Performance analysis of mobile optical wirelesssystems employing a novel beam clustering methodand diversity detection

A.G. Al-Ghamdi and J.M.H. Elmirghani

Abstract: User mobility in a diffuse optical wireless (OW) system is of particular interest as it caninduce link performance degradations. While much attention has been given in the literature to theconventional indoor diffuse optical wireless system, little has been done in terms of evaluating theimpact of user mobility in such an environment. Furthermore, there is a clear need for methods thatcan be used to reduce the link performance degradations attributed to user mobility. The paperconsiders a mobile OW system under the constraints of ambient light noise and multipathdistortion. Moreover, the authors propose an original beam-clustering method (BCM) that utilises aspot diffusing technique and evaluate the associated channel characteristics. The performance ofa mobile OW BCM system is analysed and compared with that of the conventional diffuse system(CDS) in a middle-sized room at 14 receiver locations. Our results indicate that, at the worstcommunication link, BCM can reduce the signal delay spread by nearly a factor of six and increasethe SNR by .30 dB over the CDS SNR, while resulting in improved performance over all links.

1 Introduction

Indoor OW communication systems can be classified basedon the optical signal propagation as directed or nondirected.In directed line of sight systems (LOS), the transmittedsignal reaches the receiver directly, while in nondirected(diffuse) systems, the transmitted signal reaches the receiverthrough multiple diffusive reflections (ceiling and walls) [1].OW communication links have several potential drawbacks:OW networks require installation of access points due tolight blockage through walls, ambient light noise (natural orartificial) represents major obstacle for OW applications,and multipath propagation causes pulse spread and caninduce intersymbol interference (ISI) [1–19]. One of themost efficient and simplistic techniques that can be used toovercome the destructive effects of multipath dispersion andambient light noise as well as minimise the effects of signal-to-noise ratio fluctuation is diversity. Diversity is conven-tionally considered as a method of mitigating fading effectsin radio frequency by obtaining independent replicas of thetransmitted signal. In OW systems where fading is not anissue [2], diversity serves other purposes, such as rejectingambient light noise sources [17] (even with noise asym-metry [18]), increasing signal collection, reducing signaldelay spread, and combating shadowing. OW diversitytechniques depend on employing several receivers, whereeach receiver is oriented at a specific angle. By having morethan one direction to select from, both the received opticalsignal and SNR can be significantly improved in the

receiver. The use of diversity techniques in OW environ-ments has been studied by several researchers [3–7]. One ofthe diversity techniques that has been widely studied is theangle diversity receiver configuration. A diversity receiveris usually composed of several receiver branches with arelatively small filed of view (FOV).

Transmitter beam diversity has also been presented byseveral researchers to replace the pure diffuse transmitter.Yun and Kavehrad proposed to replace this with amultibeam transmitter to create multiple diffusing spots[8, 9], which pointed in different directions toward certainreflecting surfaces, while Carruthers and Kahn used eightlaser diodes to produce eight collimated beams [10].Significant performance improvements can be achieved byusing spot diffusing techniques in conjunction with an anglediversity receiver [6, 9–13]. A multibeam transmitter isused to create multiple beams pointed in different directions,hence forming a lattice of diffusing spots. Spot diffusingtechniques can offer several advantages. They combine theadvantages of directed LOS with diffuse configurations.Several methods have been used to create multiple diffusingspots: a holographic optical element mounted on thetransmitter can be used as in [9]; a number of transmitterscan be used to produce a certain number of beams as in [10];and computer generated holograms can be used as beamsplitting elements as in [12, 13]. The angle diversity receiverutilises multiple receiving elements that are aimed indifferent directions. Employing angle diversity receiverscan offer several advantages. These receivers can spatiallyeliminate undesired signals, hence significantly reducing theeffect of ambient light noise and multipath dispersion. Themobility of spot diffusing transmitters has not been studiedin detail and previous studies have assumed a uniformdistribution of spots on the ceiling. Mobile users will violatesuch a geometry arrangement, thus producing weak cover-age in certain zones (parts of the room) as the user moves.

To reduce the performance degradation due touser mobility, we investigate the use of an originalbeam clustering method (BCM) in a mobile OW system.

q IEE, 2004

IEE Proceedings online no. 20040741

doi: 10.1049/ip-opt:20040741

The authors are with the School of Engineering, University of WalesSwansea, Singleton Park, Swansea SA2 8PP, UK

Paper first received 17th September 2003 and in revised form 19th May2004

IEE Proc.-Optoelectron., Vol. 151, No. 4, August 2004 223

We present the channel characteristics associated with anondirected communication link employing such a BCMconfiguration. BCM using angle diversity detection with acomposite receiver consisting of seven branches is simu-lated and compared with the conventional diffuse system(CDS) that employs a single wide-FOV receiver. The maingoal of this work is to improve the link performance as thetransmitter moves over the communication floor (CF)considering ambient light noise and multipath dispersion.It is noted that replacing CDS by BCM reduces the delayspread by a factor of six and increases the signal-to-noiseratio (SNR) by >30 dB compared to links utilisingCDS (at the worst CDS links) for the three mostimportant transmitter locations – room centre, room cornernear and wall.

