A Hybrid VLC-IR Optical Wireless Architecture with Load Balancing for NLoS Tunnels

This paper presents the evaluation for VLC/IR optical wireless architectures for NLoS-only underground tunnels scenarios, modeling the system as a bidirectional link with VLC uplink and IR downlink, evaluating its performance under a load-balancing approach that reallocates traffic according to link availability. Four configurations are considered, corresponding to levels of directional redundancy. The baseline 1-1 configuration is supported by a practical testbed, whereas the remaining cases are examined through system-level comparative extensions under the same traffic-allocation and link-availability, while the system-level performance is evaluated in relation with the load-balance, including throughput, unmet demand, per-link utilization, and fairness. In addition, an illustrative bit error rate (BER) analysis is used to relate physical-layer behavior with system-level robustness. The results show that increasing directional redundancy improves service continuity by mitigating throughput degradation during link DOWN events through traffic reallocation. While configurations with single-link directions exhibit single points of failure, the 2-2 configuration achieves the highest aggregate throughput, the lowest unmet-demand ratios, and the most robust behavior under partial link failures. The BER trends further indicate that parallel link availability reduces sensitivity to individual channel degradation, reinforcing the benefits of balanced uplink and downlink redundancy in tunnel environments.