Received September 16, 2017, accepted October 17, 2017, date of publication October 23, 2017,date of current version November 28, 2017.

Digital Object Identifier 10.1109/ACCESS.2017.2765336

Simple Self-Homodyne Detection Scheme forOptical OFDM With Inserted Pilot Subframesand Application in Optical Access NetworksGUO-WEI LU 1, 2, (Member, IEEE), XUN GUAN3, TAKAHIDE SAKAMOTO2, (Member, IEEE),NAOKATSU YAMAMOTO2, (Member, IEEE), ANDCALVIN CHUN-KIT CHAN4, (Senior Member, IEEE)1Institute of Innovative Science and Technology, Tokai University, Hiratsuka 259-1292, Japan2Network Science and Convergence Device Technology Laboratory, National Institute of Information and Communication Technology, Tokyo 184-8795, Japan3Center for Optics, Photonics and Lasers, Université Laval, Quebec City, QC G1V 0A6, Canada4Department of Information Engineering, The Chinese University of Hong Kong, Hong Kong

Corresponding author: Guo-Wei Lu (gordon.guoweilu@gmail.com)

This work was supported by the Grant-in-Aid for Scientific Research (C) from the Ministry of Education, Culture, Sports, Science andTechnology, Japan, under Grant 15K06033.

ABSTRACT In this paper, a simple self-homodyne detection scheme is proposed and experimentallydemonstrated for applications in optical access networks. Unmodulated pilot subframes are periodicallyinserted and interleavedwith data subframes to form an orthogonal frequency-divisionmultiplexing (OFDM)signal. Owing to the coherence between the embedded pilot and data subframes, a Mach–Zehnder delayinterferometer with a free spectral range equal to the frequency of the subframe is deployed for theself-homodyne detection of the OFDM signal. It saves the local oscillator and optical hybrid, which areusually used in a conventional coherent receiver, thus reducing the hardware complexity and implementationcost. Meanwhile, the digital signal processing (DSP) for the self-homodyne detection is free from carrier-frequency-offset compensation and carrier-phase estimation, reducing the complexity, power consumption,and latency of the system. It also allows the use of a low-cost laser source as the source for the downstreamsignal. The proposed scheme provides a cost-effective and energy-efficient downstream solution for opticalaccess networks, owing to the hardware saving and complexity reduction in DSP. A 10-Gb/s OFDMdownstream transmission over a 20-km standard single-mode fiber is experimentally demonstrated withan error-free operation, using both a 10-MHz distributed feedback laser and a 100-kHz external cavity laseras downstream laser sources.

INDEX TERMS Optical fiber communication, optical fiber networks, OFDM modulation.

I. INTRODUCTIONRecently, with the explosive multimedia-driven growth ofInternet traffic, coherent detection technologies [1], [2] havebeen extensively investigated, to address the ever grow-ing bandwidth demands from core networks to the edgeand access networks. With the deployment of digital coher-ent detection in passive optical networks (PONs), coherentPONs can effectively improve the receiver sensitivity, whichincreases the network reach and splitting ratio to support highsystem capacities for ultra-dense wavelength-division multi-plexing (WDM) applications [3]–[5]. So far, coherent PONshave been experimentally demonstrated using either intra-dyne or self-homodyne coherent detection for downstream

signals. However, intradyne detection requires a tunable localoscillator (LO), as well as carrier-frequency-offset (CFO)compensation and complicated carrier-phase estimation at thedigital signal processing (DSP), increasing the implementa-tion cost and complexity. Alternatively, self-homodyne detec-tion has been proposed for optical network units (ONUs) fordownstream detection by introducing an unmodulated pilottone through polarization multiplexing [6], [7] or mode mul-tiplexing [8] in WDM-PONs or mode-division multiplexingPONs (MDM-PONs), respectively, or by periodically insert-ing unmodulated carrier symbols [9] in the downstreamof optical orthogonal frequency-division multiplexing PONs(OFDM-PONs). The pilot tone carried in the polarization

246022169-3536 2017 IEEE. Translations and content mining are permitted for academic research only.

