Visible light communication

Received: 28 February 2018

Revised: 27 April 2018

Accepted: 2 May 2018

DOI: 10.1002/ett.3436

SPECIAL ISSUE

Visible light communication – An architectural perspective on the applications and data rate improvement strategies Muhamamd Saadi1

Touqeer Ahmad2

1 Department of Electrical Engineering, University of Central Punjab, Lahore, Pakistan 2

Department of Computer Science and Engineering, University of Nevada, Reno, USA 3 Department of Electrical Engineering, Chulalongkorn University, Bangkok, Thailand

Correspondence Lunchakorn Wuttisittikulkij, Department of Electrical Engineering, Chulalongkorn University, Bangkok, Thailand. Email: [email protected]

M. Kamran Saleem1

Lunchakorn Wuttisittikulkij3

Abstract Demand for bandwidth hungry applications and mobile services is pushing an unquenchable need for wireless capacity. Existing radio frequency networks are characterized by shared medium, inadequate spectrum, and restricted user capacity. Solid state lighting is modernizing indoor as well as outdoor illumination. The capability of quick switching of light emitting diode makes them superior to other lighting sources enabling simultaneous use as a communication and illumination device. Visible light communication (VLC) using light emitting diodes is an attractive approach for many networking scenarios and is considered as a complementary technology for future heterogeneous wireless networks. This article presents an architectural perspective on the applications and data rate improvement strategies. A wide range of VLC applications have been investigated, comparison has been drawn with radio frequency, and generalized network architecture is also proposed. Numerous applications of VLC are data rate intensive, so data rate improvement approaches have also been studied. At the end, a lucid conclusion is drawn about the applicability, acceptability, and utilization of VLC and co-VLC–based systems.

1

I N T RO DU CT ION

In the era of smart devices, Internet of Things, and ubiquitous computing, there is an increased demand of wireless connectivity. The already existing communication technologies are running out of spectrum.1 As a result of spectrum crunch in the shared radio frequency (RF) medium, network performance is deteriorated greatly because of interference and contention.2 Therefore, we need a new wireless medium.3 With the recent advances in the field of solid-state lighting, white light-emitting diodes (LEDs) are anticipated to become a key player in future lighting. Compared with conventional (incandescent and fluorescent) lighting, LEDs offer various advantages such as low power consumption, long lifetime, lower voltage requirements, smaller sizes, cooler operation, low cost, and swift switching. The quick switching property of LED into a communication carrier gives rise to a new “dual-paradigm,” ie, simultaneous illumination and communication.4 There are two methods that are commonly used for the construction of white LED. The first method is to combine red, green, and blue (RGB) lights in an appropriate ratio, and the resulting emitted light will be white in color. Such LEDs are also referred to as tri-LED (T-LED). The second method for constructing white LED is to use phosphor. On a blue LED, coating of yellow phosphor is applied. This results in the emission of yellow light. Some yellow light is absorbed by phosphor and is made to mix with non-absorbed blue light to produce desired white colored light. The former method Trans Emerging Tel Tech. 2018;e3436. https://doi.org/10.1002/ett.3436

wileyonlinelibrary.com/journal/ett

Copyright © 2018 John Wiley & Sons, Ltd.

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is not a prevalent approach for the reason that RGB lights are not equally efficient. Furthermore, this technique also has some packaging and electronic design complexities. The latter method is more widely used in the industry. Visible light communication (VLC), which falls under the umbrella of optical wireless communication, utilizes part of light that is visible to the human eye (380 nm to 780 nm) for communication. The transmitters for VLC are the LEDs and laser diodes (LDs), and the superiority of LEDs is aforementioned. In comparison with its counterpart RF, VLC offers a number of advantages such as in-built security, immunity to RF and microwave interference, providing data network in radio-restricted areas, ability to provide gigabit data rates, unregulated spectrum, huge bandwidth (400 THz), and availability of network infrastructure.5,6 The future wireless networks are expected to have many folds increased capacity, gigabit data rates, and extremely low latency. These demands can be met by incorporating dense and smaller cell size, utilization of new spectrum, and capability to integrate various networks (heterogeneous networks – HetNets).7 The aforementioned advantages of VLC makes it a favorable choice for future generation networks. Furthermore, RF and VLC can coexist in 5G networks, thus benefitting from the salient features of RF (seamless coverage) and VLC (high data rates). In this paper, a detailed investigation on the applications of VLC is conducted. What should be the network architecture of an application specific VLC system and how the designed system is superior to its competing technologies are elaborated. These applications include indoor positioning, physical layer secure network, intelligent transportation system, RF VLC coexisting networks, in-flight entertainment, underwater communication, power overline communication integrated with VLC, and electromagnetic sensitive communication networks. These applications are discussed in Section 2. Many of the VLC applications are bandwidth and data rate hungry, but the dark side of LED is that it has a limited bandwidth. Methods on improving the limited bandwidth of LED and increasing the data rates are discussed in Section 3. Two other important challenges for VLC are dimming and flickering, and these aspects are also discussed in the same section. Current challenges and future works have been mentioned in Section 4 followed by the conclusion in Section 5.

2

APPLICAT IONS OF VISIBLE LIGHT CO MMUNICATION

Visible light communication is a revolutionary technology, and in this section, we discuss its various applications.

2.1

Indoor positioning using VLC

Positioning systems that are based on RF (eg, satellite navigation) are best known for outdoor positioning system in terms of coverage and cost. The position estimation accuracy of global positioning system is definitive. However, it fails to provide desirable results in metropolitan cities (with skyscrapers) and in indoor environments because of not having clear view to the sky, signal attenuation, multipath effect, etc.8,9 In order to fill this gap, various indoor positioning systems primarily based on RF such as wireless fidelity (WiFi), Bluetooth, radio frequency identification, and camera-based positioning systems have been proposed in the works of Haute et al10 and Mautz and Tilch.11 Moreover, RF-based positioning techniques suffer from aforementioned issues, and camera-based positioning has a higher cost. Recently, utilizing light for indoor localization has become an active area of research, and many algorithms for indoor positioning have been proposed.12-20 Exploiting light for estimating the location of an object is relatively inexpensive and is inheritably safe to use. The algorithms that are used for VLC-based positioning are predominantly based on proximity, fingerprinting, triangulation, and vision-based algorithms. In literature, there exist techniques that use combinations of aforementioned algorithms to further enhance the accuracy of positioning. It is convenient to express the positioning algorithms in a form of a tree diagram as depicted in Figure 1. Proximity is the simplest and least accurate positioning algorithm in which each LED is considered as a base station with a unique ID and has certain coverage area. On the basis of the ID received by the receiver, the position is estimated by matching the position with the preinstalled database. Although, the estimated position using proximity is poor, yet it has many realistic applications. These application include navigation system for visually impaired people, navigation in super markets, locating a child in a supermarket, asset tracking in a hospital, etc.12 In fingerprinting, the relative position is estimated by comparing the online stage reading with an offline database. Such data, for the case of light, can be constructed by observing the received signal strength (RSS).13 Fingerprinting methods can further be classified into deterministic and probabilistic approaches. In the former method, the signal is treated as a scaler value, and generally, pattern matching techniques such a k-nearest neighbors, corelation, etc, are employed to it.

