Cellular tracked optical wireless demonstration link

Cellular tracked optical wireless demonstration link

F. Parand, G.E. Faulkner and D.C. O’Brien

Abstract: Line-of-sight optical wireless systems have the potential to provide extremely highbandwidth communications; the major problem is to ensure that a link can be maintained astransmitter and receiver move relative to one another, and to provide wide area coverage. Cellularsystems, or tracking systems that incorporate multiple sources and detectors, have the potential toachieve high bandwidth and provide the necessary area coverage. In the paper, the construction andtesting of a cellular tracked demonstration optical wireless link that empl‘oys a 3 � 3 array ofvertical-cavity surface-emitting lasers in the transmitter and a 3 � 3 array of silicon photodiodes inthe receiver is reported. The link operates at 32.77 Mbit/s and is demonstrated to provide coverageto a BER of 1�10�9 across an area of 0:8m� 0:8m at a link distance of 2.5 m. This is believed tobe the first reported cellular tracked link that demonstrates tracking in two dimensions.Consideration is given to extending the coverage area of the link.

1 Introduction

Optical wireless transmission has the potential to providehigh bandwidth, largely unregulated communications withmany potential advantages when compared with radiofrequency approaches [1–7]. There are various arrange-ments of optical sources and detectors which can be usedto realise communications links. They may be broadlycharacterised as either diffuse, or line-of-sight (LOS).Diffuse systems, in which the coverage volume is floodedwith optical radiation, provide the optimum in mobility andconvenience but are subject to a multipath limitedbandwidth, whilst LOS systems can provide very highbandwidth, but require alignment. In this paper, we describea link employing multiple sources in the transmitterand multiple detectors in the receiver. This results in a‘cellular tracked’ system, an architecture which hasseveral advantages over the single source single detectormethod [8–14].

Each of the sources of a multi-element transmitter emits anarrow beam, leading to reduced path loss and a moreefficient means of providing coverage. A receiver incorpor-ating a number of photodetectors has two main benefits overa single photodetector of equivalent area. Firstly, thesmaller individual detector area has reduced capacitance,which can lead to greater bandwidth and improvedsensitivity [15]. Secondly, the field of view of eachindividual detector is narrow, allowing improved rejectionof ambient light noise sources. Thus a cellular trackedsystem avoids the bandwidth limitations of diffuse linkswhilst still providing a degree of mobility unavailable witha basic single source single detector LOS system.The tracking function achieved by the combination of a

multi-element transmitter, and multi-element receiver isillustrated in Fig. 1.

Figure 2 illustrates how a cellular tracked system maybe employed in a typical office situation. A base stationin the ceiling creates a cellular intensity pattern at thedesk level by spreading the beams from several opticalsources, and a transceiver of similar design installed in aportable device maintains an LOS link to the base stationas it is moved around within the coverage area.

2 Demonstration link

To investigate the practical challenges and issues involvedin the design of a cellular tracked system, a single-duplexdemonstration link providing coverage across 0:8m�0:8m for a transmitter–receiver separation of 2.5 m hasbeen constructed and tested in the laboratory. Onlycommercially available optical and electronic componentsare used.

The use of commercial components determined to a largeextent the system parameters. The preferred configurationfor the transmitter and receiver arrays would be hexagonal,but for reasons of availability, square arrays are utilised.The initial design constraints are the output powers of thesources of the transmitter array, and the sensitivity of thereceiver subsystem. A 0.25 mm pitch, 3 � 3 square array of850 nm vertical cavity surface emitting lasers (VCSELs) ofaverage power output between 0.9 and 1.1 mW is employedin the transmitter. In the receiver, a 3 � 3 square arrayof square silicon photodiodes of area 7:29mm2 andcapacitance 9pF at the bias point is used. The schematicfor the transmitter and receiver electronic subsystems isshown in Fig. 3.

The system transmits Manchester encoded data at32.77 Mbit/s. This data rate was chosen as the transmitterintegrated circuits available would not operate at higher datarates, and the crystal oscillators required for clock recoveryoperated this specific frequency. Manchester encoding isused for a variety of reasons. The receiver integrated circuitsare AC coupled with a high pass pole at approximately90 kHz, so that Manchester coding reduces any intersymbolinterference produced, although in this case there is only asmall benefit in using Manchester coding compared with

q IEE, 2003

IEE Proceedings online no. 20030961

doi: 10.1049/ip-opt:20030961

The authors are with the Department of Engineering Science, OxfordUniversity, Parks Road, Oxford OX1 3PJ, UK

Paper first received 28th October 2002 and in revised form 20th August2003

IEE Proc.-Optoelectron., Vol. 150, No. 5, October 2003490

NRZ. However, the integrated circuit VCSEL drivers(which are experimental prototypes) have some datadependent characteristics, and Manchester coding reducesthese to acceptable levels.