Personal use is also permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

VOLUME 5, 2017

G.-W. Lu et al.: Simple Self-Homodyne Detection Scheme for Optical OFDM With Inserted Pilot Subframes and Application

FIGURE 1. Frame structure of the proposed self-homodyne OFDM signal.

or mode and the carrier symbol carried in the time domainserve as phase references for self-homodyne detection inthe ONU, which saves LO and eliminates the implementa-tion of CFO compensation and carrier-phase estimation inDSP [6]–[8].

To simplify the implementation and reduce the cost fur-ther, we propose a cost-effective self-homodyne downstreamscheme for WDM-PONs. In contrast to the conventionalOFDM frame, unmodulated pilot subframes are periodicallyinserted and interleaved with the spliced OFDM data sub-frames to form the downstream signal at the optical lineterminal (OLT). Instead of using a conventional coherentreceiver, which usually consists of an optical hybrid, an LO,and a couple of balanced photodetectors (BPDs), a singleMach–Zehnder delay interferometer (MZDI) and a followedBPD are used for single-polarization self-homodyne detec-tion at the ONU. This effectively simplifies the implementa-tion and reduces the cost. Owing to the coherence betweenthe embedded pilot and data subframes, the proposed schemerelaxes the linewidth requirement for the laser at the OLT, andallows the use of low-cost lasers as downstream laser sources.Moreover, as aforementioned, the DSP is simplified, since theestimation and compensation of CFO and common-phase-error (CPE) are not required in the self-homodyne detectionof OFDM. Owing to the savings in terms of both hardwareand DSP, the proposed self-homodyne system provides acost-effective and energy-efficient downstream scheme forWDM–PONs. Using the proposed self-homodyne scheme,we experimentally demonstrate a 10-Gb/s self-homodyneOFDM downstream system over a 20-km standard single-mode fiber (SSMF) with a <0.3 dB power penalty, which isdefined with the respect to the back-to-back sensitivity at abit-error rate (BER) of 1 × 10−3. The experimental resultsshow that nearly identical performances are obtained whenusing a low-cost distributed feedback (DFB) laser with a10-MHz linewidth and a 100-kHz external-cavity laser (ECL)as laser sources at the OLT, implying the tolerance of thescheme to laser linewidth.

II. OPERATION PRINCIPLEFigure 1 illustrates the frame structure of our proposed self-homodyne OFDM signal. To form the optical OFDM framefor the proposed self-homodyne system, the original OFDMframe is first sliced into subframes with a duration of τ ,

which is much smaller than the whole frame period, and inter-leaved with unmodulated pilot subframes that have the sameperiod [10], [11], as shown in Fig. 1. Since unmodulated sub-frames are inserted periodically in the OFDM frame, in orderto prevent errors when passing through RF circuits withcapacitive coupling or transformers, it is important to produceDC-free RF driving signals when synthesizing the proposedoptical OFDM frame. Therefore, the embedded unmodulatedpilot subframes are encoded as (1 + i) and (−1− i), alter-nately, which ensures that the DC-offset of the driving signalis close to zero. Meanwhile, since the applied alternate phasemodulations have a π phase difference between adjacentpilot subframes, no additional decoding is required after theself-homodyne detection. Assuming the duration of the sub-frame, τ , to be shorter than the coherent time of the lasersource, the coherence between successive data and pilot sub-frames is preserved, which enables self-homodyne detection.At the receiver side (ONU), a self-homodyne detector thatconsists of one MZDI and one BPD is used for detection.The MZDI has a path difference of τ , which is equal tothe subframe duration, and a 45◦ relative phase differencebetween its two arms.

FIGURE 2. Operation principle of the proposed self-homodyne detection.

As illustrated in Fig. 2, the incoming self-homodyneOFDM data frame consists of the sliced data sub-frame (D1,D2 . . . ) and the interleaved unmodulated pilot sub-frames. The signal in the upper arm experiences one subframerelative delay and 45◦ relative phase shift with respect to thatin the lower arm. As an example shown in Fig. 2, taking thephase of unmodulated pilot subframes as reference, the sliceddata subframes (D1) in the first two successive subframeslots, experiences different relative phase shift, i.e. −45◦

and +45◦, respectively. This results in the orthogonal phasedifference with respect to the pilot subframe at these twosuccessive subframes at the output of MZDI, correspondingto the I and Q components of D1, i.e. I1 and Q1, after theinterference. Through the self-homodyne detection by a sin-gle BPD, the detected orthogonal components of the OFDMsignal, with a separation of τ , are interleaved and packed