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FIGURE 1 Visible light based positioning algorithms

In the latter method, the information is stored as a probability vector, and techniques like Bayesian estimation are applied for position determination. Geometric properties of a triangle can be used for estimating the absolute position, and this method is known as triangulation. Triangulation can further be classified into lateration (based on distance) and angulation (based on geometric relationship among nodes). In lateration, time of arrival (TOA), time difference of arrival (TDOA), and RSS are used for position estimation. In angulation, the angle of arrival (AOA) from various base stations is used for positioning calculation. Moreover, TOA, which is the basis of GPS, requires the transmitted signals to be very accurately synchronized. This condition is hard to achieve for a low-cost transmitter/receiver. An alternate to TOA is TDOA, which has a lenient prerequisite that an accurate distance must be known between a pair of transmitters. Other than this, there is another condition. For an indoor environment, the distance between the transmitter and the receiver is very short, and the speed of light is very fast. Thus, the time of flight is too short, and extremely precise time measurement is required. In addition, RSS is the simplest of all in which signal strength is measured, and this parameter is used for location estimation. However, transmitted optical power is subject to blocking, shadowing, dimming, etc, thus making it possibly less reliable.14 In addition, AOA is a promising technique for VLC-based positioning; however, it is not a good choice for RF because of its dominant multipath effect. In AOA, the direction of propagation of light wave falling on circular photodiode array is determined. One such work is done by Qing et al,15 in which localization utilizing angular properties is achieved with the help of single transmitter and single receiver. Problem with multiple transmitters is that it causes intercell interference, and transmission timing scheme is required for the signals. Issue with multiple receiver other than the cost is that there should be a minimum separation distance between optical receivers, which results in larger receiver size. However, both aforementioned issues can be resolved if a single transmitter and receiver is used. There, experimental results achieved an accuracy of less than 4 cm; however, the volume of the room in which the experiment was very limited (0.6 × 0.6 × 1.135 m3 ) when compared with other reported results. Another interesting work utilizing signal arrival angle was reported in the work of Zhu et al.16 Their approach overcomes the challenges faced typically by RSS-based localization schemes, which involves database creation and online processing in which the query reading is matched with the database readings. A novel positioning system based on angle difference of arrival is used in a 3D scenario with a relaxed

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condition that receiver can be placed arbitrary tilting angle. Their experimental results show that the least mean square, which has very less computational cost, can achieve an accuracy of around 15 cm, whereas the method of exhaustion has an average error of 3.2 cm with higher computational cost. Vision-based techniques require camera(s) as a receiver(s). These techniques involve the geometric relationships between the 3D positioning of the objects and their projections on the camera. Positioning based on vision analysis are classified on the basis of how the relationship between 3D real object and its 2D projection is handled. Apart from the aforementioned approaches, hybrid algorithms utilizing RSS-TDOA, RSS-clustering, RSS-regression, RSS-proximity, RSS-AOA, etc, have also been proposed.17 Types of receiver used in VLC can be categorized into the following three categories as shown in Table 1. Visible light–based positioning can be summarized in Figure 2.

2.2

Secure communication using visible light

Security and data integrity is a major challenge to present and future wireless communication networks. Various high layered security and data encryption protocols such as access control, password protection, etc, are extensively used in wireless networks, however; there still exists risk of data stealing. Visible light communication provides an inherent security as light cannot pass through opaque materials. However, if the intruder is in the same vicinity where the communication is taking place, the VLC signal is prone to eavesdropping. In this paper, we will limit our focus within the framework of physical layer security which utilizes the channel characteristics for hiding the information from the intruder. The first theoretical work in the field of VLC's physical layer security was done by Wasinee et al, in which the concept of position-based diversity was introduced.18 The intended signal is divided into I and Q channels that are transmitted separately. In order to obtain various secured areas within the room, time-division multiple access is used, and for each TABLE 1 Types of receivers Photodiode

Type of Receivers Image Sensor

Array

Applications

• TOA • TDOA • RSS

• Suitable for low data rate applications of up to 10’s of kbps

• Suitable for hybrid algorithms • Can provide data rate higher than a single PD

Limitations

• Cannot separate light sources with the same frequency. Need to use some multiplexing or multi-access

• Cannot work for high data rate applications • Expensive

• Not suitable for vision analysis. • Design is more complicated than a

system • Vulnerable to ambient light

single PD

Abbreviations: PD, photo diode; RSS, received signal strength; TDOA, time difference of arrival; TOA, time of arrival.

FIGURE 2 Summary the visible light–based positioning algorithms. CDM, code-division multiplexing; FDM, frequency-division multiplexing; LED, light-emitting diode; LOS, line of sight; NLOS, nonline of sight; ODFM, orthogonal frequency-division multiplexing

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timeslot, adaptive transmitted power is applied. The simulated system is tested under various conditions, ie, different order of Lambert emission and position of transmitters. The results show that when the value of Lambertian source radiation lobe is one, the probability for interception is reduced to 90%. The extension to this work was done by Saadi et al, in which a practical demonstration for a physical layer secured VLC was given.19 The main idea of their approach is shown in Figure 3. Other than bringing confidentiality in the data, data integrity was improved by incorporating proposed variant of low-density parity-check codes.20 By doing so, the authors were able to establish a secure and reliable communication link. The eavesdropping scenario was investigated in detail in the work of Mostafa and Lampe.21 A single transmitter with multiple LEDs, a photodiode, and one intruder forming a multiple input single output was considered. In this work, the secrecy capacity of the scalar Gaussian wiretap channel for the case of amplitude was taken into account, and the closed-form secrecy was derived. A robust beamforming technique is then applied to improve the secrecy rates. Eavesdropping characteristics of a VLC-based communication were investigated in the work of Marin-Garcia et al.22 The results show that sniffing action can be applied to the area that is not an expected area of coverage because of the signal leakage through the windows. In a nutshell, although the VLC signal is less prone to data stealing when compared with RF, yet there is a possibility of intruder and sniffing attacks on VLC signal within the coverage area as well as close to the coverage area.

