Experimental Demonstration of an Indoor Visible Light Communication ...

2014 3rd International Workshop in Optical Wireless Communications (lWOW)

Experimental Demonstration of an Indoor Visible Light Communication Positioning System Using Dual-Tone Multi-Frequency Technique 2 1 l 3 3 4 Pengfei Luo , Zabih Ghassemlooy , Hoa Le Minh \ Ali Khalighi , Xiang Zhang , Min Zhang , Changyuan Yu l

Optical Communications Research Group, Northumbria University, Newcastle-upon-Tyne, United Kingdom Email: {oliver.1uo.z.ghassemlooy.hoa.le-minh}@northumbria.ac.uk 2 Ecole Centrale Marseille, Institut Fresnel, UMR CNRS 7249, Marseille, France

Email: [email protected] 3State Key Laboratory of Tnformation Photonics and Optical Communications, Beijing Univ. of Posts & Telecom, Beijing, China Email: {zhangxiang312.mzhang}@bupt.edu.cn 4Department of Electrical and Computer Engineering, National University of Singapore, Singapore Email: [email protected]

Abstract-In

However, such a system suffers from false position estimates due to absolute power fluctuations. Visible light communication (VLC) based IPS is another option that has attracted more attention owing to the advantage of light

this paper, a visible light communication (VLC)

based indoor positioning system (IPS) employing a dedicated algorithm and using the dual-tone multi-frequency technique is proposed. Unlike the time-division multiplexing based VLC-IPS, the proposed positioning system doesn't require synchronization

emitting

between the transmitter and receiver, which makes it a simple, robust and cost effective. To test the accuracy of the proposed diodes and a photomultiplier tube is developed. The results show that the proposed VLC-IPS is highly accurate with an average

and

VLC

technologies

RF

such

as

or outdoor),

infrastructures are upgraded LEDs, the LOS restriction will no

positioning error of about 1.6cm, which far lower than many

longer be an issue, and the VLC-based IPS will become a natural candidate for high accuracy IPS.

existing indoor position systems.

Keywords- visible light commuinication; indoor positioning system; dual-tone multi-frequency

With the popularity of smartphones and iPads, the built in cameras in these devices could potentially be used as a type of VLC receiver. Such a receiver has a distinctive ability to separate different light signals spatially from different pixels, thus making it possible to have a non-interference communications [10]. Such a capability will allow mobile devices to capture signals from LED lamps as well as determine its position by running a simple software. ByteLight [11] is the first company that offers innovative IPS solution with sub-meter and sub-second accuracy and latency, respectively by using LED lighting inrrastructure and Android or iOS equipment. However, such method requires the smartphone continuously to analyse the captured video signal

INTRODUCTION

The global positioning system (GPS) has been widely used in outdoor environment with high accuracy when employing carrier phase and differential techniques [1,2], but with limited if at all usage in indoor. There are a number of indoor applications that require the knowledge of positioning, ranging from hospitals patients [3], monitoring commercial asset locations [4], mining and underground to small-scale smart dust communication node applications [5]. The common need in these applications is to estimate the position within a fixed frame of reference. Radio frequency (RF) based indoor positioning systems (IPS) are prone to high attenuation and

which leads to high power consumption thus reduces the battery life.

multipath and can offer accuracies within 2-3 m but are

In this paper, we propose a novel algorithm for VLC based IPS using the dual-tone multi-rrequency (DTMF) technique [12]. This algorithm utilizes the time-frequency analysis to analyse the received signals in both time and rrequency

relatively costly [6]. In recent years, we have seen the growing interest in optical based IPS, which offers a number of advantageous including inherent security, smaller transceiver size, and immunity to electromagnetic interference [6]. In [7] an infrared (lR) based IPS is reported with ceiling mounted

domains. The frequency domain is used to obtain the received signal strength (RSS) rrom each lamp, while rrom the time domain the cell identification (LD) of the nearest lamp can be determined. Thus, when receiving only a signal from one

transmitters, where positioning is determined based on how proximity of the receiver to any one of the transmitters.

