  Modern 5G communication systems employ multiple-input multiple-output (MIMO)
in conjunction with orthogonal frequency division multiplexing (OFDM) to
enhance data rates, particularly for wideband millimetre wave (mmW)
applications. Since these systems use a large number of subcarriers, feeding
back the estimated precoder for even a subset of subcarriers from the receiver
to the transmitter is prohibitive. Moreover, such frequency domain approaches
also do not exploit the predominant line-of-sight component that is present in
such channels to reduce feedback. In this work, we view the precoder in the
time domain as a matrix all-pass filter, and model the discrete-time precoder
filter using a matrix-lattice structure that aids in reducing the overall
feedback while still maintaining the desired frequency-phase delay profile.
This provides an efficient precoder representation across the subcarriers using
fewer coefficients, and is amenable to tracking over time with much lower
feedback than past approaches. Compared to frequency domain geodesic
interpolation, Givens rotation based parameterisation, and the angle-delay
domain approach that depends on approximate discrete-time representation, the
proposed approach yields higher achievable rates with a much lower feedback
burden. Via extensive simulations over mmW channel models, we confirm the
effectiveness of our claims, and show that the proposed approach can reduce the
feedback burden by up to 70%.
