A Full-duplex OSSB Modulated ROF System with Centralized Light Source by Optical Sideband Reuse

Transcription

1 A Full-duplex OSSB Modulated ROF System with Centralized Light Source by Optical Sideband Reuse Fangzheng Zhang 1, Tingting Zhang 1,2, Xiaozhong Ge 1 and Shilong Pan 1,* 1 Key Laboratory of Radar Imaging and Microwave Photonics, Ministry of Education Nanjing University of Aeronautics and Astronautics, Nanjing , China. 2 National Laboratory of Microstructures and School of Engineering and Applied Sciences, Nanjing University, Nanjing , China. *pans@ieee.org Abstract A full-duplex optical single sideband (OSSB) modulated radio-over-fiber (ROF) system with centralized light source is proposed and demonstrated. The proposed system can realize light source centralization based on optical sideband reuse, which not only avoids the use of a specific light source in the base station (BS) but also reduces signal distortion due to bidirectional fiber transmission. Using a simple optical filtering scheme in the BS, OSSB modulations for both downstream and upstream signals are achieved, which solves the power fading problem induced by fiber dispersion. In addition, no frequency up/down-conversion is required in the BS, leading to a simplified configuration. An experiment is carried out. Performance of the established 18 GHz full-duplex ROF system with bidirectional transmission through 20 km single mode fiber (SMF) is investigated. The results can verify that the proposed architecture is a good candidate for future high-speed and low-cost ROF networks. Keyworks Radio over fiber (ROF), centralized light source, optical single sideband (OSSB) modulation, bidirectional fiber transmission. I. INTRODUCTION The ever-increasing mobile multimedia services need large bandwidths and high data rates, which has greatly promoted the recent development of wireless access technologies [1]. Radio-over-fiber (ROF), taking advantage of the optical fiber technologies such as low loss, large bandwidth and low cost, has been considered as a powerful solution for increasing the capacity and mobility as well as decreasing the cost for different wireless access environments such as conference centers, airports, and small offices, etc. [2-4]. To meet the requirement for larger bandwidth and higher data rates, signals in higher frequency bands are expected to be used, e.g., license free transmission bandwidth of ~7 GHz near 60 GHz has been permitted [5]. However, the high atmospheric attenuation in This work was supported in part by the National Natural Science Foundation of China ( ), the National Basic Research Program of China (2012CB315705), the Open Fund of IPOC (BUPT) (IPOC2013B003), the Natural Science Foundation of Jiangsu Province (BK ), the Fundamental Research Funds for the Central Universities (NJ , NE , NP ), the project sponsored by SRF for ROCS, SEM, the Postdoctoral Science Foundation of China (2014M550290), the Jiangsu Planned Projects for Postdoctoral Research Funds ( B) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. such frequency bands results in a small coverage area for each base station (BS). Thus, many full-duplex BSs are necessary to cover one specific access area, which requires a BS with low cost and flexible management. In this situation, it is highly desirable to avoid the use of a specific light source in each BS by applying the ROF configuration with the light sources centralized in the central office (CO) [6]. Many techniques have been proposed to realize light wave centralized systems [6-13]. For example, the intensity modulated downstream optical signal can be erased based on gain saturation in a reflective semiconductor optical amplifier (RSOA), and the obtained quasi continuous wave (CW) light is modulated by the upstream signal [6-9]. However, such systems are usually power inefficient because a low extinction ratio of the downstream signal is required to effectively remove the downstream information [14]. Another kind of method to realize centralized light source in an ROF system, is to reuse one of the optical carriers from the downstream signal as the upstream light source [10-12]. The reused carrier can be the original CW light carrier [10, 11] or an optical sideband generated by modulating a CW light at a Mach-Zehnder modulator (MZM) [12, 13]. Then, the upstream signal can be modulated onto this optical carrier by another electro-optical modulator (EOM). The main drawback of the schemes in [10-13] is that at least two EOMs are used in the CO to generate the optical sidebands as well as to load the downstream signal, which is not preferred considering the high complexity and cost. On the other side, fiber dispersion induced power fading is always a