Simple Self-Homodyne Detection Scheme for Optical OFDM With Inserted Pilot Subframes and Application in Optical Access Networks

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 for Optical OFDM With Inserted Pilot Subframes and Application in Optical Access Networks GUO-WEI LU 1, 2, (Member, IEEE), XUN GUAN 3, TAKAHIDE SAKAMOTO 2, (Member, IEEE), NAOKATSU YAMAMOTO 2, (Member, IEEE), AND CALVIN CHUN-KIT CHAN 4, (Senior Member, IEEE) 1 Institute of Innovative Science and Technology, Tokai University, Hiratsuka 259-1292, Japan 2 Network Science and Convergence Device Technology Laboratory, National Institute of Information and Communication Technology, Tokyo 184-8795, Japan 3 Center for Optics, Photonics and Lasers, Université Laval, Quebec City, QC G1V 0A6, Canada 4 Department 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 and Technology, Japan, under Grant 15K06033. ABSTRACT In this paper, a simple self-homodyne detection scheme is proposed and experimentally demonstrated for applications in optical access networks. Unmodulated pilot subframes are periodically inserted and interleaved with data subframes to form an orthogonal frequency-division multiplexing (OFDM) signal. Owing to the coherence between the embedded pilot and data subframes, a Mach Zehnder delay interferometer with a free spectral range equal to the frequency of the subframe is deployed for the self-homodyne detection of the OFDM signal. It saves the local oscillator and optical hybrid, which are usually used in a conventional coherent receiver, thus reducing the hardware complexity and implementation cost. Meanwhile, the digital signal processing (DSP) for the self-homodyne detection is free from carrierfrequency-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 downstream signal. The proposed scheme provides a cost-effective and energy-efficient downstream solution for optical access networks, owing to the hardware saving and complexity reduction in DSP. A 10-Gb/s OFDM downstream transmission over a 20-km standard single-mode fiber is experimentally demonstrated with an error-free operation, using both a 10-MHz distributed feedback laser and a 100-kHz external cavity laser as downstream laser sources. INDEX TERMS Optical fiber communication, optical fiber networks, OFDM modulation. I. INTRODUCTION Recently, with the explosive multimedia-driven growth of Internet traffic, coherent detection technologies [1], [2] have been extensively investigated, to address the ever growing bandwidth demands from core networks to the edge and access networks. With the deployment of digital coherent detection in passive optical networks (PONs), coherent PONs can effectively improve the receiver sensitivity, which increases the network reach and splitting ratio to support high system capacities for ultra-dense wavelength-division multiplexing (WDM) applications [3] [5]. So far, coherent PONs have been experimentally demonstrated using either intradyne or self-homodyne coherent detection for downstream signals. However, intradyne detection requires a tunable local oscillator (LO), as well as carrier-frequency-offset (CFO) compensation and complicated carrier-phase estimation at the digital signal processing (DSP), increasing the implementation cost and complexity. Alternatively, self-homodyne detection has been proposed for optical network units (ONUs) for downstream detection by introducing an unmodulated pilot tone through polarization multiplexing [6], [7] or mode multiplexing [8] in WDM-PONs or mode-division multiplexing PONs (MDM-PONs), respectively, or by periodically inserting unmodulated carrier symbols [9] in the downstream of optical orthogonal frequency-division multiplexing PONs (OFDM-PONs). The pilot tone carried in the polarization 24602 2169-3536 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

