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Optical Fiber Technology A radio-over-fiber system with photonic generated 16QAM OFDM signals and wavelength reuse for upstream data connection

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Optical Fiber Technology A radio-over-fiber system with photonic generated 16QAM OFDM signals and wavelength reuse for upstream data connection
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  Optical Fiber Technology 15 (2009) 222–225 Contents lists available at ScienceDirect Optical Fiber Technology www.elsevier.com/locate/yofte A radio-over-fiber system with photonic generated 16QAM OFDM signals andwavelength reuse for upstream data connection L. Chen a , b , ∗ , J. Lu a , b , J. He a , b , Z. Dong a , b , J. Yu a , b a Key Laboratory for Micro/Nano Opto-Electronic Devices of Ministry of Education, Hunan University, Changsha 410082, China b School of Computer and Communication, Hunan University, Changsha 410082, China a r t i c l e i n f o a b s t r a c t  Article history: Received 17 July 2008Revised 19 February 2009Available online 28 March 2009 Keywords: Radio-over-fiberOptical mm-waveOFDM signal We have experimentally demonstrated a wavelength reuse scheme for up-link connection in a radio-over-fiber (ROF) system with photonics generated 2.5 Gbit / s 16QAM OFDM signals. In this architecture,2.5 Gbit / s 16QAM OFDM signals are carried by the optical millimeter-wave (mm-wave) carriers whichare generated with four times frequency of the local oscillator (LO) signal. The power penalties for bothdown- and up-stream signal delivery over 20 km fiber are less than 1 dB. ©  2009 Elsevier Inc. All rights reserved. 1. Introduction The mm-wave bands would be utilized to meet the requirementfor broadband service and overcome the frequency congestion inthe future ROF-based optical-wireless network. In ROF system acenter station (CS) is connected to many functionally simple basestations (BSs) via optical fiber. Almost all processing includingmodulation, demodulation, coding, routing are performed at theCS [1–4]. The main function of the BS is to realize optical/wirelessconversion and broadcasting by antenna. Novel schemes of wave-length reuse or centralized lightwave in the central office (CO)have been proposed and experimentally demonstrated [2,5–9].Orthogonal frequency division multiplexing (OFDM) system canprovide excellent tolerance towards multipath delay spread andfrequency-dependent channel distortion. In recent research, it isdemonstrated that OFDM will become a strong candidate for trans-mission signals in the next generation long-haul and access net-works because of its high spectrum efficiency and the resistanceto chromatic dispersion and polarization mode dispersion [10–16].So the combination of OFDM and ROF is naturally suitable foroptical-wireless systems to increase the bandwidth and extend thetransmission distance of mm-wave over both fiber and air links.The generation of low-cost mm-wave for carrying OFDM signal isone of the key technologies for OFDM-ROF system [12–14]. Theoptical millimeter generation by frequency quadrupling techniquewas proposed in Ref. [9]. Because a low RF oscillator can be usedto generate optical millimeter-wave signal with frequency quadru- *  Corresponding author at: School of Computer and Communication, Hunan Uni-versity, Changsha 410082, China. E-mail address:  liliuchen12@vip.163.com (L. Chen). pling and sextupling, it has been considered a cost-effective so-lution. In this paper, we utilized a full-duplex ROF architectureas shown in Ref. [8] to transmit 2.5 Gbit / s 16QAM OFDM sig-nals on 40 GHz millimeter-wave generated by multiple double-frequency technique. The constellation diagrams of the receivedsignal before and after transmission over the fiber are obtained.Both down-stream and upstream signals transmission over 20-kmconventional single-mode fiber (SMF-28) have been experimentallydemonstrated. 2. System architecture Fig. 1 shows the principle of frequency quadrupling and wave-length reuse scheme for up-link connection in OFDM-ROF system.An intensity modulator (IM) and a cascaded optical filter are em-ployed to generate optical mm-wave and provide the lightwavesource for upstream data modulation. To realize optical mm-wavecarrier with four times of LO frequency, the IM needs to be theDC-biased at the top peak output power when the LO signal is re-moved [7]. If the repetitive frequency of the radio-frequency (RF)microwave source is f, the first-order modes are suppressed andthe frequency spacing between the second-order modes is equalto 4f. Then an optical filter is employed to separate the optical car-rier from the two second-order sidebands. The OFDM analog dataare carried by the second-order sidebands via another intensitymodulator (IM). Then the modulated mm-wave signals are com-bined with the optical carrier by using an optical coupler (OC).After transmission over the fiber, the optical mm-wave signals areseparated from the optical carrier by an optical filter. The opti-cal mm-wave signals are detected by a high-speed receiver. In thebase station, the down-converted upstream data are modulated by 1068-5200/$ – see front matter  © 2009 Elsevier Inc. All rights reserved.doi:10.1016/j.yofte.2009.02.005 转载 http://www.paper.edu.cn 国科技论文在线  L. Chen et al. / Optical Fiber Technology 15 (2009) 222–225  223 Fig. 1.  