optical wireless systems employing adaptive collaborative transmitters in an indoor channel

IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 59, NO. 1, JANUARY 2010 63

Optical Wireless Systems Employing AdaptiveCollaborative Transmitters in an Indoor Channel

Jamal M. Alattar and Jaafar M. H. Elmirghani

Abstract—We propose a novel optical wireless (OW) systembased on a power adaptive multibeam spot-diffusing transmitterserving multiple seven-segment maximum ratio combining (MRC)angle diversity receivers. A feedback link is assumed betweenthe transceivers so that each receiver conveys to the multibeamtransmitter the new beams transmit power weights to be usedto achieve the best signal quality at a given receiver location.Two cases involving three and five receivers are considered. Fourdifferent configurations for the placement of the three-receivercase in the room are also examined. The system’s performance ineach case is evaluated in terms of signal-to-noise ratio (SNR) andis compared with the single receiver scenario with and withoutpower adaptation. In the presence of one receiver, the transmitspot powers can be adjusted for optimum performance at thatreceiver location. For multiple receivers, there is conflict, and wepropose spot power adaptation based on the average requirements(power distribution in spots), i.e., transmit equal gain combining(EGC) of spot power or MRC of transmit spot powers. Theresults show that the three receivers benefit most from an adaptivetransmitter when each is placed at a corner of the room. In thiscase, an SNR increase of as much as 2.6 dB is achieved for allthree receivers at the corners by both MRC and EGC. Moreover,when all receivers are placed away from the line of diffusing spots,our proposed MRC collaborative approach is 1 dB better than thenoncollaborative system. This gain reduces the difference from theupper bound set by the single receiver adaptation, which is 3 dB.For a mobile transmitter, MRC also significantly improved theSNR for the farthest receivers at the opposite end from the trans-mitter located near one room corner.

Index Terms—Adaptive systems, communication channels,communication system performance, optical transmitters,simulation.

I. INTRODUCTION

MULTIPLE narrow-beam optical transmitters have beenproven to effectively remove the restriction of maintain-

ing direct line of sight (LOS) between the transmitter and the re-ceiver in indoor optical wireless (OW) channels while meetingthe eye-safety standards for optical emitters [1] and producinga lower delay spread. The narrow beams can be practicallyproduced using a holographic optical diffuser mounted on theface of the transmitter [2]. Holograms produced using computermethods can flexibly vary the intensity of a particular spotand/or the intensity distribution of the spots. Liquid-crystal-device-based holograms can vary the intensity among spots

Manuscript received June, 27, 2008; revised January 14, 2009 and February 20,2009. First published April 7, 2009; current version published January 20,2010. The review of this paper was coordinated by Prof. T. Kuerner.

The authors are with the School of Electronic and Electrical Engineering,University of Leeds, LS2 9JT Leeds, U.K.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TVT.2009.2020427

adaptively. The spots, which are cast on the ceiling, becomesecondary Lambertian transmitters (diffusers). Various geome-tries can be produced to improve the system’s performancebased on the specific conditions of the indoor environment,including the dimensions of the room and the placement ofthe artificial lighting fixtures, which are the dominant sourcesof background noise. Research has shown that noise fromdirective sources (natural and artificial) introduces significantimpairments on the transmitted optical signal and, thus, reducesthe SNR [3].

An effective solution to combat the influence of backgroundnoise has been proposed through the employment of anglediversity detection where reception relies on reflected com-ponents of the signal while direct noise rays are rejected [4],[5]. Increasing the number of active branches on a receiverhas been shown to improve signal detection [6]. Receiverswith multiple detecting elements have to use signal combiningtechniques to produce the resulting signal. Among the knowncombining techniques, MRC achieves the best results as it usesvariable weights assigned to the different detectors based ontheir contribution to the total SNR.

While the employment of an angle diversity receiver tacklesthe degrading effect of the noise for a mobile receiver, the equaldistribution of the transmitter power on the multiple beamsseems to waste some of the transmit power on spots that areaway from the location of the receiver. A better approach wouldbe to assign unequal transmit powers to beams based on thecontribution made by the spot (due to a particular beam) to thereceived signal at a given receiver position. This calls for an“adaptive” method to compute the optimum beam powers thatproduce the best SNR for a given transmitter and associatedreceiver location. The transmitter sets the amount of powertransmitted on each beam based on information about the signalquality fed back to it by the receiver. The need for such atransmit power adaptation algorithm is particularly strong in thecase of a fully mobile system where the mobility of the spot-diffusing transmitter may result in increased spot populationat some locations and depletion of spots in other locations.Those spots nearer the receiver should always be given highertransmit powers. Thus, the adaptive approach improves thesystem’s performance at all receiver locations since the “equalspot power case” is a special case of spot power adaptation. Theimprovement in performance is more profound at the weakestlink positions, i.e., when the distance separating the transmitterand receiver becomes largest (e.g., transmitter near one cornerof a room and the receiver near an opposite corner of a room).

In this paper, we extend the treatment of our previouslyproposed adaptive single line strip transmitter reported in [7]

0018-9545/$26.00 © 2009 IEEE

64 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 59, NO. 1, JANUARY 2010

and [8] and investigate the performance of the OW system formultiple receivers. The addition of other receivers within theOW environment followed by adaptation for one receiver loca-tion may negatively affect the quality of the received signal forsome of the users at certain locations. Therefore, the objectiveis to identify a method that can be used to enhance the SNRbased on adaptation when multiple receivers are present.

Using the same type of angle diversity receiver, we studythe effectiveness of the adaptive collaborative multibeam spot-diffusing transmitter for each receiver. First, we present a com-parison of our new technique with the single receiver adaptationmethod. Second, the performance of the collaborative adapta-tion approach is studied and compared with the performanceachieved with a nonadaptive transmitter. Third, two approachesto implement collaborative adaptation are introduced, and theirperformance differences are highlighted. For the sake of com-pleteness, our performance evaluation is not restricted to thecase of a stationary transmitter, and as such, we present resultsunder transmitter mobility.

