Performance comparison of LSMS and conventionaldiffuse and hybrid optical wireless techniques in areal indoor environment

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

Abstract: Performance evaluations were carried out for indoor optical wireless systems in threedifferent environments. Channel characteristics of infrared links have been evaluated under variousconditions including an empty room, a room with glass windows and a door, and a room that hasvery strong shadowing effects caused by mini cubical offices. The corresponding results have beenanalysed and compared for conventional diffuse/hybrid systems (CDS/CHS) and line stripmultibeam transmitter systems (LSMS) that use spot diffusing techniques. Eight directive lampsrepresented the ambient light noise and allowed further signal impairments to be taken into account.Under these conditions, results indicate that LSMS can reduce pulse spread by factors of 11 and 24compared to the maximum delay spread associated with CDS and CHS respectively. Furthermore,LSMS links can increase the SNR by more than 50 dB compared to the CDS and CHS.

1 Introduction

The growing demand for high-speed wireless communi-cation in an indoor environment has encouraged the use ofoptical wireless (OW) links as an information carrier [1].The infrared spectrum is being considered owing to itswider bandwidth and inexpensive optoelectronic devicessuch as light emitting diodes and silicon detectors.Furthermore, it is capable of offering unregulated largebandwidth and, since it is confined to the room where itoriginates, it does not interfere with other electronicequipment. OW, however, is not without drawbacks; thetwo main limitations to optical wireless transmission arebackground noise and multipath propagation that causespulse spread and intersymbol interference.

The two major OW transmission link configurations aredirect light-of-sight (DLOS) and non-directed (diffuse) links[2–5]. DLOS links improve power efficiency and minimisemultipath dispersion, but require inherent alignmentbetween transmitter and receiver in order to establishcommunication. The main drawback of this configuration isthat it is susceptible to shadowing. Non-directed (diffuse)transmission links, however, allow the system to operateeven when barriers are placed between the transmitter andthe receiver, which results in independence of the directpath, or alignment. In spite of the advantages of the non-directed transmission link, it is affected by multipathdispersion, which causes pulse spreading and severe

intersymbol interference (ISI) in addition to higher opticalpower path losses than the DLOS link.

A possible technique that can increase the receivedoptical power, mitigate the shadowing effect, and reducemultipath dispersion is the multibeam transmitter [6–8].Systems that adopt this approach possess the advantages ofthe DLOS and overcome the drawbacks of the diffuse links(that appear in a form of multipath distortion). Proposalsthat utilise this method of transmission have includeduniform distribution of multiple diffusing spots, producedby a multibeam transmitter, which cover the whole roomceiling [9]. Although an improvement in performance wasachieved, the proposed structures accomplished it at aconsiderable increase in complexity.

In this work, we extend the treatment in [10, 11] andconsider a harsh environment, which is typically encountedin real office configurations where optical signal blockage(owing to cubicles), windows, doors, furniture, ambientlight noise, and multipath propagation all exist. Forcomparison purposes two different configurations based onLOS and diffuse optical wireless links are considered andanalysed for three room scenarios: an empty room, a roomwith only windows and a door, and finally a realenvironment that consists of windows, a door, mini-cubicles, bookshelves, and other objects. The proposedtransmission configurations are LOS, which takes the formof a conventional hybrid system (CHS) (broad beamtransmitter in the ceiling and receiver on communicationfloor), and diffuse systems that consist of two separateconfigurations: conventional diffuse systems (CDS) and linestrip multibeam transmitter systems (LSMS). The difficul-ties associated with all three configurations are the ability tomaintain an acceptable optical link between transmitter andreceiver at all locations on the communication floor (CF).This paper proposes a line strip multibeam transmittersystem combined with angle diversity detection (only threereceivers). We will refer to this configuration as an LSMS.This proposed configuration produces an increase in thereceived optical power level in addition to a reduction inmultipath dispersion. This is achieved by employing a lesscomplex multibeam transmitter structure. We show that

q IEE, 2005

IEE Proceedings online no. 20045009

doi: 10.1049/ip-opt:20045009

A.G. Al-Ghamdi is with the National Information Centre, P.O. Box 64565,Riyadh 11546, Saudi Arabia

J.M.H. Elmirghani is with the School of Engineering, University of WalesSwansea, Singleton Park, Swansea SA2 8PP, UK

E-mail: agghamdi@nic.gov.sa

Paper first received 27th May 2004 and in revised form 24th February 2005.Originally published online 5th July 2005

IEE Proc.-Optoelectron., Vol. 152, No. 4, August 2005230

LSMS provides a significant signal-to-noise ratio (SNR)improvement over both conventional optical wirelesssystems (CDS and CHS). For example, the use of LSMShas demonstrated an improvement of about 40 dB overCDS systems.

