Transceiver Design in Multiuser MISO Systems with Limited Feedback

It is well known that the availability of M transmit antennas at the base station enables to serve K single-antenna users (K ≤ M), under the assumption of perfect channel state information (CSI) at the base station. However, in practical frequency division duplex (FDD) systems, the channel knowledge available at the base station is not perfect since it is obtained through a limited feedback in the uplink consisting of B bits per user. Part of the B feedback bits can be employed to quantize the channel direction information (CDI), while the rest can be used to quantize the channel magnitude information (CMI) of each user. In this work we address the transceiver design of a multiuser MISO system based on the minimum mean square error criterion and with transmit CSI obtained through a limited feedback composed of quantized CDI and CMI. We treat the case of K ≤ M users with different average channel gains and based on our findings we conclude that for uncorrelated channels it is optimum to employ all the B bits for quantizing the CDI under the assumption that the base station knows the statistics of the CMI of the users.