VOLUME 5, 2017 24603

G.-W. Lu et al.: Simple Self-Homodyne Detection Scheme for Optical OFDM With Inserted Pilot Subframes and Application

together. In the following DSP, the I and Q componentsof the OFDM frame D are recovered after de-interleavingand data stitching. Therefore, instead of separately detectingthe orthogonal components (I and Q) in the conventionalcoherent receiver using optical hybrids and two BPDs, it issufficient to adopt one set of MZDI and BPD to demodulatethe single-polarization OFDM data. To ensure stable oper-ation in the self-homodyne detection, it is suggested to usean athermal free-space Michelson interferometer [12] as analternative to the MZDI in practical applications, especiallyin PONs, since no temperature control or stabilization isrequired. Note that the same duration of the data and pilotsubframes is chosen here to allow the deployment of a MZDIto further simplify the self-homodyne detection at the receiverside.

FIGURE 3. DSP flow for the conventional (solid-line and dotted-lineboxes) and the proposed (solid-line boxes) OFDM systems.

Figure 3 shows the deployed DSP flow for the data anal-ysis. Since the data and pilot subframes are carried by thelight from a common laser source, in contrast to the DSP usedfor the conventional OFDM, it is not necessary to includethe estimation and compensation of CFO and CPE in theDSP. This can effectively simplify the DSP, thus reducing thecomplexity, power consumption, and latency of the system.Moreover, owing to the common phase noise cancellationin the self-homodyne detection based on MZDI, low-costlarge-linewidth laser sources can be used at the OLT forthe downstream signal. Because of the savings in hardwareand DSP, the proposed scheme is suitable for cost-sensitiveapplications such as optical access networks. On the otherhand, since the DSP is simplified by excluding the estimationand compensation of CFO and CPE, it also has potentialapplications in inter or intra connections in data centersbecause of its low latency. Compared with the conventionalOFDM, by periodically inserting pilot subframes, the com-plexity and cost of the proposed self-homodyne system areeffectively reduced at the expense of spectral efficiency.However, the scheme still holds other features of the conven-tional OFDM such as resilience to dispersion. The proposedself-homodyne scheme is also applicable to both single-carriermultilevelmodulation formats [11], such as 16 quadra-ture amplitude modulation (16QAM) and 64QAM, andmulti-carrier OFDM signals with subcarriers modulated inhigh-order QAMs.

FIGURE 4. Experimental setup.

III. EXPERIMENT AND RESULTSFigure 4 illustrates the proof-of-concept experimental setupof the proposed self-homodyne system for a WDM–PONdownstream. At the OLT, a continuous wave (CW) from aDFB or ECL laser emitting at 1550 nm serves as the lasersource for the downstream signal, which is then fed to anoptical in-phase/quadrature (IQ) modulator, which is drivenby the data from an arbitrary waveform generator (AWG)operating at 10 GSamples/s. The produced optical OFDMframe consists of 200 OFDM symbols. For each symbol,160 subcarriers are used for bearing data. In contrast tothe conventional OFDM, no pilot subcarriers are insertedfor phase estimation. A 256-point inverse fast Fourier trans-form (IFFT) is performed to transform the subcarriers to acomplex time-domain signal. After that, a cyclic prefix (CP)is added. The preamble in each frame includes a pseudo-random sequence for synchronization and eight symbols forchannel estimation. To construct the self-homodyne opticalOFDM signal, the OFDM data frame is sliced into ‘‘sub-frames’’ with a period of 100 ps, and interleaved with pilotsubframes, which are encoded at+45◦ and−45◦, alternately.The data and pilot subframes have the same period, i.e.,100 ps. At the ONU side, the received OFDM signal is fedto a self-homodyne receiver consisting of an MZDI and aBPD. The free spectral range (FSR) of the MZDI is 10 GHz.After the digitization using a real-time oscilloscope operatingat 50 GSamples/s, the frame synchronization and CP removalare performed. The received payload is then transferred to thefrequency domain by FFT for channel equalization. Note thatthe estimation and compensation of CFO and CPE are notincluded in the DSP.

