Using light as means of communication is not a new idea. In fact, Optical Wireless (OW) communication has been around for more than three centuries old. Techniques such as ship flags, semaphore and fire beacons were the earliest stages of OW. In early method, optical communication uses mirror for reflecting the sunlight. A better example of such a method is the heliograph, in which mirror is used to transmit flashes of light, that can be th used to refer Morse code. In the 19 century, this structure was a very active device for optical communication over a distance of 50 miles or more.

The radio and microwave message systems suffer controlled system measurements due to constrains in the radio spectrum. Nowadays, many people carry more than one wireless device at any moment, for instance, a smart phone, smart watch, tablet and smart glasse, and as a result, the data rates requested by users are in constant raise exponentially (Burns, 2004). By 2017, it is predicted that more than 11 extra bytes of data would need to be shifted through mobile links every month.

Different technologies have proposed to provide high data rate services for users. Recently, the Wireless Gigabit Alliance (WiGig Alliance) has planned for the utilization of unlicensed 60 GHz frequency band to allow a 7 Gbps small range wireless link. However, tracking algorithms and sophisticated digital beam formation are required to use it in mobile wireless networks due to high path loss of radio waves in this spectrum range (i.e., 60 GHz). Since, the RF spectrum is expensive and limited, and the complementary wireless transmission techniques are required to relieve the RF spectrum. There is a probable band of the electromagnetic spectrum offered that is able to offer tens of gigabits per second for future users, especially for indoor users. It is three times extra since the initial OW structures were planned as RF structures are needed to care high data rates.

There are two dissimilar methods to produce white light from Light Emitting Diodes (LED). The first technique is white light produced by means of a phosphor layer that is covered on blue LEDs. The bright light discharged by blue LEDs are riveted by the phosphor layer, and the blue wavelength excites the phosphor and making it to glow white, then the phosphor produces light at larger wavelengths. The second technique is by means of RGB LEDs. With equal mixture of three colors (red, green and blue) white light can be formed, as used in color TV.

Digital and analogue mechanisms are cast-off in the transmitter and receiver system. In transmitter sections, some of the digital components like data stream, baseband modulator and Digital to Analogue Convertor (DAC) are used. Similarly, in the receiver section, Analogue to Digital Convertor (ADC), demodulator and data sink are used. The analogue devices used in the transmitter part is transconductance amplifier (TCA), bias tee and LED/LD. The analogue devices used in the receiver part are PD, transimpedance amplifier (TIA) and band pass filter (BPF). transimpedance amplifier (TIA) and band pass filter (BPF).

The features of the VLC scheme network are essential to address the presentation of the system and design problems. This defines the tools used to model the VLC system communication channel in the indoor environment. The VLC signal beneath the outcome of multipath dispersion and mobility is measured for dissimilar link configurations such as Line-of-Sight (LOS) and Non-Line-of-Sight (NLOS). For an indoor VLC configuration, multipath broadcast, receiver noise, path loss and channel variation due to mobility are the major losses. They can destroy the VLC scheme presentation due to the outline of weighty alteration in the received VLC indicator.

Modern OW communication schemes overlapped with the creation of the first laser in 1960 (Bell, 1880). Though, this knowledge was not organized until the 1990s after the transmitter and the receiver mechanisms developed as obtainable at low cost (Arnon et al, 2012). OW communication includes outdoor organizations, such as free-space optical (FSO), ultra violet communication (UVC) and underwater communications and indoor organizations such as infrared data association (IrDA), infrared optical wireless communication (IROW) and visible light communication (VLC) schemes (Hussein & Elmirghani, 2015). The term ‘visible light communication' denotes to the usage of light waves in free-space broadcast in the visible band as a transmission standard for communication. The visible signal is electromagnetic radiation whose wavelengths are positioned between IR and UV. Visible radiation has wavelengths between 380 nm and 780 nm (Federici & Moeller, 2010).

