| The incessant demand for wireless capacity over the last two decades is pushing the radio frequency(RF)spectrum to its limits.As a result,visible light communication(VLC),which can offer enormous bandwidth by harnessing the vast and unregulated light spectrum,has attracted much attention in recent times.VLC is even more attractive since it can be implemented over existing lighting systems.On the other hand,VLC networks have a far smaller coverage area per access point(AP)compared to RF-based networks such as IEEE 802.11 wireless local area network(WLAN),in addition to their high susceptibility to signal blockage and potentially strong co-channel interference(CCI).Towards resolving the coverage and connectivity issues in VLC networks,the application of WLAN as a complementary network in hybrid VLC and WLAN networks is regarded as a promising solution.Apart from improving coverage in VLC-WLAN heterogeneous networks(HetNets),WLAN can be used for uplink communication,thus achieving duplex communication.The pervasiveness of WLANs makes them readily available and more suitable for backing VLC networks.Regarding the line-of-sight(LoS)component of VLC transmission,the connection to the optical wireless(OW)link may be broken due to user mobility or obstruction by an object.When the OW link is interrupted in a VLC-WLAN HetNet,a prompt vertical handover(VHO)to the existing WLAN may be needed to ensure continuity of a call or session.Nevertheless,when the link outage is frequent then waiting for link recovery will be a more efficient approach to adopt in order to avoid excessive VHO signaling cost and degradation in system performance.In all of this,the needs of various applications running in a user device(UD)should not be ignored with respect to the timing of VHO triggers.Therefore,the VHO process in hybrid VLC and WLAN networks must be well-handled to make timely and effective handover decisions when VLC link is obstructed,unavailable or encounters strong CCI.In this project,VHO in VLC-WLAN HetNets is investigated with the ultimate purpose of designing adaptive dwell-based handover mechanisms for integrating VLC networks and WLANs.This research investigates the causes of VLC link interruptions and develops VHO techniques that abate the adverse effects of VLC link interruptions.Additionally,this project seeks to address potential negative impacts of dwell timing.To this end,a channel adaptive dwell VHO(CAD-VHO)scheme based on the history of VLC accessibility is proposed in this research.The width of the dwell timer employed in this scheme is altered based on the rate of blocking and recovery of VLC-LoS channel.Simulation is carried out by setting up a dynamic network environment in an indoor setting.Results demonstrate that,the proposed CAD-VHO scheme is adaptive to the rate and extent of obstruction of VLC-LoS link due to user mobility and surrounding objects.Moreover,the proposed CAD-VHO scheme in comparison with benchmarks schemes,can reduce the number of VHOs by as much as 82% whiles achieving a higher average data rate in most cases.Subsequently,a hybrid applicationaware VHO(HA-VHO)scheme for integrated VLC and WLAN indoor networks is presented in this project.This adaptive scheme makes handover decisions based on the cause and transiency of VLC link interruptions as well as the types of applications running in UD.Handover decision is influenced by the velocity of UD or the VLC link recovery rate depending on the scenario.The HA-VHO scheme combines three handover approaches to achieve enhanced handover performance.Simulation results indicate that the proposed scheme ensures high data rate and long connection time,while substantially reducing the total signaling cost of VHOs in several cases.The application of the handover schemes presented in this project will be vital in combining VLC networks and WLANs toward fifth generation(5G)and beyond.The schemes are designed to keep UDs longer in the preferred VLC network,thus offloading the traffic from the existing RF networks. |
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