This paper presents the radiation characteristics of a circularly polarized conical beam spherical slot array antenna for applying to the mobile satellite communication subscriber. The structure of the antenna is easy to fabricate i. e. , a ring of perpendicular slot pairs cut on an outer surface of a concentric conducting spherical cavity enclosed by the conducting conical surface with the simple feeding structure, and a linear electric probe excited at the center of the inner surface of the cavity. Radiation fields of a spherical slot array antenna are calculated by superposing the patterns of all the slots. From the numerical results of the radiation pattern, in both elevational and azimuthal planes, it is obvious that the conical beam is realized. The elevational beam direction is low, which is suitable for installing in the land mobile subscriber unit located far from the equator. The tracking system is not necessary because the azimuthal pattern is omnidirectional. Directivity of the antenna for various spherical radii and angles of slot positions are illustrated as the guidelines for the design. Experimental results are in good agreement with the predictions.

We have investigated the operation of a reflection type magnetostatic wave signal-to-noise enhancer in detail. It has good enhancement characteristics, low insertion loss, and low operating power. It is also composed of a transducer using a ceramic substrate having a high dielectric constant and an LaGa-YIG film with low saturation magnetization to enable direct operation in the 400-MHz band (the IF band of current DBS receivers). Enhancement of 8 dB was achieved over a 40-MHz bandwidth. Although its operating frequency range depends critically on device temperature, we can compensate for the temperature dependence by adjusting the bias magnetic field. Experiments showed that the enhancer improved the received carrier-to-noise ratio by 2 to 3 dB, providing good noise reduction in DBS reception.

This paper discusses a method to evaluate mutual couplings of cavity-backed slot antennas using the FDTD technique. The antenna fed by the short-ended probe is considered, which is investigated as an element of the power transmission antenna, Spacetenna, for the solar power satellite SPS2000. It is found from the FDTD computation on E-plane two- and four-element array antennas that the size of the problem space should be larger for the evaluation of the mutual coupling than for the estimation of the input impedance. Since enlarging the size of the problem space requires a large amount of computer storage, it is not practical for computer simulations. In order to carry out accurate estimations of the mutual coupling with relatively small amount of computer memory, the problem space is extended only in the broadside of the array antenna and in the other directions there are ten cells between the antenna surface and the outer boundary. Computer simulations demonstrate that there are no differences between the results of the proposed problem space geometry and the problem space extended in each direction of the axis coordinate by the same number of cells. Furthermore comparisons of computed and experimental results demonstrate the effectiveness of the approach after discussing how large the size of the problem space is required to estimate the mutual coupling.

This paper proposes a novel signal transmission scheme for helicopter satellite communications. The proposed scheme is based on time diversity, and combined with a novel algorithm to suppress an influence of carrier phase slip. In the proposed scheme, carrier phase slip is detected in cross correlation processing of the received signal, and is effectively suppressed. The proposed scheme thus makes it possible to employ coherent phase shift keying modulation to achieve bit error rate performance superior to that of differential phase shift keying modulation even in the low carrier-to-noise power ratio environment.

In recent years, several global network systems using non-stationary satellites have been proposed. Some of them are announced to start services within years. We also have several experimental systems with stratospheric aircrafts. In the future, the radio communication system using stratospheric aircrafts will be one of the promising media for personal communications. The question of how to establish the optimal communication under such circumstance seems to be still open. In this paper, performance evaluations of wireless communication systems using LEO satellites and stratospheric aircrafts are proposed. We will show some proper communication parameters to improve competence of mobile communication in the such systems as well.

A decision-directed carrier phase recovery scheme for high-speed satellite communications is proposed. Since the estimation is performed in complex domain from the baseband signal, the scheme has fast acquisition performance, unlike the conventional PLL. This merit makes it applicable for various wireless systems such as wireless local area networks (LANs), wireless asynchronous transfer modes (ATMs) and local multipoint distribution systems (LMDSs) that need high-speed burst signal communications. Also, this scheme can be implemented easily because low pass filters (LPFs) are utilized in filtering the estimates in order to suppress the noise within the carrier recovery loop. Moreover it does not require any divider or voltage-controlled oscillator (VCO). The performance is analyzed through analytical methods and simulation.

QoS of a multimedia interactive satellite communications system, which uses a satellite circuit for downlinks and a terrestrial network for uplinks, has been measured and evaluated under inferior satellite circuit conditions. Our attention in this paper is focused on TCP throughput as the most significant QoS, and experiments and analysis on TCP throughput were performed. The measured results are in good agreement with theoretical values calculated by taking congestion avoidance algorithms of TCP into consideration. As a result, we were able to verify that the TCP throughput declines radically when the low signal quality of a satellite circuit causes packet retransmission to occur. In this case, congestion avoidance works to reduce network congestion. The congestion avoidance increases the value of a retransmission timer and decreases the window size even if the network is not congested. Assuming that throughput deterioration is caused by rain fade, the total time when the throughput declines is less than 4 hours a year in Tokyo, and the results show the system can be put to practical use.