A commercial bit error rate (BER) measurement system,comprising source and receiver units, is employed toprovide a pseudorandom bit-stream permitting measure-ment of the link performance. A 32.77 MHz clock obtainedfrom a pulse generator sets the output ECL NRZ data fromthe BER source unit to 32.77 Mbit/s. An encoder circuitfollows this, converting the data to a Manchester bit-stream,and an ECL–TTL converter permits this to be interfacedwith the TTL input of the laser driver. The laser driver thenmodulates the nine VCSELs of the source array with theManchester encoded signal. The encoder, converter anddriver circuits are built from commercially available ICs.

At the receiver, the nine received signals from thephotodiode array are amplified separately by a nine-channelcircuit constructed from commercially available transimpe-dance preamplifiers, and passed to a multiplexer to permitselection of the strongest signal. In general, to takeadvantage of the multiple received signals, maximal-ratio-combining is the optimal solution. However, in most casesthis only provides a modest (1 dB) improvement in receiversensitivity compared with selecting the signal with the

Fig. 3 Schematic of the demonstration link

Fig. 1 Illustration of the tracking principle when employing multiple sources and detectors

Fig. 2 Indoor cellular tracked system architecture

IEE Proc.-Optoelectron., Vol. 150, No. 5, October 2003 491

highest signal to noise ratio (SNR) [13] (so-called ‘selectbest’ combining). In the interests of simplicity, the strongestsignal is selected in this case, and in a system where noisefrom ambient light is approximately constant this will be thesignal with the highest SNR. However, there may be caseswhere ambient light produces the largest signal. In thiscircumstance, no data clock will be recovered and this canbe used as an indicator that an alternative signal is required.The selected preamplified signal is passed to the post-amplifier, clock recovery and decoding stages, all of whichare constructed around commercial integrated circuits. Linkperformance is then measured using a BER receiver unit.

With Manchester data at 32.77 Mbit/s, the measuredsensitivity of the receiver from the array to the Manchesterdecoder, in the absence of any background illumination, is231.5 dBm for a BER of 1 � 10�9: The problem of ambientlight is much reduced in a system where each channel has anarrow field of view, as a small amount of noise can becollected, and the receiver can track the line of sight signal.In this initial demonstration, the effect is mitigatedelectrically by the high pass filtering action of the receiver;future work will examine the use of optical filters to furtherreduce this effect.

A two stage method was employed to jointly optimise thetransmitter and receiver optical systems. A paraxial modelwas initially used to determine approximate parameters forthe optical systems and the coverage thus obtained for a linklength of 2.5 m. This model was based on the assumptionthat all the transmitted power incident on the aperture of thereceiver optical system is collected, with no losses, and thatthis power is shared between four of the receiver arrayphotodiodes. Based on the focal lengths and diametersobtained from the paraxial model, the transmitter andreceiver optical systems were designed using standardspherical profile lenses, and a commercially available lensdesign package. The power budget for the link is shown in

Table 1. The minimum source power and receiversensitivity are measured results. The values for surfacereflection loss are obtained from manufacturers’ data. Thepath loss value is obtained from the paraxial model.

A more accurate numerical model of the optical systemsincorporating power transfer values obtained by ray-tracking methods, and which accounted for lens aberrations,was then used to simulate the power received by thephotodiodes of the array for variable receiver position usingthe optical systems devised. Figure 4 shows a plot of thepower received for a quadrant of the coverage area when thereceiver employs the ‘select best’ method of signalcombination. It may be seen that at all positions, thesimulated power received, by one or more of the photo-diodes, is sufficient to achieve a BER of 1 � 10�9; for areceiver sensitivity of 231.5 dBm.

The final transmitter lens system is a two element designof focal length 2.4 mm and diameter 4 mm, and the receiverlens is of focal length 27 mm and collection aperture 21 mm,employing three elements. Standard spherical profile lenseswere employed in the designs.