Following the introduction, this paper is organized as fol-lows. Section II provides a review of related work in the use ofmultibeam transmitters for indoor OW communications, whileSection III describes the OW indoor environment considered.System analysis is presented in Section IV followed by resultsfor both stationary and mobile transmitters in Section V andconclusions in Section VI.

II. RELATED WORK

In 1992, Yun and Kevehard [1] proposed a new approachthat aimed at reducing the required power level of a diffuseinfrared (IR) system first proposed by Gfeller and Bapst in1978 [9] and, at the same time, maintaining similar robustness.The approach was termed spot-diffusing multi-line-of-sightconfiguration. This system had two new features. First, thetransmitter power is projected onto a small area of the reflectingsurfaces in the form of collimated or slightly diverted beams,which is a configuration referred to as spot diffusing. Second,the use of multiple narrow beams to produce the diffusing spotsresults in multiple lines-of-sight, which therefore removes therestriction of maintaining alignment between the transmitterand the receiver. This allows users to send more than one beamto geographically separated diffusing spots (useful particularlyif blockage occurs to some spots), thus keeping the broadcastfeature of the diffuse system. Moreover, another advantage ofthe availability of multiple lines-of-sight is the possibility ofemploying more than one receiver to view the diffusing spots.These receivers have narrow fields-of-view (FOVs) and couldinclude large area power concentrators.

The original approach for implementing the multispot-diffusing system created each diffusing spot from a separatelaser beam, making it hard to create a large number of dif-fusing spots. An alternative approach was later proposed byPakravan et al. [10], who used a holographic diffuser witha 3-D multiple-lobe pattern to shape the IR source radia-tion for wireless IR communications. Following the work ofMcKee et al. [11] to diffuse the radiation from the laser sourcewith a computer-generated hologram to make it safe for the

eye, McCullagh and Wisely [12] used the same technique anddemonstrated 155 Mb/s transmission in a 5-m2 cell using only40 mW of transmit power. Later, Eardely et al. [13] investigatedthe use of holographic diffusers for OW LANs, where they usedcomputerized simulated annealing (SA) techniques to designthe hologram. Simova et al. [14] combined the diffusing andsplitting functions in a single holographic optical element,while Carruthers and Kahn [15] used eight laser diodes toproduce eight collimated beams. Kavehard and Jivkova [16]employed a diffraction grating technique for transmitter patternshaping. More recently, to improve the performance of themultispot-diffusing system, Wong et al. [17] proposed a methodof optimization of the diffusing pattern by making use of a SAalgorithm. The authors showed that although the SA algorithmtook longer compared to the simple iterative approach, it re-sulted in a 19% improvement.

Adaptive OW systems have been, to date, studied for the pur-pose of achieving high bit rates under adverse and/or variableconditions. The original idea proposed by Diana and Kahn [18]was to adapt the transmission rate to achieve the best SNR atthe receiver, i.e., transmit at lower rate when the SNR at thereceiver is low and at higher rate when the SNR is high. In theirproposal, the authors described several coding and decodingschemes for rate-adaptive systems. Following this work, sev-eral other research groups proposed variations of modulationtechniques to implement rate-adaptive OW systems. These pro-posals include, in addition to simple repetition codes, rate-compatible punctured convolutional codes [19]. However, theefficient management of transmitter power in noise- anddispersion-impaired environments has not been treated. More-over, the application of a transmit power-adaptive approach tosuit an OW system involving multiple receivers comes as anatural extension of the investigations on which we reportedfor power adaptive indoor OW systems [7], [8].

III. ENVIRONMENT

A 1-W upright optical transmitter (elevation = 90◦) is placedin the center of the communication plane (CP), 1 m abovethe floor. The OW link is assumed to be within a totallyempty room (real environment studies were reported in [20])and having a size of 4 m × 8 m × 3 m (width × length ×height). Furthermore, the optical transmitter is assumed to havea holographic diffusing device mounted on its face to distributethe transmitter power into multiple narrow beams aimed at theceiling. The diffuser has the capability of varying the opticalpower associated with each diffusing spot (i.e., nonuniformpower distribution) using, for example, a liquid crystal device.The intensity of each spot can be controlled by varying theexcitation voltage applied to the liquid crystal device.

Fig. 1 shows the simulation setup of the spot-diffusing trans-mitter as well as the background noise. The optical transmitterplaced at the center of the CP [at Cartesian coordinates (2 m,4 m, 1 m)] produces 100 narrow beams projected uniformlyat the ceiling. The beams are centered along the center line ofthe ceiling. Small diffusing spots of equal intensities (initially),despite the difference in their beam angles, are cast on theceiling and act as secondary transmitters. Hence, the transmit

ALATTAR AND ELMIRGHANI: OPTICAL WIRELESS SYSTEM EMPLOYING ADAPTIVE TRANSMITTER 65

Fig. 1. Simulation setup for multibeam spot diffusing and background noise.

power of each spot is 10 mW, and their spacing is 8 cm. Thetwo end spots are 4 cm from the adjacent side walls.

An angle diversity receiver of hexagonal base that has a totalof seven detectors (one detector faces up) was employed. Theelevation angle of the side branches is set to 20◦. Six detectorsare placed on its elevated branches, making symmetric azimuthangles with the x- and y-axis of 45◦, 90◦, 135◦, 225◦, 270◦,and 315◦. With the positioning of the receiver at the centerof the CP, the FOVs of the pair of detectors that face thelarger wall (edge wall) are chosen larger than the other twoside detectors that face the shorter side wall. This allows formaximum collection of reflected signal off the surface of thewalls by these side detectors. Hence, the FOVs associated withthe six side detectors are 50◦, 60◦, 60◦, 50◦, 60◦, and 60◦.The top detector’s FOV was chosen to minimize the effectof the directive noise sources by avoiding their direct powercomponents at most locations. The noise sources (spot lights[4]) are 2 m apart on the ceiling. When placed within thedistance between two noise sources, the FOV of the detector ontop of the receiver FOVdet7 has to be large enough to includeonly all the spots that are in between the two noise sources. ThisFOVdet7 is obtained from 2 × tan−1[((1/2)D − (1/2)d)/2],where D is the distance between two spot lamps, and d is thedistance between two diffusing spots. As the spot lamps areseparated by 2 m and the distance d between the spots is fixedat 0.08 m, FOVdet7 ≤ 50.8◦. Hence, an FOV of 50◦ was chosenfor the top detector.