2 Simulation model

2.1 Room configurations

In this Section, the characteristics of the channel formed byOW systems are studied and compared with CDS and CHS.The transmitted signal propagates to the receiver throughmultiple reflections from room surfaces. The simulationmodel was developed using an example 8m� 4m roomhaving a ceiling height of 3m for three different roomconfigurations denoted as A, B, and C as shown in Fig. 1.Figure 1 shows Room A as an empty room, while Room Bhas three large glass windows and a door. Room C in Fig. 1contains a number of rectangular-shaped cubicles withsurfaces parallel to the room walls, and other furniture suchas bookshelves and filing cabinets. The walls and ceiling, inRoom A, have a diffuse reflectivity of 0.8, while the floorhas a 0.3 diffuse reflectivity. The glass windows and door in

Room B are expected not to reflect any signal hence theirdiffuse reflectivities are set to zero. In Room C, the wallssurrounding windows and the ceiling have a diffusereflectivity of about 0.8, while floor has a 0.3 diffusereflectivity. Two walls (except the door) of Room C arecovered with bookshelves and filing cabinets with a diffusereflectivity of 0.4. It is assumed that signals, which reach thecubical office partitions, are either absorbed or blocked.Furthermore, several tables and chairs within the CF areaare placed in the room with the same floor diffusereflectivity. The complexity is clearly seen in Room Cwhere physical partitions and low reflectivity objects resultin the worst reception environment where shadowing iscreated. Previous research work has shown that plaster wallsreflect light rays in a form close to a Lambertian function[4]. Therefore, reflection elements were modelled asLambertian reflectors.

A custom simulation package similar to the onedeveloped by Barry et al. [12] has been constructed andused to produce the impulse responses, power distribution,and to calculate the delay spread and path loss. To model thereflections, the room reflecting surfaces were divided into anumber of equal size square shaped reflection elements.The accuracy of the received pulse shape, and the received

Fig. 1 Three indoor scenarios, Room A, Room B, and Room C

IEE Proc.-Optoelectron., Vol. 152, No. 4, August 2005 231

optical signal power are controlled by the size of the surfaceelements. For all configurations, (the multispot channelsand the conventional diffuse=hybrid link), surface elementsof 5 cm� 5 cm for the first order reflections, and 20 cm�20 cm for the second order reflections were used. Thesedimensions have been selected in order to keep thecomputation within reasonable time and measure. Thereflecting elements have been treated as small transmittersthat diffuse the received signals from their centres in theform of Lambertian pattern with a radiation lobe modenumber n ¼ 1. Using our developed OW simulationpackage, it is found from the parameters above that thetime needed to simulate an OW system at a certain receiverposition has been significantly reduced to less than 20seconds taking into account all types of configurations withand without furniture. In all the cases studied a singlephotodiode has been located at different locations on the CF,1m above the floor, with a photosensitive area (Ar) of1 cm2, and with a wide angle of reception of 180� for theconventional system. A diversity receiver with threedetectors for the case of the LSMS was used. Thesimulations were carried out at several receiving positionswithin the room.

The proposed systems have been analysed in the presenceof directive noise sources. In order to assess the system’sperformance in addition to examining the advantages ofhaving an LSMS system in a very distorted environment,eight halogen spotlights, which result in one of the moststringent optical spectral corruptions to the received datastream [13–15], have been chosen. To evaluate the impactof ambient light, the background noise (BN) distributionpattern of an incandescent light was investigated. ‘PhilipsPAR 38 Economic’ (PAR38) was investigated. PAR38emits a power of about 65W in a narrow beamwidth whichis modelled as a generalised Lambertian radiant intensitywith order n ¼ 33:1, which corresponds to a semi-angleof 11:7�. The eight spotlights were equidistantly placedin the ceiling 2 m above the CF at coordinatesðX; Y ; ZÞ ¼ ð1; 1; 3Þ, (1, 3, 3), (1, 5, 3), (1, 7, 3), (3, 1, 3),(3, 3, 3), (3, 5, 3), and (3, 7, 3) [13]. These lamps produced awell-illuminated environment. Furthermore, simulation ofthe optical noise power along both axes of CF was carriedout in steps of 10 cm. In all cases studied, a photodiode wasplaced at different locations on the CF with a photosensitivearea of 1 cm2.

2.2 Transmitter model

For the proposed configurations, the transmitter is placed inthe middle of the CF, one metre above the floor atð4m� 2m� 1mÞ, for the case of diffuse systems (CDSand LSMS) and at the middle of ceiling at ð4m� 2m�3mÞ for the CHS configuration. It is modelled as ageneralised Lambertian emitter, with a radiant intensity(W=sr) given by

RðjÞ ¼ nþ 1

2p� Ps � cosnðjÞ ð1Þ

where Ps is the total average transmitted optical powerradiated by the laser=LED source, j is the angle ofincidence with respect to the transmitter’s surface normal,and n is the mode number describing the shape of thetransmitted beam, the higher the mode n the narrower thelight beam, which is also related to the half-power semi-angle (hps) of the transmitter pattern. For all cases studied,the transmitter emits 30 dBm total optical power with anideal Lambertian radiation pattern which corresponds to ahalf-power angle hps ¼ 60�. The amount of the power used,

in this paper, has been applied for one transmitter underdifferent configurations. The case of multiple transmitters isnot considered. The mode number (n) can be given byn ¼ �0:693= lnðcosðhpsÞÞ.

