We present analysis and proof-of-concept field demonstration of an incoherent optical code-division multiple access over an optical time-division multiple access system. The performance analysis which includes power budget and scalability calculations show scalability improvements without any significant system performance degradation

Glesk, I.

Idris, S. K.

Kwong, W. C.

Osadola, T. B.

English

University of Strathclyde Institutional Repository

Strathprints Institutional RepositoryOsadola, T. B. and Idris, S. K. and Glesk, I. and Kwong, W. C. (2012) Network scaling using OCDMAOver OTDM. IEEE Photonics Technology Letters, 24 (5). pp. 395-397. ISSN 1041-1135Strathprints is designed to allow users to access the research output of the University of Strathclyde.Copyright c© and Moral Rights for the papers on this site are retained by the individual authorsand/or other copyright owners. You may not engage in further distribution of the material for anyprofitmaking activities or any commercial gain. You may freely distribute both the url (http://strathprints.strath.ac.uk/) and the content of this paper for research or study, educational, ornot-for-profit purposes without prior permission or charge.Any correspondence concerning this service should be sent to Strathprints administrator:mailto:strathprints@strath.ac.ukhttp://strathprints.strath.ac.uk/IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 5, MARCH 1, 2012 395Network Scaling Using OCDMA Over OTDMTolulope B. Osadola, Student Member, IEEE, Siti K. Idris, Ivan Glesk, Senior Member, IEEE,and Wing C. Kwong, Senior Member, IEEEAbstract— We present analysis and proof-of-concept fielddemonstration of an incoherent optical code-division multipleaccess over an optical time-division multiple access system.The performance analysis which includes power budget andscalability calculations show scalability improvements withoutany significant system performance degradation.Index Terms— Optical code-division multiple access(OCDMA), optical networks, optical time-division multipleaccess (OTDMA).I. INTRODUCTIONOPTICAL code division multiple access (OCDMA) hasbeen shown to be a possible candidate for broadbandaccess in future optical networks [1], [2]. At present, archi-tectures used for optical data transmission in existing opticalnetworks include wavelength division multiplexing (WDM)[3] and optical time division multiplexing (OTDM) [4]. Alsohybrid high-speed networks incorporating WDM and OTDMhave been demonstrated as a means of effectively utilizing thebandwidth capacity of the optical fiber. In WDM networks,each user is assigned a different wavelength, while in OTDMits own channel (timeslot). By implementing OCDMA, eachuser is assigned a unique optical code that is generated by theuser’s OCDMA encoder. The use of optical coding has beenshown to be an attractive way of enhancing data privacy in thephysical layer of optical networks [2]. As of today, differentarchitectures have been proposed for possible implementationof either coherent or incoherent approaches to OCDMA [5],[6]. To enhance scalability and to increase the number ofsimultaneous users that can transmit effectively on the net-work, hybrid schemes such as WDM-OCDMA, and OTDMover coherent OCDMA have been investigated [1], [7], [8].In this letter, to the best of our knowledge, we present forthe first time a proof of concept with field demonstration atransmission system based on an incoherent OCDMA overOTDMA network (iOCDMA-OTDMA). Based on our overallsystem analysis which also includes scalability and powerbudget calculations, the proposed iOCDMA over OTDMAarchitecture can be used to deliver enhanced scalability anda substantial increase in the number of simultaneous users.Manuscript received August 12, 2011; revised November 1, 2011; acceptedDecember 7, 2011. Date of publication December 15, 2011; date of currentversion February 15, 2012. This work was supported in part by the GRPeGrant.T. B. Osadola, S. K. Idris, and I. Glesk are with the Electronicand Electrical Engineering Department, Center for Intelligent andDynamic Communications, University of Strathclyde, Glasgow G1 1XW,U.K. (e-mail: tolulope.osadola@strath.ac.uk; siti.idris-othman@strath.ac.uk;ivan.glesk@eee.strath.ac.uk).W. C. Kwong is with Hofstra University, Hempstead, NY 11550-1030 USA(e-mail: eggwck@hofstra.edu).