978-1-4799-6676-9/14/$31.00 ©2014 IEEE

(LED)

multiple lighting elements etc. [8]. But, the major disadvantage of the light-based IPS is the line of sight (LOS) requirement [9]. However, once the existing lighting

method, an experimental test-bed using 3 white light-emitting

I.

diode

ubiquitous coverage, static channel (unlike

lamp, the coarse location of the receiver (Rx) can be obtained by querying the received LD from the pre-stored position

55


database. In addition, the proposed algorithm will determine the accurate position of the Rx by capturing lights from more than two adjoining lamps. Without the need for network

synchronization and complex computation, the proposed algorithm is able to provide a simple, robust and cost efficient TPS service compared to both the time-division multiplexing

•

Within a building, each LED lamp is assigned a unique lD code;

•

Each lD consists of 4-6 DTMF signals (depending on the total number of lamps);

•

Each LED lamp transmits its ID at the same time periodically;

(TDM) based VLC-IPS algorithms [13] and the RF based VLC-TPS algorithms [14]. The experimental results show that

•

the proposed algorithm offers high accuracy with an average estimation error of about 1.6 cm. To the best of our knowledge the achieved 1.6 cm accuracy is the smallest error ever reported.

All DTMF signals for one ID have the same low frequency component. E.g., the DTMF signal of TD "O#D*" has the same frequency of 941 Hz;

•

The low frequency of the TD is used to distinguish adjacent lamps;

•

are used to decode the nearest lamp's ID.

the operational principle of VLC-based IPS using DTMF technique is introduced. The system implementation and positioning algorithm is developed in Section Ill. In order to evaluate the system performance an experimental test-bed is established in Section IV. Finally conclusion is given in

Tn our system, the sampling frequency for the

OPERATING PRINCIPLE

In this section the basic operating principle for the proposed algorithm is introduced; an example is also given to illustrate the system operation.

A.

In the proposed VLC-based IPS, for the purpose of separating different signals emitted from the adjacent lamps with a dense coverage of LED lamps, we have adopted the DTMF technique. The DTMF technique is a multi-frequency technique, which employs 8 different frequencies (697 Hz, 770 Hz, 852 Hz, 941 Hz, 1209 Hz, 1336 Hz, 1477 Hz, and

B. ID Acquisition Procedure

1633 Hz) in pairs to represent 16 different numbers or letters. DTMF has been widely adopted in telecommunications, such as telephone dialling, voice mail, and electronic banking systems [15].

As shown in Fig. 2, there are two LED lamps labelled as TX l and TX2 and a mobile device Rx. In this example the positioning TD ("123A" and "O*D") of Tx\ and TX 2 are

TABLE I.

Following [12] to be able to identify LED lamps with different frequencies one need at least 5 dissimilar frequencies, which are arranged as shown in Fig. 1. Note that to avoid the inter-lamp interference (ILl) and also be able to measure the RSS, every lamp is assigned a frequency, which is represented by one colour type, and surrounded by LED lamps of different frequencies.

/L1680 Hz fj2 760 Hz fj3 840 Hz fjA960 Hz fj51080 Hz

Due to the unique structure of DTMF symbol: each symbol is composed of 2 different frequencies. The procedure for setting the LED TD are as follow [12]:

DTMF

THE MODIFIED DTMF FREQUENCY TABLE

/H11200 Hz

/H21320 Hz

/H31480 Hz

I

2

3

4

5

6

symbol

7

8

9

*

0

#

F

G

H

"I"

f5

f3

f1

f4

f2

f5

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Fig. LED lamps arrangement.

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l.

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2.

�

k5

� N

/H41640 Hz

AB C E 0

1 TT"'�' 'D

f2

and the

Hz, 1480 Hz, 1640 Hz) clusters. Thus, being able to generate 5 sets of positioning TD according to 5 different low frequencies. By careful allocation of the frequencies, ILl can be eliminated or reduced [16], and the specific position of Rx can also be determined. What's more, the lowest frequency in the table is 680 Hz which is higher than the cutoff frequency of human eye (�100 Hz), therefore, the DTMF modulated light will not cause the LED bulb to flicker

DTMF Signal and 1D Code

f4

Rx,

number of discrete-time Fourier transform (DFT) point N are set to 8 kHz and to 200, respectively. Therefore only a set of discrete frequencies can be processed, in order to diminish the mismatch between transmitted frequencies and the frequencies that are able to be processed. The modified DTMF frequencies are listed in Table T. Tn the table, each element is composed of one frequency each from the low frequency (680 Hz, 760 Hz, 840 Hz, 920 Hz, 1080 Hz) and high frequency (1200 Hz, 1320

Section V. 11.

The high and low frequencies of the received DTMF signal with the highest received signal strength indication (RSST)

The rest of the paper is organized as follows. Tn Section IT,

f

TX2


generated according to the procedure of ID generation in Section IT-A. From Table I, DTMF symbols of "123A" and "O*D#" have the low frequency components of 680 Hz and

between vertical projections of each lamps and Rx can be also determined by the algorithm, which will be introduced section Ill. Finally the coordinate of the receiver can be estimated.

