Graduate School of Global Information and Telecommunication
Studies, Waseda University
Abstract of Doctoral Dissertation
Studies on Performance of Optical Wireless Systems for
RoF Signal Transmission
RoF
信号を伝送する光無線通信システムの
パフォーマンスに関する研究
PHAM Tien Dat
Optical Radio Wireless Application Engineering II
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Summary
Advanced information and communication technologies have spanned the globe, providing users with mobile and real-time access to information and services. Communication demand is increasing with higher requisition to access any desired information, at any-time, anywhere, in any-situation easily and immediately. Increasingly the services offered are becoming an indispensable part of daily life. A ubiquitous network, therefore, is necessary to satisfy the demand regardless of geographical locations. To come to a ubiquitous communication environment, however, heterogeneous wireless facilities have to be connected together and form a network spontaneously on demand. Nevertheless, in current wireless networks, operators construct and manage their own radio resources and networks separately, resulting in high system’s cost and preventing quick deployment of new wireless services, especially in the areas where broadband fiber-infrastructures have not been constructed such as in in-building, underground or remote areas.
Radio on Fiber (RoF) technology, a means of transmitting radio signals using optical fibers without changing radio format, has become a candidate for the common platform for wireless access networks. This technology provides a simplified and cost effective radio access network and supports high-speed multimedia in real-time to satisfy the increasing demand when the radio spectrum is limited. It also helps to increase transmission capacity and extend the distance of wireless communication thanks to high capability and low transmission loss of optical fiber. However, it is not always easy and feasible to use RoF systems in practice, especially in the areas where installed optical fibers (dark fibers) are not available. Installing new optical fiber will delay the network rollout and increase the whole system cost which is very important in access networks.
Recently, fortunately, free-space optics (FSO) communication systems can be used as an alternative means to transfer radio signals as an extension of RoF systems in free-space. FSO is the technology in which modulated optical signal is propagated over free-space without using optical fiber medium. FSO links provide the same capacity as optical cable in carrying radio signals while giving many other advantages including easy and quick deployment. The transmission of radio signals using FSO links combines the advantages of high transmission capacity enabled by optical device technologies and flexibility of wireless links.
Transmission of radio signals over FSO system can be applied for rapid provisioning of heterogeneous wireless services in underserved areas left out due to the high costs of installing new networks. It can also be utilized for rapid provision of broadband point-to-point communication links in both rural and urban settings, thus extending broadband connectivity to remote areas. The other potential application is that the links can effectively be used for interconnectivity of distributed antenna systems to provide links for remotely located base stations and antennas similar to RoF applications. Unlike RoF and digital FSO systems, however, the performance of radio signal over FSO links is easier to be deteriorated by deployment environment characteristics due to its analog nature, and thus, it
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requires more accurate design for the systems. To take the advantages of FSO communications and realize it as an attractive means in nowadays communications, it is necessary to investigate suitable system configuration and parameters, especially receiver schemes in order to optimize the system performance and minimize the effects of atmospheric environment. The work presented in this thesis is conducted with the aim to deal with this issue.
The first work presented in the thesis is an experiment study of the transmission of RF signals over a turbulent conventional FSO link. By transmission of W-CDMA signal as an example of RF signals, the system performance quality metrics parameters are measured and analyzed under various atmospheric influences including turbulence, heavy rain and snow conditions. The obtained results show the satisfactory performance especially in the cases with the absence of severe weather conditions. The results strongly confirm the possibility of RF signals transmission over the FSO channel with acceptable performance if the link margin is enough to compensate for atmospheric effects. In the case that transmitted signal is W-CDMA and required availability is 99%, the link margin is defined to be more than 20 dB. A simple analytical modeling of the system is also conducted in consideration of scintillation variance in the deployment field and the system specification parameters. The similarity between calculated and measured results is obtained. It is derived from the result that the effects of atmospheric turbulence on the system performance can be described by a simple model. The modeling then can be applied to evaluate other important RF signal quality metrics parameters which are difficult to measure directly, and a satisfactory performance result is obtained compared to standard required value. The results obtained from this research gave some guidelines and insights to the design, prediction, and evaluation of a newly developed Radio over FSO system.
The second work in this thesis is a study to evaluate the performance of a newly developed Radio over FSO system especially the optical path reflecting on an actual operation environment. The effectiveness of a tracking scheme utilizing a Fine Pointing Mirror (FPM) and the viability of the communication system using a specially designed optical antenna are presented. It is illustrated that the system can provide a stable connection for signal transmission, including digital and wireless signals. However, due to the use of direct connection between free-space and fiber cable at the receiver, the system is affected by a larger fading and experience new characteristics compared to the conventional systems and have not been reflected in any theory. The additional fading is observed both in optical received power and wireless signal performance. The work presented in this chapter is to quantify the fading and derive new characteristics, and from that propose a new model for system transmission loss and a design for system link margin to overcome the limitations. A consideration on applying the system effectively for different wireless services is also presented. The availability of the system for transmission of wireless signals over 1 km range is estimated based on the statistics of weather parameters and system link margin.
