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A simple communication network by microsatellites is studied. A few microsatellites pick up and deliver voice and some digital information from/to ground stations, and/or from/to satellites. 9.6 kbps and 1.2 kbps are used for the ground link and the inter-satellite link respectively with onboard digital processing techniques. It uses the formation of the microsatellites in one orbit, to widen the real-time communication area. The microsatellite is 50 kg of dumbbell shape with an expandable mast.

[Abstract] Satellite communication is expected to provide wide coverage areas and robustness for natural disaster or emergency use. It takes several years for development of a satellite and cannot be equipped with the latest technology in comparison with ground radio communication technology. Moreover satellite payload should become larger in proportion to advancement of a function, and maintenance of satellites becomes complicated when there are some troubles in the payload. The formation flight satellite system is a novel solution for the enhancement of satellite functionality and reliability. In this system, it is necessary to know the relative position between individual satellites accurately and to control a position each other. This paper proposes and evaluates a method of the location using Ultra Wide Band (UWB) signal by computer simulation. This UWB signal changes the carrier frequency of every pulse. Influence of Doppler shift is taken into account in Inter-Satellite ranging, and it was done by FM-CW radar, pulsed UWB and Multi-pulsed UWB. This calculation is based on the time of arrival (ToA) and two way ranging (TWR). Satellite location information is calculated from the ToA of the received signal, and this calculation from known location of master satellite around other satellites has to be done. This paper shows that multi-pulsed UWB is effective of ranging and location precision.

1 2 (Abstract) In recent years, the UWB system is studied as short-distance radio communications. However, sufficient consideration has not been performed as long- distance communication. In this paper, a DS-UWB system is proposed for the inter- satellite communication which is long-distance radio communications, and the influence of the Doppler effect by relative movement between satellites is evaluated by the computer simulation. In order to simplify the composition of a receiver, the effects of the down conversion of the frequency in a reception side are also evaluated. As a pulse of the UWB system using a millimeter wave, modulated Gaussian pulse is assumed. In this research, the influence of the maximum Doppler frequency shift in case the relative velocity between satellites is 1 (km/s) and 10 (km/s) is considered. As a result, it was shown that the BER characteristic hardly deteriorates by the above relative velocity, and the use of down conversion eases the requirement of carrier synchronization at the receiver.

Performance of mobile satellite communication in stratospheric platform along with low Earth orbit and quasi zenith satellite channels are examined. The throughput performance of mobile satellite communications using the slotted and unslotted CDMA-ALOHA systems is studied. Computer simulation is carried out using measured propagation data, and throughput performance for the different propagation characteristics is compared. The packet delay performance is also evaluated along with the altitude of satellite when ARQ (automatic repeat request.) scheme is introduced for error control. This paper studies the effect of transmission control of packets following the channel state monitor to improve throughput performance in measured land mobile satellite channels.

The effects of overhead on the throughput and delay performance of mobile satellite communications using slotted CDMA ALOHA systems are examined. Computer simulation is carried out using measured propagation data, and throughput performances for the different propagation characteristics are compared. The packet delay performance is evaluated along with the altitude of satellite when the ARQ (automatic repeat request) scheme is introduced for error control. Also proposed is the adaptive data rate slotted CDMA ALOHA. In this method the data rate is changed for every packet by the situation of the propagation path and is applied to slotted CDMA ALOHA. The improved throughput, delay time and data rate are shown in case of high traffic.

This paper reports land mobile satellite propagation measurements at higher elevation angles using the ETS-VI satellite at S-band (2.1 GHz) frequency. During the links between the ETS-VI and a ground earth station, the satellite is available at 50 to 67 degrees in elevation, 10 to 20 degrees higher than with geostationary satellites. Propagation data at the same test course and different elevation angles to the satellite are taken that describe elevation dependent properties of fade and non-fade duration statistics. Measured data show that shadowing statistics depend on the elevation angle and that the use of higher elevation satellites greatly enhance the availability of land mobile satellite communication systems. Conditional distributions of non-fade duration after fade state are analyzed for possible application to packet data communications.

The performance of a direct sequence spread spectrum CDMA communication system in an actual land mobile satellite channel is experimentally evaluated. Field test results using the ETS-V satellite in urban and suburban environments at L-band frequency show that this land mobile satellite channel of 3 MHz bandwidth can be seen as a non-frequency selective Rician fading channel as well as a shadowing channel. The bit error performance can be estimated using signal power measurements as if they were narrow band modulation signals. Based on the experimental results of the ETS-V, a prototype system for the personal satellite communication is proposed and field tests are planned on the S-band frequency using the ETS-VI satellite which will be launched in August 1994. >

A variable data rate CDMA system with constant envelope modulation is studied for multimedia random access satellite communications. This system utilizes a constant occupied bandwidth and has variable data rates and processing gains to mitigate communication link impairments such as fading, rain attenuation and interference as well as to handle variable data rate on demand. A proof of concept hardware system for a 6 MHz bandwidth transponder is developed for hardware simulation, which uses MSK and OQPSK-type constant envelope direct sequence spread spectrum modulation and handles data rates of 4 to 64 kbps. The receiver is designed with coherent matched filter technique to achieve fast code acquisition, AFC and coherent detection in a single matched filter circuit. This receiver structure facilitates variable data rate on demand during a call without the aid of system control earth stations. The CDMA performance of the prototype equipment is evaluated with hardware simulation of nonlinear solid state power amplifier (SSPA) transponder. Both the IF loop back hardware simulation and nonlinear SSPA test performances of the prototype equipment prove it has the potential for personal multimedia satellite applications.

Since the main problem in land mobile satellite communications is the frequent blockage of the satellite link by shadowing, this paper analyzes the propagation measurement results of land mobile communication obtained from a satellite at a high angle of elevation in different environments. From these measurements of the ETS-VI satellite, which has a sub-recurrent orbit and 10-20 degrees higher angle of elevation than a geostationary satellite, the shadowing probability is proved to become lower with increasing angle of satellite elevation. Moreover, the cumulative distributions of the fade duration (FD), which is the duration when communication is unavailable and of the non-fade duration (NFD), which is the available duration, are analyzed to verify that the use of high angle of elevation has significant advantage in land mobile satellite communications.

Recently, several activities relating high data rate satellite communications systems have been conducted by CRL in Japan. As the first step of implementation of high data rate (HDR) satellite communications network, CRL starts 155 Mbps ATM/SDH transmission experiment using NTT's new operational satellite N-STAR in December 1995 and the trans-Pacific HDR satellite communications experiment is to be started in April 1996 using Intelsat and NASA's Advanced Communications Technology Satellite (ACTS). To play a major role in the Global Information Infrastructure, satellite communications systems are required to have much higher capacity than 155 Mbps. For realizing gigabit satellite communications systems, CRL already started a study relating to required transponder technology, communications systems and networking technologies. This paper describes detail of these experiments and research project for realizing gigabit satellite communications systems.

The performance of a Direct Sequence Spread Spectrum Communication system in actual land mobile satellite channel is evaluated with experiments. Field test results in L-band with the ETS-V satellite are presented, in which the use of a multi-bitquantized Coherent Matched Filter receiver improves the initial acquisition and the bit error rate performances. The BER characteristics in urban and suburban environments are found to agree with that of nonSS BPSK on a non-selective Rician fading channel.