performance of efficient signal detection for led-id systems

Wireless Pers Commun (2011) 60:533–545DOI 10.1007/s11277-011-0307-6

Performance of Efficient Signal Detection for LED-IDSystems

In Hwan Park · Yoon Hyun Kim · Jae Sang Cha ·Yeong Min Jang · Jin Young Kim

Published online: 9 April 2011© Springer Science+Business Media, LLC. 2011

Abstract In this paper, effects of reader-to-reader interference are investigated for LEDidentification (LED-ID) system in a multi-reader environment. The LED-ID readers typicallyuse different channels to avoid collision between readers. However, in-channel collision usu-ally happens in terms of interrogation range. A reader-to-reader interference scenario isproposed, and nominal interrogation range of a desired reader is derived from this model. Inorder to evaluate the LED-ID reader-to-reader interference quantitatively, an efficient detec-tion scheme is proposed and simulated by employing spreading sequence. The spreadingsequence is inserted between each user’s frame formats. In the receiver, the desired signal isdetected by using correlation among inserted spreading sequences. From simulation results,it is confirmed that the proposed scheme is very effective to enhance reliability of LED-IDcommunication systems.

Keywords Signal detection · LED-Identification (LED-ID) ·m-sequence orthogonal frequency division multiplexing (OFDM)

I. H. Park · Y. H. Kim · J. Y. Kim (B)Department of Wireless Communications Engineering, Kwangwoon University, Seoul, Koreae-mail: [email protected]

I. H. Parke-mail: [email protected]

Y. H. Kime-mail: [email protected]

J. S. ChaDepartment of Media Engineering, Seoul National University of Technology, Seoul, Koreae-mail: [email protected]

Y. M. JangCollege of Electrical Engineering, Kookmin University, Seoul, Koreae-mail: [email protected]

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534 I. H. Park et al.

1 Introduction

Recently, LED (light emitting diode) has been emerging as a new growth technology whichis expected to replace existing illumination infrastructure. The LED is known to be moreadvantageous than the existing incandescent in terms of long life expectancy, high toler-ance to humidity, low power consumption, and minimal heat generation lighting, etc. Theirdiverse applications include numeric displays, flashlights, liquid crystal backlights, vehiclebrake lights, traffic signals and the ubiquitous power-on indicator light [1–7].

Currently, interests in LED communication using white LEDs are gradually growing asneeds for indoor communication systems increase because there are many devices using thelightings in our offices, home, the lightings on roads, traffic signals, home appliances includ-ing TVs, and etc. The typical LED has special characteristics to light on and off very fast atultra high speed. By using visible light for the data transmission, most of problems related toradio communications are resolved or relieved. The visible light communication is known tohave characteristics to be ubiquitous, transmitted at ultra high speed and harmless for humanbody and electronic devices, compared to those by radio communications. The human eyewould not be able to follow these variations, and, hence, the lighting will not be affected.As a consequence, simple off-the-shelf LEDs can be used to develop cheap transmitters.

The LED visible light communication is interpreted as a convergence communicationtechnology which is not only used as a lighting device, but also to be used as communicationdevice [8–10]. It is a kind of indoor optical wireless communication that uses ‘visible light’ray as communication medium. For example, LED-identification (ID) technology based onthe LED communication is ubiquitous information communication service that is used tosupply variable information at museum, super market, and restaurant etc.

The LED-ID system has a number of promising advantages such as low power consump-tion, no interference to radio frequency (RF) based devices, and free licensing band. However,it is also facing challenges such as using appropriate techniques to construct cheap processingunits and high brightness LEDs. Also, this system should overcome interference caused bysolar light and other forms of light.

From the implementation point of view in the LED-ID communication system, there arestill many kinds of challenging issues to be overcome. One of them include reader detectionproblem which mainly occurs in a dense reader environment where several readers try tointerrogate tags at the same time in the same area. The read results may be unsatisfactory inread times and an unacceptable level of misreads.

