On Multiuser Power Region of Fading Multiple-Access Channel with Multiple Antennas

This paper is concerned with the fading multiple-input multiple-output multiple-access channel (MIMOMAC) with multiple receive antennas at the base station (BS) and multiple transmit antennas at each mobile terminal (MT). Two multiple-access techniques are considered for scheduling transmissions from each MT to the BS at the same frequency, which are space-division multiple-access (SDMA) and time-division multiple-access (TDMA). For SDMA, all MTs transmit simultaneously to the BS and their individual signals are resolved at the BS via multiple receive antennas while for TDMA, each MT transmits independently to the BS during mutually orthogonal time slots. It is assumed that the channel-state information (CSI) of the fading channel from each MT to the BS is unknown at each MT transmitter, but is perfectly known at the BS receiver. Thereby, the BS can acquire the long-term channel-distribution information (CDI) for each MT. This paper extends the well-known transmit-covariance feedback scheme for the point-to-point fading MIMO channel to the fading MIMO-MAC, whereby the BS jointly optimizes the transmit signal covariance matrices for all MTs based on their CDI, and then sends each transmit covariance matrix back to the corresponding MT via a feedback channel. The main goal of this paper is to characterize the so-called multiuser power region under the multiuser transmit-covariance feedback scheme for both SDMA and TDMA. The power region is defined as the constitution of all user transmit power-tuples that can achieve reliable transmissions for a given set of user target rates. Simulation results show that SDMA can achieve substantial power savings over TDMA for the fading MIMO-MAC, even when the number of antennas at the BS is equal to that at each MT. Moreover, this paper demonstrates the usefulness of the multiuser power region for maintaining proportionally-fair power consumption among the MTs. Index Terms Multiple-input multiple-output (MIMO), multi-antenna systems, Gaussian multiple-access channel (MAC), fading channel, capacity region, power region, partial channel feedback, space-division multiple-access (SDMA), time-division multiple-access (TDMA), proportional fairness, resource allocation, convex optimization. Submitted to IEEE Transactions on Information Theory, September 2006, revised April 2008. This work was supported partially by NSF grant CNS-0427711. Part of this paper has been presented at IEEE International Symposium on Information Theory (ISIT), Seattle, July 2006. Please address all correspondence to Rui Zhang. Rui Zhang is with the Institute for Infocomm Research, A*STAR, Singapore. (e-mail: [email protected]) Mehdi Moshseni is with the ASSIA, Inc. (e-mail:[email protected]) John M. Cioffi is with the Department of Electrical Engineering, Stanford University. (e-mail: [email protected]) 2