2.3

Intelligent transportation system using visible light communication

According to global status report on road safety 2015, the total number of road accident deaths was around 1.25 million per year.23 Different governmental and nongovernmental organizations are making laws/awareness regarding road safety. Researchers are also putting efforts to bring intelligence to the transportation system by developing prototypes and products that can enable wireless connectivity in vehicular networking. Such solutions will not only help reducing the commuting time but also lessen the emission of hazardous gases. In addition, the feasibility of vehicle-to-vehicle communication will improve the safety situation, prevent intersection crashes, and prompt reporting in case of emergency. Dedicated short range communication is one of the proposed solutions for vehicular communication. This is an RF-based solution and operates at 5.9 GHz. Moreover, IEEE also published a standard IEEE 802.11 p for ITS (ITS) applications.22 In addition, RF-based solutions for intelligent transportation system have less positioning precision, undesirable packet collision for large number of nodes (ie, vehicles), RF channel contention, RF spectrum crunch, and RF open nature of propagation, which makes it less secure to intruder intervention.24 These reasons make RF-based vehicular networking less favorable.

FIGURE 3 Spatial based diversity transmission

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Moreover, VLC-based ITS is more reliable, less complex, and more cost effective. New cars are already equipped with LED-based head and back lights. Street and traffic lights are also LED enabled, which provides an in-built infrastructure to enable ITS. Apart from this, other advantages of VLC for ITS over RF-based solutions are improved positioning and less packet collision (as VLC is line of sight (LOS), thus limiting the participating nodes to neighborhood vehicles only and far-distance vehicles will not be contending nodes) and inherit short distance communication, which reduces the chances of intruder interception. Furthermore, VLC receivers (photodiodes) have improved reception (faster switching capability) than human eye in likely and harsh weather conditions.25 In the last decade, research in the domain of VLC-based ITS has gained good momentum, and many theoretical as well as experimental works have been done in the said field. Some experimental works have been conducted to check the feasibility of outdoor VLC for ITS. The results demonstrate an establishment of few kbps information broadcast link over a limited area (less than 50 meters).26,27 A theoretical work considering the measured headlight optical pattern and road reflection was studied, and results suggested that a 2-Mbps link can be established over a distance of 20 m.28 In another work by Luo et al, performance for car-to-car communication was analyzed while considering the effect of clean and dirty headlights in addition ambient noise. Results suggest that a successful car-to-car link can be established over a distance of 70 m providing a data rate of 50 Mbps. The received optical power for a wet road under non-LOS condition is higher than the dry road.29 These theoretical data rates and ranges are yet to be achieved in practice; thus, there still exists a huge room for research. The elements of ITS are vehicle-to-vehicle, vehicle-to-street light, and vehicle-to-traffic light communication, which are shown in Figure 4. In the system, the main distortion is caused by ambient-induced shot noise and thermal noise.30 It is very important to develop more realistic channel models for ITS. These models should take care of parameters such as optical beam patterns of head and back lights of vehicles, street and traffic lights, the effect of ambient, and primarily the effect of sun light, which is the major source of noise on outdoor VLC. Last but not the least, a realistic channel model must include the characteristics of road surface and its reflection as well as the effect of weather conditions.

2.4

Visible light communication for next generation networks

Highly anticipated 5G network promises higher data rates, better spectral efficiency, extremely low latency, low energy utilization, and higher capacity than its predecessors. Key to support high capacity lies with the femtocell size and availability of sufficient bandwidth, but the densely deployed femtocells will cause cochannel interference, cross-layer interference, and frequent handovers. To find more bandwidth, we need to explore other parts of electromagnetic spectrum that can fulfill the requirements of future generation networks. Recent studies have shown that, in the future, the wireless users will be spending most of their time (about 80%) in an indoor environment. Therefore, it is very essential to provide high-speed

FIGURE 4 Elements of intelligent transportation system. LOS, line of sight; NLOS, nonline of sight; VLC, visible light communication; V2V, vehicle to vehicle

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data access in an indoor environment.31 Talking in terms of cell size, an indoor environment typically refers to a small geographical area that harmonizes with the requirement to transfer high data rates. The future wireless networks are expected to be a heterogeneous network integration and cognition at network and device level. Moreover, mmWave (RF frequencies above 24 GHz) and VLC are considered to be a favorable complementary technologies for short-range communication for future generation networks. In addition, VLC has the potential to ease the already crowded radio spectrum in a highly localized system; VLC is considered to be the most eligible candidate to resolve the spectrum crunch issues.30-33 Here, a comparison is given in Table 2 about RF, VLC, and mmWave, which will justify the choice of VLC as a supplementary technology in 5G and beyond networks.34 In VLC, dead spots exist in all practical scenarios, which limits the full coverage, and stan-dalone VLC systems might not be very fruitful in every case. Light from a LED source is confined to a limited area and is susceptible to blockage. A hybrid integration of VLC and RF system is a ray of hope for enhanced user experience as RF can be used to provide ubiquitous coverage. The integrated network will be utilizing advantages from both technologies ie, VLC can provide a gigabit data rate but is subject to blockage and RF has ubiquitous coverage but provides moderate data rates. While the current RF networks can provide a good coverage to outdoor users (which includes fast moving users), VLC can provide a decent service to indoor users.32 The coverage scenario can be illustrated as in Figure 5. Current mobile devices are not equipped with such illuminating components that can be used as a transmitter for long time, considering the limited battery of the handheld device. Moreover, any possible uplink mechanism from the TABLE 2 Comparison chart among RF, VLC, and mmWave Attribute

RF

VLC

mmWave

Spectrum

Limited

Immense (385-790 THz)

Huge (3-300 GHz)

Spatial reuse

Low

High

High

Cochannel interference

High

Very low

Low

Data rates

High

Very high (in Gbps)

Very high (in Gbps)

LOS requirement

No

Yes

Yes

Licensing

Required

Free

Partially free

Cost

High

Low

Very high

Range

In Kms

< 10 m

Tens of meters

Human safety

Less

Yes (lenient eye safety)

Less

Infrastructure

Need to deploy

Almost already available

Need to deploy

Security

Low

High

Low

Propagation losses

Low

Very high

Very high

RF interference

Yes

No

Yes

Blockage

Low

Serious problem

High

Abbreviations: LOS, line of sight; RF, radio frequency; VLC, visible light communication.

