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It is expected that mobile-radio systems will play an important role in realizing general personal communication networks through ISDN. In the near future, the so-called mobile ISDN era, it is necessary to have come mon performance objectives between mobile radio links and fixed (or fiber) links and have a common assessment method for data transmission quality. In this paper, based on the ITU-T definition, unavailability statistics are examined in mobile-satellite systems which are characterized by low margin systems. The characteristics of signal degradation due to propagation impairments in maritime, aeronautical, and land mobile sateffite services are examined. Through this analysis, the importance of differentiation between unavailability and 'signal degradation in available time' is emphasized when the digital signal transmission quality is evaluated. [ABSTRACT FROM AUTHOR]

Onboard resource sharing (ORS) is a method wherein, for increasing effective isotropically radiated power, surplus satellite resources (radiated power or time slots in TDMA) are used to maintain communications quality in areas affected by rain attenuation. This paper analyzes simultaneous occurrence rates of rain over a wide area using Automated Meteorological Data Acquisition System (AMeDAS) data (1 hr rain rate data at approximately 1300 locations in Japan provided by the Japan Meteorological Agency). An ORS system is proposed using AMeDAS data as dynamic information for controlling satellite transmission power. Then a control simulation is performed by using actual reception intensities of satellite signals and rain rate information derived from AMeDAS data at corresponding times. The effect of the proposed ORS system is demonstrated in compensating for rain attenuation. From these results it was confirmed that the forementioned compensation provides a saving of about one-third compared to the case where unavailable time is not compensated. [ABSTRACT FROM AUTHOR]

The unavailable time percentage and the SES (severely errored seconds) occurrence rate within available time was investigated according to CCITT definition using Ku band low elevation angles satellite system data obtained mainly at the Yamaguchi station. As a result, the CCITT unavailable time percentage was found to agree with the cumulative signal attenuation time percentage as determined by the traditional statistical treatment. This agreement held even in the case of low elevation angles. Further, the ratio of SES occurrence rate within CCITT available time to the conventional cumulative signal attenuation time percentage was found to be much larger than in the case of high elevation angles, and to have a high correlation with tropospheric scintillation. An experimental formula was developed for this ratio as an index of the degree to which tropospheric scintillation affects overall signal deterioration. Using this formula a formula was developed to predict the SES occurrence rate within CCITT available time from the index and the traditional cumulative signal attenuation time percentage. Also, it was confirmed that there was good agreement between the predicted value and the experimental value. [ABSTRACT FROM AUTHOR]

Development of small marine antennas has a very important role in the maritime satellite communications with small ships, especially for Japan which has a relatively large number of small fisherboats. As the satellites become larger in size in the future, the shipborne antennas will tend to become smaller. We propose a control method for the antenna mount against ship motions to assure pointing the communication satellite. The two-axis EL/AZ mount has is near the control limit for avoiding twisting the cables. We have developed an algorithm to reduce the pointing errors within the pitching amplitudes of the ship. The principle and the experimental results of this system are described. [ABSTRACT FROM AUTHOR]

Presents a land mobile-satellite services (LMSS) propagation channel model for assessment of the effect of a satellite diversity scheme. Number of states the general fading environments for LMSS are classified into; Developmemt of a fading channel model; Validation of the model; Calculation of the satellite diversity effect based on the model.

When maritime satellite communication is conducted at low elevation angles, fading due to sea surface reflection becomes a problem. For suppression of the fading, space diversity and frequency diversity may be employed. This paper studies the correlation characteristics of multipath fading due to sea reflection in the spatial and frequency domains in L-band operation. This is important in investigating the diversity effect. First, from the theoretical model of sea surface reflection, the correlation scattering cross sections of the coherent and incoherent components of the reflected wave are obtained. Next, from numerical calculations, the correlation characteristics are obtained with the antenna elevation angle and the height over the sea as the parameters. It is found that in space diversity good diversity effects can be expected from two antennas separated in the vertical direction by about 40 cm. It is also seen that fading reduction cannot be expected with frequency diversity in the case of narrowband communications such as in maritime satellite communications. The effects of various fading reduction measures in the up and down links are considered. [ABSTRACT FROM AUTHOR]

With a view to application to array antennas for mobile stations in a mobile satellite communication system, a method is proposed for configuration of a receiving diversity array antenna acquiring a communication beam quickly and tracking with stability by means of the digital beam forming (DBF). In this method, an active array is assumed. The receiving phase difference between the antenna elements is detected by digital complex operation for the received baseband signal. Based on this phase difference, the phases of the antenna elements are made in phase in a relative sense so that equal gain or maximal ratio combining is accomplished. As a result, the main beam can be formed in the direction of the arriving wave. In addition, in the case of maximal ratio combining the radiation pattern is oriented toward the multipath arriving waves so that communication quality is improved as the beams are further combined in phase. In this paper, the effectiveness of the present method is demonstrated by the numerical simulation assuming a mobile communication environment with band-limited QPSK signals. © 1997 Scripta Technica, Inc. Electron Comm Jpn Pt 1, 80(10): 88–100, 1997 [ABSTRACT FROM AUTHOR]