To make indoor industrial cell-free massive multiple-input multiple-output (CF-mMIMO) networks free from wired fronthaul, this paper studies a multicarrier-division duplex (MDD)-enabled two-tier terahertz (THz) fronthaul scheme. More specifically, two layers of fronthaul links rely on the mutually orthogonal subcarreir sets in the same THz band, while access links are implemented over sub-6G band. The proposed scheme leads to a complicated mixed-integer nonconvex optimization problem incorporating access point (AP) clustering, device selection, the assignment of subcarrier sets between two fronthaul links and the resource allocation at both the central processing unit (CPU) and APs. In order to address the formulated problem, we first resort to the low-complexity but efficient heuristic methods thereby relaxing the binary variables. Then, the overall end-to-end rate is obtained by iteratively optimizing the assignment of subcarrier sets and the number of AP clusters. Furthermore, an advanced MDD frame structure consisting of three parallel data streams is tailored for the proposed scheme. Simulation results demonstrate the effectiveness of the proposed dynamic AP clustering approach in dealing with the varying sizes of networks. Moreover, benefiting from the well-designed frame structure, MDD is capable of outperforming TDD in the two-tier fronthaul networks. Additionally, the effect of the THz bandwidth on system performance is analyzed, and it is shown that with sufficient frequency resources, our proposed two-tier fully-wireless fronthaul scheme can achieve a comparable performance to the fiber-optic based systems. Finally, the superiority of the proposed MDD-enabled fronthaul scheme is verified in a practical scenario with realistic ray-tracing simulations.

翻译：为使室内工业无小区大规模多输入多输出网络摆脱有线前传的束缚，本文研究了基于多载波分双工的两层太赫兹前传方案。具体而言，两层前传链路依赖相同太赫兹频段内相互正交的子载波集，而接入链路则在sub-6G频段实现。该方案引出了一个复杂的混合整数非凸优化问题，涉及接入点聚类、设备选择、两层前传链路间的子载波集分配以及中央处理单元与接入点的资源分配。为解决该问题，我们首先采用低复杂度但高效的启发式方法松弛二进制变量。随后，通过迭代优化子载波集分配与AP聚类数量，获得端到端总速率。此外，针对所提方案定制了一种由三条并行数据流组成的先进MDD帧结构。仿真结果验证了所提动态AP聚类方法在处理不同规模网络时的有效性。同时，得益于精心设计的帧结构，MDD在两层前传网络中的性能优于TDD。进一步分析了太赫兹带宽对系统性能的影响，表明在充足频率资源条件下，所提两层全无线前传方案可达到与光纤系统相当的性能。最后，通过基于真实射线追踪的仿真场景验证了所提MDD前传方案的优越性。