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Introduction

1.1 Motivation

Optical wireless communications (OWC) has become an increasingly important research area. The potential of solving complicated communications problems, such as the shortage of radio frequency (RF) spectrum, interference, and the necessity of transmission at very high data rates by optical wireless systems has seen vast improvement. Optical wireless links can establish communications channels even millions of miles apart, as evidenced by the use of optical links in space exploratory missions by NASA [1]. For shorter terrestrial distances, optical wireless links in outdoor free space are a good choice for establishing pointed links a couple of miles apart. On a much smaller scale, the existence of millions of remote controls that operate using infrared light-emitting-diodes (LEDs) is a proof of the usefulness of optical wireless systems.

Apart from the various applications of OWC that are currently in use, probably the main motivating factor to focus on this area is the possibility of mitigating the increasing spectrum shortage issue. As consumption of high data rate multimedia materials is increasing day by day and the use of handheld devices is becoming more and more widespread, the precious RF spectrum range of about 1.9 GHz that is used for mobility is getting scarcer [2]. Users are encouraged to shift to the Wi-Fi bands instead of the bands used for cellular services in order to alleviate this increasing load of high data rate applications. However, there are places where even Wi-Fi bands do not operate as expected or are found to be so congested that their use becomes next to impossible, for example, heavily crowded conference halls. Also, supported data rates of Wi-Fi as well as cellular data services should be considered in this discussion. Though IEEE 802.11ac and IEEE 802.11ad standards are supposed to support high bit rates, they are not yet widespread, and so the cost issue is involved. LTE and LTE-Advanced standards are also supposed to support high bit rates, but they use the same precious cellular spectrum band and thus due to congestion cannot provide satisfactory performance. Hence, the pursuit of and research on alternatives to these radio frequency-based solutions such as optical wireless-based systems and technologies are greatly desirable [3, 4].

OWC can be both indoors and outdoors and are usually broadly divided into two categories based on the type of optical source employed. Two types of optical sources—LEDs and lasers—are currently in use as transmitters of optical links. The difference between these two sources lies in their supported bandwidth: where LEDs have a much lower electrical bandwidth than lasers, and hence if very high data rate transmission in the range of Gbps is required, lasers are the popular choice. Also, lasers emit monochromatic light signals, that is, light signals that have only one wavelength in it, whereas LEDs have a very broad spectral linewidth. LED-based communications mainly involve visible light communications (VLC) using white LEDs (WLEDs), and lasers are used only as very high-speed infrared sources. Hence, these two types of optical sources have different application scenarios. In this book, we will cover different types of applications where both LEDs and lasers are used.

The energy-saving aspect of WLEDs is probably one of the most important benefits that can be obtained using VLC. Lighting is a major source of electric energy consumption. It is estimated that one-third of the global consumption of electricity is spent for lighting purposes; therefore, development of more efficient lighting sources is important. This acknowledgment of concerns about significant consumption has generated significant activity toward the development of solid-state sources, to replace incandescent and fluorescent lights. Fluorescent lamps contain environmental pollutants, thus their elimination will remove a significant source of environmental pollution and more specifically, their replacement with highly efficient LEDs generating white light will reduce energy consumption. It is fortunate that WLEDs are already commercially available. WLEDs require roughly 20 times less power compared to conventional light sources, even 5 times less power compared to fluorescent bulbs that consume less energy. An entire rural village can be lit with less energy than that used by a single conventional 100 W light bulb. Switching to solid-state lighting would reduce global electricity use by 50% and reduce power consumption by 760 GW in the United States alone over a 20-year period. To get a clear picture of the positive impact the use of WLEDs will have, some concrete estimates can be provided. If all existing bulbs were replaced by WLED sources, within 10 years we will have the following benefits: energy savings of 1.9 × 10²⁰ J, US$1.83 trillion financial savings, 10.68 GT reduction of carbon dioxide emissions, and 962 million barrels less consumption of crude oil [4].