2.3 Single detector reception analysis in aconventional OW system

For the case of conventional diffuse=hybrid system, a wideFOV single detector is employed. Impulse responses thattake into account all relevant reflections were calculated fora range of receiver positions. In order to compute thetemporal distribution of the received signal power, whichreflects the impulse response characteristics, the test room’swalls and ceiling have been divided into equal squarereflection elements with an area dA. The reflection elementswere treated as small emitters that diffuse the receivedsignal from their centres in the form of a Lambertian pattern.The average signal power reflected by a wall and detectedby the detector can be written as

dpr ¼nþ 1

2:p2:R21:R

22

:Ps:Ar:r: cosnðWiÞ: cosðbÞ: cosðgÞ

: cosðdÞ:dA:rectðd=FOVÞ ð2Þ

where r is the reflection coefficient at the surface element, bis the angle between the direction of the ray and the normalto the surface element, g is the angle between the reflectedray and the normal of dA, d is the angle between the surfacenormal of the detector and the incident ray, R1 is thedistance between the transmitter and the surface element dA,and R2 is the distance between the surface element and thereceiver. The step function rectðd=FOVÞ describesthe relationship between the FOV of the photodetectorand the received angle. Changing the receiver’s FOV can beused to reject unwanted light, since signals must lie withinthe FOV range of angles to be received.

The total reflected light incident on the detector iscalculated from the summation over all reflecting surfaceelements, which is given by

pr ¼XNi¼1

dpri ð3Þ

where N is the total number of the reflecting elements.

2.4 Diversity receiver analysis for LSMSsystem

The proposed LSMS receiver was reported in [11], withthree photodetectors each placed at the middle of a truncatedpyramid’s face, where the centre of the pyramid’s triangularbase plane specifies the location of the diversity receiver onthe CF. The direction of each photodetector is characterisedby two major parameters: elevation angle (El) andorientation angle (azimuth angle Az). The other parametersof interest include the pyramid’s face inclination and thesize of the pyramid. While the El angle remains at 35� forthe photodetectors placed on the slope sides, the El angle forthe detector placed on the top remains at 90�. Alternatively,Az angles correspond to the truncated pyramid’s faceorientation angles and are fixed to 0�, 0�, and 180�

corresponding to the three detector locations.Compared with the single detector analysis in conven-

tional systems where the vector normal to the receiver isalso perpendicular to the CF, changes in the calculations ofthe preceding Section for the received power analysis needto be made in the case of the diversity receiver. Thereception angle (d) can be calculated by employing the

IEE Proc.-Optoelectron., Vol. 152, No. 4, August 2005232

trigonometry of rectangular triangles as shown in Fig. 2.In order to compute the reception angle (d) for any detectoron the pyramid, a point P has been defined, located on thedetector’s normal, 1m above the detector [11]. Figure 2shows light from a reflecting point (E) on a wall or ceiling,incident onto one of the detectors ðRxÞ. The power receivedowing to an element dA in the case of using a diversityreceiver is obtained as

dpr ¼nþ 1

2:p2:R21:R

22

:Ps:Ar:r: cosn Wi: cos b: cos g

:jPRx��!j2 þ jERx

��!j2 � jEP�!j2

2:jPRx��!kERx

��!j

" #:rectðd=FOVÞ:dA

ð4Þ

Since a detector can only receive a signal within its FOV,where ð�FOV=2Þ< d< ðFOV=2Þ, the incident ray iscontrolled by the rectangular function.

3 OW channel characteristics

In this Section, we investigate multipath propagation effectson three room configurations taking into account all type ofobstacles that might be seen in real environments.Comparisons are also made for three OW configurations(CHS, CDS, and LSMS) in terms of impulse response, delayspread, and path loss.

