development of radio on free space optics system for

PIERS ONLINE, VOL. 4, NO. 1, 2008 96

Development of Radio on Free Space Optics System forUbiquitous Wireless

Katsutoshi Tsukamoto1, Takeshi Higashino1, Takuya Nakamura1, Koichi Takahashi1

Yuji Aburakawa1, Shozo Komaki1, Kazuhiko Wakamori2, Toshiji Suzuki2

Kamugisya Kazaura2, Alam Mohammad Shah2, Kazunori Omae2, and Mitsuji Matsumoto2

1Graduate School of Engineering, Osaka University, Japan2GITI, Waseda University, Japan

Abstract— Radio on Fiber (RoF) with layer 1 routing capability can realize a cost effectiveuniversal platform for future ubiquitous wireless services. The layer 1 routing concept can berealize 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 Opticsor 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 futureubiquitous wireless.

1. INTRODUCTION

In ubiquitous network society, users want an environment to access any communication services atany time, any place, and any situations. In order to realize the ubiquitous networks, a combinationof IP network and broadband heterogeneous wireless access services will play an important role. Incurrent wireless networks, however, various operators independently overlaid their own radio basestations and networks. This leads redundant equipments and investments on infrastructures, andprevents the quick start of a new wireless service and employing microcellular architecture. Theseproblems are revealed especially in in-building, underground at urban areas, and rural areas wherebroadband fiber-infrastructures have not yet been constructed due to their high cost and a lowpopulation.

Radio on Fiber (RoF) technologies can realize a cost effective universal platforms for futureubiquitous wireless services. Furthermore, RoF networks can be extended to Virtual Radio FreeSpace Network with layer 1 routing realized by not only RoF but also RoFSO (Radio on FreeSpace Optics) or RoR (Radio on Radio) networks, which can provide a free space for heterogeneouswireless services in Free Space Optics or millimeter wave radio. This paper describes the conceptand features of a new advanced RoFSO system development, and furthermore, discusses softwaredefinable 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 remotestations with keeping their radio formats. Consequently, RoF link becomes a hopeful candidateof a common platform for various wireless access networks. When RoF equips photonic routingfunctions, any radio signal can be forwarded to its destination control station. We call such RoFnetworks “Virtual Radio Free Space Network (VRFSN)” [3]. By using RoF, architecture for radioaccess zones easily employs micro or pico cellular systems. A RBS receiving or transmitting radiosignals in each radio zone, equips only O/E and O/E converters. The RBS requires neither themodulation functions nor demodulation functions of radio signals. The radio signals convertedinto optical signals are transmitted via a RoF link with the benefit of its low transmission lossand broadband. Therefore, RoF links can be independent of radio signal formats and can provideuniversality for various types of radio access methods. This means that VRFSNs are very flexible tothe modification of radio signal formats, the opening of new radio services, or the accommodationof different types of radio signal formats.

VRFSNs with layer 1 routing are required for ubiquitous wireless especially in radio dead zonesat private or public spaces such as in-house, in-building, and underground at urban areas, and atrural areas where broadband fiber-infrastructures have not yet been constructed due to their highcost and a low population. The layer 1 routing concept shown in Fig. 2 can be also extended to


RoFSO (Radio on Free Space Optics) or RoR (Radio on Radio) networks, which can provide a freespace 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 convertedinto FSO or MMW signals with wideband frequency conversion. The Layer 1 routing would beimportant for the transparency not only for various protocols on Layer 2 and upper, but also forvarious 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 quickprovide of heterogeneous wireless services for not only urban area but rural area, that has a littleor no infrastructure for broadband services [4]. Fig. 3 illustrates the concept of advanced DWDMRoFSO 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, andfuture coming new wireless services by using DWDM optical wireless channels.

Figure 3: Concept of advanced DWDM RoFSO link system.

3.2. Feaures of RoFSO System

Figure 4 illustrates features of RoFSO link system. Conventional FSO system has been preparedfor 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 stable10Gbps WDM FSO transmission. Therefore, a protocol-free digital FSO link with an equivalentperformance as an optical fiber has been realized. On the other hand, the object of our developmentis 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, anddirect focusing of the received optical beam into the core of SMF, that technologies have beendeveloped in the next generation FSO system [5]. Since RoF and RoFSO are essentially analogtransmission links, higher stability and reliability will be required. One object of the developmentis to improve accuracy of the optical beam tracking system. The target is more than 1 km link


distance, DWDM with more than 4 wavelengths, transmission of more than 4 different wirelessservices including terrestrial digital TV broadcasting.

Figure 4: Advanced DWDM RoFSO link.

3.3. Development of Advanced DWDM RoFSO Link SystemFigure 5 illustrates the developed RoFSO Transmitter / Receiver with Fine Beam Tracking OpticSystem [6] to emit DWDM RoF signals directly from SMF into free space and focus a receivedoptical beam directly into a core of SMF. Objective design parameters are followings: OpticalTransmitting Power: 20 dBm/wave, Geometric Loss: 2.6 dB at 1 km (Beam angle width: 47.3 µradat 1.55µm), Equipment Loss: < 10 dB (Tx & Rx total).

