# Evolution of Hybrid LiFi-WiFi Networks: A Survey
## 1 Introduction  
Short-range wireless technologies are becoming crucial in fifth generation (5G) networking. It is predicted that there will be 17 billion mobile devices and WiFi hotspots by the end of 2030, in contrast to 549 million hotspots in 2022 [1]. Because of radio frequency (RF) limitations, WiFi access points must be densely deployed, which in return results in the congestion of available channels. Further, the demand for mobile traffic is exponentially increasing, for e.g., from 2017 to 2022, it increased by seven times to about 77.5 Exabyte monthly.  
To solve these problems with respect to the RF spectrum, researchers are employing technologies with extremely high frequencies such as light fidelity LiFi [2], which uses light as an information carrier and exploits the vast optical spectrum of about 300 THz. Its biggest advantage is the ability to integrate LiFi access points into the existing light infrastructure, which enables Access Points (APs) to serve a dual purpose (illumination and communication). In recent research of light-emitting diodes (LEDs), it was shown that a single LiFi AP could achieve data rates of 10 Gbps [3].  
Furthermore, LiFi offers:  
* license-free spectrum;
* usage in environments unfavourable to the RF spectrum (hospitals, underwater, LNG ships);
* physically secure channels (the light does pass through walls).  
* Still, there are some limitations:
* LiFi APs have a short range of coverage (of about couple metres around a single AP);
* its physical property (which is its biggest security advantage) becomes its worst drawback, because light is prone to obstructions and therefore to the loss of signal.  
The hybrid LiFi-WiFi networks (HLWNet) were first mentioned in the paper of [PERSON] et al. in 2011 [PERSON] et al. (2011). Later on, the research was extended to the integration of LiFi and femtocells (with a small low-power cellular base station designed for home use, with DSL from telecom service providers and integrated WiFi AP and router capabilities), which in fact showed that such hybrid networks achieved better performances than standalone ones [PERSON] et al. (2011). Figure 1 shows the representation of LiFi possibilities in its integration with 5G technologies. For example, outside of buildings or structures, users can connect to the internet via satellites or 5G LTE stations and inside, they can connect to HWLNet.  
To the best of the authors' knowledge, no other work in the literature provides a comprehensive review of the recent HWLNet advances and research challenges, specifically with regard to handover, load balancing, positioning, and security models for HWLNets, which is a novelty and a contribution of this manuscript.  
The contributions of this paper are as follows:  
* we present a framework for HWLNet analysis (architecture, cell deployment, modulation scheme, illumination, and design of backhaul devices);
* systematic review of the performance metrics of HWLNet is provided;
* we discuss user behaviour models and their effects on HWLNet performance;
* thorough analysis of HWLNet handover models and load balancing is presented;
* we give an introduction of HWLNet to the Internet of Things (IoT) with embassies on physical security;
* finally, we discuss future implementations of HWLNets.  
Figure 1: Vision of future LiFi network possibilities.  
The structure of this paper is the following:  
In the Section 2, we present the framework of the HWLNet architecture, while in the Section 3, we review the metrics of the hybrid networks. In the Section 4, we discuss the effect of user behaviour and then present handover models and load balancing. Furthermore, in the Section 6, we present the physical security of HWLNets and their introduction into IoT. Finally, the upcoming future implementation issues, challenges, and advancements of HWLNets are discussed in the Section 7, followed by the conclusions in the Section 8.