Performance analysis of a car to car visible light communication

system

Pengfei Luo,1,* Zabih Ghassemlooy,1 Hoa Le Minh,1 Edward Bentley,1

Andrew Burton,1 and Xuan Tang,2

1Optical Communications Research Group, NCRLab, Faculty of Engineering and Environment, Northumbria University, Newcastle-upon-Tyne, NE1 8ST, United Kingdom

2Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fujian, China*Corresponding author: oliver.luo@northumbria.ac.uk

Received Month X, XXXX; revised Month X, XXXX; acceptedMonth X, XXXX; posted Month X, XXXX (Doc. ID XXXXX);

published Month X, XXXX

This paper presents an analytical performance analysis of a car-to-car visible lightcommunications system under different communication geometries during the daytime. Amarket-weighted headlamp beam pattern model, measured dirt effects on light distributionand the road surface reflection model are employed. We consider both the line-of-sightand non-line-of-sight links, and outline the relationship between the communicationrange and the system bit error rate (BER) performance. Results show that the wet roadsurface can help to increase the received optical power and improve the BER performanceabove a certain distance, and the communications coverage range can reach up to 70 m ata data rate of 50 Mbps when a photodetector is mounted on the car at a height of 0 – 0.2m above the road surface. © 2014 Optical Society of America

OCIS codes: (060.4510) Optical communications; (120.4820) Optical systems.http://dx.doi/org/10.1364/AO.99.099999

1. IntroductionAccording to the Global status report on road safety 2013:supporting a decade of action [1] issued by the WorldHealth Organization (WHO): road traffic crashesare the leading cause of death among young peopleaged 15-29 and the eighth leading global cause ofdeath. Approximately 1.3 million people die eachyear in road accidents, with an estimated $518billion being lost globally, costing individualcountries 1-3 % of their annual gross nationalproduct. Unless urgent action is taken, currenttrends suggest that road traffic injuries willbecome the fifth leading cause of death by 2030[2]. With the growth of the number of vehicles,traffic congestion is becoming an increasinglywidespread problem. Firstly, people may spend alonger commuting time between their home andworkplace. Secondly, air and noise pollution willincrease, severely threatening the health ofurban populations. What’s more, fuel consumptionis dramatically increased, which contributesgreatly to the amount of CO2 emissionscontributing to global warming [3].

Intelligent transport systems (ITS) offer agreat potential to enhance road safety, improvetraffic flow, and address environmental concernsby monitoring driving behavior, communicatingbetween vehicles and the roadside infrastructure,thus giving warnings to drivers and providinginformation for safe driving [4]. A dedicatedshort range communication (DSRC) is considered tobe a promising technology for enablingbidirectional communications between vehicles andthe roadside infrastructures [5]. In 2010 theIEEE published the dedicated IEEE 802.11pstandard, which operates in the 5.835-5.925 GHzradio frequency (RF) band to support ITSapplications [6]. Apart from this RF-basedsystem, another approach to car to carcommunication (C2C) is via visible lightcommunication (VLC) technology, using visiblelight emitted by light-emitting diodes (LED) fordata transmission as well as for illumination[7]. For C2C communications, the VLC technologyis able to offer distinctive advantages comparedto the RF-based DSRC technologies, such as (i)low complexity and low cost, particularly inplaces where LED lamps are growingly used in

vehicles, traffic lights and street lights [8];(ii) high precision positioning, owing to thedirectional line-of-sight (LOS) propagationcharacteristics of light. VLC based positioningtechnologies are able to reduce the positioningerror to tens of centimeters [9], which are moreaccurate than the RF based positioning technology[10]; and (iii) high scalability and low level ofinterference [11], as vehicle density increases(e.g. during the rush hours), RF technologieswill typically experience undesirable packetcollisions and longer delays as well as poorpacket reception rate [12], whereas in VLCtechnologies vehicles only receive signals fromneighboring vehicles, thus leading to muchreduced signal congestion and interference. Table1 shows the comparison of VLC and RF (DSRC)technologies.In addition to the ability to provide

centimeter level positioning accuracy [9], theVLC technology will also contribute to improvedroad safety and reduced traffic congestion.Nowadays vehicles can observe their surroundingsusing a number of technologies including radar,lidar, Global Positioning System (GPS), computervision and then determine suitable navigationpaths based on this system information [13].However, sensor based