3.1 Impulse responses

In order to evaluate the overall performance of the proposedconfigurations, it is vital to study the worst receiver position,for example, room corners. Channel impulse responses forthe three OW systems are displayed in Fig. 3 for Room Aand B, and Fig. 4 for Room C, at the room corner (x ¼ 1m,y ¼ 1m, z ¼ 1m). The two axes shown in Figs. 3 and 4 arethe received power (mW) against the delay time (ns). In thispaper, the channel responses are evaluated at severalreceiver positions on the CF. As shown in Fig. 3, forRoom A and B configurations, owing to the open air pathbetween transmitter and receiver, LSMS and CHS impulseresponses are dominant by a short initial impulse owing toLOS. Also note the shorter initial delay associated withLSMS (compared to CHS) which is attributed to the shorterpath from some of the diffusing spots to the receiver.It should also be noted that the existence of windows and adoor in Room B has reduced multipath propagation (owingto elimination of some of the reflectivity surfaces) whichresults in low received power accompanied by low impulseresponse delay spread. It is also clear from Fig. 3 that thepower received by the channels in Room B is slightlydifferent from that received in Room A. The existence of thedifference in impulse responses seen in Fig. 3 for a given

system (LSMS, CHS or CDS) in Room A and Room B isdue to the existence=elimination of some of the reflectivitysurfaces. The difference seen in Fig. 3 is small and can beexplained as follows: for CHS a LOS is maintained in bothRooms (A and B) between transmitter and receiver andtherefore the reduction in reflections has little effect. ForLSMS a similar argument applies, however, note that LSMScan be more robust against shadowing owing to the largenumber of diffusing spots. The difference between theimpulse responses in Fig. 3 for CDS is also small and thiscan be explained by observing the fact that most of thereflections in CDS are due to the ceiling (CDS relies onshorter paths more than longer paths) and hence theintroduction of windows and a door on some of the wallshas a limited effect.

Figure 4 shows the channel impulse response for Room Cat the same position as in Fig. 3. The physical partitions haveformed a shadowed configuration. The effect of shadowing isevidently clear for the case of CHS (compare Figs. 3 and 4)where CHS almost entirely relies on LOS component as seenin Fig. 4. Figure 4 clearly shows that the CHS impulseresponse with a blocked LOS component typically resemblesthe corresponding impulse response of CDS.

By observing the impulse responses over the entirecommunication floor, it is found that LSMS is the only

Fig. 2 Azimuth and elevation analysis for diversity detectiontechnique used by LSMS system

Fig. 3 Channel impulse response for three OW configurationsCHS, CDS, and LSMS

Dotted lines represent the case of Room A, while solid lines represent thecase of Room B

Fig. 4 Channel impulse response for three OW configurationsCHS, CDS, and LSMS in a shadowed environment (Room C)

IEE Proc.-Optoelectron., Vol. 152, No. 4, August 2005 233

configuration among the three that can maintain a directpath component between transmitter and receiver throughits multiple diffusing spots even in the presence of officepartitions. The diffusing spots provide a large number ofpotential LOS paths in this case. It is also observed thatshadowing significantly decreases the overall powerreceived by CDS and CHS configurations (in particularnear windows and corners) typically by 11.6 dB and 9.6 dBrespectively and significantly degrades the channel impulseresponses as shown in Fig. 4. It is noted that the spotdiffusing technique that employs the LSMS configurationhas not been affected by the shadowed geometry considered(office cubicles).

Figure 4 shows that in such a highly impairedenvironment where the receiver is located near the walland inside cubical offices (as in room corner), the opticalpower associated with the LSMS is 1:024 mW, whereas it is0:0697 mW and 0:033 mW for CDS and CHS configurationsrespectively. These channel impulse response resultssuggest that LSMS links perform better than the conven-tional diffuse and hybrid links. Furthermore, Figs. 3 and 4show that the power received by first order reflection inLSMS systems contains most of the total collected power,whereas the higher order reflections comprise the lowestpower, which means better-received power and less pulsespreading for LSMS together with good mobility attributesowing to the presence of the large number of diffusing spots.

4 Delay spread analysis

Figure 5 shows channel delay spread and optical path loss ofthe three proposed configurations (CDS, CHS, and LSMS)at x ¼ 1m and along the y-axis and for the three roomscenarios. These positions have been chosen since theyrepresent the worst communication link over the entire CF(motion inside the office cubicles) and to expose thedifferences between the proposed configurations. In orderto estimate the distortion caused by pulse spread, which is aresult of multipath propagation, rms delay spread (D) can beused. Delay spread gives an indication of the intersymbolinterference (ISI) experienced in the received optical signal.The delay spread associated an impulse response is given by

D ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiRðt � mÞ2h2ðtÞdtR

h2ðtÞdt

sð5Þ

where m is the mean delay given by

m ¼Rth2ðtÞdtRh2ðtÞdt ð6Þ

The integration limits extend over all time. Since, theposition of the transmitter, receiver, and the reflectingelements are fixed, the received optical power and the delayspread can be considered as deterministic for a giventransmitter and receiver locations. For comparison pur-poses, delay spread for the proposed configurations over theentire CF are calculated.

