Poster: Vehicular VLC Experimental ModulationPerformance Comparison

Gokhan Gurbilek, Mertkan Koca, Bugra Turan, and Sinem Coleri Ergen

Department of Electrical and Electronics Engineering, Koc University, Sariyer, Istanbul, Turkey, 34450E-mail: ggurbilek13@ku.edu.tr, mkoca14@ku.edu.tr, bturan14@ku.edu.tr, sergen@ku.edu.tr

Abstract—Vehicular visible light communications (V2LC) hasrecently gained popularity as a complementary technology to ra-dio frequency (RF) based vehicular communication schemes dueto light emitting diode (LED)s readily availability on vehicles withits secure and RF-interference free nature. However, vehicularvisible light communications (V2LC) system performance mainlydepends on LED characteristics. Investigating various LED bulbsfor their frequency response and optical OFDM (O-OFDM) basedmodulation performances, it has been observed that LED andDC-bias voltage selection is key for the V2LC system modulationperformance. Experimental results indicate that, on contraryto simulation results in the literature, asymmetrically clippedoptical OFDM (ACO-OFDM) is observed to perform better thanunipolar OFDM (U-OFDM) as it inherits lower peak-to-averagepower ratio (PAPR) with lower clipping noise which is crucial forthe LED under consideration with limited linear working region.

Index Terms—Vehicular communication, visible light commu-nication, vehicle to vehicle communication

I. INTRODUCTION

V2LC, offering RF interference free and secure line-of-sight(LoS) communications is provisioned to be a strong candidatefor off-loading RF based vehicular communication schemesand providing vehicle to everything communications (V2X)at RF denied environments. Furthermore, with the increaseddeployment of LEDs on the vehicles, V2LC is also believedto be a cost-effective safety add-on to disseminate safetycritical messages. However, vehicle power LEDs are currentlydesigned solely for illumination purposes. Considering LEDsnon-linear characteristics, V2LC system design requires care-ful investigation of the modulation schemes for achievableperformance. Moreover, regulated DC-voltage fed to vehiclelights to guarantee flicker free illumination should also betaken into account as it directly affects LED lifetime.

O-OFDM is widely used in Visible Light Communication(VLC) due to its inter-symbol interference (ISI) robustnessand high data rate capability [1]–[3]. Despite the numerousO-OFDM schemes proposed for VLC, such as enhanced hy-brid asymmetrically-clipped optical OFDM (EHACO-OFDM),novel negative ACO-OFDM (NACO-OFDM) and OFDM withIndex Modulation (OFDM-IM), Direct Current biased opticalOFDM (DCO-OFDM), ACO-OFDM and U-OFDM are con-sidered to be most practical O-OFDM schemes due to theirlower implementation complexity and appealing performances.To date, DCO-OFDM, ACO-OFDM and U-OFDM perfor-mances are compared either experimentally or in simulation

environments [2], [4], [5]. However, none of the studies to dateconsidered DC-bias which is inevitable for the vehicle usecase, to evaluate modulation performances of DCO-OFDM,ACO-OFDM and U-OFDM.

Figure 1: Experimental Setup

In this work, we experimentally investigated three differentO-OFDM schemes in terms of their bit-error-rate (BER)performances with respect to varying DC-bias values. Off-theshelf automotive LED lights frequency response performancesare also compared to demonstrate the diversity of vehicleLEDs as V2LC front-ends.

II. EXPERIMENTAL SETUP

In the experimental setup, multi-beam LED headlight froma 2017 MY Ford Mondeo production vehicle is utilized asthe transmitter front-end. Pseudo-random-bit sequence (PRBS)generated with LabVIEW software at the computer is sentto Software-Defined Radio (SDR) (NI USRP 2920) equippedwith LFTX daughterboard. SDR output is amplified 19 dBusing Mini-Circuits ZFL1000+ amplifier to generate modu-lation signals considering the 1 dB compression point of theamplifier. The resulting RF signal is fed to the RF input of theMini-Circuits ZFBT-4R2GW-FT+ Bias-Tee, with the requiredDC-bias.

At the receiver side, avalanche photodiode (APD) is uti-lized to capture the transmitted signals from LED high-beamof the headlight. Received signal is directly fed into theSDR, equipped with LFRX daughterboard. Received signalis processed at the computer using LabVIEW software, tocalculate BER and signal to noise ratio (SNR). To evaluatethe BER performance at various SNR values, keeping distancefixed to 1 m, DC-bias is swept from 18.5 V to 20 V at

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Figure 2: (a) Frequency response of Ford Mondeo Headlamp High and Low Beam Lights (b) Frequency response of FordMondeo Headlamp Turn Signal and DRL LED Lights (c) SNR vs. BER graph of DCO-OFDM, ACO-OFDM and U-OFDM

the transmitter, also emulating regulated voltage provided forconstant illumination.

