2. HOW TO DESIGN THE GSCN IN 5G NR SA NETWORK: PART 3

2.4 CRB and PRB

In NR, there is concept of Common Resource Block and Physical Resource Block.

The Common Resource Blocks are the set of Resource Blocks which occupy the channel bandwidth and numbered from 0 to highest numbers, say 272 for 100 MHz BW with 30 KHz SCS.

The PRBs are numbered per bandwidth Part (BWP) i.e. PRB0 to PRB99 for 1st BWP of 100 RBs, and then PRB 0 to 99 for 2nd BWP of 100 RBs and so on.

The CRB associated with each subcarrier spacing are aligned at 'Point A' i.e. centre of the subcarriers of all numerologies are aligned but edges of the subcarriers are not aligned.

Point A is the center of the subcarrier 0 of all numerologies.

The edges of the carriers depending on the different numerologies are not aligned. So the guard band will be different for each numerology. Because of this, the 1st RB and the last RB will be unused for 30 KHz SCS.

For e.g. in 40 MHz we have 216 RBs with SCS of 15 KHz whereas 108 RBs with SCS of 30 KHz, the remaining 2 RBs, one RB in bottom of the spectrum and one RB in top of the spectrum are unused as shown below.

The UE concludes the 'Point A' using either a frequency offset relative to the SS Block for SA mode or an ARFCN for NSA mode.

·      In case of NSA mode, the position of Point A i.e. absoluteFrequencyPointA is given directly to the UE through RRC Connection Reconfiguration message during SgNB Addition procedure.

. absoluteFrequencyPointA represents the frequency-location of point A expressed as in ARFCN. AbsoluteFrequencyPointA is the Subcarrier 0 of the CRB 0.

·      In case of SA, the position of Point A is not given directly to the UE, instead a frequency offset relative to the SS Block is given to the UE through SIB1. This is because the size of the PointA ARFCN is 22 bit, which would make SIB1 very large.

·      The frequency offset is consist of 2 parts:

i> offsetToPointA: It is the Resource Block offset between CRB 0 and the CRB which overlaps with the start of the SS Block. It is provided to the UE in SIB1. It is always expressed in RBs. The numerology of the CRB depends on subCarrierSpacingCommon provided by the MIB.

ii> ssb-SubcarrierOffset (Kssb): It is the subcarrier offset between the Subcarrier 0 of the SS Block and Subcarrier 0 of the CRB which overlaps with the SS Block. The UE will read Kssb from MIB. Kssb has range 0 to 23 (5 bits) for FR1 and 0 to 11 (4 bits) for FR2.

Ø For FR1, Kssb=0 to 23 indicates that the CORESET for SIB1 is present with the current SS Block. The value of Kssb above 23 i.e. Kssb = 24 to 29 indicates that the CORESET for SIB1 is not associated with the current SS Block and provides the location of the SS Block by providing an GSCN offset which has CORESET for SIB1. Kssb = 31 indicates that the there is no SS Block with an associated CORESET for SIB1. Kssb=30 is Reserved for future use.

Ø For FR2, Kssb=0 to 11 indicates that the CORESET for SIB1 is present with the current SS Block. The value of Kssb above 11 i.e. Kssb = 12 to 13 indicates that the CORESET for SIB1 is not associated with the current SS Block and provides the location of the SS Block by providing an GSCN offset which has CORESET for SIB1. Kssb = 15 indicates that the there is no SS Block with an associated CORESET for SIB1. Kssb=14 is Reserved for future use.

·      For FR1, the SCS of Kssb is 15 KHz whereas SCSCommon can be of 15 KHz or 30 KHz. If the SCSCommon is of 30 KHz, the Kssb range has to be 0 to 23 (5 bits) since 1 SC of 30 KHz = 2 SCs of 15 KHz.

·      For FR2, SCS of Kssb is same as SCSCommon, hence Kssb range is 0 to 11(4 bits).

·      SCSCommon can be used as SCS for PDSCH carrying SIB1, MSG2/MSG4, paging, and other SIB messages.

Ø Below are the two Figures which illustrates the examples of the Resource Block and subcarrier offsets for FR1.

The Figure 1 is based upon subCarrierSpacingCommon = 15 kHz, and in this case the subcarrier offset (Kssb) ranges from 0 to 11.

The Figure 2 as shown below is based upon subCarrierSpacingCommon = 30 kHz, and in this case the subcarrier offset (Kssb) can range from 0 to 23.

Ø In case of FR2, Kssb is based on a subcarrier spacing which is equal to subCarrierSpacingCommon, the subCarrierSpacingCommon can be set to either 60 or 120 kHz, while the SS Block can use subcarrier spacing of 120 or 240 kHz. The below Figure is based upon subCarrierSpacingCommon = 30 kHz, and in this case the subcarrier offset (Kssb) can range from 0 to 23.

