Space-Time Multiple Access: Linear Growth in the Sum Rate

It is known that some of the spectacular capacity gains of using multiple antennas on a point-to-point rich scattering channel, namely linear growth with the number of antennas, can also be obtained in a multi-user environment. We give the constant of proportionality of linear growth in the sum-capacity when the number of users and antennas grow simultaneously, but with fewer users than antennas. We assume that the transmitter and receivers know the channel. Because of the linear growth in sum-capacity, we can accommodate more users simply by adding more antennas, without increasing total transmitted power or bandwidth or lowering the rate to existing users. We dub any scheme that can achieve linear growth in this fashion a space-time multiple access scheme. Channel-hardening arguments show that a “channel-inversion” technique used in point-to-point multiple-antenna links achieves a large fraction of this linear growth in a multi-user environment without excessive transmitter power. Thus, multiple-antennas offer a tremendous advantage in designing scheduling, networking, and multiple-access protocols in rich scattering environments.