960 Hz, respectively. Let's assume that both lamps have identical light intensity profile, which in practice is not the case. The strength of the signals detected at the Rx will of course depend on its position from the light sources, and therefore it will be possible to separate the signal in the

TIT.

POSITIONING ALGORITHM

For indoor LED lighting systems, LED lamps are usually arranged in a rectangular grid, see Fig. 6 (a), with the corresponding basic positioning unit shown in Fig. 6 (b). As

frequency domain.

for the Lambertian source [18], the channel gain H; of Tx; and Rx without any filtering and concentrator is given by:

Fig. 3 is the received waveform and its corresponding time-frequency graph [17]. It can be inferred from this figure that a total of 16 DTMF symbols, which are mixed together are received in 0.8 s. In our system, the time-frequency

H= ,

analysis method is adopted to analyse the received signal, which enables us to acquire the positioning ID of the nearest

{-,-m( _+_l).-:A,-,-:- R m m 2 Jrd2

m ) COS(IIf) cos ff/ ( 't'l 't'l

0:::;

IIf. 't'l

:::; \.(I . I

1f/i

o

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\.(Ic

where the order is given by = -Inz/ln(cos!P llz), where !P ll2 is the Tx semi-angle (at half power), d; is the distance from Tx; to Rx, is the photodetector (PD) physical area, !P i and 1fJ i are the radiation and incidence angles with respect to Tx; and Rx, respectively, \.(Ie is the field of view (FOV) ofRx.

LED's, which has the highest RSSI, and also get the received power of every LED lamps. In Fig. 3 (a) and (b), it can be observed that each received time domain symbol with a time

AR

duration of 0.1 s includes 4 different frequencies. This means that each received signal within 0.1 s consists of 2 DTMF symbols emitted from 2 different LED lamps. In Fig. 3 (b),

Suppose that Rx is facing vertically upward, and the

different colours are used to represent the intensity of the received signal across time. Here dark red and light red represent signal with higher and lower intensities, respectively. Thus, we are able to decode the received DTMF signal, which are listed at the button of Fig. 3 (b) along the x-axis.

ceiling height from Rx is h, then, cos(!p;) = cos(1fJ;) = hid;, as illustrated in Fig. 6 (b). Consequently, (1) can be converted as:

In order to obtain more information from the time frequency graph, we need to analyse the instantaneous spectrum of the received signal as shown in Fig. 4, which corresponds to the time of 0.05s in Fig. 3. From Fig. 4 notice that the amplitude of DTMF symbol "I" (680 Hz and 1200 Hz) and "0" (960 Hz and 1320 Hz) are 22 and 11, respectively. As the distance between ceiling and Rx is fixed we can determine the distance between vertical projections of two lamps and Rx, the detail is given in the Section TIT.

{� m( + +l)ARhff/+l

Hi = Jr(h2 1/ )(ff/+3)/2

(2)

1f/i

>

\.(Ie

Using (1) the normalized illuminance distribution of Fig. 6 (b) is plotted in Fig. 6 (c). It is clear that the distribution is axially symmetric and thus we are able to simplify the analysis with only focusing on the red triangle area in Fig. 6 (c). Suppose emitted waveform of the Tx; LED is received signal

R(t) can be expressed as: 4

S;(t),

+

(3)

where N(t) is the additive white Gaussian noise.

in Fig. 4. In this case the nearest lamp is TX1. It is evident that the best threshold level should be set to 16.5 ((11+22)/2=16.5).

Using (2) and a LED with !P1l2 = 60°

Following the threshold detection, the signal with the highest RSSI is obtained as illustrated in Fig. 5. The DTMF decoded ID is also listed in the figure, which is the same as TX1'S ID. Therefore, from this time-frequency graph, the lD of the nearest LED from the mobile can be acquired, and the distance

m(

= 1), a PD with a

2520 22 IS 680 Hz 11 121)0 Hz 105 960 H :"� �rOHZ 0 0.5 1.Fr5equency2I (kHz)2.5 3 3.5 4 Fig. 4. The frequency spectrum of the received signal at the time of 0.05s. I

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0 0.1 0.2 Ti0.m4e(s) 0.5 0.6 0.7 � 2000r- --.1500 g �I 1000500 �� 2 D A 01 2 D A Fig. gr3.aph.(a) The received waveform,and (b) its corresponding time-frequency Fig. 5. The time-frequency diagram of the highest RSSI DTMF signal. .€

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To capture the signal with the highest RSSI transited from the nearest lamp we have used threshold detection, as shown

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Fig. 8. Position estimation using trilateration. ( c ) TABLE I . Fig. nor(ma)alIinzdoor LED l am p ar r an gement , ( b ) t h e basi c uni t , and ( c ) t h e ed il uminance distribution. m DiCeisLEDtlainnceg heihalbetgfw-htpeenowerfromtwangldeto laeempsctor 2Am DetDetecteoctrodir FOV ameter 8mm DFTipoing nfrtesquency 8kHz ADC sampl o

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KEY PARAMETERS OF EXPERIMENT

6.

