HARQ RTT Timer

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Hi guys today we are focus on HARQ RTT timer in DL as well as UL for LTE and NB-IOT.


What is HARQ RTT Timer?

This parameter specifies the min amount of TTIs before a DL HARQ retransmission is expected by the UE. (3GPP TS 36.321 V8.0.0)
Simple way to understand: the HARQ Round Trip Time (RTT) timer is defined for UE to sleep during HARQ RTT. When decoding DL TB for one HARQ fails, UE can next retransmission TB after at least HARQ RTT sub frames.
While HARQ RTT timer is running, UE need not monitor PDCCH. At HARQ RTT timer expiry,
UE resumes PDCCH reception.

Two type of HARQ RTT timer is there.

HARQ RTT Timer: This parameter specifies the minimum number of subframe(s) before a DL HARQ retransmission is expected by the MAC entity. (3GPP TS 36.321 version 13.3.0 Release 13)
UL HARQ RTT Timer: This parameter specifies the minimum amount of subframe(s) before a UL HARQ retransmission grant is expected by the MAC entity. (3GPP TS 36.321 version 13.3.0 Release 13)



What happens while TB is received when HARQ RTT Timer is running?

In case of downlink spatial multiplexing, if a TB is received while the HARQ RTT Timer is running and the previous transmission of the same TB was received at least N subframes before the current subframe (where N corresponds to the HARQ RTT Timer), the MAC entity should process it and restart the HARQ RTT Timer.

What is FDD configuration for HARQ RTT Timer?

 The serving cell which carries the HARQ feedback for this serving cell the HARQ RTT Timer is set to 8 subframes.

What is TDD configuration for HARQ RTT Timer?

The serving cell which carries the HARQ feedback for this serving cell the HARQ RTT Timer is set to k + 4 subframes.
 Where k is the time interval between the downlink transmission and the transmission of associated HARQ feedback.

How does the HARQ RTT Timer work in NB-IoT? (3GPP TS 36.321 version 13.3.0 Release 13)

NB-IoT the HARQ RTT Timer is set to k+3+N+deltaPDCCH subframes.
Where,
k is the time interval between the last subframe of the downlink transmission and the first subframe of the associated HARQ feedback transmission.
N is the transmission duration in subframes of the associated HARQ feedback.
deltaPDCCH is the interval from the last subframe of the associated HARQ feedback transmission plus 3 subframes to the first subframe of the next PDCCH occasion.

How does the UL HARQ RTT Timer work in NB-IoT?

The UL HARQ RTT timer length is set to 4+deltaPDCCH subframes.
Where,
deltaPDCCH is the interval from the last subframe of the PUSCH transmission plus 4 subframes to the first subframe of the next PDCCH occasion.

What happens when HARQ RTT Timer expire?

When DRX is configured, the MAC entity shall for each subframe and if a HARQ RTT Timer expires in this subframe.
For LTE:  if the data of the corresponding HARQ process was not successfully decoded. Then start the drx-RetransmissionTimer for the corresponding HARQ process
For NB-IoT: start or restart the drx-InactivityTimer.
What happened when UL HARQ RTT Timer expire?
When DRX is configured, the MAC entity shall for each subframe and if an UL HARQ RTT Timer expires in this subframe.
For LTE:  start the drx-ULRetransmissionTimer for the corresponding HARQ process.
For NB-IoT: start or restart the drx-InactivityTimer.


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