problem in ROF systems, which can be resolved by using the optical single sideband (OSSB) modulation technique [15, 16]. Therefore, it is preferred to implement OSSB modulations for both the downstream and upstream signals in a light source centralized ROF system. In this paper, we propose and experimentally demonstrate a full-duplex ROF system with centralized light source based on optical sideband reuse, which can also realize OSSB modulations for both downstream and upstream signals. In the proposed system, only a single dual-parallel MZM (DPMZM) is applied in the CO to generate two first order sidebands and modulate the optical carrier by the downstream data. In the BS, the optical carrier and one of the first order sidebands are filtered by a wavelength division multiplexer (WDM) to obtain an OSSB modulated downstream signal, which is then used to generate the downstream RF signal. The other optical sideband is used as the upstream light source. After electro-optical 146

2 Fig.1 Schematic diagram of the proposed full-duplex ROF system with centralized light source and bidirectional fiber transmission. LD: laser diode, MZM: Mach-Zehnder modulator, OC: optical circulator, PD: photo-detector, EDFA: erbium-doped optical fiber amplifier, SMF: single mode fiber. modulation and passing through the same WDM, an OSSB modulated upstream signal is obtained and transmitted to the CO. The proposed ROF system can avoid the fiber dispersion induced power fading thanks to the OSSB modulations. Furthermore, the proposed system has a simple configuration to centralize the light source in the CO, and no frequency up/down conversion is needed in the BS. Based on the proposed architecture, a full-duplex ROF system with bidirectional transmission through 20 km single mode fiber (SMF) is established. The downstream signal is a 2 Gb/s pseudo-random binary sequence (PRBS) data carried by an 18 GHz RF carrier, and the upstream signal is a 50 Mbaud 16-QAM data also carried by an 18 GHz carrier. The system performance is investigated through bit error rate (BER) and error vector magnitude (EVM) measurements, and the results can verify the feasibility of the proposed ROF system. II. SYSTEM SETUP AND OPERATION PRINCIPLE Figure 1 shows the schematic diagram of the proposed full-duplex ROF system, where the optical or electrical spectra at different points are schematically shown for ease of understanding. In the CO, a CW light from a laser diode (LD) is modulated by a DPMZM, which is a nested MZM consisting of two sub-mzms (MZM1 and MZM2) in parallel. MZM1 is biased at its minimum transmission point to achieve optical carrier-suppressed modulation [17]. When a single-tone RF carrier is applied to MZM1, two first order sidebands are generated at the frequencies of f 0 -f and f 0 +f, respectively, where f 0 is the optical carrier frequency and f is the RF carrier frequency. At MZM2, the electrical baseband signal to be transmitted is modulated onto the optical carrier by biasing MZM2 at its linear transmission point. Then, the data-bearing optical carrier and the two first order sidebands are combined at the output of the DPMZM. The obtained signal is passed through an optical circulator (OC1) and then transmitted to the base station (BS) through the fiber. At the BS, the received downstream signal is sent to a WDM after passing two optical circulators (OC2 and OC3). It should be noted that OC2 and OC3 are inserted to measure the upstream optical signal in the experimental, and they can be removed in practice. By carefully choosing the wavelength of the CW light, the optical carrier with one of the first order sidebands can be filtered at output port1 of the WDM while the other optical sideband is filtered at output port2 of the WDM, as shown in Fig. 1. The output signal from port1 is an OSSB modulated signal, which is sent to a photo-detector (PD1) for frequency heterodyning. After PD1, an RF signal carrying the baseband data is generated with a carrier frequency of f, which is radiated to the end-user through the antenna. At port2, the filtered first order sideband is used as the upstream light source and sent to a broadband MZM through another optical circulator (OC4). The upstream RF signal (also centered at f) received by the antenna can be applied to the MZM to modulate the upstream light source without frequency down-conversion. Then, the MZM output signal is amplified by a erbium-doped optical fiber amplifier (EDFA) and sent back to port2 of the WDM by OC4. After the WDM, the optical sideband at frequency f 0 is removed, obtaining an OSSB modulated upstream signal. This OSSB signal is fed to the fiber after OC3 and OC2, and then transmitted to the CO. In the CO, the upstream signal is