2 System set-up and signal propagation

In this Section, the characteristics of a mobile channelformed by a multibeam transmitter are studied.The transmitted signal propagates to the receiver throughmultiple reflections from room surfaces. Propagationsimulations were conducted in an empty room with floordimensions of 8m� 4m ðlength� widthÞ and a ceilingheight of 3 m. Previous research work has shown thatplaster walls reflect a light ray in a form close to aLambertian function [1], and therefore the ceiling and thewalls of the room were modelled as Lambertian reflectors.It was found through previous investigations that thirdorder reflections and higher do not produce a significantchange in the received optical power [1, 2]. It has alsobeen demonstrated that the largest contribution to receivedpower is associated with the first and second orderreflections [8–11]. The surface element size used in thispaper was set to 20 cm� 20 cm for the second orderreflections and 5 cm� 5 cm for first order reflections. It isfound that reducing the element size to a level lower thanis set results in unjustifiably long processing time, whichextends over hours with the current state-of-the-art PC.Walls (including ceiling) and floor are modelled asLambertian reflectors of the first order with reflectivitycoefficients 0.8 and 0.3, respectively. Reflections fromdoors and windows are considered identical to reflectionsfrom walls.

The transmitter is placed on the CF, 1 m above the floor,pointed upwards, and emits 1 W total optical power with anideal Lambertian radiation pattern. Transmitter radiationcan be modelled by a generalised Lambertian radiantintensity (W=sr) [15, 1],

PðWiÞ ¼n þ 1

2p� Ps � cosnðWiÞ ð1Þ

where Ps is the total average transmitted optical powerradiated by the LED source, Wi is the emission angle withrespect to the transmitter’s surface normal, and n is themode number describing the shape of the transmitted beam.The higher the n, the narrower is the light beam, which isalso related to the half-power semi-angle (hps). The mode ncan be represented by n ¼ �0:693= lnðcosðhpsÞÞ:

A simulation tool similar to the one developed by Barryet al. [14] has been used to produce the impulse responsesand to calculate the delay spread. To model the reflections,the room reflecting surfaces were divided into a number ofequal sized square shaped reflection elements. The accuracyof the received pulse shape and the received opticalsignal power are controlled by the size of the surfaceelements. For all cases studied (the multispot channelsand the conventional diffuse link), the surface elements of

5 cm� 5 cm for the first order reflections and 20 cm�20 cm for the second order reflection were used. Thesedimensions have been selected to keep the computationwithin reasonable time and measurement. The reflectingelements have been treated as small transmitters that diffusethe received signals from their centres in the form of aLambertian pattern with a radiation lobe mode numbern ¼ 1: In all the cases studied a photodiode has been locatedat different locations on the CF, 1 m above the floor, with aphotosensitive area ðArÞ of 1 cm2: The simulations werecarried out at several receiving positions within the room.

To assess the system’s performance in a realisticsituation, eight halogen spotlights, which result in one ofthe most stringent optical spectral corruptions to the receiveddata stream, have been chosen to illuminate the environment.To evaluate the impact of ambient light, the backgroundnoise (BN) distribution pattern of an incandescent light wasinvestigated [17, 18]. Philips PAR 38 Economic (PAR38)was investigated. PAR38 emits a power of �65W in anarrow beamwidth in which it is modelled as havinga generalised Lambertian radiant intensity with ordern ¼ 33:1: The eight spotlights were placed 2 m above theCF and positioned equidistantly on the ceiling as shown inFig. 1a. These lamps produced a well illuminatedenvironment.

In OW communication links, intensity modulation withdirect detection (IM=DD) is the preferred choice [14].An indoor OW channel using IM=DD can be fullycharacterised by its impulse response h(t):

Fig. 1 Room model used for analysing nondirected spot diffusingcommunication links

a Eight, 65 W spot light lamps are mounted on the ceilingb BCM model using multibeam transmitter located at room centre(4 m, 2 m, 1 m)c BCM construction when transmitter located at room corner (l m, l m, 1 m)

IEE Proc.-Optoelectron., Vol. 151, No. 4, August 2004224

yðt;Az;ElÞ ¼XM

m¼1

R � SðtÞ � hmðt;Az;ElÞ

þXM

m¼1

nmðt;Az;ElÞ ð2Þ

where yðt;Az;ElÞ is the received instantaneous current in thephotodetector at a certain position due to m reflectingelements, t is the absolute time, Az and El are the directionsof arrival in azimuth and elevation (angle), M is the totalnumber of reflecting elements, S(t) is the transmittedinstantaneous optical power of the transmitter, � denotesconvolution, R represents the receiver responsivity, andn(t, Az, El) is the BN modelled as Gaussian noise indepen-dent of the transmitted signal. By evaluating the OWimpulse response (through a computer simulation), severalparameters can be obtained such as power spatial distri-bution, channel pulse response, SNR, and root-mean-squaredelay spread (D). Because of the diffuse transmission, anindoor OW system is subjected to multipath dispersion,which can cause pulse spread in time, and henceintersymbol interference (ISI) in the received signal. D isa good measure of signal pulse spread due to temporaldispersion of the incoming signal. The delay spread of animpulse response is given by [2, 16]

D ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiPðt � mÞ2h2ðtÞP

h2ðtÞ

sand m ¼

Pt h2ðtÞPh2ðtÞ ð3Þ

where t is the time delay associated with the received opticalpower h(t) and m is the mean delay. The discretisation is theresult of dividing the reflecting surfaces into small elements.Since the position of the transmitter, receiver and thereflecting elements are fixed, the received optical power andthe delay spread can be considered as deterministic forgiven transmitter and receiver locations.

