Sihua Shao

Coexistence of WiFi and LiFi toward 5G: concepts, opportunities, and challenges

Smart phones, tablets, and the rise of the Internet of Things are driving an insatiable demand for wireless capacity. This demand requires networking and Internet infrastructures to evolve to meet the needs of current and future multimedia applications. Wireless HetNets will play an important role toward the goal of using a diverse spectrum to provide high quality-of-service, especially in indoor environments where most data are consumed. An additional tier in the wireless HetNets concept is envisioned using indoor gigabit small-cells to offer additional wireless capacity where it is needed the most. The use of light as a new mobile access medium is considered promising. In this article, we describe the general characteristics of WiFi and VLC (or LiFi) and demonstrate a practical framework for both technologies to coexist. We explore the existing research activity in this area and articulate current and future research challenges based on our experience in building a proof-of-concept prototype VLC HetNet.

An Indoor Hybrid WiFi-VLC Internet Access System

Visible light communications (VLC) is emerging as a new alternative to the use of the existing and increasingly crowded radio frequency (RF) spectrum. VLC is unlicensed, has wide bandwidth, supports new levels of security due to the opacity of walls, and can be combined to provide both lighting and data communications for little net increase in energy cost. As part of a lighting system, VLC is ideal as a downlink technology in which data are delivered from overhead luminaries to receivers in the lighting field. However, realizing a symmetric optical channel is problematic because most receivers, such as mobile devices, are ill-suited for an optical uplink due to glare, device orientation, energy constraints. In this paper we propose and implement a hybrid solution in which the uplink challenge is resolved by the use of an asymmetric RF-VLC combination. VLC is used as a downlink, RF is used as an uplink, and the hybrid solution realizes full duplex communication without performance glare or throughput degradation expected in an all-VLC-based approach. Our proposed approach utilizes a software defined VLC platform (SDVLC) to implement the unidirectional optical wireless channel and a WiFi link as the back-channel. Experiments with the implemented prototype reveal that the integrated system outperforms conventional WiFi for crowded (congested) multiuser environments in term of throughput, and demonstrate functional access to full-duplex interactive applications such as web browsing with HTTP.

Design and analysis of a visible-light-communication enhanced WiFi system

Visible light communication (VLC) has wide unlicensed bandwidth, enables communication in radiofrequency-sensitive environments, realizes energy-efficient data transmission, and has the potential to boost the capacity of wireless access networks through spatial reuse. On the other hand, WiFi provides more coverage than VLC and does not suffer from the likelihood of blockage due to the line-of-sight requirement of VLC. In order to take the advantages of both WiFi and VLC, we propose and implement two heterogeneous systems with Internet access. One is the hybrid WiFi-VLC system, utilizing a unidirectional VLC channel as the downlink and reserving the WiFi backchannel as the uplink. The asymmetric solution resolves the optical uplink challenges and benefits from the full-duplex communication based on VLC. To further enhance the robustness and increase throughput, the other system is presented, in which we aggregate WiFi and VLC in parallel by leveraging the bonding technique in the Linux operating system. We also theoretically prove the superiority of the aggregated system in terms of average system delay. Online experiment results reveal that the hybrid system outperforms the conventional WiFi for crowded environments in terms of throughput and Web page loading time, and also demonstrate the further improved performance of the aggregated system when considering the blocking duration and the distance between the access point and the user device.

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.