Unlike the ‘‘noise-like’’ time-domain waveforms of theconventional OFDM signals, the embedded pilot subframeshave constant amplitudes. To investigate the influence ofthe amplitude levels of the embedded pilot subframes onthe system performance, the amplitude levels of the pilotsubframes are adjusted. Here, we define a subframe power

24604 VOLUME 5, 2017

G.-W. Lu et al.: Simple Self-Homodyne Detection Scheme for Optical OFDM With Inserted Pilot Subframes and Application

FIGURE 5. PAPR CCDF plots for the proposed self-homodyne OFDMsignals with different SFPRs (solid lines) and the conventional OFDMsignal (dotted line).

ratio (SFPR) as a ratio of the power of the pilot subframes tothe average power of the data subframes. Any reductions inPAPR are normally illustrated using a PAPR complementarycumulative distribution function (CCDF), which is defined asthe probability that the PAPR of an OFDM frame exceeds agiven reference value and is themost frequently usedmeasurefor describing PAPR reduction [13]. In order to investigatethe impact of SFPR on the PAPR reduction of the self-homodyne OFDM, PAPR CCDF curves for the proposedself-homodyne OFDM signals with different SFPRs and theconventional OFDM signals without inserting pilot sub-

FIGURE 6. Measured EVMs as a function of SFPR and the measuredconstellations at different SFPR values (a)∼(c).

frames are calculated and shown in Fig. 5. Since the averagepower is enlarged with the increase of pilot subframe’s power,it is clear that the increase of SFPR leads to the PAPR reduc-tion. Compared with the conventional OFDM signals withoutinserted pilot subframes, a self-homodyne OFDM with aSFPR of >1 shows a lower PAPR. Around 4 dB reductionin PAPR is observed for the proposed self-homodyne OFDMwith a SFPR of 4.8 compared with the conventional OFDMat a CCDF of 10−3. To further investigate the influence ofSFPR on the system performance, the corresponding errorvector magnitudes (EVMs) are measured while tuning theSFPR from 0.2 to 6.8. As shown in Fig. 6, low EVMs can beobtained when the SFPR varies from 1 to 2.3. However, toomuch power of pilot subframes in the formed self-homodyneOFDM frame may degrade the signal-to-noise ratio of theembedded data subframes when the synthesized signal passesthrough the electrical drivers and optical amplifiers, resultingin the increase of EVMs when SFPR is larger than 3. There-fore, in the following experiment, the SFPR is set as 2. Thecorresponding constellations with different SFPRs are shownin Fig. 6.

To evaluate the system performance, the BERs of the self-homodyne system are measured, and are shown in Fig. 7 with

FIGURE 7. The upper: measured BERs vs. received power with ECL andDFB lasers as laser sources with/without transmission over 20 km SSMF.The lower: measured constellations of OFDM signals after transmissionwith ECL and DFB lasers.

VOLUME 5, 2017 24605

G.-W. Lu et al.: Simple Self-Homodyne Detection Scheme for Optical OFDM With Inserted Pilot Subframes and Application

respect to the received optical power, measured just beforethe MZDI at the receiver side. The back-to-back and 20-kmtransmission performances are evaluated by measuring theBERs using laser sources with linewidths of 100 kHz (ECL)and 10 MHz (DFB), respectively. The results show thatalmost identical results are obtained with different laserlinewidths, indicating that the self-homodyne system has tol-erance against the linewidth of the laser source. This enablesthe use of cost-effective lasers at the OLT for the downstreamsignal, thus reducing the implementation cost of PON. Withrespect to the back-to-back BER, a power penalty of less than0.3 dB is obtained at a BER of 1 × 10−3 after 20-km trans-mission at the ONU. The constellations of self-homodyneOFDM at BERs of approximately 1×10−3 are also shown inFig. 7 for different laser sources. The results verify the feasi-bility of the proposed scheme. Note that improved receiversensitivity can be expected by using a preamplifier, high-sensitivity avalanche photodiode, or forward error correctioncoding.

