Differences Between Radio & Visible Light Communications A technical guide purevlc 2012
Contents Contents... 2 Abstract... 3 Introduction... 3 Historical background... 3 The differences to RF communications... 4 Link Level... 5 System Level... 6 References... 7 2 purevlc 2012
Abstract This paper shows that Optical Wireless Communications has had a long history. Wide spread deployment of solid state lighting (SSL) using LEDs is helping to drive this technology in the form of Visible Light Communication (VLC). Data from an experimental systems shows that data density s of 0.41 bits/second/hz/m 2 is being achieved from a VLC implementation This compares with 4 x 10-4 bits/second/hz/m 2 reported for wireless radio systems and illustrates that VLC can achieve three orders of magnitude improvements in data density compared with the equivalent RF system. Introduction Wireless optical communications has been used long before radio communications was first considered. However, over the last century radio communication has been the preferred means to transmit data wirelessly. Only now when we are faced with capacity shortages for wireless data communications is freespace optical communication being considered as a candidate for widespread wireless communications applications. With the widespread use of LED light bulbs, visible light communications has become the forerunner in the current optical wireless communications field. This paper first considers the historical development of visible light communications up to the present day. Then some of the fundamental differences between radio and VLC are presented. Finally results from an experimental OFDM scheme implemented in hardware are used to derive data density figures which are compared against published results for an equivalent radio system to show the significant advantage of VLC. Historical background The use of light to send messages is a very old idea [1]. Fire and smoke signalling were used in ancient civilizations. For example, the ancient Greeks used polished shields to reflect sunlight to signal in the battle and Roman records indicate that polished metal plates were used as mirrors to reflect sunlight for long distance signalling. Chinese started using fire beacons followed by the Romans and American Indians using smoke signals [2]. In the early 1800s, the US military used a wireless solar telegraph called Heliograph that signals using Morse code flashes of sunlight reflected by a mirror. The flashes are produced by momentarily pivoting the mirror, or by interrupting the beam with a shutter. The navy often uses blinking lights, i.e. Aldis lamps, to send messages also using Morse code from one ship to another. In 1880, the first example of VLC technology was demonstrated by Alexander Graham Bell with his photophone that used sunlight reflected off a vibrating mirror and a selenium photo cell to send voice on a light beam [3]. Until the late 1960s, radio and radar communications were more successful than optical communications (OC). OC started to get real attention with the invention of the light amplification by stimulated emission of radiation (laser) and the laser diode (LD) in the 1960s, followed in the 1970s by the development of lowloss optical fibres (OFs) as a medium for transmitting information using light, the invention of the OF 3 purevlc 2012
amplifier in the 1980s, and the invention of the in-fibre Bragg grating in the 1990s. These inventions formed the basis for the telecommunications revolution of the late 20th century and provided the infrastructure for the Internet. The Nobel Prize in physics 2009 went to three scientists (Charles K. Kao, Willard S. Boyle, George E. Smith) who have played important roles in shaping the modern information technology due to their groundbreaking achievements concerning the transmission of light in fibers for optical communication. Advancements in basic opto-electronic devices, such as LEDs (light emitting diodes) and LDs, p-intrinsic-n (PIN) photodiodes (PDs) and avalanche photo-diodes (APDs) and various optical components have attracted engineers to consider optical sources for wireless data transmission which has led to modern optical wireless communications (OWC). The first indoor OWC system was developed over 25 years ago. In 1979, an indoor OWC system was presented by Gfeller and Bapst [4]. In their system, diffuse optical radiation in the near-ir region was utilised to interconnect a cluster of terminals located in a room to a common cluster controller. During the last ten years, we have witnessed the emergence of visible light communications (VLC) fuelled by solid-state lighting (SSL) technology. SSL is a rapidly developing area, both in terms of commercial exploitation, and academic and industrial research. LEDs with a wide range of colours are available, including white light. The output power as well as device efficiencies are increasing rapidly. The field of applications is also expanding. White LEDs are commonly used as replacements for incandescent lamps due to more than 10 times improved energy efficiency. Therefore, LED lighting is set to revolutionise the way we illuminate our homes, offices, public buildings and streets. These SSL sources, being semiconductor devices, come with an additional feature. Their light intensity can be varied at very high speeds, and so their functionality can be extended by means of intensity modulation (IM) to also become a wireless communication device. VLC originated in Japan and the visible light communications consortium (VLCC) was established in November 2003. The VLCC has major companies in Japan on board and aims at publicising and standardising VLC technology. The formation of the VLCC has stimulated worldwide interest in VLC technology, and the first IEEE standard for VLC - IEEE 802.15.7 has emerged recently. University of Edinburgh academics have worked on VLC since 2004 and have developed enhanced modulation schemes that enable high data rates to be achieved using standard LED light bulbs. The differences to RF communications Traditionally, on-off-keying (OOK), pulse position modulation (PPM), or pulse width modulation (PWM) have been used for intensity modulation in conjunction with incoherent light sources. While these techniques are simple and robust, the main problem is that the data rates cannot be scaled with the signalto-noise (SNR) at the receiver. In a VLC setup where even minimum required levels of light intensity in a room environment often translate to SNRs in the region of 50-70 db, pulsed modulation techniques are extremely spectrally inefficient. Classical digital modulation techniques used in RF systems that would allow for higher spectrum efficiencies cannot straightforwardly be applied due to a few fundamental differences. These differences are summarised in Table 1 below. 4 purevlc 2012