3 Multipath propagation in an OW mobile system

3.1 Conventional diffuse system (CDS)

This is the basic configuration and has been widelyinvestigated [1–6]. The conventional diffuse link uses asingle beam transmitter with a Lambertian radiation patternand a single element receiver with a wide FOV. Forcomparison purposes, a conventional diffuse link has beensimulated to produce channel impulse responses, delayspread and SNR. The average signal power reflected by awall and detected by the detector (considering that there isno LOS component between transmitter and receiver) canbe generally computed using

Pr ¼nþ1

2p2R21R2

2

Ps Arr cosn W cosb cos g cos ddArectðd=FOVÞ

ð4Þwhere Pr is the optical power emitted by the light source,Ar is the photodetector area, r is the reflection coefficient atthe surface element (dA), b is the angle between thedirection of the ray and the normal to the surface element,g is the angle between the reflected ray and the normal to asurface element, d is the angle between the surface normalof the detector and the incident ray, R1 is the distancebetween the transmitter and the surface element, and R2 isthe distance between the surface element and the detector.

The distances jPRx��!j; jERx

��!j and jEP�!j are shown in Fig. 2

and represent distances between the receiver, a general pointP in space and the surface element E. To cover all possiblechannel characteristics of the proposed mobile system, three

different transmitter locations on the CF were chosen, atco-ordinates ðXt; Yt; ZtÞ ¼ ð1m; 1m; 1mÞ; ð2m; 4m; 1mÞand (2 m, 7 m, 1 m) (as shown in Fig. 1b and c for (1 m, 1 m,1 m) and (2 m, 4 m, 1 m)). Figure 3 displays the power levelsðmWÞ as a function of time (ns) in a table that shows thereceiver position (x, y), which refer to the correspondingCartesian co-ordinates of the simulation room. For com-parison purposes, the main Figure (at each location (x, y))combines six impulse responses for both beam clusteringand CDS configurations, which reflect the three transmitterlocations (1 m, 1 m, 1 m), (2 m, 4 m, 1 m) and (2 m, 7 m,1 m). Furthermore, for clarity purposes, each receiverposition in Fig. 3 includes three small Figures that showthe impulse response of the beam clustering method (whichwill be described in Section 3.2) and CDS at certaintransmitter locations. Three different Figures for threedifferent transmitter positions ðXt; Yt; ZtÞ¼ð1m; 1m; 1mÞ;ð2m; 4m; 1mÞ and (2 m, 7 m, 1 m) are displayed. Con-sidering the mobility as well as the diffuse link effects, itshould be noted from Fig. 3 that CDS has the lowestreceived optical power and the largest delay spread becauseit utilises a wide beam transmitter and a wide FOV receiver,and owing to the absence of the direct path componentbetween the transmitter and the receiver, the path loss islarger. The pulse spread is significant due to the large rangeof angles accepted by the wide FOV receiver.

3.2 Beam clustering method for an OWcommunication system

In this paper we extend the treatment and consider a novelfully mobile diffuse system that extends the work in [11].A new structure of spot distribution based on a newclustering method is proposed and examined. For this case,where 100 diffusing spots are employed and where the totalpower is to remain constant at the same level as CDS, eachspot contributes 10 mW. The BCM employs three clusters ofspots distributed as follows: 60% of the total spots on theceiling and 20% on each wall. The number of spots used inBCM has been chosen to achieve high SNR and low delay

Fig. 2 Physical structure of angle diversity receiver with sevenbranches, and azimuth and elevation parameters for diversitydetection model

IEE Proc.-Optoelectron., Vol. 151, No. 4, August 2004 225

Fig. 3 Impulse responses received by BCM and CDS

x-axis represents time (ns) and y-axis represents received optical power in mW: These charts are displayed for various receiver locations on the CF ðx=yÞ and forthree transmitter locations at (1 m, 1 m, 1 m), (2 m, 4 m, 1 m) and (2 m, 7 m, 1 m)

IEE Proc.-Optoelectron., Vol. 151, No. 4, August 2004226

spread in all receiver positions on the CF. Furthermore, theBCM has been constructed to alleviate the poor perform-ance when mobility is an issue. In a way, the BCM structurehas the ability to view and cover its surroundings throughthe three clusters of diffusing spots, which gives the receiveran option to collect the signals through the nearest diffusingspots and the shortest path. To help visualise the mobileBCM configuration, Figs. 1b and c show a limited numberof diffusing spots in an optical wireless communicationsystem. The ratio of power on the ceiling against power onthe walls can further be optimised. The ratio selected isbased primarily on the ratio of usable surface areas in theceiling and the walls given user motion and possibleshadowing. Furthermore, our work can be extended toconsider 3-D beam clustering where, in our currentapproach, the beam clusters all fall on a 2-D plane. Themultibeam transmitter is assumed to produce 100 � 1beams that form three groups of spots aimed at the threedifferent surfaces (ceiling and two walls). For simplifica-tion, BCM at the room centre has been used as a referencepoint for diffusing spots calculations and for the othertransmitter locations. A line of diffusing spots is created inthe middle of the ceiling and walls at x ¼ 2m and along they-axis as shown in Fig. 1b. The difference in distancebetween the adjacent spots is 10 cm. These spots becomesecondary distributed emitters, which emit Lambertianradiation. The ceiling and the walls of the room weremodelled as Lambertian reflectors of first order reflectionðn ¼ 1Þ:

