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NCRAMT|June 24-26, 2011|Haldia, India International Journal of Soft Computing and Engineering (IJSCE) ISSN: 2231-2307, Volume-1, Issue-NCRAMT2011, July 2011 Third Order Nonlinear Distortion Reduction in Broadband ASE Source - Based Full-Duplex ROF Transport Systems Akinchan Das, Banibrata Bag, Hai-Han Lu, A. S. Patra* network architecture and upgrade the deployment of BS since they enable full-duplex operation on one fiber. In this paper, we proposed and demonstrated a technique to reduce third order nonlinear distortion in full-duplex radio-on-fiber (ROF) transport system which is based on broadband ASE source employing RF amplifier predistorter, wavelength-division-multiplexing (WDM), optical add-drop multiplexing techniques. A data rate of 70 Mbps/10GHz (WiMAX) signal is externally modulated with broadband ASE source by an external modulator and transmitted over 120-Km long-haul fiber link. A good eye diagram and low bit error rate (BER) values were obtained in our proposed full-duplex ROF transport systems. Abstract— A technique to reduce third order nonlinear distortion in full-duplex radio-on-fiber (ROF) transport system which is based on broadband ASE source employing RF amplifier predistorter, wavelength-division-multiplexing (WDM), and optical add-drop multiplexing techniques is proposed and demonstrated. A data stream of 70 Mbps/10GHz (WiMAX) transmitted over 120 Km long-haul fiber link and good bit error rate (BER) performances with better eye-diagram were achieved in our proposed full-duplex ROF transport system. Such proposed full-duplex ROF transport systems is suitable for long-haul microwave optical link. Index Terms— Full-duplex, optical add-drop multiplexing, radio-on-fiber, third order nonlinear distortion. II. EXPERIMENTAL SETUP I. INTRODUCTION Fig. 1 shows the schematic architecture of our proposed full-duplex ROF transport system. For down-link transmission, the CS is composed of a microwave signal generator, a broadband ASE source, an external modulator, Arrayed waveguide gratings (AWG), and two erbium-doped fiber amplifiers (EDFAs). At CS, a 70 Mbps data stream is mixed with 10 GHz microwave carrier to generate data signal. The resulting microwave data signal is externally modulated with a broadband ASE source. The modulated signals are demultiplexed by AWG to select the appropriate wavelengths. To avoid the crosstalk between the channels of AWG, we choose the signals from the channels 1, 3, 5 and 7 of AWG and multiplexed them by another AWG. The central wavelengths from the channels of AWG are 1549.96 nm ( λ1), 1550.76 nm (λ2), 1551.56 nm (λ3) and 1552.36 nm (λ4), respectively. In order to transmit optical wavelengths over a long-haul fiber link, the optical power is amplified by EDFA and fed into fiber backbone. Signal is generated at the CS and then distributed to the remote BSs by using cascaded EDFAs. All the EDFAs have an output power of ~ 16 dBm and a noise figure of ~ 4.3 dB at an input power of 0 dBm. Each BS deals with by individual wavelength for an optical add-drop multiplexer (OADM) of an in-out insertion loss of 4.2 dB and >40 dB add/drop channel isolation. The down-link optical carrier is dropped to the BS, and the up-link optical carrier is added to the fiber backbone by OADM. The 70-Mbps down-link data signal is detected by photodiode (PD), passes through RF amplifier predistorter, demodulated and lastly fed into a BER tester for BER analysis at the BSs. The up-link data signal is added to the fiber backbone and transmitted to the CS. At CS, the up-link data signal is passing through an optical tunable band-pass filter (TBPF) to select the desired wavelength and the optical power level of the signal is adjusted by a variable A s the high speed optical access network technology are being rapidly developed, the interests and demands are increasing in the area of Radio-on-fiber (ROF) transport system. ROF transport system, the integration of optical fiber and wireless systems [1], is the most acceptable solution to increase the capacity and mobility of future communications with low cost because of broad bandwidth and low attenuation characteristics [2, 3]. ROF transport system is capable to simplify the configuration of the transport system due to its flexibility to link the central station (CS) and the base stations (BSs) [4, 5]. For long-distance transmission, the nonlinear distortion takes a vital role to degrade the system performances. In order to improve system performances, it is necessary to use techniques and schemes to mitigate the nonlinear distortions [6, 7]. Furthermore, to realize full-duplex ROF transport systems, a simple and cost-effective architecture is required. Considering the architecture of ROF transport systems, Wavelength-division-multiplexing (WDM) and optical add-drop multiplexing techniques can simplify the Manuscript received June 10, 2011. Akinchan Das, Department of Electronics and Communication Engineering, Haldia Institute of Technology, Haldia, India (e-mail: [email protected]). Banibrata Bag, Department of Electronics and Communication Engineering, Haldia Institute of Technology, Haldia, India (e-mail: [email protected]). Wei-Yi Lin, Institute of Electro-Optical Engineering, National Taipei University of Technology, Taiwan. Hai-Han Lu, Institute of Electro-Optical Engineering, National Taipei University of Technology, Taiwan, A.S. Patra, Department of Applied Sciences, Haldia Institute of Technology, Haldia, India (e-mail: [email protected]). *Corresponding author 31 Third Order Nonlinear Distortion Reduction in Broadband ASE Source - Based Full-Duplex ROF Transport Systems λ 1 λ Broadband ASE Source External Modulator EDFA 2 AWG MUX/DEMUX λ λ OADM EDFA 40km SMF 3 EDFA 40km SMF λ 2 λ1 λ1 EDFA OADM λ2 4 10GHz/70Mbps BS1 BS2 40km SMF BER Tester RF Amplifier Predistorter Demodulator PD EDFA VOA CS Optical TBPF 40km SMF OADM λ4 λ4 EDFA 40km SMF BS4 OADM λ3 λ3 BS3 Fig. 1. The schematic diagram of our proposed full-duplex ROF systems RF IN + RF Splitter A1 Delay Line 2nd order Nonlinearity Generator RF Combiner appropriate nonlinear coefficient to eliminate the third order nonlinear term: a 1. b 3 = - a 3. b 1 (4) Then equation (3) can be changed as v0 = (a1. b1).m (5) From equation (5), it is clear that, third order nonlinear distortion can be eliminated by proper adjusting the nonlinear coefficient. Only linear term is left in equation (3), the proposed RF amplifier predistorter is employed to cancel out the third order nonlinear distortion. RF OUT – A2 Attenuator Fig.2. A schematic diagram of the RF amplifier predistorter. optical attenuator (VOA). The received optical signal is detected by a broadband PD and fed into RF amplifier predistorter. The signal is also fed into a BER tester for BER analysis after demodulation. The architecture for RF amplifier predistorter is shown in Fig. 2. An appropriate nonlinear co-efficient is used to cancel out the third order nonlinear distortions. III. EXPERIMENTAL RESULTS AND DISCUSSION The optical spectrum of broadband ASE source and the output of multiplexed signals are shown in Fig. 3(a). The optical spectrum has a good free spectral range of 15 nm. We selected the optical wavelengths λ1, λ2, λ3 and λ4 from the odd channels of AWG to avoid the crosstalk among the channels. The optical spectrum for the selected wavelengths is illustrated in Fig. 3(b). A schematic diagram of the RF amplifier predistorter is illustrated in Fig. 2. The RF amplifier predistorter output in ROF band is given by v0 = a1.vi + a3. vi3 (1) where v0 is the output voltage, vi is the input voltage, and a1, a3 are the amplitude coefficients (a3 is coefficient characterize nonlinearity). The third order nonlinear distortion in ROF transport systems can be expressed as q = b1.m + b3. m3 (2) where q