Principle diagram of wavelength reuse scheme for up-link connection in mm-wave OFDM-ROF system. Fig. 2.  Experimental setup for OFDM-ROF system. The resolution for optical spectrum insets (i)–(iii), (v) is 0.5 nm. The resolution for optical spectrum insets (iv), (vi), (vii) is025 nm. another IM before the upstream optical signals are transmitted tothe CS. 3. Experimental setup and results Fig. 2 shows the experimental setup for OFDM-ROF system. Thelightwave generated by a distributed-feedback laser diode (DFB-LD) at 1541.54 nm is modulated by an intensity modulator drivenby a 10 GHz RF microwave signal. The optical spectrum after mod-ulation is inserted in Fig. 2 as inset (i). After modulation, it iscan be seen that the odd-order sidebands are almost suppressed,and the power of optical carrier is 12 dB larger than that of thesecond-order sidebands. The wavelength spaceing between the twosecond-order sidebands is 0.32 nm (40 GHz). The fourth-ordersidebands is 20 dB lower than the second-order sidebands. An op-tical interleavers (IL) with 50 / 25 GHz channel spacing is employedto separate the optical carrier and the second-order sidebands. Weused a tunable laser with 2 nm tunable range to enable the cor-rect separation. This interleaver has a 3 dB bandwidth of 0.15 nm,therefore, the wavelength tolerance of the interleaver cannot bevery large.The spectrum of the separated carrier is shown in Fig. 2 as in-set (ii). Then the optical mm-wave is modulated by the second IMdriven by the 2.5 Gbit / s OFDM baseband signal which are gener-ated offline by Matlab program. The OFDM is baseband signal. The 中国科技论文在线 http://www.paper.edu.cn  224  L. Chen et al. / Optical Fiber Technology 15 (2009) 222–225 Fig. 3.  The constellation diagram of the demodulated signal (a) before transmission and (b) after transmission. Fig. 4.  BER curves for (a) upstream and (b) downstream data. OFDM baseband signals are calculated offline with Matlab programincluding mapping 2 15 –1 PRBS into 256 16QAM-encoded subcarri-ers, subsequently converting the OFDM symbols into time domainby using IFFT and then adding 32 pilot signal in notch. Guardinterval length is 1 / 4 OFDM period. 10 training sequences are ap-plied for each 150 OFDM-symbol frame in order to enable phasenoise compensation. The digital waveforms are then downloadedto an arbitrary waveform generator (AWG) to generate 2.5 Gbit / selectrical OFDM signal waveform. At the output of the AWG low-pass filters (LPF) with 5 GHz bandwidth are used to remove thehigh-spectral components. The optical spectrum after modulationis shown in Fig. 2 as inset (iii). The modulated optical mm-wavesignals are combined with the separated optical carrier by usinga 3-dB OC before they are transmitted over 20-km SMF-28. If wecarefully choose the fiber length which connect the OC and inter-leaver between the two separated signal, the delay between thetwo separated signals after the interleaver does not affect the sig-nal performance because the delay is very short. The optical spec-trum after OC and EDFA is shown in Fig. 2 as insets (iv) and (v),respectively. After transmission, the optical mm-wave signals areseparated from the optical carrier by using another IL. The sep-arated optical mm-wave is detected by O/E conversion via a PINPD with a 3-dB bandwidth of 60 GHz. The converted electricalsignal is amplified by an electrical amplifier (EA) with a band-width of 10 GHz centered at 40 GHz. An electrical local oscillator(LO) signal at 40 GHz is generated by using a frequency multi-plier from 10 to 40 GHz. We use the electrical LO signal and amixer to down-convert the electrical mm-wave signal to retrievethe downlink baseband signals, while the separated optical carrieris re-modulated by a 2.5 Gbit / s upstream signal. The optical spec-tra from the two ports of the second IL are shown in Fig. 2 asinsets (vi) and (vii). The eye diagram of the upstream data aftertransmission over 20 km SMF is shown in Fig. 2 as inset (viii). Thedown-converted signals are sampled with a real-time digital os-cilloscope. The received data are processed and recovered off-linewith a Matlab program as an OFDM receiver and obtain the BER performance. Fig. 3 (a) and (b) show the constellation diagram of the received signal before and after transmission over the fiber,respectively. The effect of fiber dispersion can be neglected by us-ing the electrical OFDM signals. Compared with the B-T-B case, theconstellation diagram performance is still good. We measure theBER performance for both up- and down-stream signals in Fig. 4(a) and (b), respectively. Fig. 4 shows the up- and down-streamsignal after delivery over 20 km fiber has 1 dB and 0.5 dB powerpenalty, respectively. It should be pointed out that the BER mea- 中国科技论文在线 http://www.paper.edu.cn  L. Chen et al. / Optical Fiber Technology 15 (2009) 222–225  225 surement for OFDM signal is based on off-line processing. For apractical system, a real-time processing will be needed. 