The field of photonics starts with the efficient generation of light. The generation of efficient yet highly controllable light can indeed be accomplished using LEDs. Using a WLED instead of conventional lighting means the size, cost, and energy consumption will decrease considerably, as optical devices are smaller and simpler than electrical devices. WLEDs are semiconductor devices. About 13 000 LEDs can be formed on a substrate, which can be about 0.25 × 0.25 units in size. WLEDs use 5% of the energy of a regular incandescent bulb. An entire rural village can be lit with less energy than that used by a single conventional 100 W light bulb. By replacing the conventional lights with WLEDs and by using them for both data transmission and lighting, large amounts of energy can be saved. Undoubtedly, white light emitting solid-state devices will be the lighting sources of the twenty-first century. About 10–15 years ago, researchers came to the realization that WLED devices, in addition to being very fit for lighting the surrounding space, could also be used for wireless communications purposes. The advantages of such technology applications are many. It belongs to the green technologies category when used for lighting purposes, becoming even more environmentally friendly when it supports communication functionality compared to RF alternatives. Also, LEDs and photodetectors tend to be considerably cheaper compared to RF counterparts. OWC allows easy bandwidth reuse and improves security, as light is confined within the room it illuminates. It does not generate RF contamination, nor is it impacted by RF interference. Thus, replacing RF devices with devices using white light for communications (at least for indoor environments) will reduce interference in the RF bands. It should be pointed out that while the consumer market and the product developers will benefit from this, the technology can also make a major breakthrough in cases where RF radiation is of great concern, as in the case of hospitals, schools, airplanes, and mines. RF interference has caused accidental triggering of explosions when using remote detonator devices. Federal regulation places 1 W as the maximum acceptable RF power within mines using remotely triggered detonators. Also, baby monitoring RF signals have interfered with landing instructions of planes approaching airport runways.

1.1.1 Spectrum Scarcity Issues and Optical Wireless Communications as a Solution

Let us delve a bit deeper into the RF spectrum scarcity problem that we mentioned earlier and how OWC using either LEDs or lasers can help in this regard.

With the increasing popularity of multimedia services supplied over the RF networks and services such as web browsing, audio and video on demand, it is for sure only a matter of time before users will face extreme congestion while trying to connect to avail themselves of these aforementioned services. Advancements in displays, battery technology, and processing power have made it possible for users to afford and carry around smart phones and tablets. As we are entering a new era of always on connectivity, the expectation from users for not only ubiquitous but also seamless voice and video services presents a significant challenge for today’s telecommunications systems. The prospects for the delivery of such multimedia services to these users are crucially dependent on the development of low-cost physical layer delivery mechanisms.

According to market research published by Cisco Systems, Inc. [5], the largest manufacturer of networking equipment, mobile data consumption is going to explode in the next 5 years, largely due to the proliferation of mobile video and mobile web applications. Cisco market research includes the Visual Networking Index (VNI). The VNI research predicts mobile data use to expand from 2.5 to 24.3 EB monthly. This is an increase of a factor of 10 in 5 years, or about 57% cumulative annual growth rate (CAGR). This is an enormous growth in mobile data, a very large portion of which is growth due to the proliferation of mobile video (66%). Much of this mobile data growth (about 70%) will be consumed by laptops and other mobile ready portables such as pico-projectors, wireless reading devices, digital photo frames, and smart phones. These mobile devices can generally be thought of as in-building networked devices that are used to share information (video) within a classroom, conference, or meeting room. The report predicts that a greater amount of traffic will migrate from fixed to mobile networks.