It can be clearly seen from Fig. 5a for Room A and B thatCDS and CHS configurations have a clear delay spreadincreasing trend towards the room corners. For the CDScase, where a single wide FOV receiver is used, the delayspread is clearly larger than that of the CHS (single widebeam transmitter and wide receiver FOV) and LSMS overthe entire communication floor. This is a result of the factthat the CDS features many signal propagation pathsbetween transmitter and receiver. Figure 5a also shows,for the CDS case, that there is a direct relation between the

delay spread and the distance from the transmitter. Sincewindows do not reflect optical signals, the level of delayspread is expected to be less for Room B as can be seen inFig. 5a. It is also clear that both CDS and CHS suffer frommultipath propagation especially at room corners. At theselocations, LSMS has a lower delay spread, even more whenwindows are considered as in Room B since the receiverlocation is always close to a diffusing spots.

Since Room C represents the worst communicationenvironment among the considered rooms, it was used toevaluate the proposed configurations. Figure 5c shows thedelay spread performance of CDS, CHS, and LSMS systemsin Room C. The results demonstrate that the delay spread ofLSMS, where a diversity receiver is employed, is the lowestcompared to the CDS and CHS configurations. This is due tothe LSMS structure where it can maintain LOS componentsin all receiver positions and owing to the presence of spotdiffusing transmission points near the room corners and thelower contribution of the far spot points.

For the CHS case (Fig. 5c), where a single wide FOVreceiver is used and the LOS component is blocked (byoffice cubicles), the delay spread is clearly larger than that ofthe CDS (single beam transmitter and wide receiver FOV)and LSMS over the entire CF except at location x ¼ 1m,y ¼ 4m, and z ¼ 1m where CHS establishes a LOS path.Diffuse configurations, such as CDS, do not rely on a directlink between transmitter and receiver, but instead, relymainly on the many propagation paths that cover the entireenvironment. Therefore, CDS is far less susceptible tocubical partitions (shadowing) than CHS as shown in Fig. 5c.It is found that delay spread values for CDS depend on thereceiver position, which is a departure from the behaviour ofCDS systems reported in the literature. This behaviour is aresult of the presence of windows, office cubicles, andbookshelves in the environment considered. For example inmoving from location (x ¼ 1m, y ¼ 1m, z ¼ 1m) referredto as (1, 1, 1) to (1, 2, 1) the delay spread associated withCDS increases. At (1, 1, 1) a significant propagation of thesignal is contributed by the 20 cmwall strips surrounding thewindows. These contributions diminish in moving to (1, 2,1) and hence an associated increase in delay spread isobserved. The CDS delay spread remains constant as theoffice cubicles are swept towards y ¼ 6m and along x ¼1m and z ¼ 1m. The effect of the bookshelves is observedat (1, 7, 1) where the CDS delay spread increases comparedto the symmetric point at (1, 1, 1). This is attributed to theincreased reflections by bookshelves (reflectioncoefficient ¼ 0:4) and the lack of such reflections by thewindows near (1, 1, 1). The presence of reflectivity in the20 cm wall strips near the bookshelves is also observed.

The delay spread associated with CHS is shown in Fig. 5c.The transmitter is mounted in the centre of the room ceiling.At (1, 1, 1) the delay spread is lower than that at (1, 2, 1)owing to the 20 cm wall strips round the windowscontributing a strong first order reflection signal at (1, 1,1) compared to (1, 2, 1). A signal is also received from theceiling and other walls through second order reflections.This latter contribution is weaker and fairly invariant as faras (1, 1, 1) and (1, 2, 1) are concerned. The CHS delayspread drops significantly at (1, 4, 1) where a LOScomponent is established. The trend beyond y ¼ 4m isfairly symmetric, however symmetry is broken by thepresence of the bookshelves at the y ¼ 8m plane. Thereforeat (1, 7, 1) the delay spread drops owing to strong first orderreflections received from the bookshelves with a reflectioncoefficient of 0.4.

The LSMS system maintains a low delay spread through-out the environment as the receiver is able to observe a

IEE Proc.-Optoelectron., Vol. 152, No. 4, August 2005234

diffusing spot at all locations and is able to establish a LOSwith at least one such diffusing spot. In contrast to the CDSand CHS configurations, the delay spread variation, for thecase of LSMS, is small (towards the room corners), which isdue to the presence of spot diffusing transmission points nearthe room corners and the lower contribution of the far spotpoints. Comparing the proposed configurations under a veryshadowed environment (Fig. 5c), it is to be noted that thelowest delay spread values are obtained by the proposedLSMS. The largest delay spread associated with the LSMS is0.16 ns which is a factor of 11 and 30 lower than themaximum delay spread associated with conventional diffusesystem and conventional hybrid system respectively. Also,the delay spread associated with LSMS is small and evenlydistributed all over the receiver positions.

