Benhaddou, Driss

Ghassemlooy, Zabih

Hashemi, Seyyedkamal

Lu, Chao

English

Northumbria Research Link

Orthogonal Frequency Division Multiplexing forIndoor Optical Wireless Communications usingVisible Light LEDsS. K. Hashemi1, Z. Ghassemlooy1, L. Chao2, and D. Benhaddou31Optical Communications Research Group, NCRLab Northumbria University, Newcastle upon Tyne, UK2Department of Electronic and Information Engineering, Hong Kong Polytechnic University3Engineering Technology Department, College of Technology, University of Houston, USAE-mail:seyyedkamal.hashemi@unn.ac.uk, fary.ghassemlooy@unn.ac.uk, enluchao@polyu.edu.hk, dbenhaddou@uh.eduAbstract— Visible light LEDs are being used for indooroptical wireless systems as well as room lighting. Inindoor diffuse optical wireless links multipathdispersion limits the maximum transmission datarates. In this paper we investigate OFDM system wheremultipath induced intersymbol interference (ISI) isreduced, enabling higher data rates. Pilot signals areuniformly inserted into data symbols (subcarriers) and areextracted at the receiver for channel estimation. Predictedand simulated results for the symbol error rate (SER) foran OFDM employing BPSK, QPSK and M-QAM for line ofsight (LOS) and diffuse links are presented for a range ofpilot signal.I. INTRODUCTIONThe demand for a wireless broadband communicationlink capable of supporting multimedia applications athigh data rates is growing. The existing RF and wiredbased technologies available (such as Wi-Fi, WiMax andWLAN) suffer from the bandwidth bottleneck but offermobility and flexibility. In number of applications wherehigher data throughputs is more of requirement than themobility, transmission link based on optical wirelesswould be one of the best options as outlined in [1-4].Recently, white LEDs have been used for bothillumination and indoor wireless communications [1-4].Limited mobility can be provided using a diffuse opticalwireless link but at the cost of the much reduced data ratedue to multipath dispersion that gives rise to inter-symbol interference (ISI) in high-speed systems. For aparticular IR systems, the degree of multipath is complexto predict, and depends on many parameters such asroom geometry, materials, the position of the transmitter,receiver and objects within the room, and system design.A number of techniques have been proposed tocombat ISI. In [5] guard slots are inserted into eachdifferential pulse interval modulation scheme, whereas in[6] spread spectrum technique has been used to combatISI but at the expense of reduced bandwidth efficiency.Angle diversity scheme based on multibeam-narrowFOV transceiver has also been employed to overcomemultipath dispersion since a small fraction of the delayedlight normally lies within the same field-of-view (FOV)as the primary signal as in [7]. In [8] an adaptivedecision equalizer is used to combat ISI. OFDM has beenused in mainly RF based wireless communicationschemes offering high data rates capability as well ashigh bandwidth efficiency and effectively dealing withthe multipath induced ISI [9] and has been proposed inoptical wireless communications by researchers [10]. InOFDM a large number of closely-spaced orthogonal sub-carriers, each modulated at a low symbol rate, isemployed to offer overall data rates similar toconventional single-carrier modulation schemes withinthe same channel bandwidth [9]. Since the duration ofeach symbol is long compared with the channelmultipath time-spreading, then by inserting a guard slotbetween the symbols, it is feasible to eliminate the ISI.The guard slot is much shorter than the symbol lengthbut longer than the channel multipath time-spreading. Inthis paper we investigate OFDM system where multipathinduced ISI and the effects of background noise arereduced by taking the advantage of narrowbandfrequency interference, which affects only one of thefrequency subbands when compare to