Effective Capacity Analysis for Hybrid FSO/THz Backhaul Link in 6G Networks

The hybrid free-space optical (FSO)/terahertz (THz) backhaul link has emerged as a promising solution for high-capacity wireless backhaul networks, particularly in 6G systems. However, the statistical delay performance of such hybrid links for ultra-reliable low-latency communication remains an open problem. This letter investigates, for the first time, the delay-constrained performance of a hybrid FSO/THz backhaul link under hard and soft switching mechanisms, using the effective capacity (EC) framework. Closed-form analytical expressions for EC are derived for each switching strategy, enabling the quantification of achievable throughput under delay constraints. The EC performance of the hybrid link is compared against FSO-only and THz-only baselines. Results demonstrate that the hybrid link significantly outperforms standalone links, achieving approximately 0.7 dB and 0.9 dB throughput gains for hard and soft switching, respectively, along with a 1.6 dB improvement in reliability. These findings highlight the advantages of hybrid FSO/THz links for robust, high-capacity backhaul communications.