Published in Reviews in Physics 3 (2018) 26–31
Harald Haas, LiFi Research and Development Centre, The University of Edinburgh, King’s Buildings, Edinburgh, EH9 3JL, UK
A B S T R A C T
In this paper we will first explain what Light-Fidelity (LiFi) is and argue that it is a 5th Generation (5G) technology. Peak transmission speeds of 8 Gbps from a single light source have been demonstrated, and complete cellular networks based on LiFi have been created. We will discuss numerous misconceptions and illustrate the potential impact this technology can have across a number of existing and emerging industries. We also discuss new applications which LiFi can unlock in the future.
LiFi is a wireless communication technology that uses the infrared and visible light spectrum for high speed data communication. LiFi, first coined in  extends the concept of visible light communication (VLC) to achieve high speed, secure, bi-directional and fully networked wireless communications . It is important to note that LiFi supports user mobility and multiuser access. The size of the infrared and visible light spectrum together is approximately 2600 times the size of the entire radio frequency spectrum of 300 GHz (see Fig. 1). It is shown in  that the compound annual growth rate (CAGR) of wireless traffic has been 60% during the last 10 years. If this growth is sustained for the next 20 years, which is a reasonable assumption due to the advent of Internet-of-Things (IoT) and machine type communication (MTC), this would mean a demand of 12,000 times the current bandwidth assuming the same spectrum efficiency. As an example, the industrial, scientific and medical (ISM) RF band in the 5.4 GHz region is about 500 MHz, and this is primarily used by wireless fidelity (WiFi). This bandwidth is already becoming saturated, which is one reason for the introduction of Wireless Gigabit Alliance (WiGig). WiGig uses the unlicensed spectrum between 57 GHz–66 GHz, i.e., a maximum bandwidth of 9 GHz. In 20 years from now, the bandwidth demand for future wireless systems would however, be 12,000 × 500 MHz which results in a demand for 6 THz of bandwidth. The entire RF spectrum is only 0.3 THz. This means a 20 times shortfall compared to the entire RF spectrum, and a 667 times shortfall compared to the currently allocated bandwidth for WiGig. In comparison, the 6 THz of bandwidth is only 0.8% of the entire IR and visible light spectrum. One could argue that a more aggressive spatial reuse of frequency resources could be adopted to overcome this looming spectrum crunch. This approach has been used very successfully in the past and has led to the ‘small cell concept’. In fact, it has been the major contributor towards the improvements of data rates as illustrated in Fig. 2. The cell sizes in cellular communication have dramatically shrunk. The cell radius in early 2 G systems was 35 km, in 3G systems 5 km, in 4G systems 100 m, and in 5G probably about 25 m in order to reuse the available RF spectrum more efficiently and to achieve higher data densities. However, further reductions in cell sizes are more difficult to achieve due to the high infrastructure cost for the backhaul and fronthaul data links which connect these distributed access points to the core network. Moreover, with a smaller cell size the likelihood of line-of-sight between an interfering base station and a user terminal increases. The resulting interference can significantly diminish data rates and may cause a major problem in cellular networks . Therefore, WiFi access points have been mounted under the seats in stadia to use the human body as an attenuator for the RF signals and to avoid line-of-sight interference links. Clearly, this is not a viable solution for office and home deployments. For these reasons, it is conceivable that the contributions for the future mobile data traffic growth will stem from more spectrum rather than spatial reuse. In particular, the optical resources are very attractive as they are plentiful as shown in Fig. 1 and they are license-free….SNIP
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