detected by another PD (PD2) and demodulated at the upstream receiver (Rx). It should be noted that, the maximum RF carrier frequency allowed in the system is determined by the bandwidth of each output port of the WDM. While, the minimum RF carrier frequency is related to the transmission spectrum of the WDM, i.e., the edge slop of the WDM transmission spectrum should be sharp enough to separate the optical carrier and the modulation sideband. III. EXPERIMENTAL DEMONSTRATION To investigate the performance of the proposed ROF system, an experiment is carried out based on the setup shown in Fig. 1. In the experiment, an 18 GHz (f=18 GHz) full-duplex ROF system with bidirectional transmission through 20 km single mode fiber (SMF) is established. In the CO, the CW light source is at nm and has a power of 12 dbm. It is modulated by a DPMZM (Fujitsu FTM7962EP) which has a 3-dB bandwidth of 28 GHz. An 18 GHz RF carrier generated by a microwave signal source (Agilent 8257D) is applied to one RF port of the DPMZM to achieve optical carrier suppressed modulation by biasing the corresponding sub-mzm (MZM1) at the minimum transmission point. To investigate the carrier suppression properties, the other sub-mzm (MZM2) is also biased at the minimum transmission point to generate an optical carrier suppressed spectrum at the DPMZM output. Fig. 2(a) shows the 147

3 optical spectrum measured by an optical spectrum analyzer (OSA) with a resolution of 0.02 nm. It is seen that, the optical carrier is suppressed by up to 20 db compared with the two first order sidebands which are apart from the optical carrier by ±18 GHz, respectively. Then, MZM2 is biased at its linear transmission point, and the downstream baseband data, which is a 2 Gb/s PRBS signal with a pattern length of , is applied to MZM2. At the DPMZM output, an optical carrier bearing the 2 Gb/s PRBS data and two first order sidebands are obtained, with the spectrum shown in Fig. 2(b). This optical signal, having a power of 1.5 dbm, is fed to a 20 km SMF for downstream transmission. At the BS, the downstream optical signal sent to the WDM has a power of -5.3 dbm. The WDM has a bandwidth of 50 GHz and an edge slop of 190 db/nm for each channel. In the experiment, the wavelength of the CW light in the CO has been carefully chosen such that the optical carrier and one of the first order sidebands pass through port1 of the WDM, while the other first order sideband is obtained at port2 of the WDM. Figs. 2(c) and (d) show the optical spectra measured at port1 and port2 of the WDM, respectively. As can been seen, at port1, the optical carrier and one of the first order sidebands are obtained with the other sideband well suppressed. At port2, the residual optical carrier is suppressed by ~33 db compared to the filtered first order sideband. When the optical signal from port1 is sent to a 20 GHz PD (PD1), an 18 GHz RF signal carrying the 2 Gb/s baseband data is generated. Meanwhile, the optical sideband from port2 is sent to a 40 GHz MZM (Fujitsu FTM7938EZ-A) and intensity modulated by the upstream signal, which is an 18 GHz RF signal bearing a 50Mbaud 16-QAM data generated from a vector signal generator (Agilent E8267D). The obtained optical signal from the MZM is boosted by an EDFA. Fig. 2(e) shows the measured spectrum of the optical signal after the EDFA. When this optical signal passes through the WDM again, one of the sideband at nm is removed, and an OSSB modulated upstream signal is generated. This OSSB modulated signal is sent to the fiber for upstream transmission after passing OC3 and OC2, respectively. The optical power of the upstream signal is measured to be -1 dbm and its spectrum is shown in Fig. 2(f), through which the OSSB modulation can be observed with the unwanted sideband suppressed by about 40 db. In the CO, the received upstream optical signal has a power of -8 dbm, and it is converted to the electrical domain at another 20 GHz PD (PD2). Fig. 3(a) shows the electrical spectrum of the downstream signal after PD1, which is measured by a signal analyzer (Agilent N9030A). As can be seen, an RF signal centered at 18 0 Fig. 2 Measured optical spectra of (a) the carrier suppressed signal, (b) the signal after DPMZM, (c) the signal at port1 of the WDM, (d) the signal at port2 of the WDM, (e) the signal after the EDFA, and (f) the OSSB modulated upstream signal Fig. 3 Measured electrical spectra of (a) the 18 GHz downstream stream signal carrying a 2 Gb/s PRBS data, (b) the demodulated 2 Gb/s baseband signal and (c) the 18 GHz upstream signal carrying a 50Mbaud 16-QAM data. 148

