A 10-Gbps optical WiMAX transport system

Transcription

1 A 1-Gbps optical WiMAX transport system Ying-Pyng Lin, Hai-Han Lu,* Po-Yi Wu, Chia-Yi Chen, Tai-Wei Jhang, Sheng-Siang Ruan, and Kuan-Hung Wu Institute of Electro-Optical Engineering, National Taipei University of Technology, Taipei, 16, Taiwan Abstract: A 1-Gbps optical worldwide interoperability for microwave access (WiMAX) transport system employing vertical cavity surface emitting laser (VCSEL) and spatial light modulator (SLM) with 16- quadrature amplitude modulation (QAM)-orthogonal frequency-division multiplexing (OFDM) modulating signal is proposed. With the assistance of equalizer and low noise amplifier (LNA) at the receiving site, good bit error rate (BER) performance, clear constellation map, and clear eye diagram are achieved in the proposed systems. An optical WiMAX transport system, transmitting 16-QAM-OFDM signal over a 6-m free-space link, with a data rate of 1 Gbps is successfully demonstrated. Such a 1-Gbps optical WiMAX transport system would be attractive for providing services including Internet and telecommunication services. Our proposed system is suitable for the free-space lightwave transport system in visible light communication (VLC) application. 14 Optical Society of America OCIS codes: (6.65) Free-space optical communication; (3.61) Spatial light modulators; (5.76) Vertical cavity surface emitting lasers. References and links 1. H. T. Lin, C. L. Lai, and Y. C. Huang, Dynamic bandwidth allocation with QoS support for integrated EPON/WiMAX networks, IEEE 14th International Conf. on High Performance Switching and Routing (HPSR) (13).. N. Cvijetic and T. Wang, WiMAX over free-space optics - evaluating OFDM multi-subcarrier modulation in optical wireless channels, IEEE Sarnoff Symposium 1 4 (6). 3. S. I. Chakchai, R. Jain, and A. K. Tamimi, Scheduling in IEEE 8.16e mobile WiMAX networks: key issues and a survey, IEEE J. Sel. Areas Comm. 7(), (9). 4. F. M. Wu, C. T. Lin, C. C. Wei, C. W. Chen, Z. Y. Chen, and H. T. Huang, 3.-Gb/s WDM visible light communication of a single RGB LED employing carrier-less amplitude and phase modulation, Conf. on Opt. Fiber Commun. (OFC) OTh1G4 (13). 5. Y. Wang, Y. Wang, N. Chi, J. Yu, and H. Shang, Demonstration of 575-Mb/s downlink and 5-Mb/s uplink bi-directional SCM-WDM visible light communication using RGB LED and phosphor-based LED, Opt. Express 1(1), (13). 6. C. H. Yeh, Y. F. Liu, C. W. Chow, Y. Liu, P. Y. Huang, and H. K. Tsang, Investigation of 4-ASK modulation with digital filtering to increase times of direct modulation speed of white-light LED visible light communication system, Opt. Express (15), (1). 7. C. Y. Chen, P. Y. Wu, H. H. Lu, Y. P. Lin, J. Y. Wen, and F. C. Hu, Bidirectional 16-QAM OFDM in-building network over SMF and free-space VLC transport, Opt. Lett. 38(13), (13). 8. W. Y. Lin, C. Y. Chen, H. H. Lu, C. H. Chang, Y. P. Lin, H. C. Lin, and H. W. Wu, 1m/5 Mbps WDM visible light communication systems, Opt. Express (9), (1). 9. Y. C. Chi, Y. C. Li, H. Y. Wang, P. C. Peng, H. H. Lu, and G. R. Lin, Optical 16-QAM-5-OFDM transmission at 4 Gbit/s by directly modulating a coherently injection-locked colorless laser diode, Opt. Express (18), (1). 1. W. I. Way, Broadband Hybrid Fiber/Coax Access System Technologies (Academic, 1999), pp H. Henniger and O. Wilfert, An introduction to free-space optical communications, Radioengineering 19(), 3 1 (1). 1. J. Carpenter, B. C. Thomsen, and T. D. Wilkinson, Degenerate mode-group division multiplexing, IEEE /OSA, J. Lightwave Technol. 3(4), (1). 1. Introduction Optical worldwide interoperability for microwave access (WiMAX), in which combining the advantages of optical communication and wireless communication systems, has provided a (C) 14 OSA 1 February 14 Vol., No. 3 DOI:1.1364/OE..761 OPTICS EXPRESS 761

