MAC-Contention Resolution Timer

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What is MAC-ContentionResolutionTimer?

The timer which states the number of consecutive subframe/subframes during which MAC entity will monitor the PDCCH after the Msg3 is transmitted. (3GPP TS 36.321 version 13.3.0 Release 13) 
The UE will start mac-Contention resolution timer from the immediate subframe after transmitting Msg3. 

As UE is waiting for the contention resolution which is through PDCCH reception on C-RNTI (in connected mode), the UE should be monitoring PDCCH. So that the time duration in which mac-Contention resolution timer is running, is also considered as Active Time.

Which timer is started after msg3 is transmitted?

Following two timers are started Once,  msg3 is transmitted:

1. T300 : Transmission of RRCconnectionRequest
2. Contention Resolution Timer: broadcasted in SIB2. If the UE doesn't receive msg4 (Contention Resolution message) within this timer, then it go back to msg1 i.e. transmitting RAP. If there is a HARQ NACK for msg3 (RRCconnectionRequest) and it has to be re-transmitted then this Contention Resolution Timer will be re-started.

What if two UE attempted RA with the same RA preamble sequence on the same RA channel?

If multiple UEs attempt RA with the same RA preamble sequence on the same RA channel (RA-RNTI) then the eNodeB will receive msg1 from multiple UEs.
The eNodeB will detect the preamble transmission and estimate the uplink transmission time of the UEs.

The eNodeB will derive the RA-RNTI from the timeslot number in which the preamble is received. Using this information the eNodeB will transmit the RA Response on the DL-SCH channel. The message carries the timing and uplink resource allocation information for Preamble. The message also contains the backoff indicator MAC header for which controls the backoff duration in the event of a random access procedure.

All UEs receive the message as they were expecting the message on the same RA-RNTI. Then UE’s saves the Temporary C-RNTI from the MAC data for Preamble. After applying the correction, the UE is synchronized in the return direction and can transmit data bursts to the eNodeB. It is likely that its transmission will not be received at the eNodeB as it is transmitting with a timing advance that was not intended for the UE.

The eNodeB receives Msg3 (RRC Connection Request) from both the UEs. Here, the eNodeB will send msg4 (Contention Resolution) with MAC CRI (Contention Resolution Identity) to both the UEs. This CRI will carry a reflection of the RRC Connection Request as generated by one of the UEs. The MAC layer of the UE will match the CRI (as received from msg4) with the CRI embedded in the RRC Connection Request. If it matches, then the UE will proceed to decode RRC Connection Setup and the other UE will back off and return to Msg1, i.e. start the RA procedure again.

Type of contention Resolution process?

Contention Resolution process is again of two types:

1. MAC based Contention Resolution
=> uses the DCCH logical channel 
=> used in HO scenarios
==> here the rule: if the UE have a valid C-RNTI and UE is going for RA procedure then it can say that it will be a MAC based Contention Resolution procedure

2. L1 based Contention Resolution
=> CRI (Contention Resolution Identity) on DL-SCH based 
=> Contention Resolution is addressed to T-CRNTI
=> uses CCCH logical channel
=> here the rule: if the UE don't have a valid C-RNTI and UE is going for RA procedure then it can say that it will be L1 based Contention Resolution procedure.

Why the macCRT timer is required?

This timer is to check if the eNB has sent the Contention Resolution Identity MAC CE in response to msg3 to complete the contention resolution procedure (for contention based mechanism). If no such message from eNB during this timer, then the new RACH procedure happened.

How does ContentionResolutionTimer work in NB-IOT?

If the UE is an NB-IoT UE, the MAC entity shall use the macContentionResolutionTimer for the corresponding enhanced coverage level if it exists.

Which msg body in you can find macContentionResolutionTimer ?

Sib 2

What happened when macContentionResolutionTimer is expire?

When the macContentionResolutionTimer is expire then tow think happened.
discard the Temporary C-RNTI
Consider the Contention Resolution not successful.

What is the start and end activity of macContentionResolutionTimer?

Once Msg3 is transmitted, the MAC entity will start mac-ContentionResolutionTimer and restart mac-ContentionResolutionTimer at each HARQ retransmission.
Regardless of the possible occurrence of a measurement gap or Sidelink Discovery Gap for Reception, monitor the PDCCH until mac-ContentionResolutionTimer expires or is stopped.
If the MAC PDU is successfully decoded then mac-ContentionResolutionTimer stop.


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