Random Multiple Access Communications on Multipacket Reception Channels
Chan, Douglas S.
By applying multiuser detection techniques, receivers today can decode multiple packets transmitted simultaneously over a channel. Known as multipacket reception (MPR), this presents a physical layer model significantly different from the one that practical design and theoretical analysis of many network protocols have traditionally assumed. The principal objective of this study is to consider the design of efficient multiple access communication systems for this new model. Specifically, we investigate the maximum stable throughput attainable with decentralized control using channel sense multiple access (CSMA), which has not previously been studied with MPR. We show CSMA provides throughput gain over slotted ALOHA (S-ALOHA), the non-channel-sensing protocol of choice. However, we also find that this gain diminishes as the physical layer strength increases, thereby diminishing the need for channel sensing. Searching for improvements, we investigate the generalized CSMA protocols which allow new transmissions to begin even when the channel is already in use, provided the usage level is below capacity. In addition, we also analyze the effects of implementing collision detection (CD). We find that generalized CSMA provides a moderate improvement in throughput compared to the significant improvement that CD affords. This is attributable to CD's shortening of the time that corrupted transmissions occupy the MPR channel, thus increasing any new transmission's success probability. As generalized CSMA/CD offers little improvement over the simpler classical CSMA/CD, we conclude the latter suffices for satisfactory improvement. We also present a novel CD method applicable not only to wireline but also to wireless networks. Finally we investigate the general multiaccess protocol's capacity with MPR. We find that, if the expected number of successes is maximized in the limit of infinitely many simultaneous transmissions, then S-ALOHA and CSMA are optimal. We also establish an upper bound on the throughput of decentralized multiaccess protocols operating on channels not satisfying this condition. The gap between the channel capacity and said upper bound is informative about the cost of decentralization. This gap also narrows as the physical layer's ability to separate packets gets stronger, thus corroborating existing theories that suggest there is less need for multiple accessing for strong MPR channels.
Multiple access communications; multipacket reception; MPR; CSMA; network capacity; collision detection; carrier sense multiple access
dissertation or thesis