Experimental Demonstration of Electronic GVDCompensation in Optical CDMA Networks

Miguel Pimenta and Izzat Darwazeh, {m.pimenta,i.darwazeh}@ee.ucl.ac.ukUniversity College London, Torrington Place - London, WC1E 7JE

Abstract— We experimentally demonstrate a flexible electroniccompensator for Group Velocity Dispersion induced time skewingin multi-wavelength Optical CDMA systems. The compensatoruses a distributed transversal filter already proposed in thecontext of Optical Communications. Results show improvementin the ability to detect the autocorrelation peak in presence ofMulti-Access Interference.

I. INTRODUCTION

Wavelength-Hooping Time-Spreading (WHTS) OpticalCode Division Multiple Access (OCDMA) allies traditionaladvantages of OCDMA with increased code design flexibilityand improved network performance [1]. A key drawback ofmulti-wavelength OCDMA systems is the dispersion-inducedtemporal skewing among wavelength channels which compro-mises the detection process [2]; if the network induces relativetemporal shifting between the wavelengths, the code patternis distorted giving rise to errors in the receiver. This problemhas been addressed before [2] [3] and proposed solutions relyon a combination of Arrayed Waveguide Gratings (AWGs) andprecisely configured optical delay lines [2]; these solutions arecostly and difficult to implement in practice.

Electronic post-detection processing is a potential solutionand has traditionally been ignored as it is believed to be unableto cope with the ever increasing speed of Optical CDMAproposals. In fact, it is empirically known that conventionalIntegrated Circuit (IC) topologies rarely operate at frequencieshigher than 40% of the transistors cut-off frequency [4].However, recent advances in IC technologies (CMOS, GaAs,InP, etc.) in conjuction with distributed IC topologies [5], allowthe design of ICs operating across a wide range of frequenciesfrom near DC up to (and beyond) OCDMA chiprates ofpractical interest. A structure with such characteristics is theelectronic distributed amplifier, which may be configured as asimple multi GHz amplifier/transversal filter/compensator [5]and has been reported for CDMA applications [6] over opticalfibre.

We propose a front end receiver structure based on thedistributed transversal filter in order to correct the correla-tion peak distortion resulting from temporal skewing amongwavelength channels. This allows not only tunability of com-pensation parameters but also tunability across different codewords, thereby resulting in a flexible, adaptable and efficientsolution.

II. CONCEPT

Analog transversal filters have been used in a variety of ap-plications to combat optical system impairments [5]. However,

Fig. 1: Optical Orthogonal Codewords

analog electrical signal processing has never been proposedin the context of Optical CDMA networks. In this work theOptical CDMA signal is decoded in the optical domain usingtraditional methods (for example, an array of Fiber BraggGratings [1]) after which it is received with a photodetector.The compensator is an electronic distributed transversal filterwith the frequency response described by the equation 1 [5]:

HTF (ω) =N−1∑k=0

Gkexp(−jωkT ) (1)

where N is the number of stages of the transversal filter, Tis the temporal spacing between transversal filter taps, Gj isthe gain coefficient of the stage j.

III. EXPERIMENTAL SETUP

Fig. 2 shows the proof-of-concept prototype; a Fiber BraggGrating based OCDMA network for 2.5 GBit/s transmission(20 GChip/s). The system comprises five CW SemiconductorDistributed Feedback Lasers with central wavelengths sepa-rated by 200 GHz (1548.20 to 1554.50 nm). The pulses areobtained with a Mach-Zehnder interferometer set to modulatean alternate sequence of zeros and ones (effectively repeatinga pulse every 800 ps). The optical signal is then dividedin a 1:3 splitter. Three FBG arrays are used to encode theWHTS codewords shown in Fig. 1. After encoding, a 3:1coupler is used to couple the three users to the optical fibre.The transmission medium is a 40.5 km Single Mode Fibre(SMF) followed by a 5 km Dispersion Compensation Fibre(DCF). The receiver module comprises a 30 GHz receiver anda trans-impedance amplifier. A five-stage Gallium ArsenideDistributed Transversal Filter with 18 ps of delay betweenadjacent stages is used to compensate residual dispersion [7].The results are observed using an Optical Oscilloscope.

IV. EXPERIMENTAL RESULTS

To assess the performance improvement, we introducethe parameter auto-correlation intensity peak to the cross-correlation level ratio (P/C ratio). Two scenarios are analysed.

