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Photon Netw Commun

DOI 10.1007/s11107-015-0507-1

Indoor visible light communication without line of sight:investigation and performance analysis

Maged A. Esmail1,2· Habib A. Fathallah1,2

Received: 19 January 2015 / Accepted: 15 April 2015

© Springer Science+Business Media New York 2015

Abstract Visible lightcommunication (VLC)using indoor

LED lighting generally assumes the existence of line of sightlink in addition to multipath, delayed, lower power reflec-

tions. In this paper, we investigate the possibility to establish

VLC links in shadowed areas, i.e., where the line of sight

is blocked or unavailable. First, we study the system perfor-

mance in terms of received power, SNR, BER, and rms delay

spread. The results show acceptable, yet promising perfor-

mance of BER = 1.2 × 10−3 in worst non-line of sight

case. Second, we define three configuration scenarios for

receiver’s PD orientation in which line of sight is absent.

For each scenario, we evaluate the link performance. Our

analysis shows that the system performance can be poten-

tially improved when MIMO is considered for locations that

have low performance with single photodetector.

Keywords Visible light communication · Optical

communication · Shadowing · Non-line of sight · LED ·

Reflection

1 Introduction

During the last decade, visible light communication (VLC)

observed an increasing importance in various research cir-

cles [1,2]. The interest in this new technology is motivated

by the rapid development in LED technology. Compared to

other lighting sources, LEDs have a number of advantages.

B Maged A. Esmail

[email protected]

1 Electrical Engineering Department, King Saud University,

P.O. Box 800, Riyadh 11421, Saudi Arabia

2 KACST Technology Innovation Center in Radiofrequency

and Photonics for the e-Society (RFTONICS), King Saud

University, P.O. Box 800, Riyadh 11421, Saudi Arabia

For example, longer life expectancy, mercury free, smaller

size, high energyefficiency, low power consumption, etc. [3].By the end of this decade, it is expected that LED will dom-

inate the lighting market [4]. LED is a semiconductor, and

it can be modulated (switched ON and OFF) at high speed.

This key specification motivated many researchers to exploit

LEDs in data communication with very limited additional

cost. In addition to the use of LEDs for lighting and indoor

communications, LEDs have found many new applications

that appeared during the last few years. This includes indoor

(2D and 3D) positioning and navigation systems, advertising,

vehicle-to-vehicle communication, underwater communica-

tion, aircraft and hospital communication, etc. [5].

Many studies have been conducted on indoor VLC system

to characterize its channel and improve its performance. Most

of these studies haveassumed theexistence of theline of sight

(LOS) between the transmitter and the receiver. Furthermore,

the link performance was always evaluated mainly based on

the power carried by the LOS, i.e., reflected light paths are

usually neglected [1,3,5]. However, in reality, the LOS sig-

nal can be blocked permanently or temporally by obstacles

between the LED and the photodetector (PD). Therefore, the

non-LOS (NLOS) link that is produced by light reflections

can be used to maintain communication. In [6], the authors

investigated the performance of the NLOS communication

system by measuring the received power at few receivers’

locations in the room. Then they calculated theoretically the

SNR, interference, and BER at specific values of the mea-

sured received power.

In this paper, we investigate the possibility of establishing

VLC when the LOS signal is blocked, relying on reflected

lights from the walls. For this purpose, we exploited com-

mercial LED data to produce reliable results. The system

performance is evaluated using different metrics such as

the received power strength, SNR, interference, BER, and

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transmitted data rate. These metrics are calculated not at spe-

cific locations, but in every point in the room at receiver

height to determine the limitations of the system perfor-

mance. Moreover, we investigate the performance of the

system when increasing the number of detectors in the

receiver (MIMO system). Three different receiver config-

urations are used and compared. For each configuration, we

calculated the rms delay spread, SNR, BER, and data rate.The results show improvement in the system performance

when MIMO system is used.

