Time-of-arrival-based retransmission scheduling for unslotted random access channels

Abstract

Time-of-arrival-based retransmission scheduling algorithms for unslotted ALOHA-type random access channels with high propagation delay are introduced and analysed in this paper. The techniques described are motivated by the fact that collision bursts on asynchronous channels can be analysed to create a partial time-ofarrival based ordering between the conflicting packets. Packets successfully ordered by collision burst analysis are then retransmitted in a locally synchronous, scheduled manner, thus eliminating potential further conflict between a large fraction of retransmitted messages. Collision resolution algorithms appropriate for channels with continuous signal detection, signal and collision detection, or signal and modified collision detection are described and evaluated in terms of throughput using a simple Poisson traffic model. The maximum throughput is shown to be 0.41, 0.47 and 0.509 for the three cases, thus demonstrating that the proposed time-of-arrival algorithms are competitive with the best slotted collision resolution algorithms. The practicality of the techniques described is also established by showing that maximum throughput is relatively insensitive to inaccuracies in channel feedback for a typical implementation of modified collision detection on a radio frequency channel.