The UE calculates the ‘Point A’ from the centre of the SS Block based on two parameters, offsetToPointA and ssb-SubcarrierOffset (Kssb).

Interview Questions:

1.  For n78 band, ssb-SubcarrierOffset (Kssb) = 2, subCarrierSpacingCommon = 30 KHz, ARFCN for GSCN frequency= 634080, offsetToPointA= 36, offsetToCarrier =0. Calculate the frequency offset from the Point A to the centre of the SS Block.

=> The SS Block SCS = 30 KHz

Offset from Point A to the Subcarrier 0 of the SS Block = offsetToPointA (in RBs) + Kssb = 36*12 15 KHz + 2 15 KHz.

Offset from Point A to the Centre of the SS Block =    36*12 15 KHz + 2 15 KHz + 120 * 30 KHz = 10.11 MHz i.e. Point A is 10.11 MHz away from the centre of the SS Block.

Now, calculate the centre of the SS Block. We know that

So, the frequency of the Point A = 3511.2 – 10.11 = 3501.09 MHz //

i.e. the starting carrier frequency is 3501.09 MHz and end carrier frequency is 3599.39 MHz of the allocated spectrum ( 3500 MHz to 3600 MHz)

2. Compute the frequency offset from the Point A to the centre of the SS Block for the below configuration.

Here OffsetToPointA = 246,  OffsetToPointA is measured in RBs with SCS of 15 KHz i.e. 246*12* 15 KHz

ssb_Subcarrier_offset (Kssb) = 4

Frequency Offset from Point A to the Subcarrier 0 of the SS Block = 246*12*15 KHz + Kssb* 15 KHz = 246*12*15 KHz + 4* 15 KHz

Frequency Offset from Point A to the Centre of the SS Block =246*12*15 KHz + 4* 15 KHz + 120 * 30 KHz = 47.94 MHz //

offsetToCarrier indicates Offset in frequency domain between Point A (lowest subcarrier of CRB 0) and the lowest usable subcarrier on this carrier in number of PRBs.

Here offsetToCarrier =0 i.e. the starting usable subcarrier is the Point A.

locationAndBandwidth parameter specify the set of contiguous CRBs belonging to the Initial Downlink Bandwidth Part and expressed using Resource Indication Value (RIV) and calculated using below formulas:

Here locationAndBandwidth=1099 i.e. Initial BWP contains full carrier bandwidth of 273 RBs; RBstart =0 and RBallocated = 273 RBs.

3. Compute the frequency offset between the SSB Centre frequency and the Point A for the below ENDC configuration.

=>  Here absoluteFrequencySSB = 627936, which is the ARFCN of the SS Block.

Carrier Bandwidth = 162 RBs (60 MHz) and Subcarrier Spacing = 30 KHz.

locationAndBandwidth = 31624 i.e. RBstart =0 and RBallocated = 162 RBs.

 Gap between centre of the SSB and Point A = 3419.04 - 3370.86 = 48.18 MHz i.e. SSB towards the end of the carrier.//

 4. Why GSCN is required in 5G SA mode but not in NSA mode or What are the major differences in SSB parameter configurations for SA and NSA network.

=> In case of NSA mode, the centre frequency of the SS Block (absoluteFrequencySSB) as well as starting frequency of the carrier bandwidth (absoluteFrequencyPointA) is informed to the UE by MeNB using dedicated signaling (RRC Connection Reconfiguration message) during SgNB addition procedure. The parameter ‘absoluteFrequencySSB’ indicates the ARFCN for the centre frequency of the SS Block. The UE simply goes there and read the PSS, SSS and PBCH and synchronizes with the network.

In LTE, the location of PSS and SSS is fixed and always located at the central 62 subcarriers of the carrier bandwidth i.e. 31 subcarriers above the DC carrier and 31 subcarriers below the DC carrier.

The step size for locating the PSS and SSS in LTE is 100 KHz which is also called Channel Raster.

But in case of SA, the location of the SSB is not fixed and can be anywhere within the carrier bandwidth. The UE has to locate the SS Block and read PSS and SSS for synchronization. If the UE needs to search SSB based on ARFCN raster, it would take very long time to detect the SSB since ARFCN raster is very narrow and bandwidth of 5G can be as large as 400 MHz (for FR2). The ARFCN raster is 5 kHz, 15 kHz and 60 kHz for low band, mid band and high band respectively. So, 3GPP has introduced a new concept called GSCN which has wider granularity for locating SSB. GSCN uses a granularity of 1.2 MHz for low band, 1.44 MHz for mid band and 17.28 MHz for high band. Furthermore, unlike NSA the UE has to compute ‘Point A” using the parameter ‘Kssb’ and ‘offsetToPointA’ which are informed to the UE in MIB and SIB1 respectively.

