IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 17, SEPTEMBER 1, 2013 1687

2.7 Mb/s With a 93-kHz White Organic LightEmitting Diode and Real Time ANN Equalizer

Paul Anthony Haigh, Zabih Ghassemlooy, Ioannis Papakonstantinou, and Hoa Le Minh

Abstract— This letter presents new experimental results on a2.7-Mb/s organic light emitting diode (OLED) based visible lightcommunications (VLCs) system. We also demonstrate for thefirst time an online artificial neural network for any VLC system(including inorganics). The major mitigating factors for OLED-VLC systems are the baseline wander phenomenon causingthreshold detection to fail and bandwidth limitation caused bylow transport mobility, introducing inter-symbol interference intothe link and thus the requirement for an equalizer. The onlinefiltering is implemented using the TI TMS320C6713 DSP boardand compared with offline results.

Index Terms— Artificial neural networks, equalizers, organiclight emitting diodes, visible light communications.

I. INTRODUCTION

MEGABITS per second (Mb/s) data communications hasso far not been demonstrated in OLED based VLCs,

due to the lack of suitable devices with high-speed capabilities.Both OLEDs and VLC are technologies in their infancy,which makes this a very exciting subject for research anddevelopment. There is a common desire to drive up data ratesas far as possible in VLC and so far the results for solidstate devices have been impressive, with data rates in thehundreds of Mb/s up to 3.4 Gb/s being reported dependingon the modulation scheme and complexity of technique beingused [1], [2].

The major aim of VLC is to provide simultaneous illu-mination and data communications; however it has beenlargely assumed that conventional LEDs will be the auto-matic choice of light source. This is because LEDs have ahigh optical power output and a wide bandwidth of severalMHz [1] and lack of alternative light sources. While LEDscan offer very high data rates, the production is expensivein comparison to OLEDs and they are a point source –which means they are required to adhere to the eye safetyregulations [3] while OLEDs do not cause any damage to

Manuscript received April 24, 2013; revised June 24, 2013; acceptedJuly 16, 2013. Date of publication July 19, 2013; date of current versionAugust 5, 2013. This work was supported in part by the EU COST ActionIC 1101 and Northumbria University.

P. A. Haigh, Z. Ghassemlooy, and H. Le Minh are with theSchool of Computing, Engineering and Information Sciences, OpticalCommunications Research Group, Northumbria University, Newcastle-upon-Tyne NE1 8ST, U.K. (e-mail: paul.haigh@northumbria.ac.uk;z.ghassemlooy@northumbria.ac.uk; hoa.le-minh@northumbria.ac.uk).

I. Papakonstantinou is with the Department of Electronic and ElectricalEngineering, University College London, London WC1E 7JE, U.K. (e-mail:i.papakonstantinou@ucl.ac.uk).

Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LPT.2013.2273850

the eye because of much softer light output. In comparisonto LEDs, the major advantage of OLEDs is the low cost ofproducing large area panels [4]. State-of-the-art OLEDs withbrightness approximately equal to conventional LEDs [5] offera natural alternative for VLC, which is the motivation forthis letter. Compared to incandescent and fluorescent lightsources, white OLEDs can emit light that is brighter and moreuniform with higher power efficiencies (90 lm/W compared to10–17 and 60–70 lm/W for incandescent and fluorescentsources, respectively) [6]. Thus OLEDs have the potentialto emerge as an ultra-efficient light source for displays andgeneral lighting.

The Orbeos CMW-031 OLED is used in this letter and has aluminous efficacy of 23 lm/W and a radius of 79 mm and mostimportantly a low bandwidth of 93 kHz. In OLEDs, the totalstack thickness is typically between 1–200 nm [4]. OLEDscan be modeled as a first order low-pass filter with a cut-offfrequency fc = (2πRC)−1 where R is the series resistance andC = ε0εr A/d is the plate capacitance where ε0 and εr are therelative permittivity of a vacuum and the dielectric constant ofthe organic layer, respectively [4]. The device photoactive areais given by A and the device thickness is given by d . Clearlythe plate capacitance causes a limit on the device bandwidththat depends on A; large A implies a small bandwidth andvice-versa.

Other causes towards low bandwidth in OLEDs includelow charge transport properties which cause a fundamentalrestriction on the recombination time of the electron-holepairs [7] and charge traps that impede the progress of chargecarriers across the semiconductor [8]. These two character-istics can be controlled by careful selection of the emissivelayer and high quality processing, although in the best case thecharge transport mobility of organics is around three orders ofmagnitude lower than amorphous silicon [4].

