Cross-layer design of a time-division multiple access protocol with cooperative diversity

This paper addresses the medium access control (MAC) and physical (PHY) cross-layer design of a time-division multiple access protocol (TDMA) where users with good channels are allowed to relay the packets of other users with less favorable conditions. All signals are combined at the central node using signal processing tools in order to achieve diversity. The work is focused on an asymmetrical scenario where users experience different channel and queuing states. Two performance metrics are used: packet throughput and access delay. To facilitate the analysis, a packet reception model which represents more efficiently the cooperative PHY layer is also proposed. A multi-objective optimization is used to derive the boundaries of the throughput region, i.e., the set of all achievable user throughput values. Sketches of the two-user case show that the region is always convex and larger than the region of its non-cooperative counterpart. The results also show that cooperation is useful when some users with good channels experience low/medium traffic loads and thus can help other users with bad channels and higher traffic loads. Evaluation of access delay is also obtained by using the expressions of a modified TDMA system.