IV. CONCLUSIONWe proposed and experimentally demonstrated a cost-effective and energy-efficient OFDMdownstream scheme forWDM-PONs using a simple self-homodyne system. In con-trast to the conventional OFDM coding, unmodulated pilotsubframes were periodically embedded into the downstreamdata, and interleaved with the OFDM data. At the receiverside, a simple self-homodyne detector consisting of an MZDIand a BPD was deployed to perform self-homodyne detec-tion, without using the optical 90◦ hybrid or LO, reducingthe hardware complexity. Since the embedded pilot sub-frames and the data subframes originated from the same lasersource, it freed the DSP from the estimation and compensa-tion of CFO and CPE, simplifying the DSP implementationat ONU. Besides, the proposed scheme exhibited toleranceagainst laser linewidth, enabling the use of cost-effectivelaser sources at the OLT and providing a cost-effective solu-tion for access networks. A 10-Gb/s OFDM self-homodynedownstream transmission over a 20-km SSMF was experi-mentally demonstrated with an error-free operation. Similarperformances were observed when both a 10-MHz DFB anda 100-kHz ECL were deployed as laser sources for the down-stream data, implying a phase-noise-tolerant scheme.

ACKNOWLEDGMENTG.-W. Lu would like to thank Yang Hong for useful discus-sions. X. Guan was with the Department of Information Engi-neering, The Chinese University of Hong Kong, Hong Kong.

REFERENCES[1] K. Kikuchi, ‘‘Fundamentals of coherent optical fiber communications,’’

J. Lightw. Technol., vol. 34, no. 1, pp. 157–179, Jan. 1, 2016.[2] E. Ip, A. P. T. Lau, D. J. F. Barros, and J. M. Kahn, ‘‘Coherent detection in

optical fiber systems,’’ Opt. Exp., vol. 16, no. 2, pp. 753–791, 2008.[3] E.Wong, ‘‘Next-generation broadband access networks and technologies,’’

J. Lightw. Technol., vol. 30, no. 4, pp. 597–608, Feb. 15, 2012.[4] N. Cvijetic, ‘‘OFDM for next-generation optical access networks,’’

J. Lightw. Technol., vol. 30, no. 4, pp. 384–398, Feb. 15, 2012.

[5] S. J. Savory, ‘‘Digital coherent optical access networks,’’ in Proc. IEEEPhoton. Conf. (IPC), vol. 1. Sep. 2013, pp. 125–126, paper MG2.

[6] R. S. Luís et al., ‘‘Ultra high capacity self-homodyne PON with simplifiedONU and burst-mode upstream,’’ IEEE Photon. Technol. Lett., vol. 26,no. 7, pp. 686–689, Apr. 1, 2014.

[7] Z. Vujičić et al., ‘‘Self-homodyne detection-based fully coherent reflectivePON using RSOA and simplified DSP,’’ IEEE Photon. Technol. Lett.,vol. 27, no. 21, pp. 2226–2229, Nov. 1, 2015.

[8] Y. Chen et al., ‘‘Novel MDM-PON scheme utilizing self-homodyne detec-tion for high-speed/capacity access networks,’’ Opt. Exp., vol. 23, no. 25,pp. 32054–32062, 2015.

[9] R. Hu et al., ‘‘Delayed self-homodyne detection for OFDM-PON down-stream,’’ in Proc. Conf. Opt. Fiber Commun. Exhib., vol. 18. Mar. 2014,pp. 1–3, paper W2A.

[10] G.-W. Lu, M. Nakamura, Y. Kamio, and T. Miyazaki, ‘‘40-Gb/s QPSK and20-Gb/s PSK with inserted pilot symbols using self-homodyne detection,’’Opt. Exp., vol. 15, no. 12, pp. 7660–7666, 2007.

[11] G.-W. Lu, X. Guan, T. Sakamoto, N. Yamamoto, and C. C. Chan, ‘‘Sim-plified self-homodyne detection for optical OFDMwith inserted pilot sub-samples and its application in downstream of optical access networks,’’ inProc. Conf. Lasers Electro-Opt., 2017, paper STh1O.5.

[12] X. Liu, A. H. Gnauck, X. Wei, J. Hsieh, C. Ai, and V. Chien, ‘‘Athermaloptical demodulator for OC-768 DPSK and RZ-DPSK signals,’’ IEEEPhoton. Technol. Lett., vol. 17, no. 12, pp. 2610–2612, Dec. 2005.

