Takeshi Higashino

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.

Direct optical switching code-division multiple-access system for fiber-optic radio highway networks

Recently, microwave photonics techniques have significantly progressed to realize a new type of communication system enhancing the synergetic effect of wireless easy access and photonic broadbandness. This paper describes the concept of radio highway networks configured by radio on fiber links, which are transparent for various types of radio formats. Also as an effective candidate for a multiple-access method of radio signal into the radio highway network, this paper reports direct optical switching code-division multiple-access system and shows its performance theoretically and experimentally.

Convergence of WDM Access and Ubiquitous Antenna Architecture for Broadband Wireless Services

This paper proposes a novel network architecture for Giga-bit throughput in broad- band ubiquitous networks. A convergence of WDM access and RoF ubiquitous antenna archi- tecture can realize universality of remote base stations for various types of air interfaces and the scalabilities of WDM technologies are expected to improve the throughput in wireless service area covered by RoF-MIMO antenna. We discusse distribution of MIMO antenna, method of RoF MIMO signals over WDM PON, and conflgurations of center station and remote base stations.

An experimental investigation of applying MIMO to RoF ubiquitous antenna system

This paper proposes the application of MIMO to RoF ubiquitous antenna system. As a result of locating remote antennas ubiquitously, it is experimentally found that the effect of MIMO is maintained and throughput performance is improved.

A new position detection method using leaky coaxial cable

Position detection services tend to be important in ubiquitous wireless communication systems. A new position detection method which utilizes an LCX (Leaky CoaXial cable) is proposed. Proposed method is based on a TOA (time of arrival) of both direct wave and reflected wave from the end of LCX. Position detection error is experimentally investigated.

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.

Radio access considerations for data offloading with multipath TCP in cellular/WiFi networks

Mobile network operators are facing a huge increase in data traffic. Moreover, several mobile traffic forecasts indicate that this trend will continue to increase in upcoming years. Nowadays, delivering adequate quality of service is already an important challenge for operators, and it will also be in the future. A critical issue is the availability of radio resources, which are licensed, intrinsically finite and usually costly. Alternatively, there are also unlicensed bands used by technologies like WiFi, which can be exploited. Indeed, mobile network operators are already using WiFi hotspots for offloading the data traffic from their macro networks. However, available offloading strategies cannot provide continuous connectivity across different networks, due to intrinsic limitations of current Transmission Control Protocol (TCP). Multipath TCP is a recent protocol that enables more flexible connectivity by extending conventional TCP, and could be used for offloading data traffic. The aim of this paper is not to propose a new solution, but to summarize the points which should be considered on the radio access when implementing data offloading with Multipath TCP.

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.

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.