How to trigger groupB preambles in a TC

Hi All.
Wanted to understand how to trigger groupB preambles in a TC.
MSG3 size is usually 56 bits and groupA Msg3 size cannot be set less than 56 bits.
So how do we make UE send more than 56 bits in MSg3 during RACH?

Keep the UE at the cell edge.

Okay.
But how would this increase Msg3 size?

Are You using UE simulator or real UE?

I am using Simulator.

Simulator even more easy:
Just change the Preamble ID in Rach Configuration.
Preamble ID from Group B,

I guess yes.
But I later want to try it with a real UE also.
How do You think we can make real UE trigger a msg3 size greater than 56 bits?
Lets say a QC UE.

I tested it keeping UE at cell edge.
Don’t know how the size of msg3 is increased…

Okay.
Was this initial RACH?
Because if it was a CCCH based RACH, pathloss should not come into the picture and power will not have affect on selecting group A or B.
It would be just msg3 size.

If anyone knows the answer it will be good to know for me also. :wink:

I hope you are taking about 5G:

The received MAC PDU (total 56 bits) contains MAC header (8 bits) and contention resolution identity (48 bit).

Following are the 48 bits in RRC setup request message:

  • CCCH Message type
  • UE Identity
  • Spare bit
  • Establishment cause
  • Message extension

Further extension of the number of alternative PDU types is facilitated using the messageClassExtension alternative in the outer level CHOICE. (IE: UL-CCCH-MessageType)

Please correct me if I am wrong…

Yes, You’re right.
The problem is the minimum I can set for triggering group B preambles is also 56 bits.
So I need a way to make UE send Msg3 greater than 56 bits.

Group A selected by UE for small tx and large path loss.
I think we don’t have parameter to trigger selection of group B.
It’s on basis of message size and path loss.