Kazuhiko Wakamori

Transmission Analysis of OFDM-Based Wireless Services Over Turbulent Radio-on-FSO Links Modeled by Gamma–Gamma Distribution

Radio-on-free space optical (RoFSO) communication systems are rapidly gaining popularity as an efficient and cost-effective means of transferring high data rates and radio frequency (RF) signals with the same capacity as optical fiber. However, the performance of those systems depends strongly on the atmospheric conditions and the nonlinear characteristics of the optical link. In this paper, we introduce an analytical model for the transmission of orthogonal frequency division multiplexing (OFDM)-based signals over freespace optics (FSO) links. Further, we derive a closed-form bit error probability (BEP) and outage probability expressions, taking into account the optical noises, the laser diode nonlinear distortion, and the atmospheric turbulence effect on the FSO channel modeled by the gamma-gamma distribution. This paper reports the most significant parameters that degrade the transmission performance of the OFDM signal over FSO links and indicates the cases that provide the optimal operating conditions for the link. The obtained results can be useful for designing, predicting, and evaluating the RoFSO systems ability to transmit wireless services over turbulent FSO links under actual conditions.

RoFSO: A universal platform for convergence of fiber and free-space optical communication networks

The demand on capacity and quality offered over wireless communication links has pushed researchers to innovate new design methodologies and concepts over wireless systems and networks with the ultimate aim towards achieving a Next Generation Network (NGN). Among the emerging technologies is the Radio on Free Space Optics (RoFSO) system described in this paper. With this technology it is possible to simultaneously transmit multiple RF signals comprised of various wireless services over FSO links using WDM technology. The technology can be applied as a universal platform for providing convergence of fiber and free-space optical communication networks extending broadband connectivity to underserved areas. We present the design concept and highlight some experimental results obtained from performance evaluation of the RoFSO system we have developed. The results demonstrate a satisfactory performance in terms of reliability and stability based on the quality metric parameters defined for the different RF service signals transmitted over the RoFSO system. Considering the potential of the RoFSO technology we propose a study for standardization work in the ITU as an initiative which can lead to its rapid adaptation.

Performance Evaluation of Next Generation Free-Space Optical Communication System

Free-space optical communication systems can provide high-speed, improved capacity, cost effective and easy to deploy wireless networks. Experimental investigation on the next generation free-space optical (FSO) communication system utilizing seamless connection of free-space and optical fiber links is presented. A compact antenna which utilizes a miniature fine positioning mirror (FPM) for high-speed beam control and steering is described. The effect of atmospheric turbulence on the beam angle-of-arrival (AOA) fluctuations is shown. The FPM is able to mitigate the power fluctuations at the fiber coupling port caused by this beam angle-of-arrival fluctuations. Experimental results of the FSO system capable of offering stable performance in terms of measured bit-error-rate (BER) showing error free transmission at 2.5 Gbps over extended period of time and improved fiber received power are presented. Also presented are performance results showing stable operation when increasing the FSO communication system data rate from 2.5 Gbps to 10 Gbps as well as WDM experiments.

A Universal Platform for Ubiquitous Wireless Communications Using Radio Over FSO System

In this paper, we present the design concept and performance evaluation results of a newly developed DWDM Radio over Free-Space Optics (RoFSO) system capable of transmitting multiple wireless service signals simultaneously. The system performance is evaluated over 1-km free-space transmission under effects of atmospheric turbulence and attenuation. Effectiveness of a proposed tracking scheme used to mitigate beam angle-of-arrival (AOA) effect and the viability of the communication system using a specially designed optical antenna are illustrated. The properties of free-space propagation under influences of atmospheric turbulence and attenuation are characterized. We then present the performance of wireless signals after being transmitted over the system. We verify that the system can provide a stable connection for wireless services especially with the absence of strong atmospheric turbulence and heavy rain. Finally, we present a consideration on the system application by indicating the possible applicable link range based on the system designed power margin and the tracking system efficiency. The proposed system is expected to be an alternative or be combined with wireless over fiber (WoF) to realize a universal platform for ubiquitous and high throughput wireless communications.

Enhancing performance of next generation FSO communication systems using soft computing based predictions

The deterioration and deformation of a free-space optical beam wave-front as it propagates through the atmosphere can reduce the link availability and may introduce burst errors thus degrading the performance of the system. We investigate the suitability of utilizing soft-computing (SC) based tools for improving performance of free-space optical (FSO) communications systems. The SC based tools are used for the prediction of key parameters of a FSO communications system. Measured data collected from an experimental FSO communication system is used as training and testing data for a proposed multi-layer neural network predictor (MNNP) used to predict future parameter values. The predicted parameters are essential for reducing transmission errors by improving the antennas accuracy of tracking data beams. This is particularly essential for periods considered to be of strong atmospheric turbulence. The parameter values predicted using the proposed tool show acceptable conformity with original measurements.