Table 1. Comparison of VLC and RF (DSRC)technologies

Type VLC [14] RF (DSRC) [15]

Communicationmode

LOS: Point-to-point

Broadcasting:Point-to-multipoint

Carrierfrequency 400-790 THz 5.85-5.925 GHz

Mobility Low - medium HighCoverage area Low and narrow Long and wide

technologies are complex, costly and not green (ahigh level of power consumption). Instead ofmonitoring their surroundings for information,vehicles can receive and transmit trafficinformation such as speed, position, anddirection from and to nearby vehicles androadside infrastructures using the C2C VLCtechnology. This technology is less complex, morecost effective and green where added valuefeatures can be implemented.Some theoretical and experimental studies on

C2C VLC have been reported in the literature. In[16], the feasibility of a road-to-vehiclecommunication system using a LED array and ahigh-speed camera was studied. A hierarchicalcoding scheme was used to allocate data todifferent spatial frequency components dependingon their priorities. According to the results ofboth static and driving field trials, there wasan improvement in the bit error rate (BER)

performance following adaptation of hierarchicalcoding. In [17], an outdoor VLC system for theITS application was investigated, where a directsequence spread spectrum (DSSS) scheme replacedon-off keying (OOK) and/or pulse positionmodulation (PPM) to minimize the effect ofambient noise. The system achieved a low datarate transmission of 20 kpbs over a distance morethan 40 m in the presence of ambient light (i.e.the sun etc.). In [11] VLC for vehicle safetyapplications has been studied, offering a datarate of 10 kbps over a distance of 20 m.Although the Lambertian model has been widely

used for indoor VLC systems, for vehicle’sheadlamps this model is inappropriate due to thenature of lighting application [18]. In thispaper, we carry out an accurate performanceanalysis of a C2C VLC by adopting a market-weighted headlamp beam pattern model. Here weconsider both the LOS and non-line-of-sight(NLOS) link configurations. The received opticalpower plots are obtained for different types ofroad surfaces. Finally, we determine the systemBER distributions for a series of verticalreceiver planes under different communicationranges and road surfaces by using both clean anddirty lamps during the daytime.The rest of the paper is organized as follows.

In Section 2, we describe the C2C VLC systemconfiguration, which includes the market-weightedheadlamp beam pattern model, the reflectionproperties of road surfaces and the ambient lightnoise analysis. Based on the mathematical modelsin Section 2, the received optical power and theBER performances are investigated in Section 3.Finally a conclusion and suggestions for thefuture work are given in Section 4.

2. System ModellingThis section focuses on the modeling of the C2CVLC system, which includes the model of market-weighted headlamp beams, road surface reflection,and the ambient noise. A typical C2C VLC scenariois shown in Fig. 1., where the car 1 on the leftcommunicates with the car 2 on the right usingits headlamps. As shown, the projected beampattern is unique, and the received light is acombination of the LOS and NLOS (i.e. reflected)

Fig. 1. (Color online) A basic VLC-based C2Ccommunication system.

beams. Note that the reflected rays from the roadsurface, which depend on the road surfacematerials, the angle of incidence as well as theweather condition (fog, rain, snow etc.), willlead to multipath induced inter-symbolinterference (ISI), thus leading to reduced datarate. However, in C2C VLC since the typical datarate is not too high then inter-symbolinterference (ISI) may not be of a major concern.In Fig. 1., the major noise sources are thesunlight during the day time and light from othercars (e.g. car 3) and road side infrastructuresduring the night time, which will affect the linkperformance.