The objectives of this research are to formulate a reader interference scenario and providean efficient reader detection scheme for LED-ID deployment. In this paper, we propose thesignal detection method using spreading sequence for LED-ID system in multiuser indoorwireless environments. The LED-ID readers typically use different channels to avoid colli-sion between readers. However, in-channel collision may happen in terms of interrogationrange. As a spreading sequence, m-sequence is chosen due to its many advantageous featuressuch as highly peaked autocorrelation and minimum cross-correlation [11,12]. The salientfeature of the autocorrelation is ratio of peak value to modulus of the highest sidelobe. Apartfrom noise or interference, this is a key parameter which determines probabilities of detectionand false alarm.

This paper is organized as follows. In Sect. 2, reader interference scenario of LED-ID sys-tem and its system modle are described. In Sect. 3, signal detection scheme based on spreadingsequence is proposed for LED-ID system. In Sect. 4, simulation results are presented, andfinally, the conclusions are drawn in Sect. 5.

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Performance of Efficient Signal Detection for LED-ID Systems 535

Fig. 1 Interference modelof LED-ID sysetm

2 LED-ID System Model

In this section, the system model of LED-ID system using LED lights is described. TheLED-ID technology is a kind of green technology which allows very low consumption tag tocommunicate identification information to an interrogation reader located at some distance.This seems to be similar to RFID system in a conceptual manner, however, it is not only usedas a lighting device, but also used as a communication means. Furthermore, the LED-ID sys-tem allows quantitative advanced characteristics; (1) the reader can have line-of-sight to thetag, (2) the tag can store and communicate many more bits of information, (3) multiple tagscan be interrogated by the same reader, and (4) the reader only allow secured communication

2.1 LED-ID System

There are several candidates in modulation schemes for LED-ID system. OOK (on-off-keying), pulse code modulation (PCM) and pulse position modulation (PPM) are some ofthe more popular modulation modes used in conjunction with LED-ID systems [10]. Weconsider OFDM (orthogonal frequency division multiplexing) transmission scheme, whichuses multiple carriers overlapped in the frequency domain. The OFDM systems are able tosupport high data rates without need of channel equalizers as aggregate throughput is distrib-uted over the set of subcarriers. The inherent robustness of OFDM against multipath effects,the possibility to combine it with any multiple access schemes and the possibility to easilycombine OFDM with any higher order modulation scheme makes it an excellent choice alsofor LED-ID systems.

Figure 1 shows a rough concept on how the interrogation between reader A and reader Band the tag occurs. In this paper, an interference scenario of LED-ID readers involve onlytwo readers and the each reader transmit and receive in the LED-ID wireless optical channel.Because both the reader A and the read B are in the FOV (field of view) of the tag, the tagreceives signals of the reader A and B together and the signals acts as interference each other.

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Fig. 2 Geometry of transmitter and receiver

2.2 LED-ID Channel Model

For rigorous analysis of the proposed system, a suitable channel model is highly requiredfor exact estimate of system performance in LED-ID systems. Background noise is assumedto be AWGN (additive white Gaussian noise). In LED-ID systems, the LEDs are usuallyinstalled in a ceiling and they have has large superficial area. Therefore, LED-ID systemhas particular impulse response other than that from infrared communication. To considerreflection effect correctly, both reflex and diffusion characteristics are also taken into accountfor more practical approach. Lambertian reflector model has been known be a well-fitting onefor modeling of indoor diffusion characteristics of representative materials such as plasterwall, acoustic-tiled walls, carpets, unvarnished woods, and etc. [13]. Therefore, the wall orceiling can be interpreted as Lambertian reflector in LED-ID systems.