FIGURE 1. Frame structure of the proposed self-homodyne OFDM signal. or mode and the carrier symbol carried in the time domain serve as phase references for self-homodyne detection in the ONU, which saves LO and eliminates the implementation of CFO compensation and carrier-phase estimation in DSP [6] [8]. To simplify the implementation and reduce the cost further, we propose a cost-effective self-homodyne downstream scheme for WDM-PONs. In contrast to the conventional OFDM frame, unmodulated pilot subframes are periodically inserted and interleaved with the spliced OFDM data subframes to form the downstream signal at the optical line terminal (OLT). Instead of using a conventional coherent receiver, which usually consists of an optical hybrid, an LO, and a couple of balanced photodetectors (BPDs), a single Mach Zehnder delay interferometer (MZDI) and a followed BPD are used for single-polarization self-homodyne detection at the ONU. This effectively simplifies the implementation and reduces the cost. Owing to the coherence between the embedded pilot and data subframes, the proposed scheme relaxes the linewidth requirement for the laser at the OLT, and allows the use of low-cost lasers as downstream laser sources. Moreover, as aforementioned, the DSP is simplified, since the estimation and compensation of CFO and common-phaseerror (CPE) are not required in the self-homodyne detection of OFDM. Owing to the savings in terms of both hardware and DSP, the proposed self-homodyne system provides a cost-effective and energy-efficient downstream scheme for WDM PONs. Using the proposed self-homodyne scheme, we experimentally demonstrate a 10-Gb/s self-homodyne OFDM downstream system over a 20-km standard singlemode fiber (SSMF) with a <0.3 db power penalty, which is defined with the respect to the back-to-back sensitivity at a bit-error rate (BER) of 1 10 3. The experimental results show that nearly identical performances are obtained when using a low-cost distributed feedback (DFB) laser with a 10-MHz linewidth and a 100-kHz external-cavity laser (ECL) as laser sources at the OLT, implying the tolerance of the scheme to laser linewidth. II. OPERATION PRINCIPLE Figure 1 illustrates the frame structure of our proposed selfhomodyne OFDM signal. To form the optical OFDM frame for the proposed self-homodyne system, the original OFDM frame is first sliced into subframes with a duration of τ, which is much smaller than the whole frame period, and interleaved with unmodulated pilot subframes that have the same period [10], [11], as shown in Fig. 1. Since unmodulated subframes are inserted periodically in the OFDM frame, in order to prevent errors when passing through RF circuits with capacitive coupling or transformers, it is important to produce DC-free RF driving signals when synthesizing the proposed optical OFDM frame. Therefore, the embedded unmodulated pilot subframes are encoded as (1 + i) and ( 1 i), alternately, which ensures that the DC-offset of the driving signal is close to zero. Meanwhile, since the applied alternate phase modulations have a π phase difference between adjacent pilot subframes, no additional decoding is required after the self-homodyne detection. Assuming the duration of the subframe, τ, to be shorter than the coherent time of the laser source, the coherence between successive data and pilot subframes is preserved, which enables self-homodyne detection. At the receiver side (ONU), a self-homodyne detector that consists of one MZDI and one BPD is used for detection. The MZDI has a path difference of τ, which is equal to the subframe duration, and a 45 relative phase difference between its two arms. FIGURE 2. Operation principle of the proposed self-homodyne detection. As illustrated in Fig. 2, the incoming self-homodyne OFDM data frame consists of the sliced data subframe (D1, D2...) and the interleaved unmodulated pilot subframes. The signal in the upper arm experiences one subframe relative delay and 45 relative phase shift with respect to that in the lower arm. As an example shown in Fig. 2, taking the phase of unmodulated pilot subframes as reference, the sliced data subframes (D1) in the first two successive subframe slots, experiences different relative phase shift, i.e. 45 and +45, respectively. This results in the orthogonal phase difference with respect to the pilot subframe at these two successive subframes at the output of MZDI, corresponding to the I and Q components of D1, i.e. I1 and Q1, after the interference. Through the self-homodyne detection by a single BPD, the detected orthogonal components of the OFDM signal, with a separation of τ, are interleaved and packed VOLUME 5, 2017 24603

