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Abstract-Over the past few years, a lot of interest has focused on Orthogonal Frequency Division Multiple Access (OFDMA) and Multiple Input Multiple Output (MIMO) systems thanks to the tremendous gain on system capacity they offer. Transmit beamforming is a low complexity technique that helps in achieving the full diversity afforded by the multiple antenna environment. MIMO-OFDMA systems using opportunistic beamforming are a promising solution to satisfy the growing demand in terms of data rate and Quality of Service (QoS).An important practical issue in MIMO-OFDMA systems is the feedback load. As a large number of carriers (e.g. 2048 for WiMax) is usually used in such systems, feeding back full Channel State Information at the transmitter (CSIT) for each carrier is prohibitive. In this paper, the problem of feedback reduction in MIMO-OFDMA opportunistic beamforming is addressed. We present different partial CSIT schemes that reduce significantly the feedback overload at little expense of system throughput.

Next generation wireless networks are being developed to support a wide variety of data services with a broad range of Quality of Service (QoS) requirements. We consider the problem of downlink scheduling wherein a base station (BS) with multiple antennas serves a multiuser system. Assuming that different users can be served by the transmitter antennas at each slot, we allocate each BS antenna to users based on certain priority functions. We study a QoS-based allocation scheme, wherein the priority functions capture the user QoS demands quantified in terms of throughput and delay. We attempt to identify scheduling schemes that provide a good trade-off between (i) higher system capacity by exploiting inherent multiuser diversity, (ii) fairness among users based on their instantaneous channel conditions relative to average channel conditions, and (iii) tolerable latency requirements specified by the user applications. Simulation results evaluate the performance of the above allocation scheme and show that multiple antennas and delay-aware scheduling rules can be efficiently used to provide QoS at little expense of throughput.