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.2011.2179924Our iOCDMA implementation and iOCDMA-OTDMA sys-tem analysis is based on implementation of two dimensionalwavelength-hopping time-spreading (2D-WH/TS) codes whichuse multiwavelength picosecond pulses. The iOCDMA encod-ing/decoding was done using fiber Bragg gratings (FBG) basedencoders/decoders [6].II. SYSTEM ARCHITECTUREThe iOCDMA over OTDMA network architecture is con-ceptually shown in Fig. 1 where M-users iOCDMA systemis being implemented in each of N time slots (channels)of OTDMA network. This approach allows N groups ofM-users iOCDMA system transmit data over OTDMA net-work while reusing the same OCDMA hardware. This alsoallows OCDMA code reuse which greatly simplifies theiOCDMA hardware implementation. This hardware configu-ration and code reuse are made possible by way of assign-ing each individual OCDMA transmitting group a separateOTDMA timeslot (channel). By so doing, the scalability ofthe resulting system is N times enhanced when comparedto the stand alone iOCDMA. In view of this, the maximumnumber of users (K) of the proposed iOCDMA-OTDMAsystem depends on the number of simultaneous users of theiOCDMA system M as well as the number of OTDMAtimeslots (channels) N, respectively and can be written as K =M ∗ N. For a given transmission rate 1/T, the number ofOTDMA channels N = T/τ , where τ is a channel width andT is an OTDMA bit period. To optimize the performance τis set to be equal to the OCDMA code length. In general,the iOCDMA hardware reuse is not required. Each iOCDMAgroup transmitting into its assigned OTDMA time slot mayuse a different OCDMA hardware uniquely tuned to supportany targeted application.Figure 2(a) presents results of our calculations showing therelationship between probability of error and the number ofsimultaneous users for M-user stand alone iOCDMA systemwith the code weight 4 and 8, respectively. Fig. 2(b) shows theimprovement in the number of simultaneous users if M-useriOCDMA system is overlaid over N-channel OTDMA system.In our field demonstrations the OCDMA code length was400ps therefore to deliver OC-12 data rates four OTDMAchannels were available, i.e. N = (OC-12)/400ps = 4. SeeFig. 1 for more detailed diagram of the M-user iOCDMAsystem.III. EXPERIMENTAL DEMONSTRATION AND RESULTSThe experimental demonstration of the proposed iOCDMAover OTDMA system was done in a field-based transmissiontestbed which is a 17km long bidirectional fiber optic link1041–1135/$26.00 © 2011 IEEE396 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 5, MARCH 1, 2012Transmitter OTDM MUX OTDM DEMUX ReceiverEDFAUser 1To ChN from ChNUser MModulatorFBG encoderSCCUser 1User MModulatorFBG encoderCAtt(N − 1)τ(N − 1)τNX11XNM-usersiOCDMAtransmitting toOTDM Ch1M-usersiOCDMACh1Ch1Ch2Ch2OC-12400 psChNTGRFRFTGChNM-usersiOCDMAM-usersiOCDMAM-usersiOCDMAtransmitting toOTDM ChNM-usersiOCDMAtransmitting fromOTDM Ch1M-usersiOCDMAtransmitting fromOTDM ChNM-Users iOCDMAtransmitting to OTDM ChNM-users iOCDMAtransmitting from OTDM ChN1 OTDM frame contains Nchannels (Ch)ττFig. 1. Incoherent OCDMA over OTDMA network architecture. TG: time gate; EDFA: erbium-doped fiber amplifier; FBG: fiber Bragg grating; Att: attenuator;RF: radio frequency; SC: supercontinuum; ChN: channel N.(8, 47) Pe = 10−4(8, 47) Pe = 10−9(4, 47) Pe = 10−9(4, 47) Pe = 10−4iOCDMA-OTDMA  users (K) 3002001000Number of OTDMA channels (N)(b)14868  (8, 47) (4, 47) Number of simultaneous users per iOCDMA system (M)(a)10010−410−910−12Pe (log) 0 5 10 2017 30 37 401 2 3 4 5 6 7 8Fig. 2. Showing scaling potential of the iOCDMA over OTDMAnetwork architecture. (a) Performance curve for a standalone OCDMA system.