is system’s output voltage detected from PD, m is system’s input voltage, and b1, b3 are the amplitude coefficients (b3 is coefficient characterize nonlinearity). q is equal to vi, substituting equation (2) into equation (1) and neglecting higher order nonlinear terms, then yields v0 = (a1. b1).m + (a1. b3 + a3. b1). m3(3)In ROF transport systems, IMD3 is the major nonlinear distortion induced in the system. While achieving linearity means cancelling out the nonlinear terms, a RF amplifier predistorter would have to cancel out the third order nonlinear term. Setting the Fig. 3 (a). The spectrum of broadband ASE source and the multiplexed optical signals. Fig. 3 (b). The optical spectrum of selected Wavelengths for transmission. The down-link BER curves of 10-GHz (f1)/70-Mbps transmitted data signal from CS to BS3 (120 km fiber link) are measured with and without the RF amplifier predistorter and are presented in Fig. 4(a). For a BER of 10-9; the received 32 NCRAMT|June 24-26, 2011|Haldia, India International Journal of Soft Computing and Engineering (IJSCE) ISSN: 2231-2307, Volume-1, Issue-NCRAMT2011, July 2011 optical power is -7.6 dBm with the RF amplifier predistorter; and without the predistorter, the received optical power is -2.4 dBm. Due to the employment of RF amplifier predistorter, the receiver sensitivity is improved by 5.2 dB. And further, the measured up-link BER curves of 10-GHz (f1)/70-Mbps data channel from BS2 to CS (120 km fiber link) are presented in Fig. 4(b). For a BER of 10-9; with and without the RF amplifier predistorter, the received optical powers are -7.3 dBm and -2.2 dBm respectively. A 5.2 dB receiver sensitivity is improved when the RF amplifier predistorter technique is employed. IV. CONCLUSION We have proposed a novel technique to reduce third order nonlinear distortion in full-duplex radio-on-fiber (ROF) transport system which is based on broadband ASE source employing RF amplifier predistorter, wavelength-division-multiplexing (WDM), optical add-drop multiplexing techniques. We have experimentally observed and demonstrated the feasibility of our proposed system with WiMAX signal and obtained low BER values with very good eye-diagram over a long-haul fiber link. Our proposed full-duplex ROF transport systems have the potential to transmit signal over long-haul microwave optical link. with RF Amplifier Predistorter (λ3 ) without RF Amplifier Predistorter (λ3 ) 10 -5 Down-Link BER 10 -6 10 -7 10 -8 10 -9 10 -10 10 (a) -11 -10 -8 -6 -4 -2 0 Received Optical Power ( dBm ) Fig.4.a. The experimental BER for down-link transmission. with RF Amplifier Predistorter (λ2 ) without RF Amplifier Predistorter (λ2) 10 -5 Up-Link BER 10 -6 (b) Fig. 5. The eye-diagram for 120 Km SMF transmissions (a) with and (b) without RF amplifier predistorter. 10 -7 10 -8 10 -9 ACKNOWLEDGMENT 10 -10 10 -11 -10 The authors would like to thank Haldia Institute of technology and National Taipei University of technology. -8 -6 -4 -2 0 Received Optical Power ( dBm ) Fig. 4.b. The experimental BER for up-link transmission. REFERENCES [1] H. H. Lu, A. S. Patra, W. J. Ho, P. C. Lai, M. H. Shiu, “A full-duplex More than 5 dB BER curve power penalty improvement is obtained for both the uplink and downlink transmissions when the RF amplifier predistorter scheme is implemented. RF power degradation improvement is observed for this technique. So this scheme reduces the RF power degradation, causing system with higher SNR value, finally leading to an improvement BER performance. Cross talk from down-link optical signal to up-link optical signal may influence the BER performance because 10 GHz carrier is used both at CS and BS in the experiment. However, since the OADM has a high add/drop channel isolation (>40 dB), such a high channel isolation characteristic provides excellent add/drop ability to prevent crosstalk. The clear eye-diagrams of RF data signal with and without the