MDD-Enabled Two-Tier Terahertz Fronthaul in Indoor Industrial Cell-Free Massive MIMO

To liberate indoor industrial cell-free mas-sive multiple-input multiple-output (CF-mMIMO) net-works from wired fronthaul, this paper proposes amulticarrier-division duplex (MDD)-enabled two-tierterahertz (THz) fronthaul scheme. In our scheme, twolayers of fronthaul links rely on the mutually orthogonalsubcarrier sets in the same THz band, while accesslinks are implemented over sub-6G band. However, theproposed scheme leads to a complicated mixed-integernonconvex optimization problem incorporating accesspoint (AP) clustering, device selection, the assignmentof subcarrier sets and the resource allocation at boththe central processing unit (CPU) and APs. Hence, inorder to address the formulated problem, we first resortto the low-complexity but efficient heuristic methodsthereby relaxing the involved binary variables. Then,the overall end-to-end optimization is implementedby iteratively optimizing the assignment of subcarriersets and the number of AP clusters. Furthermore, anadvanced MDD frame structure consisting of three par-allel data streams is tailored for the proposed scheme.Simulation results demonstrate the effectiveness of theproposed dynamic AP clustering approach in dealingwith the networks of varying sizes. Moreover, benefit-ing from the well-designed frame structure, MDD iscapable of outperforming TDD in the two-tier fronthaulnetworks. Additionally, the effect of the THz band-width on system performance is analyzed, and it isshown that empowered by sufficient bandwidth, ourproposed two-tier fully-wireless fronthaul scheme canachieve a comparable performance to the fiber-opticbased systems. Finally, the superiority of the proposedMDD-enabled fronthaul scheme is verified in a practicalscenario with realistic ray-tracing simulations.