5 Path loss analysis

The main goal of any OW system is to achieve a lowprobability of error or high SNR at the receiver. SNR in OWsystems is based mainly on the square of received opticalsignal power. Furthermore, owing to power consumptionand eye safety regulations, the transmitter optical power islimited. Therefore, path loss and the average receivedoptical power represent some of the main OW systemparameters. However, because of the nature of optical signalpropagation (1=r2 rule) multipath propagation, lossy reflec-tion from the ceiling, walls, cubical partitions, other objects,

and finite photodetector collection area, optical powerattenuation is experienced, which leads to optical path lossdefined as

path lossðdBÞ ¼ �10 log10

ZhðtÞdt

� �ð7Þ

Therefore, an increase in optical power path loss inconjunction with an increase in delay spread result in apoor system performance.

Figure 5b shows optical path loss distribution for RoomsA and B at x ¼ 1m and along the y-axis. It is clear that thepath loss levels increase towards the room corners wherethe amount of the received optical power decreases. TheLSMS path loss is almost evenly distributed over the y-axis,owing to the large number of distributed diffusing spotcontributions. For the CDS and CHS configurations, nearthe room corners, the signal power is small owing to thelarge distance between transmitter and receiver. Thereforefor the CDS and CHS configurations the highest path loss isexperienced near the room corners. It should be noted thatthe performance of the LSMS system in Rooms A and B iscomparable. The presence or absence of windows has littleeffect as the LSMS system does not rely on reflections, butmakes use of the direct contributions of the spots. The CHSsystem is impacted to a larger extent than the LSMS systemespecially near the room corners where the presence ofwindows causes a larger signal loss and hence a larger pathloss near the room corners in Room B. The effect is even

Fig. 5 Delay spread and optical power path loss distribution for CDS, CHS, and LSMS configurations

a Delay spread for Rooms A and Bb Optical path loss for Rooms A and Bc Delay spread for Room Cd Optical path loss for Room C

IEE Proc.-Optoelectron., Vol. 152, No. 4, August 2005 235

more marked on the CDS system that relies to a large extenton reflections. In the CDS system, the path loss in Room B(with windows) is larger than that in Room A at all locationsand more so near the room corners.

Figure 5d shows the optical power path loss distributionfor Room C which represents the worst communicationroom setup. By comparing the proposed configurations, it isclear that LSMS has a low, evenly distributed power pathloss over all receiver positions at x ¼ 1m and along they-axis. It is to be noted that this line represents the worstzone that can be scanned in Room C. It is also to be notedthat among the proposed configurations, LSMS generallyhas the lowest path loss level, since it can maintain manydirect paths between a number of diffusing spots in the linestrip and the receiver. Furthermore, it is observed that owingto channel blockage caused by the cubical partitions, theCDS and the CHS systems suffer from multipath dispersionand higher optical losses than LSMS.

Figure 5d clearly shows that CHS has its lowest value ofpath loss at receiver position (x ¼ 1m, y ¼ 4m, z ¼ 1m),which is the closest position to the transmitter location. Thissignificant drop in path loss, at this point, is due to theestablishment of a LOS component. The high path loss levelat the other points is related to the obstruction of the LOScomponent. Optical path loss distribution is related to thereceiver physical locations, where locations near toshadowing areas (caused by cubical partitions) create ahigh path loss. Consequently, in the presence of shadowingas in Room C, CDS and CHS channels impulse responseexhibit the largest path losses, for instance, at the worstcommunication link point (room corner) typically 74 dB and76 dB respectively. While LSMS has a path loss level ofabout 59 dB. The CHS system path loss increases in movingfrom (1m, 1m, 1m) to (1m, 2m, 1m) owing to the increasein distance between the 20 cm reflecting strips around thewindows and the receiver position as the receiver movesfrom (1m, 1m, 1m) to (1m, 2m, 1m). At both locations,and also at (1m, 3m, 1m), there are no LOS components.However at (1m, 3m, 1m) the path loss decreases as thereflecting strips in that office cubicle are closer to the CHStransmitter mounted in the centre of the ceiling. The non-symmetry in the CHS path loss curve is due to the presenceof windows at one end of the room and the presence ofbookshelves at the other end.

A similar pattern is observed in Fig. 5d for the CDSsystem where the path loss is lowest near the room centre(closest to transmitter) and higher near the walls with someasymmetry owing to the effects explained above.

6 Performance analysis

In this Section, we investigate the performance in termsof SNR for the three room configurations. Additionally,the systems are assessed and evaluated for the threeroom scenarios. Furthermore, the evaluation of systemperformance was done in a BN environment and under themost stringent conditions. Neither optical filter nor opticalconcentrator has been used. Additionally, CDS, CHS, andLSMS are compared.