RF preamplifier predistorter are observed in our experiment and shown in Fig. 5(a) and (b) respectively. radio-over-fiber transport system based on FP laser diode with OBPF and optical circulator with fiber bragg grating,” IEEE Photon. Technol. Lett., vol. 19, pp. 1652–1654, 2007. [2] C. T. Lin, J. Chen, P. C. Peng, C. F. Peng, W. R. Peng, B. S. Chiou, S. Chi, “Hybrid optical access network integrating fiber-to-the-home and radio-over-fiber systems,” IEEE Photon. Technol. Lett., vol. 19, pp. 610-612, 2007 [3] P. C. Won, and J. A. R. Williams, “Third-order intermodulation products generated on transmission through nonlinear radio-on-fiber link,” Electron. Lett., vol. 40, pp. 1290–1291, 2004. [4] M. G. Larrode, A. M. Koonen, J. J. V. Olmos, and A. Ng’Oma, “Bidirectional radio-over-fiber link employing optical frequency multiplication,” IEEE Photon. Technol. Lett., vol. 18, no. 1, pp. 241–243, Jan. 1, 2006. [5] H. H. Lu, A. S. Patra, W. J. Ho, P. C. Lai, M. H. Shiu, “A Full-Duplex Radio-Over-Fiber Transport System Based on FP Laser Diode With OBPF and Optical Circulator With Fiber Bragg Grating,” IEEE Photon. Technol. Lett., vol. 19, no. 20, pp. 1652–1654, Oct.15, 2007. 33 Third Order Nonlinear Distortion Reduction in Broadband ASE Source - Based Full-Duplex ROF Transport Systems [6] Y. Okamoto, R. Miyamoto, and M. Yasunaga, “Radio-on-fiber access network systems for road-vehicle communication,” in Proc. IEEE Intell. Transport. Syst. Conf., 2001, pp. 1050–1055. [7] H. H. Lu, S. J. Tzeng, and Y. L. Liu, “Intermodulation Distortion Suppression in a Full-Duplex Radio-on-Fiber Ring Network” IEEE Photon. Technol. Lett., vol. 16, no. 2, pp. 602–604, Feb., 2004. Akinchan Das has completed B.Sc. with Electronics (Hons) from Vidyasagar University in 2005, M.Sc. with Electronics from Vidyasagar University in 2007, and M.Tech. with Modern Communication Engineering Under WBUT (West Bengal University of Technology)in 2009.Curently working as an Assistant Professor(from 2010) in the Dept. of Electronic and Communication Engineering, Haldia Institute of Technology, Haldia , WB, India. His research interest includes Wireless networking and Optical fiber communication. He is working towards his Ph.D. degree. Banibrata Bag has completed B.E. in Computer Science and Engineering from Burdwan University in 2004 and M.Tech. Under WBUT (West Bengal University of Technology) in 2009. He has worked as a software Developer (from 2005 to 2010), and currently working as an assistant professor (from 2010) in the Dept. of Electronic and Communication Engineering, Haldia Institute of Technology, Haldia, WB, India. His research interest includes networking, Optical fiber communication and processor architecture. He is a professional member of ACM. He is working towards his Ph.D. degree. Prof. Hai-Han Lu was born in Taipei, Taiwan, on June 1963. He received the B.S. in the Electro-Physics Department of the National Chiao-Tung University, Taiwan, in 1987. He received the M.S. and Ph.D. degree in the Institute of Optical Sciences of the National Central University, Taiwan, in 1991 and 2000, respectively. He joined the Institute of Electro-Optical Engineering, National Taipei University of Technology as an Associate Professor in 2001. His current research interests include the applications of the lightwave communication and hybrid DWDM systems. Dr. Ardhendu Sekhar Patra is working as an Assistant professor at Department of Applied Sciences, Haldia Institute of Technology, Haldia, India. He received his Ph.D. degree from Indian Institute of Technology. He has worked as a post doctorate fellow at Institute of Electro-optical Engineering, National Taipei University of Technology, Taiwan and Center for semiconductor technology and Optoelectronics (ZHO), University of Duisburg-Essen, Duisburg, Germany. His current research interests include the applications of the lightwave communication and WDM-PON systems. 34