A. Headlamp ModelAccording to the Economic Commission of Europe(ECE) [19] and Federal Motor USA Vehicle SafetyStandards (FMVSS) [20] regulations, in order toinsure that vehicles provide acceptable roadillumination while not causing any glare forother road users, the headlamp, reflectivedevices, and associated equipment must meetspecific requirements as in [21]. Therefore, tomodel a vehicles headlamp pattern one cannotsimply use the standard well known Lambertianmodel. The main purpose of both high and low beamheadlamps is to provide a safe and comfortablelighting environment for the drivers and otherroad users, during the day and night times and inall weather conditions. In detail, the high beamsare used for long-distance visibility with nooncoming cars, whereas the low beams, with anasymmetrical light pattern, provide maximumforward and lateral illumination while minimizingthe glare toward oncoming cars and other roadusers [21].In this work we utilize a market-weighted

headlamp beam pattern model for C2C VLCs, as amore realistic and reliable mathematical systemmodel. This headlamp model, provided byTransportation Research Institute of theUniversity of Michigan, [22] is measurementbased, providing a database for both high and lowheadlamps for top-selling vehicles in USA. Forthis market-weighted database 25 tungsten halogenheadlamps were randomly selected from the top 90%of vehicle sold in USA in 2010. Followingphotometric data measurement using agoniophotometer, the collected data were weightedby the current sales figure for the correspondingvehicle.According to the luminous intensity (candela) I

table given by this model, we are able tocalculate the corresponding illuminance value atany location of interest using the followingexpression [23]:

11\*MERGEFORMAT ()

where E is the illuminance (lx), Φ is theluminous flux (lumen), S is the area of the lightreceiving surface (m2), Ω is the solid angle(sr), I(ζ, ξ) is the luminous intensity (cd), ζ andξ are the horizontal and vertical angle (inrelation to the headlamp axis), respectively, dis the distance between the light source and the

Fig. 2. (Color online) Illuminance calculation.

Fig. 3. (Color online) Isocandela and isoilluminancediagrams of the road surface from a pair of high-beamheadlamp, luminous intensities at the 50th percentile(Lamp mounting height: 0.62 m. Lamp separation: 1.12 m).

Fig. 4. (Color online) Isocandela and isoilluminancediagrams of the road surface from a pair of low-beamheadlamp, luminous intensities at the 50th percentile(Lamp mounting height: 0.66 m. Lamp separation: 1.20 m).

Horizontal angle (degree)

Vertical angle (degree

)

(a). Isocandela diagram (cd)

-30 -20 -10 0 10 20 30-20

-10

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10

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300

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10000

30000

1 1 1

1 11

2 2 2

22

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3 3

333

5 555

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20203050

Lateral distan

ce(m)

Longitudinal distance(m )

(b). Iso-illum inance diagram (vertical lx)

0 20 40 60 80 100 120 140 160 180 200-20-1001020

Horizontal angle (degree)

Vertical angle (degree)

(a). Isocandela diagram (cd)

-30 -20 -10 0 10 20 30-20

-10

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Lateral distance(m)

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(b). Iso-illum inance diagram (vertical lx)

0 10 20 30 40 50 60 70 80 90 100

-10

0

10

small area dS, and γ is the angle between thelight receiving surface normal and the incidentdirection, see Fig. 2..According to [22] and (1), the isocandela and

isoilluminance diagrams of the road surface froma pair of high-beam headlamps (luminousintensities at the 50th percentile, lamp mountingheight: 0.62 m, lamp separation: 1.12 m), and apair of low-beam headlamps (luminous intensitiesat the 50th percentile, lamp mounting height: 0.66m, lamp separation: 1.20 m) are plotted in

0 500 1000 1500 20000

500

1000

1500

2000

Input lum inouis intensity (cd)

Output Lum

inouis inten

sity (cd)

clean conditiondirty condition

Fig. 5. (Color online) The measured output luminousintensities for the right headlamp before and after thewinter/salty drive [24].

Fig. 6. (Color online) (a) ideal Lambertian (diffuse)reflection, (b) diffuse and specular combined reflection[25].