Indoor OW communications links are strongly impairedby the shot noise induced by ambient light noise. Thereceived pulse shapes corresponding to the configurationspresented have been considered in calculating Ps1 and Ps0,the power associated with logic ‘0’ and logic ‘1’respectively. The probability of error (Pe) of the indoorOW communication system can be written as

Pe ¼ QðffiffiffiffiffiffiffiffiffiSNR

pÞ ð8Þ

where

QðxÞ ¼ 1ffiffiffiffiffiffi2p

pZ 1

xe�ðz=

ffiffi2

pÞ2dz;

is the Gaussian function which assumes a value of 6 atprobability of error Pe ¼ 10�9, corresponding to SNR of 36(15.6 dB). Taking Ps1 and Ps0 into account (hence ISI) theSNR is given by

SNR ¼ R� ðPs1 � Ps0Þst

� �2

ð9Þ

whereR ¼ 0:5A=W is the photodetector responsivity, and stis the total noise rms value. Equation (9) takes into accountthe impact of the pulse spread caused by ISI where Ps1 andPs0 account for the eye closure at the sampling instant.The pulse response was found through convolution of theimpulse response with a rectangular transmitted pulse of20 ns duration. This corresponds to a 50Mbit=s bit rate andthe receiver used had a bandwidth of 70MHz [16], whichensures that it does not cause an extra dispersion at the bitrate chosen. Furthermore, evaluation of the pulse responseswas carried out over the entire communication floor.

The total noise that is shown 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 and shot noise current ðssoÞ when a ‘0’ is received,and finally receiver noise that is normally generated in thepreamplifier components ðsprÞ.

sbn ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2� q� Pbn � R� BW

pð10Þ

where q, Pbn, and BW are the electron charge, receivedbackground optical power, and receiver bandwidth, respect-ively. The preamplifier used in this study is the PIN-BJTdesign proposed by Elmirghani et al. [16]. This preamplifierhas a noise current density of 2.7 pA=

ffiffiffiffiffiffiHz

p. Therefore,

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

p¼ 0:023 mA ð11Þ

Note that s2t isffiffiffiffiffis20

pþ

ffiffiffiffiffis21

p, which represent the noises

associated with the signal and can be obtained as

s20 ¼ s2pr þ s2bn þ s2s0 and s21 ¼ s2pr þ s2bn þ s2s1 ð12Þ

The assumption of Gaussian noise statistics holds in ourcase, since thermal and shot noise can be accuratelymodelled as Gaussian processes. In this paper, we considerthe diversity receiver in Fig. 2 with selection combing forprocessing the three received optical signals where thedetector that has the highest SNR is chosen. Substituting of(12) into (9) and considering the diversity receiver withthree branches

SNR ¼ maxi

R� ðPs1 � Ps0Þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffis2pr þ s2bn þ s2s0

qþ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffis2pr þ s2bn þ s2s1

q0B@

1CA

2

;

1 � i � 3

ð13Þ

In this Section, we report SNR performance results based onpulse propagation simulations. The simulations were carriedout for the three different room configurations (including therealistic Room C with high impairments) and for CDS,CHS, and LSMS systems in each room.

IEE Proc.-Optoelectron., Vol. 152, No. 4, August 2005236

SNR calculations were performed for the proposedsystems in seven different locations along the y-axis atconstant x ¼ 1m where this scans the office cubicles inRoom C and scans in all rooms the peaks and troughs ofambient light sources (background noise). Note that thepeaks of BN occur underneath the light sources (x ¼ 1m,and y ¼ 1m, y ¼ 3m, y ¼ 5m, y ¼ 7m). A summary ofthe SNR results is given in Fig. 6 where SNR is plottedagainst receiver positions for CDS, CHS, and LSMSsystems and for the three room scenarios.In the case of the conventional systems (CDS and CHS),

it is found that the SNR distribution (as seen in Fig. 6) isalmost symmetric around the room centre. Moreover, sincethe transmitter is located at the middle of the room (on theCF for the CDS and on the ceiling for CHS), it results in asymmetric SNR set of values about y ¼ 4m in particular forRoom A, as shown in Fig. 6. Figure 6 also shows that theSNR has its highest value at the centre of the y-axisðy ¼ 4mÞ. This is attributed to two facts: the distancebetween the transmitter and the receiver is the minimumpossible compared to the other locations, resulting in astrong received signal. Secondly, the noise distribution has a

very low value at this location because the receiver at thatlocation is not underneath a spotlight lamp. Since there isalways a LOS path between the noise sources (lamps) andthe receiver (in conventional systems), BN power is at themaximum value it can assume for a given location. Thiseffect can be evidently seen at locations underneath thenoise sources as seen in Fig. 6a. As an example, the receivednoise powers for the conventional systems underneatha noise source are the same and equal to 8.82mW. Thisimplies that conventional system performance dependsmainly upon the information bearing signal strength.

Figure 6a also demonstrates that noise owing to back-ground light has a significant effect on the systemperformance. The impact is more pronounced when thereceiver is placed directly under a light source and towardsthe room corners where the distance between the transmitterand the receiver is large. This can be seen easily at x ¼ 1mand y ¼ 1m, 3m, 5m, and 7m, where SNR for CDS andCHS has a value of 21.9 dB and 22.3 dB at the cornerswhen the Room A configuration is used and about 24.8 dBand 24 dB when Room B is used for the same respectiveorder. This results in very poor system performance.