3 Experimental results

For the purpose of measuring the link BER performance, thetransmitter and receiver are arranged 2.5 m apart across thelaboratory (Fig. 5), rather than from ceiling to desk level.The transmitter is mounted on a translation stage, whichallows it to be moved in the vertical direction, and the

Table 1: Link power budget

Minimum source power 0.94 mW 2 0.3 dBm

Reflection loss of 1.5% for 8

lens surfaces (coated lenses)

20.5 dB

Reflection loss of 4% for 4

surfaces (uncoated lens & receiver

array package window)

20.7 dB

Power shared between four photodiodes of

receiver array

26 dB

Path loss (receiver aperture of 21 mm radius) 223 dB

Received power 230.5 dBm

Receiver sensitivity 231.5 dBm

Margin þ1.0 dB

Fig. 4 ‘Select best’ received power across upper quadrant ofcoverage areas

Values at each grid point indicate the highest simulated power received byany of the photodiodes when the receiver is at that position in the coveragearea. The individual measured transmit powers of the separate VCSELs areused in the simulation

Fig. 5 Arrangement used to vary relative positions of transmitter and receiver in two dimensions


receiver is positioned on a table 2.5 m away, allowingmovement in the horizontal direction.

Figure 6a shows the key used to identify the individualVCSEL sources and the corresponding receiver planecoverage cells. Figure 6b shows the key for the individualphotodiodes of the receiver array.

From consideration of the receiver optical design, it maybe predicted that if, for example, the receiver is positionedin the upper right quadrant of the coverage area, where thecoverage cells are produced by VCSELs 1, 2, 4, and 5, thephotodiodes 1, 2, 4 and 5 will provide the strongest receivedsignals.

It is impractical to measure the BER produced by each ofthe nine photodiodes resulting from each of the nineVCSELs as this would require 81 readings for each position.However, for many combinations, no significant power willbe received and the majority of these readings may beneglected. Ideally, the significant measurements would berecorded for a two dimensional grid across the whole of thecoverage area to produce a map of the BER. This would,however, require many months of measurement. It istherefore only possible for certain areas of the overallcoverage area to be examined in detail. Figure 7 illustratesthe positions in the coverage area for which BERmeasurements were taken. There is particular emphasis onthe upper right quadrant because this is the weakest area interms of power and thus should provide the most significantmeasurements.

Figures 8–11 show the BERs recorded for all relevantcoverage cell and photodiode combinations along the line ofDiagonal 1. There are sixteen in total. Taking the upper rightquadrant as an example, the BER produced in photodiodes1, 2, 4 and 5 due to VCSEL 1 must first be measured. Thenthe BER produced in photodiodes 1, 2, 4 and 5 by VCSEL 2,in photodiodes 1, 2, 4 and 5 by VCSEL 4, and inphotodiodes 1, 2, 4 and 5 by VCSEL 5.

Owing to the limitations of the BER test equipment usedto measure the BER, only values between 1 � 10�9 and9 � 10�3 may be recorded. In the following BER plots,measured values of less than 1 � 10�9 are shown as 9 �10�10 and measured values of greater than 9 � 10�3 areshown as 1 � 10�2:

Fig. 7 Lines and areas of the coverage profile for which BERmeasurements were taken

Each spot corresponds to a number of measurements corresponding to allrelevant combinations of VCSELs and photodiodes

Fig. 6 Key for identificaion

a VCSELs of the transmitter array and corresponding coverage cellsb Photodiodes of the receiver array

Fig. 8 BER along diagonal 1 for signal received by photodiodes1, 2, 4 and 5

a Resulting from VCSEL 1b Resulting from VCSEL 2


Figures 8–11 show the expected correspondence betweenposition and the strongest received signals, and alsoillustrate the degree of overlap between cells. TakingFigs. 9 and 10 as an example, it may be seen that at distancesclose to the centre of the coverage profile (<15 cm), thereis sufficient power received by the centre photodiode (no. 4)to achieve a BER of better than 1 � 10�9 from VCSELs 5, 6and 8, but not VCSEL 9, which produces coverage furthertowards the corner of the coverage area. At the least powerpoint, approximately 20 cm from the centre, there areseveral possible signal path combinations, all of whichproduce the required BER, such as that between VCSEL 6and photodiode 7, between VCSEL 6 and photodiode 6, andbetween VCSEL 9 and photodiode 5. As expected, atpositions close to the corner of the coverage area, thestrongest signal is produced in photodiode 6 by VCSEL 9.The overall indication given by Figs. 8–11 is that sufficientpower is received for at least one combination of VCSELand photodiode at all positions along the line of diagonal 1,to approximately 54 cm in either direction. The results for aBER sweep along the diagonal 2 are similar, and alsoindicate coverage at all points.

The handover between cells can be more clearlyillustrated by the BER readings taken along the horizontaland vertical lines shown in Fig. 7. These are shown in Figs.12 and 13, with only the strongest signal paths shown.

