Optical Code Division Multiplexing (OCDM) and Its ......Optical code division multiplexing (OCDM) is...

2616IEICE TRANS. FUNDAMENTALS, VOL.E82–A, NO.12 DECEMBER 1999

INVITED PAPER Special Section on Spread Spectrum Techniques and Applications

Optical Code Division Multiplexing (OCDM) and Its

Applications to Photonic Networks

Ken-ichi KITAYAMA†, Hideyuki SOTOBAYASHI††, and Naoya WADA††, Members

SUMMARY Optical code division multiplexing (OCDM) isthe other class of multiplexing techniques than time division mul-tiplexing (TDM), wavelength division multiplexing (WDM) andspace division multiplexing (SDM). OCDM has been proposedin mid ’70s. It has been long since OCDM remains outside themainstream of research community of optical communications,however, possible scarcity of the wavelength resource in futurephotonic networks, the simple access protocol as well as versa-tility of optical codes motivate recent growth of OCDM researchactivities. In this paper, first, fundamentals of OCDM conceptare presented, highlighting optical encoding and optical time gatedetection which realize time spreading/despreading. Next, cur-rent research activities of OCDM are reviewed by focusing par-ticularly on the optical implementations and the proof-of-conceptexperiments. It is devoted to three categories; high bit rate point-to-point transmissions, gigabit multiple access, followed by opti-cal path networks using optical code. Finally, future issues arebriefly summarized.key words: optical communications, code division multiplexing,wireless CDMA, multiple access, optical network

1. Introduction

Optical fiber communication systems in a wide com-mercial usage have been based upon time divisionmultiplexing (TDM). There is a wide consensus thatwavelength division multiplexing (WDM) [1] will comenext to realize terabit/s per fiber transmission capac-ity. WDM technology migrate from the point-to-pointtransmission to higher layer of network hierarchy torealize so-called photonic networks [2]. In photonic net-works, the node transferring functions will be eventu-ally performed in optical domain without optical-to-electrical conversion, thereby enhancing the throughputand minimizing the transfer delay of the network.

Optical code division multiplexing (OCDM) isthe other class of multiplexing techniques than TDM,WDM, and space division multiplexing (SDM) [3].OCDM has been proposed in mid ’70s [4]. It has beenlong since OCDM remains outside the mainstream ofresearch community of optical communications, how-ever, possible scarcity of the wavelength resource infuture photonic networks, the simple access protocol

Manuscript received July 14, 1999.†The author is with the Department of Electronics and

Information Systems, Osaka University, Suita-shi, 565–0871Japan.

††The authors are with Communications Research Labo-ratory, Ministry of Posts and Telecommunications of Japan,Koganei-shi, 184–0015 Japan.

as well as versatility of optical codes motivate recentgrowth of OCDM research activities [5].

Characteristics of OCDM systems are summarizedin Table 1 by contrast with wireless cellular CDMAsystems [6], [7]. OCDM inherits unique features fromthe wireless counterpart; accommodation of a numberof channels on a single carrier frequency, tell-and-goaccess capability [8], freedom from network wide tim-ing synchronization, soft capacity, and security. Ow-ing to progresses in photonic device technologies, therehave been growing activities in their optical imple-mentations. The applications range from point-to-point transmission, multiple access, optical path net-work, and optical secure communications using chaosCDMA [9].

In this paper, the first is on the fudamentals ofOCDM concept, highlighting optical encoding and op-tical time gate detection which realize time spread-ing/despreading. Next, current research activities ofOCDM are reviewed by focusing particularly on theoptical implementations and the proof-of-concept ex-periments. It is devoted to three categories; high bitrate point-to-point transmissions, gigabit multiple ac-cess, followed by optical path networks using opticalcode. Finally, future issues are briefly summarized.

2. Fundamentals of OCDM

2.1 Principle

As shown in Fig. 1 (a), in spread-spectrum wirelessCDMA one uses matched filtering for the detectiontechnique. The data signal is spread by a pseudo-noise (PN) code sequence in radio frequency domainand transmitted. The received signal is despread bythe matched filtering, followed by narrow-bandpass fil-tering, and only desired data are recovered. The ratioof data bit rate to total bandwidth is defined as theprocessing gain.

