Linear Precoding in the Downlink of Limited Feedback Multiuser MIMO Systems

Abstract

The downlink of a <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">multiuser</i> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">multiple-input</i> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">multiple-</i> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">output</i> system is considered. Compared to nonlinear precoding strategies which are designed to achieve the channel capacity in case of perfect <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">channel</i> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">state</i> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">information</i> (CSI) at the transmitter, we focus on systems which are restricted to use only linear precoders due to complexity constraints and where only partial CSI is available at the transmitter due to limited feedback. Here, we compare two different feedback approaches: in the first, users feed back the best entry of a precoder codebook, and in the second, users provide quantized channel information based on a channel codebook. The presented transmitters exploit the available feedback information to select the users for transmission ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">scheduling</i> ) and to compute the corresponding precoder. Sum rate investigations show that channel codebook based feedback should be preferred in systems with a very low feedback rate.