3- fundamental analysis for indoor visible light positioning system

7/30/2019 3- Fundamental Analysis for Indoor Visible Light Positioning System

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Fundamental Analysis forIndoor Visible Light Positioning System

Penghua Lou a,b, Hongming Zhang a,b, Xie Zhang a,b, Minyu Yao a,b, Zhengyuan Xu a,ba Department of Electronic Engineering, Tsinghua University, Beijing, China

b Tsinghua National Laboratory for Information Science and Technology (TNList), Beijing, China

[email protected]

AbstractThis paper describes the prototype of an

indoor positioning system (IPS) using LED

identification (LED-ID) technology. LED sources

transmit unique ID codes which identify users' position.

An optical link budget is analyzed and simulated. Our

lab prototype can satisfy the required accuracy in some

coarse location environments.

Keywordsindoor positioning system, LED

identification, visible light communication, effective

positioning radius.

I. INTRODUCTIONIn the past decades, outdoor positioning has been

studied extensively and systems like Global

Positioning System (GPS) has been studied and

developed. In general, GPS needs to receive signals

from at least 4 satellites, which is impossible inside

some buildings [1]. Thus, other technologies for indoor

positioning are being developed such as RF based

positioning and infrared positioning. However, those

positioning systems are not widely popular so far, due

to high power consumption and lower resolution and

accuracy.

Meanwhile, light-emitting diode (LED) has been

considered as the most potential lighting technology

of the 21st century for its high brightness, affordable

cost, low power consumption and minimal heat

generation [2]. Additionally, LEDs can also be

modulated at relatively high rate which is proper for

transmitting data signal. Therefore, LEDs could be

utilized for both illumination and communication,

such as LED-identification (LED-ID) based

communication [3][4]. In this paper, we apply LED-ID

technology to supply variable position information for

indoor positioning system.

In the paper, we first demonstrate the indoor

positioning system prototype. Some numericalanalyses for the proposed system are performed, and a

MATLABbased simulation is made to study the

effectiveness and accuracy of positioning algorithm,

the probability distribution of detection and the

acceptable movement speed. Thereafter, it is

concluded that our lab prototype basically satisfies the

required accuracy in some coarse location

environments.

The paper is organized as follows. In section II,

the design of indoor positioning system is described,

and then we discuss the transmitter and channel

models in a hallway scenario. In section III, asimulation is presented to analyze the performance of

positioning algorithm. Finally, conclusions are in

section IV.

II. SYSTEM DESIGN AND ANALYSISIn this section, we introduce an indoor positioning

system using LED-ID technology. The system model

is shown in Fig. 1, which consists of transmitter,

optical wireless channel and receiver. The details are

as follows.

Fig. 1. System Model

The 1st International Workshop on Optical Wireless Communications in China (OWCC'12)

978-1-4673-2997-2/12/$31.00 2012 IEEE 59


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LED sources of visible light ID are modulated

with baseband On-Off Keying of 200kbps data rate,

in conjunction with Direct Detection (DD) for data

demodulation at the receiver. Since it is desired tomaintain visibility and flicker-free operation, a

symmetric Manchester encoder/decoder composed by

microprocessor is applied. At the transmitter, we

design a constant-current drive circuit using power

MOSET and low noise Operational Amplifier to

achieve 220mA driving current for 3W LED sources.

At the receiver, the signal preprocessing circuits

combine with weak signal pre-amplifier and signal

shaping circuit. Both of the transmitter and receiver

circuits create USB, SPI, UART interfaces to realize

communication between our system and Personal

Digital Assistant (PDA).

We next introduce simplified models for

transmitter and optical channel in a hallway scenario,

where coarse location is applicable.

A. Transmitter ModelIn order to satisfy the standardfor lighting design

in aforementioned scenario, we choose 3W LED as

transmitter sources in the prototype. The power

spectrum distribution ( )tS of the used LED is

shown in Fig. 2, which was measured by an

integrating sphere.