5. What the recommended location of the SS Block in the operating bandwidth for ENDC.

=> In case of ENDC, most of the OEMs are keeping the SS Block around the centre of the operating bandwidth (for e.g., n78 band and 100 MHz BW). So, the radio condition measured over SSS is closed to average.

6. How does GSCN work for synchronization?

·      The gNB transmits SS Blocks periodically (typically every 20 ms).

·      The UE uses the GSCN to determine the center frequency of the SS Blocks based on the operating band and the frequency range. The UE scans a set of GSCN values as per the GSCN raster.

·      The UE read the PSS and SSS and computes the PCI.

·      The UE also decodes the PBCH in the SS/PBCH blocks to obtain the Master Information Block (MIB), which contains essential system information such as the system frame number, the subcarrier spacing common, Kssb etc.

7. Does UE need to make full band search to detect the SSB location in case of SA or How does the UE locate the SS Block in SA network?

=> In SA, when the UE is powered on, it will scan all the possible GSCNs to locate the centre frequency of the SSB which can be used by the UE for system acquisition. In NR, the SSB can be anywhere in the effective bandwidth. The UE will start searching the SSB from the beginning of the bandwidth in step size given as per GSCN raster. The GSCN raster can be 1.2 MHz for < 3 GHz, 1.44 MHz for 3 GHz to 24.25 GHz and 17.28 MHz for >24.25 GHz.

8. What is the advantage of having flexible SSB position in NR?

=> In NR SA, SSB can be defined anywhere in the allocated bandwidth. However in most of the current deployment, SSB is normally defined within the 1st half of the allocated band width.

Advantages: SSB closer to start of bandwidth will save SSB detection time and hence accessibility time will reduce. Most impact will be observed mainly for high BW scenario (400 MHz).

9. Design GSCN for the spectrum allocation 3400 MHz to 3500 MHz (100 MHz BW).

=> We know that n78 band uses SSB SCS of 30 KHz.

SSB occupies 20 RBs i.e. the BW of SSB = 20*360 = 7.2 MHz i.e. 3.6 MHz below the SSB centre frequency and 3.6 MHz above the SSB centre frequency.

·      For frequency range 3 GHz to 24.25 GHz, the GSCN raster size is 1.44 MHz which is 48 times of channel raster (30 KHz). The SSB centre frequency has to be multiple of 1.44 MHz.

·      Secondly, we have to check the CORESET#0 configuration as well which is provided to the UE in MIB using the parameter ‘pdcchConfigSIB1’( Detail is explained in Chapter 3)

The frequency domain configuration of CORESET#0 consist of 3 parameters:

:- Number of RBs occupied by CORESET#0, Number of Symbol occupied by CORESET#0 and an Offset which represent the RB offset between Subcarrier 0 of the CRB which is overlapping with the SS Block and the Subcarrier 0 of the CORESET#0.

If the OEMs uses 16 RBs as an offset as using CORESET#0 index as 12 i.e. there should be gap of at least 5.76 MHz (16 RBs*0.36 MHz) between the lower edge of the SSB and the Point A assuming Kssb=0.

Hence offsetToPointA = 16 RBs

In 100 MHz BW of NR, the effective bandwidth is 98.28 MHz i.e. the remaining 1.72 MHz is used as a Guard band. i.e. Starting Frequency = 3400.86 MHz, End Frequency =3499.14 MHz, Effective BW = (3499.14 – 3400.86) = 98.28 MHz

So, Point A = 3400.86 MHz

Let’s us assume the centre of the SSB is 3410.4 MHz which is as per GSN raster since (3410.4 – 3000) % 1.44 MHz =0. Furthermore 3410.4 MHz is closer to the start of the bandwidth.

Secondly, the gap between the lower edge of the SSB and the Point A is 3410.4 – 120*0.03 – 3400.86 = 5.94 MHz which greater than RB offset (16 RBs =5.76 MHz)

SS Block Centre Frequency = 3000 MHz + N * 1.44 MHz

=> 3410.4 = 3000 + N*1.44

=> N = (3410.4 – 3000)/1.44 = 285

Hence, GSCN = 7499 + N =7499 + 285 = 7784//

Now we need to calculate the offsetToPointA and ssbSubcarrierOffset (Kssb).

We know that

Frequency Offset from the Centre of the SS Block to Point A = offsetToPointA*12*15 KHz + Kssb* 15 KHz + 120 * 30 KHz

=> (3410.4 – 3400.86)* 1000 KHz = offsetToPointA*180 KHz + Kssb* 15 KHz + 3600 KHz

=> offsetToPointA*180 KHz + Kssb* 15 KHz = 5940 KHz

From the above equation, we can conclude that offsetToPointA = 32 and Kssb = 12.

10. What is the difference between CRB and PRB in 5G?

=> CRB is the no of RBs available in the whole bandwidth whereas PRB is no of RBs available in a specific Bandwidth Part (BWP).