Sequential Hardware-Aware Precoding for CF-mMIMO-OFDM with Serial Fronthaul

Scalable and cost-effective solutions are of the foremost importance for the deployment of distributed/cell-free massive MIMO (CF-mMIMO) networks. Moreover, in the quest for cheaper hardware, power amplifiers (PAs) are expected to be operated on highly non-linear operation regimes, where strong distortions are impacting the system's performance. In this paper, we first analyze the impact of the hardware impairment (HWI) caused by the PAs on an orthogonal frequency division multiplexing (OFDM) based CF-mMIMO system. Then, we introduce a novel sequential precoding scheme aiming at cancelling the PA-induced HWI by taking advantage of a serial fronthaul topology. Building on the so-called Bussgang decomposition, we derive a HW-aware precoding, in the form of a closed-form linear system equation. It takes advantage of a serial fronthaul connecting multiple APs, which offers the possibility of communicating HWI information between successive APs in a branch. Numerical results show that the proposed method is able to provide substantial improvement of the achievable spectral efficiency (SE) when the the PA is operated near its saturation region, where its power efficiency is high. Most importantly, it shows promising outcomes to reach high energy efficiency in CF-mMIMO system while maintaining network scalability.