the single-carriermodulation. This paper is organized as follows. Insection 2, we introduced the proposed optical OFDMsystem, whereas in section 3 channel estimation detailsare given. Performance analysis and simulation areoutlined in section 4, and finally concluding remarks arepresented in section 5.II. OPTICALOFDMThe Baseband model of an optical OFDMtransmission system is depicted in Fig. 1. An input binarybit stream b[i] is grouped by passed through a serial-to-parallel (S/P) converter to generate parallel streamsbefore being modulated. Here the modulation schemesadopted are the BPSK, QPSK, and M-QAM. Themodulated output symbols are definedas ? ?110 ,,, ?? dNd XXX? ? . Pilot signals (tones) areuniformly inserted into all symbols (sub-carriers) forchannel estimation in diffuse links. For LOS links thereare no pilot signals. The Inverse Fast Fourier Transform(IFFT) block is used to implement the OFDM, which to978-1-4244-1876-3/08/$25.00 ©2008 IEEECSNDSP08 - 174 - ProceedingsFig 1. Baseband model of an optical OFDM system.transform the data sequence X(k) of length N into acomplex time-domain signal. The transmittedinformation symbols X(k) are the frequency coefficientsand the output of the IFFT stage )(nx is the time domainof the input samples: ))}(({)( nkXIFFTnx N? , where nand k = 0, 1, 2, …, N-1. In optical system with intensitymodulation, only real-value and positive signals are used,therefore in OFDM, the subcarriers should haveHermitian symmetry to produce real value. To mitigatethe effects of multipath induced ISI, OFDM symbol ispreceded by a guard interval of G-sample (or cyclicprefix (CP) as shown in Fig. 2 The length of G-sampledepends on the channel delay spread and is normallyconsidered to be grater than or equal to the channellength (impulse response time) to ensure prevent ISI [9].Fig 2. OFDM symbol structure with a cyclic prefix.In optical transmission with IntensityModulation/Direct Detection (IM/DD), a DC bias isusually added to the signal to ensure that the signals areall positive [10]. Finally, the OFDM signal is passedthrough a parallel-to-serial converter (P/S) to generate adiscrete-time OFDM symbol s(n) of durationGTNTTs ?? , where NBf /?? is the frequencyspacing between the subcarriers within the bandwidth B,prior to being applied to the optical source.The received OFDM signal propagated through thechannel h(n) is given by:][][][.][ nwnhnsnyr ???? (1)where w[n] is the additive white Gaussian noise(AWGN), ? is the photodetector responsivity, and? denotes the circular convolution.As for the channel impulse response, Barry [11]proposed a recursive algorithm to take into accounthigher order reflections, whereas in [12] a MIMOmethod for speedy calculation of the impulse response ispresented. In [13,14] a Mont Carlo ray-tracing algorithmand its modified version uses both the Lambert’s diffuseand the Phong’s models to approximate surfaces with astrong specular component. A detailed derivation of theimpulse response for the Sphere model was given in [15]and was practically measured by using the Ulbricht’sintegrating sphere in [16]. In [17] a fast iterative modelcapable of calculating high-order reflections is outlined.Due to the CP, the discrete linear convolution of s[n] andh[n] becomes a circular convolution hence simplifyingthe equalization process at the receiver. Thus at thereceiver, the signal is passed through S/P and its CP fromeach received symbol is removed before being applied tothe N-point FFT module for the following operation:)},({)( nyFFTkY N? (2)where 1......,,2,1,0 ?? NkIII. CHANNEL ESTIMATIONIn OFDM system due to multi-amplitudemodulation schemes, channel estimation is required forequalization and decoding provided there is noknowledge of the channel. In one dimensional channelestimation schemes pilot insertion could be done in (i)block-type and (ii) comb-type. The former uses the leastsquare (LS) or the minimum mean-square error (MMSE)to perform estimation in a slow fading channel [18],whereas the latter uses the LS with interpolation and themaximum likelihood (ML) to perform estimation in arapidly changing channel [19]. Here, we have adoptedthe latter for the indoor optical wireless channel. In thecomb-type pilot based channel arrangement,pN pilotsymbols )}1(,),1(),0({ ??? ppppP NXXX ? areuniformly inserted into Xd according to the followingexpression:Copy and paste last G symbols11 ???? NGNGN xxx ? 