It is worthy to note that NLOS VLC could serve as an

excellentbackup link (even with reduced capacity and perfor-

mance) when the LOS is interrupted. This helps to maintain

continuous communication between the end user mobile and

the emitting LEDs. Moreover, it can be easily noted that

indoor building designers frequently fix the lighting diodes

in hidden corners and parts of the rooms. In that case, no

LOS may exist.

The remaining of the paper is organized as follows. In

Sect. 2, we study the system model including the channelcharacteristics when no LOS is available. In Sect. 3, we

evaluate the system performance in terms of SNR, BER,

and transmitted data rate. We also show how the perfor-

mance improves when the number of detectors is increased

in Sect. 4. Finally, we conclude in Sect. 5.

2 VLC system model

The transmitted signal in indoor wireless channel encounters

reflections from walls and objects. This causes time disper-

sion and power fluctuation in the received signal. Specularreflection can occur from mirrors and other shiny objects.

However, most reflections are diffuse in nature and most are

well modeled as Lambertian [7].

2.1 Illuminance

Since the preliminary objective for LED installation is light-

ing, theVLC shouldnot affect theillumination. Theluminous

intensity generated by an LED that has a Lambertian radia-

tion pattern is given by

I (φ) = I (0) cosm

(φ) (1)

where I (0) is the center luminous intensity of the LED andφ

is the angle of irradiance [8]. The order of Lambertian emis-

sion, m = −1/ log2(cosϕ1/2), is a measure of the directivity

of the light beam and is related directly to semi-angle at half

power (ϕ1/2) of the LED. The horizontal illuminance at a

point ( x , y) is given by

E hor( x , y) = I (φ) cos(θ)/d 2 (2)

400 450 500 550 600 650 700 750 0

0.2

0.4

0.6

0.8

1

0

1.2

2.4

3.6

4.8

6 x10

P o w e r S p e

c t r a l D i s t r i b u o n

S p e c t r a l R e fl e c t a n c e

Wavelength (nm)

Plaster

Φ(λ)

Plasc

-3

Fig. 1 PSD(leftside—solid line), and spectralreflectance for different

materials (right side—dashed lines)

where θ is the incidence angle and d is the distance between

the transmitter and the illuminated surface [8].

2.2 Channel impulse response

The light produced by the LED for lighting purpose cov-

ers the visible band (380–780nm) and other wavelengths.

Therefore, data transmission by LED covers wide range of

wavelengths. This is not the case with IR where single wave-

length is used for data transmission. Therefore, the radiated

power in VLC is wavelength dependent and distributed over

the visible band. This dependency is described by the power

spectral distribution (PSD) Φ(λ) which describes the radi-

ant power per unit wavelength. Moreover, the reflectivity

of objects in indoor environment is a function of the sig-

nal wavelength. Figure 1 shows the PSD for high-power

commercial LED device (LCW CQAR.EC) from OSRAM

Opto Semiconductors Company [9]. Also the figure shows

the spectral reflectance measured by the authors in [10] for

different materials in the visible band.

The multi-bounce power delay profile (PDP) in indoor

channel is given as [10]

h(t ) =

N LED

n=1

∞

k =0

h(k )(t ;Φn) (3)

where N LED is the number of LED sources and h(k ) is the

PDP due to the k th reflection. If the LOS signal is lost due

to shadowing, then the received signal is the sum of reflec-

tions only. In [10], it is shown that in NLOS VLC, the first

reflection (k = 1) has clearly the largest contribution to the

received signal which contains about 80 % of the total power

as compared to subsequent order reflections. In our analy-

sis of the impulse response, we have noticed that including

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1φ

2φ

1θ

2θ FOV

1d

2d

PD

nth LED

LOS blocker

Shadow area

Blocked LOS

Wall Reflected Ray

Fig. 2 Geometry of the single reflection of the nth LED

the contribution of higher-order reflections (k > 1) to the

received signal requires complex computation. Therefore,we

primarily focus most of our attention to single reflection as

shown in Fig. 2, which is used to be the second most impor-

tant optical path after the LOS. The channel response after

the first reflection to the nth LED is given as

h(1)(t ;Φn) =

s

L1 L2Γ (1)