Fig. 3. and Fig. 4., respectively. It is apparentthat for high-beam headlamps, a narrow and flatbeam is projected in a horizontal direction a fewdegrees to the left, providing a symmetricalillumination pattern on the road. However, thelow-beam headlamps provide an asymmetrical

pattern designed to offer adequate forward andlateral illumination, in addition to controllingglare by limiting light being directed towardsthe eyes of other road users.According to [24], dirt deposits do affect the

low beam headlamps distribution, which leads to areduction in the total amount of emitted lightand an increase in scattered light. Fig. 5. showsa measured light intensity change before andafter a specified winter/salty road drive. Thegreen solid line is the best fitting linear modelof measurement, which can be expressed as:

22\* MERGEFORMAT()It is clear that dirt affects tend to increase

light output in the direction where the lightintensity is less than 400 cd—near or above thehorizontal level, see Fig. 4. (a), whereas itreduces the light intensity below the horizontallevel (or larger than 400 cd). The effects causedby dirt on the light distribution will beassessed by means of the BER performance.

B.Road Surface ReflectionIn C2C VLCs, the received optical power isusually composed of LOS and NLOS components. Foran ideal Lambertian diffuse surface, the radiantintensity of a Lambertian surface R() isexpressed as [26]:

33\* MERGEFORMAT()

where is the diffuse reflectivity, which varieswith different road surfaces, and is the polarangle of the scattered light.However the Lambertian model is not an accurate

model to be adopted for the real-world roadsurface [25]. As is well known, the reflectanceof road surface depends on its nature andphysical state, and it also changes with the roadsurface and weather

Fig. 7. (a) angles for the calculation of luminance coefficient q; (b) the luminance-coefficient indicatrix [27].

conditions. For wet or moist road surfaces alarge amount of specular reflection may occur,thus resulting in greater luminance non-uniformity reflections [28].Fig. 6. illustrates the reflection patterns for

an ideal Lambertian surface and an arbitrary roadsurface. As we can observe, for the ideal case,the reflected radiant intensity is proportionalto the cosine value of polar angle. However, forthe arbitrary surface both diffuse and specularreflections will take place, and the reflectedlight will be emitted in all directions of theupper hemisphere, which is challenging todescribe mathematically [25]. Therefore, a table(r-table) of luminance coefficients for thearrange of angles has been developed fordifferent road surface classifications based on alarge number of photometric measurements [29].According to [27], the reflective properties of

a typical road surface can be characterized bythe luminance coefficient q, which is expressedas:

44\*MERGEFORMAT ()

where L is the road surface luminance (cd/m2), αis the angle of observation, is the anglebetween vertical plane of incidence and thevertical plane of observation, see Fig. 7. (a).By convention α is fixed at 1º for road lightingcalculations [27]. Generally the value of q isreasonably constant when 0.5º < α < 1.5º [30],therefore, q depends only on and γ. A moreintuitive figure is plotted in Fig. 7. (b), wherethe luminance coefficient q corresponding to theobservation position O and a direction ofincidence of the light (, γ) is indicated by aline segment in the direction of (, γ) whoselength is proportional to q. The end points of

these lines describe the surface of theindicatrix B [27].For convenience, the reduced luminance

coefficient r(, tan γ) is employed instead of q(,γ) [27], which is given by:

55\*MERGEFORMAT ()Once the complete reflection data of a

particular road surface are measured, an r-tablecan be created. From the r-table the reflectiveproperties from any direction (, γ) for a givenroad surface can be simply obtained usinginterpolation.In addition to the detailed reflective property

for a range ofincidence angle, to describe the generalreflection properties of a road surface weintroduce two more parameters, they are thespecular factor S1 and the average luminancecoefficient Q0, which is defined as [28]:

66\* MERGEFORMAT()

where Ω0 is the solid angle, which is measuredfrom the point on the surface, containing allthose directions from which light is incidentthat are taken into account in the averagingprocess [28].