OCDM draws an analogy from the spread-spectrum wireless CDMA. By contrast to the frequencyspread/despread, the time spread/despread techniqueis adopted in OCDM as shown in Fig. 1 (b). An opti-cal short pulse, having much higher frequency spectrumthan the data bandwidth, is spread over one bit dura-tion T by the encoding. The decoding time-despreadsthe encoded signal, reconstructing the original short

KITAYAMA et al: OPTICAL CODE DIVISION MULTIPLEXING (OCDM) AND ITS APPLICATIONS2617

Table 1 Comparison of OCDM with wireless CDMA.

Fig. 1 (a) Principle of spread-spectrum wireless CDMA system on frequencyspread/despread basis and (b) principle of OCDM system on time spread/despread basis.

pulse if the codes between the encoder and decodermatch. While, on the other hand, unmatched codesremain randomly spread over T after the decoding. Dis-

crimination is achieved by the threshold of the output.The processing gain N of OCDM is expressed as theratio of one bit time duration T to the time window


Fig. 2 Time gate having a time-window g(t) and the frequency spectrum G(ω).

∆τ . The optical time gate is detailed in 2.4.

2.2 Optical Encoding

Optical encoding schemes are classified into incoher-ent and coherent regimes depending upon the degreeof coherence of the light source. Most of the works in80’s have dealt with incoherent optical code but theyhave revealed the limited system performances [10], [11].Recently, coherent approach based upon optical phaseand/or amplitude encoding scheme has become feasi-ble, which preserves a commonality to direct sequenceof wireless CDMA [12]–[17]. Furthermore, the wave-length hopping has also been realized [18], [19]. Theseprogresses are due to the fact that perceived difficul-ties of optical phase encoding have been overcome inrecent years. Hereafter, a focus will be on the coherentapproach.

There are two encoding schemes; encoding in timedomain and that in frequency domain. As shown inFig. 1 (b), the encoder generates binary phase-shift key-ing (BPSK) signals with N time-resolved chip pulsecode within one bit time duration in which the phaseshift of each chip pulse represents a code sequence [12]–[14]. On the other hand, the frequency encoder cre-ates N space-resolved code [15]–[17]. The time domainencoding has a difficulty in generating a long code se-quence, while, on the other hand, the space encoding isonly available in pico- or subpico-second regime.

2.3 Matched Filtering

Matched filtering in optical domain is a basis of thedespreading. In general, matched filtering is a detectiontechnique which maximizes the signal-to-noise ratio ofthe received signal. The impulse response of matchedfilter hd(t) along with its Fourier spectrum Hd(ω) is thecomplex conjugate of optical encoder output, and it isgiven by

Hd(ω) = He(ω)∗ε−jωt0 (1)hd(t) = he(t0 − t) (2)

where the impulse response of optical encoder is he(t),and He(ω) is its Fourier spectrum. Then, the outputof matched filter is expressed by the convolution of the

impulse responses of the encoder and the matched filter

Output =∫ ∞

−∞He(ω)Hd(ω)εjωtdf

=∫ ∞

−∞|He(ω)|2εjω(t−t0)df

=∫ ∞

−∞he(t′)he(t′ − t + t0)dt′

≡ Ψ(t − t0) (t0: arbitrary) (3)

where Ψ(t) represents the auto-correlation function ofthe input optical code he(t). The matched filtering re-sponse can be physically realized in optical domain bytime-reversing the in/output of the optical encoder.

It is noteworthy that in OCDM systems a photode-tector having a square-law of the electric field is usedas the OE converter. Therefore, the output of the de-tector varies with the degree of coherence of the lightsource. In the incoherent encoding scheme the outputis reduced to a half in the worst scenario compared tothe coherent regime, while, on the other hand, the in-terferometric noise becomes significant in the coherentregime.