(b) Number of users achievable with iOCDMA- OCDMA while maintainingthe same performance.installed between the University of Strathclyde CIDCOMlaboratory and University of Glasgow Optics laboratory andis compensated for chromatic dispersion with sub-picosecondaccuracy. The conceptual diagram of the iOCDMA-OTDMAtransmission demonstration is shown in Fig. 1. In our proofof concept experiment we used four-user iOCDMA systemper each of the four OTDMA channels. The user data ratewas OC-12. All users’ data were uniquely encoded by FBG-based iOCDMA encoders generating 2D-WH/TS codes. TheiOCDMA systems used 400ps long 2D-(4,47) WH/TS codesconsisting of four wavelengths (λ1 = 1551.72nm, λ2 =1550.92nm, λ3 = 1552.52nm, λ4 = 1550.12nm) based on100GHz ITU grid accordingly positioned into 47 time chipseach of ∼8ps duration. Every code uses all four wavelengths.As an example, chips occupied by the user1 code are (1-λ3,9-λ2, 28-λ4, 31-λ1). Wavelengths are obtained by spectralslicing of a 3.2nm wide optical super continuum (SC) result-ing from pulse compression of 1.8ps laser pulse from amode-locked Erbium doped fiber laser. Generated 2D-WH/TSOCDMA codes are then modulated by a Mach-Zehnder mod-ulator driven by a 231-1 pseudo-random bit sequence (PRBS)data from an Agilent N4903A series bit error rate (BER) tester.Transmitted power levels from each user were equalized by500 ps/div500 ps/divCh4 (a) (b)Ch1 Ch4 Ch1 Fig. 3. Examples of OCDM traffic over OTDM channels as seen onbandwidth limited oscilloscope (a) traffic in Ch2 only and (b) traffic in Ch1and Ch2.optical attenuators placed at each encoder output (see Fig. 1).Modulated data from all users within each OTDMA channelare first combined by a power combiner and then the trafficfrom all OTDM channels is time division multiplexed into OC-12 OTDM frame having 400ps channel duration. The resultingsignal is amplified by the EDFA and lunched into a field-basedoptical testbed.Figure 3 shows a temporal trace of the combined iOCDMAover OTDMA traffic. Here, Fig. 3(a) is a snapshot of OCDMAcodes traveling in OTDMA Ch2, while Fig. 3(b) in channelCh1 and Ch2, respectively. Because of the bandwidth limitedoscilloscope used code weight of 4 is not properly resolved.At the receiver the traffic is first de-multiplexed in timedomain into the individual OTDMA channels by Mach-Zehnder based gates, TG (see Fig. 1) placed at 1xN powersplitter outputs and as such presented for OCDMA decodingand photo-detection. To decrease the guard band needed bythe Mach-Zehnder gate, all optical approach [9] could beimplemented. Since OTDMA is a synchronous multiplexingtechnique, the self clocking enables the synchronization.The autocorrelations representing received OCDMA databy the user 1 in Ch1 and the user 1 in Ch2 are shown inFig. 4. For clarity of explanation, a snapshot of data pattern“1” is shown in Fig. 4 instead of PRBS pattern. Figure 4(a)shows autocorrelations of received data over Ch1 by user1 when transmitting alone on OTDM Ch1; Fig. 4(b) showsthe autocorrelation of the OCDMA user 1 who alone istransmitting on OTDMA Ch2.Fig. 4(c) shows received autocorrelation from user 1 ofiOCDMA system transmitting on Ch1 and user 1 of iOCDMAsystem transmitting on Ch2. Fig. 4(d) shows a scenario similarto one in Fig. 4(c) but with added transmission by all userson all four OTDMA channels.OSADOLA et al.: NETWORK SCALING USING OCDMA OVER OTDM 397User 1 Autocorrelation (Ch1)User 1 Autocorrelation (Ch1 + Ch2)User 1 Autocorrelation User 1 Autocorrelation (Ch2)Ch1 + Ch2 + MAICh1 Ch4 500 ps/div Ch1 Ch4 500 ps/divCh1 Ch4 500 ps/divCh1 Ch4 500 ps/div(a) (b)(c) (d)Fig. 4. (a) Autocorrelations representing received OCDM data by user 1when transmitting alone only on OTDM Ch1, (b) only on OTDM Ch 2,(c) both users 1 are transmitting on their channels Ch1 and Ch2, respectively,and (d) case in (c) when all other users also transmit on all four OTDMAchannels.1 iOCDMA user on OTDM Ch11 iOCDMA user on OTDM Ch24 iOCDMA users on OTDM Ch14 iOCDMA users on OTDM Ch2Power (dBm)BER   (LOG)−2210−210−410−810−10−20 −18 −14 −12 −10Fig. 5. BER performance of iOCDMA-OTDMA system. The inset showsan eye diagram for iOCDMA user 1 transmission on OTDMA Ch2.In order to quantitatively evaluate the overall iOCDMA-OTDMA system performance the decoded traffic from Ch1and Ch2, respectively were first sent via an optical attenuatorwith built in optical power meter (Agilent 8157A) to a bit errorrate tester with an 11GHz optical receiver (Nortel PP10G)as its frontend. Obtained bit error rate results are shown inFig. 