EFFECT OF CAPTURE ON THROUGHPUT OF VARIABLE-LENGTH PACKET ALOHA SYSTEMS

The effect of the capture phenomenon on the throughput of a radio communications system using the Aloha random access protocol with variable length packets is examined. In a packet radio communications system, the base station typically receives stronger signals from near users than from far users, leading to the so-called 'near/far effect'. The packet capture phenomenon, in which a packet captures the base station receiver when its signal-to-interface ratio exceeds some threshold, is a consequence of the near/far effect. System throughput with capture is obtained analytically in terms of a tagged packet's conditional capture probability using the most general packet length distribution, i.e. packet length is taken to be exponentially distributed. The maximum throughput and the channel traffic rate corresponding to the maximum throughput of a variable packet Aloha system with capture are both considerably higher than that of a comparable system without capture. Depending on the threshold, the maximum throughput and the channel traffic rate corresponding to maximum throughput of a variable packet Aloha system with capture can be as much as 67% and 120% higher (resp.) than that of a comparable system without capture. Consequently, not only throughput but also the stability of the system is enhanced by the capture effect.