9 ARQ and HARQ
36.3003GPPEvolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN)Overall descriptionRelease 17Stage 2TS
9.0 General
E-UTRAN provides ARQ and HARQ functionalities. The ARQ functionality provides error correction by retransmissions in acknowledged mode at Layer 2. The HARQ functionality ensures delivery between peer entities at Layer 1.
9.1 HARQ principles
The HARQ within the MAC sublayer has the following characteristics:
– HARQ transmits and retransmits transport blocks;
– In the downlink:
– N-process Stop-And-Wait;
– Asynchronous adaptive HARQ;
– Uplink ACK/NAKs in response to downlink (re)transmissions are sent on PUCCH or PUSCH (except for NB-IoT and short TTI);
– For BL UEs or UEs in enhanced coverage, uplink ACK/NAKs are sent in response to transmission bundles;
– For NB-IoT, Uplink ACK/NAKs in response to downlink (re)transmissions are sent on NPUSCH;
– For short TTI, Uplink ACK/NAKs in response to downlink (re)transmissions are sent on SPUCCH or (sub)slot-PUSCH;
– PDCCH, MPDCCH, NPDCCH or SPDCCH signals the HARQ process identifier (except for NB-IoT when only one HARQ process is configured) and if it is a transmission or retransmission;
– Retransmissions are always scheduled through PDCCH, MPDCCH, NPDCCH or SPDCCH.
– In the uplink:
– N-process Stop-And-Wait;
– Synchronous HARQ (except for the cases below;
– Asynchronous adaptive HARQ for NB-IoT, BL UEs, UEs in enhanced coverage, HARQ processes scheduled with (sub)slot based PUSCH, HARQ processes scheduled with SPT, or for HARQ processes not configured with AUL operation for SCells configured with uplink LAA or for SCells configured with PUSCH enhancement mode;
– Autonomous HARQ for HARQ processes configured with AUL operation for SCells configured with uplink LAA;
– Maximum number of retransmissions configured per UE (as opposed to per radio bearer) for synchronous HARQ;
– For asynchronous adaptive HARQ, HARQ process identifier is either signalled by PDCCH, MPDCCH, NPDCCH or SPDCCH, except for NB-IoT when only one HARQ process is configured the HARQ process identifier is fixed (see clause 5.4.2.1 in TS 36.321 [13]);
– For Autonomous HARQ, UE selects HARQ process identifier from a pool of configured HARQ processes;
– Downlink ACK/NAKs in response to uplink (re)transmissions are sent on PHICH except for asynchronous adaptive HARQ and autonomous HARQ;
– For autonomous HARQ, downlink ACK/NAKs in response to uplink (re)transmissions are sent on PDCCH;
– HARQ operation in uplink is governed by the following principles (summarized in Table 9.1-1) except for asynchronous adaptive HARQ:
1) Regardless of the content of the HARQ feedback (ACK or NACK), when a PDCCH for the UE is correctly received, the UE follows what the PDCCH asks the UE to do i.e. perform a transmission or a retransmission (referred to as adaptive retransmission);
2) When no PDCCH addressed to the C-RNTI of the UE is detected, the HARQ feedback dictates how the UE performs retransmissions:
– NACK: the UE performs a non-adaptive retransmission i.e. a retransmission on the same uplink resource as previously used by the same process;
– ACK: the UE does not perform any UL (re)transmission and keeps the data in the HARQ buffer. A PDCCH is then required to perform a retransmission i.e. a non-adaptive retransmission cannot follow.
– For asynchronous adaptive HARQ, HARQ feedback is not sent, except for BL UEs and UEs in enhanced coverage. The UE follows what the PDCCH, MPDCCH, NPDCCH or SPDCCH asks the UE to do i.e. perform a transmission or a retransmission. For BL UEs or UEs in enhanced coverage, downlink ACK in response to uplink (re)transmissions may be sent in the DCI with C-RNTI or SPS C-RNTI scheduling MPDCCH.
– In the sidelink:
– No HARQ feedback;
– Retransmissions are always performed in a pre-defined/ configured number.
– Measurement gaps and sidelink discovery transmission during a sidelink discovery gap for transmission are of higher priority than HARQ retransmissions: whenever an HARQ retransmission collides with a measurement gap or sidelink discovery transmission during a sideink discovery gap for transmission, the HARQ retransmission does not take place.
Table 9.1-1: UL synchronous HARQ Operation
|
HARQ feedback seen by the UE |
PDCCH |
UE behaviour |
|
ACK or NACK |
New Transmission |
New transmission according to PDCCH |
|
ACK or NACK |
Retransmission |
Retransmission according to PDCCH (adaptive retransmission) |
|
ACK |
None |
No (re)transmission, keep data in HARQ buffer and a PDDCH is required to resume retransmissions |
|
NACK |
None |
Non-adaptive retransmission |
9.2 ARQ principles
The ARQ within the RLC sublayer has the following characteristics:
– ARQ retransmits RLC PDUs or RLC PDU segments based on RLC status reports;
– Polling for RLC status report is used when needed by RLC;
– RLC receiver can also trigger RLC status report after detecting a missing RLC PDU or RLC PDU segment.