2.4 Optical Time Gate

The time gating can realize in a strict sense the ex-pected processing gain N(= T/∆τ) [5]. As shown inFig. 2, the time gate opens the time-window g(t) for theduration of ∆τ at the bit rate 1/T at the moment whenthe auto-correlation mainlobe passes. The Fourier spec-trum of the time gate has discrete frequency compo-nents with the frequency interval of 1/T and the band-width of 1/∆τ . It samples only the data-bearing frac-tion of each bit, thereby rejecting the sidelobes of auto-correlation and the interference codes that fall outsidethe gate interval. This operation is somewhat equiva-lent to narrow-bandpass filtering in the wireless CDMA.Placing the optical time gate before the photodetectorhas a substantial advantage because the requirementfor the detector bandwidth is significantly relaxed tothe bit rate of 1/T . Had an electical time gate afterthe photodetector, it would require the detector band-width of chip rate 1/∆τ .

Assumed that the signal powers of the channels


Fig. 3 (a) Temporal waveform of 8-chip equi-amplitude BPSK code sequence after thepropagation in a long dispersion shifted fiber, its auto-correlation before and after timegating and (b) eye diagrams of 10 Gb/s IM-DD in RZ format, and (c) eye diagrams of200 Gb/s IM-DD in RZ format. The calculations are made under the following condi-tions; wavelength dispersion D = −0.285 ps/nm/km and the loss = 0.2 dB/km. Fibernonlinearity is ignored.

are equal, the signal-to-interference power ratio at thereceiver is given by

SNR =S

(K − 1)S + σ2=

1(K − 1) + σ2/S

(4)

where K is the number of channels, and σ2 is the vari-ance of additive noise power including the amplifiedspontaneous emission (ASE) noise of optical amplifier,shot noise, and thermal noise. The above assumptionof equal signal power is actually realized in optical fibertransmission, while, in wireless cellular CDMA the sig-nal power is controlled to realize it. The bit energy-to-interference density ratio Eb/N0 [20] after time gatingis obtained by multiplying S in the numerator by T andby multiplying the noise by the time-window ∆τ

Eb

N0=

ST

(K − 1)S∆τ + σ2∆τ

=N

(K − 1) + σ2/S(5)

Comparing with Eq. (4), the bit energy-to-interferencedensity ratio is enhanced by the processing gain N(=T/∆τ). The signal power after the time gating is ex-pressed by using the time-gated auto-correlation func-tion given by

S =∫ ∆τ/2

−∆τ/2

Ψ(t)dt ≤ Ψ(0) (6)

Note that the equality hold good only for the case withthe auto-correlation having no sidelobe. The additivenoise is also subject to the time gating. The noise re-duction will be actually more efficient by taking into ac-count the discrete spectrum of the time gate as shownin Fig. 2. More detail will be a subject in forthcom-ing paper. From Eq. (5), the number of channels K is


Fig. 4 Experimental setup for 10 Gb/s OCDM transmission with time gate detection.

expressed by

K = 1 +N

Eb/N0− σ2

ASE

S(7)

Equation (7) shows that K is interference limited. Theoptical time gate has been realized by using a semicon-ductor optical amplifier (SOA) based upon four-wavemixing (FWM) [21] as described in 3.1. This will bethe first accomplishement to the authors’ knowledge.

2.5 Wavelength Dispersion Effects

It is crucial for OCDM systems using a pulse code se-quence to characterize the effects of wavelength disper-sion and nonlinearity. Surprisingly, there has been littlethorough investigation [22]. Here, a focus is on the dis-persion effects, and the simulation results of the propa-gations of picosecond pulse code sequence in dispersion-shifted fiber will be presented. In Fig. 3 (a), tempo-ral waveform of 8-chip equi-amplitude BPSK code se-quence after the propagation in a long dispersion shiftedfiber, its auto-correlation before and after time gatingare shown. The cross-correlation with an interferencecode is also shown in the time-gated output. After50 km long propagation, the code sequence severely de-forms, however, the eye opening demonstrates a gooddiscrimination from the interference. This will guaran-tee the error-free transmission over 50 km but no longerthan 100 km. As a result, an appropriate compensationscheme to overcome the dispersion effect would be re-quired for high bit rate OCDM systems in the similarmanner as TDM systems. It is interesting to compare