To simulate the proposed system under the mobilityeffect, the transmitter was placed at various locations on theCF. For each transmitter movement, simulation compu-tations were carried out for 14 different receiver positions onthe CF. The received multipath profiles were stored for eachspot at each location. The resultant power profile is the sumof the powers due to the 100 impulse responses that reflectthe 100 spots.

In contrast to the preceding Sections, where a singlewide-FOV receiver is employed, in this Section the receiveris a collection of narrow-FOV receivers oriented in differentdirections, forming an angle diversity configuration. Theoptical signal power received in the various receivers can betreated separately, and can be processed using severaltechniques such as combining or selection methods.Furthermore, to combat background noise as well asmultipath dispersion, diversity detection is an appropriatechoice to achieve significant performance improvements.The diversity detectors are designed especially for the BCMsystem and are investigated in this work. By using such aconfiguration, and by optimising the FOV, directionalinterference and multipath dispersion can be minimised.The receiver diversity system considered consists of sevenphotodetector branches. Each face bears a certain directionthat can be defined by two angles: azimuth (Az) andelevation (El) angles. While the El of six branches remainsat 20�; the seventh branch faces up with an El of 90�; and theAzs for the seven branches of the receiver are fixed at 0�;55�; 90�; 125�; 235�; 270� and 305�. While the photosen-sitive area of each photodetector remains 1 cm2; their FOVswere restricted to 12�; 30�; 25�; 30�; 30�; 25� and 30�;respectively. FOV can be set through careful design of theoptical concentrator at the front of the detector. Selectioncombining was used and therefore fair comparison ispossible between the diffuse system and BCM diversitysystem even when each branch of the latter uses a 1 cm2

detector. The Az, El and FOVs were chosen through anoptimisation [3, 17] to achieve the best SNR consideringthe transmitter locations as well as the system motion.

Moreover, the angle diversity receiver is designed so that atleast five diffusing spots are always positioned within thereceiver FOV, providing a robust link against diffusing spotblockage. Using such a configuration also leads to anadditional degree of freedom that can be used to eliminate alarge amount of the background interference as well as toreduce the multipath dispersion effects.

Owing to the inclination of the receiver branch normals,the single detector with a wide FOV analysis and simulations,which assume an upwards-facing detector, has to bemodified. Compared to the optical signal analysis that wasused in Section 3.1, where the vector normal to the receiver isalso perpendicular to the CF, changes in the calculations forthe received power analysis need to be considered in the caseof the diversity receiver. The reception angle d can becalculated by employing the trigonometry of rectangulartriangles, in which Az and El angles for each detector areconsidered. To compute the reception angle d for anydetector, a point P has been defined, located on the detector’snormal, 1m above the detector. Furthermore, light from areflecting point (E) on a wall can be received by one of thedetectors ðRxÞ as shown in Fig. 2. Consequently, (4) can berewritten taking into account the detector orientation, and canbe given as

Pr ¼n þ 1

2p2R21R2

2

Ps Ar r cosn W cos b cos g

�PRx��!��� ���2þ ERx

��!��� ���2� EP�!��� ���2

2 PRx

��!��� ���ERx

��!��� ���264

375 rectðd=FOVÞ dA ð5Þ

To be able to receive the incident ray in (4), d must lie withinthe detector’s FOV, i.e. the detector will receive power if andonly if the reception angle lies within the FOV. This processis explained by the rectangular function rectð:Þ as in (5).

Similarly to the CDS configuration, the transmitteris located at co-ordinates ðXt; Yt; ZtÞ ¼ ð1m; 1m; 1mÞ;ð2m; 4m; 1mÞ and (2 m, 7 m, 1 m), while the receiver ismoved over 14 positions at co-ordinates x ¼ 1m and x ¼ 2mand along the y-axis. Owing to the various mobilityscenarios, diffusing spot locations and their beam angleshave been computed with respect to the transmitter locationat the room centre, which has been considered as a referencepoint for the BCM mobile system. Throughout this paper, allcomputations of spot incidence angles W; spots height(especially on the wall) and space between spots are carriedaccording to the reference transmitter point, which assumes adownwards-facing spot. To include spots mobility. furtheranalysis should be considered. The spot incidence angle Wcan be calculated by employing the trigonometry ofrectangular triangles, in which spot angles at the referencepoint are taken into account for each transmitter movement.Owing to transmitter mobility, the spot heights hs as well astransmission beam angles a have to be modified based on spotlocations on the ceiling and spot incidence angle. Therefore,a is given by

beam angle � a ¼ tan�1 d

hs

�ð6Þ

where d is the horizontal separation distance between thenormal projection of the transmitter and the diffusing spot.For the reference case (transmitter at the room centre),transmitter beam angle a and spot incident angle W are equal,as shown in Fig. 4. As the transmitter starts moving, the spotpositions and heights change accordingly. At certainlocations on the floor, some of the spots on the wall start to

