What are the difference between CRB and PRB in 5G?

5G NR
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1

Hi All
What are the difference between CRB and PRB in 5G?

2

CRB refer to common resource block, i.e. BW of 5G divided into multiple CRB used for BWP.
PRB refer to block assigned to user by scheduler.

In 5G we have VRB to PRB mapping which depends on Downlink resource allocation type, it can be interleaved or non inter leaved.

3

That understood, thanks.
But question is: why we need to divide BW with CRB?
Are configured CRB sub carrier spacing in our gNB?

4

CRB is just common reference point.
Every frequency domain configuration is defined with respect to CRB.

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5

Thanks. That is also understood that Point A, SSB, Kssb all are the reference to CRB and CRB has fixed SCS 15khz for FR1 and 30Khz for FR2.
My question is: what are the challenges to define frequency domain configuration with respect to PRB what is need to introduce CRB?

6

Yes, your question is right.
But most of the time we get answer of “What” and not Why".
Even 3GPP doesn’t tell many time why.

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7

Yes, but they mentioned that based on numerology my SCS will change, so it’s difficult to define all the parameter with different configuration, that is the reason given by 3gpp to introduce CRB.
It seems logical, but we have only 2 option in each band FR1 or FR2 then what is the challenge to define the reference with PRB.

8

What do you mean by 2 option, for FR1 we can have SCS 15,30,60 similarly for FR2 60,120,240. So there are many options na? So to have common base, 15 Khz finalized and name given as CRB.

9

3 options for FR1 as you said, and SCS of CRB is fixed at 15 kHz now if we don’t use CRB what are the challenges?

10

In case of NSA, absolute frequency of SSB (absoluteFrequencySSB), and point A (absoluteFrequencyPointA),offsetToCarrier directly given to UE.
So position of SSB with respect to channel bandwidth is readily known to UE.
But in SA this is not true.
In SA UE first detect SSB and then after reading MIB/SIB UE estimate Point A.
Parameter Kssb gives duration in between SSB symbol 0 and symbol 0 of CRB overlapping to SSB (Kssb SCS is 15 Khz in FR1, 60/120 for FR2).
And then offsetToPointA gives distance between CRB0 subcarrier 0 and subcarrier 0 of CRB overlapping SSB, this distance in terms of RB.
And RB size is taken from CRB subcarrier spacing.

11

CRB - Total B/W available.
PRB - Specific to each BWP.

GSCN are sparse to reduce UE time to scan network , and therefore are not at center of the total bandwidth. CRB acts as a reference to move UEs to appropriate BWP.