Parameters

60°

diameter of 8 mm and a FOV 'Pc = 180°, we have plotted the DC channel gain as a function of r as depicted in Fig. 7. Therefore, based on the above analysis, if Hof an unknown position is detennined as a function of the received optical I power, then r can then be calculated using H -i .

12

1800 200

As shown in Fig. 6, if all LED lamps mounted on the ceiling have the same emission angle and emission power,

then the same Hvs. r plot can be shared among them. By measuring the received light powers from different LED lamps, different ri can be calculated. If there are more than 2 r;

this fitted curve is given by a Fourier model as: H 0.492 + 0.508 x cos(0.009489 x r ) =

being obtained, the position of Rx can subsequently be estimated by trilateration [19], which is the process of detennining absolute or relative locations of points by measurement of distances, using the geometry of circles, spheres or triangles. To simplifY the calculations, we consider

-0.04062 x sin(0.009489 x r )

As mentioned in the Section TIT, owing to the symmetry, the area inside the red square in Fig. 6 (b) can be represented by the red identical isosceles right triangle as shown in Fig. 6 (c). Therefore, the basic unit shown in Fig. 6 (b) and (c) can be simplified into 3 LEDs at position of Txz, TX l and TX4 and the area inside of the red triangle. For the experimental setup, we installed 3 RL534CWC220 LEDs at the 3 positions with the same h = 240 em, and divide the red triangle into small grids

EXPERIMENTS AND DISCUSSION

It is known that different LEDs and PDs will have different plots for Has a function of r. In order to obtain the real relationship between Hand

r

(4)

We estimate r from the measured H by means of the inverse function of (4) with high accuracy.

only a two-dimensional geometry. Obviously the object with unknown position should be located at the intersection of the 3-circle, as presented in Fig. 8. TV.

Value

for our system, we have

with a side length of 5 em where each element of the grid

located a LED (RL534CWC220, ip llz=600) at the centre of a ceiling of an empty room with a height h of 240 em, and employed a head-on type PMT module (HAMAMATSU H9656), which has high sensitivity, large photocathode area, large FOV and good spatial uniformity. The key parameters are listed in Table IT.

indicates a test spot. The schematic diagram is depicted in Fig. 10, where the DTMF symbols of the positioning IDs of TX l, TXl and T� have the same frequency (680 Hz 760 Hz and 840 Hz). When the Rx is irradiated with 3 LED lamps, the 3 normalized Hi and ri can be calculated by using the time frequency analysis and the proposed algorithm. As a result, the location of the Rx can be estimated by using trilateration. Suppose the coordinate of the test spot is (Mx, My) and the estimated coordinate is (Mx', My'), hence the two-dimension

Following normalization, the measured channel gain Hat each measured points with an incremental distance of 5 em is plotted in Fig. 9 along with a fitted curve. The expression of

58


�

be improved, in particular, different h, variable orientation of the receiver as well as user mobility. The experimental system will be completed by more transmitters. In addition, the real time implementation will also be carried out.

l.0 ;t----L,--"'_E'=' �xp�e�nll· ml trJe:Iln�t s� 0.8��-+_�-+_��--+_ 1 6 30 60 90 120 Fig. 9. Measured normalized channel gai(em) n versus DTMF 123AI23AsymbolDC --1 � � IX �� DTMF --1 Txo 56B456B4SymbOIDC � DTMF 9C789C78symbolDC o---i � � l� Algorithm Fig. 10. Experimental setup of the proposed VLC-based IPS. 60 40 § 20 il! 20x(cm)40 60 Fig. II. Experimental results. rw-.......

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ACKNOWLEDGEMENTS

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This work is supported by EU cLINK project (GN.3722421-2012-I-UK-ERA), EU Cost Action IC 1101, the National Natural Science Foundation of China (NSFC NO: 61101110)

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and the Doctoral Scientific Fund Project of the Ministry of Education of China (No. 20120005110010).