S1 defines the specularity of the road surface,which is generally large for specular reflection,and is defined as [31]:

77\* MERGEFORMAT ()

where r (0, 2) and r (0, 0) are the reducedluminance coefficients when = 0º, γ = 63.4º(tan(63.4º) = 2) and = 0º, γ = 0º respectively.

Considering a certain level of tolerance, CIEhas classified the road surfaces into differentclasses for a range of road surface materials andwet conditions according to values of Q0 and S1[27, 32]. Each class is represented by astandard r-table, which are based on differentroad surface materials and measurements carriedout in the 1960’s and 1970’s [27]. Although newroad surface materials have been used since then[33], in this paper, we will use the standard CIEr-tables as a proof of concept. Table 2 [34]shows 8 road surfaces classification defined interms of the r-table, Q0 and S1. Note R1 is themost diffuse surface while W4 is the mostspecular surface.

Table 2. Road surface classificationsStandardtable Q0 S1 Description

R1 0.10 0.247 Mostly diffuse

R2 0.07 0.582 Mixed (diffuse andspecular)

R3 0.07 1.109 Slightly specularR4 0.08 1.549 Mostly specularW1 0.11 3.152 Wet road surfaceW2 0.15 5.722 Wet road surfaceW3 0.20 8.633 Wet road surfaceW4 0.25 10.842 Wet road surface

Fig. 8. (Color online) Reflection of headlamps by (a)dry road, (b) wet road, (c) “wet” snowy road.

Fig. 8. illustrates three real life roadconditions with reflections: dry, wet, and “wet”snowy road. It is notable that the degree ofreflection changes dramatically as the roadsurface condition changes.A typical C2C VLC geometric configuration is

depicted in Fig. 9.. Since both the left andright headlamps have almost identical outputlight distribution [35], only rays from the rightheadlamp are illustrated in this figure andconsequently detailed mathematical analysis forthe right side headlamp (RSH) will be outlined inthis paper. It can be observed from Fig. 9. thatowing to the reflection from the road surfacereceived beams are composed of both LOS and NLOScomponents. We first analyze the received power from NLOS

link. According to (1), the illuminance ERSH-A (lx)at the location A with an area of dS is given by:

88\*MERGEFORMAT ()

where dRHS-A is the NLOS path length from RSH to thepoint A.Therefore, based on Fig. 2., Fig. 7. and 4, the

road surface luminance LRHS-A (cd/m2) caused by theRSH at the point A is expressed as:

99\* MERGEFORMAT ()

where q(A, γA) is the luminance coefficient withangles of A and γA, and hTX is the height of thelamp above road surface, see Fig. 9..By combining 5 and 9 we have:

1010\*MERGEFORMAT ()

Consequently, the reflected luminous intensityIA-ref (cd) from the road surface is given as:

1111\*MERGEFORMAT ()

By considering the small area of road surfaceas a secondary light source with a luminousintensity of IA-ref, the illuminance captured at thePD surface is defined as:

1212\*MERGEFORMAT ()

where ψNLOS is the angle of incidence from theview of PD, and dA-Rx is the NLOS path between A andPD, see Fig. 9..According to [36], we consider the luminous

efficacy of radiation (LER) of a high powerphosphor-coated WLED to be 250.3 lm/W. Thus thereceived optical power dPRx-RHS-NLOS (W) from thereflected surface A is expressed as:

1313\*MERGEFORMAT ()

where Ar is the detection area of the PD.As a result, for RSH, the total received

optical power PRx-RHS-NLOS from all reflected paths isexpressed as:

1414\* MERGEFORMAT ()

where Ψ is the half angle of PD’s field of view(FOV), and S is the entire area of road surfacethat has been illuminated by the right side lamp.For RSH the received optical power PRx-RHS-NLOS for

the LOS link is defined as [37]:

1515\* MERGEFORMAT ()

where IRHS(ζB, ξB) is the luminous intensity for theangle pair of (ζB, ξB) as is defined in Fig. 2.,dRHS-Rx is the distance between RHS and the receiver,and ψLOS is the angle of LOS incidence.Therefore, the total received optical power PRx-RHS

from RSH is given by: 1616\*

MERGEFORMAT ()Finally, the total received optical power PRx is

expressed as: 1717\*

MERGEFORMAT ()where PRx-LHS is the total received optical powerfrom the left side headlamp (LHS), which has thesame mathematical expressionsas RSH except a different lateral position.

