Constructive Multi-User Interference for Symbol-Level Link Adaptation: MMSE Approach

Unlike the general concept of eliminating or avoiding inter-user interference in the multi-user downlink multiple-input single-output (MISO) system, the data-aided precoding scheme attempts to exploit the constructive interference at symbol level among the multiple users. A positive interference can be constructed to enhance the received signal gain by predicting the phase and magnitude of inter-user interference. Thus, we propose constructive interference optimization that maximizes the signal-to-interference and noise ratio (SINR) of users by predicting the constructive interference. Towards this end, we formulate a minimum mean square error (MMSE) problem, which is relaxed over the constructive interference region with the minimum required cumulative constrictive interference gain (CCIG) under a total power constraint. We then derive the optimal transmit signal vector as a function of Lagrange multipliers obtained by Karush-Kuhn-Tucker (KKT) conditions. Our numerical results demonstrate that the proposed data-aided precoding scheme provides more CCIG than other state-of-the-art schemes, which turns out to be useful under a mm-wave band channel with few multi-paths or non-homogeneous environment.