FIGURE 5 Visible light communication and radio frequency (RF) transmitters with their footprints in an indoor environment. Weak or dead spots can be seen for visible light communication coverage

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mobile device might not be in LOS with the receiver at all times, thus deteriorating the throughput. However, a hybrid VLC and RF network can serve the purpose. Such a study has been conducted in the work of Shao et al,32 in which the down streaming was done through VLC link, and uplink was done through conventional WiFi (such networks can be called as LiWiFi, ie, light wireless fidelity).35 Results show that in a multiuser environment, the proposed mechanism outperformed the conventional WiFi in terms of throughput. Furthermore, results from other studies suggest that VLC systems supplementing WiFi increases the capacity and overall system throughput.36 A proposed hybrid VLC and RF system can be illustrated as in Figure 6.

2.5 Miscellaneous applications of visible light communication 2.5.1 In-flight entertainment using visible light In-flight entertainment is becoming more like a requirement for airplane manufacturing companies. Offering in-flight data access is now considered as an attraction for the passengers. There are several proposals and solutions for providing high internet access for in-flight passengers. Some companies are offering WiFi-based solutions for in-flight data access; however, the WiFi spectrum may interfere with the aeronautical navigation system.37 Another solution is the ground-to-plane data link via airband radio or satellite (connection between aircraft and internet service provider). Optical wireless communication finds its room in this area as well because of some advantages such as reduction of overall weight induced by wires, no RF interference, and availability of reading lamp above the passenger seat making LOS with the passenger sitting area. Kavehrad et al theoretically investigated the use of power lines and LED-enabled reading lamp in the airplane to realize data network; however, it lacks the presence of uplink channel to become a real network alternative.38 Jimenez et al explored VLC-based data networking for passengers in which light was used for downlink and infrared (IR) was used for uplink. A successful communication link was established over a distance of 3 m with a field of view of 30◦ .37 Quintana et al developed a low-cost portable passenger data adapter containing IR transmitter and a photodiode to send and receive signals from a modified LED lamp containing an IR receiver,39 thus overcoming the shortcomings of Kavehrad et al.38 Krichene et al proposed an aeronautical network architecture that was based on VLC using two different wavelength assignment methods. They also incorporated multiaccess system and all optical scheme based on free space optics.40 A generalized architecture for in-flight data access is shown in Figure 7.

2.5.2

Power line communication with visible light communication

Power line communication (PLC) enables to use the power lines for the purpose of data transfer without affecting the electrical system performance. Fortunately, in houses and offices, electrical wires are already installed. These wires can act as data networks and the electrical outlets as ports. All devices that are connected with the power lines can communicate through these lines; however, battery-driven devices such as mobile phones need to have some wireless connectivity mechanism. Komine et al proposed the first PLC-based VLC system and analyzed its basic performance.41 Later, the same group improved their system and proposed a narrow band orthogonal frequency-division multiplexing (OFDM) on integrated system of PLC and VLC. Their results show that the signal of integrated OFDM system does not affect function

FIGURE 6 A hybrid radio frequency (RF) and Visible light communication–based system for future networks

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FIGURE 7 Architecture for in-flight data access. IR, infrared; FSO, free space optics; VLC, visible light communication

FIGURE 8 Power line communication (PLC)–based visible light communication (VLC). DC, direct current; LED, light-emitting diode

of lighting. Integrated OFDM gets enough SNR inside a large room for establishing a datalink of more than 200 kbps. Their proposed architecture is shown in Figure 8.

2.5.3

Underwater communication with visible light communication

Underwater communication is very different from terrestrial communication since RF wave attenuation is very high, especially for sea and ocean water, because of its salty nature. Therefore, RF cannot be a good choice for underwater communication. On the other hand, acoustic systems can work from few meters to kilometers in range, but they suffer from extremely low bit rate (in kbps) and high latency. When the number of nodes in an acoustic network increases, the latency is further augmented because of the possible collision and retransmission of the packet(s). With the rise in human underwater activities, it is becoming crucial to develop some high-speed data communication network for underwater. Communication via light is a feasible option as light can travel in water reasonably well. In a laboratory environment, a data rate of 1 Gbps was achieved using laser as a transmitting source; however, data rates achieved through LEDs are comparatively low.42 Studies have shown that a data rate of few Mbps can be achieved using light emitted from commercially available LEDs.43,44 An experimental demonstration was given in the work of Cossu et al,44 where a successful error-free communication link of 6.25 Mbps was established over a distance 2.5 m utilizing a low-cost circuit of blue LED array and avalanche photodiode. A long-distance underwater VLC system with a single photon avalanche diode was demonstrated in the work of Wang et al.45 Simulation results for blue/green light band of 430 ∼ 550 nm transmitters show that a communication link can be established for a distance of 500 m with a data rate of 50 Mbps in pure sea water. The reason for limited range of underwater VLC is absorption, scattering, and turbulence, which adversely affect the photons' propagation underwater, thus causing fading and intersymbol interference (ISI) on received optical strength.46 A general architecture for their work is shown in Figure 9. Such experiments guarantee that VLC can be effectively used for underwater communication.