Simulation for the channel impulse response as well as thebackground noise was carried out using ray tracing up to thesecond-order reflection. Reflections beyond the second orderare highly attenuated and have been found to insignificantlycontribute to the received signal [21]. The walls and ceilingof the simulation room are covered with plaster. Reflectionoff the painting on the plastering is taken as ideal Lambertian(i.e., with a unity mode number n). Therefore, reflected raysoff the centers of small surface elements (size: 5 cm × 5 cmfor the first-order reflection and 20 cm × 20 cm for thesecond-order reflection) have been taken to follow a Lambertianpropagation model. Eight spot lights [4] were used to evaluatethe interference induced by background noise within the indoorenvironment. These noise sources were placed on the ceilingseparated by 2 m starting 1 m from a wall. Therefore, as shownin Fig. 1, four lamps are on each side of the center line ofthe ceiling. Spot lights were chosen as they produce highly

Fig. 2. Transmitter and receivers positioning on CP. (a) Receivers on edgeline. (b) Receivers on center line. (c) Receivers on diagonal line. (d) Receiversat three corners. (e) Transmitter moved to a corner, receivers at three corners.(f) Five receivers. (g) Transmitter at a corner with five receivers.

concentrated lighting and the most damaging form of noisewhen the receiver is underneath such a light source. We haveconsidered other light sources [22]. The work in [22] can beused to extract parameters for a statistical noise model, whichis beyond this work.

Binary data are transmitted with a 1-W pulse of 20 ns dura-tion (corresponding to 50 Mb/s bit rate). The pulse response ofthe channel was obtained through convolution of the impulseresponse with the transmitted pulse. The simulated peak levelsof the background noise as well as power levels from thereceived pulse were used to compute the SNR at a givenreceiver position. The performance evaluation was carried outacross the following two lines within the room: 1) the edge line(x = 1 m) and 2) the center line (x = 2 m) due to symmetry inthe shape of the room.

Two scenarios were investigated. The first was with a station-ary transmitter (positioned at the center of the CP). In this case,four of the many possible variations for the positioning of thethree receivers were studied. The studied configurations werechosen based on the following criteria:

1) cases including a receiver position with no separationdistance from the transmitter, i.e., receiver is at the sameposition as the transmitter—the configurations shown inFig. 2(b) and (c);

2) a case having all receivers at maximum distancesfrom the transmitter as in the configuration shown inFig. 2(d);

3) cases in between these two extremes, i.e., with one re-ceiver close to the transmitter and the others at maximumdistance from the transmitter, as in the configurationshown in Fig. 2(a);

4) a case that represents transmitter mobility [compareFig. 2(d) and (e)];

5) a case that represents transmitter mobility, but withmore receivers [five receivers instead of three; compareFig. 2(f) and (g)].

There are many other configurations possible, and a moreextensive study may be conducted. We presented this investi-gation to validate our cooperative adaptive concepts, and weintend to proceed to produce wider conclusions. In the secondscenario, collaborative adaptation of the last case (receiversat three corners) was studied with the multibeam transmitterrelocating to a corner, as shown in Fig. 2(e).


IV. SYSTEM ANALYSIS

To achieve collaborative transmit power adaptation, the ap-proach for the adjustment of beams powers has to be defined.As this adaptive approach uses the computed SNRs from eachdetector of the receiver, this section begins with an explanationof the SNR calculations. These SNRs are combined accord-ing to MRC to form the multidetector receiver’s SNR. Theadaptive approach that performs the adjustment of the beamsweights is explained in Section IV-B. Once the weights arecomputed for each of the receivers in the OW system, ournovel collaborative algorithm is then performed to calculate thefinal set of weights to be adopted by the transmitter. The twoweights’ combining methods used for the purpose of perfor-mance comparison are presented in Section IV-C. With thesenew adjusted beam weights, the OW system’s performance isevaluated in terms of its most important parameter: the SNR.Finally, the effect of transmitter mobility on the system perfor-mance resulting from the change in spot locations is given inSection IV-D.

A. SNR Calculation

The OW system employed on–off keying modulation due toits simplicity. The SNR is computed as [24]

γ =(

R × (Ps1 − Ps0)σt

)2

(1)

where Ps1 and Ps0 denote the power levels of the signal wheneither logic 1 or logic 0 is received, respectively, and R is thereceiver responsivity (0.5 A/W). The total noise in the receiverσ2

t consists of the following three parts:

1) shot noise induced by background noise that is generatedby the artificial lighting sources σbn;

2) preamplifier noise σpr generated in the preamplifierelectronics;

3) signal noise σs.

The background noise σnb is given by

σbn =√

2 × q × Pbn × R × BW (2)

where q is the electron charge, Pbn is the collected backgroundnoise, and BW is the receiver bandwidth. The receiver was as-sumed to have the p-i-n bipolar junction transistor preamplifierproposed by Elmirghani et al. [23]. This receiver was designedto have a large bandwidth of 70 MHz. The noise current densityassociated with this preamplifier is 2.7 pA/

√Hz. Therefore, the

preamplifier shot noise is

σpr = 2.7 × 10−12√

70 × 106 = 0.023 μA. (3)

The noise induced by the power levels of the received signalσs is very small in this work and, therefore, can be neglected.When an optical transmitted pulse of 1 W passes through thechannel, the received pulse at the receiver was seen to be on theorder of a few microwatts. Hence, the noise that is associatedwith this signal, which is in the form of shot noise current in

the receiver electronics, is also small. The background noisesources have a power of 65 W per spot light, which inducesa larger shot noise component in comparison with the signal-dependent shot noise.