together. In the following DSP, the I and Q components of the OFDM frame D are recovered after de-interleaving and data stitching. Therefore, instead of separately detecting the orthogonal components (I and Q) in the conventional coherent receiver using optical hybrids and two BPDs, it is sufficient to adopt one set of MZDI and BPD to demodulate the single-polarization OFDM data. To ensure stable operation in the self-homodyne detection, it is suggested to use an athermal free-space Michelson interferometer [12] as an alternative to the MZDI in practical applications, especially in PONs, since no temperature control or stabilization is required. Note that the same duration of the data and pilot subframes is chosen here to allow the deployment of a MZDI to further simplify the self-homodyne detection at the receiver side. FIGURE 4. Experimental setup. FIGURE 3. DSP flow for the conventional (solid-line and dotted-line boxes) and the proposed (solid-line boxes) OFDM systems. Figure 3 shows the deployed DSP flow for the data analysis. Since the data and pilot subframes are carried by the light from a common laser source, in contrast to the DSP used for the conventional OFDM, it is not necessary to include the estimation and compensation of CFO and CPE in the DSP. This can effectively simplify the DSP, thus reducing the complexity, power consumption, and latency of the system. Moreover, owing to the common phase noise cancellation in the self-homodyne detection based on MZDI, low-cost large-linewidth laser sources can be used at the OLT for the downstream signal. Because of the savings in hardware and DSP, the proposed scheme is suitable for cost-sensitive applications such as optical access networks. On the other hand, since the DSP is simplified by excluding the estimation and compensation of CFO and CPE, it also has potential applications in inter or intra connections in data centers because of its low latency. Compared with the conventional OFDM, by periodically inserting pilot subframes, the complexity and cost of the proposed self-homodyne system are effectively reduced at the expense of spectral efficiency. However, the scheme still holds other features of the conventional OFDM such as resilience to dispersion. The proposed self-homodyne scheme is also applicable to both singlecarrier multilevel modulation formats [11], such as 16 quadrature amplitude modulation (16QAM) and 64QAM, and multi-carrier OFDM signals with subcarriers modulated in high-order QAMs. III. EXPERIMENT AND RESULTS Figure 4 illustrates the proof-of-concept experimental setup of the proposed self-homodyne system for a WDM PON downstream. At the OLT, a continuous wave (CW) from a DFB or ECL laser emitting at 1550 nm serves as the laser source for the downstream signal, which is then fed to an optical in-phase/quadrature (IQ) modulator, which is driven by the data from an arbitrary waveform generator (AWG) operating at 10 GSamples/s. The produced optical OFDM frame consists of 200 OFDM symbols. For each symbol, 160 subcarriers are used for bearing data. In contrast to the conventional OFDM, no pilot subcarriers are inserted for phase estimation. A 256-point inverse fast Fourier transform (IFFT) is performed to transform the subcarriers to a complex time-domain signal. After that, a cyclic prefix (CP) is added. The preamble in each frame includes a pseudorandom sequence for synchronization and eight symbols for channel estimation. To construct the self-homodyne optical OFDM signal, the OFDM data frame is sliced into subframes with a period of 100 ps, and interleaved with pilot subframes, 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 fed to a self-homodyne receiver consisting of an MZDI and a BPD. The free spectral range (FSR) of the MZDI is 10 GHz. After the digitization using a real-time oscilloscope operating at 50 GSamples/s, the frame synchronization and CP removal are performed. The received payload is then transferred to the frequency domain by FFT for channel equalization. Note that the estimation and compensation of CFO and CPE are not included in the DSP. Unlike the noise-like time-domain waveforms of the conventional OFDM signals, the embedded pilot subframes have constant amplitudes. To investigate the influence of the amplitude levels of the embedded pilot subframes on the system performance, the amplitude levels of the pilot subframes 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 OFDM signals with different SFPRs (solid lines) and the conventional OFDM signal (dotted line). ratio (SFPR) as a ratio of the power of the pilot subframes to the average power of the data subframes. Any reductions in PAPR are normally illustrated using a PAPR complementary cumulative distribution function (CCDF), which is defined as the probability that the PAPR of an OFDM frame exceeds a given reference value and is the most frequently used measure for describing