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Homepage / Publications & Opinion / Archive / Articles, Lectures, Preprints & Reprints![]() Optical Wireless David J T Heatley, David R Wisely, Ian Neild, Peter Cochrane IEE COMSOC Mag, Vol 36/12, pp 72 82, December 1998 Abstract 1 Prologue As you enter your car it creates an in-car infra-red link with your laptop and interrogates your diary for the day. Your car learns that your first journey is to your office in the city so it accesses traffic information via its GSM link and displays on the dashboard navigation system, or speaks, a recommended route for you to follow. On your way to work you pull into a gas station. As you do so your car enters an infra-red cell and is instantly linked to the high speed network. Your car uploads its performance data to an agency which will contact you later if your car needs attention. At the same time your laptop uses the in-car infra-red system to establish a connection between itself and the external cell. This done, it takes the opportunity to contact your office to update your dairy and to up/download files from your desktop machine that it knows you will need. By the time you reach your office your laptop has exchanged Gbytes of data via a variety of wireless infra-red links and you are ready to face the challenges of the day. During the day you make several personal calls on clients, all of whom have infra-red cells at their location, so you are on-line the instant you arrive. The buildings that you travel between are all located in the down town business area and are interconnected via high capacity line-of-sight infra-red systems that overlay the conventional cabled network. This enables you to up/download vast amounts of data very quickly, thereby maximising your effectiveness while with your clients. And so on throughout the day. Although this vision lies in the future, much of the implicit optical wireless technology is in fact available today, and in this paper we call upon our own work and that of others to present a broad overview of the field. We begin by describing the underlying principles of optical wireless, paying particular attention to the intrinsic benefits and limitations. We then examine a broad selection of systems, some experimental, others now available commercially, that are tailored to a variety of operational requirements. Performances and deployment issues are compared and contrasted, and we offer informed opinions on optimum solutions for a variety of applications. 2 Design fundamentals 2.1 Transmitter : 2.2 Eye safety :
Table 1. Laser safety classifications for a point-source emitter. ( note that Class 2 only applies to visible light sources ) Outdoor point-to-point systems generally use high power lasers that operate in the Class 3B band to achieve a good power budget. The safety standard recommends that these systems should be located where the beam cannot be interrupted or viewed inadvertently by a person. Roof top locations or high walls are usual for this type of system. Indoor systems pose a particular challenge because the safety standard recommends that they must be Class 1 eye safe under all conditions. Table 1 shows that for systems employing laser sources, launch powers must not exceed 0.5mW at the short wavelengths where most low cost devices operate. Indoor systems that use lasers therefore find it difficult to achieve a good power budget. However, by using LED's instead of lasers a much higher launch power can be used and still remain Class 1 eye safe. This is because LED's are not 'point source' devices as are lasers, they are large area devices, so if viewed the image on the retina of the eye covers a large area, and hence the power is diffused. Indeed, arrays of LED's can produce substantial launch powers and yet be Class 1 eye safe. This and cost are the two primary reasons why indoor systems mostly use LED's as the emitter, either singly or in arrays. Lasers operating inside the Class 3B band can in fact be rendered Class 1 eye safe by passing their beam through a hologram which is sealed within the overall laser enclosure. The hologram breaks up the wavefront in the optical beam which diffuses the image of the laser spot on the retina of eye. The same hologram also shapes the beam to create a definable footprint, e.g., square or rectangular. We have demonstrated that a 40mW Class 3B laser can be rendered Class 1 eye safe [2], and with certain refinements to the hologram the same outcome could be achieved with a 100mW laser. This technique, although proven in the laboratory, has yet to be taken up by the industry. 2.3 Atmospheric loss : Free space loss defines the proportion of optical power arriving at the receiver that is usefully captured within the receiver's aperture (Fig 1). A typical figure for a point-to-point system that operates with a slightly diverging beam would be 20dB, whereas an indoor system using a wide angle beam could have a free space loss of 40dB or more. ![]() Fig 1. Schematic representation of free space loss. Clearly free space loss is experienced by all forms of optical wireless systems, from short distance indoor systems to long distance outdoor systems. However, the remaining sources of atmospheric loss described below are entirely the domain of the latter. Clear air absorption (the equivalent of absorption loss in optical fibres) is a wavelength dependent process that gives rise to low loss transmission windows centred on 850nm, 1300nm and 1550nm [3]. This process is essentially the same as that present in optical fibres, and hence the low