10 Gb/s OCDM system with TDM 10 Gb/s IM-DD(intesnsity-modulation-direct detection) RZ (return-to-zero) transmission. In Fig. 3 (b), the eye closure even-tually appears at the longer distance than 100 km. Tobe fair, however, a comparison has to be made with asingle-channel bit rate of TDM system which is equiv-alent to the total transmission capacity of 10 Gb/sOCDM system. The time interval of 5 ps in the codesequence is the same as TDM 200 Gb/s IM-DD in RZformat. As shown in Fig. 3 (c), impairment in the tem-poral profile of the pulse train becomes critical onlyafter a few kilometer. The eye closure is seen around5 km.

3. Applications to Photonic Networks

3.1 Point-to-Point Transmission

Since mid 90’s gigabit rate OCDM transmissions havebeen experimentally demonstrated [5], [23]. Recently,a long distance transmission of 10 Gb/s OCDM us-ing BPSK pulse code sequence has been realized byintroducing time-gating detection [24]. As shown inFig. 4, the experimental setup consists of a transmit-ter, optical encoder/decoder, 100 km dispersion-shiftedsingle-mode fiber (DSF) link of three different pieces,and time-gate detector. In the time-gating detector,the clock pulse is optically extracted from the de-coded signal by an injection-locked mode-locked laserdiode (MLLD), and the SOA time gate using FWMis driven by the extracted clock to gate the auto-correlation mainlobe. The monolithically integrated


all-optical 8-chip BPSK encoder/decoder consists ofvariable tapped delay-lines with thermo-controlled op-tical phase shifters. This is a key device to the excel-lent phase stability against the environmental fluctua-tion [25]. In Fig. 5 the auto-correlation waveforms withand without interference code are shown. Although,each chip pulse is not clearly resolved due to the lim-ited bandwidth of the detector. Only the mainlobe ofauto-correlation is time-gated. Measured bit error rates(BERs) for pseudo-random bit stream (PRBS) 1023−1at 10 Gb/s signal in Fig. 6 confirms error-free transmis-sion over 100 km regardless of an interference code.

The simulation supports the experimental results.In Fig. 7 the waveforms of a code and its auto-correlations are simulated by varying the input optical

Fig. 5 Auto-correlation waveforms with and without interfer-ence code.

Fig. 7 Simulated waveforms of a code and its auto-correlations. The calculations are un-der the following conditions; (Di, dDi/dλ, Li) = (−0.285 ps/nm/km, 0.0904 ps/nm2/km,39 km), (−0.285 ps/nm/km, 0.0935 ps/nm2/km, 26 km), (0.158 ps/nm/km, 0.0677 ps/nm2

/km, 33 km), the nonlinear coefficient = 2.63 × 10−3 W−1, and the loss = 0.2 dB/km.

power. As increasing the input power, the 8-chip codewaveform (on the top) severely distorts mainly due tothe dispersion and the nonlinear effect. On the otherhand, the time-gated output (on the bottom) main-tains up to 400 mW but collapses for the input powerof 1250 mW (0.13 W in average). Based upon the simu-lation result, the experimental error-free 100 km trans-mission is mainly due to the fact that the frequencyfiltering shortens the pulse width of the chirped pulse,which is broadened by the normal dispersion and self-phase modulation.

Fig. 6 Measured BERs at 10 Gb/s signal vs. optical receivedpower.


3.2 Multiple Access

OCDM/WDM [26]–[28] can be preferably applied to ac-cess networks in a ring topology shown in Fig. 8 [28].Node#1 has an add/drop multiplexer (ADM) whichdrops λ1 and adds all the wavelengths. Access sta-tion ST#1 of Node#1 has a wavelength-tunable trans-mitter/tunable encoder (TT-TE) and Wavelength λ1-fixed receiver/Optical Code(OC)1-fixed decoder (FR-FD). The same code sequences can be reused for allthe nodes. The OCDM access protocol is simply tell-and-go without a prior coordination in the followingmanner. From ST#1 of Node#1 to;

1. ST#2 of the same Node#1; The sender tunes itsown transmitter to λ1 and encodes the data withOC2. The sender resends the data until it receivesthe acknowledgement.

