IEEE Standard 802.15.7: Short – Range Wireless Optical Communication Using Visible Light – A Brief Overview

As an IEEE standard, 802.15.7 is a voluntary and open standard. It has been created in order to fulfill the increasing societal needs for communication that today's standards will soon not be adequately to serve. It also has direct influence on the market by creating for instance new applications and thus the necessary hardware or by broadening and upgrading already existing ones like LED market which should make available LEDs for wireless data transmission. As VLC are located in a free, unlicensed band they suffer minimum political interventions unlike mobile communications for example which are operating in licensed bands (800 or 900MHz, 1.9GHz, etc) and they are subject to political decisions regarding that part of the spectrum. On the other hand precautions are taken for health hazards by restricting the maximum power emission. The 802.15.7 standard has been technically kept as simple as possible and also based on a structure which allows that while some other widely used and mature standards such as IEEE Std 802.15.4 – 2006, ITU – T I.432.1, ANSI/INCITS 373 are fundamental for its application.

Furthermore, some critical points that hold for all IEEE standards and IEEE itself are presented below as were taken from an IEEE standard and give some more insights to the nature of the standards:

• IEEE develops its standards through a consensus development process, approved by ANSI which brings together volunteers representing varied viewpoints and interests to achieve the final product

• Volunteers are not necessarily members of the Institute and serve without compensation

• Use of an IEEE standard is wholly voluntary

• The existence of an IEEE standard does not imply that there are no other ways to produce, test, measure, purchase, market or provide other goods and services to the scope of the IEEE standard

• The viewpoint expressed at the time a standard is approved and issued is subject to change brought about through developments in the state of the art and comments received from users of the standard

• Every IEEE standard is subjected to review at least every five years for revision or reaffirmation, or every ten years for stabilization

These points reveal exactly that IEEE is open to everyone and acceptable to new ideas for developing new standards. Its products (standards) are available for everyone and it is up to every individual to use them and in what way or not adopt them at all.

The 802.15.7 (2011) standard is the first IEEE standard that involves wireless optical communication technology using visible light. It defines both physical layer (PHY) and media access control layer (MAC). The architecture is based on defining several layers and sub – layers to simplify the standard and offer services and logical links from lower layers to higher ones. It takes into consideration many factors thus adding characteristics and functionalities like diming and visibility support, color function and color – stabilization support. The standard is designed for supporting multimedia data transfer and other services. The scope of the standard as it is stated in the document itself is as follows: 

“This standard defines a PHY and MAC layer for short-range optical wireless communications using visible light in optically transparent media. The visible light spectrum extends from 380 nm to 780 nm in wavelength. The standard is capable of delivering data rates sufficient to support audio and video multimedia services and also considers mobility of the visible link, compatibility with visible-light infrastructures, impairments due to noise and interference from sources like ambient light and a MAC layer that accommodates visible links. The standard adheres to applicable eye safety regulations.”

The 802.15.2 standard describes how visible light can be used for wireless personal networks (WPAN) while offering the illumination functionality at the same time. In a WPAN each device is given a short 16 – bit address or an extended 64 – bit address. Light emitting diodes (LED) and laser diodes (LD) can be used as light sources and find applications in several places like lighting, signboards, streetlights, vehicles, traffic signals.

In the standard three different topologies are defined. In figure 1 the peer – to – peer, star and broadcast are depicted. Moreover three classes of devices are defined, namely infrastructure, mobile and vehicle which are shown in the table of figure 2 along with some of their properties.

In a peer – to – peer topology two devices are communicating and one of the two handles the communication administration thus becoming the coordinator. In a star topology all devices have a bidirectional communication with the coordinator. The third topology is broadcast in which users only receive data from a transmitter. In the framework of these topologies three types of data transfer transactions are used. The first comprises the data sent from a device to the coordinator, the second the data send from the coordinator to the device and the third the data transferred between the devices in a peer – to – peer connection. In the latter all three types of data exist but in a star topology only the first two. 

Figure 1

Figure 2

 

The three classes of devices (fig.2) are infrastructure, mobile and vehicle. According to their physical properties and capabilities - limitations like physical mobility, power supply and of course their applications, their specifications such as range and data rates are defined. For instance infrastructure has “unlimited” power supply while vehicle moderate and mobile terminals very limited. These yield higher power light sources for infrastructures and vehicles and furthermore potentially higher range. Regarding mobility, only the infrastructure type has no physical mobility. Based in their applications vehicle devices need low data rates for exchanging information about traffic for example while mobile and infrastructure devices can reach much higher rates for exchanging multimedia like high definition videos, online gaming etc.

The physical layer of communication in the 802.7.15 standard supports three types. PHY I finds outdoor use for low data rates in the order of tens to hundreds kb/s (up to 266.6kb/s) and uses OOK (ON – OFF keying) and VPPM (variable pulse position modulation). On the other hand PHY II is intended for indoor use and delivers data rates of tens of Mb/s (up to 96Mb/s). PHY III achieves the same high data rates as PHY II but instead of OOK and VPPM it deploys CSK (color shift keying) as it is destined for applications with multiple sources and detectors. In figure 3 the spectrum usage is shown. PHY I uses lower frequencies (longer wavelengths) while PHY II and III use higher ones (shorter wavelengths). They in fact use the same part of the spectrum. Furthermore PHY II and III accommodate higher data rates thus are in need of broader slice of the spectrum.

Figure 3

As already mentioned there are two types of devices that present mobility (mobiles and vehicles). In order to support this mobility there are some provisions in the standard. Two types of mobility are defined, the physical and the logical, and are depicted in figure 4. The physical mobility occurs when a mobile device is physically moving and the logical one when the terminal happens to be in the light beam of two different light sources and a hand – over must take place. In this case, after the hand – over has occures the mobile even though physically remains in the same place it appears to be in a different place from the source perspective.  

Figure 4

Lastly in this post the security of the personal network will be dealt. VLC are inherently safer than RF wireless networks as they do not penetrate walls and are more directive. In principle if an unwanted receiver interferes can be easily recognized. Nevertheless, security algorithms are still implemented for data confidentiality, authentication and replay protection. The limited resources though, such as computing power, available storage and power drain, impose limitations in the level of security as a high – quality random number generator is not always the case for example.