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.

The Consultative Committee of Space Data Systems (CCSDS) proposes a packetized telemetry scheme for the convenience of data exchange and networking in space activity. This paper describes the outline of the telemetry scheme and the on-orbit experiment which was carried out to show the applicability of the proposed CCSDS packet telemetry scheme using the Japan's satellite "Hiten" in a highly elliptical orbit. The telemetry data which are generated by the onboard instruments are packetized in Hiten, and reformed to the original data in earth stations successfully. The experimental results show that the standardized scheme is helpful for tracking cross-support between organizations, and that the concatenated code is quite effective to transmit data in a low C/N condition.

This paper proposes a self frequency preset (SFP) PLL synthesizer to realize a simple frequency preset PLL synthesizer with temperature-resistant and shorter frequency settling time than the conventional temperature un-compensated phase and frequency preset (PFP) PLL synthesizer. Since the proposed synthesizer employs a simple frequency locked loop (FLL) circuit to preset the output frequency at each frequency hopping period, the synthesizer eliminates the need to store f-V characteristic of the VCO in ROM. The frequency settling time of the proposed synthesizer is theoretically and experimentally analyzed. The theoretical analysis using the realistic f-V characteristic of a IF band VCO show that the frequency settling time of the proposed synthesizer is 130µs shorter than that of the conventional PFP PLL synthesizer at 40MHz hopping in the 200MHz band for all temperatures. Furthermore, the experimental results confirm that the frequency acquisition time of a prototype FLL circuit is accordant with the calculated results. Thus, the proposed SFP PLL synthesizer can achieve faster frequency settling than the conventional PFP PLL synthesizer for all temperatures and its simple configuration allows to be easily implemented with existing CMOS ASIC devices.

This paper proposes a superposed SSMA (Spread Spectrum Multiple Access)-QPSK (Quadrature Phase Shift Keying) signal transmission scheme over high speed QPSK signals to achieve higher frequency utilization efficiency and to facilitate lower power transmitters for SSMA-QPSK signal transmission. Experimental results show that the proposed scheme which employs the coding-rate of one-half FEC (Forward Error Correction) and a newly proposed co-channel interference cancellation scheme for SSMA-QPSK signals can transmit twenty SSMA-QPSK channels simultaneously over a nonlinearly amplified high speed QPSK signal transmission channel and achieve as ten times SSMA channels transmission as that without co-channel interference cancellation when the SSMA-QPSK signal power to the high speed QPSK signal power ratio equals -30dB. Moreover, cancellation feasibility generation of the interference signals replica through practical hardware implementation is clarified.

This paper addresses onboard processing architecture employing direct regeneration. The advantage of direct regeneration is its hardware simplicity, even though the bit error rate performance is slightly inferior to that of demodulation-remodulation scheme with coherent detection. The channel filtering schemes as well as achievable capacities are examined by computer simulation. It is found that the system with direct regeneration has advantage in channel capacity and transmit earth station e.i.r.p. for small earth stations. A possible configuration of direct regeneration onboard in future satellite systems is proposed.

This paper proposes a suitable combination of the digital modulation schemes and the coding-rate of forward error correction (FEC) schemes for satellite digital video communication networks. The comparative study is carried out by computer simulation considering non-linearly amplified, narrow bandwidth satellite channels with adjacent channel interference signals. The proposed system employs an offset QPSK modulation scheme supported by the coding-rate of 7/8 convolutional encoding and Viterbi decoding to realize high-quality and compact spectrum characteristics in non-linear channels. By employing a 32Mbps DPCM video codec, the developed prototype system achieves a post demodulated S/N ratio of higher than 52dB. Moreover, it achieves high protection ratio against co-channel interference than conventional analog FM systems. The optimized digital video transmission system makes it possible to transmit high-quality NTSC video signals over non-linearly amplified narrow bandwidth satellite channels, for example 27MHz or 36MHz bandwidth transponders, with high-security digital encryption.