IEE Proc.-Optoelectron., Vol. 151, No. 4, August 2004 227

appear on the ceiling and vice versa; for instance, at locationsclose to the corner (transmitter at room corner ðXt; Yt; ZtÞ ¼ð1m; 1m; 1mÞÞ; some of the wall spots appear on the ceilingas shown in Fig. 1b. Spots height Zs on the wall can becalculated, taking into account transmitter beam angle asfollows:

Zs ¼d

tan að7Þ

Owing to BCM movement as well as the presence ofdiffusing spots on the wall, the conventional spot distributionanalysis as in [6, 11], which assumes downward facing spots,has to be modified considering the new spot locations.

The impulse response h(t) of the spot diffusing link(considering the angle diversity receiver) differs signifi-cantly from that of the CDS, in particular with reference tothe received power. In this paper, we consider a selectioncombining for processing the received optical signal, wherethe best received signal (SNR) was selected among thesignals that were received by the diversity receiver at agiven position. Because of different LOS paths betweenthe line diffusing spots and the receiver, h(t) consists ofmany peaks, as shown in Fig. 3. The BCM channel impulseresponses shown in this figure contain the LOS componentsbetween diffusing spots and receiver. For comparisonpurposes, the transmitter and the diversity receiver wereplaced at different locations on the floor as in Fig. 3, whichinclude one of the worst communication links, as in corners.For comparison, another of the worst communication linkswas chosen. The transmitter and the receiver locations werechosen at the room corner ðXt; Yt; ZtÞ ¼ ð2m; 7m; 1mÞ andðXr; Yr;ZrÞ ¼ ð1m; 1m; 1mÞ; respectively, to examine thesystem quality at the worst reception case. At this location,the impulse response of the CDS has the lowest power,whereas the BCM system has a LOS component betweenone of the spot clusters and the receiver, thus offering higherpower at lower delay spread. Furthermore, it is noted thatfor the CDS, as the distance between the transmitter and thereceiver becomes large, the power of the collected opticalsignal decreases rapidly and thereby the total coverage isreduced. Impulse responses, however, at different locationson the CF give a very good visualisation of the powerdistribution, where the received optical power clearlydecreases towards the room corners as shown in Fig. 3.

It is clearly seen that the BCM structures are significantlybetter than the CDS in terms of received power and signalspread. This is due to the fact that the impulse response of thisconfiguration contains many peaks corresponding to thedifferent direct LOSs between the diffusing spots and thereceiver. Generally, the results have also shown that most of

the collected signal is in the first order reflection, concen-trated within a very short time period due to direct LOS.Furthermore, regardless of the transmitter and receiverpositions, BCM in all locations leads to a higher receivedpower compared to the wide diffuse transmitter system.

4 Simulation results

In this Section, we investigate the performance of anondirected mobile OW system in terms of delay spreadand SNR. Additionally, the system is assessed and evaluatedin the presence of background noise. To evaluate the systemperformance in a BN environment and under the moststringent conditions, neither optical filter nor opticalconcentrator have been used. BCM in conjunction withangle diversity is discussed and compared with CDS.

4.1 Delay spread analysis

Delay spread calculations were performed for both BCMand CDS in 14 different locations along the y-axis atconstants x ¼ 1m and x ¼ 2m: A better perspective on howdelay spread varies with receiver position within a room canbe obtained by plotting delay spread values at differentlocations, as in Fig. 5. Figures 5a and 5b show the delayspread performance of a mobile nondirected BCM con-figuration as well as a mobile CDS at three transmitterlocations (1, 1, 1), (2, 4, 1) and (2, 7, 1) considering areceiver positioned on CF; x ¼ 1m and x ¼ 2 and along they-axis. For the CDS case, where a single wide FOV receiveris used, the delay spread is clearly larger than that of theBCM over the entire communication floor. Figure 5 alsoshows, for the CDS case, that there is a direct relationbetween the delay spread and the distance from thetransmitter. Moreover, Fig. 5 shows that the delay spreadgradually increases towards the room corners and when thereceiver moves away from the transmitter, which applies forall transmitter locations.

