Slotted ALOHA with MIMO Spatial Multiplexing

Many contention protocols in wireless networks such as carrier sense multiple access with collision avoidance (CSMA/CA) incorporate methods that minimize probability of collision. However there is another possible way that collisions can be avoided, that is by getting some help from physical layer technologies. It has been proved that multiple-input-multiple-output (MIMO) technology with spatial multiplexing (SM) operating in a rich scattering environment can resolve up to M different data streams where M is the minimum number of transmitting or receiving antennas. In this paper, we evaluate the performance of slotted ALOHA access protocol employing MIMO with SM. The purpose of this analysis is to gain insight and directions for developing access protocol to fully utilise benefits of MIMO technology from the network point of view. We evaluate an upper bound performance in terms of efficiency (success rate) and corresponding capacity utilisation efficiency, collision rate, and an average number of successful transmissions per slot. We find a substantial gain in efficiency in comparison with the conventional slotted ALOHA. However, we also discover that an additional gain is diminished when more antennas at the receiver are included. Mathematical derivations followed by numerical simulations and discussion are presented.