4. Conclusion We have proposed and experimentally demonstrated a wave-length reuse scheme for up-link connection in a radio-over-fiber(ROF) system with photonics generated 2.5 Gbit / s 16QAM OFDMsignals. The 2.5 Gbit / s electrical OFDM signals is transmitted overthe 40 GHz optical millimeter wave signals which are generatedby using multiple double-frequency techniques. In this scheme, therepetitive frequency of the RF source and the bandwidth of opticalmodulator are largely reduced. The separated high power opticalcarrier is re-modulated in the base station; hence the all opticalpower can be efficiently utilized. The power penalty of down-stream signal delivery over 20 km fiber is less than 1 dB. The effectof fiber dispersion can be neglected by using the OFDM signals. Webelieve that this multiple double-frequency technique to generatemm-wave signal to carry OFDM signal is a practical scheme forfuture broadband ROF network.  Acknowledgments This work is partially supported by the National “863” high-tech research and development program of China under Grant2007AA01Z263, the Hunan Provincial Natural Science Foundationof China (Grant No. 06JJ50108) and the Open Fund of Key Lab-oratory of Optical Communication and Lightwave Technologies,Ministry of Education, PR China (Beijing University of Posts andTelecommunications). References [1] J. Ma, J. Yu, C. Yu, Z. Jia, G.K. Chang, The influence of fiber dispersion onthe code from of the optical mm-wave signal generated by single sidebandintensity-modulation, Opt. Commun. 271 (2006) 396–403.[2] L. Chen, H. Wen, S. Wen, A radio-over-fiber system with a novel scheme formillimeter-wave generation and wavelength reuse for up-link connection, IEEEPhoton. Technol. Lett. 19 (2006) 2056–2058.[3] J. Ma, C. Yu, Z. Zhen, J. Yu, Optical mm-wave generation by using external mod-ulator based on optical carrier suppression, Opt. Commun. 68 (2006) 51–57.[4] L. Arthur, A. Jean, Orthogonal-frequency-division multiplexing for dispersioncompensation of long-haul optical systems, Opt. Express 14 (2006) 2079–2084.[5] Z. Jia, J. Yu, G.K. Chang, A full-duplex radio-over-fiber system based on opticalcarrier suppression and reuse, IEEE Photon. Technol. Lett. 18 (2006) 1726–1728.[6] J. Yu, Z. Jia, W. Ting, G.K. Chang, A novel radio-over-fiber configuration usingoptical phase modulator to generate an optical mm-wave and centralized light-wave for uplink connection, IEEE Photon. Technol. Lett. 19 (2007) 140–142.[7] A. Kaszubowska, L. Hu, P. Barry, Remote downconversion with wavelengthreuse for the radio/fiber uplink connection, IEEE Photon. Technol. Lett. 18(2006) 562–564.[8] J. Yu, Z. Jia, T. Wang, G.K. Chang, Centralized lightwave radio-over-fiber sys-tem with photonic frequency quadrupling for high-frequency millimeter-wavegeneration, IEEE Photon. Technol. Lett. 19 (2007) 1499–1501.[9] J.J. O’Reilly, P.M. Lane, Fibre-supported optical generation and delivery of 60GHz signals, Electron. Lett. 30 (1994) 1329–1330.[10] G. Qi, J. Yao, J. Seregelyi, S. Paquet, C. Béisle, Generation and distribution of a wide-band continuously tunable millimeter-wave signal with an optical ex-ternal modulation technique, IEEE Trans. Microwave Theory Tech. 53 (2005)3090–3097.[11] J.E. Mitchell, Performance of OFDM at 5.8 GHz using radio over fiber link, Elec-tron. Lett. 40 (2004) 1353–1354.[12] W. Shieh, X. Yi, Y. Tang, Transmission experiment of multi-gigabit coherentoptical OFDM systems over 1000 km SSMF fiber, Electron. Lett. 43 (2007) 183–184.[13] H. Bao, Transmission simulation of coherent optical OFDM signals in WDM sys-tems, Opt. Express 15 (2007) 4410–4418.[14] T. Yan, S. William, Y. Xingwen, E. Rob, Optimum design for RF-to-optical up-converter in coherent optical OFDM systems, IEEE Photon. Technol. Lett. 19(2007) 483–485.[15] A. Kim, H.J. Yong, K. Yungsoo, 60 GHz wireless communication systems withradio-over-fiber links for indoor wireless LANs, IEEE Trans. Consum. Elec-tron. 50 (2004) 517–520.[16] L. Chen, J. He, Y. Li, et al., Simple ROF configuration to simultaneously realizeoptical millimeter-wave signal generation and source-free base station opera-tion, ECOC 2 (2007) 45–46. Lin Chen  was born in 1968. He received the Ph.D. degree in optical com-munications from the Beijing University of Posts and Telecommunications,Beijing, China, in June 2004. He is currently a Professor at Hunan Uni-versity, Changsha, China. He has authored or coauthored over 40 journalpapers, His current research interests include polarization mode dispersioncompensation, new modulation format techniques, and radio over fiber. 中国科技论文在线 http://www.paper.edu.cn
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