2.5.4

Entertainment visible light communication

Researchers from Disney Research have materialized the idea that a toy or a network of toys can be connected to smart phones via visible light. Nowadays, toys are equipped with LEDs and smartphones are enabled with camera(s) and

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FIGURE 9 Experiment setup for underwater communication. ADP, area-delay product; DC, direct current; LED, light-emitting diode; VLC, visible light communication

flash light(s), and these are the components that can be used to establish a VLC link. Although there exist methods for communicating toys over a short distance with the help of license-free radio spectrum or through IR, but VLC, which is a radio-free communication, makes it an attractive and safe option. Usually, the data rate requirements for toy-to-toy communication are extremely modest, so a low-cost microcontroller can serve the purpose. Disney Research has classified their system design into three modes, ie, LED-to-LED communication, smartphone-to-LED communication, and LED-to-camera mode. On the basis of these three classifications, they have developed various entertaining products.47 These products include communication network for toys, toys without radio emission, toy communication with smartphones, and interactive fashion and fabrics.48 Thus, it is safe to state that VLC has a very high demand, yet it is a less explored horizon. In the domain of entertainment and toy-to-toy communication, the aforementioned concept can be expanded to inexhaustible applications, including Internet of Things.

2.5.5 Visible light communication in electromagnetic interference sensitive environments There are many practical scenarios where RF communication is not preferred or prohibited. For instance, it is prohibited to use RF-based services in intensive care units, during takeoff and landing of an aircraft, and in intrinsically safe environments such as petrochemical industries and coal mines. Medical devices in a critical care setup are found to be affected by high levels of RF energies. It has been found that the operation of heart pacemaker is influenced if an operative handheld device is directly placed over it. Furthermore, there have been studies showing that the interference is caused between cellphones and hearing aids.49,50 Passengers are advised to turn off electronic equipment before takeoff and landing to avoid possible electromagnetic interference between electronic device and airplane navigation and communication system. Moreover, RF-prohibited areas such as underground mines lack sophisticated communication mechanism. Presence of inflammable gases in coal mines and petrochemical industries like methane and ethylene may cause ignition or heating when exposed to higher level of RF.51 In addition, VLC can bridge the gap in this area as well; intensive care units are equipped with ceiling lights that are always on, aircrafts are equipped with LED lamps, and intrinsic safe environments also have lighting system. Thus, a VLC-based communication system can be deployed on already existing infrastructure to materialize decent and safe communication mechanism.

3 TECHNIQUES FOR DATA RATE IMPROVEMENT IN VISIBLE LIGHT CO MMUNICAT ION Light-emitting diodes offer various advantages over conventional lighting sources; however, phosphor-based white LED modulation has low intrinsic bandwidth, which limits the available data rate to few MHz. Although, there exist methods for constructing LEDs without yellow phosphor such as devices that use separate RGB emitters or ultraviolet emitters in conjunction with RGB phosphor, yet the first method is preferred over the rest because of its higher efficiency, lower

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FIGURE 10 Data rate improvement methods for visible light communication

complexity, and cost effectiveness. There are various methods in order to improve the bandwidth and the data rate of a VLC-based system, which can be viewed in Figure 10, and the detail of each technique is discussed in the subsequent section.

3.1

Data rate improvements through filtering

The limited modulation bandwidth of white LED can be increased from 3 MHz to 20 MHz if only the blue part of emitted spectrum is detected. This can be easily achieved by blue filtering but at the cost of reduced signal strength at the receiver. The filter can be placed at the receiver or at the transmitter; however, placing it at the receiver is preferred for obvious reasons. The blue filtering method is generally used in conjunction with a modulation technique or with an equalizer or both. The first experimental use of blue filter was done by Grubor et al,52 in which a 40-Mbps link was established using blue filtering along with on-off keying (OOK). The data rate was further improved to 100 Mbps by utilizing multilevel modulation.

3.2

Data rate improvements through equalization

Another widely used method for data improvement is through equalization. Zeng et al employed equalization, which not only helps improving the data rates but also establishes a highly reliable communication channel.53 Their results show that the data transmission rate can be doubled for equalized channel when compared with an unequalized channel utilizing nonreturn-to-zero (NRZ)–OOK, and the reported bit error rate (BER) was 10−6 . Another interesting work was done by Minh et al, in which a pre-equalization (first-order circuit) technique was used to increase the modulation bandwidth

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of an LED.54 The results show that the equalized bandwidth of 45 MHz for a single blue LED was achieved, and data link utilizing NRZ-OOK achieved a data rate of 80 Mbps. The same authors extended the concept of multiple resonant equalization for a single LED to a group of resonantly modulated LEDs.55 By creating a resonant electrical response of LED with the external components, each LED was made to produce a different peak output frequency. By intelligently selecting the frequencies, an overall broader bandwidth channel was created. In their experiment, they used 16 LEDs, which were modulated by using a resonant driving circuit generating a bandwidth of 25 MHz, and a 40-Mbps NRZ-OOK link was established with acceptable BER levels. A 100-Mbps NRZ-OOK VLC link was established by means of a postequalized white LED. A first-order (RC) equalizer circuit was used at the receiver, which achieves a bandwidth of 50 MHz and data link operating at 100 MHz.56 When LED is used both for illumination and communication, the system has much higher power levels when compared to IR communication link because the same transmitters are to be used for lighting as well. Moreover, BER performance for an indoor VLC system can be significantly affected by ISI. Unlike IR, the impulse response of VLC system is dynamic and unknown to the receiver because of user mobility. To address the aforementioned issue, an adaptive equalization system was proposed and evaluated in the work of Komine et al.57 The performance of an adaptive equalizer with least mean square algorithm yields a data rate of more than 200 Mbps. Decision feedback equalizer, which predicts and eliminates the effect of ISI in future symbols by first detecting the information symbol, is found to be very effective for the links, and the achieved data rate is more than 700 Mbps. The use of equalizer with active circuit component along with the blue filter was introduced by Li et al, which enables to achieve a bandwidth of 151 Mbps and allows NRZ-OOK transmission at a data rate of 340 Mbps.58 In this work, the postequalization was not done in a single stage but in a cascaded passive, active, and passive equalizer stages. Passive equalizer consists of a combination of resistors, and a capacitor and active equalizer consists of an amplifier with the combination of resistors and capacitors. A 4.5-Gbps RGB-LED–based wavelength division multiplexing VLC system employing carrier-less amplitude and phase modulation and recursive least square–based adaptive equalization were demonstrated experimentally in the work of Wang et al.59 An aggregate data rate of 6.36 Gbps of RGB 2 × 2 multiple-input multiple-output (MIMO) VLC system is demonstrated, and to the best of our knowledge, this is the highest ever achieved data rate using RGB LED.60 We can summarize the discussion on data rate improvement utilizing equalization with the help of Figure 11. The green dotted line highlights pre-equalizer and postequalizer blocks. The blue dotted line blocks are optional, which can be used to further enhance the performance of the system.