n rec

θ 2

FOV

× δ

t −

d 1 + d 2

c

d Aref (4)

where

L1 = Aref (m + 1) cosm φ1 cos θ 1

2πd 21,

L2 = APD

cos φ2 cos θ 2

πd 22

The integration in (4) is performedoverthe whole surfaces

of the reflectors. A ref is the reflector area, which is the wall in

Fig. 2. APD

is the photodetector area, c is the speed of light,

and FOV is the field of view of the detector. The photodiode

in Fig. 2 detects light if the incidence angle θ 2 ≤ FOV. The

rectangular function in (4) is given as

rec( x ) =

1, | x | ≤ 1

0, | x | > 1 (5)

The power of single reflection is given by

Γ (1)n =

λ

Φnρ1(λ)dλ (6)

where ρ1(λ) is the spectral reflectance of the reflector. In this

paper, we assume the room is empty so the signal is reflected

by walls only.

2.3 System model

The received optical signal at the PD input is given by

Pr = H (0)Pt (7)

where H (0) is the channel DC gain and Pt is the transmitted

power. The PD photocurrent is expressed as

Y (t ) = R X (t )⊗ h(t )+ N (t ) (8)

where X (t ) is the transmitted optical pulse, R is the detector

responsivity, and N (t ) is the noise. The desired signal power

is given as

Prs =

T

0

N LEDi=1

hi (t )⊗ X (t )dt (9)

and the intersymbol interference (ISI) power contribution is

given by

Prisi =

∞T

N LED

i=1

hi (t )⊗ X (t )dt (10)

2.4 SNR with intersymbol interference

The quality of the VLC communication system is determined

by SNR with interference. The SNR is given by

SNR =( R Prs)

2

σ 2shot + σ 2therm + ( R Prisi)2 (11)

where σ 2shot is the shot noise variance, and σ 2therm is the ther-

mal noise variance. The shot noise variance is given by

σ 2shot = 2q R(Prs + Prisi) B + 2q I bg I 2 B (12)

where q is the electron charge, B is the equivalent noise

bandwidth, I bg is the background current, and I 2 is the noise

bandwidth factor. The thermal noise is given as [7]

σ 2therm =8πkT k

Gη A

PD I 2 B2 +

16π2kT k Γ

gm

η2 A2PD

I 3 B3 (13)

where k is the Boltzmann’s constant, T k is the absolute tem-

perature, G is the open-loop voltage gain, η is the fixed

capacitance of PD per unit area, Γ is the FET channel noise

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Table 1 Parameters used in simulation

Parameter Value

Transmitter

Transmitted power 0.7 W

Semi-angle at half power 60◦

Center luminous intensity 57cd

Number of LED lamps 4Number of LEDs/Lamp 49

Multi-transmitter position (1, 1, 3); (1,4, 3); (4, 1, 3); (4, 4, 3)

Receiver

PD area 1 cm2

FOV 60◦

Optical filter gain 1

Refractive index of a lens 1.5

Responsivity 0.54 A/W

Time resolution 0.2 ns

Data rate 40 Mbps (OOK NRZ)

Noise

Equivalent noise bandwidth 40Mb/s

Background current 5100 µA

I 2 0.562

Absolute temperature 298 K

Open-loop voltage gain 10

Fixed capacitance 112 pF/cm2

FET channel noise factor 1.5

FET transconductance 30 mS

I 3 0.0868

factor, gm is the FET transconductance, and I 3 is a constant.The first term in (13) represents the feedback resistor noise,

while the second term represents the FET channel noise. The

values of the noise parameters are taken from [8] and listed

in Table 1.