4 GHz and carrying a 2 Gb/s baseband data is generated. Due to the hardware constraints, wireless transmission is not implemented in the experiment. To investigate the fiber transmission performance, the downstream 18 GHz RF signal from PD1 is down-converted by mixing with an 18 GHz local oscillator (LO) and the baseband signal is filtered by an electrical low-pass filter having a 3-dB bandwidth of 2.24 GHz. The obtained electrical spectrum is shown in Fig. 3(b). To further evaluate the system performance, BER measurements of the demodulated 2 Gb/s baseband data are implemented. The results for both back-to-back and 20 km transmission systems are shown in Fig. 4, where the eye diagrams of the demodulated baseband 2 Gb/s signal under certain conditions are also provided. It is known from Fig. 4, for back-to-back condition, the receiver sensitivity at BER of 1x10-9 is dbm. After 20 km SMF transmission, the receiver sensitivity is.5 dbm when the upstream signal is not launched to the fiber, indicating the unidirectional fiber transmission causes a power penalty of 1.1 db. When bidirectional fiber transmission is enabled, the receiver sensitivity is slightly degraded to be.1 dbm. In the proposed ROF system, the downstream OSSB modulated signal at port1 of the WDM has no spectral overlap with the upstream OSSB modulated signal, thus the downstream signal distortion due to backward Rayleigh scattering (RS) is wakened, and a very small power penalty (i.e., 0.4 db) is observed compared with the unidirectional transmission system. Log (BER) Back to back 20km without upstream 20km with upstream confirmed that for both the downstream and upstream signals, despite of the slight degradation caused by bidirectional fiber transmission, good system performance is still achieved based on the proposed ROF architecture upstream back to back upstream 20km Fig. 5 Measured EVM vs. received optical power for the upstream signal (insets: constellations of the demodulated 16-AQM signal). IV. CONCLUSIONS We have proposed a full-duplex OSSB modulated ROF system with centralized light source based on optical sideband reuse. In the proposed ROF system, only one DPMZM is used in the CO and no light source and frequency up/down-conversion are needed in the BS, which leads to a simple structure with low complexity and cost. Besides, fiber dispersion induced power fading problem can be solved by the proposed ROF system since OSSB modulations are implemented for both downstream and upstream signals. Performance of the proposed ROF system is investigated experimentally for an 18 GHz full-duplex ROF system with bidirectional transmission through 20 km SMF. The results can confirm the good performance of the proposed technique, which may find wide applications in future light wave centralized high-speed ROF systems. Fig. 4 BER vs. received optical power for the 2 Gb/s downstream signal (insets: eye diagrams of the demodulated 2Gb/s baseband signals) Then, the recovered upstream 18 GHz RF signal carrying the 50 Mbaud 16-QAM data is analyzed by the signal analyzer. Fig. 3(c) shows the electrical spectrum, which is centered at 18 GHz with a bandwidth of 50 MHz. The EVM values at different received optical power are measured with each measurement employing 2000 symbols to calculate the EVM. The results are shown in Fig. 5, where several constellations of the demodulated 50 Mbaud 16-QAM data are also provided for comparison. In Fig. 5, as the received optical power increases from -33 dbm to dbm, the EVM gets smaller for both the back to back system and the 20 km transmission system, since a higher optical power corresponds to a higher signal-to-noise ratio (SNR). For back-to-back system, the minimum EVM reaches 3.9%, while the minimum EVM is 5.1% after 20km upstream transmission. From the experimental results, it can be REFERENCES [1] Eizmendi, G. Prieto, G. B. Eriz, I. Pena, M. M. Velez, P. Angueira, Next generation of broadcast multimedia services to mobile receivers in urban environments, Signal Process-Image Communication, vol. 27, no. 10, pp , [2] N. Ghazisaidi and M. Maier, Fiber-wireless (FiWi) access networks: Challenges and opportunities, IEEE Network, vol. 25, no. 1, pp , [3] J. Zhang, J. Yu, N. Chi, Z. Dong, X. Li, and G.-K. Chang, Multichannel 120-Gb/s data transmission over 2x2 MIMO fiber-wireless link at W-band, IEEE Photon. Technol. Lett., vol. 25, no. 8, pp , [4] D. Wake, A. Nkansah, and N. J. Gomes, Radio over fiber link design for next generation wireless systems, J. Lightw. Technol., vol. 28, no. 16, pp , [5] IEEE cWorking Group Homepage. [Online] Available: [6] Y. Y. Won, H. C. Kwon, and S. K. Han, 1.25-Gb/s wavelength-division multiplexed single-wavelength colorless radio-on-fiber systems using reflective semiconductor optical amplifier, J. Lightwave Technol., vol. 25, no. 11, pp ,

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