Interpolation based unitary precoding for spatial multiplexing MIMO-OFDM with limited feedback

Spatial multiplexing with linear precoding is a simple technique to take advantage of the high spectral efficiency provided by multiple-input multiple-output (MIMO) systems, and it can be simply implemented over frequency selective channels by using orthogonal frequency division multiplexing (OFDM). This requires channel state information (CSI) at the transmitter in the form of the precoding matrices for all OFDM subcarriers, which can be achieved using feedback. To reduce the feedback requirements, this paper proposes a new precoding method incorporating quantization of a fraction of precoding matrices followed by a special interpolation at the transmitter. Since the precoding matrices are unitary, a new interpolator is proposed on the space of unitary matrices for which the outputs are also unitary. The parameters of the interpolator are optimized based on a mean square error (MSE) or mutual information criterion. Simulations show that performance near that of complete CSI can be achieved with a reasonable amount of feedback.