Therefore achieving a high enough bandwidth is a con-siderable challenge to encounter if OLEDs are to permeateinto the VLC domain. The only Mb/s link to date with anOLED transmitter and P-I-N photodetector was at 1.4 Mb/susing discrete multi-tone modulation (DMT) as reported in [9].DMT requires a significant amount of signal processing at boththe transmitter and receiver so is undesirable for real timeapplications. Pulse modulation schemes such as on-off keying(OOK) and pulse-position modulation (PPM), which relativelysimple to implement and also have compatibility with lineartransversal equalizers in order to eliminate inter-symbol inter-ference (ISI). In [4] an OLED-VLC link with OOK, whichis bandwidth efficient,was reported with a data rate limited to

1041-1135 © 2013 IEEE

1688 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 17, SEPTEMBER 1, 2013

VOLED

VPD

High Z

OLED

PD

TIA

Data

Received Data

AFG3022 DSO9254A

LV

Fig. 1. Experimental test setup.

550 kb/s using a multilayer perceptron (MLP) ANN equalizer.However, to the best of our knowledge no work on PPM basedOLED-VLCs has been reported in the literature.

Pulse modulation schemes with DC levels will experi-ence the baseline wander (BLW) phenomenon due to opticalfiltering and the use of a bias tee, which will affect thesystem performance owing to failure of threshold detection.The BLW is a well-known problem in telecommunicationsand closed form expressions can be found in [10]. PPM isan attractive modulation format due to the low average powerlevel, offering a natural protection to BLW as well as softdecision demodulation method offering an electrical signal-to-noise ratio (SNR) gain of 1.5 dB, however at the cost ofincreased bandwidth requirement. Therefore in this letter weonly use the lower orders of PPM (i.e. 2 and 4-PPM). The slotrate and amplitude of 4-PPM is set to twice that of 2-PPM andtherefore 4-PPM will require twice the bandwidth of 2-PPM(and four times that of OOK). Soft decision demodulation isnot covered here as it is well-known and can be referred toin [11]. The performance of all other pulse modulation formatslays somewhere between the bandwidth efficient OOK andthe power efficient PPM [12] and this is the reason that weimplement these two formats only.

In this letter we make several contributions; firstly weimplement OOK, 2 and 4-PPM using a new driving circuit toinvestigate the suitability of each modulation format with noequalizer and BLW. Secondly, we equalize each signal in theMATLAB domain and finally we implement the equalizers inreal time using a TI TMS320C6713 digital signal processing(DSP) board that will offer a true perspective of how theequalizing filters will perform in a real world scenario. Usingonline and offline filtering we achieved 2.65 and 2.7 Mb/susing the 4-PPM modulation format, which represents a newworld record data rate. This is the first time that any of thesetopics are confronted for OLED-VLC and this is the firsttime that online filtering has been reported in any VLC link,including inorganics.

II. EXPERIMENTAL SETUP & ARTIFICIAL NEURAL

NETWORK

The experimental test setup employing the LabVIEW envi-ronment (LV) is shown in Fig. 1. A programmable arbitrarywaveform generator (Tektronix AFG2022) is used to generatea pre-defined pseudorandom binary sequence (PRBS) andshape them using a rectangular pulse shaping filter with unit

Fig. 2. Measured SNR (red) (left), system bandwidth (BW) (blue) (right)and noise floor (black) (right).

height. APRBS datastream is applied to the OLED drivecircuit via the NAND gate with a high output impedance Z .Employing a bias-tee, which uses a coupling capacitor toblock the DC path back to the data source and an inductorthat blocks the AC data path to the DC supply is a morepopular option. However using the NAND circuit has twoadvantages; (i) no signal voltage is dropped in the bias-teecomponents therefore the modulation depth can be increasedto 100% from <10%, and (ii) neither the DC nor low frequencycomponents are removed, thus no BLW phenomena, thus thedata rate can be extended in comparison to [4]. Furthermore itis entirely and easily scalable to incorporate multiple OLEDsto increase the light output if desired. The OLED bias voltageVO L E D is mixed with the PRBS signalto intensity modulatethe OLED, depicted mathematically in [13] and not here in aneffort to save space. The link distance was set to 0.10 mas in [4] and [14] to give a light level of 400 lux. Thisdistance could be improved by scaling the number of OLEDsto increase the light level. At the receiver the incident lightis detected by the ThorLabs PDA36A (5 MHz bandwidth @20 dB transimpedance gain, 2.1 pW/

√Hz noise equivalent

power) module composed of a P-I-N photodetector and abuilt-intransimpedance amplifier (TIA). The output of thereceiver is then captured by the real time oscilloscope (AgilentDSO9254A) and acquired by LabVIEW for further signalprocessing.