PAPR reduction [13]. In order to investigate the impact of SFPR on the PAPR reduction of the selfhomodyne OFDM, PAPR CCDF curves for the proposed self-homodyne OFDM signals with different SFPRs and the conventional OFDM signals without inserting pilot sub- FIGURE 6. Measured EVMs as a function of SFPR and the measured constellations at different SFPR values (a) (c). VOLUME 5, 2017 frames are calculated and shown in Fig. 5. Since the average power is enlarged with the increase of pilot subframe s power, it is clear that the increase of SFPR leads to the PAPR reduction. Compared with the conventional OFDM signals without inserted pilot subframes, a self-homodyne OFDM with a SFPR of >1 shows a lower PAPR. Around 4 db reduction in PAPR is observed for the proposed self-homodyne OFDM with a SFPR of 4.8 compared with the conventional OFDM at a CCDF of 10 3. To further investigate the influence of SFPR on the system performance, the corresponding error vector magnitudes (EVMs) are measured while tuning the SFPR from 0.2 to 6.8. As shown in Fig. 6, low EVMs can be obtained when the SFPR varies from 1 to 2.3. However, too much power of pilot subframes in the formed self-homodyne OFDM frame may degrade the signal-to-noise ratio of the embedded data subframes when the synthesized signal passes through the electrical drivers and optical amplifiers, resulting in the increase of EVMs when SFPR is larger than 3. Therefore, in the following experiment, the SFPR is set as 2. The corresponding constellations with different SFPRs are shown in Fig. 6. To evaluate the system performance, the BERs of the selfhomodyne system are measured, and are shown in Fig. 7 with FIGURE 7. The upper: measured BERs vs. received power with ECL and DFB lasers as laser sources with/without transmission over 20 km SSMF. The lower: measured constellations of OFDM signals after transmission with ECL and DFB lasers. 24605

respect to the received optical power, measured just before the MZDI at the receiver side. The back-to-back and 20-km transmission performances are evaluated by measuring the BERs using laser sources with linewidths of 100 khz (ECL) and 10 MHz (DFB), respectively. The results show that almost identical results are obtained with different laser linewidths, indicating that the self-homodyne system has tolerance against the linewidth of the laser source. This enables the use of cost-effective lasers at the OLT for the downstream signal, thus reducing the implementation cost of PON. With respect to the back-to-back BER, a power penalty of less than 0.3 db is obtained at a BER of 1 10 3 after 20-km transmission at the ONU. The constellations of self-homodyne OFDM at BERs of approximately 1 10 3 are also shown in Fig. 7 for different laser sources. The results verify the feasibility of the proposed scheme. Note that improved receiver sensitivity can be expected by using a preamplifier, highsensitivity avalanche photodiode, or forward error correction coding. IV. CONCLUSION We proposed and experimentally demonstrated a costeffective and energy-efficient OFDM downstream scheme for WDM-PONs using a simple self-homodyne system. In contrast to the conventional OFDM coding, unmodulated pilot subframes were periodically embedded into the downstream data, and interleaved with the OFDM data. At the receiver side, a simple self-homodyne detector consisting of an MZDI and a BPD was deployed to perform self-homodyne detection, without using the optical 90 hybrid or LO, reducing the hardware complexity. Since the embedded pilot subframes and the data subframes originated from the same laser source, it freed the DSP from the estimation and compensation of CFO and CPE, simplifying the DSP implementation at ONU. Besides, the proposed scheme exhibited tolerance against laser linewidth, enabling the use of cost-effective laser sources at the OLT and providing a cost-effective solution for access networks. A 10-Gb/s OFDM self-homodyne downstream transmission over a 20-km SSMF was experimentally demonstrated with an error-free operation. Similar performances were observed when both a 10-MHz DFB and a 100-kHz ECL were deployed as laser sources for the downstream data, implying a phase-noise-tolerant scheme. ACKNOWLEDGMENT G.