2.5 Channel delay spread and maximum transmitted

data rate

The rms delay spread (Drms) is a critical criterion for system

performance. It determines the upper bound of data trans-

mission rate. The Drms is calculated using the mean excess

delay (µ) and is given by

Drms = µ2 − (µ)2 (14)

where

µ =

M i=1 Prs,i t i +

N j=1 Prisi,i t i

Prtotal

,

01

23

45

1

2

3

4

5

300

400

500

600

x(m) y (m)

I l l u m i n a n c e ( l x

)

Fig. 3 Illuminance distribution on the room floor, Min=300 lx;

Max= 550 lx; Avg.= 480 lx

µ2 = M

i=1 Prs,i t 2

i

+ N j=1 Prisi,i t 2

iPrtotal

Using (14), the maximum data rate that can be transmitted

through the channel without equalizing is given by [3]

Rb ≤1

10 Drmsbps. (15)

3 Simulation results and discussion

In this section, we evaluate the system performance when

LOS signal is absent. We consider an empty room with

dimensions 5 m × 5 m × 3 m. Four LED lamps are used.

Each lamp consists of 7× 7 LED chips. The LED chip has a

PSD asshownin Fig. 1. Thewallsare assumed to be plastered

with spectral reflectance also shown in Fig. 1. The transmitter

parameters used in simulation arefor the commercial product

LED (LCW CQAR.EC) and listed in Table 1.

3.1 Illuminance on room floor distribution

First, we investigate the horizontal illuminance at the room

floor. Thesufficient illuminance as definedby ISOis between

300 and 1500 lx. In Fig. 3, we show the illuminance at the

room floor assuming no additional light reflected from sur-

rounding walls. We notice that the illuminance is high at the

center and decreases at the edges. On average, the illumi-

nance on the floor is 482 lx. The minimum at the edge is

300 lx and the maximum at the center is 550 lx. Therefore,

the LED lamps with the parameters used in this simulation

can guarantee sufficient illuminance.

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x (m)

y ( m

)

85-120 120-17044-85

1 2 3 4

1

2

3

4

Fig. 4 Data rate distribution in Mbps at PD level: Min =44Mbps,

Max= 170 Mbps, Avg.= 84Mbps

3.2 Maximum transmitted data rate

In Fig. 4, we calculated the maximum data rate distribution

that can be transmitted through the channel without need for

equalizers using (15). When the PD is located at equal dis-

tances from the transmitters, the Drms effect is lowest and

then the data rate is the highest. The maximum data rate that

can be achieved is at the middle of the room where all trans-

mitters apart with equal distance. This is equal to 170 Mbps

with 0.6ns Drms. The minimum data rate is 44Mbps with

2.26ns Drms. Therefore, for the rest of this paper, the datarate is simulated at 40 Mbps.

3.3 Received power distribution

The distribution of the received power with a PD oriented

toward ceil is shown in Fig. 5 where four regions are defined.

The PD is located on a table at height 0.85m. Due to the

symmetry of the room and also the transmitters’ location, the

power distribution appears symmetric. High received power

is achieved at points located between two walls where more

power can be collected. The maximum received power is

19.2 µW. The amount of received power decreases as the PD

location becomes far from the walls and the transmitters. The

worst case is when the PD is located far from the transmitter

in the middle of the room. The minimum received power by

a single detector is 1.7 µW. This small power still can be

detected by a PD having sensitivity around −30dBm. The

average of the received power over all points is 7 .2 µW. It is

also worthy to note that power distribution through the room

surface in our NLOS case is largely different from what is

used to be studied when LOS exists. For more details about

x (m)

y ( m )

2-5 5-101.7-2

1 2 3 4

1

2

3

4

10-19.2

Fig. 5 Received power distribution in µW at PD level (height=

0.85m), Min = 1.7 µW;Max = 19.2 µW;Avg. = 7.2 µW

x (m)

y ( m

)

14-18 18-28.512.7-14

1 2 3 4

1

2

3

4

Fig. 6 SNR in dB distribution at PD level: Min= 12.7dB, Max=

28.5 dB, Avg.= 19dB

power distribution in LOS system, the reader is referred to

[3,8].