The measured system SNR, bandwidth and the noise floorusing a frequency sweeping sine wave and an Agilent MXAN9010A electrical spectrum analyzer as can be seen in Fig. 2.

The bandwidth and noise amplitudes referred to the righthand axis were measured directly by the electrical spectrumanalyzer with the OLED on (bandwidth) and off (noise).The SNR was measured by subtracting the bandwidth andnoise amplitudes and indicates a high quality signal withSNR > 30 dB for frequencies <1 MHz. For frequencies>1 MHz the SNR quickly degrades and at ∼3 MHz the signalhas descended into the noise floor.

III. EQUALIZATION

It is possible to observe equalization as a classificationproblem rather than an information theory problem and thususing an ANN as a system response equalizer. ANNs are

HAIGH et al.: WHITE OLED AND REAL TIME ANN EQUALIZER 1689

the best performing transversal linear equalizer for bothchannel and system equalization [15]. This is because theyare capable of mapping any input-output sequence (linearor otherwise) and nullifying the negative effects that otherequalizers struggle to cope with such as large spectral nulls.The multilayer perceptron (MLP) ANN is implemented herein order to maximize the available data rates. The MLP isnot the best performing ANN; however it is the least complexand supports the Levenberg-Marquardt back-propagation (BP)algorithm, which is very popular due to the ease of hard-ware implementation [15]. The BP algorithm is a supervisedtraining method based on a gradient descent on the errorcost function. That is, the difference between the receiveddata and known data at the receiver for a certain numberof training symbols. An adaptive algorithm is used to selectthe number of neurons and the learning rate parameter [16].The MLP-ANN equalizer is very well known so not fullycovered here also to save space. For a more detailed overviewincluding the mathematical analysis the reader should see[4], [15], [17]. The MLP-ANN is implemented offline in theMATLAB domain first and followed by the DSP based MLP-ANN. A field programmable gate array (FPGA) could havebeen implemented for a fully real time result; however afull clock recovery and synchronization method would havebeen required. In order to ensure that it is the filters thatare being examined, we selected a DSP board where thesynchronization can be performed in the MATLAB domain.The clock speed of the DSP is 225 MHz, which is clearlywell in excess of the requirements for this system. The DSPboard is programmed using the TI Code Composer Studio(CCS) software and the MLP-ANN is implemented with thesame number of taps as in the offline case for a like-for-likecomparison. Since the adaptive algorithm was used it is notpossible to give a general figure. The output of the DSP boardis then recovered and transferred to the MATLAB domain forcomparison with the offline DSP. It should also be noted thatthe data used in the offline and online cases is not the samedata in order to ensure the thorough examination of the onlinefilters and ensure the comparisons are not influenced by thedata sequence.

IV. RESULTS

The discussion of results will be performed in order ofdemodulation method and the impact on the system bit errorrate (BER) for each modulation scheme. Firstly, thresholddetection is examined followed by soft decision demodulationand then the MLP-ANN equalization. At least 10 × 106

symbols were captured using the LabVIEW script. The BERwas calculated by comparing the transmitted and received bitssymbol-by-symbol.

The BER performance of OOK, 2 and 4-PPM with harddecision decoding using a threshold detector is illustrated inFig. 3 demonstrating transmission of 250, 150 and 50 kb/sdata rates. In comparison to the 93 kHz system bandwidthbottleneck introduced by the OLED it was expected that OOKwould offer the best performance with no equalization asit has half the bandwidth requirement of 2-PPM and fourtimes less than 4-PPM. Furthermore, as expected 2-PPM also

Fig. 3. Unequalized BER performance of each modulation scheme where250, 150 and 50 kb/s can be transmitted by OOK, 2-PPM and 4-PPM,respectively.

Fig. 4. Soft decision BER performance of 2-PPM and 4-PPM where 400and 200 kb/s can be recovered, respectively.

outperforms 4-PPM and this is also attributed to the lowerbandwidth requirements. The measured performance data forOOK in comparison to [4] has improved by a factor of aroundthree times (from 80 kb/s to 250 kb/s), which can be attributeddirectly to the simple drive circuit and the elimination of theBLW phenomenon. The 250 kb/s data rate is around half ofthe maximum data rate reported with the MLP-ANN in [4].