2 very promising candidate for free-space lightwave transport systems due to its high-speed data rate characteristic [1,]. Based on the IEEE 8.16e global standard, the mobile WiMAX technology leads a new revolution of wireless communications providing high-speed mobile access with wide coverage [3]. As optical WiMAX penetrates into the optical wireless network, more benefits like higher transmission data rate and longer free-space transmission distance can be achieved by the introduction of visible light communication (VLC) scheme [4 6]. Recently, VLC has attracted a lot of attention as a potential option for the indoor wireless communication because VLC has several advantages over traditional radiofrequency (RF)-based wireless communication. It offers the advantage of a communications channel in an unregulated, unlicensed part of the electromagnetic spectrum. VLC has potential applications in a number of areas, such as communication link in particular areas in which RF wireless communication is restricted [7,8]. VLC system employs modulated light wavelength emitted by a variety of light sources, the light source can be modulated at highspeed, providing a data channel in addition to the illumination. However, the performance of VLC system has been limited mostly due to the limited modulation bandwidth (BW) of the light source. A VLC link consists of an optical transmitter, the free-space propagation channel, and an optical receiver. In view of the fact, the optical receiver is in general more robust than optical transmitter with respect to modulation BW in a VLC link. To overcome the modulation BW limitation, vertical cavity surface emitting laser (VCSEL) is proposed to employ in VLC system. In recent years, the VCSEL technology has advanced enough that it can be designed to operate in the 68-nm visible light range. VCSEL, with high modulation BW feature, is expected to have good performances in VLC system. In this paper, a 1-Gbps optical WiMAX transport system employing VCSEL and spatial light modulator (SLM) with 16-quadrature amplitude modulation (QAM)-orthogonal frequency-division multiplexing (OFDM) modulating signal over a 6-m free-space link is proposed. OFDM is a method of digital modulation in which a signal is divided into several narrowband channels on multiple carriers. It is a brilliant technology which has very high spectrum efficiency and robust dispersion tolerance to improve the transmission performance of systems [9]. To adapt the SLM in a 1-Gbps optical WiMAX transport system, the transmitting light can be focused into a point to extend the free-space transmission distance. To be the first one of employing VCSEL and SLM in a 1-Gbps optical WiMAX transport system, the transmitting light is successfully modulated with 16-QAM-OFDM signal. With the assistance of equalizer and low noise amplifier (LNA) at the receiving site, good bit error rate (BER) performance, clear constellation map, and clear eye diagram are achieved in the proposed systems. An optical WiMAX transport system, delivering 16-QAM-OFDM signal over a 6-m free-space link, with a data rate of 1 Gbps is successfully demonstrated. Such an optical WiMAX transport system would be attractive for providing services including Internet and telecommunication services. It is shown to be a prominent one not only to present its convenience in free-space lightwave transport system, but also to reveal its possibility for the real implementation.. Experimental setup The experimental configuration of our proposed 1-Gbps optical WiMAX transport system employing VCSEL and SLM with 16-QAM-OFDM modulating signal over a 6-m free-space link is present in Fig. 1. The VCSEL, with 3-dB modulation BW/wavelength range/color of 5.GHz/ nm/red, is directly modulated by a 1Gbps/5GHz 16-QAM-OFDM signal. The 16-QAM-OFDM signal is generated offline by MATLAB program and uploaded into a Tektronix arbitrary waveform generator (AWG). Such 16-QAM-OFDM signal is represented by 18 subcarriers, 51 FFT size, 1G samples per second, and 5 GHz intermediate frequency (IF), respectively. After emitted from the VCSEL, the light is diverged, launched into the convex lens, transmitted in the free-space, fed into the SLM, and focused on the high BW photodiode (PD). The convex lens is used to transfer the divergent beam into the parallel beam, and the SLM is used to focus the parallel beam into a point. The SLM (LCOS-SLM) is based on liquid crystal on silicon (LCOS) technology in which liquid crystal is controlled by a direct and accurate voltage. It has translucent liquid (C) 14 OSA 1 February 14 Vol., No. 3 DOI:1.1364/OE..761 OPTICS EXPRESS 76