1110 ???? NGNGN xxxxx ??OFDM SymbolsCyclic PrefixBinaryDatab[i]FFTCPRemovalS/PChannel h(n)P/SX(k)S/PPilotInsertionx(n)CPInsertiony(n)YdChannelInterpolationChannelEstimationpH?dH?Decoding/EqualizationDemapdX?][ib?P/S AWGNw(n)Photodetectors[n]yr[n]IFFTMap:BPSKQPSKM-QAMXdOpticalSource +DC Biasb?? YpOutputDataProceedings - 175 - CSNDSP080123450123450123Height[m]Length[m]Width[m]2.5m(3.5,3.5)m(1,1)m(3.5,1)m(1,3.5)mLEDs interval: 1cmFig 3. Positions of LEDs within a room with a size3355 m??})),/((),1(),1)/(()0(),0({ ?? pNdNFloordXpXpNdNFloordXdXpX ?before performing IFFT.Information on the exact locations of the pilot and datasymbols as well as the values of the pilot symbols arealready available at the receiver. The received pilotsymbols Yp are extracted from the received OFDM signalafter the FFT block for channel estimation. The estimatedchannelpH? at pilot subcarriers for the LS estimation(where there is no need for the prior knowledge of thevariance of the noise) is given as:1,,1,0)()()(? ??? pppp NmmXmYmH ? (3)There are a number of interpolation schemes includingthe linear interpolation, the second-order interpolation,the spline cubic interpolation and the time domaininterpolation which are used in the one dimensionalmodel to extract channel information on the data sub-carriers Hd [18]. In [21, 23] detailed information on thenumber of pilots used and how to located them asoptimal pilot design techniques are given.IV. PERFORMANCEANALYSIS AND SIMULATIONThe proposed scheme is simulated with the followingconditions. Optical sources used are four sets of whitelight LEDs located at a height of 2.5 m from the floor asshown in Fig. 3. We assumed the user terminal is at adesk height of 85 cm. Both LOS and diffuse links areconsidered. The properties of photometric andradiometric are employed for the distribution of luminousintensity. The radiation pattern is assumed to beLambertian with the order of semi-angle half illuminanceof an LED equal to ))ln(cos(2ln 21???n at 70 degreeand the following distribution:]2,2[)(cos..21)( ?????????? forPnR nS (4)where n is the Lambert’s order expressing directivity of012345012345200400600800100012001400x[m]Distribution of horizantal illuminance [lx]y[m]Illuminance[lx]Number of LEDs60 x 60 (4 set)Fig 4. Distribution of illuminance for the simulation parameters.the source beam,sP is the total optical power and 0?? isthe angle of maximum radiated power.To provide sufficient lighting according to ISO, theilluminance must be between 300 to 1500 lx. All thesimulation parameters adopted are listed in Table I. Thedistribution of illuminance is illustrated in Fig. 4, wherethe maximum and minimum values of illuminance are at1210 lx and 334 lx, respectively.Figure 5 depicted the predicted SER performanceagainst the energy-to-noise ration (Eb/N0) for the BPSK,QPSK and 16-QAM modulation schemes for the LOSlinks showing a close match with the simulated data.Note that no pilot signal is used for the LOS links. Theanalytical OFDM system performance for M-QAM andQPSK modulation over AWGN channel is given by [24]:??????????????? ??obQAMM NEMQMP13112,? ?2, 11 MQAMM PP ??? , MMb PP 2log? (5)for QPSKTable I. Simulation parametersParameters ValuesTransmitted optical power Ps 20.0 mWCentre luminance intensity of LED 730 mcdField of view (FOV) 60 DegreePhotodetector surface area Ar 1 cm2Lambert mode 0.6461Refractive index of theconcentrator nr1.5Photodetector responsivity 0.53 (A/W)FFT size 