In this paper, we consider medium access control (MAC) policies for emerging systems that are equipped with fully digital antenna arrays which are capable of adaptive multi-beam directional communications. With this technology, a user can form multiple simultaneous transmit or receive beams, allowing for greater spatial reuse and higher network throughput. The enabling technology that we consider is the ability to use digital post-processing to form multiple receive beams in real-time without a priori knowledge of the time and angle-of-arrival of the transmission. We present a novel unslotted, uncoordinated ALOHA-like random access MAC policy for multi-beam directional systems that asymptotically achieves the capacity of the network. Such an approach is particularly useful for systems where propagation delay makes the overhead associated with any sort of coordination prohibitive. We also consider the impact of numerous practical considerations including power constraints, latency, and beamwidth on the performance of our MAC policy.
