Development of Radio on Free Space Optics System for Ubiquitous Wireless

Transcription

1 PIERS ONLINE, VOL. 4, NO. 1, Development of Radio on Free Space Optics System for Ubiquitous Wireless Katsutoshi Tsukamoto 1, Takeshi Higashino 1, Takuya Nakamura 1, Koichi Takahashi 1 Yuji Aburakawa 1, Shozo Komaki 1, Kazuhiko Wakamori 2, Toshiji Suzuki 2 Kamugisya Kazaura 2, Alam Mohammad Shah 2, Kazunori Omae 2, and Mitsuji Matsumoto 2 1 Graduate School of Engineering, Osaka University, Japan 2 GITI, Waseda University, Japan Abstract Radio on Fiber (RoF) with layer 1 routing capability can realize a cost effective universal platform for future ubiquitous wireless services. The layer 1 routing concept can be realize by not only RoF but also RoFSO (Radio on Free Space Optics) or RoR (Radio on Radio) networks, which provide a free space for heterogeneous wireless services in Free Space Optics or millimeter wave radio. Development of a new advanced RoFSO system has been started. This paper describes its concept and features, and furthermore, discusses about its role in future ubiquitous wireless. 1. INTRODUCTION In ubiquitous network society, users want an environment to access any communication services at any time, any place, and any situations. In order to realize the ubiquitous networks, a combination of IP network and broadband heterogeneous wireless access services will play an important role. In current wireless networks, however, various operators independently overlaid their own radio base stations and networks. This leads redundant equipments and investments on infrastructures, and prevents the quick start of a new wireless service and employing microcellular architecture. These problems are revealed especially in in-building, underground at urban areas, and rural areas where broadband fiber-infrastructures have not yet been constructed due to their high cost and a low population. Radio on Fiber (RoF) technologies can realize a cost effective universal platforms for future ubiquitous wireless services. Furthermore, RoF networks can be extended to Virtual Radio Free Space Network with layer 1 routing realized by not only RoF but also RoFSO (Radio on Free Space Optics) or RoR (Radio on Radio) networks, which can provide a free space for heterogeneous wireless services in Free Space Optics or millimeter wave radio. This paper describes the concept and features of a new advanced RoFSO system development, and furthermore, discusses software definable radio networks [1, 2] from a viewpoint of its role in future ubiquitous wireless. 2. LAYER 1 ROUTING WITH RADIO ON FIBER, RADIO, AND FREE SPACE OPTICS Radio on Fiber (RoF) links shown in Fig. 1 have a function of transmitting radio signals to remote stations with keeping their radio formats. Consequently, RoF link becomes a hopeful candidate of a common platform for various wireless access networks. When RoF equips photonic routing functions, any radio signal can be forwarded to its destination control station. We call such RoF networks Virtual Radio Free Space Network (VRFSN) [3]. By using RoF, architecture for radio access zones easily employs micro or pico cellular systems. A RBS receiving or transmitting radio signals in each radio zone, equips only O/E and O/E converters. The RBS requires neither the modulation functions nor demodulation functions of radio signals. The radio signals converted into optical signals are transmitted via a RoF link with the benefit of its low transmission loss and broadband. Therefore, RoF links can be independent of radio signal formats and can provide universality for various types of radio access methods. This means that VRFSNs are very flexible to the modification of radio signal formats, the opening of new radio services, or the accommodation of different types of radio signal formats. VRFSNs with layer 1 routing are required for ubiquitous wireless especially in radio dead zones at private or public spaces such as in-house, in-building, and underground at urban areas, and at rural areas where broadband fiber-infrastructures have not yet been constructed due to their high cost and a low population. The layer 1 routing concept shown in Fig. 2 can be also extended to