3.4 SNR and BER distribution

Using the signal-to-noiseratio model in (11), we illustrate the

SNR distribution in the room at all possible PD locations as

shown in Fig. 6. Three regions are defined in Fig. 6. Since the

walls operate now as secondary light sources, the minimum

obtained SNR is 12.7dB at the middle of the room (farthest

location to the walls). The highest SNR is observed close

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x (m)

y ( m

)

10-6

- 10-4

10-4

- 1.2×10-3<10

-6

1 2 3 4

1

2

3

4

Fig. 7 BERdistribution at PD level: Min= 1.2 × 10−3,Max <10−6

to the walls (secondary light sources) and specifically in the

corners where two walls’ reflections dominate the received

signal. In effect, the maximum SNR 28.5dB is observed at

the corners. On average, 19dB SNR is achieved through all

locations in the room.

In Fig. 7, we show the BER distribution for the system

obtained through Monte Carlo simulation of the NLOS sys-

tem. The BER results are consistent with those of SNR in

Fig. 6 confirming that the quality of the communication may

very quickly change when the receiver changes its location

in the room for few tens of centimeters or less. A BER lessthan 10−4 is observed almost everywhere in the room; 10−6

is mainly in the corners, and 10−3 in the center. Recall that

these regions resulted from the location we have fixed for the

LED lamps. The regions are very sensitive to the placement

plan of the LEDs. When observing the current general trend

of deploying LED lighting technology, it seems to develop

toward more distributed architectures through ceil surface,

rather than centralized. This gives more uniform illumina-

tion in the space and reduces energy consumption. Note this

is in the benefit of better uniformity of SNR and BER through

the room surface.

4 Multi-input multi-output (MIMO) system

In previous section, we investigated the performance of VLC

system with NLOS. Four LED lamps are used and only a

receiver with single LED. This system is called multi-input

single-output (MISO) system. In this section, we explore the

performance of VLC system when multi-PDs are used in the

receiver.

Tx1

LOS

blocking

Conf. #2

Tx2 Tx3

Tx4

PD

Conf. #1 Conf. #3

Fig. 8 A simulation for a room shows PD orientation

We define three operation configurations for thePDs in the

receiver. In configuration 1, single PD is used as discussed

in previous section. In configuration 2, four PDs oriented to

the room ceil are used, while in configuration 3 four PDs

are oriented to each wall in the room. The receiver device is

located on a table at 0.85m height. The room configuration

and receiver position is shown in Fig. 8.

In configuration 1, the PD is placed at the center of the

receiver. In configurations 2 and 3, the distance between the

PDs and the receiver’s center is 10 cm. Selection combining

technique is used where only one PD’s signal is selected at

a time that has the best SNR [11]. The system performance

is investigated at two receiver positions as shown in Table 2.

The first position corresponds to locations having minimum

SNR for configuration 1. The second position corresponds to

location having maximum SNRfor configuration 1. Forthese

two positions, we investigate how MIMO system improves

the performance. The system performance for the three con-

figurations in terms of SNR, BER, Drms, and data rate are

calculated and listed in Table 2.

The results show that when configuration 1 shows low

performance at location (2.3, 2.5, 0.85), especially in terms

of BER, the MIMO system in configuration 2 was able to

improve the system performance in terms of SNR, BER,

Drms, and data rate. The BER is improved by one order

of magnitude, while the data rate is improved by 9 Mbps. For

configuration 3, the performance was better in terms of SNR

and shows very low BER. However, the Drms was higher

which results in lower transmitted data rate.