Fig. 9. (Color online) Configuration of C2C VLC system (only rays from the right headlamp are illustrated).

C.Noise AnalysisWith regarding to outdoor VLC systems, there aretwo main ambient light noise sources, thebackground solar radiation and artificial light(i.e. street lights, other vehicle lights, staticneon sign boards, and advertising screens) [38].The background solar radiation consists of directand scattered radiation. From the view of VLCs

the direct solar radiation is the dominant noisesource depending on the weather conditions,months, time of the day, the direction of the PDand the position of the sun both during the dayand throughout the year [39]. The scatteringradiation modelling is complex due to thesurrounding environment. According to [7] themeasured electrical power spectrum of the solar

radiation is almost a direct current (DC), whichcan easily be removed by AC coupling. However,the shot noise induced by the solar radiationremains the main source of noise for C2C VLCsystems during the daytime [7, 38].From [38] we know that, artificial light

induced interference is mainly located in thelow-frequency portion of the communicationspectrum (i.e. below a few hundreds of Hz). Theselight intensities are quite small compared to thesolar radiation. Therefore they have negligiblecontribution during the daytime when the solarradiation is the dominant noise source. Inaddition to the ambient light noise, we also haveto consider the thermal noise associated with thereceiver. Here, we mainly consider the shot noiseinduced by solar radiation and the thermal noise,which both are considered as the additive whiteGaussian noise (AWGN). The total noise varianceis expressed as:

1818\*MERGEFORMAT ()According to [40], the shot and thermal noise

variances are given as:1919\*

MERGEFORMAT ()

2020\*MERGEFORMAT ()

where, e is the electronic charge (1.602×10−19 C),R is the average responsivity of the PD, PRx-Signal isthe average received optical power of the desiredsignal, Bs is the system bandwidth, Ibg is thereceived background noise current, k isBoltzmann’s constant, TK is absolute temperature,G is the open-loop voltage gain, η is the fixedcapacitance of PD per unit area, Γ is the field-effect transistor (FET) channel noise factor, gm

is the FET trans-conductance, I2 is the noisebandwidth factor for the background noise [41],and the noise bandwidth factor I3 = 0.0868 [42].

3. Performance of C2C VLC SystemIn this section, the relationship between thereceived optical power and transmission distanceis analyzed, and then the BER performance of theC2C VLC system is obtained. According to [43],the typical channel delay for a C2C VLC link isabout 10 ns whereas the VLC data rate is about afew Mbps, therefore the ISI introduced by themultipath environment is considered to benegligible.

A. Received Optical Power PerformanceFor mathematically modelling the received opticalpower against the communication distance, we haveadopted the following: optical channel

configuration as shown in Fig. 9. and low-beamlamps (50% luminous intensity) for the daytime.Table 3 gives the key parameters for the systemmodelling, here we have chosen R1 and W4 road forthe purpose of maximizing the contrast. Therelationships between the received optical powersfrom both LOS and NLOS links and distance areillustrated in Fig. 10.. The results arecalculated based on the analysis from Section 2when the PD is mounted at different heights hRx

from 0.2 – 0.8 m at the lateral position of -0.6m, which is the position of RSH of the car inorder to obtain the maximum optical power asshown in Fig. 4..The results show that for LOS links, there is

no difference in received optical power for bothR1 and W4 road surfaces for a range of PDpositions. However, for the NLOS link, there is asignificant difference between the two types ofroad surfaces when the PD is mounted at a heightof 0.2 m, but the difference becomes smaller asPD mounting height increases. We can also

Table 3. Modelling parameters

Parameters Symbol Value

Absolute temperature TK 298 (K)Average responsivity of PD γ 0.35 (A/W)

