Performance of a Concurrent Link SDMA MAC under Practical PHY Operating Conditions

Abstract: Space Division Multiple Access (SDMA) based Medium Access Control (MAC) protocols have been proposed to enable concurrent communications and improve link throughput in Multi-Input Multi-Output (MIMO) Ad Hoc networks. For the most part, the works appearing in the literature make idealized and simplifying assumptions about the underlying physical layer as well as some aspects of the link adaptation protocol. The result is that the performance predicted by such works may not necessarily be a good predictor of actual performance in a fully deployed system. In this paper we look to introduce elements into the SDMA MAC concept that would allow us to better predict their performance under realistic operating conditions. Using a generic SDMA-MAC we look at how the network sum throughput changes with the introduction of the following: $(a)$ use of the more practical MMSE algorithm instead of the zero-forcing or SVD based nulling algorithms used for receive beamnulling; $(b)$ impact of channel estimation errors; $(c)$ introduction of link adaptation mechanism specifically designed for concurrent SDMA MACs; $(d)$ incorporation of TX beamforming along with RX beamnulling. Following on the transmission window during which concurrent transmissions are allowed by the MAC, we qualify the impact of each of these four elements in isolation. At the conclusion, the performance of a system that incorporates elements $a-d$ is presented and compared against the baseline system, showing an improvement of up to 5x in the overall network sum throughput.