Along with radio waves, organizations have also used infrared waves for communication purpose. It is more than three decades since OW schemes are being applied for industrial communication (Zhang & Kavehrad, 2013). Table 1 shows the studies of OW and their parameters. OW has a number of benefits over Radio Frequency (RF), such as profusion of tolerant bandwidth, no multipath fading and has effective data transfer rate close to 100 Mbps. To a great extent, RF and OW are considered to be inexpensive media. For illustration, if a wireless LAN is essential for a large area, users can roam freely and continue to be linked to the system at all times, then radio frequency is the only cost-effective standard which can be best utilized. Though, a wireless LAN is essential to cover a more defined area, but send innovative high bandwidth multimedia system facilities such as video conferencing and video are required upon request, then OW is the only medium which really has the bandwidth offered to deliver this.

Table 1. Studies on OW and their Parameters

Until now, commercially available OW organizations have not come close to sending the high data rates which can be potentially presented from the OW spectrum, the details of which are more to do with the restricted variety, difficultly to operate outdoor, high power condition and cost limitations sooner than any important restrictions of the core intelligence.

Furthermore, OW links must be accomplished to operate in situations where strong ambient light levels exist, which destroys connection performance in two ways. Initially, the usual power of the background radiation produces shot noise in the receiver, which is self-determining of the transmitted signal, and artificial foundations of ambient light produce a periodic interfering signal, which may contain harmonics into the MHz area for fluorescent lamps using by electronic ballasts.

The core purpose of the VLC source is to change an electrical signal into an optical form and then propagate it as optical signal into the free space. Figure 1 shows the basic block diagram of VLC system. LEDs are presently used for VLC broadcast. White LEDs are available at low cost and they are identified to be eye-safe, even at moderate to high powers. This is owing to their adequately great surface area to emit light over a moderately wide spectral range. In addition, they are more reasonable and consistent when compared to the glowing light bulbs. Therefore, LEDs are a better source of light for indoor applications. In spite of LEDs taking several advantages, they also have some disadvantages: 

Figure 1. Basic Block Diagram VLC System

• Low modulation bandwidth (typically tens of MHz)
• Low electro-optic power conversion efficiency (typically 10 to 40%)
• Non linearity

Considerable damage due to the long-distance transmission of VLC is observed and they are measured as path-loss, conservation noises, and Signal to Noise Ratio (SNR). Many modulation systems are cast-off in VLC. The core variance in VLC and RF technique is the VLC might not be frequency encrypted as well as voltage level. It suggests that phase and amplitude encryption cannot be employed in VLC. In VLC system encrypting is done by the strength of light waves. The demodulation depends on the data receiver. The strength of the modulation is named as direct detection modulation. The modulation systems from extra kinds of digital infrastructures such as higher data rates, quality factor and assembling are the requirements of visible light to humans, which are realized by VLC.

In On-Off Keying (OOK), the data bit 1 is denoted as LED ON and 0 is denoted as LED OFF. In off state, the strength of light is reduced but not entirely turned off. In early days, OOK modulations are usually cast-off for VLC with white LED. OOK suggests dissimilar structures such as simple and feasible application when keeping various dimming levels and its reduced data rates.

Overall, optical communication is a scheme which is cast-off daily by lots of users. VLC is a novel concept in the ongoing knowledge evolution to exploit the organization to use light for information exchange. Nevertheless, as the improvement of internal wireless information exchange is increasing rapidly Wi-Fi cannot handle the increasing complications of the bandwidth. Through a long process, a user's data can be controlled individually via Wi-Fi.

Studies show that in future, more than eighty five percent of wireless data communication would be used indoors. The VLC directs data back and forth from base station to mobile users for wireless data communication structure. The incandescent light sources or fluorescent light sources are not used since it might not be controlled very competently. Non-coherent and coherent modulations are carried by LEDs. The source of light from LED can be measured at a very utilitarian rate. It is not essential for matching carrier, phase, etc., as only a few megahertz (MHz) bandwidth is needed for larger spectrum of communication.

The commercial market for visible light communication seems to have just begun. There are continuous researches and inventions going on. Markets are supposed to exploit this excellent opportunity to extend these facilities to their customers. Many progressive requests of VLC are likely to venture into the market successfully in the near future.

