資料５－ＤCommission D (Electronics and Photonics) Activity ReportAugust 2009-November 2009
November 10, 2009
Tadao Nagatsuma

Commission D discussed the study areas, on which the current 14 committee members shouldespecially focus, and they are as follows:(1) Device and circuit technology for wireless and fiber-optic systems(2) Advanced electronic/photonic devices and their integration(3) Novel materials and their applications(4) Microwave photonics technology(5) Terahertz-wave technology
Scientific sessions organized by Commission D at AP-RASC 2010 were proposed based on theabove areas.
In Activity Report, Commission D will report each time regarding recent significant progress inthe above areas especially in Japan. This time, Professor Tsukamoto of Osaka University reviews thearea (4); fusion of radio-wave and light-wave system technologies.

Mobile Network Backhaul Applications of Free Space Optical Link Technology
1. Introduction
As optical technologies are becoming more advanced, free-space optical links (FSOL) applications
have been considered as a potential medium to provide short range mobile backhaul links using
simple and compact equipment. Free-space optical transmission has advantageous aspects such asrapid deployment and relocation, while it has to be operated under limited link lengths due to thepropagation impairments. This report describes the recent development of free-space optical links
including radio on free space optical links (RoFSOL).

2. Concept of Mobile Network Backhaul with FSOL

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Operational aspects of FSOLs are considered in Report ITU-R F.2016 [1]. Mobile network
backhaul is recommended as one of the basic applications of FSOL [2]. Figure 1(a) illustrates anapplication of FSOLs extending a mobile infrastructure, where digital data from a base station for amobile network are transmitted through a FSOL to a sub-mobile base station for a spot service area.
At the optical transmitter (Tx), which includes an electrical-to-optical converter (E/O), opticalcarriers are intensity-modulated by these digital data. At the optical receiver (Rx), which includes anoptical-to-electrical converter (O/E), a received optical signal is detected. The data are input to aradio modem in the base station equipment.
Recent high-speed FSOL systems have achieved some terabits/sec class transmission rates.
Reference [3] has reported a 1.28-Tb/s (32 * 40 Gb/s) wavelength-division-multiplexed (WDM)
signal was successfully transmitted over a free-space optic (FSO) link. The system used a novel pairof FSO terminals, transparently connected to optical fibers, to transmit/receive the WDM channels
over a double-pass FSO path between two buildings (2 * 210 m).
Another type of FSOL, Radio on Free-Space Optical Links (RoFSOL) are also effective as a shortlink where a conventional wireless/wired link is disadvantageous [4][5]. In RoFSOLs, some Radioon Fiber (RoF) signals are directly transmitted from an optical fiber into air. This can provide afree-space link for various types of wireless services in an optical wireless beam similar to RoF.
Since RoFSOLs can transmit some radio-frequency signals independent of their radio interfaces,
network operators commonly and flexibly use RoFSOLs as a backhaul network for mobile and otherwireless services. RoFSOL technology has been also developed in order to realize a universal
backhaul for heterogeneous wireless services.
As shown in Figure 1(b), RoFSOLs can provide another method to extend mobile network
infrastructure, where radio signals from a base station analog-modulate an optical signal at aradio-to-optical signal converter (R/O) using a laser diode or an optical external modulator. The
modulated optical signal is transmitted through an optical fiber, emitted to the RoFSOL at an opticalantenna (Tx), fed into an optical fiber at a receiver optical antenna (Rx), and finally detected at a
photodiode in an optical-to-radio converter (O/R). At the output of the O/R, we can again obtain theoriginal radio signals in the radio-frequency band. In RoFSO, therefore, radio signals can betransmitted without any modification in their signal formats. The flexibility and transparency ofRoFSOL come from these operations, and from the various types of radio-signal formats that can betransmitted by RoFSOL.

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(a)

(b)
Figure 1: (a) Free-space optical links (FSOLs) and (b) radio over free-space optical links (RoFSOLs)
for extension of mobile network infrastructures.

3. Radio on Free Space Fiber (RoF), Radio on Free Space Optic (RoFSO), and Radio on Free
Space Radio (RoR)
Radio on fiber (RoF) technologies could be a cost effective universal platform for ubiquitous
wireless services. RoF networks can be treated as a virtual free space to transport the data link,
network, transport and application layers of various wireless services [6]. This layer 1 routing and its
benefit can be realized not only by using RoF but also by using radio on free-space optical links(RoFSOLs), which can provide free space for heterogeneous wireless services in an optical wirelesslink. The same routing can be realized by radio on radio (RoR) networks, in which wireless service
signals are frequently converted into microwave, millimeter wave, THz bands and transmitted with
non-regenerative repeaters.
By using this transparency for radio signal, radio on fiber links have been applied for mobilenetworks as feeder links to remote antennas in radio dead zones such as public underground area,indoor, tunnels, and so on[7, 8]. RoFSOLs and RoRs are effective for these feeder links at places
where a fiber construction is difficult. A combination of RoF, RoFSOL and RoR is called "hybrid
radio access network (H-RAN)"[9] as shown in Fig. 2. H-RAN can be used as an universal and3flexible mobile backhaul network for different types of air-interfaces. Air interfaces tends to rapidlychange as demands for broadband wireless services increase. H-RAN can easily accommodate a newcoming wireless service only by installing a new BTS equipment at center station. Considering
trends of future femto-cell operations and fixed mobile convergence (FMC) services, software
definable radio network (SDRN) has been also proposed to connect various types of wireless accesslinks, H-RAN and Internet Protocol (IP) based core network [6]As seen above, RoFSOLs and RoRLs are effective as same as RoF in mobile backhaul networks.
Compared with RoRs, the advantages of RoFSOLs are license-free operation and a higher
transmission capacity. Moreover, employment of WDM technology can provide mutuallyindependent free space in a optical beam for each of the heterogeneous wireless services. It can alsooffer an extremely large and transparent transmission capacity to a wireless service signal.