3.3

Data rate improvements through modulation

As mentioned previously, the limited bandwidth of LED is well taken care off by using equalization and blue filtering techniques. However, there exist issues with the flickering and dimming of LED in order to achieve higher data rates. Flickering refers to the change in brightness level, and dimming control is required to accommodate different levels of light intensities. A study on flickering reveals that flickering of light can cause grave detrimental physiological disorders in humans.61 Moreover, IEEE 802.15.7 standard also suggests that the flickering should be more than 200 Hz to avoid human eye inconvenience and long runs of 0's and/or 1's can be avoided with the use of suitable light coding schemes. Dimming control issue arises as human eye perception against various levels of illumination is not linear. The perceived light by human eye and the actual measured light can be expressed by Equation (1) as follows62 : √ Lightmeasured (%) Lightpreceived (%) = 100 × . (1) 100

FIGURE 11 Data rate improvement through equalization

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Therefore, it is important to modulate light in such a manner that any application/need oriented dimming level is suited for communication as well. IEEE 802.15.7 standard also comprehends a high data rate communication link with the contemplation of flickering and dimming.63 The major difference between VLC and RF is that VLC data cannot be encoded with the amplitude and phase of light because for an optical wireless communication link, it is hard to gather significant power in a single electromagnetic mode. Thus, amplitude and phase modulation schemes cannot be used in VLC. For such links, the preferred method is to modulate the message signal onto the instantaneous power of the carrier (known as intensity modulation), and at the receiver, a photodetector can be used to generate current on the basis of the instantaneous received power (direct detection). In VLC with intensity modulated direct detection, the wavelength of light is expressively lesser than the effective area of the receiver (which, in this case, is a photodetector), and there is no fast fading because of effective spatial diversity. However, slow fading appears in the form of path loss and log-normal shadowing; OOK, pulse modulation (PM), OFDM, and color-shift keying (CSK) are the popular modulation schemes that are used in modulation, and details of each modulation scheme are given below.

3.3.1

On-off keying

The easiest type of intensity modulated direct detection scheme is OOK, in which light is made to turn on and off. Turning on the light corresponds to 1 and turning off the light corresponds to 0, and this process is done at such a speed that is unnoticeable to human eye. Most of the early work done to achieve higher data rates utilizing VLC were through OOK in conjunction with appropriate line coding scheme like NRZ. The prominent work done in the field of VLC employing OOK is tabulated in Table 3.

3.3.2

Pulse modulation methods

In addition, OOK is indeed a widely used modulation scheme for VLC, which can overcome the shortcomings of flickering and dimming; however, the data rates achieved by OOK are limited to few hundred MHz. This limitation pushes the researchers to employ some more sophisticated modulation schemes. Pulse modulation is also a popular modulation scheme that is used both in RF and VLC, and it has many variants as well. Pulse width modulation, which is a subclass of PM, achieves modulation by varying the width of the pulses in response to the required level of dimming. A vital advantage of pulse width modulation is that any dimming level from 0 to 100% can easily be achieved without changing the intensity levels of the pulse.67 Other variants of PM are pulse position modulation (PPM), overlapping PPM, differential PPM, expurgated PPM, variable PPM, and pulse amplitude modulation (PAM). Table 4 gives a brief overview and comparison of these modulation schemes. TABLE 3 Visible light communication employing OOK Work

Data Rate

Minh et al56

100 Mbps

Blue component from the white LED was detected. With the help of first-order analogue equalizer and NRZ-OOK, a data rate of 100 Mb/s was achieved.

Methodology and Contribution(s)

Vuˇcic´ et al64

230 Mbps

The data message is amplified to increase the modulation depth and superimposed to LED bias current with the help of bias tee, which is then fed to LED module. The signal was detected by avalanche and pin photodiode, and the achieved data rates were 230 Mbps and 125 Mbps, respectively. No equalization or blue filtering was used.

Fujimoto and Mochizuki65

477 Mbps

Designed LED driver, which can supply a large and high speed current to LED; NRZ data is fed to CML circuit followed by an emitter follower transistor and pre-emphasis circuit. The error-free data rate of 477 Mbps was reported in this study.

Fujimoto and Mochizuki66

613 Mbps

Line coded NRZ-OOK data pre-emphasized and duo-binary transmission technique was applied to break the bottleneck of LED operating speed. At the receiver, duo-binary decoding and post equalization are done. The achieved data rate for the study was 613 Mbps, and to the best of our knowledge, this is the highest ever data rate achieved using OOK modulation.

Abbreviations: CML, current mode logic; LED, light-emitting diode; NRZ, nonreturn to zero; OOK, on-off keying.

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TABLE 4 Variants of pulse modulation scheme Scheme

Principle of Operation

Advantages/Limitations

PPM68

Symbol duration is divided into slots of equal duration and one pulse is transmitted in each slot.

Low spectral efficiency.

OPPM69

More than one pulse is to be transmitted during a symbol duration.

Higher spectral efficiency than OOK and PPM.

MPPM70

More than one pulse transmission during a symbol duration, however, pulses within the symbol need not to be continuous.

Higher spectral efficiency than OOK, PPM and OPPM.

DPPM71

Same as that of PPM but off time (zero) is deleted and is replaced by next symbol.

Significant less power consumption than PPM.

EPPM72

MPPM is abridged to maximum inter-symbol distance.

Very useful to mitigate the effect of flickering and dimming.

VPPM63

Bits are encoded by choosing different positions of pulse but width of the pulse can be varied.

Easy to get different dimming levels.

SPAM73

Different light intensities are transmitted simultaneously and with the help of single receiver, the transmitted signal can be recovered.

low cost, insensitive to nonlinearity of LED

Abbreviations: DPPM, differential pulse position modulation; EPPM, expurgated pulse position modulation; LED, light-emitting diode; MPPM, multipulse position modulation; OPPM, overlapping pulse position modulation; OOK, on-off keying; PPM, pulse position modulation; SPAM, superposed pulse position modulation; VPPM, variable pulse position modulation.