Fig. 2. Measured spectrum distribution of LED source

The optical power tP of such LED is obtained

from ( )tS as[5]

( )H

L

t tP S d

= (1)

The total luminous flux tF is given as[5]

780

380

683 ( ) ( )

nm

t t

nm

F S V d = (2)

where ( )V is the relative luminous efficiencyfunction defined by CIE and it can be approximated

by a Gaussian curve fitting [6] as follows:

22 85 .4 ( 0.5 59 )( ) 1.019V e (3)

The luminous flux tF is also a spatial integral of

spatial luminous intensity, and we have the following

relation [7]

max

0

0

2 ( )sint t

F I g d

= (4)

where 0I is the axial intensity, max is the maximum

half angle and ( )tg is the normalized spatial

luminous intensity distribution.

The spatial distribution ( )tg is claimed to be a

Lambertian radiation pattern [7][8], which would be

written as:

( ) cos ( )m

tg = (5)

where m is the order of Lambertian radiation. Thus,

a simplified formula of the flux tF is obtained.

max

0

0

0

2 cos ( )sin

2 ( 1)

m

tF I d

I m

=

= +

(6)

Fig. 3 shows that the LED source in our prototype

can be assumed as an ideal Lambertian radiation

pattern with m=14.

-50 -40 -30 -20 -10 0 10 20 30 40 500

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Angle ()

NormalizedAmplitude

Measured pattern

Ideal Lambertian pattern(m=14)

Fig. 3. Comparison between the actual and ideal Lambertianradiation pattern

B. Channel ModelIn an optical wireless link, the signal path loss is

determined as follows:

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max

2

0

2

( ) cos( )

2 ( )sin

( 1)cos ( ) cos( )

2

t

t

m

g AL

d g d

m A

d

=

+ =

(7)

where is the angle between source beam axis and

source-receiver line and is the angle between

receiver normal and source-receiver line.

Now we can obtain the received optical spectral

density as

( ) ( )r tS L S = (8)

The received optical power can be written as

( ) ( ) ( )rH rH

rL rL

r r tP S d L S d

= = (9)

At the receiver, we use a photodiode to convert

optical power to electric power rI , which can be

derived as

( ) ( ) ( ) ( )rH rH

rL rL

r r tI S R d L S R d

= = (10)

where ( )R is the photodiode responsivity. Fig. 4

shows the responsivity of photodiode (THORLABS

FDS100) that we used.

300 400 500 600 700 800 900 1000 1 1000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Wavelength (nm)

Responsivity(A/W)

FDS100

Fig. 4. The responsivity of photodiode (FDS100)

By calculating the integral of the spectrum

distribution ( )tS (Fig. 2) and the photodiode

responsivity ( )R , the electric power can be

simplified as

( ) ( ) 0.131rH

rL

r tI L S R d L

= = (11)

The electrical signal-to-noise ratio (SNR) at

receiver is expressed as [9]

2 2

2 2 2

r r

tatal thermal shot

I ISNR

= =

+(12)

where2

thermal is the variance of thermal noisestemming from the pre-amplifier, and

2

shot is the

variance of shot noise stemming from ambient light,

which is a dominant one in the wireless optical

communications. The shot noise variance is given by[10]

2 22 2

0(10 )

shotN B A Hz B = (13)

where 0N is the noise power spectral density and B

is equivalent noise bandwidth, which is equal to the

bit rate at OOK.

III. SIMULATION RESULTS AND DISCUSSIONIn this section, we will discuss about the effective

positioning radius, the probability distribution of

detection and the acceptable movement speed.

The following simulation and discussion is based

on the assumption that the size of the scenario is 10m

2.5m3m, a 3W LED is located at the coordinate

of (4, 1.25, 3) and the height of the receiver is 0.85m

from the floor.

A. The Effective Positioning RadiusIn the system, the available LED-ID received at

the receiver is derived as the present location.

Therefore, the effective illumination radius reflects

the system positioning accuracy. Fig. 5 shows the

SNR distribution as a function of distance. To achieve

BER=10-6 it requires SNR=13.6dB in OOK

communication system.

