# Visible Light Communication: A System Perspective--Overview and Challenges
## 4 Hybrid VLC
### Pdc-Vlc  
For VLC to provide a fully functional indoor network, it requires connectivity among VAPs and external networks. The easy and straightforward solution is to connect VAP via separate cables. However, this will increase the cost of cabling, installation, labour and design. PLC utilises the already installed electrical power lines for broadband access [76]. PLC modems are typically plugged into power sockets to provide the interface between data and power network. For over a decade, PLC has been successfully used to transmit radio programs, home automation (remote control of lighting and appliances), home networking, internet access, utility company control switching and automated meter reading [77; 78]. Thus like VLC, PLC has the advantage of having the readily available infrastructure. [PERSON] et al. [79] proposed PLiFi, a Hybrid WiFi-VLC that promises low-cost connectivity to the internet, interconnectivity between the VAPs and seamless integration of WiFi for the uplink transmission. The user mobility and change in device orientation is investigated. A smaller and distributed VAP along with a MAC protocol coordinated multi-point transmission (CoMP) is proposed to address the co-channel interference due to adjacent LEDs. In the experimental setup, PLC modems were used to synchronise VAPs. [PERSON] et al. [80] have developed a cascaded PLC and VLC channel model of hybrid PLC-VLC systems. The influence of PLC channel and VLC transmitters on the propagation of the signal is analysed. The available channel frequency response is used to determine the transfer function to characterise the channels over time. The cascaded channel path of PLC-VLC can have many sources of impairments including but not limited to attenuation and impulsive noise. The proposed model claims to analyse the cascaded channel attenuations accurately. However, noise is ignored in their analysis which can be a source of impairment in the PLC-VLC environment. [PERSON] et al. [81] have performed experiments with three-LED lamps to form a single frequency network (SFN) that showed the feasibility of VLC and PLC hybrid system for the indoor environment. Their system promises high-speed communication and greater signal coverage with less modification to the existing infrastructure. A similar experiment is performed by [PERSON] et al. [82]. They have implemented a full duplex decode and forward (DF) hybrid PLC-VLC system with minimal protocols or medium modifications. A data rate of 5 Mbps was demonstrated with their experimental test bed, which can be extended theoretically up to 30 Mbps. The research work as mentioned above provides some theoretical models as well as some practical implementations for setting up a PLC-VLC hybrid system. It provides an investigation into some essential components of hybrid systems such as channel modelling, noise removal, interference mitigation, and basic networking. However, hybrid systems are not evaluated for their benefits such as energy efficiency and scalability. The cost of electrical devices (e.g., PLC modems) is much higher compared to VAP, and the effect of adding these devices to the system have been ignored. According to [PERSON] et al. [57] data rates of up to 1 GB are possible by careful analysis of the PLC channel and appropriate selection of modulation schemes. For the current data requirements, 1 Gbps would suffice. However, VLC is capable of providing a data rate of up to 15 Gb/s [30], which is far higher than PLC. Thus, it would lead to a bottleneck. Future network application would require a backbone network that can support higher data rates. A viable solution might be to connect VAP through fibre optic, which will increase the system cost. Such VAP would potentially provide an ultra-high data rate for specific applications, where a user wants an ultra-high data rate. In [83] optical fibre link has been used to connect central station (CS) to radio access points (RAP) for indoor applications.