User capacity analysis of space division multiple access channel

Space division multiple access (SDMA), which uses beamforming techniques to exploit the spatial diversity of different users, can significantly increase system capacity as well as the communication range between users and the basestation. In this paper, we consider the SDMA uplink channel where a pool of users can communicate with a basestation using a fixed quality of service (QoS) requirement. We analyze the SDMA average user capacity, defined as the average maximum number of random users that can be scheduled for simultaneous transmission, while meeting their respective QoS requirements. Using a standard probabilistic model involving narrow-band and far-field assumptions for the random users, we characterize the average user capacity in terms of the number of antennas, the QoS requirement, number of users and the maximum SNR (signal to noise ratio) among all users. These results act as a system design guide, offering insight into the tradeoff between hardware cost and system performance.