3.3.3

Orthogonal frequency-division multiplexing

Orthogonal frequency-division multiplexing, which is widely used in RF communication, is also the most popular modulation technique in VLC order to achieve higher data rates. Morever, OFDM is well known for its exceptional response against multipath effect and ISI; OFDM symbols are generated by taking the inverse fast Fourier transform of a block of symbols from a modulation scheme; however, the resulting signals are complex-valued bipolar time domain samples that are unfit to be used for intensity modulation. A real bipolar signal can be realized by using Hermintian symbols, and to obtain a unipolar signal, two methods are commonly used, ie, direct current biased optical OFDM and asymmetrically clipped optical OFDM. In the former method, a bias value is added to all samples resulting in unipolar samples. However, the disadvantages of this technique is that the power consumption is increased.74 In the latter method, symmetric time domain signal is achieved by exploiting the properties of fast Fourier transform and OFDM frame structure, and only odd subcarries are modulated. The advantage of this technique is that no biasing is required.75 A comparison between direct current biased optical OFDM and asymmetrically clipped optical OFDM is given in the work of Dissanayake and Armstrong.76 The feasibility of OFDM for VLC was first investigated by Tanaka et al77 Plural lights were used, and optical path difference was investigated from the transmitter to the receiver. Computer simulations show that optical path difference was reduced by employing OFDM; thus, the attribute of inherit robustness of OFDM against multipath effect was validated. The first experiment work was done by Afgani et al, and they was showed that high crest factor that plagues OFDM RF equipment is no longer a disadvantage in VLC; however, their proposed system was not optimized.78 The improvement proposed by Afgani et al78 were addressed in the work of Elgala et al,79 and BER was improved from 2 × 10−3 to 2 × 10−5 . An experimental work on optical MIMO-OFDM system was proposed in the work of Azhar et al80 with imaging receiver. Equalized four-channel MIMO LED links were used to create a gigabit with a coverage area of 25 cm2 .81 Communication capabilities of Gallium Nitride micro LEDs were used along with OFDM and equalization to achieve a data rate of 3 Gb/s. To the best of our knowledge, this is the highest possible achieved data rate using a single LED. The use of OFDM is not limited to LEDs but has also been used by LD as well. Moreover, LDs are considered as a very promising alternative to LEDs for better utilization of the visible light spectrum for communication. Chi et al used a Gallium nitride blue laser diode with 64-quadrature amplitude modulation (QAM) and 32-subcarrier OFDM communication at 9 Gbps over a 5-m free-space link.82 A 100-Gb/s MIMO visible laser light communication system employing vertical-cavity surface-emitting lasers with 16 QAM-OFDM modulating signals is experimentally demonstrated.83 With the help of low-noise amplifier and equalizer at the receiving site, an acceptable BER was achieved. A system of eight 16-QAM-OFDM channels over 5-m free-space links with a total data rate of 100 Gbps is successfully achieved.

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3.3.4

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Color-shift keying

The previously discussed modulation schemes were first designed and implemented on RF, and later, they were adopted for VLC and found to be constructive. Moreover, IEEE 802.15.7 standard proposed a new modulation scheme that was specifically designed for VLC, and it is gaining a lot of attention in the research community.84-87 As mentioned in the introduction section, there are two methods for the construction of white LED out of which one was of creating T-LEDs. In addition, CSK modulates the signal using the intensity of the three colors in T-LED source. Color-shift keying is similar to frequency shift keying, and in CSK, bit patterns are encoded to color. The standard divides the spectrum into seven color bands to support multiple LED color combinations for communication. The pronounced advantage of CSK is that all CSK combination lights have a constant power that guarantees that the average optical power from light sources is kept the same. Thus, CSK handles flickering issues in the best way. The dimming support in CSK is straightforward. Driving current of LED is varied, which results in varying brightness of resultant white light. Various studies have been conducted to optimize higher-order CSK constellation such that the luminary functions with a desired operating color, allowing designed constellations to meet industrial quality metrics of lighting.84,85 Singh et al proposed a novel CSK scheme in which a four-color system is used instead of three.86 The proposed scheme results show that the four-color system is superior to the three-color scheme in terms of electrical SNR by 4.4 db over an additive white Guassian noise channel. Furthermore, in terms of power efficiency and reliability, this scheme was better under various loss models. However, these multilevel CSK systems undergo LED nonlinearity effects. To overcome this shortcoming, Murata et al proposed a digital CSK with multicolor LED array. Moreover, CSK symbols are represented by digitally controlling the multicolor LEDs to avoid nonlinearities of LED. The results show that the same BER can be achieved as that of conventional CSK with an advantage that conventional CSK symbols are expressed with the total amount of each nonvarying on-off intensity of multiple LEDs.86

3.4

Data rate improvements through multiple input multiple output

As mentioned earlier, LEDs have limited bandwidth, and filtering, equalization, and modulation techniques can help in enhancing the data rate of the link. An attractive way to enable broadband link is to employ multiple LEDs (which coincides with a typical indoor environment light fixture distribution) with multiple receivers. Parallel transmission offers a linear capacity gain with the number of channels in an ideal cross-talk–free configuration. However, for all practical scenarios, mechanical alignment between transmitter and receiver is hard to achieve; MIMO relaxes this alignment condition. Furthermore, supporting multiple users simultaneously with higher data rates can cause multiple-user or multiaccess interference. In addition, MIMO, in this regard, can help not only to limit multiaccess interference but also to improve SNR. One of the pioneer work on MIMO VLC was done by Zeng et al,88 which rigorously analyzed the performance of imaging and nonimaging MIMO under various conditions (different receiver positions, imaging receiver diversity, different type of transmitters, and effect of misalignment). Simulation results show that a data rate of several hundred MHz can be achieved. An experimental work supporting gigabit/s indoor wireless transmission using MIMO-OFDM VLC was reported in the work of Azhar et al.81 The system consists of four MIMO links with each transmitter transmitting at 250 Mbps using OFDM. Utilizing nine-channel imaging diversity receiver, a BER of 10−3 was achieved at a range of 1 m. Precoding the multiuser data and then incorporating MIMO is also a promising method to achieve higher SNR with high data rates and acceptable BER of 10−6 .89 Spatial modulation, which enables spatial multiplexing gain without interchannel interference and synchronization, is a very useful tool for next generation networks. On such work, determining the performance of spatial modulation on the basis of multiple-input–single-output optical wireless communication system was reported by Wang et al.90 In the proposed system, spatial modulation and finite alphabets are employed, and source data were divided into two parts that are used for channel selection and symbol selection, respectively. The results show that there is a strong impact of input-dependent noise on system performance. Multiuser MIMO VLC using spatial multiplexing was proposed in the work of Lian and Brandt-Pearce,91 which was robust again shadowing, dimming, background radiation, and LED nonlinarites. Furthermore, it was showed that the computation burden of decentralized power allocation algorithms is less when compared with centralized power allocation algorithms. Recently, a fixed-scale pixelated MIMO VLC was proposed in the work of Han and Hranilovic.92 The transmitter consists of array of emitters with collimating lens, which enables space to angle mapping of data. Focusing the receiver at infinity, a sequence of time-varying images were captured