-W. Lu would like to thank Yang Hong for useful discussions. X. Guan was with the Department of Information Engineering, 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. IEEE Photon. Conf. (IPC), vol. 1. Sep. 2013, pp. 125 126, paper MG2. [6] R. S. Luís et al., Ultra high capacity self-homodyne PON with simplified ONU 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 reflective PON 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 detection 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 downstream, 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 and 20-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, Simplified self-homodyne detection for optical OFDM with inserted pilot subsamples and its application in downstream of optical access networks, in Proc. Conf. Lasers Electro-Opt., 2017, paper STh1O.5. [12] X. Liu, A. H. Gnauck, X. Wei, J. Hsieh, C. Ai, and V. Chien, Athermal optical demodulator for OC-768 DPSK and RZ-DPSK signals, IEEE Photon. 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 power ratio reduction techniques for multicarrier transmission, IEEE Wireless Commun., vol. 12, no. 2, pp. 56 65, Apr. 2005. GUO-WEI LU (M 05) received the Ph.D. degree in information engineering from The Chinese University of Hong Kong (CUHK), Hong Kong, in 2005. From 2005 to 2006, he was a Post-Doctoral Fellow with CUHK. From 2006 to 2009, he was an Expert Researcher with the National Institute of Information and Communications Technology (NICT), Tokyo, Japan. From 2009 to 2010, he was an Assistant Professor with the Chalmers University of Technology, Göteborg, Sweden. From 2010 to 2014, he was a Researcher with NICT. Since 2014, he has been with Tokai University as an Associate Professor. He has authored or co-authored over 167 peer-reviewed journal and conference publications. His current research interests include advanced optical modulation formats, photonic signal processing, and optical parametric amplifiers. XUN GUAN received the B.Eng. degree from the Huazhong University of Science and Technology, China, in 2012, and the Ph.D. degree from The Chinese University of Hong Kong, Hong Kong, in 2016. He is currently a Post-Doctoral Fellow with Université Laval, Canada. His research interests include novel fiber applications, high-spectrum-efficiency optical connections, 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 from the University of Tokyo, Tokyo, Japan, in 1998, 2000, and 2003, respectively. Since 2003, he has been with the Communications Research Laboratory (now National Institute of Information and Communications Technology, NICT), Tokyo, where he is involved in the area of optical-fiber communications. From 2010 to 2012, he was a Visiting Scholar with the Department of Electrical and Computer Engineering, University of California at Davis, Davis, supported by the Japan Society for the Promotion of Science. He is currently a Senior Researcher with the Lightwave Devices Laboratory, Photonic Network Research Institute, National Institute of Information and Communications Technology. His current research interests include fiberoptic devices and subsystems for optical modulation/demodulation and signal processing. He is a member of the Institute of Electronics, Information and Communication Engineering, Japan. 24606 VOLUME 5, 2017

NAOKATSU YAMAMOTO received the Ph.D. degree in electrical engineering from Tokyo Denki University, Tokyo, Japan, in 2000. In 2001, he joined the National Institute of Information and Communications Technology (NICT). In 2008, he was with Tokyo Denki University as a Visiting Professor. From 2012 to 2013, he was with the Ministry of Internal Affairs and Communications as a Deputy Director. Since 2016, he has been managing the Network Science and Convergence Device Technology Laboratory, NICT, and also has been the Director of Advanced ICT Device Laboratory. His current research interests include heterogeneous quantum dot laser with silicon photonics, and a convergence device technology of photonics and wireless. Recently, he has also been interested in the use of a 1-µm waveband (thousand-band, T-band) as a new optical frequency band for 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 Chinese University of Hong Kong. He joined the Department of Electronic Engineering, City University of Hong Kong, as a Research Assistant Professor before he joined Bell Laboratories, Lucent Technologies, Holmdel, NJ, as a Member of Technical Staff. In 2001, he joined the Department of Information Engineering, The Chinese University of Hong Kong, where he is currently serves 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 international 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