  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.