For LOS (line-of-sight) case, we assume there are no reflections and source and receiverseparation squared is much greater than the receiver area. In Fig. 2, the channel impulseresponse can be approximated by a scaled and delayed Dirac delta function given by

h(t; S, R) ≈ m + 1

2πcosm(ψ) · d� · rect

(θ

FOV

)δ

(t − R

c

), (1)

where m is the mode number associated with the directivity of the source and calculated fromthe source half-angle, d� is defined as solid angle subtended by receiver’s differential areagiven by

d� ≈ cos(θ)AR

R2 , (2)

θ is angle between nR and (rS − rR)given by

cos θ ≈ nR · (rS − rR)

R, (3)

R is distance between the source and receiver given by

R = ‖rS − rR‖ , (4)

ψ is angle between nS and (rR − rS) approximated by

cosψ ≈ nS · (rR − rS)

R, (5)

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and, rectangular function is defined by

rect(x) ={

1 for |x | ≤ 10 for |x | > 1

. (6)

The channel of LED-ID systems can be modeled with an additive white Gaussian noise(AWGN) model. In optical channels, quality of transmission is typically dominated by shotnoise because receiver employs a narrow band optical filter. However, the system can neglectthe shot noise caused by signals. Accordingly, the received signal can be expressed as

y(t) = r · x(t)⊗ h(t)+ Gn, (7)

where y(t) represents received signal, x(t) embodies transmitted optical pulse, Gn depictsAWGN noise, the symbol ⊗ denotes convolution, and r denotes an optical/electric (O/E)conversion efficiency.

In this paper, we employ impulse response channel with bounces of ninth times. Consid-ering reflected signal by reflectors, the impulse response can be written as

h(t; S, R) =∞∑

k=0

h(k)(t; S, R), (8)

where h(k)(t) is response of the reflected impulse signals k times.At k = 0, Eq. (8) is the same with Eq. (1). Higher order terms, at k > 0, can be calculated

recursively. It is given by

h(k)(t; S, R) =∫S

h(0)(t; S, {r, n,π

2, dr2} ⊗ h(k−1)(t; {r, n, 1}, R). (9)

Using Eq. (1), the Eq. (9) can be rearranged and be written as

h(k)(t; S, R) = m + 1

2π

∫S

ρr · cosm(ψ) · cos(θ)

R2 · rect

(θ

FOV

)

×h(k−1)(t − R

c; {r, n, 1}, R)dr2. (10)

3 Proposed Signal Detection Scheme

In this section, the proposed signal detection scheme is described for LED-ID systems. Blockdiagram of the proposed LED-ID system is illustrated in Fig. 3. The serial data stream of thei th user is mapped to data symbols with a symbol rate of 1/Ts , employing the signal con-stellation scheme of 4 quadrature amplitude modulation (QPSK). And the resulting symbolstream is demultiplexed into a vector of Nc data symbols. The parallel data symbol rate is1/ (Nc × Ts).

The parallel symbol duration is Nc times longer than the serial symbol duration Ts . Then,inverse fast Fourier transform (IFFT) of the data symbol vector is computed. The resultsconstitute an OFDM signal of the i th user, xi (n), which is given by

xi (n) =(

1

Nc

) K∑k=−K

Xik exp

(j

2πkn

Nc

), (11)

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DataSource

Serialto

Parallel

SignalMapper IFFT

Parallelto

Serial

GuardIntervalInsertion

Digitalto

Analog

LED-IDChannel

h(n)

Parallelto

Serial

SignalDemapper FFT

Serialto

Parallel

GuardIntervalRemoval

Analogto

Digital...

...

......

...

...Signal

Detection

Inserted SequenceRemoval

InsertSequence

DataOutput

Fig. 3 Block diagram of the proposed LED-ID sysetm

(i+1)th OFDM frame

M-seq.

0 Nm-sqe-1

(i+2)th OFDM frame ith OFDM frame

0 Nc-1

Fig. 4 Procedure of inserting m-sequence in the OFDM frame

where n = 0, 1, . . . , Nc − 1 and Nc ≥ 2K + 1. After the IFFT, an orthogonal m-sequenceof the i th user is combined with the OFDM signal in the time domain. This sequence isused for frame synchronization at the receiver. The multipath effect causes the inter-symbolinterference (ISI) in time dispersive channels. And the orthogonality of the OFDM signal isdistorted. In order to maintain the orthogonality of the OFDM signal in multipath channel, aguard interval is inserted in front of each OFDM block. The last Ng samples of the OFDMsignal are copied and appended as a preamble to compose an OFDM frame. This is knownas a cyclic prefix. Then, the resulting signal is transmitted.

