Optical Wireless Communications for Broadband Access in Home Area Networks Klaus-Dieter Langer1, Jelena Grubor1, Olivier Bouchet2, Mamdouh El Tabach2, Joachim W. Walewski 3, Sebastian Randel 3, Martin Franke4, Stefan Nerreter4, Dominic C. O'Brien5, Grahame E. Faulkner5, Ioannis Neokosmidis6, Georgia Ntogari 6 and Mike Wolf 7 1 Fraunhofer Institute for Telecommunications, Heinrich-Hertz-Institut, Berlin, Germany 2 France Télécom, Orange Labs, Cesson-Sévigné, France 3 Siemens AG, Corporate Technology, Information & Communications, Munich, Germany 4 Siemens AG, Corporate Technology, Materials & Microsystems, Berlin, Germany 5 University of Oxford, United Kingdom 6 University of Athens, Greece 7 Technische Universität Ilmenau, Germany Tel: (4930) 31002 457, Fax: (4930)31002 250, e-mail:
[email protected] ABSTRACT As a part of the EU-FP7 R&D programme, the OMEGA project (hOME Gigabit Access) aims at bridging the gap between mobile broadband terminals and the wired backbone network in homes. To provide Gb/s connectivity a combination of various technologies is considered. Beside radio frequencies, the wireless links will use infrared and visible light. Combined with power-line communications this enables a home area network (HAN) that meets the vision of broadband home networking ‘without new wires’. A technology-independent MAC layer is foreseen to control such network and to provide services as well as connectivity to any device the user wishes to connect. Moreover, this MAC layer should allow the service to follow the user from device to device in any room of a building /apartment. The contribution presents ideas and approaches for broadband optical wireless (OW) communications using infrared Gb/s hotspots and 100 Mb/s information broadcasting by means of interior lighting based on white-light LEDs. Important issues concerning the physical layer are discussed. Keywords: Home area network, optical wireless, infrared communication, visible light communication, broadband, LED, laser. 1. INTRODUCTION The rapid deployment of broadband access technologies such as fibre to the home offers the potential to deliver data rates of 100 Mb/s and more to customers’ homes. In order to fully use this bandwidth home access networks (HANs) that can operate a factor of ten faster than this will be required, and research in gigabit home networks is a rapidly growing area. For widespread acceptance, wireless networks are required, and the OMEGA project aims to develop gigabit home networks ‘with no new wires’ [1]. Figure 1 shows a schematic of the system envisaged. Such networks will use RF and optical wireless (OW) communications together with (local) power line communications. The technologies will be combined using an ‘InterMAC’ layer, which will ensure that devices are ‘always best-connected’. Optical wireless links will provide high-speed (Gb/s) line-of-sight (LOS) data transmission at wavelengths in the near-infrared (IR) range. In addition, novel visible-light communication (VLC) will be used to broadcast data at bit rates of 100 Mb/s while providing illumination within the home. Figure 2 shows schematics of the link geometries. The LOS IR system consists of a base station, located within the coverage area, and a terminal, each with identical transceivers. These consist of a number of data links with narrow fields of view (FOV), which together create a connection that can operate over a wide FOV and has the potential of transmitting to more than one terminal at a time. The VLC system uses the white LEDs of ceiling lights that can also be modulated to enable data communication. Bridge Office (Mesh) radio
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Figure 1. Potential configuration of a HAN considered in the OMEGA project.
In this paper, we outline the techniques required to implement these OW systems and the challenges faced. In section 2, eye and skin safety are discussed. Section 3 details the high-speed LOS system and section 4 the proposed VLC system. Future work such as the integration of the OW into the OMEGA HAN and conclusions are discussed in section 5. white-LED panel
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Figure 2. Hybrid wireless optics proof-of-concept links. Left panel: Gb/s line-of-sight infrared link; right panel: visible-light communications. 2. EYE AND SKIN SAFETY The international standard IEC 60825-1 is the primary standard for laser safety. This standard is currently under revision, and a new edition of the UK and European standard equivalent has recently been published (BS EN 60825-1:2007, Edition 2) [2]. In the new edition, the measurement conditions for diverging sources are more relaxed leading to less stringent limits for class 1 sources, which are harmless even if viewed using an optical instrument like a magnifying glass. Figure 3 shows the permitted radiant intensity (in mW/srad, measured in the main direction) for diverging laser sources, which are class 1 conform according to the new edition 1. The permitted radiant intensity depends on the wavelength, and for wavelengths ≤ 1400 nm where the radiation reaches the retina also on the apparent source diameter D. Especially for point sources with D = 0, the permitted radiant intensity is much larger compared to previous versions of the standard. The data shown in Fig. 3 can be used to determine the type of source (point source with D = 0 or extended, "diffused" source with D > 0) that is required to meet the link requirements for a particular wavelength.