In contrast to the CDS configuration, the delay spreadvariation, for the case of the spot diffusing technique whereBCM is employed, is small (towards the room corners),which is due to the presence of spot diffusing transmissionpoints near the room corners that appeared on the nearest walland the lower contribution of the far spot points. Further-more, the delay spread values increase slightly in the areaswhere the diffusing spot illumination is low. Comparing thecases of the BCM technique and the CDS when a single widereceiver is used, it is to be noted that the lowest delay spreadvalues are obtained by the proposed BCM with anglediversity receiver. The smallest delay spread associated withthe BCM and seven narrow branches receiver is 0.19 ns,

Fig. 4 Two cases of spots distribution at two different places (ceiling and wall) and for two different transmitter positions

IEE Proc.-Optoelectron., Vol. 151, No. 4, August 2004228

0.359 ns and 0.19 ns at transmitter locations (1 m, l m, 1 m),(2 m, 4 m, 1 m) and (2 m, 7 m, 1 m), respectively, which are afactor of about 10.4, 40 and 10 lower than the smallest delayspread associated with the CDS when a single wide-FOVreceiver is used, and at the same transmitter locations.Additionally, the maximum delay spread associated with theBCM and angle diversity receiver is also low and has valuesof 1.28 ns, 2.4 ns and 1.6 ns at transmitter locations (1 m, 1 m,1 m), (2 m, 4 m, 1 m) and (2 m, 7 m, 1 m), respectively. This isabout 2, 1 and 2, respectively, lower than the maximum delayspread associated with the CDS. At the worst reception pointin a CDS, for example when transmitter is placed at (1 m, 1 m,1 m) and the receiver at (1 m, 7 m, 1 m), the delay spread isreduced by nearly a factor of six. Consequently, the mobileBCM with angle diversity structure has resulted in asignificant improvement over the CDS configuration in thevarious metrics considered, shown in the impulse responsesin Fig. 3. The redistribution of the diffusing spots in the formof beam clustering (ceiling and walls distribution), inparticular when accompanied by narrow directive FOVreceivers, can achieve improvement, in all transmitterlocations, in both delay spread and power reception.

4.2 Signal-to-noise ratio calculations for amobile OW communications link

Indoor OW communications links are strongly impaired bythe shot noise induced by ambient light noise. Under theseconditions, the thermal noise at the receiver can be neglectedcompared to the shot noise [2]. The received pulse shapes forthe presented mobile configurations have been considered incalculating Ps1 and Ps0; the power associated with logic 0 andlogic 1, respectively. The probability of error ðPeÞ of theindoor OW communication system can be written as

Pe ¼ QðffiffiffiffiffiffiffiffiffiSNR

pÞ ð8Þ

where

QðxÞ ¼ 1ffiffiffiffiffiffi2p

pZ 1

xe� z=

ffiffi2

pð Þ2

dz

is the Gaussian function which assumes a value of 6 ata probability of error Pe ¼ 10�9; and the SNR of 36(15.6 dB) taking Ps1 and Ps0 into account (hence ISI) isgiven by

SNR ¼ R � ðPs1 � Ps0Þst

�2

ð9Þ

where R ¼ 0:5A=W is the photodetector responsivity and s2t

is the total noise variance. Equation (9) has considered theimpact of the pulse spread caused by the ISI, where Ps1 andPs0 account for the eye closure at the sampling instant. Thepulse response was found through convolution of the impulseresponse with a rectangular transmitted pulse of 20 nsduration. This corresponds, to a 50 Mbit=s bit rate and thereceiver used had a bandwidth of 70 MHz [19], whichensures that it does not cause an extra dispersion at the bitrate chosen. Furthermore, evaluation of pulse responses wascarried out over the entire communication floor.

The total noise variance given in (9) can be classified intothree categories: background light-induced shot noise ðsbnÞ;noise induced by the received signal power, which consistsof two components; shot noise current ðss1Þ when a ‘1’ isreceived, shot noise current ðss0Þ when a ‘0’ is received, andreceiver noise that is normally generated in the preamplifiercomponents ðsprÞ:

sbn ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2 � q � Pbn � R � BW

pð10Þ

spr ¼ 2:7 � 10�12 �ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi70 � 106

p¼ 0:023 mA ð11Þ

Fig. 5 RMS delay spread distribution of a mobile OW system for BCM and CDS at constant x

Receiver position x ¼ (a) l m and (b) 2 m, along y-axisTransmitter at: (i) (1 m, 1 m, 1 m); (ii) (2 m, 4 m, 1 m); (iii) (2 m, 7 m, 1 m)

IEE Proc.-Optoelectron., Vol. 151, No. 4, August 2004 229

where q, Pbn and BW are the electron charge, received back-ground optical power and receiver bandwidth, respectively.The preamplifier used in this study is the PIN-BJT designproposed by Elmirghani et al. [19]. This preamplifier has anoise current density of 2:7 pA=

ffiffiffiffiffiffiHz

pand a bandwidth of

70 MHz.Hence, st is s0 þ s1; which represent the noises

associated with the signal and can be obtained as

s0 ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffis2

pr þ s2bn þ s2

s0

qand s1 ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffis2

pr þ s2bn þ s2

s1

qð12Þ

The assumption of Gaussian noise statistics holds in ourcase, since thermal and shot noise can be accuratelymodelled as Gaussian processes. It is to be noted that thenoise is spatially asymmetric and therefore the SNR isposition-dependent. The signal-dependent noise is small inmagnitude compared to the background induced shot noiseand therefore the Gaussian noise assumption holds. Asmentioned, we consider a selection combining technique forprocessing the received optical signal where the detectorthat has the highest SNR is chosen. Substitution of (12) into(9) and considering the diversity receiver with sevenbranches gives

