  Relay-aided transmission is envisioned as a key strategy to combat severe
path loss and link blockages emerging as unique challenges in millimeter-wave
(mmWave) communications. This work considers a relay-aided multiuser mmWave
communications scenario aiming at maximizing the sum rate through optimal
transmit and relay precoder design. We propose a novel joint precoder design
strategy, which exploits weighted minimum mean-square error (WMMSE) using its
equivalency to sum-rate maximization. We obtain closed form expressions of
transmit and relay precoders, and propose to compute them through
alternating-optimization iterations without having to resort to complicated
numerical optimization techniques. Numerical results verify the superiority of
the proposed precoding strategy as compared to conventional precoding schemes.