5. The inset is an example of an eye diagram obtainedfor OCDMA user 1 transmitting in the OTDMA Ch2 at thepresence of multi-access interference (MAI). Observed wideeye opening indicates good fidelity of the received signal. Thefirst set of BER curves corresponds to the transmission whenonly user 1 in OTDMA Ch1 and user 1 in the adjacent Ch2transmit (see Fig. 4(c)). The second set of curves reflects BERof the user 1 in Ch1 and user 1 Ch2 with the presence of allOCDMA users in all four OTDMA channels (see Fig. 4(d)).IV. CONCLUSIONWe have proposed and demonstrated a novel approachfor increasing the number of simultaneous network users byway of overlaying M-USER incoherent OCDMA system overN-channel OTDMA architecture. The field demonstration ofthis hybrid system was successfully done over a 17km fiberoptic link connecting University of Strathclyde and Glasgowwith achieved BER better then 10−9, no error floor wasobserved.From results we have obtained, the following was con-cluded.1) Within the experimental error the obtained BER curvesare nearly identical for the demonstrated transmissionon OTDMA Ch1 and Ch2. This outcome indicates no orvery low crosstalk between OTDMA channels and highON/OFF contrast ratio achieved by Mach-Zehnder gates,TG performing demux operation of OTDMA signal intoindividual channels.2) ∼7dB of power penalty per user was incurred as theconsequence of detrimental effects of MAI on iOCDMAusers transmitting on OTDMA channel.Based on our system scalability calculation results (seeFig. 2) it can be observed that a 17-users “standalone” (8, 47)iOCDMA system will achieve BER = 10−9; if now overlaidover four-channel OTDMA, will support up to 68 users.Thanks to OCDMA soft blocking properties if the same(8, 47) iOCDMA is scaled up (designed and build) to support37 simultaneous users by allowing BER to drop to 10−4(which in turn can be corrected back to 10−9 by forwarderror correction (FEC)) then the iOCDMA-OTDMA conceptwith four OTDMA channels will be able to support up to 148simultaneous users at the OC-12 data rates. Our power budgetcalculations which include splitting losses, transmission lossover the demonstrated distance, and the losses incurred byindividual devices indicate that an EDFA with 30dB gainand 10dBm saturation power and a photo-receiver with the−30dBm sensitivity will be sufficient to support 64-usersbased on (8, 47) iOCDMA over 4-channel OTDMA system,(BER = 10−9 no FEC) and 128-users with (8, 47) iOCDMAover 4-channel OTDMA system when adopts the FECapproach. The drop in the number of simultaneous usersresulted from adopting power of two splitting “rule” whichis important in system design.If the iOCDMA-OTDMA system employs a (4, 47)iOCDMA over 4 channels OTDMA and adopts the FECapproach it will scale up from 18 to 64 simultaneous users.REFERENCES[1] I. Glesk, Y.-K. Huang, C. S. Brès, and P. R. Prucnal, “Design anddemonstration of a novel optical CDMA platform for use in avionicsapplications,” Opt. Commun., vol. 271, no. 1, pp. 65–70, Mar. 2007.[2] M. B. Othman, J. B. Jensen, X. Zhang, and I. T. Monroy, “Performanceevaluation of spectral amplitude codes for OCDMA PON,” in Proc. 15thInt. Conf. ONDM, Feb. 2011, pp. 1–5.[3] G.-K. Chang, et al., “Key technologies of WDM-PON for future con-verged optical broadband access networks [invited],” J. Opt. Commun.Netw., vol. 1, no. 4, pp. C35–C50, Sep. 2009.[4] S. Kimura, “10-Gbit/s TDM-PON and over-40-Gbit/s WDM/TDM-PONsystems with OPEX-effective burst-mode technologies,” in Proc. OFC2009, Mar., pp. 1–3.[5] C.-S. Brès, Y.-K. Huang, I. Glesk, and P. R. Prucnal, “Scalable asyn-chronous incoherent optical CDMA,” J. Opt. Netw., vol. 6, no. 6, pp.599–615, Jun. 2007.[6] S.-G. Park and A. M. Weiner, “Performance of asynchronous time-spreading and spectrally coded OCDMA systems,” J. Lightw. Technol.,vol. 26, no. 16, pp. 2873–2881 Aug. 15, 2008.[7] L. Zhou, X. Cheng, Y.-K. Yeo, and L. H. Ngoh, “Hybrid WDM-TDM PON architectures and DWBA algorithms,” in Proc. 5th Int. ICSTCHINACOM 2010, Aug., pp. 1–6.[8] S. Yoshima, N. Nakagawa, J. Nakagawa, and K. Kitayama, “10G-TDM-OCDMA-PON systems,” in Proc. 15th OECC 2010, Jul., pp. 724–725.[9] I. Glesk, K. I. Kang, and P. R. Prucnal, “Demonstration of ultrafast all-optical packet routing,” Electron. Lett., vol. 33, no. 9, pp. 794–795, Apr.1997.

Network scaling using OCDMA Over OTDM

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Network scaling using OCDMA Over OTDM

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