Overhead and Spectral Efficiency of Pilot-Assisted Interference Alignment in Time-Selective Fading Channels

The spectral efficiency achievable by interference alignment (IA) in a K-user multiple-input-multiple-output interference channel is studied in the face of time-selective continuous fading explicitly estimated through pilot-symbol observations. The robustness of IA in such operationally relevant conditions is assessed through a joint optimization of the pilot overhead and the IA update interval, which are characterized - in high-power conditions - as solutions of a fixed-point equation. Variations of the formulation are given for both frequency-division duplexing (FDD) and time-division duplexing, the former requiring explicit feedback of the fading estimates and the latter relying on fading reciprocity. For the FDD variation, analog feedback is considered. In addition to arbitrary numbers of users and antennas and arbitrary temporal fading correlation functions, the derivations accommodate forward and reverse links with asymmetric power levels.