Optical wireless link enhanced wireless mesh networks with bottleneck connections

Bottleneck link connections may seriously decrease the overall system throughput of wireless mesh networks (WMNs) in which the user nodes are interconnected in a distributed way. With the increasing demands for higher data rates among the WMN user nodes, traditional mesh network using radio frequencies (RF) can easily break down if one or more critical links are selected to serve many parallel traffics or overwhelmed by other co-channel wireless interferences. These bottleneck links are shown to introduce severe end-to-end delays, and hence become a big barrier for the real deployments of WMN. To tackle the bottleneck link problem in RF-based WMNs, we propose a novel system architecture by introducing another dimension of transmissions. Specifically, we establish optical wireless communications (OWC) links on top of the original bottleneck links whose negative effects on the system throughput are mitigated, and we call the system O-WMN which is designed to make the best of both RF and OWC links’ superior properties. In O-WMN, the RF links are used to initialize the basic routing tables for each node using the optimized link state routing (OLSR) protocol. And then, the bottleneck links are identified and broadcasted to all nodes in O-WMN. After that, the OWC links are activated for the bottleneck links. However, since the OWC links are vulnerable to the line-of-sight (LoS) connections, some links might be blocked due to the next topology. To address the OWC link blockage issue in O-WMN, we further design a link selection method to leverage the routing tables, which ensures that OWC links are deployed in a proper way, such that the throughput of the whole network can be improved. Specifically, a new routing protocol named parallel OLSR (P-OLSR) is proposed, so that new OWC link based routing can offload traffics in the bottleneck RF links. To find an ideal route for O-WMN, a new link quality metric based on expected transmission count (ETX) is employed to determine routing path. Simulation results demonstrate that P-OLSR in O-WMN can achieve a considerable network throughput gain compared with traditional OLSR in WMN given the same network topology. To verify the effectiveness of O-WMN, we further conduct experiments using six sets of Raspberry PIs in an indoor environment with different rooms to collect the physical-world link data statistics. Based on the link data collections, we perform experiment-assisted simulations and the results show that the O-WMN with P-OLSR can achieve higher throughput gain compared with RF-only WMNs, showing the effectiveness of our proposed hybrid network architecture.