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Table 2 Simulation results for different PD positions

Position Worst SNR Best SNR

(2.3, 2.5, 0.85) (0.2, 0.2, 0.85)

MISO Mode#1

Drms (ns) 0.73 1.48

SNR (dB) 12.7 28.5

BER 1.2 × 10−

3 <10−

6

Data rate (Mbps) 135 67

MIMO Mode#2

Drms (ns) 0.64 1.65

SNR (dB) 14.5 28.1

BER 10−4 <10−6


MIMO Mode#3

Drms (ns) 2.0 3.2

SNR (dB) 18.9 31

BER <10−6 <10−6


When configuration 1 shows high performance at location

(0.2, 0.2, 0.85), the MIMO system in configuration 2 shows

close performance to configuration 1. The performance of

configuration 1 was better because its location in the cen-

ter of the receiver was better than the location of the PDs

in configuration 2. This enables configuration 1 to collect

more power and achieves a little higher SNR. Configura-

tion 3 at this location shows higher SNR, but it increases the

Drms which results in lower data rate. Therefore, under Non-

LOS condition, single PD can work well in some locations

only. Therefore, MIMO system with PDs oriented towardceil is the best solution to achieve better performance in any

receiver position in the room. MIMO system with PDs ori-

ented toward walls can achieve higher SNR and lower BER

than configurations 1 and 2, but it undergoes higher Drms

and lower data rate.

5 Conclusion

In this paper, the performance of VLC system under NLOS

condition is studied. The simulation results showed that VLC

system with NLOS signal has acceptable performance. Westudied three configurations for PD orientation in the VLC

receiver. Theresult showedthat in general, theMIMO system

with PDs oriented to ceil can improve the system perfor-

mance better than using single PD. The MIMO system with

PDs oriented toward walls has high Drms witch reduces the

maximum transmitted data rate.

Acknowledgments This research is supported by the KACST Tech-

nology Innovation Center in Radio Frequency and Photonics for the

e-Society (RFTONICS), King Saud University.

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[9] Lcw, CQAR.EC, OSRAM opto semiconductors. http://www.osr

am-os.com/osram_os/en/products/product-catalog/leds-for-gen

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[10] Lee, K., Park, H., Barry, J.: Indoor channel characteristics forvisible light communications. IEEE Commun. Lett. 15(2), 217–

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Maged A. Esmail received his B.E. degree

in electronic engineering from Ibb Univer-

sity in 2006 and M.Sc. degree (with first

class honors) in electrical engineering from

KSU University in 2011 where he is cur-

rently followinghis Ph.D.studies. From2009

to 2013, he joined Prince Sultan AdvancedResearch Technologies Institute (PSATRI) as

a researcher. From 2014, he joined Kacst-

Technology Innovation Center RTONICS as

an assistant researcher. His research interests include fiber-optic com-

munications, PON and long-reach PON, network management and

protection, sensor networks, free space and visible light communica-

tions.

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Habib A. Fathallah (S’96, M’01) received

the B.S.E.E. degree (with Honors) from the

National Engineering School of Tunis, 1994

and the M.A. and Ph.D. degrees in electri-

cal and computer engineering from Laval

University, Qc, Canada, in 1997 and 2001,

respectively. He initiated the use of Bragg

gratings technology for all-optical/all-fiber

coding/decoding in Optical CDMA systems.

He was the founder of Access Photonic Net-

works (2001–2006) and adjunct professor with the Electrical and Com-

puter Engineering Department of Laval University (Quebec, Canada).

He is currently with Electrical Engineering Department, College of

Engineering of the King Saud University (Riyadh, KSA), co-founder

and Photonics Thrust leader of Kacst-Technology Innovation Center

RTONICS. His research interests include coherent optical commu-

nications systems, mode and multicore space division multiplexing

(SDM), Free Space Optical and Visible light communications; Long

reach PONs, hybrid fiber wireless (FiWi) systems, and OCDMA.

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