[13] S. H. Han and J. H. Lee, ‘‘An overview of peak-to-average powerratio reduction techniques for multicarrier transmission,’’ IEEE WirelessCommun., vol. 12, no. 2, pp. 56–65, Apr. 2005.

GUO-WEI LU (M’05) received the Ph.D. degree in information engineeringfrom The Chinese University of Hong Kong (CUHK), Hong Kong, in 2005.From 2005 to 2006, he was a Post-Doctoral Fellow with CUHK. From2006 to 2009, he was an Expert Researcher with the National Institute ofInformation and Communications Technology (NICT), Tokyo, Japan. From2009 to 2010, he was an Assistant Professor with the Chalmers Universityof Technology, Göteborg, Sweden. From 2010 to 2014, he was a Researcherwith NICT. Since 2014, he has been with Tokai University as an AssociateProfessor. He has authored or co-authored over 167 peer-reviewed journaland conference publications. His current research interests include advancedoptical modulation formats, photonic signal processing, and optical paramet-ric amplifiers.

XUN GUAN received the B.Eng. degree from the Huazhong University ofScience and Technology, China, in 2012, and the Ph.D. degree from TheChinese University of Hong Kong, Hong Kong, in 2016. He is currently aPost-Doctoral Fellow with Université Laval, Canada. His research interestsinclude novel fiber applications, high-spectrum-efficiency optical connec-tions, and passive access networks.

TAKAHIDE SAKAMOTO (S’98–M’03) was born in Hyogo, Japan, in 1975.He received the B.S., M.S., and Ph.D. degrees in electronic engineering fromthe University of Tokyo, Tokyo, Japan, in 1998, 2000, and 2003, respec-tively. Since 2003, he has been with the Communications Research Labora-tory (now National Institute of Information and Communications Technol-ogy, NICT), Tokyo, where he is involved in the area of optical-fiber commu-nications. From 2010 to 2012, he was a Visiting Scholar with the Departmentof Electrical and Computer Engineering, University of California at Davis,Davis, supported by the Japan Society for the Promotion of Science. Heis currently a Senior Researcher with the Lightwave Devices Laboratory,Photonic Network Research Institute, National Institute of Information andCommunications Technology. His current research interests include fiber-optic devices and subsystems for optical modulation/demodulation and sig-nal processing. He is a member of the Institute of Electronics, Informationand Communication Engineering, Japan.

24606 VOLUME 5, 2017

G.-W. Lu et al.: Simple Self-Homodyne Detection Scheme for Optical OFDM With Inserted Pilot Subframes and Application

NAOKATSU YAMAMOTO received the Ph.D. degree in electrical engi-neering from Tokyo Denki University, Tokyo, Japan, in 2000. In 2001, hejoined the National Institute of Information and Communications Tech-nology (NICT). In 2008, he was with Tokyo Denki University as a Vis-iting Professor. From 2012 to 2013, he was with the Ministry of InternalAffairs and Communications as a Deputy Director. Since 2016, he hasbeen managing the Network Science and Convergence Device TechnologyLaboratory, NICT, and also has been the Director of Advanced ICT DeviceLaboratory. His current research interests include heterogeneous quantumdot laser with silicon photonics, and a convergence device technology ofphotonics and wireless. Recently, he has also been interested in the use ofa 1-µm waveband (thousand-band, T-band) as a new optical frequency bandfor short-range communications.

CALVIN CHUN-KIT CHAN (S’93–M’97–SM’04) received the B.Eng.,M.Phil., and Ph.D. degrees in information engineering from The ChineseUniversity of Hong Kong. He joined the Department of Electronic Engi-neering, City University of Hong Kong, as a Research Assistant Professorbefore he joined Bell Laboratories, Lucent Technologies, Holmdel, NJ, as aMember of Technical Staff. In 2001, he joined the Department of InformationEngineering, The Chinese University of Hong Kong, where he is currentlyserves as a Professor and the Chairman. He holds two issued U.S. patents.He has authored or co-authored over 280 technical papers in refereed inter-national journals and conferences, two book chapters, and one edited book.His research interests include optical transmission systems, access networks,and optical performance monitoring. He is a Senior Member of the OSA.

VOLUME 5, 2017 24607