VLC systems cater from the indoor to outdoor applications. Indoor applications could be information communications within organization, location estimation within indoor limits, and guiding directions and positioning for visually challenged people. The outdoor applications are passage and place information, information broadcast using the traffic light structure based on JEITA CP-1222 standard, environmental hazards warning, security and defense applications, and regulator of robots and air travel.

Like IROW structures, VLC structures can be used to transmit information within workplaces. Electrical appliances can be switched on and off by predictability based on human entry or exit in the indoor situation using white LEDs and hallways can be irradiated by LEDs with a single ID for each LED.

Lately, several methods for VLC indoor locating arrangements have been considered. Initially, it is seen that there is a well locating precision (few millimeters) related to radio wave structures, as VLC suffers less from nosiness and multipath properties. Furthermore, VLC locating structures can be best used in situations where radio locating structures are controlled, such as in hospitals.

White LEDs can be used to transmit digital information in the locomotive ground (car to car) communication with the help of interconnection between head and tail LED lights of the cars. Also, there are opportunities to interconnect cars and the traffic lights setup. In 2010, the Geo Spatial Information (GSI) Authority of Japan offered position data using visible light, the idea is to give a single documentation number to each area of 3m × 3m in Japan (Matsumura et al., 2004).The restricted area, such as oil rigs and mines, safe communication and lighting can be provided by VLC structures using LEDs. Today, airplanes use white LEDs for lighting, which can also be used to offer media facilities to travelers instead of cables. This will decrease the weight and the cost of the airplanes.

VLC structures have both advantages and disadvantages that need to be carefully studied. VLC and RF systems are matching transmission systems, with dissimilar requests preferring one standard over the other. According to the sample, RF is favored in requests, where transmission has greater distance or area has hindrances. On the other hand, VLC is preferred in short to medium linkage requests, where combined structure capacity must be exploited at negligible cost or receiver signal processing difficulty must be diminished.

VLC is the source of present wireless communication structures. Nevertheless, the development in request for more frequencies, lower cost mechanisms, higher data charges and better Quality of Service (QoS), have required investigators to prefer choices such as VLC. VLC has numerous advantages over RF. As for the sample, it has fast arrangement, low start-up working costs and high bandwidth for fibre optic. The VLC's unrestricted bandwidth (380~780 nm) is wide spectrum, which can reduce the infrastructure cost. The accessibility of simple front-end devices at small cost, the facility of energy productivity, hundreds of tera hertz of license-free bandwidth, are being harmless for humans, which can add extra electronic devices and also it is easy to carry.

A VLC structure is not without disadvantages. VLC access need to be organized via optically wired backbone as light cannot enter through walls from one room to another. Furthermore, the spread in the received pulse owing to multipath diffusion damages the SNR. Multipath diffusion is qualified for reflective sides, such as walls, holes, gates and ceilings. Since these reflective obstacles act as minor emitters of light that diffuse the signal in the form of a Lambertian design, the transmitted information reaches at the receiver from multiple dissimilar paths, which has the possibility of transmitting pulses spread. Besides, the obtainable present modulation bandwidth of the transmitters (LEDs) is associated with the VLC spectrum, which means transmission bandwidth is restricted by the LED bandwidth. In addition, a one-way communication (downlink), apparently through white LEDs for communication, is useful for such requests as associated music transmission, consequently, providing an uplink to a transferrable transmitter assembly can be a big challenge.

In this paper, a general introduction on VLC and their potential applications have been reviewed. The review highlighted the important tasks in VLC, such as small modulation bandwidth of transmitters in LEDs, multipath dispersion, photodetector high capacitance and the facility of an uplink. Further, it has conversed some of the methods that can be used to moderate the shortcomings in the present technology. This would direct the system level investigation and application of the VLC system. In future, researches may focus on the use of Orthogonal Frequency-Division Multiplexing (OFDM) modulation to expand the data rates in the VLC system.