Fig. 5. SNR distribution as a function of distance

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From Fig. 5, the effective positioning radius of

1.3m is available, which is mainly dependent on the

order of LED Lambertian radiation m . Suppose that

the orderm is small, then the beam is more divergentand the effective positioning radius will become

larger. However, it is important to note that we should

consider avoiding overlapping of two adjacent LEDs

while selecting the radiation pattern of LED sources.

B. The Probability Distribution of DetectionThe SNR distribution is not only a function of the

distance between the receiver and the transmitter, but

also a function of the pitch angle of the receiver. In

practical applications, the receiver attitude is random,

which leads to the probability distribution of detection.

Fig. 6 shows the SNR distribution withradius=0.5m as the pitch angle varies. The probability

reaches 89%.

Fig. 6. SNR distribution as a function of the pitch angle

Fig. 7 shows the detection probability variation

trend as the radius increases.

0 0.2 0.4 0.6 0.8 1 1.2 1.40

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Radius (m)

Probability

Fig. 7. The detection probability trend as a function of radius

From Fig. 7, we can find that the probability

decreases slowly within the range of 0.8m, and then

start decreasing rapidly that is resulted from an

increasingly important part of the pitch angle in theSNR function. When the horizontal distance between

transmitter and receiver is within 1.08m, the

probability of detection is beyond 50%.

C. The Acceptable Movement SpeedAs every LED source has an effective radius, the

receivers' movement speed should be considered. The

case when the receiver is moving parallel to the

ground is studied.

In the system, the bit rate is 200kbps, and one

frame is made up of 4 bytes. Thus the time that LED-

ID sends once is 160s. If the effective positioningradius 1.3m is applied, the maximum movement

speed is calculated as 1.3m2/160s=16.25km/s,

that far exceeds the walking speed 0.85~1m/s.

However, if this system is extended on the vehicle

positioning applications, the bit rate should be

increased to guarantee steady positioning.

IV. CONCLUSIONSIn this paper, the LED-ID technology based on

visible light communication is developed into a kind

of indoor positioning system. We introduce an indoor

localization prototype that has been built, and thenreport the simulation results for indoor scenarios

based on MATLAB. Starting from the standardized

requirements for hallway illumination, we select a

3W LED source with Lambertian order m=14.

Concluded from the above discussions and

simulations, we may state the following:

The positioning accuracy, which is related to

the effective positioning radius, reaches 1.3m. It is

less than that most of wireless location or infrared

location systems.

If the receiver position is fixed, the detection

depends on the pitch angle of the receiver, which is

defined as the probability distribution detection. The

probability is beyond 50% while the distance between

the receiver and the vertical projection of LED source

is within the range of 1.08m.

The normal movement speed in interior

environment is completely acceptable in our

positioning system.

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In summation, our system prototype can meet the

location accuracy in indoor environment to locate

both moving and static objects.

ACKNOWLEDGMENTS

This work was supported by Tsinghua National

Laboratory for Information Science and Technology

(TNList) Cross-discipline Foundation (2011Z02289),

National Natural Science Foundation of China (Grant

No.61171066, 60977003).

REFERENCES

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[3] Yoshinori Matsumoto, Takaharu Hara and Yohsuke Kimura,Integrated CMOS photo-transistor array for visual light

identification (ID), ISDRS 2007, December 12-14, 2007.

[4] http://www.naka-lab.jp/index_e.html.[5] R. Roberts and Z. Xu, Update on VLC link budget work,

IEEE 802.15.7 VLC Standard Meeting, document #: 802.15-

09-0635-01-0007, September 2009.

[6] http://ww.optics.arizona.edu/Palmer/rpfaq/rpfaq.htm.[7] J. M. Kahn and J. R. Barry, Wireless infrared

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[8] F. R. Gfeller and U. Bapst, Wireless in-house datacommunication via diffuse infrared radiation, Proc. of

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[9] Toshihiko Komine, and Masao Nakagawa, FundamentalAnalysis for Visible-Light Communication System using

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[10] J. Grubor, O. C. Gaete Jamett, J. W. Walewski, S. Randel, K.-D. Langer, High-Speed Wireless Indoor Communication

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3- fundamental analysis for indoor visible light positioning system

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