Beam Domain Optical Wireless Massive MIMO Communications with Transmit Lens

We present beam domain optical wireless massive multiple-input multiple-output communications, where base station with massive transmitters communicates with a number of user terminals through transmit lens simultaneously. We focus on LED transmitters, and provide an optical channel model, where the light emitted from one LED passes through the transmit lens and generates a narrow beam. Based on this channel model, we analyze the performance of maximum ratio transmission and regularized zero-forcing linear precoding schemes, and propose concave-convex-procedure based transmit covariance matrix design to maximize the sum-rate. Then, we design the transmit covariance matrix when the number of transmitters goes to infinity, and show that beam division multiple access (BDMA) transmission achieves the asymptotically optimal performance for sum-rate maximization. Compared with the case without transmit lens, BDMA transmission can increase the sum-rate proportionally to $2K$ ($K$ is the number of users) and $K$ under total power and per transmitter power constraints, respectively. Motivated by the asymptotic results, for the non-asymptotic case, we prove the orthogonality conditions of the optimal power allocation in the beam domain and propose beam allocation algorithms satisfying the conditions. Numerical results confirm the significantly improved performances of our proposed beam domain optical massive MIMO communication approaches.