At the receiver, the transmitted data is obtained after removing the orthogonal m-sequenceof the i th user and the cyclic prefix, and demodulating the Nc samples of each frame usingthe FFT. The requirement for the guard interval in time dispersive environments certainlyreduces the overall efficiency of OFDM transmissions by a factor of Nc/

(Nc + Ng

).

After the IFFT, an orthogonal m-sequences of the i th user are added to the OFDM signalin the start position of a new frame of xi (n), as shown in Fig. 3. The resulting time domainsignal of the i th user, yi (n), is given by

yi (n) = xi (n)+ √PM Mi (

n − Ng − j(Nc + Ng

))+√

PM Mi (n − Ng − j

(Nc + Ng

) − NM), (12)

where PM is the power of the orthogonal m-sequence, Mi (n) is the orthogonal m-sequenceof the i th user whose value is ±1, Ng is the guard interval, and NM is the one orthogonalm-sequence length. The start position of an OFDM frame is equal to that of the orthogonalm-sequence as in Fig. 4.

The added orthogonal m-sequence is not allowed to affect the performance of the OFDMsystem except for signal detection.

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(j+1)th

OFDM frame (j+2)th

OFDM frame jth

OFDM frame guard

intervalguard

intervalguard

interval

n

Observation Interval

Signaldetection

M-sequence

Fig. 5 Observation interval of r(n)

The received signal, r (n), is given by

r (n) =I−1∑i=0

yi (n)⊗ hi (n)+ Gn, (13)

where hi (n) is the LED-ID channel impulse response for the i th user, ⊗ is the convolutionoperation, and Gn is complex additive white Gaussian noise (AWGN) with zero mean andvariance σ 2

n . Then, the received signal can be written as

r (n) =I−1∑i=0

[xi (n)⊗ hi (n)

+√PM Mi

(n − Ng − j

(Nc + Ng

) ⊗ hi (n)

+√PM Mi (


) − NM) ⊗ hi (n)

]+ Gn

)

=I−1∑i=0

[xi (n)⊗ hi (n) + √

PM Mi (n − Ng − j

(Nc + Ng

)) ⊗hi (n)]

+ n (n) .

(14)

In order to find the start position of the OFDM frame of the i th user, we observe 3(Nc + Ng

)consecutive samples of r (n) as in Fig. 5. In the case of Nc � 1, the OFDM signals in thetime domain are Gaussian distributed

with zero mean and variance σ 2x . Besides power level of the I OFDM signals is much

higher than that of AWGN, that is, I × σ 2x � σ 2

n . Then the received signal in (14) can berewritten as

r (n) =I−1∑i=0

√PM Mi (


)) ⊗ hi (n)+ n0 (n), (15)

where n0 (n) is AWGN with zero mean and variance I × σ 2x + σ 2

n ≈ I × σ 2x .

When the OFDM signals are received, a log-likelihood function of the synchronizationposition is given by

L (S) = ln p (r (n)), (16)

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Table 1 Simulation parameters Number of FFT points 512Number of data sub-carriers 256The receiver O/E conversion efficiency 0.53 (A/W)Detector physical area of PD 1.0 (cm2)Transmitted optical power 1 (W)FOV at the receiver 60 (deg.)SNR 10 (dB)Channel LED-ID channel

Fig. 6 Cross-correlationat 3 m distance

Fig. 7 Cross-correlationat 3 m distance

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Fig. 8 Autocorrelation at 3.75 mdistance

where p (r (n)) denotes the probability density function of r (n). Substituting (15) in (16),the maximum likelihood (ML) estimation of S is given by

S = arg

{max

SL (S)

}

= arg

{max

S

∑n∈Sync

ln (p (r (n)))

}

= arg

{max

S

S+NM−Seq−1∑n=S

r (n)Gi (n − S)

}.

