Efficient Reordering-Reassembly PDCP and RLC Window Management Algorithm in 5G and Beyond

To support diverse use cases, 5G has changed the design and functionalities of its different protocol layers, like in Packet Data Convergence Protocol (PDCP) and Radio Link Control (RLC) Layers. 5G RLC and PDCP layer supports Automatic Repeat Request (ARQ) and reordering mechanisms respectively, in Acknowledge Mode (AM). In AM mode, RLC receiver window maintains T-reassembly timer to support retransmissions and to update ACK/NACK status to transmitter side. Similarly, PDCP receiver window maintains T-reorder timer to support in-sequence delivery to upper layer [1]. The RLC window doesn't move its lower bound unless packet is received completely while the PDCP window updates its lower bound after T-reorder timer expiry. As these PDCP and RLC windows are updated independently, (i.e., both are not in coordination) there is a high chance that PDCP layer update its start window even with some missing packets in between while RLC tries continue recovering such missing packets. These missed packets will be considered as out of window (OOW) at PDCP due to its start window movement and are dropped even though they are delivered by RLC using ARQ mechanisms. This paper provides a new idea to minimize retransmissions of such unnecessary PDCP OOW packets at RLC layer itself by coordinating window movements intelligently between PDCP and RLC layer. Simulation results show that percentage of retransmissions saved at RLC is around 9.7% in moderate signal coverage and around 36% in poor or cell edge scenario.