The detected signals from the multiple detectors of thereceiver are processed using the MRC scheme, which furtheroptimizes the system’s output/SNR. The SNR using the MRCmethod is given by [24]

γMRC =

(J∑

i=1

(wi · Ii))2

J∑i=1

w2i · σ2

i

(4)

where Ii is the electrical current produced in branch i, and σi

is the standard deviation of the total noise in that particularbranch. Maximum SNR can obtained by setting the weight wi

proportional to the SNR [25], i.e., wi = Ii/σ2i .

Therefore, the output SNR with MRC is reduced to [24]

γMRC =J∑

i=1

(Ii

σi

)2

. (5)

B. Transmit Beam Power Adaptation

The adaptive algorithm used to calculate the transmit powerof each of the 100 beams can be described as follows:

1) Turn spot j beam on and all other spots off.2) Compute the SNR in each detector face separately.3) Calculate the weights using MRC.4) Calculate the SNR at the seven-detector receiver output

after combining with the MRC weights.5) Use the calculated SNR in 4) as the SNR associated with

spot j.6) Repeat steps 1) to 5) for all 100 spots.7) Set the power of each spot according to the SNR ratio

among the spots.

C. Collaborative Weights Combining

When multiple receivers are present, the spot powers adaptedbased on a given receiver location may not be optimum for theother receiver [two other locations in Fig. 2(a) and (e)]. Instead,here, we compute the optimum spot powers as requested byeach receiver. These requests are either 1) weighted equally,and therefore, the final set of spot powers is the average of thepowers requested by each receiver, i.e., a form of equal gaincombining (EGC), as in the following:

PjCollEGC =

n∑i=1

Pi,j

n(6)

where n is the number of existent receivers, and Pi,j is thepower requested by receiver i for spot j under the singleadaptive transmit power algorithm in Section IV-B, or 2) the


Fig. 3. Mobile spot diffusing transmitter analysis.

final set of spot powers is set according to the following MRCrule: In the MRC approach, the ratios are not the same butare computed based on the SNR to which a given spot powerallocation leads in the receiver. Therefore, for the adaptation ofthe power of spot j for the n receivers, the factor x is used as amultiplier

PjColl.MRC =n∑i

(Pi,j

γi

)× x (7)

where γi is the computed SNR for receiver i if this power isused. Using a transmitter power of PS , the multiplier x is foundfrom (8) as follows:

PS =∑

j

PjColl.MRC =∑

j

(n∑i

(Pi,j

γi

)× x

). (8)

Noting that∑

j Pi,j = PS , then

x =1

n∑i

(1γi

) . (9)

For the simple case of only two receivers

x =(

γ1 × γ2

γ1 + γ2

). (10)

D. Mobile Transmitter Analysis

The analysis for a mobile spot-diffusing transmitter is de-tailed in [8]. As shown in Fig. 3, the vertical location of a spotZs can be found from

Zs = Yt1 · tan(αi) (11)

where Yt1 is the new distance of the transmitter—on they-axis—from the wall to which it is moving, and αi is the angleof beam i with respect to the CP given by

tan(αi) =hs

dyi(12)

where hs is the vertical separation of a spot from the CP.

Fig. 4. Spot power distribution profile for single receiver and collaborativesystems (receivers on edge line).

The distance dyi is found from the difference between they-coordinates of the transmitter and spot i.

For the spots to the left of the transmitter (see Fig. 3), dyi isfound from

dyi =L

2−

(L

2Ny+

(L

Ny

)· i

)1 ≤ i ≤ Ny

2(13)

where L is the room length (dimension along the y-axis), thefirst spot is assumed to be at half of the spacing between thespots, i.e., 1/2(L/Ny) from the wall, and Ny is the number ofspots along the y-axis.

For the other half of the spots to the right of the transmitter,we have

dyi =((

L

Ny

)· i − L

2Ny

)− L

2Ny

2+ 1 ≤ i ≤ Ny. (14)

V. RESULTS

The results in Figs. 4–17 were obtained by first carrying out apropagation simulation as in Section III to identify the impulseresponse and noise due to spot lights at a given transmitter andreceiver(s) location. The analysis in Section IV is carried out,including our collaborative adaptive algorithm, leading to theidentification of SNR in the different configurations considered.

We also present the distribution of the powers among thespots for each receiver, where the power allocation is com-puted based on the spot contribution to the SNR. This distri-bution shows the amount of power adapted above or belowthe original uniform distribution of power among the spots.While a statistical analysis of power allocation among spotsmay be interesting, in practice, the adaptive algorithm willallocate power depending on a number of parameters, includingroom geometry, impairments, mobility, and receiver structure.Such a statistical presentation may also mask special casesof interest and will be applicable mainly to the configurationconsidered. We therefore attempt to treat the most importantcases/configurations identified.

A. Stationary Transmitter

1) Receivers on Edge Line: Fig. 4 shows the power distrib-ution on the 100 spots for each receiver as computed with the


Fig. 5. SNR comparison for receivers on edge line.

adaptive approach. The solid curve in the figure representingthe power distribution without the adaptive approach is drawnto show the amount of power above or below the initial figuregiven to each spot as a result of the adaptation. It can beseen from the plots that the power distributions for the tworeceivers located at the opposite room corners [at (3 m, 1 m,1 m) and (3 m, 7 m, 1 m)] are mirror images of one another.For these receivers, the transmit powers of the middle 40 spotsis increased as the spot location approaches the position ofthe receiver. However, when the spot is near a noise source,the spot is assigned less power due to its reduced contribution(note that an angle diversity receiver is used) as a result of thedegrading effect of the noise on the received signal. This canclearly be seen in the two gradual decreases in the spot powerstoward the receiver placed near the corner. In addition, if thelocation of a spot near the receiver is also closer to the wall, itstransmit power is increased. This increase reflects the enhancedcontribution due to multipath reflection effect.