(17)

The receiver has no idea from whom the signal is coming. But it knows that what kind of theorthogonal m-sequence is added on the OFDM frame. Therefore, by correlating the receivedsignal with the known orthogonal m-sequence, the ML estimation of the synchronizationposition of the signal which is transmitted by the i th user is obtained. The ML estimation ofS finds the maximum value of the correlator outputs.

4 Simulation Results

In this section, the proposed the signal detection scheme is simulated for LED-ID system.The simulation parameters of the OFDM transmission system are listed in Table 1.

To verify the signal detection performance of the proposed detection scheme, we eval-uate detection performance as the distance of between tag and reader in LED-ID channelinterfered by the unwanted signal.

The OFDM frame detection performances with m-sequence method over the LED-IDchannel as distance of between tag and reader are shown in Figs. 6, 8 and 10. The results inFigs. 6, 8 and 10 show autocorrelation values for varying distance between reader and tag.

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Fig. 9 Cross-correlation(m-sequence) at 3.75 m distance

Fig. 10 Autocorrelation at 4 mdistance

Figures 6, 8 and 10 show autocorrelation values for varying distance between reader andtag. We can see that the signal detection performance is gradually improved as the distancebetween reader and tag becomes closer. The reason of this is that the signal power is inverselyproportional to a square of distance.

Figures 7, 9 and 11 represent cross-correlation values with m-sequence using two identicalsequences in the LED-ID channel, respectively. It is shown that the peak cross-correlationlevels of the unwanted signal are very low. In order to perform the desired signal detection,we need to find the points in which the autocorrelation level is peak or exceeds predeterminedthreshold. However, in a multi-user environment, there exist not only the autocorrelation val-ues but also the cross-correlation ones. Therefore, a gap for establishing threshold becomesnarrow. Then, the probability of signal detection is enhanced.

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Fig. 11 Cross-correlation at 4 mdistance

5 Simulation Results

In this paper, we have proposed the improved signal detection scheme based on m-sequencefor LED-ID system. A m-sequence sequence has property that highly peaked autocorrelationand minimum cross-correlation. Therefore, we superimposed the m-sequence directly overthe OFDM signals in the time domain before transmitting the signals.

Then, by correlating the m-sequence at the receiver with the received signal, we coulddetect the desired signal of OFDM frame although there were several signals of multi-user,who had their own m-sequences. From the simulation results, it was confirmed that theproposed detection scheme is very effectively in detecting the desired signal. The proposedscheme of this paper can be applied to detection module of the OFDM based LED-ID systems.

Acknowledgments This work was, in part, supported by the IT R&D program of MKE/KEIT. (10035264,Development of Home Network Tech. based on LED-ID), and in part, supported by Kwangwoon Universityin 2011.

References

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Author Biographies

In Hwan Park received the B.Sc. degree in electrical engineering fromthe Department of Wireless Communications Engineering, Kwang-woon University, Seoul, Korea, in 2010. His research interests includevisible light communication, MIMO, OFDM, cooperative communica-tion, interference cancellation, channel coding, and compatibility anal-ysis between radio communication services. He is currently workingtoward the master course at Kwangwoon University, Seoul, Korea.

Yoon Hyun Kim received the B.Sc. and M.Sc. degrees in electricalengineering from the Department of Wireless Communications Engi-neering, Kwangwoon University, Seoul, Korea, in 2006 and in 2008,respectively. His research interests include next generation communi-cation systems and their applications, such as VLC, UWB, MIMO,OFDM, CDMA, cooperative communication, interference cancellation,and channel coding. He is currently working toward the Ph.D. degreeat Kwangwoon University, Seoul, Korea.