Table 1: Fundamental differences between radio frequency and incoherent optical wireless communications System Information Signal Radio frequency conveyed through electric field Complex valued bipolar Incoherent optical wireless Conveyed through optical intensity Real valued unipolar (nonnegative) Therefore new digital modulation techniques have to be developed in order to achieve high data rates for intensity modulated systems. Link Level Harnessing high crest factor in OFDM and exploiting high SNR for high data rate transmission OFDM enables very high data rate transmission with low computational complexity at the receiver since it is robust to multi-path propagation. OFDM entirely eliminates the need for complex algorithms to cope with inter-symbol interference (ISI) which typically gets worse with higher data rates. However, a standard OFDM transmitter produces a complex-valued signal. Through a simple mathematical trick, this signal can be converted to a real-valued signal whose amplitude greatly varies in time (as depicted in Figure 1). As a consequence, the peak-to-average-ratio (PAR), or crest factor is high. This causes concerns in RF communications because of the detrimental impact on system performance due to power amplifier nonlinearities. For optical wireless communications this effect, however, can be turned into an advantage as the high PAR signal can be exploited for intensity modulation [5]. Given that the minimum illumination for reading purposes is 400 lx, and that this already translates into a SNR greater than 30 db, OFDM combined with higher order modulation techniques such as M-level quadrature amplitude modulation (QAM) result in a powerful transmission technology for incoherent visible light sources. The D-Light team at the University of Edinburgh have demonstrated real-time data transmission using off-the-shelf LEDs of 130 Mbps. 5 purevlc 2012
Figure 1: Real-valued OFDM signal [1]. System Level More than three orders of magnitude higher data density OFDM results in high link-level data rates, but what happens if multiple transmitters are deployed which together form an optical cellular network? In a recent publication [6] the area spectral efficiency (ASE) of future interference limited wireless systems has been determined. The ASE is a measure for the maximum data rate per unit and per Hertz bandwidth. It assumes a wireless network that is composed of multiple randomly deployed access points where each access point uses the same transmission resource/bandwidth. Basically, if there are many access points, this means a high reuse of the same transmission resource and thus high data rate per unit area, but at a certain point this gain is outweighed by increased interference which results in a drop in ASE. On the other hand, if there are only a few access points, this means a low resource reuse and hence low data rate per unit area, but also low interference. Therefore, there is an optimum point for the ASE. This optimum ASE for an indoor environment is found to be 4 x 10-4 bits/second/hz/m 2. In VLC, there will be no interference from one room inside a building to another as rooms are typically separated by walls and light does not propagate through walls (illustrated in Figure 2) as opposed to RF signals. If we assume a typical room of the size 4 m x 4 m, i.e., 16 m 2, and a VLC transmitter that is capable of delivering 130 Mbps with an off-the-shelf LED lamp of 20 MHz bandwidth, as demonstrated by the D- Light team, this would result in ASE of: 130 x 10 6 [bits/second] / (20 x 10 6 [Hz] x 16 m 2 ) = 0.41 bits/s/hz/m 2 When we compare this to the maximum 0.0004 bits/second/hz/m 2 for state-of-the-art wireless systems, we can observe a 1025 times higher ASE. This essentially means that VLC technology has the potential to provide wireless Giga-bit services inside buildings using standard off-the-shelf LEDs. This results in a massive RF spectrum relief which frees up RF resources for the provision of better services in areas were VLC technology is difficult to use such as in remote areas. If we take this further and exploit the particular LED light radiation characteristics and allow different light sources in a room to co-exist, the improvement of ASE could be well beyond a factor of 2000 and more. 6 purevlc 2012
Figure 2: Interference-free reuse in VLC enables high data rates per unit area References [1] H. Elgala, "A Study on the Impact of Nonlinear Characteristics of LEDs on Optical OFDM," PhD Thesis, 2010. [2] G. Holzmann and B. Pehrson, "The Early History Of Data Networks," ed, 1994, p. 292. [3] A. G. Bell, "Selenium and the Photophone," in Nature vol. 22, ed, 1880, pp. 500--503. [4] F. R. Gfeller and U. Bapst, "Wireless In-House Data Communication Via Diffuse Infrared Radiation," in Proceedings of the IEEE vol. 67, ed, 1979, pp. 1474--1486. [5] M. Afgani, H. Haas, H. Elgala, and D. Knipp, "Visible Light Communication Using OFDM," presented at the 2nd International Conference on Testbeds and Research Infrastructures for the Development of Networks and Communities (TRIDENTCOM), 2006. [6] Y. Kim, T. Kwon, and D. Hong, "Area Spectral Efficiency of Shared Spectrum Hierarchical Cell Structure Networks," IEEE Trans. Veh. Technol., vol. 59, pp. 4145-4151, October 2010. 7 purevlc 2012