Hybrid automatic repeat request (HARQ) protocol typically operates in multi-process stop-and-wait (SAW) mode to fully utilize channel capacity. A prior packet, due to retransmission(s), may arrive at the receiving side later than the packets that follow it but use other HARQ processes, thus leading to out-of-order reception. Medium access control (MAC) layer is responsible for packet reordering operations so as to offer in- sequence delivery to upper layer. An arriving packet should be temporarily stored in the reordering buffer, if there is at least one prior packet that has not arrived yet. Nevertheless, protocol stalling may occur if a packet is discarded at the transmitter due to excessive failed retransmissions, or if a negative acknowledgement (NACK) signal is inversed as an acknowledgement (ACK). A timer-based mechanism is suggested by 3GPP to handle protocol stalling. In this paper, we propose an adaptive timer- based stalling avoidance mechanism as the enhancement to the current 3GPP specifications. In our proposal, the duration of the timer is set or updated through dynamic monitoring on the status of each HARQ process. We show with illustrative examples and simulations that the proposed mechanism reduces considerably extra packet delay incurred during reordering operations.
['Yong Li', 'Wenbo Wang', 'Jiangfeng Ji', 'Mugen Peng']
On the Enhancement to Timer-Based Stalling Avoidance Mechanism in HARQ Protocols