Fig. 6 Signal-to-noise ratio distribution for indoor optical wireless system considering three room scenarios: Room A, Room B, and RoomC where windows, door, mini cubical offices with physical partitions (shadowing) are included for receiver located at x ¼ 1m and alongy-axis

a Room A and B configurationsb Room C configuration

IEE Proc.-Optoelectron., Vol. 152, No. 4, August 2005 237

In contrast, Fig. 6a shows that LSMS exhibits an evendistribution in SNR over all receiver locations owing to twoeffects: noise level over the CF is significantly low becauseof (i) the LSMS system employs a diversity receiverstructure that selects the receiver that has best SNR, and (ii)the received optical signal is strong because of thepossibility of maintaining LOS components between thetransmitter and receiver at all receiver locations. Forexample, for Room B, the LSMS SNR at the room corneris about 21.3 dB, which is about 26 dB SNR improvementover the conventional systems (CHS=CDS).

Figure 6b displays SNR distribution for Room C where areal office environment is considered. It shows that for theconventional systems, BN has a significant effect on thesystem performance, since a single wide FOV (180�)receiver is used (recognised by peaks and the troughs).The LOS components that should be established by the CHSsystem have been affected by the physical cubical partitionsthat cause shadowing effects, hence blocking the LOScomponents, therefore reducing the total received poweryielding system degradation. Figure 6b also illustrates thatat the location close to the room centre (1m, 4m, 1m), theSNR has a high peak value for CDS and CHS. This peak forCHS is attributed to the LOS component between thetransmitter and the receiver at this location where the opticallink has not been affected by the shadowing.

System performance improvement is clearly observedwhen LSMS is used. Figure 6b shows SNR improvement inmost of the receiver locations (including room corners), andalong the y-axis where directional interference exist. This isattributed to the fact that the noise levels at these locationsare reduced owing to diversity and owing to reduction in theFOVs. Also note that the results in Fig. 6 are in agreementwith the general observations made in Fig. 5. For examplefor a CHS system in Room C the path loss is lowest at (1m,4m, 1m) and so is the delay spread (Figs. 5c and d),resulting in a high SNR at this location in Fig. 6b.Furthermore, Fig. 5c shows a comparable delay spread forCHS and LSMS at (1m, 4m, 1m), but Fig. 5d shows alower path loss for CHS at (1m, 4m, 1m) compared toLSMS. This is reflected in the higher SNR achievement byCHS at that one location in Fig. 6b.

Similarly, the peaks and troughs of SNR in Fig. 6a followa convex envelope (rather than rising and falling within auniform envelope). This can be understood by observing theconcave envelope of the delay spread and path loss curvesfor CHS and CDS in Figs. 5a and b respectively.

The improvement obtained by using the proposedstructure can be seen; a significant SNR improvementover both CDS and CHS systems. Comparing LSMS andthe conventional systems, it is to be noted that the largestSNR values are obtained by the proposed LSMS. In viewof the fact that the weakest point in a communication linkis the criterion of the system quality, the minimum SNR ofthe proposed configurations have been compared. Thesmallest SNR associated with the LSMS is about 21 dB,whereas it is about 230 dB and 234 dB for the CDS andCHS respectively.

7 Conclusions

In this paper we proposed an indoor optical wirelesssystem in three different environments. Employing a spotdiffusing technique in the form of LSMS and a threeelement diversity receiver has improved the systemperformance in the real environment considered. Thisimprovement has been achieved in presence of very

directive noise sources in addition to impaired propagationwhere shadowing exists. Furthermore, three different roomconfigurations have been studied. The third configuration(room C), which deals with a real environment, hascovered various types of reflectivity, taking into accountpartition design, cubical design, window availability, andcabinets. Therefore, in this paper, most of the importantaspects of a real environment have been considered.Based on the results, it can be concluded that complicatedroom design can play an important role in changing theimpulse response uniformity in addition to SNR distri-bution. Comparisons were carried out between conven-tional systems (CHS where LOS can be established andCDS) with LSMS system. Such a system combines theadvantages of both DLOS links and diffuse systems. Theimprovement in performance achieved is a result of thesignificant reduction in background noise in addition toreduction in ISI effects. Using the multiple diffusing spotcharacteristics produced by the LSMS configuration, itwas demonstrated that a significant improvement can beachieved including an extensive drop in the noise powerlevel, a strong received signal, reduced delay spread, andevenly distributed SNR over the communication floor. Ourresults indicated that LSMS can reduce pulse spread by afactor of 11 and 30 lower than the delay spread associatedwith CDS and CHS respectively. Furthermore, LSMSlinks can increase the SNR by more than 50 dB and about55 dB compared to the CDS and CHS respectively.

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