Local PAPR-Aware Precoding for Energy-Efficient Cell-Free Massive MIMO-OFDM Systems

Orthogonal frequency-division multiplexing (OFDM) based Cell-free massive multiple-input multiple-output (CF-mMIMO) is the most promising technology that, compared to cellular technology, provide uniformly better quality of service for users in sub-6GHz bands. To make OFDM based CF-mMIMO more cost-effective and power-efficient, the access points (APs) should be equipped with low-cost and low-quality transceiver hardware. However, transmitting OFDM signals with high peak-to-average power ratio (PAPR) via such hardware causes severe hardware impairments (HWI), damaging the system performance. In this paper, we present an energy-efficient downlink transmission scheme adequate to the emerging CF-mMIMO-OFDM technology. Specifically, we introduce a smart PAPR-aware precoding scheme, referred to as localPAPRfree, adequate for cell-free architecture. Interestingly, the proposed technique can be implemented in a distributed and scalable manner, demonstrating benefits of CF-mMIMO-OFDM. The PAPR-aware precoding scheme is formulated as a simple convex optimization problem solved via an efficient and steepest iterative algorithm. In addition, we present a theoretical study to analyze the downlink (DL) spectral-efficiency (SE) and energy-efficiency (EE) performance of CF-mMIMO-OFDM under power amplifier (PA) non-linearity, providing important insights into the practical impact of nonlinear PA. Numerical results show that the proposed localPAPRfree algorithm offers excellent PAPR reduction performance, guaranteeing high EE.