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by a high-speed camera. This system has an advantage that it does not require refocusing and range estimation in order to decode the subchannel in addition to achieve significant multiplexing gains.

4

CURRENT CHALLENGES AND FUTURE WORKS

Visible light communication is one of the promising technology for the future secured, efficient, and high-speed data communication networks. Presently, work in the domain of VLC is being carried out in various directions. Moreover, VLC transmitters are getting smart and are easily available with the projected LED lighting market to be more than $US 63.1 billion by 2020.93 Contrarywise, the annual anticipated global mobile data traffic will be 587 Exabyte by 2021, out of which 99% of the traffic will be from smart devices,94 which pushes researchers to include other parts of electromagnetic spectrum for the purpose of communication. Researchers have developed state-of-the-art prototypes for various VLC-based applications, and the commercialization of VLC products is in early stage. PureLiFi95 and Fraunhofer-Gesellschaft96 are the pioneers in commercializing VLC products. Other than that, the major stakeholders for commercializing VLC are Apple, ByteLight, Casio, Intel, LVX system, Philips, and Samsung (written in alphabetical order). In VLC, enhancing the data rate, improving the mobility and range, is one of the biggest challenge. This is severely dependent on the ability to reject the parasitic light in close proximity of VLC transmitter or receiver. In VLC communication links, it is generally assumed that the field of view (FOV) of both transmitter and receivers are aligned (LOS). Moreover, most LED sources have Lambertial beam distribution,97 which means that the intensity drops as the cosine of the incident angle. In most practical scenarios, movement/orientation of transmitter or receiver is common, resulting in sufficient reduction in received optical power. Furthermore, the data rate is also sufficiently reduced by shadowing events.98 Thus, it is necessary to design techniques that can guarantee high data rate and quick adaption to changes in received power due to FOV misalignment or shadowing. Higher data rate and capacity can be achieved by dense deployment of LEDs. However, for VLC, this small cell architecture brings challenge of handling the intercell interference. Especially, closely spaced LEDs in an indoor environment can cause severe interference to each other. The solution to this interference is to employ MIMO99,100 and joint transmission networks.101,102 Furthermore, this interference can also be mitigated by rearranging the LEDs such that their mutual interference is minimum.103 Most of the research in VLC network focuses on downlink traffic. The currently available LEDs on mobile devices cannot be used efficiently for uplink. Main reason is that constantly operating the LEDs will sufficiently consume power and it is also difficult to maintain FOV of both transmitter and receiver LEDs because of the constant movement of Rx (receiver). To handle these challenges, use of hybrid VLC techniques can be employed. These hybrid VLC techniques consist of RF32,104 or infrared105 systems to handle uplink traffic. The application of diverse technologies give rise to heterogeneous networks.106-108 This imposes supplementary practical challenges such as throughput asymmetry issues for transport layer, network management for multi homed clients etc. There is limited research done on asymmetric and heterogeneous networking. In order to build robust high speed heterogeneous network of RF and VLC, it is crucial to address these challenges.109

5

CO N C LU S I O N

Visible light communication is a green communication technology that has the potential to not only resolve the current challenges faced by the already congested RF spectrum but also become a supplementary candidate for next generation networks. In this paper, we have reported various applications and contributions of VLC. Since its inception, VLC finds numerous applications in a wide range of scientific, industrial, commercial, and entertainment fields. Moreover, RF-based technologies fail to provide accurate indoor positioning because of signal attenuation, multipath effect, etc; however, VLC can be used for precise location estimation. Various state-of-the-art visible light positioning algorithms are discussed along with their pros and cons. After that, security and data integrity in current and future wireless communication systems have been debated, and the techniques related to physical layer security, utilizing channel characteristics for hiding the information, are discussed. Next, the need of intelligent transportation system is discussed, along with the possible network architecture for ITS engaging VLC, which can deliver an active information about traffic updates and dynamic danger

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targets and facilitates autonomous driving. We have further compared various attributes of RF, VLC, and mmWave and argued why VLC be a part of 5G and beyond networks. Apart from these major applications, we have also discussed various other applications such as in-flight entertainment with the PLC communication in conjunction with VLC network, underwater VLC network, and VLC for entertainment purposes. Another very critical and much needed application of VLC is its availability in RF-sensitive and intrinsically safe environments. Some of the aforementioned applications are data rate critical, ie, higher data rates are required such as VLC integration to 5G and beyond. Therefore, in this paper, we have highlighted reasons for limited data rates for LED-based communication and the strategies that can be opted to improve the data rate of VLC-based network. These strategies include blue filtering, equalization, and modulation, and through comparison, it is shown that OFDM and CSK will be the future modulation schemes for VLC. Appropriate pre-equalization/postequalization techniques will guarantee gigabit data rate and mitigate the effect of flickering, dimming, and ambient noise, which are the major challenges in VLC. In a nutshell, it is safe to state that VLC is no more a theoretical technology; rather, it has been standardized by international bodies (IEEE, JEITA). Various prototypes have been developed utilizing this technology, and it is surging toward commercialization.

ORCID Muhamamd Saadi

http://orcid.org/0000-0001-7901-7435

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How to cite this article: Saadi M, Ahmad T, Kamran Saleem M, Wuttisittikulkij L. Visible light communication – An architectural perspective on the applications and data rate improvement strategies. Trans Emerging Tel Tech. 2018;e3436. https://doi.org/10.1002/ett.3436