loss windows coincide with those in fibres. This in turn means that the same opto-electronic devices can be used, which from a commercial point of view is crucial. Scattering and refraction due to water droplets refers to the attenuating effect of rain, fog, mist and snow on the power reaching the receiver. This attenuation is continually in a state of flux so is most meaningfully quantified in terms of the percentage of time that it exceeds a given value (usually taken to be the point at which the error rate in the recovered data is just acceptable). Over a period of a year we undertook detailed measurements of attenuation over a 100m path in a rural area. Fig 2 shows some of the results obtained. ![]() Fig 2. Seasonal atmospheric attenuation over a 100m path for a 1 year period. Not surprisingly the worst performance was achieved during the winter months due to the high prevalence of rain, fog, mist and snow. Corresponding charts for an urban area show similar trends but with rather less variability between seasons and higher average attenuations. A further study used data provided by the UK Weather Centre, spanning a period of 8 years from Jan'75 to Dec'83 [3]. Fig 3 shows some of the results obtained for three major UK cities. ![]() Fig 3. Atmospheric attenuation averaged over 9 years. It is apparent that atmospheric attenuation is consistently low for all three cities for 99% of the time. Put another way, a free space link spanning say 1km in any of these cities would achieve an availability (i.e., up time) of 99%. Fig 3 suggests that this figure could be improved to 99.5% given a suitable power budget etc, however 99.9% could be difficult to achieve. Of course, by reducing the link length and/or increasing the power budget, the availability would improve correspondingly. Scintillation is the result of solar energy heating small pockets of air to slightly different temperatures, thereby creating regions of varying refractive index along the propagation path. This causes the optical signal to scatter preferentially at very shallow angles in the direction of propagation, whereupon multiple signals, all phase shifted relative to each other, arrive simultaneously at the receiver. This in turn causes the amplitude of the received signal to fluctuate rapidly by as much as 30dB if conditions are unfavourable. The power spectral density of these fluctuations typically spans 0.01-200Hz and hence can give rise to long bursts of data errors. Scintillation also distorts the wavefront of the received optical signal, causing the focused image at the photodiode to dance around the surface of the device. This requires a larger than normal photodiode to be used in order to ensure that the signal is never lost. Scintillation is generally not significant over distances less than 500m but increases rapidly with distance. 2.4 Receiver : ![]() Fig 4. Receiver sensitivity (at 155Mbit/s) in relation to photodiode type and area. We have developed a variety of bootstrap receivers using PIN and avalanche photodiodes (APD) of different dimensions. The sensitivities of some of these receivers are summarised in Fig 4, all for a 155Mbit/s data rate. As expected the sensitivity improves (i.e., reduces in numerical value) as the photodiode area reduces because of the correspondingly lower capacitance. However, small area photodiodes incur a greater coupling loss due to the small aperture they present to the incoming beam, so a careful trade off between these factors is necessary to optimise the final performance. Fig 4 also shows that a receiver with an APD gives a 10dB sensitivity advantage over a corresponding PIN receiver, which is consistent with optical fibre receivers. APD receivers however are more costly and require high operating voltages, hence are predominantly used in specialist systems where performance is key. Systems in which economy is a priority, such as most indoor applications, favour PIN receivers. 2.5 Interference from ambient light : First, by placing a narrowband infra-red filter over the photodiode the level of ambient light relative to the wanted beam will be significantly reduced. Consequently, photonic noise in the receiver will predominantly originate from the wanted signal, which is the optimum condition. Infra-red filters may be fabricated in glass or plastic, depending on the optical quality required and the application. Secondly, the power spectral density of ambient induced noise extends from DC to typically a few 10's of kHz, exceptionally a few 100's of kHz, depending on the type of source. By conveying the data over the optical beam on a high frequency subcarrier, or more commonly by applying a line code that contains no low frequency components, interference is entirely avoided. The strong DC content of ambient light also means that the dynamic range of the receiver could be impaired. By using the above infra-red filter and a receiver design which cancels or blocks any DC from the photodiode, this form of impairment is avoided. 