Boltzmann’s constant K 1.38×10−23 (J/K)Distance between Tx and Rx dTx-Rx 0-70 (m)

Electronic charge e 1.6×10−19 (C)FET channel noise factor Γ 1.5FET transconductance gm 30 (mS)

Fixed capacitance of PD perunit area η 112 (pF/cm2)

FOV of the PD Ψ 35ºLuminous efficacy of

radiation [36] LER 250.3 (lm/W)

Noise bandwidth factor I2 0.562Open-loop voltage gain G 10PD effective active area Ar 150 (mm2)

PD lateral position - 0.6 mReceived background noise

current Ibg 5100 (μA)

VLC system bandwidth Bs 2 (MHz)

Fig. 10.(Color online) Received optical power.

clearly observe that for NLOS links each plot ofthe received optical power displays a peak, whichchanges both in amplitude and position with thePD mounting height. This is due to combination oflow beam lamp and road reflections. The level ofthe received optical power for the NLOS link isabout 1/10 of that of the LOS link. The BERperformance of the proposed system underdifferent communication distance is outlined inthe following section.

B.C2C VLC BER PerformanceIn the proposed system, we have adoptedintensity-modulation direct-detection (IM/DD)employing OOK and a PD. Once the system bandwidthBs and the relative position of Tx and Rx arefixed, the received optical power and the noisevariances which includes ambient light andthermal noise can be obtained from (17), (19) and

(20). Therefore, for the AWGN communicationsystem the electrical signal-to-noise ratio (SNR)at the receiver side is given by [26]:

2121\* MERGEFORMAT()

where γ is the responsivity of PD which changeswith wavelength.Consequently, the BER is given as:

2222\*MERGEFORMAT ()

where, Q(x) is the Q-function used to calculatethe tail probability of the standard Gaussiandistribution, and is given by:

2323\*MERGEFORMAT ()

0 10 20 30 40 50 60 7010-7

10-6

10-5

10-4

10-3

10-2

Distance between two vehicles (m )

Received Optical Pow

er (W

)

(a). PRx of R1 and W 4 road surfaces (hRx = 0.2m )

PRxx

from LOS link (W 4)

PRx from NLOS link (W 4)

PRx from LOS link (R1)

PRx from NLOS link (R1)

0 10 20 30 40 50 60 7010-7

10-6

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Distance between two vehicles (m )

Received Optical Pow

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(b). PRx of R1 and W 4 road surfaces (hRx = 0.4m )

PRx

from LOS link (W 4)

PRx from NLOS link (W 4)

PRx from LOS link (R1)

PRx from NLOS link (R1)

0 10 20 30 40 50 60 7010-7

10-6

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Received Optical Pow

er (W

)

(c). PRx of R1 and W 4 road surfaces (hRx = 0.6m )

PRx

from LOS link (W 4)

PRx from NLOS link (W 4)

PRx from LOS link (R1)

PRx from NLOS link (R1)

0 10 20 30 40 50 60 7010-7

10-6

10-5

10-4

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Distance between two vehicles (m )

Received Optical Pow

er (W

)

(d). PRx of R1 and W 4 road surfaces (hRx = 0.8m )

PRx

from LOS link (W 4)

PRx from NLOS link (W 4)

PRx from LOS link (R1)

PRx from NLOS link (R1)

C. BER ResultsThe BER distributions at a data rate of 50 Mbpson a vertical plane for three differenttransmission spans of 20 m, 40 m and 70 m, andemploying clean and dirty low beam headlamps aredepicted in Fig. 11. and Fig. 12., respectively. It is notable that as the distance increases

the system BER performance decreases evidently.We can see from both Fig. 11. and Fig. 12. thatthe lowest contour lines for the BER degrade from10-80 at 20 m to 10-4 at 70 m. This is because whendTx-Rx changes from 20 m to 70 m, the receivedoptical power reduces by more than 10 times (e.g.it decreases from 3.79×10-5 W to 2.15×10-6 Waccording to Fig. 10. (a)). It can also be