2 PIERS ONLINE, VOL. 4, NO. 1, RoFSO (Radio on Free Space Optics) or RoR (Radio on Radio) networks, which can provide a free space for heterogeneous wireless services in Free Space Optics or millimeter wave radio. Therefore, RoF links can be replaced with RoFSO or RoR links. In each network, radio signals are converted into FSO or MMW signals with wideband frequency conversion. The Layer 1 routing would be important for the transparency not only for various protocols on Layer 2 and upper, but also for various types of air interfaces. Figure 1: Radio on Fiber (RoF) links. Figure 2: Layer 1 routing and Radio on Free Space concept. 3. DEVELOPMENT OF ROFSO SYSTEM 3.1. Concept and Object The development of DWDM RoFSO link system has been started to realize an effective and quick provide of heterogeneous wireless services for not only urban area but rural area, that has a little or no infrastructure for broadband services [4]. Fig. 3 illustrates the concept of advanced DWDM RoFSO link system. An object of the development is to realize an advanced RoFSO link system, which can transparently transfer cellular phone, wireless LAN, terrestrial digital broadcasting, and future coming new wireless services by using DWDM optical wireless channels. Figure 3: Concept of advanced DWDM RoFSO link system Feaures of RoFSO System Figure 4 illustrates features of RoFSO link system. Conventional FSO system has been prepared for each of digital data transmissions with different data rates such as Ethernet, cellular entrance, digital CATV, and so on. Recently, next generation FSO system [5] has successfully realized a stable 10 Gbps WDM FSO transmission. Therefore, a protocol-free digital FSO link with an equivalent performance as an optical fiber has been realized. On the other hand, the object of our development is to realize RoFSO link that has an equivalent capacity for heterogeneous wireless services as RoF. We will employ the direct optical amplification and emission of RoF signal into free space, and direct focusing of the received optical beam into the core of SMF, that technologies have been developed in the next generation FSO system [5]. Since RoF and RoFSO are essentially analog transmission links, higher stability and reliability will be required. One object of the development is to improve accuracy of the optical beam tracking system. The target is more than 1 km link

3 PIERS ONLINE, VOL. 4, NO. 1, distance, DWDM with more than 4 wavelengths, transmission of more than 4 different wireless services including terrestrial digital TV broadcasting. Figure 4: Advanced DWDM RoFSO link Development of Advanced DWDM RoFSO Link System Figure 5 illustrates the developed RoFSO Transmitter / Receiver with Fine Beam Tracking Optic System [6] to emit DWDM RoF signals directly from SMF into free space and focus a received optical beam directly into a core of SMF. Objective design parameters are followings: Optical Transmitting Power: 20 dbm/wave, Geometric Loss: 2.6 db at 1 km (Beam angle width: 47.3 µrad at 1.55 µm), Equipment Loss: < 10 db (Tx & Rx total). Figure 5: Developed RoFSO Transmitter / Receiver with fine beam tracking optic system which emits DWDM RoF signals directly from SMF into free space and focuses a received optical beam directly into a core of SMF Development of Seamless Connecting Equipments between RoF and RoFSO To design RoFSO link, we have to examine a total performance through RoF, RoFSO and wireless access links. Since different wireless services have different data-rate, modulation formats, and sensitivity, we have to investigate the influence of distortion or disturbances suffered on RoF and RoFSO links for each wireless services. Fig. 6 shows indoor short-range experiment setup to confirm FSO loss compensation effect with optical amplifier. A RoF signal with 1.55 µm wavelength modulated by WLAN signals at 2.4 GHz is amplified at EDFA, transmitted directly from SMF into FSO channel, directly focused into a core of SMA, amplified at post EDFA up to 0 dbm and then received at PD. Fig. 6 shows experimental results of RF output power versus RF input power of RoFSO link when the optical loss between Tx and Rx was 50 db, and received optical power, Popt R was 3.75 dbm. In this figure, experimental results for RoF when Popt R = 3.3 dbm are also shown for comparison. It is seen from the figure that booster and post EDFA can compensate the loss of RoFSO link without any IMD3 increase, and realize RoFSO link equivalent to RoF link.

