Delay Analysis of Unsaturated Heterogeneous Omnidirectional-Directional Small Cell Wireless Networks: The Case of RF-VLC Coexistence

The coexistence of omnidirectional small cells (OSCs), such as RF small cells, and directional small cells (DSCs), such as visible-light communication cells, is investigated. The delay of two cases of such heterogeneous networks is evaluated. In the first case, resource allocated OSCs, such as RF femtocells, are considered. In the second case, contention-based OSCs, such as WiFi access point, are studied. For each case, two configurations are evaluated. In the first configuration, the non-aggregated scenario, any request is either allocated to OSC or DSC. While in the second configuration, the aggregated scenario, each request is split into two pieces, one is forwarded to OSC and the other is forwarded to DSC. For the first case, under Poisson request arrival process and exponential distribution of request size, the optimal traffic allocation ratio is derived for the non-aggregated scenario and it is mathematically proved that the aggregated scenario provides lower minimum average system delay than that of the non-aggregated scenario. For the second case, the average system delay is derived for both non-aggregated and aggregated scenarios, and extensive simulation results imply that, under certain conditions, the non-aggregated scenario outperforms the aggregated scenario due to the overhead caused by contention.