3 Long distance systems (i.e., 100m to 5km). A high power Class 3B semiconductor laser can be used as the source, emitting perhaps 100mW (+20dBm). Alternatively, a lower power device can be coupled to an erbium doped fibre amplifier (EDFA) to achieve the same end result. We utilised the latter approach in a trial 155Mbit/s link that spanned 4km between Imperial College and University College in London [4]. The use of an EDFA dictated a laser wavelength of 1550nm, which enabled the system to be Class 1 eye safe while still affording a useful launch power of 10mW (Table 1). The overall system design is depicted in Fig 5. ![]() Fig 5. Experimental 4km point-to-point system. A good power budget requires that the overall propagation loss is minimised. Little can be done to alleviate the various atmospheric losses, however steps can be taken to minimise free space loss (Fig 1). In our experimental system we achieved this by using an astronomical telescope at each end (Schmidt-Cassegrain, 20cm aperture). The resulting beam diameter at the receiver was typically 2m, which equates to a free space loss of 20dB. It was possible to reduce the beam diameter to only 0.5m (8dB free space loss) but then maintaining beam alignment became more difficult. Beam alignment is a critical factor in long distance point-to-point systems because it will drift with temperature changes and fatigue in the anchorage assemblies. Some means of automatically maintaining alignment by mechanically steering the beams is desirable, particularly in commercial installations where maintenance intervention must be kept to a minimum or even avoided altogether. A long distance system developed and marketed by Canon [5] embraces this requirement to the fullest extent. After crude manual alignment, each end of this system locks on to the other and maintains alignment even in the event of the transmitter/receiver units being jolted or moved off-axis by several degrees. Of course, such robustness comes at a premium. Receiver sensitivity is the final factor in achieving a good power budget. Since the best sensitivity is delivered by an APD fronted receiver (Fig 4), these are generally used in long distance systems. The relatively high cost of these receivers is offset by the commercial advantage afforded by the greater reach. In any case, receiver cost tends to be a small factor in the cost equation for long distance systems. ![]() Fig 6. Capacities and distances for various point-to-point systems. The date rate -v- distance of our 4km system and others we have developed are plotted in Fig 6, together with commercial systems from Canon [5], AirOptics [6] and CableFree [7]. It is unlikely that long distance point-to-point systems will afford reliable spans much beyond 4-5km because of the extreme vagaries of atmospheric loss. Systems such as the Canon and our own are indeed capable of longer distances (e.g., 8km has been reported) but only when the weather, season and time of day combine to produce ideal conditions. This might equate to an availability of only 1-10% averaged over a year, whereas users require at least 99%. Indeed, some applications in telecommunications require 99.99% or even 99.999% (referred to as 5 nines). As Figs 4 and 5 show, obtaining that extra percentage point requires a significant improvement in the power budget parameters, which in turn will be reflected in a higher cost. We therefore believe that the largest market for long distance point-to-point systems will involve sub-1km spans, with the >1km market largely confined to specialist applications. Of course in countries such as the USA which enjoy a predominantly dry climate and contain large open expanses, the balance and size of these markets will be different. 4 Short distance systems (i.e., less than 100m). 4.1 Point-to-point systems : Indoor point-to-point systems do not differ from the outdoor variety in their operating principles, however in practice their designs are invariably very different. Firstly, they must be Class 1 eye safe, which generally means that the optical source is an LED. This in turn limits the capacity to typically a few Mbit/s. On the other hand, indoor systems need none of the weather proofing that outdoor systems require and only operate over short distances, hence they can be produced very cost effectively. A good example of such a system is manufactured by JVC [9]. Here a capacity of 10Mbit/s can be conveyed over a span of around 20m (Fig 6). Such systems could be used, for example, to extend a 10Mbit/s LAN port to a different part of an office where no convenient port exists, or to link two separate offices via a link corridor. 4.2 Telepoint systems : ![]() Fig 7. Optical telepoint. We have experimented with a number of telepoint systems, creating cell diameters of 10m down to 0.5m. The larger cells might be deployed in open offices and public areas such as foyers, libraries, waiting rooms, hospital wards, etc. The smaller cells (e.g., 1m or less) are particularly suited to applications in which a cell is dedicated to a single user, for example a desktop or a telephone booth. A good example of a commercial telepoint system, again manufactured by JVC [9], generates a cell of about 10m diameter, which is sufficient for up to perhaps 6 simultaneous users. Each cell delivers a capacity of 10Mbit/s (Fig 6) shared between the users via conventional LAN protocols. Line of sight paths are required between the base station and all the user terminals. As we will shortly discover, line of sight propagation is in fact vital if capacities more than a few Mbit/s are to be reliably achieved. ![]() Fig 8. Diffuse optical wireless system. 