3 crystal pixels with pixel pitch of 8 μm, and an active area of mm. To modulate the amplitude or phase of the light with the SLM, each pixel is controlled through a control computer. By means of SLM, the parallel light is focused into a point. The distance between the SLM and the focal point is the focal length of Fresnel lens. The distance between the VCSEL and the SLM is 5 m, and the distance between the SLM and the high BW PD is 1 m. It means that the modulated light is transmitted over a free-space distance of 6 m (5 + 1 m), and then reached to the high BW PD. The high BW PD has the detection wavelength range of 3-1 nm, with an active area diameter of around.4 mm and a responsivity of.43 ma/mw (at 68 nm). The received signal is then passed through an equalizer for data signal equalization, and amplified by a LNA. The LNA has a 3-dB BW of 8 GHz, a small signal gain of db (measured at 5 GHz), and a low noise figure of around 3.8 db. Finally the 16- QAM-OFDM signal is analyzed by an OFDM analyzer, captured by a communication signal analyzer (CSA), and processed off-line with a MATLAB program to evaluate the BER performance and corresponding constellation map. Furthermore, in order to analyze the eye diagram, a 1Gbps/5GHz data stream generated from a signal generator is employed at the transmitting site to modulate the VCSEL directly. Over a 6-m free-space link, the detected data stream is down-converted by mixing with a 5 GHz sinusoidal signal and then analyzed by a digital communication analyzer (DCA). Fig. 1. Experimental configuration of our proposed 1-Gbps optical WiMAX transport system employing VCSEL and SLM with 16-QAM-OFDM signal over a 6-m free-space link. 3. Experimental results and discussions The output optical power of VCSEL at different driving currents and the optical spectrum of VCSEL at a driving current of 1 ma are shown in Figs. (a) and (b), respectively. And further, the output optical powers and RIN values of VCSEL under different driving currents are listed in Table 1. It is clear that as the driving current is 1 ma, the maximum optical power of 4.5 mw is obtained. In order to have the maximum free-space transmission distance, the driving current of VCSEL is operated at 1 ma. The relative intensity noise (RIN) of VCSEL is given by [1]: (C) 14 OSA 1 February 14 Vol., No. 3 DOI:1.1364/OE..761 OPTICS EXPRESS 763

4 where i is the noise current spectral density, and i RIN = (1) I p I p is the detected DC photocurrent. Output optical power increases with an increase in the detected DC photocurrent, in which leading to VCSEL with low RIN value. Such a VCESL is operated as high as possible to simultaneously obtain high output optical power and low RIN value. Nevertheless, for a fixed 16-QAM- OFDM driving signal power, when the driving current is increased, the optical modulation index (OMI) is decreased, and consequently the carrier-to-noise ratio of the received 16- QAM-OFDM signal is degraded. In view of the fact that the digital signal is in general more robust than analog signal with respect to noise and nonlinear distortion. Although the format of 1Gbps/5GHz 16-QAM-OFDM signal is a RF passband format, yet the format of 1 Gbps signal is a digital bassband format. Such a 1 Gbps digital bassband signal has a high tolerance to noise and nonlinear distortion. High output optical power and low RIN value of VCSEL are worthy of obtaining due to high tolerance to noise and nonlinear distortion for 1 Gbps digital bassband signal. Output Optical Power (mw) (a) Driving Current (ma) Power (dbm) (b) Wavelength (nm) Fig.. (a) The output optical power of VCSEL at different driving currents. (b). The optical spectrum of VCSEL at a driving current of 1 ma. Table 1. Output optical powers and RIN values of VCSEL under different driving currents Moreover, the frequency response of the VCSEL at different driving currents is present in Fig. 3. As the driving current is 1 ma, the 3-dB BW of VCSEL is around 5. GHz. It can be concluded that as the VCSEL driving current is 1 ma, not only maximum optical power of 4.5 mw is obtained, but also low RIN value of 13 db/hz and sufficient 3-dB BW of 5. GHz are achieved. (C) 14 OSA 1 February 14 Vol., No. 3 DOI:1.1364/OE..761 OPTICS EXPRESS 764

