MIMO downlink joint processing and scheduling : a survey of classical and recent results

Using M antennas at the base station can boost the downlink throughput by a factor M (multiplexing gain), even though the receivers have a single antenna and do not cooperate. In this semi-tutorial paper, we discuss some lowcomplexity alternatives to achieve a downlink throughput very close to the optimal sum capacity. We review and compare these alternatives and we show that, in the simple setting of independently fading channels, conventionalinear beamforming achieves the best tradeoff between performance and complexity . This calls for a reconsideration of the emphasis that some recently proposed non-linear downlink precoding techniques have been given both in the research literature and in the technology development. We address also the problem of joint downlink procesing and scheduling in a packet data system (cross-layer design) and compare recently proposed scheme that require very simple CSIT feedback. I. OPTIMAL DOWNLINK PRECODING A downlink channel with one base station equipped with M transmit antennas and K users with one antenna each is the simplest form ofMulti-Input Multi-Output Gaussian Broadcast Channel (MIMO-GBC), whose sum capacity and more recently the whole capacity region have been fully char acterized in a series of recent papers (see [1] and reference s therein). A time sample of this channel is characterized by

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