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REFERENCES [I] H.posiLitiuo,niH.ng Darabi , P. Baner techniques and systjee,eandms,"J. Liu, "Survey of wirevolless. 37,indoorpp. -1080,ng,2007."Performance Evaluation of Automotive Radars Using [2] Z.Car1067rShuqi ier-Phase Dif erentvolial. 59,GPS,"pp. 2732-2741,2010. [3] L."DistribZhuang, W. tLirackiu, nJ.g B.usiZhang, D.lessH.sensor Zhang,netandwork,"Kamaj aya, ut e d asset n g wi r e i n pp. I11651Soo168. Jin, and P. Pil June, "Patient Monitoring System [4] usiS.2008,nDong k , H. g315.Sensor Network Based on the ZigBee Radio," in. 2007,pp. 313[5] ofG. Dynami A. RollcinSmar gs andt DustD. W.NetCorwornke,s,""Inintel igent2008,Operpp.ato477-rs fo4r82.Localisation [6] A.WitArh aafaDi,fJ.eXirentan,ialandPhotR.osensor Klukas,," "Wireless Indoor Optical Positioning 24,pp. 10271H.029,2012. [7] lJ.volocal.Kemper and Linde, pp."Chal63-le7nges0. of passive infrared indoor i z at i o n," i n [8] Z. Ghassemlooy, W. Popoola, and S. Rajbhandari, CRC [9] Z.iPrndooreWuss,2012.fandree T.spaceLit lopte,i"cNaletwsystorkem,"soluitnions for the LOS prNewcast oblemleofuponnew Uni,S.tedItoki,nK.gdom,Yasut2010, pp.K. 582-Kagawa, 587. M. Andoh,and S. Kawahito, [10] Tyne, Takai o mi , "LEDemandforCMOS Systpp.6801418Aut6801418,2013. omotImiageve ApplSensoricatBased ions," Optical Wireless Communivolcat.io5,n bout [[1II]2] P.BytLuo, eLightM.. (2Zhang, 014). X. Zhang,AvaiG. lCaiable,:D.httpHan,:/ www.andbytelLiig,ht."cAom/n inadoor vifresquency ible lighttechnicommuni cnatIiWonOW2013, positioninpp.g syst25-2e9.m using dual-tone multi q ue," i [13] lZ.ightZhou, M.ingKavehr avid,siandble lP.ightDeng,communi "Indoorcatiposions,"tioning algorithm using e mi t di o de [14] ivolH.ndoor-S. 51,p.085009,2012. . Kivimsib,leD.l-iRght. Kicommuni m, S.-Hcat. iYang, Y.tio-Hni.ngSon,systeandm usiS.n-gK.a RFHan,carr"Aienr o n posi allocat144,2013. ion technique," vol. 31, pp. 134[15] Avai Wikilpaediblea:.ht(t2p014):/ en..wikipedia.org/wikilDtmf [16] ofH.-SIn. tKier-mC,elD.l -IRnt.eKirfemre,nceS.-Hand. Yang,CroY.sst-aHlk. Son,in CarrandieS.r-KAl. lHan, "An nalBasedysis o cat i o [17] M.fVioursiBaribelretLietrlasgnsfhtandoCommuni S.rm-bLiasednfocotatf,iroe"n,"quency Anianaln yresicogni sLosof rtAngel eioaln tialmegs,eori2012, mithplms,"ementJTh2A.inatio5n0.of [18] Readi J. M.ng,KahnUniteandd kinJ.gdom,R. Bar2004,ry,pp.vol"W.128-i85,relepp.1ss32.265-infrar2ed98,1997. communications," [19] lZ.ocalYang, .netLiw,ork"Bs,"eyond trilateration: On the izabilityY.volof.Liwi18,ur,epp.leandss1806adX.hoc-1Y814,2010.

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In order to test the accuracy of the proposed method, we performed the measurement at 91 points as shown in Fig. 11, which is corresponding to the red identical isosceles right triangle shown in Fig. 6 (c). And due to the symmetry this area can be used to represent the whole the area inside the red square in Fig. 6 (b). According to (5), the average estimated error is l.565 cm, the largest and the smallest errors are 5.2 cm

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CONCLUSION AND FUTURE WORK

In this paper, a VLC-based IPS using DTMF technique has been proposed and tested. With the proposed algorithm and system, the inter-lamp interference can be mitigated, and the unknown position can be estimated with high accuracy. The results showed that the proposed algorithm is able to provide an average estimated error of less than 2 cm in a simple and easy way.

Dual-tone multi-frequency signaling.

OFC,

ISCE,

PROCEEDINGS OF THE IEEE,

IEEEIACM Transactions on

Networking,

As far as future work concerned, the proposed model will

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