Optimal Precoder Design for MIMO-OFDM: Understanding the Role of Power Amplifiers and Nonlinear Distortion Noise

This paper investigates the optimal design of multiple-input-multiple-output (MIMO) precoders for orthogonal frequency-division multiplexing (OFDM) systems in the presence of nonlinear distortions due to the high power amplifier (HPA). In traditional approaches, the designer lets the input backoff (IBO) of the HPA be large enough to neglect the unavoidable nonlinear effects of the amplifier. However, this hypothesis may severely hinder the efficiency of the HPA and, consequently, the global energy efficiency of the link. On the other hand, making full use of the available power gives rise to unbearable nonlinear effects with consequent strong rate degradations. In this work, we take advantage of the Bussgang theorem to unveil the impact of the nonlinear distortion noise on the optimal precoding and to derive a power allocation algorithm that achieves the optimal tradeoff between HPA efficiency and rate degradation. We also provide a sufficient condition for the concavity of the information rate objective function in this nonlinear scenario. Finally, numerical results shows that the proposed algorithm outperforms conventional fixed-IBO precoding strategies.