5 1 5 Response (db) Frequency (GHz) Fig. 3. The frequency response of the VCSEL at different driving currents. A SLM is an optical device which can modulate optical amplitude or phase in each pixel. The function of the SLM is to extend the free-space transmission distance. The SLM has focused the light into a point on the PD, in which resulting in the extension of free-space transmission distance. The received optical power P r depends on the transmitted optical power P t, transmitted gain G t, received gain G r, and the free-space range loss R l [11]: Pr = Pt Gt Gr Rl () Since the SLM has focused the light into a point, yet a large received gain G r is achieved. It can be seen that, from Eq. (), a large G r is related to a large received optical power P r. As a result, the free-space transmission distance is extended. A more 1-m free-space transmission distance is obtained due to the result of employing SLM. By means of Fresnel lens function, the SLM can be operated as a dynamic convex lens by electrical controller [1]. The configuration of employing the SLM as a dynamic convex lens to extend the free-space transmission distance and enlarge the coverage area is present in Fig. 4. By using the SLM as a dynamic convex lens, not only the free-space transmission distance can be extended, but also the mobility problem of line-of-sight (LOS) system can be overcome. VLC system can be split into two categories: the diffused system and the LOS one. The former utilizes the diffused beam (such as LED light source) to cover the entire service area and to provide the mobile service to the user. However, it is difficult to obtain high-speed data rate and long free-space link due to finite BW and irradiance decline with distance. On the other hand, the latter utilizes a narrow laser beam (such as VCSEL light source) to obtain high-speed data rate and long free-space link. However, no mobility is provided in LOS system even though it can transmit high-speed data rate. It means that as blocking occurs, a rapid performance degradation happens in the LOS system. However, with the rapid progress of VLC system, the increasing requirements raise the needs for high-speed data rate and long free-space link. The LOS system employing VCSEL light source is the potential candidate to meet the demands. As to the mobility problem, the SLM employed as a dynamic convex lens can be added in systems to overcome it. The SLM is worth employing due to the success of extended free-space transmission distance and large coverage area. (C) 14 OSA 1 February 14 Vol., No. 3 DOI:1.1364/OE..761 OPTICS EXPRESS 765

6 Fig. 4. The configuration of employing the SLM as a dynamic convex lens. A block diagram of the equalizer is illustrated in Fig. 5(a), in which including first (1st) and second (nd) equalized functions. The equalizer is used to render the frequency response. When the signal has been equalized, the frequency domain attributes of the signal at the input are faithfully reproduced at the output. The PD is followed by an equalizer to equalize the 1Gbps/5GHz 16-QAM-OFDM data signal. The equalizer is an active high-pass filter with two adjustable parameters. Two stages of the adjustable equalized functions are cascaded to equalize the data signal. The first parameter (V g ) controls the low frequency gain and the second parameter (V z ) controls the cut-off frequency value of the equalizer. And further, the circuitry of the equalizer is shown in Fig. 5(b). The passive resistor Re is used to give a proper minimum DC-gain to prevent an undesired low DC-gain, as M 1 is off. The high cut-off frequency is controlled by varying the MOS capacitors M and M 3. Figures 6(a) and 6(b) show the electrical spectra of the 1Gbps/5GHz 16-QAM-OFDM data signal before and after an equalizer. For 1Gbps/5GHz 16-QAM-OFDM signal, it has a bandwidth of.5 GHz; it means that the lowest frequency of 1Gbps/5GHz 16-QAM-OFDM signal is 3.75 GHz (5.5/ = 3.75), and the highest frequency of 1Gbps/5GHz 16-QAM-OFDM signal is 6.5 GHz (5 +.5/ = 6.5). The electrical spectrum of the 1Gbps/5GHz 16-QAM-OFDM data signal after an equalizer is more flat than that before an equalizer. The equalizer is employed to compensate for the frequency response, and the frequency response of the 1Gbps/5GHz 16-QAM-OFDM data signal is compensated after through an equalizer. The better flat electrical spectrum we get in systems, the better transmission performance we obtain in ones. To evaluate the transmitted 16-QAM-OFDM signal performance, the measured BER curves and constellation map at a data signal of 1Gbps/5GHz are present in Fig. 7. Here, the equalizer works at the same parameters such as the low frequency gain and the cut-off frequency value. At a free-space transmission distance of 6 m; without employing equalizer and LNA, the BER is about 1 ; with employing equalizer and LNA, the BER is reached down to 1 6. As equalizer and LNA are simultaneously employed, low BER value and clear constellation map are obtained. Error free transmission is achieved to demonstrate the possibility of establishing a 1-Gbps optical WiMAX transport system. To show a more direct association with equalizer and LNA, we remove one of the schemes and measure the BER value. It is obvious that the BER performance improvement is restricted as only one improvement scheme is employed. With employing LNA only, the BER is around 1 3 ; with employing equalizer only, the BER is around 1 4. Such results mean that equalizer and LNA play important roles for errors correction. They can further improve the signal-to-noise ratio (SNR) of the optical WiMAX transport system, in which resulting in the BER performance improvement. (C) 14 OSA 1 February 14 Vol., No. 3 DOI:1.1364/OE..761 OPTICS EXPRESS 766

