We develop a framework that exploits network coding (NC) and multiple-input/multiple-output (MIMO) techniques, jointly together, to improve throughput of downlink broadcast channels. Specifically, we consider a base station (BS) equipped with multiple transmit antennas that serves multiple mobile stations (MSs) simultaneously by generating multiple signal beams. Given the large number of MSs and the small number of transmit antennas, the BS must decide, at any transmission opportunity, which group of MSs it should transmit packets to, in order to maximize the overall throughput. We propose two algorithms for grouping MSs that take advantage of NC and the orthogonality of user channels to improve the overall throughput. Our results indicate that the proposed techniques increase the achievable throughput significantly, especially in highly lossy environments. 1. Introduction In recent years, multiple-input/multiple-output (MIMO) has been recognized as a key enabling technology for improving the performance of wireless communication systems. Unlike traditional communications, MIMO techniques rely on multiple antennas to transmit and/or receive signals. The number of antennas that a device can be equipped with can be limited due to spatial constraints. For example, in a cellular network, while there may be no limit on the number of antennas that the base station (BS) can be equipped with, there is a limit on that number when it comes to mobile station (MS) due to size and/or cost constraints. MIMO capabilities can be exploited to enable the spatial division multiple access (SDMA) technique, which basically allows multiple simultaneous transmissions from the BS to multiple MSs, thereby achieving higher overall data throughput [1–3]. Specifically, the BS exploits MIMO to generate radiation patterns that simultaneously target different groups of MSs [4, 5]; this is known as beamforming (BF). Mathematically, the signal beam from each antenna is coded and multiplied independently by a BF weight vector to control its shape and direction. A BF weight vector is determined by MSs locations as well as the characteristics of channels between the BS and MSs. Assuming that the BF weight vectors are available, an optimal SDMA scheme consists then of selecting the set of MSs whose signal-to-interference plus noise ratios (SINRs) are maximized. A BS with transmit antennas can form and transmit a signal with at most beams [6, 7]. Therefore, assuming that one beam is allocated for each MS, when the number of MSs, , exceeds the number of transmit antennas, , the BS selects
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