronization Signal Block

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,

• PSS uses 127 REs (Subcarrier No. 56 to 182) in OFDM Symbol 0.
• SSS uses 127 REs (Subcarrier No. 56 to 182) in OFDM Symbol 2.
• PBCH and its DMRS together use 576 REs in OFDM Symbol 1, 2 & 3 (from 0 to 239 in Symbol 1 & 3) and in OFDM Symbol 2 (from 0 to 47 & 192 to 239).
• Remaining 130 REs are not used and referred as “set to 0”.

Out of 576 REs of PBCH and its DMRS

• DMRS uses 1/4th of the REs i.e., 576/4 = 144 REs
• PBCH uses 3/4th of the REs i.e., 576*3/4 = 432 REs

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,

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,

• 0+3, 4+3, 8+3,….236+3 in Symbol 1
• 0+3, 4+3, 8+3,…...44+3 in Symbol 2
• 192+3, 196+3,….236+3 in Symbol 2
• 0+3, 4+3, 8+3,….236+3 in Symbol 3

Contents of SSB:

#Synchronization Signals: PSS and SSS are DL Synchronization Signals used by UE for DL Synchronization.

• PSS is used by UE to get radio frame boundary.?
• SSS provides Subframe Boundary i.e., Position of 1st?Symbol in?a Subframe

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.