The results of Figs. 8–13 indicate that it is likely thatsufficient power is received for a BER of better than 1 �10�9 at all positions of the coverage area. For furtherconfirmation, the BER for positions shown by the two gridsreferred to as ‘edge’ and ‘weak’ in Fig. 7 were measured.These indicate those areas most likely to suffer from lack ofcoverage. The results obtained confirmed that, at allpositions measured, at least one combination of sourceand detector provided a signal path sufficient to achieve aBER of 1 � 10�9:

4 Scaling

The coverage of the demonstration link is somewhat limitedfor application to, for example, an office environment, andconsideration may be given to extending the coverage. Themain difficulties encountered relate to the design of thetransmitter and receiver optical systems.

The receiver optical system must collect as much light aspossible over as wide a range of input angles as possible.The maximum available optical ‘gain’, i.e. the ratio of thecollection area of the optical system to the area of thefocused spot on the receiver array, cannot exceed that set byconstant radiance considerations [16]. As the light isincident on the receiver optical system at greater angles tothe optical axis, the available gain decreases and tends to

Fig. 9 BER along diagonal 1 for signal received by photodiodes1, 2, 4 and 5


Fig. 10 BER along diagonal 1 for signal received byphotodiodes 3, 4, 6 and 7



unity, in which case the photodetector area governs the lightcollected.

In practice, it is difficult to design lens systems thatapproach this constant radiance limit, and designing lensesfaster than around f/# 0.5 is difficult, so this might be takenas a practical limit. Given these two constraints anapproximate receiver design can be achieved; first, therequired receiver aperture can be calculated consideringthe amount of power that needs to be collected, and theminimum f/# then sets the focal length of the system. Therequired detector array size can then be estimated using amaximum optical gain calculated according to constantradiance considerations.

To illustrate the issues involved, extending the 0.8 m by0.8 m coverage area of the demonstration link may beconsidered. One possible method is to reduce the focallength of the transmitter to provide the wider coverage area,and increase the radius of the receiver lens aperture toenclose the required power, whilst maintaining the rest ofthe parameters of the demonstration link constant. Thisrapidly leads to impractically large receiver optical systemsand detector arrays: for a 3 m by 3 m total coverage area, forexample, a receiver lens aperture of radius 69 mm isrequired to enclose sufficient power, together with a photodetection area of approximately 45 mm radius.

Increasing the transmitter power, or improving receiversensitivity are required to scale this system in a practicalfashion. Operating at a wavelength longer than 1400 nmwould allow a transmit power of 10 mW, increasing thesystem margin by approximately 10 dB. For a total coveragearea of 3 m by 3 m, an individual coverage cell radius of 1 mis required. With the transmitter array pitch of 0.25 mm, thiswould then require a transmitter lens of focal lengthapproximately 0.6 mm. For the useful power from thesources to be able to pass through the transmitter lens,it must be of diameter greater than 1.06 mm. The f/# ofsuch a lens is then approximately 0.57. Assuming sucha transmitter lens could be designed with adequatelycontrolled aberrations, and employing sources of power10 mW, the receiver lens must have an aperture of 21 mm toenclose the required power. If it is then assumed that thebest possible receiver lens that may be designed is of f/# 0.5,its focal length becomes 21 mm. The restrictions of constantradiance [16] then require that the minimum radius of the

Fig. 11 BER along diagonal 1 for signal received byphotodiodes 3, 4, 6 and 7


Fig. 12 BER sweep along horizontal line through centre ofcoverage area

Results are shown only for the combination of VCSEL and photodiodeyielding the strongest signals

Fig. 13 BER sweep along vertical line through centre ofcoverage area

Results are shown only for the combination of VCSEL and photodiodeyielding the strongest signals


detection area be approximately 14 mm. Applying thisradius as the exscribed radius for a square array, to providethis detection area with the photodiodes employed in thedemonstration link, a receiver array of 7 by 7 elements isrequired. Such a system would be practical, albeit withoptical design and electronic integration challenges; oncethe practical limitations discussed here inhibit larger cellradii, the number of base stations may be increased toprovide further coverage.

5 Conclusion

The results presented in this paper demonstrate thefeasibility of the cellular tracked architecture for a focusedcoverage area. The demonstration link is able to providedata transfer at 32.77 Mbit/s to a BER of 1 � 10�9 forall positions within an area of 0.8 m square, for a transmitterto receiver separation of 2.5 m. To the best of ourknowledge, this is the first such link demonstrating cellulartracking in two dimensions. Modelling indicates anincreased link margin is required to scale to a practicalsystem size, either using increased transmitter power or amore sensitive receiver. Given this, and careful opticaldesign, systems can scale to provide practical coverageareas. Work on systems that can do this is underway.

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Cellular tracked optical wireless demonstration link