Furthermore, in order to characterize frequency responseof high-beam, low-beam, turn signal and Day Time RunningLight (DRL) LEDs within the headlight assembly, vectornetwork analyzer (VNA) is also utilized, where DC-bias pointsare selected to be optimum for each bulb.

III. PERFORMANCE EVALUATION

Fig.2 (a) and (b) demonstrates the frequency responses ofLED bulbs within the headlight assembly under considerationof different DC-bias conditions. The optimum DC-bias valuein terms of maximum frequency response gain is determinedas 23 V for the high beam, 12 V for the low beam, 3.2 Vfor DRL LED. On the other hand, for the Turn Signal LED,frequency response gain is higher for 2.8 V bias up-to 316.8kHz, whereas, increasing DC-bias to 3.8 V yields higher gainat frequencies above 316.8 kHz. It can be concluded that, eachLED has different characteristics, thereby constitutes a majorlimitation for V2LC performance.

Three different O-OFDM schemes are implemented forperformance comparison. DCO-OFDM is known to be spec-trally efficient within all compared schemes, where ACO-OFDM is superior with its lower power consumption whencompared to DCO-OFDM, and U-OFDM is favorable for itsdemodulation process with similar BER performance to ACO-OFDM [5]. In order to compare BER performances of themodulation schemes, same throughput yielding modulationorders are selected. Therefore, 4-Quadrature Amplitude Modu-lation (QAM) DCO-OFDM is compared with 16-QAM ACO-OFDM and 16-QAM U-OFDM. The fast fourier transform(FFT) size, number of guard carriers and cyclic prefix areselected as 256, 76 and 16 respectively.

In Fig.2 (c), BER performances are observed to be differentfrom the work in [5]. Despite the subtle difference betweenU-OFDM and ACO-OFDM, U-OFDM is demonstrated to per-form slightly better than ACO-OFDM in simulation environ-ment. However, experiment results reveal that, ACO-OFDMperforms slightly better than U-OFDM as the limited linearworking region of LEDs is not considered in the simulations.[5] also states that the PAPR of U-OFDM signal is higher than

of ACO-OFDM while PAPR of DCO-OFDM is the highest.Therefore, the effects of clipping noise and high PAPR of U-OFDM makes it perform worse than simulated U-OFDM andexperimental ACO-OFDM. On the other hand, both experi-mental and simulation results are consistent regarding DCO-OFDM BER performance. Both in simulations and in theexperiment, BER performance of DCO-OFDM is worse thanACO-OFDM and U-OFDM.

IV. CONCLUSION

This work presents experimental investigations of thethree low-complexity O-OFDM modulation schemes targetingV2LC. Implementation and measurement results indicate that,DC-bias plays a key role at the modulation selection ofa V2LC system, where it is generally required to providecontinuous illumination. Our results reveal that, for the par-ticular automotive LED utilized, ACO-OFDM outperforms U-OFDM by 0.336 dB for target BER 2.5x10−3 and outperformsDCO-OFDM by 4.357 dB for same target BER. Moreover,LED frequency response measurements demonstrate that, LEDselection is also important to obtain better performance withthe utilized modulation scheme.

REFERENCES

[1] H. Elgala, R. Mesleh, and H. Haas, “Practical considerations for indoorwireless optical system implementation using ofdm,” in Telecommunica-tions, 2009. ConTEL 2009. 10th International Conference on, pp. 25–29,IEEE, 2009.

[2] J. Armstrong and B. J. Schmidt, “Comparison of asymmetrically clippedoptical ofdm and dc-biased optical ofdm in awgn,” IEEE CommunicationsLetters, vol. 12, no. 5, 2008.

[3] J. Armstrong, “Ofdm for optical communications,” Journal of lightwavetechnology, vol. 27, no. 3, pp. 189–204, 2009.

[4] S. D. Dissanayake, K. Panta, and J. Armstrong, “A novel technique tosimultaneously transmit aco-ofdm and dco-ofdm in im/dd systems,” inGLOBECOM Workshops (GC Wkshps), 2011 IEEE, pp. 782–786, IEEE,2011.

[5] D. Tsonev, S. Sinanovic, and H. Haas, “Novel unipolar orthogonal fre-quency division multiplexing (u-ofdm) for optical wireless,” in VehicularTechnology Conference (VTC Spring), 2012 IEEE 75th, pp. 1–5, IEEE,2012.