Adaptation of spots’ power for the case of the third receiver[placed in the middle of the edge line at (3 m, 4 m, 1 m)]results in the 30 spots at each end being assigned higher powersthan the 40 spots in the middle. The receiver at this locationis midway between two noise sources at y = 3 and 5 m. Eachof these noise sources affects the contribution of the spots onboth of its sides. Although much of the noise at this location isavoided with the rejection of direct rays by the receiver’s topdetector, direct as well as reflected rays from the noise sourceare still collected by the side detectors. Hence, the effect ofthese noise sources is seen to lower the contribution in the SNRof the nearby spots.

The adaptive power distribution for the three receivers to-gether (collaborative) using both MRC and EGC is shown inFig. 4 (solid curve). It is noted that MRC increases the powersassigned to the middle 30 spots, while the powers of the endspots are reduced compared with the respective powers givenby the EGC method. This distribution by the MRC produces theflat SNR levels seen in the solid line in Fig. 5. In contrast, withthe EGC, the SNRs at the room corners are 1.7 dB below thosecloser to the transmitter on the edge line. The figure also showsthat if the transmitter adapts its beam powers for that receiveronly, the SNR will be raised by 2.7 dB at the expense of a loss of2.5 dB for the receivers at the two corners. Similarly, adaptingfor a receiver at one corner produces a 1.7-dB gain in SNR at

Fig. 6. Spot power distribution profile for single receiver and collaborativesystems (receivers on center line).

the opposite corner, which is equivalent to the loss at the centerof the edge line. For the configuration shown in Fig. 2(a) and thereceivers at (3 m, 1 m, 1 m) and (3 m, 7 m, 1 m), single receiveradaptation (an upper bound) is 3 dB better than the nonadaptivesystem. Our proposed MRC collaborative approach is 1 dBbetter than the noncollaborative system under these conditionsand is therefore closer to the upper bound, which is a goodimprovement. Moreover, the leveled SNR distribution obtainedwith the collaborative MRC adaptive approach is favored froma user’s point of view, which means that the user can changelocation without impact on the received signal.

2) Receivers on Center Line: The power distribution whenthe three receivers are placed on the center line of the CP(directly underneath the line of diffusing spots) is presented inFig. 6. For each receiver, the 30 spots directly on top are givenhigher powers that increase toward the position of the receiver.Moreover, higher powers are assigned to the spots on top of thereceiver located near a side wall [at (2 m, 1 m, 1 m) or (2 m,7 m, 1 m)] compared with the corresponding spot powers forthe receiver at the center of the room. This, again, arises fromthe enhanced contributions of the spots near the side wall dueto the multipath reflection effect.

The results of power adaptation in the three receivers in thislayout show reduced power for the spots on top of a receivercoupled with increased powers for the spots on top of the otherreceivers’ locations. Similar to the previous case, the MRCcollaborative adaptation slightly increased the powers of themiddle spots as their assigned weights were lower than thoseon top of the other two receivers. This leads to a higher SNRfor the receiver in the middle of the center line (a 0.7-dB in-crease) compared with its level with the EGC, where a uniformSNR distribution for the three receivers is obtained as shownin Fig. 7.

The SNR levels on this center line are higher than thosein the previous case. This difference is due to the strongersignal received directly underneath the line of spots as well asthe lower noise collected as the receivers are away from thelocations of the noise sources. Furthermore, adaptation for asingle end receiver only greatly degrades the signal received bythe receiver at the other end. This is seen in the figure as a hugedecrease in SNR of over 10 dB for each of the end receivers


Fig. 7. SNR comparison for receivers on center line.

Fig. 8. Spot power distribution profile for single receiver and collaborativesystems (receivers on diagonal).

when the other receiver is being adapted for. This drop in SNRis split on the two end receivers when the adaptation is donefor the middle receiver. Moreover, the levels obtained withthe collaborative adaptive approach are only 1.9 and 3.3 dBless than those obtained with a nonadaptive transmitter for asingle receiver at the room center and its edges, respectively.Therefore, no adaptation is better in this configuration becausethe receiver follows the unadapted line of diffusing spots.

3) Receivers on Diagonal Line: In this layout, the powerdistributions of the receivers at opposite corners of the roomthat are also on both sides of the line of the diffusing spotsare mirror images of one another with increasing spot powertoward the receiver that is also away from the center of theroom. In contrast, for the receiver at the center of the room,the highest powers are given to the middle 30 spots (38–63) andthen decreases toward the edges of the room, as shown in Fig. 8.Power adaptation for this receiver produces a 6.9-dB increasein SNR as well as a 2.9-dB improvement in SNR for the twoopposite corner receivers (see Fig. 9).

Adaptation with EGC of the weights increases the SNR lev-els at the opposite corners; however, it results in a considerabledecrease in SNR for the receiver at the center of the room. Thepower distribution computed with the MRC is seen to improvethe weights for spots of low power away from the center whilelowering the powers immediately around the center (solid linein Fig. 8). Effectively, this redistribution improved the SNRat the center by 8.1 dB, with a very minimal decrease of its

Fig. 9. SNR comparison for receivers on diagonal line.

Fig. 10. Spot power distribution profile for single receiver and collaborativesystems (receivers at three corners).

level at the corners (0.1 dB) compared with the respective levelsobtained by the EGC collaborative case, as shown in the SNRplots in Fig. 9. Moreover, with MRC, the SNR of the receiversat the corners are 2.3 dB higher than with a nonadaptivetransmitter and only 1.4 dB lower than with single receiveradaptation. In contrast, the EGC resulted in a 2.5-dB SNRgain for the receivers at the corners at the expense of as muchas a 9.9-dB decrease for the receiver at the center. Therefore,MRC collaborative adaptation is better than EGC as it improvesthe SNR at the opposite ends of the diagonal line (farthest awayfrom the transmitter) while slightly affecting the receiver placedat the center.

