Kazunori Omae

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.

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.

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.

An alternative access technology for next generation networks based on full-optical wireless communication links

In order to provide telecommunications services in the envisaged next generation network (NGN), a packet based network utilizing multiple broadband, QoS-enabled transport technologies and in which the service related functions are independent of the underlying transport related functions is required. Optical wireless communication (OWC) systems will play an important role as alternative broadband access technology in the emerging NGNs. We have developed a revolutionaiy new full-optical wireless communication (F- OWC) system which operates by directly propagating an optical beam from a fiber termination point through free-space using a F-OWC transceiver. At the receiving end a corresponding transceiver directly couples the free-space propagated optical beam into an optical fiber connection port. In this paper we present the design concept and highlights of this new F-OWC system utilizing an innovative technology for seamless coupling a free-space propagated optical beam to a fiber. Experimental results, based on a prototype system, demonstrate that the system is capable of offering stable, error-free transmission at multiple giga-bit per second data rates over extended periods of time.

Investigation of suitability of RF signal transmission over FSO links

Transmission of modulated RF signal using FSO communications links is receiving a lot of research interest recently. We are developing a new advanced DWDM RoFSO link system capable of transmitting multiple RF signals. In this paper, we present results from analytical modeling reflecting on the design parameters of the RoFSO system to evaluate and predict its performance. The parameters of interests include the atmospheric turbulence characterization parameter, Cn 2, as well as another important parameter in evaluating RF signals, SFDR. Results show that for W-CDMA signal transmission using RoFSO in presence of relative strong atmospheric turbulence, the SFDR value might occasionally drop below the required threshold of 60 dB. A 10 dB improvement of SFDR can be achieved when an optical power amplifier is employed. The paper also shows preliminary experimental results of a RF-FSO link performance in terms of CNR and ACLR when transmitting W-CDMA signal. Combination of theoretical and preliminary experimental works will give a helpful insight to the performance of the new RoFSO system.

Mitigation of atmospheric effects on terrestrial FSO communication systems by using high-speed beam tracking antenna

When a free-space optical beam propagates through the atmosphere it experiences deterioration and deformation of its wave-front caused from small scale, randomly localized changes in the atmospheric index of refraction. This results in beam wander and scintillation effects which can reduce the link availability and may introduce burst errors. This paper outlines experimental work on a free-space optical (FSO) communication system which connects an optical beam directly to a single-mode fiber (SMF) without any optical-to-electrical (O-E) conversion. In order to effectively couple the 1550 nm transmitted optical beam to a SMF it is necessary to be able to track and control the beam angle-of-arrival (AOA) changes. To achieve this, we have developed an optical antenna which uses a fine positioning mirror (FPM) capable of performing high-speed beam tracking and steering thus reducing to a great extent the optical power fluctuations of the received beam coupled to the SMF. This optical power fluctuation is partly a result of beam angle-of-arrival fluctuations caused by atmospheric turbulence. In our experiments we have tried to measure and quantify the magnitude of atmospheric turbulence experienced by an optical beam propagating through the atmosphere. First we demonstrate the relation between the AOA fluctuations and the frequency characteristics of the scintillation effects for a free-space optical beam propagating through a turbulent atmosphere. We use this information to determine the optimum antenna FPM tracking speed for improved performance and error free transmission. The results in the improved fiber received power as well as continuous error free transmission are presented.

Link design of Radio on Free Space Optic system for heterogeneous wireless services

Radio on free space optics (RoFSO) is being developed to realize a cost effective and universal platform for heterogeneous wireless services as same as RoF. The developed RoFSO transceiver can directly transmit some RoF signals from an optical fiber and can receive an RoFSO signal from air into a fiber core. It can achieve an free space that is transparent for various types of wireless services in an optical wireless beam. This paper describes the features of developed equipments, and propose the link budget design for WLAN and terrestrial digital broadcasting services.

Experimental demonstration of next-generation FSO communication system

Free-space optical communication has emerged as a competitive and viable technology for offering high data rates, improved capacity, cost-effective and an easy to deploy solution for providing connectivity between two points which are up to a few kilometers apart. In this paper we present experimental work which demonstrates the practicality of next generation free-space optical (FSO) communication systems suitable for short-haul, high-speed and robust data links. This experimental system is placed between two buildings in the Waseda University campus area for a communication link spanning a distance of 1 km. We outline the design of the optical antenna which uses 1550 nm wavelength and directly coupling a freespace optical beam to a single-mode fiber without the need for OE/EO conversion, to offer a communication link with data rates from 2.5 Gbps to 10 Gbps. The antenna is capable of overcoming most common limitations inherent in FSO communication systems, such as atmospheric induced beam wander and scintillation effects. A high-speed tracking mechanism which utilizes a fine positioning mirror (FPM) capable of tracking and controlling the received beam and focusing/steering most of beam power into the fiber is presented. This FPM is capable of suppressing the frequent power fluctuations caused by beam angle-of-arrival (AOA) variations. This paper presents experimental results of the FSO communication system capable offering stable performance in terms of measured bit-error-rate (BER). Performance results showing increasing the systems data rate from 2.5 Gbps to 10 Gpbs are also presented.

Project of Radio on Free Space Optic System Development for Heterogeneous Wireless Services

Radio on free space optics (RoFSO) is being developed to realize a cost effective and universal platform for heterogeneous wireless services as same as RoF. The developed RoFSO transceiver can directly transmit some RoF signals from an optical fiber and can receive an RoFSO signal from air into a fiber core. It can achieve an free space for various types of wireless services in an optical wireless beam like a fiber. This paper describes the features of developed equipments, and propose the link budget design for WLAN and terrestrial digital broadcasting services.

Characterization of RF signal transmission using FSO links considering atmospheric effects

Radio on Free-Space Optics (RoFSO) communication systems have attracted a considerable attention for a variety of applications where optical fibers are not feasible, especially in rural areas, to provide ubiquitous wireless services quickly and more effectively. RoFSO links can be used to transmit signals like cellular W-CDMA, terrestrial digital TV or WLAN signals. In spite of its potential, such links are highly dependent on the deployment environment characteristics in particular the weather conditions. Severity and duration of the atmospheric effects have direct impact on the availability of the links as well as on the quality of RF signal transmitted over it. Thus, the necessity of investigating the effects of various weather conditions on RF signal transmission using FSO links. In collaboration with several institutions, we are currently developing an advanced Dense Wavelength Division Multiplexing (DWDM) RoFSO antenna capable of transporting multiple RF signals. As preliminary work, we are conducting experiments on a 1 km link using an off-the-shelf Radio Frequency - FSO (RF-FSO) antenna, with the objective of obtaining and characterizing performance related parameters of RF-FSO transmission in operational environment. As an example, we examine the influence of atmospheric turbulence on the transmission quality of W-CDMA signal. Among the performance metric of interest is the Adjacent Channel Leakage Power Ratio (ACLR) which will be measured, analyzed and correlated with the weather conditions. An atmospheric fluctuation model for estimating the communication quality of RF signal transmission on FSO links is being developed. Also the obtained results will be used for the deployment environment characterization as well as baseline for the design and performance evaluation of new advanced DWDM RoFSO communication systems we are currently developing.