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Jae Sang Cha received the B.Sc. And M.Sc. degrees from the Schoolof Electrical Engineering, Sungkyunkwan University in Korea in 1991and 1997, and the Ph.D. degree from the School of Electrical Engineer-ing, Tohoku University in Japan in 2000, respectively. He was Memberof Research Staff at the Electronics and Telecommunications ResearchInstitute (ETRI), from 2000 to 2002. He was Assistant Professor atthe Dept. of Information and Communication Engineering, Seo KyeongUniversity, Seoul, Korea, from 2002 to 2005. He is currently AssociateProfessor at the Dept. of Media Engineering, Seoul National Universityof Science and Technology, Seoul, Korea. His research interests includedesign and implementation of wireline/wireless multimedia communi-cation systems for applications to spread-spectrum, digital broadcastingtransmission technology, ultrawideband (UWB), LED communicationapplications.

Yeong Min Jang received the B.E. and M.E. degree in ElectronicsEngineering from Kyungpook National University, Korea, in 1985 and1987, respectively. He received the doctoral degree in Computer Sci-ence from the University of Massachusetts, USA, in 1999. He workedfor ETRI between 1987 and 2000. Since 2002, he is with the Schoolof Electrical Engineering, Kookmin University, Seoul, Korea. He hasorganized several conferences such as ICUFN2009 and ICUFN2010.He is currently a member of the IEEE and KICS (Korea Informationand Communications Society). He received the Young Science Awardfrom the Korean Government (2003–2006). He had been the directorof the Ubiquitous IT Convergence Center at Kookmin University since2005. He has served as the executive director of KICS since 2006.He has served as a founding chair of the KICS Technical Committeeon Communication Networks in 2007 and 2008. His research interestsinclude IMT-advanced, RRM, femtocell networks, Multi-screen con-vergence networks, and VLC WPANs.

Jin Young Kim (S’91–M’95–SM’08) received the B.Sc. M.Sc. andPh.D. degrees from the School of Electrical Engineering, Seoul NationalUniversity (SNU), Seoul, Korea, in 1991, 1993, and 1998, respectively.He was Member of Research Staff at the Institute of New Media andCommunications (INMC) and at the Inter-university SemiconductorResearch Center (ISRC) of the SNU from 1994 to 1998. He was Post-doctoral Research Fellow at the Department of Electrical Engineering,Princeton University, NJ, USA, from 1998 to 2000. He was PrincipalMember of Technical Staff at the Central Research and DevelopmentCenter, SK Telecom, Korea, from 2000 to 2001. He was Associate Pro-fessor at the School of Electronics Engineering, Kwangwoon Univer-sity, Seoul, Korea, from 2001 to 2010. He had his sabbatical leave asVisiting Scientist at the LIDS (Laboratory of Information and DecisionSystems), Massachusetts Institute of Technology (M.I.T), MA, USA in2009. Now, he is currently Professor at the School of Electronics Engi-neering, Kwangwoon University, Seoul, Korea.His research interests include design and implementation of wire-

line/wireless multimedia communication systems for applications to spread-spectrum, cognitive radio,ultrawideband (UWB), space communication, optical communication and powerline communication systemswith basis on modulation/demodulation, synchronization, channel coding, and detection/estimation theory.He received the Best Paper Awards from several academic conferences and societies including Jack Neb-auer Best Systems Paper Award from IEEE VT Society (2001), the Award of Ministry of Information andCommunication of Korea Government (1998), the Best Paper Award at APCC’00 (2000), the Best PaperAward at IEEE MoMuC’97 (1997), and the many other Best Paper Awards from conferences of IEEK’08,KITFE’08, KITS’08, and KITS’09 (2008–2009). He was listed in the Marquis Who’s Who in the World,Marquis Who’s Who in Science and Engineering, ABI and IBC throughout from 2001 to 2009 Editions. Heis now Senior Member of IEEE, Regular Member of IET, IEICE, and Life Member of IEEK, KICS, KEES,KITFE, KITS and KOSBE.

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performance of efficient signal detection for led-id systems

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