Asynchronous mmWave Cell-free Massive MIMO-OFDM with Per-Beam Timing Advance

Cell-free massive multiple-input-multiple-output (CF-mMIMO) is regarded as one of the promising technologies for next-generation wireless networks. However, due to its distributed architecture, geographically separated access points (APs) jointly serve a large number of user-equipments (UEs), and there will inevitably be differences in the arrival time of transmitted signals. In this paper, we investigate the downlink millimeter-wave (mmWave) CF-mMIMO orthogonal frequency division multiplexing (OFDM) systems with asynchronous reception in a wide area coverage scenario, where asynchronous timing offset is far beyond the length of cyclic prefix (CP). We present a comprehensive asynchronous beam-domain downlink signal transmission model for mmWave CF-mMIMO-OFDM systems, incorporating phase offset, inter-carrier interference (ICI) and inter-symbol interference (ISI). To address the issue of asynchronous reception, we propose a novel per-beam timing advance (PBTA) hybrid precoding architecture and obtain the spectral efficiency in the beam domain for downlink asynchronous reception. Both scalable fully centralized and distributed implementations are taken into account. The simulation results demonstrate that the performance can be severely limited by ICI and ISI, and our proposed PBTA hybrid precoding architecture can effectively mitigate asynchronous interference.