4.3 Diffuse Systems : Interference from ambient light is a particular issue for diffuse systems because of the extremely wide field of view of the receivers. However, as outlined earlier, the use of infra-red filters and robust signal formats minimises any performance penalty that may arise. A good example of a diffuse system is manufactured by Spectrix [11]. A single base station can comfortably irradiate a room 10 metres on a side and deliver 4Mbit/s, which is shared by the users within the cell. This system, like the JVC system described above, utilises LED emitters to be Class 1 eye safe and to keep cost low. 5 Tracking architecture for indoor systems. ![]() Fig 9. Concept behind the tracking system. We developed an experimental system, operating at 155Mbit/s, to investigate the practicality of this concept [12]. The emitter in each transmitter was a laser array, and similarly the receivers used a PIN photodiode array (Fig 9). Device availability dictated that only a single base station and user station could be constructed, and the laser and photodiode arrays were one dimensional rather than the ideal two. Upon arrival of the user in the cell, pilot LED's on the base and user stations enable the down and up-beams to locate and lock on to the corresponding receivers. Thereafter alignment is maintained by monitoring the power detected by adjacent photodiodes in the array then re-aligning the beam as required by activating the appropriate laser in the array. At this point a single laser in each array would be active, the particular choice depending on the required launch angle of each beam. This process would be repeated for other users entering the cell, resulting in additional lasers in the base station array being activated. As a user begins to move within the cell the detected beams would migrate from one PIN to the adjacent one in the array. This data, when fed into a tracking algorithm, would cause different lasers to be activated so that the beams remain directed at the corresponding receivers. This process would continue until the user was again stationary or had left the cell. Of course the converse approach to a tracking architecture is to flood the floor area with stationary overlapping beams, with each cell wide enough to accommodate only a single high capacity user. Roaming is then achieved by handing over from cell to cell, akin to cellular mobile telephony. We are investigating this technique for use in indoor locations such as finance centres and trading rooms where there are many mobile users, each requiring a very high capacity link. These experiments have proven that very high capacities can be delivered to multiple users in an indoor, Class 1 eye safe environment. Commercial systems with these capabilities have yet to appear for reasons that include the immaturity and high cost of some of the devices, particularly the arrays for the tracking architecture. 6 The IrDA standard for very short distance point-to-point systems (e.g., 1m or less). The IrDA standard presently specifies the following parameters :-
All of these features ensure that a useful data rate is available over a wireless link that requires only the crudest of alignment. 7 Comparison with radio systems. Today's radio (or wireless) LAN systems can deliver 2Mbit/s to multiple users over distances of typically 50-100m depending on terrain, with the promise of 8Mbit/s in the near future. Standards are being produced for next generation systems to deliver 10's of Mbit/s. Radio systems will almost always afford a greater reach and wider coverage than optical wireless systems because a higher transmitter power can be used and the receivers can take full advantage of sensitive heterodyning techniques. On the other hand, radio will always be a narrower bandwidth medium than optical, although this is not apparent in today's commercial systems because optical wireless manufacturers have yet to fully exploit the available bandwidth. 8 Seamless hand over. Clearly it would be desirable for the workers in such an office to access the network via whichever wireless link delivers the highest capacity to their laptops (or some other portable device) at that time. Furthermore, it would be equally desirable to have sufficient intelligence within these laptops to automatically and seamlessly hand over from one wireless scheme to the other as the workers roam throughout the room, thus maintaining a link at all times. A typical sequence might then be: a worker at their desk accesses the network from their laptop via 10Mbit/s telepoint or 4Mbit/s IrDA; upon leaving the cell their laptop hands over to the office-wide radio cell which delivers a 40kbit/s link (from 2Mbit/s shared between 50 users); upon returning to a telepoint or IrDA cell the laptop hands over and the original data rate is restored; and so on. The intelligence within the laptop, as well as managing the hand overs, would also take account of the different capacities by tailoring the quality of service delivered to the user. For example, streamed video (vision and sound) at the desk via telepoint or IrDA might be reduced to sound only in the radio cell, then back to vision and sound upon return to a desk. Of course, hand over need not be confined to indoor wireless technologies. GSM can also be included to great effect. A typical sequence might then be: a worker at their desk accesses the network from their laptop via 10Mbit/s telepoint or 4Mbit/s IrDA; while roaming the office their laptop hands over to the radio cell which delivers a 40kbit/s link; upon leaving the office or even the building their laptop hands over to the 9.6kbit/s channel provided by the GSM network; and so on. We are investigating a number of hand over schemes that can support these scenarios. For example, we have demonstrated seamless hand over between IrDA and GSM, and IrDA and DECT. By building on this work hand over schemes that have the ubiquity envisaged above and in the prologue to this paper will become practicable. Closing remarks References
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