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Figure 4. Link geometry shown for one transmitter (Tx) and one receiver (Rx) element.
Apart from the eye, human skin tissue is also susceptible to damage by optical radiation. A Maximum Permissible Exposure (MPE) level is specified for the skin, and this level must be measured at the position where the exposure level is highest. For a portable appliance, this is with the skin in direct contact with the emitter output window, and if the beam is not sufficiently expanded at this position skin, rather than eye, exposure can limit the available output power. This is likely to mean that a minimum output window size, where the window is fully illuminated, will be required. This is currently being investigated.
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For a diverging source with a half intensity angle not less than a few degrees, the radiant intensity in the main direction gives a good estimate of the accessible emission at the output of the measurement aperture defined in the standard.
3. HIGH-SPEED LINE-OF-SIGHT COMMUNICATIONS 3.1 Basic Design Considerations Wireless optical transmission at 1 Gb/s and beyond requires an unobstructed LOS-path between the Tx and the Rx. In this case, a reasonable link budget as well as immunity against multipath dispersion can be attained by means of angle diversity transmitters and receivers [3], which consist of a number of switchable elements that point in different directions. In the optimum case, only a single pair of Rx and Tx elements have to be activated to establish a link, where each element covers only a small fraction of the overall required angular range. This leads to an "antenna" gain being advantageous both from the link budget as well as from the multipath dispersion point of view. Clearly, such an angle diversity system requires a tracking-mechanism as the terminal’s position changes. The data rate attainable is limited by the Tx and Rx components, rather than the available channel spectrum. In the next sections, the parameters of such angle-diversity transceivers are discussed from the link budget point of view. 3.2 Path Loss and Receiver Noise Figure 4 shows the basic geometry to estimate the path loss, where only one Tx- and one Rx-element needed to establish the LOS-link is shown. It is assumed that each Rx-element consists of a photodiode (PD), which is equipped with an ideal optical concentrator of refraction index nconc. If Adet is the active area of the bare PIN- or avalanche photodiode (APD), the effective detector area for an angle of incidence φ rx equal to the sub-FOV Ψsub 2 of the Rx-element is Aeff = Adet nconc cos ( Ψ sub ) sin 2 ( Ψ sub ) , where φrx = Ψ sub denotes the worst case situation for Aeff. With I (φ ) denoting the angle dependent radiant intensity of the source, the received power Prx at a distance d from the Tx is then simply Prx = I (φtx ) Aeff d 2 . The radiant intensity already contains the Tx "antenna" gain, since a low power source with a small half-intensity angle may exhibit the same radiant intensity as a high power source with a large half-intensity angle, cf. Fig. 5. In order to estimate the required radiant intensity I (φ tx) in the direction φ rx of the Rx, we need to determine the Rx sensitivity, i.e., the minimum required value of Prx. Our analyses for a PIN-diode based On-Off Keying Rx operating at a data rate Rb = 1 Gb/s show, that the majority of the noise is f 2 noise and not background-light induced shot noise. The power of this f 2 noise depends not only on the (squared) PD capacitance, which is much larger than in fibre-optical systems, but also on the third power Rb3 of the data rate. This shows drastically that the f 2-part of the noise variance at 1 Gb/s is 1000 times larger than at 100 Mb/s. Data transmission at 1 Gb/s is therefore very challenging. 3.3 Initial Link Budget for Detectors Based on Silicon PIN-Diodes A detailed estimation of the Rx sensitivity is beyond the scope of this paper. However, our analyses show that a well designed bootstrapped transimpedance amplifier [4] with a cascode as the amplifying stage could offer a Rx sensitivity of -30 dB at a bit error ratio (BER) of 10-3, if state of the art SiGe bipolar transistors are used and if the PD capacitance does not exceed 5 pF. Figure 6 shows the required radiant intensity in the direction of the Rx as a function of the receiver’s sub-FOV Ψsub for different areas Adet. The distance d was assumed to be 200 cm, nconc. was set to 1.8. With respect to the PD, the responsivity and the capacitance per unit area where chosen to 0.54 A/W and to 5 pF/mm2. This capacitance value is only attainable with silicon diodes and can be seen as a typical lower bound value, which ensures the required response time at a reasonable reverse biasing with 30 V.