An architecture is presented for efficient and reliable delivery of multimedia contents from a primary center (PC) to secondary centers (SCs). Requested contents are delivered from the PC to the SCs through a satellite broadcast channel, or from one SC to another SC through a terrestrial channel. Cycling methods are presented that enable sharing of the contents directory of each SC. Several fundamental models and algorithms are introduced for possible consideration during the planning and design of a contents-delivery system. Simulation has shown that using both satellite broadcast and terrestrial channels for contents delivery is superior in terms of cost to the conventional use of only a satellite network.

Backfire quadrifilar helical antennas combined with parasitic loops are investigated in detail, focusing on clarifying the function of parasitic loops. First, the basic property is examined for the case of one parasitic loop, and it is found that the loop behaves as a director when the circumferential length of the loop is nearly 0. 9λ, and a reflector when the circumferential length of the loop is nearly 1. 2λ provided the distance between the parasitic loop and the top plane of helical antennas is approximately 0. 1λ, where λ is the wavelength. Next, the function of the parasitic loop is investigated by comparing the current distributions on the helices and the loop with those on a monofilar helix with a ground plane. It is found that the function of the parasitic loop is quite different from that of the ground plane. Then, the case of two parasitic loops is examined, and it is shown that the use of two parasitic loops is very effective and simple measures to control the radiation pattern and gain of the backfire quadrifilar helical antennas. Finally, for this type of antennas with two parasitic loops, an example of structural parameters suited to the use in satellite communications is presented.

Unequal error protection (UEP) is a very promising coding technique for satellite broadcasting, as it gradually reduces the transmission rate. From the viewpoint of bandwidth efficiency, UEP should be achieved in the context of multilevel coded modulation. However, the conventional mapping between encoded bits and modulation signals, usually realized for multilevel block modulation codes and multistage decoding, is not very compatible with UEP coding because of the large number of resulting nearest neighbor codewords. In this paper, new coded modulation schemes for UEP based on unconventional partitioning are proposed. A linear operation referred to as interlevel combination is introduced. This operation generalizes previous partitioning proposed for UEP applications and provides additional flexibility with respect to UEP capabilities. The error performance of the proposed codes are evaluated both by computer simulations and a theoretical analysis. The obtained results show that the proposed codes achieve good tradeoff between the proportion and the error performance of each error protection level.

This paper develops a prediction model for evaluating the influence of propagation attenuation due to aircraft's flying across the earth-satellite link. This prediction model is based on the Aperture-field method of Huygens-Fresnel wave theory. Considering arriving and taking off course around airport, attenuation impairment is calculated for different types of aircrafts and flight directions. In order to verify this model's accuracy, numerical results are compared with measurement values. The calculations agree well with the measurements. Ground antenna directivity and anticipated impairment to digital broadcasting system such as Perfect TV are also discussed.

The results of experiments on the effect of the height and diameter of the cup on cup microstrip antennas are presented. The results show that the optimum height of the cup for the narrowest beamwidth and the highest gain is about 1/3 λ, and that the beamwidth decreases and the gain increases as the diameter of the cup increases.

Future satellite communication systems will be developed at Ka-band (20/30 GHz) owing to the relatively wide frequency allocation and current freedom from terrestrial interference for multimedia services. A serious disadvantage of the Ka-band, however, is the very high atmospheric attenuation in rainy weather. Quasi-synchronous CDMA drastically reduces the effect of self-noise with several interesting features of CDMA for mobile communications such as flexible frequency reuse, the capability of performing soft-handover and a lower sensitivity to interference. This paper evaluates the performance of a quasi-synchronous CDMA return link for a Ka-band geostationary satellite communication system. For a fixed satellite channel whose characteristics depend on weather conditions, the signal envelope and phase for this channel is modeled as Gaussian. The bit error and outage probability, and the detection loss due to imperfect chip timing synchronization is analytically evaluated and the system capacity degradation due to the weather condition is estimated. Two cases of general and worst conditions are evaluated, in which i) rain attenuation ii) nonlinearity of transponder are considered. The two cases consist of the general case in which all users are affected by rainy weather, and the worst case in which only the user of interest, not multiple access interferers, is affected by rain attenuation. The results for the two cases of rainy weather clearly show that quasi-synchronous CDMA eases the power control requirements and has less sensitivity to imperfect power control. When dealing with the impact of the satellite transponder nonlinearity in addition to the rain attenuation, the shift of optimum amplifier operating point is shown so that [Eb/N0]sat, defined as the sum of the Eb/N0 value required to obtain a BER equal to Pb at a given output backoff (OBO) and the value of the OBO itself, tends to decrease, and higher BER impairment is given, since the rain attenuation results in the same effect as the additive input backoff (IBO) at the satellite transponder input. As the BER increases, the optimum [Eb/N0]sat and IBO decrease that result in the shift of optimum operating point.