In the past few years, we have witnessed rapid growth in technologies producing low-cost communications devices, using the RF license-free bands: ISM (2.4–2.4835 GHz), UNII (5.15–5.25 and 5.35–5.825 GHz). As technology advances, the service capability of such devices will strengthen. However, uncontrolled deployment of devices using the same spectrum allocation can generate interference beyond the level that these systems can afford, thus leading to service quality deterioration. The IEEE 802.15.2 working group was formed to address this growing problem; however, without controlling the number of devices operating within certain areas, the problem cannot be solved, unless more bandwidth becomes available. The 57–64 GHz band has been added to license free bands; however, the design of communication systems at these extremely high frequencies is very challenging. It will take some years for products of reasonable cost and satisfactory performance to be introduced in the market. Also, adding bandwidth does not address the problem at its root. What is needed is a broadband, interference-free, or at least interference-resistant technology, allowing easy frequency reuse made available to the customer at an affordable cost [4]. Considering the rapidly growing wireless consumer devices, it is evident that the need for such technology is quite urgent.

The wireless handheld devices require ever-increasing bandwidth, and along with that, explosive growth in interdevice wireless communications is already creating huge demands on spectrum resources, which can be resolved only by near-zero-sum allocation decisions, made through a mixture of bidding and politics.

In economy, the game theoretic Nash equilibrium (named after John Forbes Nash, who proposed it) [6] is a solution concept of a game involving two or more players, in which each player is assumed to know the equilibrium strategies of the other players, and no player has anything to gain by changing only his own strategy unilaterally. If each player has chosen a strategy and no player can benefit by changing his or her strategy while the other players keep theirs unchanged, then the current set of strategy choices and the corresponding payoffs constitute Nash equilibrium. The practical and general implication is that when players also act in the interests of the group, then they are better off than if they acted in their individual interests alone.

Unfortunately, with spectrum usage, Nash equilibrium may result in a spectrum crunch [2], if the participants do not cooperate. An example of this was the Cellular Digital Packet Data (CDPD). This was a wide-area mobile data service, which used unused bandwidth normally used by AMPS mobile phones between 800 and 900 MHz to transfer data. Speeds up to 19.2 Kbps were possible. The service was discontinued in conjunction with the retirement of the parent AMPS service; it has been functionally replaced by faster services such as 1xRTT, EV-DO, and UMTS/HSPA. Developed in the early 1990s, CDPD was large on the horizon as a future technology. However, it had difficulty competing against existing slower but less-expensive Mobitex and DataTac systems, and never quite gained widespread acceptance before newer, faster standards such as GPRS became dominant. CDPD had very limited consumer offerings. Though AT&T Wireless first offered the technology in the United States under the PocketNet brand, they eventually refused to activate the devices. Despite its limited success as a consumer offering, CDPD was adopted in a number of enterprises and government networks. It was particularly popular as a first-generation wireless data solution for telemetry devices (machine-to-machine communications) and for public safety mobile data terminals. In 2004, major carriers in the United States announced plans to shut down CDPD service. In July 2005, the AT&T Wireless and Cingular Wireless CDPD networks were shut down. Equipment for this service now has little to no residual value [7].

Another example of co-existence with already existing services over radio spectrum (a form of bandwidth sharing) is the idea of ultra wideband (UWB) [8] that proposed to use direct-sequence spread spectrum sharing bands over 7 GHz of already allocated radio spectrum. This technology did not go too far either, although a huge amount of resources was spent on demonstrating the feasibility of the technology through research and development. The developed technologies work perfectly according to the specifications, but there is no public acceptance in adopting these techniques.

Some views on bandwidth sharing, be it through cognitive radios or dynamic spectrum allocation (DSA) [9], are given here. The wireless/mobile environment is very dynamic. To capture when a piece of spectrum is free and available (known as white space) in order to reallocate it, many accurate energy sensors have to be installed to identify these available bands. Then a command has to be sent to a cloud (database) at a distance in order to make the availability of the idle bands known to users in order to reallocate these available bands. This is a very difficult and expensive proposition in a densely populated metropolitan area where bandwidth sharing is needed the most. There might be several available portions of bands idle in rural areas; however, bandwidth and channel borrowing concepts only work over short distances. In dense metropolitan areas, by the time sensing is done and a reallocation decision is reached, the spectrum availability status may be