A non-profit satellite communication network is desired to be configured by using low cost earth stations in the field of education, research and health in the Pacific region. This paper proposes the following concept as one of the tools to realize such a network: (a) A hitchhiker transponder dedicated to the network, and (b) The volunteer group prepares earth stations. A preliminary system design shows that the S band hitchhiker payload is most appropriate and has the weight of about 3kg. The feasibility of manufacturing earth stations by a volunteer group is examined through the experiment using ETS-V satellite. The parameters of the hitchhiker payload are re-examined on the basis of the experience of the experiment.

NICT has developed a test model of an optically controlled beam-forming network (OBF) for a future multiple-beam antenna. The OBF test model consists of an electro-optic converter unit, an OBF unit, and an optoelectronic converter unit. A Ka-band OBF test model was manufactured to demonstrate the OBF. Radiation patterns obtained from the measured OBF data confirmed agreement between the expected and calculated results. Communication tests of the bit error rate (BER) for the digital communication link were performed. The results confirmed the OBF had no serious degradation below 1 dB of Eb/N0 on BER performance at 110-8.

The progress of multimedia applications for education, research, social welfare and commerce is generating a lot of interest in the potential of a combination of satellite networking and Internet technology. The combination is particularly attractive as a low cost solution in regions which are large and sparsely populated. In 1991, aiming at networking the Pan-Pacific region, the PARTNERS (Pan-Pacific Regional Telecommunications Network Experiment and Research by Satellite) project was initiated. In this project, the major target was to construct a satellite-based network infrastructure to support education, research and so on in the Pan-Pacific region. As a part of PARTNERS the MEISEI-NET (Multimedia EducatIon System using satellite ETS-V and InterNET) project was started to evaluate the utility of satellite networking for education and reserch and, to investigate the feasibility of expanding the reach of the Internet using the PARTNERS infrastructure. MEISEI-NET focussed on (1) low start-up cost, (2) open access to the rich information resources on the Internet, (3) use of network to support education and research, and , (4) development and distribution of software for MEISEI-NET users. The construction of MEISEI-NET will be detailed followed by a report on its usage and the effects of this network. To support and manage MESEI-NET operations, we developed and deployed SNMP-based intelligent network management system. It offered fault detection and notification. This made the MEISEI-NET robust and practical despite of the satellite's (ETS-V) drift-problem. Students and researchers of universities from different countries participated in and benefited from MEISEI-NET until March 1996.

The performance of a digital matched filter receiver with multi-bit quantized correlators for Direct Sequence Spread Spectrum (DSSS) mobile satellite applications is evaluated with experiments. A Coherent Matched Filter (CMF) technique is used to accomplish fast code acquisition, automatic frequency control and coherent detection of data with a single matched filter circuit to satisfy the requirements for the mobile satellite applications. Loop-back test and field test results in suburban areas with the ETS-V satellite are presented, in which the use of multi-bit quantized correlators improve the initial acquisition and the bit error rate performances. The dynamic performance of the CMF receiver is confirmed to be satisfactory for the mobile satellite fading channels.

For a LEO constellation system, it is important to optimize the orbit parameters to maximize the quality of communication service. At the Next-generation LEO System (NeLS) Research Center, the LEO parameters were evaluated for a mobile satellite communication system. A 2π constellation was selected to maintain a stable inter-satellite link (ISL). An elevation angle above 20 degrees was required for a service area of 70 degree of latitude. The performance of optical ISL terminals has since improved as a result of key technological developments at the NeLS Research Center. As a consequence, the constellation parameters for ISL have become more flexible. Furthermore, the ability of ground station to access two satellites has improved communication quality. In this paper, we address the optimum constellation parameters for dual-satellite coverage. An equation for determining the optimum inclination angle was derived from the constellation parameters. Moreover, by using the new constellation parameters, we found that the satellite network consists of a bi-directional Manhattan Street Network (MSN), and the ISL network structure was improved.