Transmission analysis of digital TV signals over a Radio-on-FSO channel

Recently, radio-on-free-space optical (RoFSO) technology is regarded as a new universal platform for enabling seamless convergence of fiber and FSO communication networks, thus extending broadband connectivity to underserved areas. In this article, an experimental demonstration of the newly developed advanced RoFSO system capable of transmitting the Japanese integrated services digital broadcasting- terrestrial (ISDB-T) signals over a 1 km FSO link is presented. Our innovative system combines a new generation full optical FSO system with radio over fiber technology. Important performance quality metrics like the modulation error ratio and bit error rate are measured and analyzed to characterize the deployment environment influencing the transmission of ISDB-T signals over the FSO link. The obtained results can be used for designing, predicting, and evaluating the RoFSO system capable of transmitting multiple wireless services over turbulent FSO links.

Design and evaluation of optical antenna module suitable for radio-on free-space optics link system for ubiquitous wireless

We present initial results on research and development of an optical antenna module suitable for Radio-on Free-Space Optics (RoFSO) links. This new optical communication system is envisaged to be an effective means of realizing a ubiquitous society and therefore eliminating the digital divide. The RoFSO system is a trial system applying Radio on Fiber (RoF) technology for transmission through free space. Based on the results of research of next generation high-speed free-space optical communication system conducted in the past two years at Waseda University, we have developed an optical antenna module with efficient laser receiving characteristics as well as simple adjustment. The tracking system adopts two phases including rough tracking by the beacon light at 0.85 μm wavelength and fine tracking using communication light at 1.55 μm wavelength to improve compensation precision for the atmospheric turbulence at the time of beam propagation. We present results on the evaluation of performance characteristics (static characteristics) of the separate functions for RoFSO antenna module we have developed and confirmed the coupling efficiency and fine tracking characteristics which were set as goals at the beginning.

Bit error rate performance analysis of optical CDMA time-diversity links over gamma-gamma atmospheric turbulence channels

When we use the free space optical(FSO) communication in the near-ground, it will be great influenced by the atmospheric turbulence. There are many studies on the effects of atmospheric turbulence in FSO. Recently gamma-gamma is a widely accepted model for the optical propagating through the atmosphere in normal and strong turbulence. In order to enhance bit error rate(BER) performance influenced by turbulence-induced fading, we promote a new time-diversity scheme which use optical code division multiple access(OCDMA) as channel division code. As we know, time-diversity is an effective method to enhance the throughput performance of wireless communication system. In order to provide a higher performing and cost-efficient solution OCDMA time-diversity scheme is proposed. In this paper, we analyze the performance of the new scheme with gamma-gamma model. We first present the structure of the OCDMA time-diversity scheme. Then we present the temporal covariance of gamma-gamma model to get the independent time-interval of atmospheric turbulence. We use experiment data to confirm theoretical result. At last we give the theoretical analysis on the BER of new scheme, and use numerical results to prove the theoretical analysis. We find that the new OCDMA time-diversity scheme can enhance the BER for several orders in strong turbulence. Furthermore, the performance of the new scheme in strong turbulence is even better than the traditional scheme in normal turbulence.

Studies on characterizing the transmission of RF signals over a turbulent FSO link.

In this paper, we present an experimental study on transmission of RF signals over turbulent free-space optics (FSO) channel by using off-the-shelf Radio Frequency - FSO (RF-FSO) antennas. The results demonstrate potential of utilizing FSO links for transmission of RF signals and are used as a guideline in the design, prediction and evaluation of an advanced Dense Wavelength Division Multiplexing (DWDM) RoFSO system we are developing capable of transmitting multiple RF signals. An analytical modeling of the system is also conducted to identify key parameters in evaluating the performance of RF signal transmission using FSO links. The results confirm that the effect of scintillation on RF-FSO system performance can be estimated by using a simple estimation equation and satisfactory result are obtained from comparing the experimental and theoretical derived data under weak to strong turbulence condition.

A study on transmission of RF signals over a turbulent free space optical link

In a ubiquitous network society, a universal platform is necessary to satisfy customerspsila increased demand on communication at any-time, anywhere and any-situation. Radio on fiber (RoF) technologies can realize a cost effective universal platform for ubiquitous wireless services. RoF networks can also be extended to radio on free space optics (RoFSO) providing communication links for heterogeneous wireless services where it is not easy or feasible to install optical fiber. In this paper, we present an experimental study on transmission of RF signals over a turbulent FSO channel by using off-the-shelf radio frequency - FSO (RF-FSO) antennas. The performance results show the potential of FSO links for transmission of RF signals and are also used as a benchmark in designing, predicting and evaluating an advanced dense wavelength division multiplexing (DWDM) RoFSO antenna capable of transporting multiple RF signals we are developing. An analytical modeling of the system is also conducted to confirm the suitability of the system and a good result is obtained compared with experimental data under weak turbulence condition.