Figure 5: Developed RoFSO Transmitter / Receiver with fine beam tracking optic system which emitsDWDM RoF signals directly from SMF into free space and focuses a received optical beam directly into acore of SMF.

3.4. Development of Seamless Connecting Equipments between RoF and RoFSOTo design RoFSO link, we have to examine a total performance through RoF, RoFSO and wirelessaccess links. Since different wireless services have different data-rate, modulation formats, andsensitivity, we have to investigate the influence of distortion or disturbances suffered on RoF andRoFSO links for each wireless services. Fig. 6 shows indoor short-range experiment setup to con-firm FSO loss compensation effect with optical amplifier. A RoF signal with 1.55µm wavelengthmodulated by WLAN signals at 2.4GHz is amplified at EDFA, transmitted directly from SMFinto FSO channel, directly focused into a core of SMA, amplified at post EDFA up to 0 dBm andthen received at PD. Fig. 6 shows experimental results of RF output power versus RF input powerof RoFSO link when the optical loss between Tx and Rx was 50 dB, and received optical power,PoptR was −3.75 dBm. In this figure, experimental results for RoF when PoptR = −3.3 dBm arealso shown for comparison. It is seen from the figure that booster and post EDFA can compensatethe loss of RoFSO link without any IMD3 increase, and realize RoFSO link equivalent to RoF link.


Figure 6: Indoor short range experiment setup of RoFSO link for 2.4 GHz and UHF band radio signals andexperimental results of RF output power versus input power.

3.5. Long-term Demonstrative Measurements of RoFSO LinkIn outdoor environments, an atmospheric turbulence has a significant impact on the quality ofthe free-space optical beam. Optical intensity fading due to beam broadening, angle-of-arrivalfluctuations, and scintillation will reduce the received optical beam power, and causes burst errorsin transmitted RF signals. We have started constructing a statistical model of scintillation andperformed some simulations. Also, a long-term measurement of scintillation has been started usinga measurement setup with conventional 800 nm FSO system. Fig. 7 shows the experiment setupat Waseda University, Tokyo, and some examples of measurement results of scintillation varianceversus time in a day in May or September. It is found that a large variation in temperaturecauses a large variation in scintillation variance. After long-term measurement of scintillation, andevaluation of its influences on four types of RF signals transmitted on RoFSO link, then we willconduct 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 FSOsystem 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 forvarious types of wireless services, and SDRGW (Software Definable Radio Gateway), which pro-vides 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 PhotonicIntensity (1 dimension). In the SDRN, any radio signals in a RoFSx network with routing functionsare transmitted to their desirable SDRGW. At SDRGW, air-interface conversion or IP packeteri-zation of wireless data and control signals are executed. The later function is that the datagram inany radio signal are converted to IP packet, which are transferred to its SDRCS altogether with thecontrol channel signal (Wireless service over IP [7]). This can realize a cross layer platform on the


IP network for heterogeneous wireless services networks. RoFSx networks with several distributeduniversal BS can also achieve SDMA to improve frequency efficiency (Heterogeneous Radio SmartSpace Construction [8]).

Figure 8: Software Definable Radios.

5. CONCLUSIONS

We have described the concept and object of advanced DWDM RoFSO system development. Weare developing RoFSO link equipments transferring more than 4 different wireless services withDWDM 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 CommunicationsTechnology (NICT) of Japan.

REFERENCES

1. Komaki, S., “Microwave technologies for software radio networks,” Proceedings of APMC2003,Vol. 3, No. FC7-1, 1780–1785, November 2003.

2. Tsukamoto, K., “Software definable radio networks for the ubiquitous networks,” XXXIIIthGA of URSI, Vol. 1, 205, October 2005.

3. Komaki, S., et al., “Proposal of radio high-way networks for future multimedia-personal wirelesscommunications,” IEEE ICPWC’94, Vol. 1, No. 1, 204–208, August 1994.

4. Tsukamoto, K., K. Nakaduka, M. Kamei, T. Higashino, S. Komaki, K. Wakamori, Y. Abu-rakawa, 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 systemfor ubiquitous wireless,” Technical Digest of AP-MWP 2007, Vol. 1, No. 1, 1, April 2007.

5. Kazaura, K., et al., “Experimental performance evaluation of next generation FSO communi-cation system,” AP-MWP 2006, Vol. 1, 289–292, April 2006.

6. Takahashi, K., et al., “Development of radio on free space optics system for ubiquitous wirelessservices — (2) Basic design of optical system —,” Proc. of 2007 IEICE General Conf., C-14–6,257, Japanese, March 2007.

7. Tajima, K., et al., “A consideration on handover delay for wireless service over IP networksusing wireless LAN,” IEICE Tech. Report, Vol. 105, No. 80, 7–12, Japanese, May 2005.

8. Hai, L. H., et al., “Performance improvement of radio-on-fiber ubiquitous antenna system usingsub-carrier resource management,” Proc. of SPIE — Broadband Access Commun. Technols,Vol. 6390, B1–B11, Oct. 2006.

development of radio on free space optics system for

Documents