Large Intelligent Surfaces for Energy Efficiency in Wireless Communication

The adoption of Large Intelligent Surfaces (LIS) in assisting downlink multi-user communication from a multi-antenna base station is investigated in this paper. We present efficient designs for both the transmit power allocation and the coefficients of the surface reflecting elements that target at maximizing either the energy or the spectral efficiency subject to individual link budget guarantees for the mobile users. Since both problem formulations are non-convex, we propose two computationally affordable approaches, which both capitalize on alternating maximization. To tackle the more demanding energy efficiency problem, we present an algorithm based on gradient descent for obtaining the LIS phase coefficients, while the optimal transmit power allocation is provided by a fractional programming method. For the energy efficiency maximizing LIS tuning, we also propose an efficient algorithm based on the majorization-minimization approach. Computer simulation results under a realistic outdoor system set up and power consumption model for LIS are presented, where LIS-assisted communication is extensively compared with the case of multi-antenna amplify-and-forward relaying. The results corroborate the merits of LIS incorporation using both our proposed design algorithms, and as it is shown that LIS-assisted communication can provide up to 300% higher energy efficiency than the conventional relay-assisted one.