2. ST#2 of different Node#2; The sender tunes itsown transmitter to λ2 and encodes the data withOC2. The sender resends the data until it receivesthe acknowledgement.

Gigabit OCDM/WDM multiple access experimentwas conducted using two wavelengths with two op-tical codes at 1.24 Gb/s. This allows to accommo-date two nodes having two access stations. The ex-

Fig. 8 OCDM/WDM ring network.

Fig. 9 Experimental setup gigabit OCDM/WDM multiple access.

perimental setup is simplified into two decoders asshown in Fig. 9. The optical frequency interval be-tween two optical carriers are 60 GHz and 120 GHz,respectively. 4-chip binary-shift keying (BPSK) pulsecode sequence with 100 ps interval is used. Optical en-coders 1 and 2 impress different codes OC1 and OC2,respectively, onto each bit of Data 1 and 2. Thus,four data signals with different combinations of opti-cal code and the wavelength are generated. At thereceiver, the signal is first wavelength-demultiplexedby the arrayed waveguide grating (AWG) and directedto the decoder. The optical spectra before and afterwavelengh-demultiplexing and the temporal waveformsof auto-correlation function of (OC1, λ1) in the pres-

Fig. 10 (a) Optical spectra before and after wavelengh-demultiplexing and (b) the temporal waveforms of auto-correlation function of (OC1, λ1) in the presence of interferencecode of (OC2, λ1).


ence of interference code of (OC2, λ1) are shown inFigs. 10 (a) and (b), respectively. In Fig. 11 the mea-sured BERs of Data1 (OC1, λ1) are shown. By com-paring with the back-to-back BER of single-code on a

Fig. 11 Measured BERs of Data1 (OC1, λ1) vs. optical re-ceived power.

(a) (b)

Fig. 12 (a) WDM paths and (b) optical paths in the OCDM/WDM network.

Fig. 13 Experimental setup for all-optical code and wavelength conversion.

single wavelength,

1. Interference code on a same wavelength causes themajor power penalty,

2. Wavelength crosstalk is negligible due to the nar-row signal bandwidth compared to the wavelengthinterval.

3.3 Optical Path Networks

It is only natural that OCDM/WDM concept isextended to the optical code-wavelength path net-works [29], [30]. The optical code-wavelength path isdefined as an end-to-end logical path in which eachlink is represented by a combination of an optical codeand a wavelength. In Fig. 12, the optical paths inthe OCDM/WDM network are compared with WDMpaths. The optical code-wavelength conversion will berequired in the OCDM/WDM network, for example,to establish Optical path 5 at Node C. Advantages ofoptical path over conventional SDH (synchronous dig-ital hierarchy) electrical path include path restorationin optical domain, minimum transfer delay, and highthroughput node.


All-optical code and wavelength conversion of10 Gb/s has been experimentally demonstrated [31].The wavelength and BPSK optical code can be simulta-neously converted via four-wave mixing (FWM) usingan SOA. Suppose that the idler wave i is a convertedsignal, the input signal s is arbitrarily converted to theidler wave having a desired wavelength and optical codeby injecting the optical pump pulse sequence p. Thephase and the wavelength of pump is preset so as tosatisfy the following conditions;

ωi = 2ωp − ωs (8)φi = 2φp − φs (9)

where ω and φ denote the frequency and phase, respec-tively. For example, the signal BPSK codes of [0 0 π π]is converted to the BPSK codes of [0 π π 0] by usingQPSK codes of [0 π/2 0 π/2] as the pump. Figure 13shows the experimental setup. The signal and pumppulse streams are generated at the center wavelengthsof (λs =)1552.5 nm and (λp =)1547.5 nm, respectively.The signal and pump pulse streams are combined to-gether and directed into a semiconductor optical ampli-

Fig. 14 Optical spectra of the input (lower trace) and the out-put (upper trace) of SOA.

