On Transmit Beamforming for MISO-OFDM Channels With Finite-Rate Feedback

Abstract

With finite-rate feedback, we propose two feedback methods for transmit\nbeamforming in a point-to-point MISO-OFDM channel. For the first method, a\nreceiver with perfect channel information, quantizes and feeds back the optimal\ntransmit beamforming vectors of a few selected subcarriers, which are equally\nspaced. Based on those quantized vectors, the transmitter applies either\nconstant, linear, or higher-order interpolation with the remaining beamforming\nvectors. With constant interpolation, we derive the approximate sum achievable\nrate and the optimal cluster size that maximizes the approximate rate. For\nlinear interpolation, we derive a closed-form expression for the phase rotation\nby utilizing the correlation between OFDM subcarriers. We also propose a\nhigher-order interpolation that requires more than two quantized vectors to\ninterpolate transmit beamformers, and is based on existing channel estimation\nmethods. Numerical results show that interpolation with the optimized cluster\nsize can perform significantly better than that with an arbitrary cluster size.\nFor the second proposed method, a channel impulse response is quantized with a\nuniform scalar quantizer. With channel quantization, we also derive the\napproximate sum achievable rate. We show that switching between the two methods\nfor different feedback-rate requirements can perform better than the existing\nschemes.\n