Why do we need HARQ in 5G when RLC and PDCP do retransmissions?

Retransmissions in 5G NR are possible at three layers: MAC, RLC and PDCP. RLC and PDCP being higher layers, the feedback signalling is slower. HARQ retransmissions at MAC react faster to channel conditions and improve performance for delay-sensitive applications. Applications requiring 100Mbps may need highly reliable connections to avoid triggering TCP congestion-avoidance mechanism. A combination of HARQ’s fast retransmissions and RLC’s reliable packet delivery achieves this.

RLC retransmissions are limited to logical channels in Acknowledged Mode (AM). HARQ offers retransmission capability for RLC UM and TM. 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.

PDCP retransmissions are useful during inter-gNB handovers. Undelivered packets are forwarded at the PDCP layer from old to new gNB. RLC retransmissions won’t work since new RLC entities are established in the new gNB.

HARQ retransmissions won’t work since HARQ buffers are flushed during handover.

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