In addition, the system being used in the experiment is a prototype one and has not been fully optimized yet. The performance is therefore expected to be improved when the system design is optimized. On the other hand, further performance improvement may be realized if the system is
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combined with other atmospheric turbulence mitigation techniques. The third work in the thesis is to deal with this issue. A system using a reception diversity scheme is introduced, modeled and numerically evaluated. To compare accurately the performance of systems employing different numbers of receivers, a new analysis considering the impact of receiving aperture size on signal fading and received power is presented. It is clarified that by using this model, the system performance is significantly enhanced, especially under strong turbulence conditions. Based on the model, optimal values for system including receiver aperture size, optical pre-amplifier gain, transmitting power and number of receivers are also derived. This might be helpful for future design to minimize the effects of atmospheric turbulence and optimize the system performance.
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List of academic achievements
Category (Subheadings)
Paper Title
Articles in
refereed journals
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P. T. Dat, A. Bekkali, K. Kazaura, K. Wakamori, M. Matsumoto, “A Universal Platform for Ubiquitous Wireless Communications using Radio over FSO System,” IEEE/OSA Journal of Lightwave Technology, Vol. 28, Iss. 16, pp. 2258–2267, Aug. 2010.○
P. T. Dat, A. Bekkali, K. Kazaura, K. Wakamori, T. Suzuki, M. Matsumoto, T.Higashino, K. Tsukamoto, S. Komaki, “Studies on Characterizing the Transmission of RF Signals over a Turbulent FSO Link,” OSA Journal of Optics Express, Vol. 17, No. 10, pp 7731- 7743, May 2009.
Presentations at International conferences
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P. T. Dat, C. Naila, P. Liu, K. Wakamori, M. Matsumoto, and K. Tsukamoto, “Next Generation Free Space Optics System for Ubiquitous Communications,” 2011 Progress in Electromagnetics Research Symposium (PIERS), Marrakesh, Morocco, 20-23 March, 2011○
P. T. Dat, A. Bekkali, K. Kazaura, K. Wakamori, T. Suzuki, M. Matsumoto, T. Higashino, K. Tsukamoto, and S. Komaki, “Performance Evaluation of an Advanced DWDM RoFSO System for Heterogeneous Wireless,” 2009 IEEE GLOBECOM, Honolulu, Hawaii, USA, Dec. 2009○
P. T. Dat, A. Shah, K. Kazaura, K. Wakamori, T. Suzuki, K. Takahashi, M. Matsumoto, Y. Aburakawa, T. Nakamura, T. Higashino, K. Tsukamoto, and S.Komaki, “A Study on Transmission of RF Signals over a Turbulent Free Space Optical Link,” 2008 IEEE Topical Meeting on Microwave Photonics, Gold Coast, Queensland, Australia, Oct. 2008.
P. T. Dat, A. Shah, K. Kazaura, K. Wakamori, T. Suzuki, K. Takahashi, M. Matsumoto, Y. Aburakawa, T. Nakamura, T. Higashino, K. Tsukamoto, and S. Komaki, “An Innovative Technology for Ubiquitous Communication using Radio on FSO Links,” International Conference on Advanced Technologies for Communications, Hanoi, Vietnam, Oct. 2008.
P. T. Dat, A. Shah, K. Kazaura, K. Omae, T. Suzuki, H. Watanabe, M. Matsumoto, T. Higashino, Y. Aburakawa, K. Tsukamoto, S. Komaki, T. Nakamura, K. Wakamori, “On the Challenges of Transporting RF Signals over an FSO Channel”, OECC/IOOC, Yokohama, Japan, Jul. 2007.
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Presentations at domestic conferences
P. T. Dat, A. Bekkali, K. Kazaura, K. Wakamori, T. Suzuki, M. Matsumoto, T.
Higashino, K. sukamoto, and S. Komaki, “Development of Radio on Free Space Optics System for Ubiquitous Wireless Services − (4) Optical Path Link Margin and
Applicable Link Range of a RoFSO System,” IEICE General Conference, C-14-4, Mar. 2009.
P. T. Dat, A. Bekkali, K. Kazaura, K. Wakamori, T. Suzuki, M. Matsumoto, T. Higashino, K. Tsukamoto, and S. Komaki, “Development of Radio on Free Space Optics System for Ubiquitous Wireless Services − (5) Angle-of-Arrival
Characterization Measurement,” IEICE Society Conference, C-14-14, Sept. 2008. P. T. Dat, A. Bekkali, K. Kazaura, K. Wakamori, T. Suzuki, M. Matsumoto, T. Higashino, K. Tsukamoto, and S. Komaki, “Development of Radio on Free Space Optics System for Ubiquitous Wireless Services − (4) Link Margin Analysis for a Given RF-FSO System,“ IEICE General Conference, C-14-4, March, 2008.
Others P. T. Dat, K. Wakamori, M. Matsumoto, “Studies on the Transmission Platform for Heterogeneous Wireless Communications Using Free- Space Optical Technology,” Exchange seminar Tsinghua – Waseda University, Beijing March 12nd, 2011 P. T. Dat, K. Wakamori, M. Matsumoto, “Studies on the Transmission Platform for
Heterogeneous Wireless Communications Using Free- Space Optical Technology,” Exchange seminar Hanyang – Waseda University, Tokyo November 27th, 2010 P. T. Dat, K. Wakamori, M. Matsumoto, “Studies on Transmission of Wireless Signals
over Free Space Optics Systems,” GITS PhD Academy workshop, Tokyo, October 6th, 2010
P. T. Dat, K. Wakamori, M. Matsumoto, “Next Generation Free Space Optical System Characteristics and Performance Enhancement using
Reception diversity, ” High-tech project research festival, School of Science and Engineering, Waseda University, October 16th 2010.
