A stochastic gradient algorithm for transmit antenna weight adaptation with feedback

It is generally accepted that the downlink of next generation cellular systems will require greater capacity than the uplink, due to the asymmetry of data traffic patterns. The use of transmit adaptive antenna arrays at the base station is a promising area for downlink capacity improvement. This paper describes a gradient algorithm utilizing mobile to base feedback in order to achieve some of those possible gains. A simple algorithm for adaptation of the complex baseband weights of a transmit antenna array utilizing feedback from the receiving unit is presented. A dedicated pilot signal is transmitted from the transmitting unit with a pair of perturbed test weights; a feedback bit is transmitted from the receiving unit indicating which weight vector provided the larger power; the transmitting unit adjusts a base weight to this selection and continues the process. This approach is shown to be a coarse stochastic gradient algorithm. The settling time and steady state characteristics of the algorithm are investigated analytically and verified by simulation, and the algorithm is shown to converge to those weights which maximize the power delivered to the receiver.