On multiple access communications using spatial diversity

Information-theoretic results have shown significant gains in transmission rate over fading channels using multiple transmitter and receiver antenna (spatial) diversity. We review some of these results and show that these gains could also be achieved using simpler receiver structures. We then examine the rates achievable in a multiuser environment, i.e. a multiple access fading channel. The symmetric rate per user is studied and we show that even with simple linear detectors one could obtain non-zero symmetric rates for fading channels (when we have a large number of users). Next we observe that the symmetric rate decreases with the number of users sharing the spectrum. Therefore, one can define a "user capacity" which refers to the number of users who can be supported at a desired symmetric rate per user. We evaluate the user capacity as a function of the number of transmit and receive antennas and observe the benefits of using spatial diversity, from this point of view.