Current trend in telecommunications is "broadband" and "ubiquitous." To achieve this goal, satellite communications systems are expected to play an important role in cooperation with terrestrial communications systems. Along with the advancement of optical fiber transmission systems, the role of satellite communications was dramatically changed from long distance transmission to various applications utilizing unique features of satellite communications. This paper overviews recent Japanese R&D in satellite communications.

In inter-vehicle networks, it is required that collision warning messages be distributed over multiple hops with significantly short delay. Cut-through packet forwarding is known to be an effective method to reduce forwarding latency. However, it suffers from the reliability issue, which is especially severe in highly mobile scenario, due to multi-path fading. This paper tries to establish a reliable cut-through forwarding scheme by applying diversity combining method to collision warning messages. By exploiting the nature of the multi-hop forwarding of a broadcast packet in CDMA wireless networks, the diversity combining scheme does not incur any additional communication overhead or delay. The simulation results show that diversity combining schemes, especially Maximal Ratio Combining (MRC), can effectively reduce BER by orders of magnitude.

In Mobile Ad hoc Networks (MANET) geographic routing is characterized by local forwarding decision. Links with a long progress are preferred under the greedy forwarding rule. However in a real system long links tend to have a high packet loss rate due to multipath fading. A sub-optimal solution may separately exploit path diversity or rate adaptation. In this paper we study channel efficiency of multi-hop forwarding and try to jointly optimize rate adaptation and forwarder selection in geographic routing by the tradeoff between progress and instantaneous rate. We define a new metric -- Bit Transfer Speed (BTS) -- as the ratio of the progress made towards the destination to the equivalent time taken to transfer a payload bit. This metric takes overhead, rate and progress into account. Then we propose a packet forwarding scheme that Opportunistically exploits both long Progress and Adaptive Rate (OPAR) by a cross-layer design of routing and MAC. In OPAR each node selects for a packet the forwarder with the highest BTS. The forwarder changes as local topology (progress), packet size (overhead ratio) or channel state (data rate) varies. Simulation results show that compared with the normalized advance (NADV) [7] scheme and contention-based forwarding (CBF) [17] scheme, OPAR has lower packet loss and can effectively reduce channel occupation time by over 30% in the scenario with moderate mobility speeds.

The increasing demand for navigation and automation has led to the development of a number of accurate and precise navigation applications that make use of the Global Navigation Satellite System (GNSS) and additional sensors. One of the precise navigation techniques in GNSS, the real-time kinematic (RTK) technique, is well known. In this method, once the correct integer ambiguities are found in the carrier phase observation data, position can be determined to within 10cm. In particular, the advent of QZSS and BeiDou satellites can increase the availability of RTK-GNSS (relative to RTK using only GPS). It is understood that the increasing availability of RTK-GNSS will improve the performance of the integration of GNSS with additional sensors because the errors due to the inertial measurement unit (IMU) accumulate as time goes on. On the other hand, GNSS tends to suffer from multipath errors, especially in urban environments. To overcome this problem, a method was developed for improving RTK-GNSS using a low-cost IMU and conventional vehicle speed sensors. In this study, the quality of the complete observation data was assessed based on the carrier-to-noise ratio and satellite elevation angle, and the least-squares ambiguity decorrelation adjustment method and the ratio test were used to obtain fixed positions. We used speed information obtained from Doppler measurements as an alternative source of information; information from the IMU and vehicle speed sensor (integrated with the RTK-GNSS via a Kalman Filter) was used when there were no visible satellites. We also used the IMU and vehicle speed sensors to detect wrong fixes in the RTK-GNSS. A position and orientation system for land vehicles (Applanix) was used to estimate the reference positions. During GNSS outages, it is important to accurately determine the last heading of the car for precise navigation. In this study, it was found that GNSS Doppler-based direction data are required to obtain better direction information. The results of the experiment demonstrate that our proposed method is, to some extent, beneficial as an alternative to the conventional RTK-GPS in an urban environment.