Fig. 15 Auto-correlations of generated idler signals, I[0 0 0 0] and I[0 0 π π] where theinput signal is encoded with S[0 0 0 0].

fier (SOA) to generate FWM. Figure 14 shows the op-tical spectra of the input (lower trace) and the output(upper trace) of SOA. In Fig. 15 the auto-correlationsof generated idler signals, I[0 0 0 0] and I[0 0 π π] areshown where the input signal is encoded with S[0 0 00].

4. Conclusion

From Table 1, a major stumbling block of OCDM sys-tems is the interference, and the future task would beto establish the countermeasure to it. Wireless cellu-lar CDMA system has already solved the problem byintroducing multisectored antenna and multibeam. Al-though not elaborated, the interference of OCDM willbecome even worse in coherent regime due to the inter-ferometric effect. This also has to be overcome.

The raison d’etre of OCDM/WDM will often bequestioned by contrast with dense WDM approach.This issue has to be carefully addressed from variousviewpoints of the target application, time table as wellas system cost.

Finally, a basis of OCDM, matched filtering intime domain can be preferably applied to recognition oftemporal profiles, so-called photonic processings. Pho-tonic switching based upon optical spread spectrum hasbeen demonstrated [32]. More recently, Internet Proto-col (IP) header processing using optical codes has beenproposed [33], and this will potentially overcome thespeed bottleneck of current electrical IP router.

Aknowledgement

The author would like to thank H. Yokoyama of NECOptoelectronic and high frequency Laboratory, M.Okuno of NTT Photonics Laboratory, D.D. Sampsonof The Univercity of Western Australia, and R. Griffinof University College London for invaluable discussions.They would also like to express their thanks to Y.


Furuhama and T. Itabe of CRL for their encourage-ment.

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Ken-ichi Kitayama received theB.E., M.E., and Dr.Eng. degrees in com-munication engineering from Osaka Uni-versity, Osaka, Japan, in 1974, 1976, and1981, respectively. In 1976, he joined theNTT Electrical Communication Labora-tory. In 1982–1983, he spent an academicyear as a visiting scholar at the Univer-sity of California, Berkeley. In 1987–1995,he was with the NTT Optical NetworkSystems Laboratories. In 1995, he joined

the Communications Research Laboratory, Ministry of Posts andTelecommunications, Japan. Since 1999, he is with Oasaka Uni-versity. He is currently a Professor of the Department of Elec-tronic, Information Systems and Energy Engineering, GraduateSchool of Engineering. His research interests include photonicnetworks, radio-on-fiber communications, and photonic signalprocessings. He has published over 140 papers in refereed journalsin English, written two book chapters, translated one book. Hehas been awarded about 30 patents. Dr. Kitayama received the1980 Young Engineer Award from the Institute of Electronic andCommunication Engineers of Japan and the 1985 Paper Awardof Optics from the Japan Society of Applied Physics. He is amember of the Japan Society of Applied Physics, the Optical So-ciety of Japan, and the Optical Society of Japan, and a SeniorMember of the IEEE.

Hideyuki Sotobayashi was bornin Kyoto Prefecture, Japan, on April 2,1969. He received the B.S. degree in elec-tronic engineering in 1992 and the M.S.and Ph.D. degrees in electrical engineer-ing from the University of Tokyo, To-kyo, Japan, in 1994 and 1997, respec-tively. In 1997, he joined Communica-tions Research Laboratory, Ministry ofPosts and Telecommunications, Tokyo,Japan, where he has been engaged in re-

search on optical communications and photonic networks. Dr.Sotobayashi is a member of the Institute of Electrical and Elec-tronics Engineers (IEEE), and the Optical Society of America(OSA).

Naoya Wada received the B.E.,M.E., and Dr.Eng. degrees in electron-ics from Hokkaido University, Sapporo,Japan, in 1991, 1993, and 1996, respec-tively. In 1996, he joined Communica-tions Research Laboratory, Ministry ofPosts and Telecommunications, Tokyo,Japan. His current research interests arein the area of optical code-division mul-tiplexing based photonic network. Dr.Wada is a member of IEEE, the Japan

Society of Applied Physics and the Optical Society of Japan.

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