4 PIERS ONLINE, VOL. 4, NO. 1, Figure 6: Indoor short range experiment setup of RoFSO link for 2.4 GHz and UHF band radio signals and experimental results of RF output power versus input power Long-term Demonstrative Measurements of RoFSO Link In outdoor environments, an atmospheric turbulence has a significant impact on the quality of the free-space optical beam. Optical intensity fading due to beam broadening, angle-of-arrival fluctuations, and scintillation will reduce the received optical beam power, and causes burst errors in transmitted RF signals. We have started constructing a statistical model of scintillation and performed some simulations. Also, a long-term measurement of scintillation has been started using a measurement setup with conventional 800 nm FSO system. Fig. 7 shows the experiment setup at Waseda University, Tokyo, and some examples of measurement results of scintillation variance versus time in a day in May or September. It is found that a large variation in temperature causes a large variation in scintillation variance. After long-term measurement of scintillation, and evaluation of its influences on four types of RF signals transmitted on RoFSO link, then we will conduct a long-term and pragmatic demonstration of the developed RoFSO link. Figure 7: Experiment field at Waseda Univ., Tokyo and measurement setup with conventional 800 nm FSO system and example of measurement results of scintillation variance versus time in a day in May or September. 4. SOFTWARE DEFINABLE RADIO NETWORKS Figure 8 illustrates the concept of extended VRFSN, called Software Definable Radio Network (SDRN) [1, 2]. The SDRN is composed with Radio on Free Space (RoFSx) networks such as RoF, RoFSO and RoR, universal RBS and software definable control server (SDRCS) in IP network for various types of wireless services, and SDRGW (Software Definable Radio Gateway), which provides a seamless connectivity between local RoF networks and IP network. RoFSx (Fiber, Optics, Radio, LCX etc) networks can transparently connects multi-dimensional radio spaces with Photonic Intensity (1 dimension). In the SDRN, any radio signals in a RoFSx network with routing functions are transmitted to their desirable SDRGW. At SDRGW, air-interface conversion or IP packeterization of wireless data and control signals are executed. The later function is that the datagram in any radio signal are converted to IP packet, which are transferred to its SDRCS altogether with the control channel signal (Wireless service over IP [7]). This can realize a cross layer platform on the

5 PIERS ONLINE, VOL. 4, NO. 1, IP network for heterogeneous wireless services networks. RoFSx networks with several distributed universal BS can also achieve SDMA to improve frequency efficiency (Heterogeneous Radio Smart Space Construction [8]). Figure 8: Software Definable Radios. 5. CONCLUSIONS We have described the concept and object of advanced DWDM RoFSO system development. We are developing RoFSO link equipments transferring more than 4 different wireless services with DWDM techniques, and we will conduct a long-term demonstrative measurement in the next phase. Furthermore, we have described the concept of SDRN and its availability for ubiquitous wireless. ACKNOWLEDGMENT This work is supported by a grant from the National Institute of Information and Communications Technology (NICT) of Japan. REFERENCES 1. Komaki, S., Microwave technologies for software radio networks, Proceedings of APMC2003, Vol. 3, No. FC7-1, , November Tsukamoto, K., Software definable radio networks for the ubiquitous networks, XXXIIIth GA of URSI, Vol. 1, 205, October Komaki, S., et al., Proposal of radio high-way networks for future multimedia-personal wireless communications, IEEE ICPWC 94, Vol. 1, No. 1, , August Tsukamoto, K., K. Nakaduka, M. Kamei, T. Higashino, S. Komaki, K. Wakamori, Y. Aburakawa, T. Nakamura, K. Takahashi, T. Suzuki, K. Kazaura, K. Ohmae, M. Matsumoto, S. Kuwano, and H. Watanabe, Development of DWDM radio on free space optic link system for ubiquitous wireless, Technical Digest of AP-MWP 2007, Vol. 1, No. 1, 1, April Kazaura, K., et al., Experimental performance evaluation of next generation FSO communication system, AP-MWP 2006, Vol. 1, , April Takahashi, K., et al., Development of radio on free space optics system for ubiquitous wireless services (2) Basic design of optical system, Proc. of 2007 IEICE General Conf., C-14 6, 257, Japanese, March Tajima, K., et al., A consideration on handover delay for wireless service over IP networks using wireless LAN, IEICE Tech. Report, Vol. 105, No. 80, 7 12, Japanese, May Hai, L. H., et al., Performance improvement of radio-on-fiber ubiquitous antenna system using sub-carrier resource management, Proc. of SPIE Broadband Access Commun. Technols, Vol. 6390, B1 B11, Oct