The system BER performance using soft decision demodu-lation as a function of the bit rate is shown in Fig. 4. It is notpossible to use the soft decision algorithm with OOK so only2 and 4-PPM with bit rates of 400 kb/s and 200 kb/s,respectively are used in this letter. There is still a dispar-ity between 2-PPM and 4-PPM caused by their respectivebandwidth requirements. Using soft decision demodulationoffers a significant improvement in the available data ratein comparison to threshold detection. More specifically, theimprovement in data rates for 2 and 4-PPM are 250 kb/s and150 kb/s, respectively. This means that 2-PPM has been able todouble its capacity while 4-PPM has improved by four times,which is remarkable. These data rates obviously outperformOOK with threshold detection are just slightly short of the550 kb/s as reported in [4] using an MLP-ANN equalizer.Using the DSP board for soft demodulation yielded extremelyclose agreement to the BER performance carried out in theMATLAB domain.

1690 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 17, SEPTEMBER 1, 2013

Fig. 5. Equalized BER performance of 2-PPM, OOK and 4-PPM inconjunction with the MLP-ANN in the MATLAB (M/L) domain, where datarates of 2.7, 2.15 and 1.6 Mb/s can be achieved, respectively. Significantly,using the DSP MLP-ANN, data rates of 2.65, 2.15 and 1.5 Mb/s can beachieved for the same modulation schemes which offer extremely goodagreement in each case.

Using MLP-ANN as an equalizer, it is possible to sig-nificantly improve data rates well into the Mb/s region asillustrated in Fig. 5. In every single case we record a higherdata rate than the 1.4 Mb/s reported in [9] using a less complexsystem and modulation scheme. Firstly considering the offlinecase, 2-PPM, OOK and 4-PPM can transmit 2.7, 2.15 and1.6 Mb/s, respectively. This is a significant improvementover the soft decision demodulation (for 2 and 4-PPM) andthreshold detection (for OOK). The most unexpected andsignificant result in Fig. 5 is that 4-PPM offers the highest datarate, followed by OOK and then 2-PPM, especially as 4-PPMwas the worst performing in each of the previous demodulationmethods. Since the system is-band limited, it was expected thatOOK would outperform 4-PPM due to the additional band-width requirement; however the experimental measurementsshow a contrary result. The cause of this is attributed to thefact that P(0) = 0.75 for 4-PPM and P(0) = 0.5 for OOKwhere P(.) is the probability. Equalized 2-PPM and 4-PPMshould have similar optical power penalties (OPPs). 4-PPM hasa shorter pulse duration than 2-PPM so the OPP for 4-PPMshould be higher as more power is required to reach the sameaverage energy. However, the probability of occurrence of twoconsecutive pulses is much lower in 4-PPM than in 2-PPM,thus the significant improvement in BER performance [18]. Atdata rates > 2.7 Mb/s the presence of severe ISI in combinationwith degradation of SNR causes the MLP-ANN to fail.

The DSP MLP-ANN can offer similar data rates. For4-PPM, OOK and 2-PPM data rates of 2.65, 2.15 and 1.5 Mb/scan be transmitted, respectively. This is a significant resultas it qualifies the offline results and indicates that in afully real time system including clock recovery it would befeasible to achieve Mb/s data rates. The difference betweeneach case is 0.05, 0 and 0.1 Mb/s for 4-PPM, OOK and2-PPM, respectively which in terms of percentage is 1.85%,0% and 6.25%, indicating a very small difference in eachcase. It should also be noted that in each case the MATLABMLP-ANN outperforms the DSP based MLP-ANN.

V. CONCLUSION

We have experimentally demonstrated the first 2.7 Mb/sVLC link with an OLED as transmitter, both offline and online(2.65 Mb/s). In comparison to previously reported works, wehave improved the record data rate by ∼ two fold, from1.4 Mb/s [9] to 2.7 Mb/s for any state-of-the art organicVLC system. In terms of pulse modulation the improvementis ∼ four fold from 550 kb/s [4] to 2.15 Mb/s (for OOK)and ∼ five times from 550 kb/s to 2.7 Mb/s (for 4-PPM).Furthermore, we have demonstrated for the first time onlinefiltering for any VLC system to the best of our knowledge anddemonstrated the agreement with the offline results.

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