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Fig. 2: Experimental setup of the Optical CDMA system

(a) Non-compensated (Time: 100ps/div)

(b) Compensated (Time: 200 ps/div)

Fig. 3: Snapshot of the received signal with no coincidence incross-correlation

(a) Non-compensated (b) Compensated

Fig. 4: Snapshot of the received signal with coincidence incross-correlation (Time: 100 ps/div)

In the first scenario, the cross-correlation function from userstwo and three do not have any chip coincidence. This is doneby adjusting the tunable optical delay lines after the encoder.In the second scenario analyzed in this section, the cross-correlation functions are deliberately delayed so that they havetwo chips coinciding in time. This corresponds to the worstcase scenario because the cross-correlation peaks achieve theirmaximum value and the P/C ratio is the worst possible. Thetransversal filter gains were set manually to maximize the P/Cratio.

Figures 3 and 4 show the compensation results for the firstscenario and second scenario, respectively. In both cases, 50%improvement is evident; P/C ratio improves from 2 to 3 in thefirst scenario and from 1 to 1.47 in the second. Note that theextent of the autocorrelation peak compensation is ultimatelylimited by the interactions between the cross-correlation chips.This is especially obvious in figure 4(b); if the gain coefficientscontinue to rise, the peak value of cross-correlation will rise atfaster pace than the autocorrelation peak. Therefore, the idealcoefficients provide the best balance between cross-correlationpeaks and the autocorrelation level.

V. CONCLUSION

The use of electronic post-detection processing is proposedand experimentally demonstrated in the context of OpticalCDMA networks for the first time. Results show 50% im-provement in the autocorrelation peak in relation to the cross-correlation level and therefore potential improvement the per-formance of the system.

ACKNOWLEDGMENT

The experimental work took place at Instituto das Teleco-municacoes (IT), Aveiro, Portugal. The authors are grateful toProf. Rogerio Nogueira and Mr. Carlos Marques for the FBGarray fabrication and Prof. Paulo Monteiro and Dr. MiguelMadureira for the transversal filter provisioning and generoushelp with the experimental setup. The authors would like tothank the Portuguese FCT for funding the PhD work of MiguelPimenta.

REFERENCES

[1] H. Fathallah, L. Rusch, and S. LaRochelle, “Passive optical fastfrequency-hop CDMA communications system,” Journal of LigthwaveTechnology, vol. 17, no. 3, pp. 397–405, March 1999.

[2] C. Zuo, W. Ma, H. Pu, and J. Lin, “The impact of group velocity onfrequency-hopping optical code division multiple access system,” Journalof Ligthwave Technology, vol. 19, no. 10, pp. 1416–1419, Oct. 2001.

[3] E. Ng, G. Weichenberg, and E. Sargent, “Dispersion in multiwavelengthoptical code-division multiple-access systems: impact and remedies,”IEEE Transactions on Communications, vol. 50, no. 11, pp. 1811–1816,Nov. 2002.

[4] R. Amaya, N. Tarr, and C. Plett, “A 27 GHz fully integrated CMOSdistributed amplifier using coplanar waveguides,” in Proc. Digest ofPapers Radio Frequency Integrated Circuits (RFIC) Symposium 2004IEEE, 6–8 June 2004, pp. 193–196.

[5] A. Borjak, P. Monteiro, J. O’Reilly, and I. Darwazeh, “High-SpeedGeneralized Distributed-Amplifier Baser Transversal-Filter Topology forOptical Communication Systems,” IEEE Transactions on MicrowaveTheory and Techniques, vol. 45, pp. 1453 – 1457, 1997.

[6] M. Pimenta and I. Darwazeh, “Novel Encoder and Correlator for OpticalCode Division Multiple Access Networks,” in IEEE Lasers and Electro-Optics Society Annual Meeting, Oct. 2006, pp. 422–423.

[7] P. M. Monteiro, M. A. Madureira, D. Fonseca, R. L. Aguiar, A. V. Car-taxo, R. Sousa, and M. Violas, “Optical Channel Impairments Mitigationon 40 Gb/s Systems Resorting to Post-Detection Adjustable ElectricalCompensation,” in Proc. 9th International Conference on TransparentOptical Networks ICTON ’07, vol. 1, 1–5 July 2007, pp. 20–23.

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