Space-time precoding with mean and covariance feedback: implications for wideband systems

This paper focuses on optimizing the capacity of a cellular "downlink" in which the base station (BS) is equipped with multiple antennas, while the mobile has a single antenna. The BS has access to both the first and second order statistics of the channel, obtained from mean feedback and covariance feedback, respectively. Our results apply to wideband orthogonal frequency division multiplexing (OFDM) systems, where accurate covariance feedback is obtained without overhead by averaging uplink measurements. Mean feedback is obtained from uplink measurements using reciprocity for time division duplex (TDD) systems, and would require explicit feedback for frequency division duplex (FDD) systems. Since mean feedback degrades rapidly with feedback delay for mobile channels, and requires feedback overhead for FDD systems, our purpose is to quantify the tradeoffs in using covariance feedback alone versus using both covariance and mean feedback.