7 (a) Equalizer V g Data Signal Iutput Equalized 1st Equalized nd Data Signal Output V z V cc (b) R c R c Data signal + Output - Vg Data signal Iutput Q 1 Q Data signal Iutput M 1 M M 3 V z R e Biasing M 4 M 5 Fig. 5. (a) A block diagram of the equalizer, in which including 1st and nd equalized functions. (b). The circuitry of the equalizer. RF Level (dbmv) (a) RF Level (dbmv) (b) -1 Start 1.75GHz 5MHz/D Stop 6.75GHz -1 Start 1.75GHz 5MHz/D Stop 6.75GHz Fig. 6. (a) The electrical spectrum of the 1Gbps/5GHz 16-QAM-OFDM data signal before an equalizer. (b). The electrical spectrum of the 1Gbps/5GHz 16-QAM-OFDM data signal after an equalizer. (C) 14 OSA 1 February 14 Vol., No. 3 DOI:1.1364/OE..761 OPTICS EXPRESS 767

8 1 without Equalizer and LNA with LNA only with Equalizer only with Equalizer and LNA -Log 1 (BER) Free-Space Transmission Distance (m) Fig. 7. The measured BER curves and constellation map at a data signal of 1Gbps/5GHz. Figures 8(a) and 8(b) display the eye diagrams of 1-Gbps optical WiMAX signal under a 6-m free-space link for the conditions of without employing equalizer and LNA [Fig. 8(a)] and with employing equalizer and LNA [Fig. 8(b)], respectively. The amplitude and phase fluctuations in the signal are obviously observed in the case of without employing equalizer and LNA [Fig. 8(a)]. The signal distortion will give an increase in power penalty. For the case of employing equalizer and LNA [Fig. 8(b)], however, a clear eye diagram is achieved due to amplitude and phase fluctuations suppression. 1 (a) (b) Fig. 8. The eye diagrams of 1-Gbps optical WiMAX signal under a 6-m free-space link: (a) without employing equalizer and LNA, (b) with employing equalizer and LNA. To have a more association with the transmitted 16-QAM-OFDM performance and the location of the LNA, we change the location of the LNA. If the LNA is placed before the VCSEL, then the RF level of the amplified 16-QAM-OFDM signal is too high to drive the VCSEL. Such a high RF level of the 16-QAM-OFDM signal will degrade the transmission performance. Furthermore, if the LNA is placed before the equalizer, then the PD is followed by a LNA to amplify the 16-QAM-OFDM signal, and the amplified 16-QAM-OFDM signal is equalized by an equalizer. However, the RF level of the amplified 16-QAM-OFDM signal is too high to be equalized, due to the limitation of the equalizer RF level input. Thereby, the optimum location of the LNA is placed after the equalizer. (C) 14 OSA 1 February 14 Vol., No. 3 DOI:1.1364/OE..761 OPTICS EXPRESS 768

9 4. Conclusions We proposed and demonstrated a 1-Gbps optical WiMAX transport system employing VCSEL and SLM with 16-QAM-OFDM modulating signal. With the assistance of equalizer and LNA at the receiving site, low BER operation, clear constellation map, and clear eye diagram are achieved. A system of transmitting 16-QAM-OFDM signal over a 6-m free-space link with a data rate of 1 Gbps is successfully demonstrated. Optical WiMAX has provided a very potential candidate for the free-space lightwave transport system due to its high-speed data rate. Employing VCSEL and SLM in a 1-Gbps optical WiMAX transport system is a promising option, an attractive feature that can accelerate the optical WiMAX deployment. Acknowledgment The authors would like to thank the financial support from the National Science Council of the Republic of China under Grant NSC 1-1-E-7-67-MY3, NSC 11-1-E-7-4 -MY3, and NSC 1-18-E-7-. (C) 14 OSA 1 February 14 Vol., No. 3 DOI:1.1364/OE..761 OPTICS EXPRESS 769