Figure 5. Radiant intensity of sources with a total Txpower of 100 mW. A Lambertian source with a half power angle φhp is compared to an idealized source with a constant radiant intensity within its cone.
Figure 6. Required radiant intensity of the transmitter in the direction of the receiver for a distance d = 2 m.
It should be noted that the results do not include any link margin and that I (φ tx) denotes the required radiant intensity in the direction of the Rx. Hence, if φ tx would be equal to the half-intensity angle of the Tx-element, a radiant intensity of 2 I (φ tx) would be required in the direction of the optical axis, as is shown in Fig. 5. Figure 6 shows clearly that the required radiant intensity and the Tx-power, respectively, cannot be reduced if the PINdiode area exceeds approximately 1 mm2. For larger areas, the increased f 2-noise completely compensates the (collection) gain due to the increased detector area. The overall FOV of the collection of links used should approach 90° (full-angle), which can be (approximately) achieved using seven links with a 15° sub-FOV. Such link requires a radiant intensity of about 100 mW/Sr in Rx direction, which is just equal to the ‘class 1’ point source limit at 850 nm. Taking into account that a Lambertian source operating at its half intensity angle would require 200 mW/Sr on-axis, and other losses in any practical system it is clear that extended sources are required. A diffuser with an apparent source diameter D = 2 mm extents the permitted radiant intensity by more than 10 dB (to more than 1000 mW/Sr), which would allow sufficient link margin in practical situations. 3.4 Improving Link Margin Several methods can be used to improve link margin. At the Tx, the beam profile can be designed to optimise the use of power. Lambertian sources provide good overlap between each link, but significant amounts of power fall outside the sub-FOV. Assuming that an intensity of 500 mW/Sr is required at the Rx an ideal diffuser with a cutoff angle of 15°, that exhibits a constant radiant intensity within its cone, can produce the 500 mW/srad with approximately 107 mW of Tx power. This compares with approximately 300 mW for the case of a Lambertian diffuser producing the same intensity at the same 15° angle. Rx sensitivity can be improved if APDs are used instead of PIN PDs, and a preliminary analysis shows that a typical APD might improve sensitivity by 4.5 dB. The choice of wavelength also affects the link performance. For 1300 and 1500 nm point sources can emit more than 2000 mW/Sr at 1300 nm and 1275 mW/Sr at 1550 nm. In addition, PD responsivities are up to twice as large as for silicon devices, offering a potential 3 dB advantage. However, for a particular capacitance, the available device area is typically less than 20% of that for silicon, and in the case of APDs the devices are noisy when compared with silicon. The net effect of the area reduction is usually greater than the responsivity increase, which makes the use of long wavelengths unattractive. It should be noted that this is a preliminary analysis, and further work is required to finally quantify these differences. 4. VISIBLE LIGHT COMMUNICATIONS 4.1 Basic Design Considerations A special case of OW is communication via visible light, based on rapidly advancing high-power white LED sources. These LEDs are becoming the most promising candidate for future general illumination, due to some important advantages, which they offer over the traditional light sources. These include long life expectancy and the continually increasing power efficiency. Another distinct characteristic that distinguishes LEDs from other light sources is their considerable modulation bandwidth, which enables wireless high-speed VLC. The pioneering work of the Japanese VLC consortium [5], [6] has led to increasing research interest, including the formation of a study group within the IEEE [7], which aims to standardise this technique. VLC allows sources to provide both illumination and data transmission and it can provide information broadcast by use of ceiling-based lighting in public and private areas, such as offices, airports, trains, railway stations and convention centres. In addition, it might be used to provide high-speed download to mobile devices such as PDAs and mobile phones. Within the OMEGA project, a VLC system as shown in Fig. 2 is under consideration. VLC transmission at 100 Mb/s will serve as a wide-area high-speed back up for wireless Gb/s hot spots. However, providing transmission speeds of 100 Mb/s faces some challenges. The most widely available white-light LEDs consist of a blue LED chip overlaid by yellow phosphor. It has been shown in [8] that the long response time of the phosphor limits the available modulation bandwidth, but this can be mitigated by detecting only the blue peak of the emission spectrum of the white LED [9]. This is achieved by using a blue optical filter, and Fig. 7 shows that the resulting modulation bandwidth is approximately an order of magnitude larger than without filtering. The design of the system illustrated in Fig. 2 needs to be guided by the requirements for proper illumination of the coverage area. Depending on the application, there is a certain range of recommended brightness of the illuminated surfaces [10]. The illuminance (or brightness) E of a given surface depends on its luminous intensity in direction θ from the orientation axis, including radiation pattern I (θ ), and the distance to the surface in question r [11]. This results in E = I (θ ) r 2 = I ( 0 ) cos m (θ ) r 2 , where m is the order of the Lambertian emission pattern which is typical for LEDs. Figure 8 illustrates the relevant geometrical parameters. In [8] calculations were performed to determine the requirements for proper illumination of a small office room with realistic LED parameters. It is shown that using less than 700 LEDs (with I(0) = 9.5 cd and m = 1) in four lamps on the ceiling, provides 400 -800 lux of horizontal illuminance at the desk surface, which is sufficient for an office environment.
