# Visible Light Communication: A System Perspective--Overview and Challenges
## 3 Applications
### Hospitals  
VLC operates in 430-770 THz, and due to nanometer wavelength, it cannot penetrate in objects, therefore making it ideal for applications where confidentiality of data is of utmost importance. The inherent security and safety feature of VLC provides an alternate of wireless communication that could be used to minimise health risks associated with radio frequency radiations. One such area is hospitals, where VLC can be utilised for monitoring patients, machine to machine communication, record keeping of patients and all other networking applications [53]. Newborn babies are fragile and vulnerable to even small RF radiations of the ISM band and cellular frequencies. Implementation of VLC in newborn babies wards would be a natural choice for monitoring and indoor connectivity to the rest of the internal and external networks. To replace the existing wireless communication in hospitals, a broadband powerline and VLC with orthogonal frequency division multiplexing (OFDM) is demonstrated [53]. [PERSON] et al. have proposed another hybrid system consisting of VLC and 60 GHz RF for e-health applications [54]. The rationale of VLC for these applications as it is easily available in hospitals and can reduce communication system cost. As VLC offers only downlink communication, therefore, 60 GHz RF to provide uplink communication is used. Apart from uplink, 60 GHz can be used for localisation as well. It is concluded that this hybrid system could be used efficiently to collect a wide range of information from the patients.  
[PERSON] et al. [25] have demonstrated secure data transmission that can have applications in a hospital environment. In [25] authors have proposed a CMOS camera-based VLC system which encrypts emitted light. The transmitter consists of an encryption module which receives an optical signal that is encrypted using a private key or another advanced encryption scheme and is then emitted. The CMOS-based mobile camera is used as a receiver. The rolling shutter feature of the CMOS camera is used to enhance data rates. An additional Otsu thresholding module is used to get effective BER. The proposed scheme gives good results in terms of BER and encryption.  
A communication system inspired by barcoding technology is proposed [55]. Unlike VLC, this system considers that without explicitly modulating a light source, the disturbances in the natural (e.g., sun) can be used to convey information and thus it is named as a passive communication system. It involves the design of a communication channel in which information is captured from the mobile elements with wear patterns consisting of distinctive surfaces. A user can be reduced to a \"tiny-box\" equipped with a commercial off-the-shelf (COTS) photodiode. Massive deployment of these tiny boxes can infer significant pieces of information from the surrounding. Such systems are deployed in hospitals where emergency, treatment and housing trolleys can report their physical locations. A key aspect of this system is sustainability in terms of infrastructure (as it needs of minimum infrastructure changes), energy efficiency (as the system uses only photodetectors which are energy efficient compared to the camera used in similar scenarios) and cost (as photodetectors are inexpensive). The passive communication system is demonstrated in the parking area and reported encouraging results.