Figure 2: Concept of hybrid radio access network (H-RAN).

4. Radio on Free Space Optical Link Technology
RoFSOLs using WDM techniques system have bee developed to create an effective and quick
provision of heterogeneous wireless services for not only urban but also rural areas that have little orno infrastructure for broadband services [4],[5]. In rural areas or developing countries, rich fiber
infrastructures have not yet been constructed due to their high cost or the region’s low population.
While broadband wireless technologies are rapidly progressing and new services are being offered tourban areas quickly, these new services have been under-represented in rural areas for a long time.
To solve such a gap in broadband services, RoFSOLs without fiber or a combination of RoF andRoFSOLs are suitable as a universal backhaul link and a universal access point for heterogeneous
wireless services. The latter combination is applicable for a relatively longer backhaul link, wherefiber construction often becomes difficult due to rivers, roads, or some other geographical conditions.

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In this case, an RoFSOL is effective to relay between two RoFs or to extend RoF without any signal
conversions, because RoFSOLs can transmit an optical signal from an optical fiber into air, anddirectly feed it into another fiber.
Figure 3 illustrates the technical features of a RoFSOL system. Conventional FSOL systems
shown in Fig. 3 (a) have been adapted for each of the digital data transmissions with different datarates such as Ethernet, mobile phone entrance, digital CATV, and so on. Recently, next generation
FSOL systems shown in Fig 3 (b) [10], have realized a stable 10 Gbps WDM FSO transmission.
Therefore, a protocol-free digital FSOL with the almost same performance as an optical fiber has
been realized.
RoFSOLs make it possible to extend FSOL flexibility and transparency from the physical layer tothe application layers, thus enabling heterogeneous wireless services. As shown in Fig 3 (b), theRoFSOL is capable of the transparent transmission used for cellular phones, two types of wirelessLAN, and digital terrestrial broadcasting by using WDM optical wireless channels. Future wirelessservices can be accommodated by attaching their modems to the central station. The opticaltransceiver employs direct optical amplification, the emission of a RoF signal into free space, andthe direct focusing of the received optical beam into the core of a single-mode fiber. An optical finebeam tracking system achieves a stable and reliable FSOL against weather conditions andscintillation at an atmospheric channel of more than 1 km. Similar systems have been reported in[11] and [12].

Figure 3: Features of RoFSOL system.

55. Summary
This report introduced the use of FSOLs in relation to backhaul applications as well as its
technology trend, including the recent development of RoFSOLs. FSOLs are becoming useful media
for backhaul applications for particular cases in which immediate deployment of a conventionalwireless/wired link is disadvantageous. RoFSOLs using WDM technology can provide sharedbackhaul for flexible deployment of various wireless services operating in different radio frequency
bands.

References[1]“Fixed Service Applications using Free-space Optical Links”, ITU-R, Report ITU-R F.2106,2007.

[2]

K. Tsukamoto, A. Hashimoto, Y. Aburakawa, and M. Matsumoto :''The Case for Free Space Explorng the Prospect of Free-space Optical Links for Backhaul Applications-'', IEEE
Microwave Magazine, Vol. 10, No.5, pp. 84-92, August 2009.

[3]

E. Ciaramella, Y. Arimoto, G. Contestabile, M. Presi, A. D’Errico, V. Guarino, and M.
Matsumoto, "1.28-Tb/s (32 40 Gb/s) Free-Space Optical WDM Transmission System," IEEEPTL, Vol 21, No. 16, pp. 1121-1123, 2009.

[4]

K. Tsukamoto, et. al. ,"Development of DWDM Radio on Free Space Optic Link System for
Ubiquitous Wireless'', Technical Digest of AP-MWP2007, Vol.1, No.1, April 2007.

[5]

K. Kazaura, et. al. ,"Experimental Evaluation of a Radio-on-FSO Communication System for
Multiple RF Signal Transmission", Proc. of SPIE, Vol. 7199, pp. 719907-1-12 , January 2009.

[6]

S. Komaki :''Microwave Technologies for Software Radio Networks'', Proceedings ofAPMC2003, Vol. 3, No. FC7-1, pp. 1780-1785, November 2003.

[7]

Y. Ito and Y. Ebine, “Radio on Fiber System for Triple Band Transmission in Cellular Mobile
Communication,” IEEE International Topical Meeting on Microwave Photonics (MWP2000)
Tech. Dig., TU1.2, pp. 35-38, Sep. 2000.

[8]

Y. Horiuchi, “ROF Application to 3G Mobile Systems in Offices and Outdoors,” International
Topical Meeting on Microwave Photonics (MWP2005) Tech.Dig., p. 3, Oct. 2005.

[9]

S.Komaki, ”Software Definable Radio Networks for Ubiquitous Network Applications,”
ASTAP Wireless JEG (MMCS-EG) 9, ASTAP08-FR14-EG-MMCS-04_RadioNetworks , Kobe,June 2008.

[10] K.

Kazaura, et. al. ,"Experimental Performance Evaluation of Next Generation FSO

Communication System," AP-MWP 2006, Vol. 1, pp. 289-292, April 2006.

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[11] Hakki H.Refai, et al.,“The Transmission of Multiple RF Signals in Free Space Optics Using

Wavelength Division Multiplexing”, Proceedings of SPIE, Vol.5793, pp 136-143, 2005.
[12] G. Katz, S. Armon, P.Goldgeier, Y. Hauptman and N. Atias, “Cellular over Optical WirelessNetworks”, IEE Proc.-Optoelectron.,Vol. 153, No.4, pp 19.

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