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Figure 7. Measured modulation bandwidth of a phosphorescent whitelight LED [12]. The electrical gain of the detector is shown on the yaxis. The solid horizontal line denotes the 3-dB modulation bandwidth.
Figure 8. Illustration of parameters relevant for brightness calculation.
The same design was used to investigate the communication channel. Due to high optical power distributed via many LOS links it is found that the channel is flat over the bandwidth of interest (with f3dB > 88 MHz), leading to the conclusion that the LED modulation bandwidth rather than the free space channel limits the achievable data rate. Figure 9 shows a contour map of the illumination levels and bandwidth within the considered room. y y 5m
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Figure 9. Horizontal brightness and bandwidth of the pure OW channel. The contour plots correspond to one quarter of the considered ceiling (dashed area). Such a bandwidth limitation requires the use of spectrally efficient modulation schemes (such as pulse amplitude modulation, PAM, or discrete multitones, DMT, with quadrature amplitude modulation, QAM), to reach the desired high transmission speeds. While PAM requires relatively complex decision feedback equalization and is susceptible to the ambient light noise, which is particularly strong around DC, DMT is less power efficient and requires a highly linear transmission system. The choice of the modulation scheme will therefore depend on the results of a cost-benefit analysis to be performed within the project. 4.2 Some Further Technical Challenges In the case of discrete multi-tone modulation, special attention must be paid to the nonlinear behaviour of the LED. However, operation of the LED in the nonlinear region results in an interaction between the carriers, leading to the generation of intermodulation products. In fact, the frequencies of these products may coincide with the frequencies of the initial carriers. Thus, it is expected that the performance of the system will degrade due to the nonlinear nature of the LED [13]. Within OMEGA, the influence of the nonlinear interaction between the carriers will be investigated. However, this is not a straightforward procedure due to the complicated expression giving the nonlinear distortion noise. Unfortunately, the probability density function of the photocurrent (symbol estimates) cannot be approximated by the Gaussian distribution since the noise components are not independent. Hence, one must resort to Monte Carlo simulations in order to estimate the performance of the system. To start with, the DC current – power characteristic will be modelled with a polynomial 2 Pout = a0 + a1 ( I in − I DC ) + a2 ( I in − I DC ) , where α0, a1 and a2 are the DC term, the linear gain and second-order nonlinearity coefficient, respectively, while IDC is the DC offset. In order to characterize the nonlinearity of the LED – PD system, that is to evaluate the coefficients a0, a1 and a2, experimental data for the Iin-Pout characteristic can be fitted using a second-order polynomial. This model is not fully accurate since the frequency response of the LED is not taken into account. However, it is expected that the obtained results will offer a first insight of the nonlinear effect of the LED. In order to incorporate both the nonlinear and the frequency response of the LED, a rate equation model [14] must be used. Another important point under study within OMEGA is dimming. Pulse-width modulation (PWM) in which the lamp is switched on and off for predefined periods, seems to be the most probable representative for dimming purposes. Thus, a lot of attention must be paid in combining the PWM dimming signal and the DMT