SNR ¼ maxi

RðPs1 � Ps0Þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffis2

pr þ s2bn þ s2

s0

qþ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffis2

pr þ s2bn þ s2

s1

q0B@

1CA

2

;

1 � i � 7

ð13Þ

Figure 6 shows the detected SNR, for the mobile OWsystem when BCM and CDS are used. The performance

evaluation was carried out for three transmitter locations(1 m, 1 m, l m), (2 m, 4 m, 1 m) and (2 m, 7 m, 1 m) and over14 receiver positions for each transmitter location. Further-more, calculations were considered under the constraints ofbackground noise (eight directed spotlights) and multipathdispersion, at the x co-ordinate that contains the weakest andthe strongest received optical signal power along the y-axis.Note also that neither optical concentrator nor optical filterwas used. In addition, Fig. 6 shows that for CDSconfiguration, SNR is maximum (along the y-axis) at pointsfar away from the noise sources and close to the transmitter.For example, in Fig. 6b (CDS) when the transmitter is at(1 m, 1 m, l m) the SNR has a clear decaying trend as yincreases from y ¼ 1m to y ¼ 7m with clear peaks andtroughs that reflect the background noise due to spotlightlamps. When the transmitter is placed at (2 m, 7 m, 1 m),almost the opposite trend is observed. Finally, when thetransmitter is at (2 m, 4 m, 1 m) (room centre) the SNR isbest near the room centre and decays towards the walls withthe district peaks and troughs due to background noise. Thisis attributed to two facts: firstly, that the distance betweenthe transmitter and the receiver is minimum (at this receiverlocation) compared to other locations, resulting in a strongreceived signal; and secondly, the noise distribution has avery low value at certain locations on the CF where thereceiver is not underneath a noise source. Since the noisesources are very directed ðn ¼ 33:1Þ; their impact is mostlyconcentrated underneath the source, which might result indata burn-out. The effect of those sources are clearly shownat points close to the noise sources such as underneath thelamps at (1 m, 1 m, l m), (1 m, 3 m, l m), (1 m, 5 m, 1 m) and(1 m, 7 m, 1 m) as shown in Fig. 6. Therefore, the SNRexperiences very low levels in particular at those points.

Fig. 6 SNR performance under eight spotlights and multipath dispersion effects for a BCM and CDS at constant x

Receiver position x ¼ (a) 1 m and (b) 2 m, along y-axisTransmitter at: (i) (1 m, 1 m, 1 m); (ii) (2 m, 4 m, 1 m); (iii) (2 m, 7 m, 1 m)

IEE Proc.-Optoelectron., Vol. 151, No. 4, August 2004230

The delay spread in Fig. 5 for the transmitter at (1 m, 1 m,1 m) is a maximum at y ¼ 5m;whereas SNR is a minimum aty ¼ 6m in Fig. 6. This apparent discrepancy can beexplained as follows: the SNR is a function of backgroundnoise, delay spread and path loss. The effect of backgroundnoise is minimised through the use of diversity detection.Therefore, path loss and delay spread play the mostsignificant rules. Figure 1c shows the (1 m, 1 m, 1 m)transmitter beam clustering and spots. Along x ¼ 1m andat y ¼ 7m; the receiver is close to the spot cluster on they ¼ 8m wall. At y ¼ 5m; the receiver is close to the maincluster and a minimum power is received at y ¼ 6m; givingrise to the low SNR at this point. By comparing the resultsshown in Fig. 6, for the mobile BCM and CDS systems, it canbe seen that, in spite of moving the transmitters over threedifferent locations, the improvement in signal reception,when BCM is used, is clearly visible over the selectedlocations (14 different positions) on the CF. This is attributedto the small differences in distance between the transmitter(spots on walls and ceiling) and the receiver. In view of thefact that the weakest point in a communication link is thecriterion of the system quality, the minimum SNRs of the twoconfigurations have been compared. Comparing the cases ofthe BCM technique and the CDS, it is to be noted that the bestSNR values are obtained by the proposed BCM with anglediversity receiver. The smallest SNRs associated with theBCM and seven narrow branches receiver are �16 dB; and17 dB and 16 dB at transmitter locations (1 m, 1 m, l m), (2 m,4 m, 1 m) and (2 m, 7 m, 1 m), respectively, while for the CDSthey are ��14:5 dB; �2 dB and �13:9 dB when a singlewide-FOV receiver is used. Additionally, the maximum SNRassociated with the BCM is also significant and has values of37.6 dB, 27.8 dB and 38 dB at transmitter locations (1 m, 1 m,1 m), (2 m, 4 m, 1 m) and (2 m, 7 m, 1 m), respectively, whilefor the CDS they are �27:1 dB; 27.5 dB and 27.8 dB.

Consequently, BCM in a mobile OW environment can beemployed to improve the overall system performance.An angle diversity detection technique would also seem tobe an appropriate choice for reducing the BN and multipathdispersion effects as it selectively confines the range ofreception angles.