4) Receivers at Three Corners: The power distributions ofthe spots with the adaptive transmitter are identical for thetwo receivers at the corners on each side of the line of thediffusing spots and flipped as mirror images for the thirdreceiver at the other opposite corner, as shown in Fig. 10.Consequently, and due to symmetry, the SNR levels achievedwith the single receiver adaptive approach are equal. Hence,the power distributions due to the collaborative transmitteremploying MRC shows no difference compared with the powerdistribution computed with EGC. This also leads to the identicalSNR levels of the two methods, as illustrated in Fig. 11. Thisresult implies that this system configuration can use the simplerEGC, which reduces the computation of the weights to justone summing and one averaging operation. Furthermore, thecollaborative approach produces an SNR gain of as much as


Fig. 11. SNR comparison for receivers at three corners.

2.6 dB for the two receivers at opposite corners compared withthe case of the nonadaptive transmitter. This is a very highgain, which reduces the difference from the upper bound setby the single receiver adaptation to only 0.8 dB (compared with3.8 dB). Moreover, it is noted in Fig. 10 that the power distrib-ution of the 100 spots with the collaborative adaptation (MRCor EGC) is no longer symmetric. Higher powers are given tothe spots to the left of the center than at the right side. This,in turn, results in higher SNRs at each of the two receivers onthe left side of the room [at (1 m, 1 m, 1 m) and (3 m, 1 m,1 m)] than for the one at the right side corner [at (1 m, 7 m,1 m)]. The plot in Fig. 11 shows a slight difference of 0.5 dBbetween these SNR levels. It is clear that this difference isrelated to the difference in the number of receivers on eachside for which the collaborative system is adapting. Therefore,if placed at equal distances from the transmitter, the largerconcentration of receivers at one side of the transmitter resultsin slightly higher SNRs with the collaborative system than atthe other side with fewer receivers.

It is also noted from the curves of the single receiver adap-tation that as the transmitter adapts for one receiver at a corner,the SNRs at the other corners are improved as well. However,the comparable levels of the SNR at the other two cornersare reduced to almost a flat distribution with the collaborativeadaptation. This means that three users can take positions at anyof the corners and get almost equal quality signals, which aremuch better than the conventional nonadaptive noncollabora-tive transmitting approach. Moreover, both forms of collabora-tive adaptation are better than the nonadaptive system as evidentwith the SNR increase at all three receivers at the corners.

5) Five-Receiver Configuration: This configuration isshown in Fig. 2(f). In this configuration, two receivers areadded to the last configuration of three receivers at roomcorners: one at the center of the edge line at (1 m, 4 m, 1 m)and the other at the center of the side line at (2 m, 7 m, 1 m).Therefore, as shown in Fig. 12, the spot power profiles forthe three receivers at the corners are the same as are thosefor the other two receivers that were shown in the first twoconfigurations (i.e., three receivers on the edge line and thecenter line in Figs. 4 and 6, respectively).

For a clearer comparison of the SNR figures obtained foreach receiver with the various methods, the computed SNRsare listed in Table I. The numbers in parentheses are the SNR

Fig. 12. Spot power distribution profile for five receivers. (a) Single receiveradaptive and nonadaptive. (b) Collaborative system with MRC and EGC.

Fig. 13. SNR comparison for five receivers and collaborative systems (at threecorners and center of edge and side lines).


Fig. 14. Spot power distribution profile for single receiver and collaborativesystems for receivers at three corners [transmitter near room corner at (1, 7, 1)].

Fig. 15. SNR comparison for receivers at three corners [transmitter near roomcorner at (1, 7, 1)].

gain or loss compared with the nonadaptive scenario. It can beseen that at the position of the receiver with the highest SNR,EGC collaborative resulted in a considerable reduction of theSNR for that receiver location while producing a significantSNR gain (at least 0.6 dB) for each of the three receiversat the corners. On the other hand, MRC produced a betterperformance for the three receivers at the corners. As expectedfrom the MRC, the difference between the highest and lowestSNRs has been reduced from 3.8 (22.3–18.5 dB) to 4.7 dB(from 23.0 to 18.3 dB) obtained with the EGC. Moreover,despite the large distance of the receivers at the corners fromthe transmitter at the room center, collaborative transmissionwith MRC beam power adaptation produced an SNR increaseof nearly 1 dB over the nonadaptive system. This improvementis, however, only 0.6 dB if EGC was to replace MRC in transmitbeam power adaptation. Furthermore, collaborative adaptationwith MRC achieved 2.9 dB of the SNR level that could beproduced with single receiver adaptation for those receiversat the farthest distance from the transmitter. For the receiveraway from the line of spots, the difference was 3.3 dB. Whileobtaining the high SNR figures of the single receiver adaptationfor all five receivers together is impossible, the gain producedwith the MRC collaborative approach is a good achievement.

Single receiver adaptation for only the receiver at the cornerat (1 m, 7 m, 1 m) produced comparable performance with thatobtained by collaborative adaptation for the five receivers withMRC. In particular, the SNR at that receiver is improved by

Fig. 16. Spot power distribution profile for five receivers with mobile trans-mitter [at room corner (1, 7, 1)]. (a) Single receiver. (b) Collaborative systemwith MRC and EGC.

3.8 dB, and the SNR gain of the two receivers at the oppositecorners is nearly doubled (from 0.9 to 1.7 dB). While adaptationyielded this much improvement, it, however, resulted in afurther loss in SNR for the receiver that is located midwaybetween the two corners at (1 m, 4 m, 1 m) and midway betweentwo noise sources, which is also twice that obtained by MRC.Furthermore, adaptation for either receivers at the oppositecorners [at (1 m, 1 m, 1 m) or (3 m, 1 m, 1 m)] produced amuch lower SNR for the receiver located at the center of theside line (2 m, 7 m, 1 m).

B. Mobile Transmitter

1) Three Receivers: The result of the multibeam transmitterrelocating from the room center to the corner at (1 m, 7 m,1 m) is that 36 of the spots closer to the side wall drop to appearon the wall, thus creating a depletion area at the other end(y < 3 m). Hence, signal reception is tremendously enhancedaround the room corner to which that the transmitter moved.This is evident from the higher level of SNR at that cornercompared with those at the other far-end corners [at (1 m, 1 m,1 m) and (3 m, 1 m, 1 m)], as shown by the dotted line in Fig. 14.

