Performance of Quantized Feedback Beamforming in MIMO-OFDM Links over Time-Varying, Frequency-Selective Channels

MIMO beamforming, which can improve signal reception and simplify receiver design, has been extensively studied and reported in the literature. Previously, we studied the different quantized feedback schemes proposed by IEEE 802.11n in terms of their feedback overhead and performance degradation. These comparisons, however, assume idealized conditions such as flat Rayleigh fading, no feedback delay, and perfect channel estimation. In practical scenarios, channels are both time-varying and frequency-selective, and feedback delays are inevitable. All these factors degrade the performance of feedback beamforming, and the aim of this paper is to quantify their impact on MIMO-OFDM links. First, we look at a single subchannel with time variations, for two different Doppler spectra (Cauchy and Jakes), and we show how much performance degrades as a function of the product of the Doppler and the feedback delay. Then, we look at the multicarrier case, taking delay spread into account, using specific indoor channel models with different power delay profiles.