Almost all broadcasting systems and their equipment would be digitalized in the near future. In Japan, investigation of digital broadcasting has been going on for a long time, aiming at a realization of improvement of picture quality, new services, system flexibility, etc. Japanese digital broadcasting systems under development have a lot of technical merits, for example, a high transmission capacity and a hierarchical transmission scheme for satellite, and mobile reception for terrestrial digital broadcasting systems, compared to conventional digital systems.

This paper proposes a new method to improve cooperation in concurrent systems within the framework of Multi-Agent Systems (MAS). Since subsystems work concurrently, achieving appropriate cooperation among them is important to improve the effectiveness of the overall system. When subsystems are modeled as agents, it is easy to explicitly deal with the interactions among them since they can be modeled naturally as communication among agents with intended information. Contrary to previous approaches which provided the syntax of communication protocols without semantics, we focus on the semantics of cooperation in MAS and aim at allowing agents to exploit the communicated information for cooperation. This is attempted by utilizing more coarse-grained communication based on the different perspective for the balance between formality and richness of communication contents so that each piece of communication contents can convey more meaningful information in application domains. In our approach agents cooperate each other by giving feedbacks based on the metaphor of explanation which is widely used in human interactions, in contrast to previous approaches which use direct orders given by the leader based on the pre-defined cooperation strategies. Agents show the difference between the proposal and counter-proposals for it, which are constructed with respect to the former and given as the feedbacks in the easily understandable terms for the receiver. From the comparison of proposals agents retrieve the information on which parts are agreed and disagreed by the relevant agents, and reflect the analysis in their following behavior. Furthermore, communication contents are annotated by agents to indicate the degree of importance in decision making for them, which contributes to making explanations or feedbacks more understandable. Our cooperation method was examined through experiments on the design of micro satellites and the result showed that it was effective to some extent to facilitate cooperation among agents.

In this paper, we propose a teletraffic model and evaluate the performance in the terrestrial/satellite integrated mobile communication networks having hierarchical structure consisting of terrestrial microcell and satellite spot beam. In the proposed teletraffic model, both the subscriber terminal mobility in the terrestrial microcell and the satellite mobility in the LEO satellite spot beam are considered. The overflow traffic from microcell can be accommodated by the satellite spot beam and is analyzed by IPP (Interrupted Poisson Process) which is often used to analyze non-random overflow traffic. Various reservation channels for handoff calls in terrestrial microcell and satellite spot beam are considered. New calls initially directed to the satellite spot beam are also considered for reservation channels. Carried traffic, blocking, forced termination and non-completion probabilities are evaluated for the overall integrated networks.

Satellite beam-switching problems are studied where there are m up-link beams, n down-link beams and multiple carriers per beam. By augmenting a traffic matrix with an extra row and column, it is possible to find a sequence of switching modes ((0,1)-matrices with genrally multiple unit entries in each row and column) that realize optimal transmission time. Switching modes generated are shown to be linearly independent. The number of switching modes required for an mn matrix is bounded by (m1)(n1)1. For an augmented (m1)(n1) matrix, the bound is then mn1. The bounds on the number of switching modes and the computational complexity for a number of related satellite transmission scheduling problems are lowered. In simplified form, the results (particularly the linear independence of permutation matrices generated) apply to algorithmic decomposition of doubly stochastic matrices into convex combinations of permutation matrices.

This paper proposes an FFT (Fast Fourier Transform) interference detection for interference suppression which combines notch filtering and FEC (forward error correction) to improve the Pe (probability of error) performance degradation due to co-channel interference in digital satellite communication systems. The proposed FFT interference detection scheme can determine the co-channel interference carrier frequency, power, and bandwidth precisely by using the power detection threshold suitable for the desired signal power spectrum, and the notch filter characteristic can be set according to the results. The interference suppression with the proposed scheme achieves the degradation in required Eb/No to only 1.0 dB at a Pe of 10-4 compared to that with the optimum notch filter (ideal detection) in unknown CW (continuous wave) and FM (frequency modulation) co-channel interference environments. Moreover, the proposed scheme improves the required Eb/No by 6.5 dB compared to that without a notch filter in an FM interference environment with interference carrier frequency offset normalized by the desired signal clock rate of 0.52, desired to undesired (interference) signal power ratio of 3 dB and interference bandwidth at 10 dB down power point from the peak normalized by the desired signal clock rate of 0.25.

A multicast (point-to-multipoint) protocol of a satellite broadcast channel by a source and many destinations is presented and its performance characteristics are analyzed. We propose a new time-domain multicast scheme for packet satellite channels, retransmission via collisions protocol (called RVCP). RVCP is classified to the automatic repeat request (ARQ) of the multi-selective-repeat scheme and does not require individual channels for each receiving station to request for broadcast packets that have been received incorrectly. Our analytical models show that RVCP performs considerably better than the other protocols, particularly in the situation that packet error rate is less than 10-4 or there are a large number of destinations. It is an excellent characteristic of RVCP that the equipment of the source station need not increase in proportion to the number of destinations, too. And since RVCP is a relatively simple protocol, it is easy to be implemented.

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.