Power gain of large multi-user and multi-antenna systems under adaptive ZF transmission

In this paper, we investigate adaptive zero-forcing (ZF) transmission for multi-user multi-antenna (MUMA) systems in fading environments. We assume that the number of antennas at the base station (BS) is greater than the number of users with each having single antenna, and power control can be done at the transmitter(s) as channel condition changes. Under constraints of individual rates and maximum transmit powers, we adopt the optimal transmit strategy of minimizing the average total transmit power (ATTP). We derive and give closed-form ATTP expressions under short-term rate constraints for real-time communications. When the individual maximum transmit powers are allowed to be infinite, we analyze deeply relationship among the ATTP, the numbers of BS antennas and users in order to get an insight into the power benefit provided by the MUMA system over a single-user and single-antenna (SUSA) system. Our theoretical analysis shows that a large MUMA system can obtain stable and satisfying performance by dynamically changing the number of antennas at the BS. Moreover, we present a simple adaptive transmission scheme with trinary power control satisfying short-term rate constraints.