TABLE OF CONTENTS In the preceding sections, we have assumed that the channel is perfectly known at the transmitter. In practice, this knowledge is available only within some inevitable estimation errors. In this section, we relax this assumption and we review the methodologies proposed for optimizing the transmission strategy, in the presence of partial channel information.
Information about the channel at the transmitter side can be acquired in two basic manners. In systems employing TDD, transmission and reception occur over the same frequency band. In such a case, the reception channel coincides with the transmission channel. Then, by reciprocity, the same channel estimate carried out at the receiver to retrieve the transmitted information can be used to design the transmission strategy. The situation is more complicated with FDD systems, where transmission and reception occur over non-overlapping frequency bands. In such a case, the only possibility for the transmitter to know the channel is that the receiver sends its channel estimate back to the transmitter.
In practice, the channel estimate available at the transmitter does not coincide with the true channel, for the following reasons: 1) any channel estimate, in the presence of noise, is inevitably affected by estimation errors; 2) in case of FDD systems, the feedback channel is itself affected by transmission errors and the channel estimate needs to be properly quantized before transmission; 3) in case of time-varying channel, the channel behavior to be used to optimize the transmission, must be predicted...
