The Importance of Transmitter Side Information and the Value of Training∗

Coherent communication over fading wireless channels requires channel state information (CSI). Therefore, CSI has value. Communication systems that require CSI must devote some system resources to obtaining it. CSI has associated cost. When the cost can be used to train either the transmitter or the receiver, then the optimal solution is to allocate the training where it will have the highest value. In this work, we show that wireless communication systems that require minimum outage probability should allocate their training resources to the transmitter. To that end, we derive the associated bound on mutual information and use it to develop a precoding scheme. At high SNR, we show that transmitter training yields infinite diversity for any amount of training while receiver training offers only finite diversity. Finally, we demonstrate that the outage performance provided by even minimal transmitter training is significantly superior to infinite training of the receiver over all SNR.