The combined MIMO with Adaptive Modulation and Coding (AMC) technology can provide high spectral efficiency and link robustness. While most existing adaptive algorithms focus on physical layer, cross layer analysis on the queuing behavior of MIMO AMC systems is necessary, but remains open. In this paper, under the conditions of unsaturated traffic and finite length buffer, we investigate the queuing characters of two representative categories of MIMO systems, namely the BLAST system and the space time block coding (STBC) system. We model the service processes of both STBC and BLAST coupled with AMC, which is the most challenging part of the queuing analysis. We observe a new tradeoff between diversity and multiplexing in terms of link layer packet loss rate and queuing delay, based on which we propose a cross layer design of diversity multiplexing switching scheme to optimize the QoS of the MIMO AMC systems.
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