With the power adaptive approach, those spots that fall onthe wall are given higher powers, which decrease with respectto every increase in spot height on the wall (increased directdistance to the receiver). In contrast, the other end spots are


Fig. 17. SNR comparison for five receivers and collaborative systems (atthree corners and center of edge and side lines) with a mobile transmitter [atroom corner (1, 7, 1)]. (a) Five receivers: single receiver adaptation. (b) Fivereceivers: adaptive, nonadaptive, and collaborative.

TABLE ISNR WITH SINGLE ADAPTIVE AND COLLABORATIVE (MRC AND EGC)

DATA FOR THE FIVE RECEIVERS

assigned higher powers for the other two receivers at theopposite corners. This approach yielded increases in SNR of2.2, 8.3, and 0.7 dB for the three receivers at the positionsshown in the figure, respectively, compared with the case ofthe nonadaptive transmitter. The SNR for the receiver at samelocation as the transmitter [at (1 m, 7 m, 1 m)] is highest dueto the increased spot population on top of it as a result oftransmitter movement. Moreover, this receiver benefits mostfrom the adaptive approach as it gets an 8.3-dB increase in itsSNR. On the other hand, the receiver that is farthest from thetransmitter on the opposite room corner [at (3 m, 1 m, 1 m)]gets the lowest SNR. Although this receiver is at the maxi-mum distance [the length of the room diagonal (about 9 m)],

a significant increase in SNR is obtained when the systemadapts for this receiver. As the MRC collaborative adaptiveapproach produced about 0.3-dB SNR improvement for thisreceiver, it also improved the SNR level for the other cornerreceiver [at (1 m, 1 m, 1 m)] by 1.5 dB of the 2.3 dB that wouldhave been obtained if the transmitter power was adapted for thatother corner receiver. These improvements are significant whencompared with the results of the EGC collaborative adaptiveapproach. The performance of the collaborative adaptation withEGC is poor since it produced much more improvement for thereceiver already enjoying a high SNR than those receivers thatare starved for better signals. This poor performance is linkedto the effect of transmitter movement, thus creating depletionof diffusing spots near these “far away” receivers in additionto the influence of the spot lamps (the background noise) ontop of the receivers. These effects are diminished with theMRC collaborative adaptive approach. Hence, MRC showedits effectiveness through the comparable improvement of theSNR at the two far-end corners when the three receivers coexistand the transmitter is at the farthest/opposite end of the room.Although the SNR gain for the receiver at the farther cornerfrom the transmitter is less than that at the other end with theMRC collaborative, a 1-dB increase in SNR is obtained forthe far-end receiver. On the other hand, the EGC collaborativeapproach produced better SNR for the receiver at the sameposition as the transmitter [at (1 m, 7 m, 1 m)] and poorerperformance for the other two receivers at the opposite corners.

2) Five Receivers: This configuration is shown in Fig. 2(g),where the multibeam transmitter is relocated at the corner at(1 m, 7 m, 1 m), and the locations of the five receivers are thesame as in Fig. 2(f) for which collaborative performance wasexplained in Section V-A5. The result of the transmitter movingto the room corner is the same as explained in Section V-B1(with three receivers). In contrast to the spot power distributionsof the collaborative system with the nonmobile transmitter (inA.5), the two combining techniques show distinct differences.As shown in Fig. 16(b), MRC assigns higher powers to the first20 spots at the expense of lowering the transmit powers of thelast 20 spots compared with the respective transmit power levelsassigned by EGC. The result of this is that the SNRs of thetwo receivers at the far-end corners significantly increase withthe collaborative adaptation of the transmitter’s power. MRCsignificantly increased the SNR at the farthest corner from thetransmitter [at (3 m, 1 m, 1 m)] by almost 1 dB, while EGCresulted in an SNR decrease of 0.5 dB. Moreover, at the othercorner that is on the same line as the transmitter [at (1 m, 1 m,1 m)], MRC produced an SNR gain of 2.4 dB, whereas a 0.9-dBdecrease in SNR was obtained with EGC. These improvementsare produced despite the receivers being directly under spotlights as well as the transmitter having moved to the worsttransmission location (near a room corner). In producing thesegains for the distant receivers, the two receivers that are closerto the transmitter [the one at (1 m, 7 m, 1 m) and that at(2 m, 7 m, 1 m)] whose SNRs are highest are slightly affectedby an SNR reduction of only 5%–7%. It is to be noted that theSNRs obtained by MRC for the five receivers are very close tothose that result when the transmitter adapts its spot transmitpowers only for the receiver at the center of the edge line


TABLE IISNR WITH SINGLE ADAPTIVE AND COLLABORATIVE (MRC AND EGC)

DATA FOR THE FIVE RECEIVERS WITH A MOBILE

TRANSMITTER [AT ROOM CORNER (1, 7, 1)]

[at (1 m, 4 m, 1 m)], as seen from the figures in Table II.Therefore, it appears that the location of this receiver actedas a median between the five receivers resulting in balancingthe transmit powers of the multibeam transmitter between thenearby and the far-end receivers.

VI. DISCUSSIONS AND CONCLUSION

A novel collaborative adaptive power transmitter for indoorOW systems has been introduced. The transmitter adjusts thepower of each of its narrow beams that produce the diffusingspots based on the coexistence of multiple receiving users inthe channel. The objective is to improve the performance ofall receivers concurrently, if possible. The performance of thesystem was evaluated for three receivers placed in four differentconfigurations in the room as well as a case of five receivers.Particular attention was given to the case where the receiversare located at the weakest link positions at the room corners.Combining the transmit spot powers from the three receiverswith an MRC-based method proved effective, particularly atthe weaker links. The collaborative adaptive system has beenextended to study mobile transmission.

