In both downlink and uplink, UE supports up to 16 HARQ processes per cell. Each process can have one packet pending an ACK. 5G NR HARQ solves this by allowing multiple concurrent HARQ processes. Due to the round trip time, this results in underutilization of radio resources. Another packet is not sent while waiting for feedback on the current packet. The method employed to combine different versions of the packet is called Incremental Redundancy. All versions are used to improve decoding. Receiver doesn't discard erroneous packets but stores them in its buffer. Retransmissions can use different coding rates and redundancy bits. Asynchronous HARQ is also more suited for unlicensed bands.ĥG NR HARQ is adaptive.

They're not pre-determined according to some fixed pattern. Transmissions and retransmissions are explicitly signalled. Source: Malhotra 2020.ĥG NR HARQ is asynchronous. What are some technical details of 5G NR HARQ?ĥG NR uses multiple stop-and-wait HARQ processes.HARQ retransmissions won't work since HARQ buffers are flushed during handover. RLC retransmissions won't work since new RLC entities are established in the new gNB. Undelivered packets are forwarded at the PDCP layer from old to new gNB. PDCP retransmissions are useful during inter- gNB handovers. Moreover, RLC transmissions can potentially happen on a different component carrier or cell but HARQ retransmissions happen on the same cell as the original transmission. HARQ offers retransmission capability for RLC UM and TM. RLC retransmissions are limited to logical channels in Acknowledged Mode ( AM). A combination of HARQ's fast retransmissions and RLC's reliable packet delivery achieves this. Applications requiring 100Mbps may need highly reliable connections to avoid triggering TCP congestion-avoidance mechanism. HARQ retransmissions at MAC react faster to channel conditions and improve performance for delay-sensitive applications. RLC and PDCP being higher layers, the feedback signalling is slower. Retransmissions in 5G NR are possible at three layers: MAC, RLC and PDCP. Why do we need HARQ in 5G when RLC and PDCP do retransmissions?.Better performance can be achieved but at the cost of increased feedback signalling, higher power or lower spectral efficiency. HARQ typically achieves a residual error rate of 0.1-1%. The use of HARQ in 5G NR benefits URLLC and eMBB use cases. PHY layer at the receiver combines one or more transmissions to increase the chances of correct decoding. In 5G NR, HARQ operates at both MAC and PHY layers. The receiver attempts to decode the packet based on current and previous transmissions.

Where correction is not possible, errors are detected and packet retransmission is requested. Source: EventHelix 2017.ĥG NR supports Hybrid ARQ, which is a combination of retransmissions and error correction.

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