2. HOW TO DESIGN THE GSCN IN 5G NR SA NETWORK: PART 2

2.3 Synchronization Raster and GSCN

Synchronization Raster indicates the centre frequency of the SS block that can be used by the UE for system acquisition when explicit signalling of the synchronization block position is not present.

An SS Block consist of 20 RBs (from RB0 to RB19), so the Subcarrier 0 of the RB10 (i.e. 11th RB) is considered the centre of the SSB.

·      A center frequency belonging to the synchronization raster is called Global Synchronization Channel Number (GSCN), i.e. the GSCN is equivalent to the NR-ARFCN used by the channel raster.

·      In case of ENDC, UE does not need to do blind search for SSB since the frequency, subcarrier spacing etc. are all configured by LTE RRC Connection Reconfiguration message.

·      But in SA, UE need to blindly detect SSB since UE need to identify this before it receives MIB/SIB. If UE needs to search SS Block based on ARFCN raster, it would take long time since ARFCN raster is very narrow. GSCN is used to define a SSB searching frequency in wider steps.

The three main requirements for designing the SSB center frequency are:

·      The Resource Blocks occupied by the SS Blocks must be within the channel bandwidth

·      The SS Blocks must be subcarrier aligned with the Resource Blocks belonging to the channel bandwidth.

·      The SS Block is defined as per the GSCN raster.

For carrier frequency 0 to 3 GHz, there are 3 rules to calculate the GSCN.

Rule 1: Compute N by Floor (Carrier Center Frequency / 1.2 MHz) assuming SSB is at center. The carrier center frequency would be as per 100 kHz raster.

Rule 2: Compute M (1 or 3 or 5) such that SSB Frequency is orthogonal to carrier center frequency i.e. the difference between the centre frequency of the carrier and the centre frequency of the SSB is integer multiple of 15 kHz.

Example 2:  Suppose an operator is allocated from 778 MHz to 788 MHz within operating band n28 (NR 700). The centre frequency is 783 MHz. Assume, the SSB centre frequency is around the the centre of the channel bandwidth. Calculate the GSCN.

·      SSB Centre Frequency = N*1200 KHz + M*50 KHz

Now we need to calculate the value of N assuming SSB centre frequency as 783 MHz.

N = Floor (Centre Frequency/1.2 MHz)

    = Floor (783 MHz/1.2 MHz)

    = 652

For N=652 and M=1, SSB Centre Frequency = 652*1200 KHz + 1*50 KHz = 782.45 MHz, here the difference between the centre of the carrier and the SSB centre frequency is (783-782.45) =0.55, which is not integer multiple of 15 KHz and is not subcarrier aligned.

Now consider For N=652 and M=3, SSB Centre Frequency = 652*1200 KHz + 3*50 KHz = 782.55 MHz, here the difference between the centre of the carrier and the SSB cenre frequency is (783-782.55) =0.45, which is Integer multiple of 15 KHz and is subcarrier aligned. So this candidate frequency is selected as SSB centre frequency.

GSCN = 3N + (M-3)/2 = 3*652 + (3-3)/2 = 1956//