An improvement of 1 dB is achieved over the nonadaptivesystem for the two receivers at the room corners. This SNRgain brings the system closer to the upper bound of 3-dBimprovement attained if the adaptation was for single receivers.On the other hand, the single receiver adaptation producedhigher SNRs when all receivers were underneath the line ofspots (strong signal and very little noise). The collaborativesystem with MRC adaptation had better results for the receiversat the opposite ends of the diagonal line than with EGC. Thisresult clearly showed that the collaborative system with MRCadaptation overcomes the following three degrading factors:1) the effect of background noise; 2) the signal weaknessresulting from increased transmitter–receiver separation; and3) the mutual effect of other existing users. When all threereceivers were on different corners (with equal distances tothe transmitter), both MRC and EGC adaptation produced thesame high improvement of nearly 3 dB. This is a very highgain, which reduces the difference from the upper bound setby the single receiver adaptation to only 1.2 dB. Moreover, theresults of this case showed that as the number of receivers atone side of the transmitter increase, the collaborative system

producing slightly higher SNRs for those receivers than at theother side at equal distances from the transmitter. Of particularsignificance is the gain achieved for the case involving theworst transmitter–receiver scenario, i.e., receiver at a cornerand transmitter at the other corner. A higher SNR than thatachieved with the single receiver adaptation has been producedwith the collaborative system for the receiver at the farther op-posite corner from the transmitter. Therefore, the collaborativesystem is also effective with a mobile transmitter. Among allthe cases studied, the case having receivers at three differentcorners (hence equal distances from the transmitter) showedno difference in the results obtained with either EGC or MRCweights combining. Moreover, the MRC shows its advantagesfor those receivers located further away from the transmitterthan those closer or at the same location of the spot-diffusingtransmitter. Furthermore, the advantages are clear as uniformSNR levels are produced for the three receivers along the roomedge line, where an SNR gain of as much as 1.2 dB over thesingle receiver at the corners has been achieved. Finally, whenthe collaborative adaptation method was applied to a mobilemultibeam spot-diffusing transmitter, a significant SNR im-provement was obtained for the receivers that are far awayfrom the transmitter compared with the nonadaptive case. Ourfurther investigation will focus on deriving a model for thedependence of received power at each receiver on its distancefrom the transmitter.

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Jamal M. Alattar was born in Khan Younis-GazaStrip, Palestine, in April 1965. He received the B.Sc.degree in computer science from Mississippi StateUniversity, Starkville (USA), in August 1989 andthe M.Sc. degree in communication systems, fundedby the Chevening scholarship program of the BritishCouncil in Jerusalem, from the University of Wales,Swansea, U.K., in September 2001. He is currentlyworking toward the Ph.D. degree in optical wirelesscommunication with the University of Leeds, Leeds,U.K., through a scholarship granted by the Ministry

of Higher Education, Palestine, in 2003.From 1993 to 2000, he took several administrative and academic posts at

the College of Science and Technology (CST), Khan Younis. Promoted frombeing the College Registrar in July 1995, he was the Director of Registrationand Admissions until August 2000. In September 2001, he returned to CST toswitch to an academic career. He is the author of a number of published papersboth at the national and international levels. In particular, his novel work onindoor optical wireless (OW) communication appears in the IEEE proceedingsof London Communications Conference (LCS), International CommunicationsConference (ICC), Global Communications Conference (GlobeCom), and Op-tical Networks Design and Modelling (ONDM) conference. Moreover, he hasauthored papers published in the IET Optoelectronics journal and the IEEETRANSACTIONS ON VEHICULAR TECHNOLOGY. He also simulated multipathpropagation in indoor channels. His research interests include adaptive tech-niques for OW, OW systems design, and indoor OW networking.

Jaafar M. H. Elmirghani joined the University ofLeeds, Leeds (UK), in 2007, and prior to that, hefounded, developed, and directed the Institute ofAdvance Telecommunications, University of Wales,Swansea, U.K. He has provided outstanding lead-ership in a number of large research projects overthe past five years and led a number of multisitecollaborative projects funded by the Engineering andPhysical Sciences Research Council, (EPSRC), De-partment of Trade and Industry (DTI)/TechnologyStrategy Board, and the European Union (EU). He

was a Principal Investigator (PI) of the following European Regional Devel-opment Fund (ERDF) projects: Institute of Advanced Telecommunicationsand projects Intelligent Radio-Fibre Telematics Scout (IRIS), and Radio-FibreEnabled ACcess Highway (REACH). He was/is a co-PI/WP leader of theEPSRC/DTI projects Heterogenous Internet Protocol Networks (HIPNet), TheIntelligent Airport (TINA), Village eScience for Life (VESEL), and INte-grated STorage Area NeTworks (INSTANT) and of EU FP6 PHOSPHOROUS(Lambda user controlled infrastructure for European research). The followingprojects involved him, forming important industrial links: Laing O’Rourke,British Airports Authority, Motorola, Ericsson, Red M, Boeing, Tyco, Zinwave,Innovision, and Arup (TINA); Ericsson, Freescale, and Emerson (HIPNet);Motorola and O2 (Vesel, IRIS); BT and Agilent (REACH); ALPS and Xyratex(INSTANT); and Hitachi and Nortel (PHOSPHOROUS). He has published over300 journal and conference papers. He has research interests in communicationsystems and networks.

Mr. Elmirghani is a Fellow of the Institute of Engineering Technology(IET), a Fellow of the Institute of Physics, and the Director of the Institute ofIntegrated Information Systems within the School of Electronic and ElectricalEngineering, University of Leeds. He received the IEEE CommunicationsSociety Hal Sobol Award for achievements in communications, the IEEEComsoc Chapter Achievement Award for excellence in chapter activities (bothin international competition in December 2005), the IEEE ComSoc 2009 signalprocessing and communication electronics Award, and the University of WalesSwansea “Outstanding Research Achievement Award” in 2006.

optical wireless systems employing adaptive collaborative transmitters in an indoor channel

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