Integrated Access and Backhaul in Cell-free Massive MIMO Systems

One of the major challenges with cell-free (CF) massive multiple-input multiple-output (MIMO) networks is providing backhaul links for a large number of distributed access points (APs). In general, providing fiber optics backhaul for these APs is not cost-effective and also reduces network scalability. Wireless backhauling can be a promising solution that can be integrated with wireless access links to increase spectrum efficiency. In this paper, the application of integrated access and backhaul (IAB) technique in millimeter-wave (mmWave) CF massive MIMO systems is investigated. The access and backhaul links share a frequency spectrum in the mmWave bands, and in both, hybrid beamforming techniques are adopted for signal transmission. The bandwidth allocation (division) parameter between the two link types as well as the beamforming matrices are optimized to maximize the end-to-end data-rate. This leads to a non-convex optimization problem for which an efficient solution method is proposed. The simulation results show the effectiveness of the IAB technique and our proposed scheme in CF massive MIMO systems. These simulations also compare the proposed hybrid beamforming method with a fully digital solution in terms of the number of radio frequency (RF) chains and the volume of backhaul traffic. Finally, the effect of increasing the number of APs on the users data rates in terms of wireless access and backhaul links constraints is also examined.