The Effect of Capture on Collision-Resolution Algorithms

In many communication systems, the stronger of two or more overlapping packets might capture the receiver and thus be received without error. The effect of capture on collision-resolution algorithms in a slotted ALOHA type broadcasting network is investigated here. Extensions to the algorithms are suggested for both the situations in which the receiver can or cannot distinguish between success slots and capture slots. In particular, we present a class of retransmission schemes for packets that have been transmitted during capture slots but have not been received correctly. The performance analysis is confined to a simplified model in which the nodes of the network are divided into two groups and only packets sent by the nodes of one of them might be captured. For this simplified model and for each extended algorithm, explicit recursive equations are given, from which the average conditional collision-resolution interval length, as well as the maximal throughput, can be determined. As expected, we show that in the presence of capture, the performance of the network is improved and the maximal attainable throughput is increased. Extensions of the simplified model such as dividing the nodes into groups instead of two, or considering the situation that capture depends on relative distances and transmission powers, are also discussed. For the latter situation we give simulation results.