# Adaptive Software Defined Equalization Techniques for Indoor Visible Light Communication
## 2 Advances in Visible Light Communications  
Visible light communication is slowly surfacing as a fully-fledged alternative or at least a supplementary technology to today's widespread wireless fidelity (WiFi). WiFi tends to lack free channels in highly urbanized areas or flat complexes [40]. Visible light communications provide an alternative, which does not penetrate walls, thus ensuring high reusability of the channels (Figure 3). As the technology itself operates in the free band, which is currently not limited by strict standardization, it is possible to develop practically any custom system; see [41; 42; 43]. Many companies focusing on the manufacturing of LED light sources or even smart devices (such as smart phones) are already exploring the possibilities of light fidelity (LiFi) or have claimed multiple patents in this field. Both major smartphone players, Samsung and Apple, are currently developing their own versions of LiFi. Samsung holds multiple patents in this field [44]. On the other hand, Apple actively cooperates with PureLifi company, based around professor [PERSON], who pioneered  
Figure 2: VLC prototyping research concept. one of the first experiments with LiFi itself [45]. PureLifi is experimenting with LiFi modules integrated into laptops as well, reaching up to one gigabit [46]. In the case of conventional WiFi, the new standard called 802.11 ad was recently released, and active network elements are slowly surfacing on the market. This standard moves WiFi from 5 GHz to 60 GHz, also called the mm-wave region. However, the path loss will significantly increase since it is proportional to the square of the carrier frequency. It is estimated that either the transmit power has to be increased, the distance between individual access points has to be decreased, or the whole system has to employ advanced beamforming to focus transmissions [47]. As the current regulations for 4G/5G systems have reached their ceiling, it is not likely that the transmit power will increase in the enhanced 5G or possibly 6G network. Thus, a different approach has to be chosen such that the cell size of 5G/6G will be reduced, which will inevitably lead to the increase of manufacturing costs, since the beamforming, which requires multiple transmitting/receiving antennas, will be a necessary technology. On the other side are VLC technologies with the unlicensed spectrum of approximately 350 THz. However, the VLC systems suffer from high path loss; thus, advanced optical components are practically a must to develop a long-distance system. Currently, VLC is mainly used in free space optical (FSO) wireless links, which have advanced collimators or even automatic fine-tuning capabilities and are often used as the backbone of Internet providers. [PERSON] et al. differentiated between VLC, optical camera communications (OCC), free-space optical communications (FSO), and light fidelity (LiFi) [48]. According to him, VLC should be used for IoT or machine-to-machine communications providing ultra-reliable low latency communication (URLLC) for 5G, B5G, and Industry 4.0. OCC is similar to VLC, but can also use a display as the transmitter and a camera image sensor as the receiver. This whole concept stands on recent advances in image sensor and camera technologies, offering higher resolutions and/or frame rates. Furthermore, all the sensors are currently much smaller and can be embedded into smartphones or laptops. Other than data transmissions, this technology can be also used for localization [49]. As mentioned before, FSO links based on laser diodes are often used for backhaul connections, where optical fibers can be reliably used (or the solution would be pricy). Finally, LiFi is a completely wireless network solution, serving as a complete alternative or supplementary technology to WiFi. This technology uses visible light for receiving data and the infrared band for transmissions. The coverage of LiFi is severely limited by the illumination area of the light source.