5 Conclusions

A novel beam clustering has been presented and shown to bea promising means for improving the performance of mobilenondirected optical wireless links. The BCM in conjunctionwith an angle diversity receiver has reduced the impact ofmultipath dispersion and ambient light noise, enabling high-speed applications. Furthermore, BCM links performancehas been evaluated in terms of delay spread and signal-to-noise ratio. A composite receiver with seven branches isassumed with the BCM. Although CDS with a single wide-FOV receiver improves the system reception in the absenceof noise, it is very sensitive to ambient light noise andmultipath distortion. In contrast, BCM provides robust

communication links tolerant to background noise andmultipath dispersion. Replacing CDS by BCM reduces thedelay spread by a factor of six and increases the SNR by>30 dB at the worst communication links and producesimprovements at other transmitter locations. It is expectedthat the performance of BCM will improve if a diversityreceiver with more branches is used.

6 References

1 Gfeller, F.R., and Bapbst, U.H.: ‘Wireless in-house data communicationvia diffuse infrared radiation’, Proc. IEEE, 1979, 67, (11),pp. 1474–1486

2 Kahn, J., and Barry, J.: ‘Wireless infrared communications’, Proc.IEEE, 1997, 85, (2), pp. 265–298

3 Al-Ghamdi, A.G., and Elmirghani, J.M.H.: ‘Performance evaluation ofthe triangular pyramidal fly-eye diversity detector for optical wirelesscommunications’, IEEE Commun. Mag., 2003, 41, (3), pp. 80–86

4 Carruthers, J.B., and Kahn, J.M.: ‘Angle diversity for nondirectedwireless infrared communication’, IEEE Trans. Commun., 2000, 48,(6), pp. 960–969

5 Djahani, P., and Kahn, J.M.: ‘Analysis of infrared wireless linksemploying multibeam transmitters and imaging diversity receivers’,IEEE Trans. Commun., 2000, 48, (12), pp. 2077–2088

6 Al-Ghamdi, A.G., and Elmirghani, J.M.H.: ‘Spot diffusing techniqueand angle diversity performance for high speed indoor diffuseinfrared wireless transmission’, IEE Proc. Optoelectron., 2004, 151,(1), pp. 46–52

7 Tang, A.P., Kahn, J.M., and Ho, K.: ‘Wireless infrared communicationslinks using multi-beam transmitters and imaging receivers’. Proc.IEEE Int. Conf. on Communications, Dallas, TX, USA, June 1996,pp. 180–186

8 Yun, G., and Kavehrad, M.: ‘Spot diffusing and fly-eye receivers forindoor infrared wireless communications’. Proc. 1992 IEEE Conf. onSelected Topics in Wireless Communications, Vancouver, BC, Canada25–26 June 1992, pp. 286–292

9 Jivkova, S., and Kavehard, M.: ‘Multispot diffusing configuration forwireless infrared access’, IEEE Trans. Commun., 2000, 48, (6),pp. 970–978

10 Carruthers, J.B., and Kahn, J.M.: ‘Angle diversity for nondirectedwireless infrared communication’. Proc. IEEE Int. Conf. on Communi-cations, Atlanta, USA, 1998, pp. 1665–1670

11 Al-Ghamdi, A.G., and Elmirghani, J.M.H.: ‘Line strip spot-diffusingtransmitter configuration for optical wireless systems influenced bybackground noise and multipath dispersion’, IEEE Trans. Commun.,2004, 52, (1), pp. 37–45

12 Jivkova, S., and Kavehrad, M.: ‘Receiver designs and channelcharacterization for multi-spot high-bit-rate wireless infrared com-munications’, IEEE Trans. Commun., 2001, 49, (12), pp. 2145–2153

13 Jivkova, S., and Kavehrad, M.: ‘Indoor wireless infrared localaccess, multi-spot diffusing with computer generated holographicbeam-splitters’. Proc. IEEE Int. Conf. on Communications, 1999,vol. 1, pp. 604–608

14 Barry, J.R., Kahn, J.M., Krause, W.J., Lee, E.A., and Messercgmitt,D.G.: ‘Simulation of multipath impulse response for indoor wirelessoptical channels’, IEEE J. Sel. Areas Commun., 1993, 11, (3),pp. 367–379

15 Carruthers, J.B., and Kannan, P.: ‘Iterative site-based modeling forwireless infrared channels’, IEEE Trans. Antennas Propag., 2002, 50,(5), pp. 759–765

16 Carruthers, J.B., Carroll, S.M., and Kannan, P.: ‘Propagation modellingfor indoor optical wireless communications using fast multireceiverchannel estimation’, IEE Proc. Optoelectron., 2003, 150, (5),pp. 473–481

17 Al-Ghamdi, A.G., and Elmirghani, J.M.H.: ‘Optimisation of atriangular PFDR antenna in a fully diffuse OW system influenced bybackground noise and multipath propagation’, IEEE Trans. Commun.,2003, 51, (12), pp. 2103–2114

18 Boucouvalas, A.C., and Barker, P.: ‘Asymmetry in optical wirelesslinks’, IEE Proc. Optoelectron., 2000, 147, pp. 315–321

19 Elmirghani, J.M.H., Chan, H.H., and Cryan, R.A.: ‘Sensitivityevaluation of optical wireless PPM systems utilising PIN-BJTreceivers’, IEE Proc. Optoelectron., 1996, 143, (6), pp. 355–359

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