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Most multiuser pre-coding techniques require accurate Transmitter Channel State Information (CSIT) to maintain orthogonality between the users. Such techniques have proven quite fragile in time-varying channels because the CSIT is inherently imperfect due to estimation and feedback delay, as well quantization noise. An alternative approach recently proposed by Maddah-Ali and Tse (MAT) allows for significant multiplexing gain in the Multi-Input Single-Output (MISO) Broadcast Channel (BC) even with transmit CSIT that is completely stale, i.e. uncorrelated with the current channel state. With K users, their scheme claims to lose only a log(K) factor relative to the full K Degrees of Freedom (DoF) attainable in the MISO BC with perfect CSIT for large K.
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