signal. In fact, the frequency of the PWM signal must be larger than the bandwidth of the DMT signal to prevent signal distortion. However, this approach is not energy efficient and is limited to data rates below 1 Mb/s for modulation bandwidths of ~ 20 MHz. Thus, one must use lower PWM signal frequencies along with MAC concepts for ensuring proper data transmission by monitoring. Moreover, a prerequisite for such approach is packet-based transmission over the VLC link and one or several data queue(s) within the OW data transmission modulator, depending on how many traffic classes have to be differentiated. In addition, the Rx subsystem needs to handle challenging operating conditions such as the high dynamic range of the room illumination and disturbing influences from external (unmodulated) light sources such as light bulbs or sunlight. To meet the requirements for high quality indoor data transmission the development of a highly efficient Rx optoelectronic subsystem is in progress. First experiments with Si-PIN diodes show promising results, with bandwidths of more than 80 MHz achieved for large area detectors and high bias voltages. 5. CONCLUSIONS AND OUTLOOK The OMEGA project aims at bridging the gap between mobile broadband terminals and the wired backbone of a realistic hybrid home area network. Besides radio, optical wireless communication is an important technology to reach this goal. In this paper, we discussed the basic physical layer issues and major challenges for high-speed optical wireless communications in such scenario. Two systems have been considered: one using IR light and the other using visible light from lamps based on white LEDs, which illuminate the considered room. The IR system aims at providing Gb/s hot spots with a large FOV. It has been shown that this is demanding, but the use of diffused sources and typical receivers should allow Gb/s operation. Nevertheless, there are challenging implementation issues, such as source modulation. The VLC system aims at 100 Mb/s information broadcast and wide area back-up of the IR hot spots. We have shown that the target bit rate can be achieved while the room is illuminated according to office needs. As the system is clearly limited by the modulation bandwidth of the LEDs appropriate modulation formats such as PAM or DMT are considered. As important aspects that will influence the choice of modulation, we have briefly addressed the nonlinear behaviour of LEDs and the function of dimming commonly used in lighting engineering. The IR and VLC systems will be connected to a novel ‘InterMAC’ control layer and this will require a means of quality reporting and reacting link to the inter-MAC. These concepts will be analyzed in more detail during the course of the OMEGA project. In the first phase, this will result in a hybrid optical network, and will lead in the second stage to an overall HAN. ACKNOWLEDGEMENTS The research leading to these results has received funding from the European Community's Seventh Framework Programme FP7/2007-2013 under grant agreement n° 213311 also referred as OMEGA. The authors would like to acknowledge the contributions of their colleagues. This information reflects the consortiums view, the Community is not liable for any use that may be made of any of the information contained therein. REFERENCES [1] OMEGA project. http://www.ict-omega.eu [2] European Standard EN 60825-1: Safety of laser products — Part 1: Equipment classification and requirements, edition 2, 2007. [3] J. Carruthers and J. Kahn, “Angle diversity for nondirected wireless infrared communication,” IEEE Trans. on Commun., vol. 48, no. 6, pp. 960-969, 2000. [4] M.J. McCullagh and D.R. Wisely, “155 Mbit/s optical wireless link using a bootstrapped silicon APD receiver,” Electronics Letters, vol. 30, no. 5, pp. 430-432, 1994. [5] http://www.vlcc.net [6] S. Haruyama, “Japan's Visible Light Communications Consortium and its standardization activities,” Presentation at IEEE 802.15 SGvlc, Jan. 2008. [7] https://mentor.ieee.org/802.15/file/08/15-08-0214-01-0vlc-ig-vlc-closing-report.ppt [8] J. Grubor et al., “High-speed wireless indoor communication via visible light,” ITG Fachbericht, vol. 198, pp. 203-208, 2007. [9] J.W. Kaehler, K.K. Foo, “Devices and methods for intradevice optical communication of data,” United States Patent 20070152950, 2007. [10] European standard EN 12464-1: Lighting of indoor work places, 2003. [11] E.F. Schubert, “Light-emitting diodes,” Cambridge University Press, 2003. [12] http://www.osram.de (DOT-it). [13] T.P. Lee “The nonlinearity of double-heterostructure LED’s for optical communications,” IEEE Proceedings, vol. 65, no. 9, pp. 1408-1410, 1977. [14] R. Windisch et al., “Large-signal-modulation of high-efficiency light-emitting diodes for optical communication,” IEEE Journal of Quantum Electronics, vol. 36, no. 12, pp. 1445-1453, 2000.