observed that the zones with the lowest BER tendto be smaller and shorter as the distanceincreases, and their positions incline to be moreskewed to the right. This becomes more apparentwhen examining lines for the BER of 10-30 in Fig.11. (c) & (d) and 10-4 in Fig. 12. (c) & (d),since the adopted low-beam headlamp model isdesigned for USA cars with the right hand drive.This phenomenon can also be found in Fig. 10.,where the communication distances increases, thereceived optical power from the LOS link (NLOSlink is not considered in this situation due tolow power levels compared to LOS link), see Fig.10. (a)

Fig. 11.(Color online) BER (Log10) distribution on a vertical plane at 3 different distances with clean low beamlamps.

with hRX of 0.2 m, which is less steep than Fig.10. (b) and (c) with hRX of 0.4 m and 0.6 m,respectively, and is larger than Fig. 10. (d).Therefore, as the distance increases, the heightof highest received optical power decreases.As we know from Section 2 that W4 has higher Q0

and S1 compared to R1. Thus W4 road reflects morelight than R1 road, which increases receivedlight from NLOS link. From the BER profiles oftwo road surfaces (R1 and W4) in Fig. 11. andFig. 12., we see an improvement in the BERperformance for the W4 road surface compared tothe R1 road surface, especially for the link spanof 40 m.When observing the BER difference between two

types of low beam lamps (clean and dirty), forexample the contour lines for the BERs of 10-30,10-20 and 10-10 in Fig. 11. (d) and Fig. 12. (d)for the W4 road surface, we notice deteriorationin the BER performance. However replacing a cleanlamp with a dirty lamp under the R1 road surfacecondition, we hardly observe any changes in BERcontour lines. The reason is that, dirt on thedirty headlamps results in reduced light belowthe horizontal line and increased light above

horizontal. Additionally, with reference to Fig.5., the higher the luminous intensities thehigher the loss will be. Due to the fact thatmost LOS light captured by a PD located at theback of a front car is from the horizontaldirection and NLOS light captured is from belowthe horizontal direction, the light attenuationfor LOS link is less than that of NLOS link. As aresult, the dirt has a larger impact on W4 road.It should be noted that in the proposed model

we have only used a data rate of 50 Mbps, whichis far more than what may be required for C2C.However, considering the low data ratetransmissions in C2C and according to 21 if thebandwidth of both LED and PD are not alimitation, a longer transmission span can beachieved.

4. Conclusion and Future WorkIn this paper the potential of using LED-basedheadlamps for C2C communications during thedaytime was investigated. In order to consider amore practical scenario we have adopted a modelbased on the measured headlamp beam pattern

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instead of the ideal Lambertian model and a roadsurface reflection model. The ambient noise wasnumerically analyzed and the link BER performancewas investigated for the clean and dirtyheadlamps. Results obtained indicated that thereceived optical power level for the NLOS linkfor a wet road surface is higher than the drysurface for a link span longer than 11 m, thusoffering improved BER performance. The resultsalso showed that if the PD is mounted at a height

in a range of 0 – 0.2 m then a transmission linkdistance up to 70 m could be reached at a datarate of 50 Mbps.As far as future work is concerned,

experimental work will be carried out to validatethe mathematical model. In particular, differenttemperature and weather conditions as well as theeffects of multipath interference will beinvestigated for various

Fig. 12.(Color online) BER (Log10) distribution on a vertical plane at 3 different distances with dirty low beamlamps after winter/salty drive.

data rates.The ambient noise characteristics and the

effect of dust accumulation on the headlamps andPD surfaces will be investigated experimentally.In addition, the frequency response of a typicalLED-based headlamp and the outdoor SNR vs. thefrequency curve will be determinedexperimentally.

AcknowledgementsThis work is supported by the EU cLINK project

(GN. 372242-1-2012-1-UK-ERA) and the EU CostAction IC 1101. One of the authors (AndrewBurton) receives a PhD scholarship fromNorthumbria University.

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