256Number of data subcarriers 128Number of pilots 16,32,64Cyclic prefix (CP) ratio 1/4CSNDSP08 - 176 - Proceedings0 5 10 15 20 2510-410-310-210-1100Eb/No(dB)SEROFDM over AWGN ChannelBPSK SimulationBPSK TheoryQPSK SimulationQPSK Theory16-QAM Simulation16-QAM TheoryFig 5. SER performance of LOS optical wireless OFDM linkemploying BPSK, QPSK and 16-QAM.QAMMQPSKMPP,,2? , 2/,, QAMMQPSKM PP ?and for BPSK over AWGN channel?????????obbNEerfcP21 . (6)The OFDM scheme employing the BPSK shows thebest performance requiring 3.5 dB and 10.5 dB lowergains than QPSK and 16-QAM at the BER of 10-4, but atthe cost of lower data rates.Next we considered a diffuse channel with 16 taps (i.e.L = 16). Since the number of data and pilot symbols isless than the subcarriers, we have used zero padding onboth sides of the input symbols, which also provideimmunity to the inter-symbol and inter-carrierinterferences particularly at high data rates. Without loseof generality, we assumed the number of pilots is evenwhich simplifies zero padding into the IFFT.The channel impulse response in time domain isestimated by least-square error (LSE) as [20]:Gyh ?? (7)WhereHHpppHpppHpp FXDFXDXDFG ))(())()((1 ?? ?? (8)and NL? matrix )/)1)(1(2exp(][ , NnljF nl ???? , fnis the nthcolumn of F and L is the length of the channel,Fp= [fn1 ,…,fnp] , H stand for the Hermitian transpose, Dis a diagonal matrix andp? is the transmitted power perpilot symbol. Having h? we can obtain the frequencyresponse of the estimated channel on each subcarrier,which is used to equalize the received symbols. For thediffuse links we considered a frequency selective channelthat has 16 zero-mean uncorrelated complex Gaussianrandom taps. Figure 6 illustrated the simulated symbolerror rates (SER) versus the energy to noise ratio forOFDM system employing QPSK for a range of pilotsymbols. As expected the performance improves whenhigher numbers of pilot symbols are used.0 5 10 15 20 2510-310-210-1100Eb/N0(dB)SERNpilot=16Npilot=32Npilot=64Fig 6. SER versus Eb/No for diffuse optical wireless OFDM linkemploying QPSK and LS channel estimation and for a number of pilotsymbols.V. CONCLUSIONOFDM for indoor optical wireless communications(LOS and diffuse) using visible light LEDs have beenpresented. For diffuse links, pilot signals uniformlyinserted into all symbols (subcarriers) were extracted atthe receiver for channel estimation. The symbol error rateresults for an OFDM employing BPSK, QPSK and M-QAM for LOS and diffuse links were presented for arange of pilot signal. It was shown that to achieve alower SER high numbers of pilot signals will be need.REFERENCES[1] T. Komine, M. Nakagawa, “A Study of Shadowing on IndoorVisible-Light Wireless Communication Utilizing Plural WhiteLED Lightings,” Int. Sympo. On Wireless Commun., pp. 36-40,2004.[2] M.Z. Afgani, H. Haas, H. Elgala, D. Knipp, “Visible lightcommunication using OFDM,” Proc. IEEE Symp. on WirelessPervasive Computing, TRIDENTCOM 2006.[3] T. Komine, M. Nakagawa, “Fundamental Analysis for Visible-Light Communication System using LED Lights,” IEEE Trans.on Consumer Electron., vol. 50, no., pp. 100-107, 2004.[4] M. Kavehrad, “Broadband room service by light,” Scint.American, pp. 82-87, July 2007.[5] Z. Ghassemlooy, A.R. Hayes, and B. Wilson, "Reducing theeffects of intersymbol interference in diffuse DPIM opticalwireless communication", IEE Proc. Optoelectron, vol. 150, no.1, pp. 445-452, 2003.[6] K.K Wong,, T. O’Farrell, and M. 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A Study of Shadowing on Indoor Visible-Light Wireless Communication Utilizing Plural White

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Orthogonal frequency division multiplexing for indoor optical wireless communications using visible light LEDs

Orthogonal frequency division multiplexing for indoor optical wireless communications using visible light LEDs

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