Under the conditions of Poisson arrivals and single copy transmission, we designed a minimum delay protocol for packet satellite communications. The approach is to assume a hybrid random-access/reservation protocol, derive its average delay and minimize the delay with respect to all tunable system parameters. We found that for minimum average delay,1) a spare reservation should normally but not always be made for each packet transmission.2) all unreserved slots (i.e. Aloha slots) should be filled with a packet rate of one per slot whenever possible. In other words, the utilization of Aloha slots should be maximized.3) an optimum balance between transmitting packets and making reservations before transmission should be maintained.

A group-based random access communication system which consists of two groups of many users is considered. The two different groups share a common random multiple access channel. Users from a group are allocated a high transmitting power level and have a high probability of correct reception among overlapping packets. We set a threshold, θ, which is such that the group with the high power level will occupy the channel if less than or equal to θ packets are transmitted from the group with the low power level. We obtain a two-dimensional Markovian model by tracing the number of backlogged users in the two groups. The two-dimensional Markov chain is shown to be not ergodic and thus the system is not stable. A two-dimensional retransmission algorithm is developed to stabilize the system and the retransmission control parameters are chosen so as to maximize the channel throughput. An equilibrium point analysis is performed by studying the drift functions of the system backlog and it is shown that there is a unique global equilibrium point. The channel capacity for the system is found to be in the range from 0.47 up to 0.53, which is a remarkable increase compared to the conventional slotted ALOHA system.

This paper proposes a feedback-loop type transmission power control (TPC) scheme coupled with first and second order prediction methods and analyzes the optimum control period and residual control error. In order to minimize residual control error, the three main factors contributing to residual control error are analyzed. First, to detect accurately up-link rain attenuation, a channel quality detection method is proposed and analyzed experimentally for puseudo-error detection. Second, rain attenuation rates in Ka band are measured and analyzed statistically. Finally, the optimum control period of the proposed TPC scheme is analyzed. The simulation results on the prototype TPC system show a maximum of 4.5 dB residual control error is achievable with an optimum control period of about 1 second to 1.5 seconds.

Link conditions of a low-earth orbit (LEO) satellite communications system were evaluated, to provide the information necessary for designing a broadband LEO-SAT communications system. The study was made both for optical intersatellite and user/satellite links. For the optical intersatellite link (ISL), we examined several ISL configurations in a circular polar orbit, and found that when the satellites are in the same orbital plane, the link parameters are quite stable, that is, the link between adjacent satellites can be regarded as fixed and, therefore, suitable for broadband transmission via an optical link. However, the link conditions between adjacent orbits change very quickly and over a wide range. To overcome this and extend the network path between satellites in adjacent orbital planes, we proposed intermittent use of the link between satellites in co-rotating adjacent orbital planes at the low latitude region, i.e., only during the period of stable conditions. The optical intersatellite link budget also sets link parameters that are realistic, given present optoelectronic technologies. From quantitative evaluations of the user/satellite link, we believe that both the satellite altitude and minimum elevation angle are critical, both in defining the quality of the service of the LEO-SAT system and in their impact on the other transmission parameters. The link loss, the visible period and the required number of satellites vs. satellite altitude and elevation angle are also indicated. These are important considerations for future system design.

With the arrival of B-ISDN, point-to-point routing alone is no longer adequate. A new class of computer and video related services, such as mass mailing, TV broadcasting, teleconferencing, and video 900 service, requires the network to handle multiple destination routing (MDR). Multiple destination routing enables widespread usage of multipoint services at a lower cost than networks using point-to-point routing. With this in mind, network providers are researching more into MDR algorithms. However, the MDR problem itself is very complex. Furthermore, its optimal solution, the Steiner tree problem, is NP-complete and thus not suitable for real-time applications. Recently, various algorithms which approximate the Steiner tree problem have been proposed and, in this invited paper, we will summarize the simulation results of these algorithms. But first, we will define the MDR problem, the issues involved, and the benchmark used to compare MDR algorithms. Then, we will categorize the existing MDR algorithms into a five-level classification tree. Lastly, we will present various published results of static algorithms and our own simulation results of quasi-static algorithms.

A slotted tube resonator (STR) having a shield of conducting circular cylinder which is used as a probe for the magnetic resonance imaging (MRI) is analyzed by using the variational method and the dyadic Green's function of a circular waveguide. Three surface current modes are employed to expand the currents on the STR. Quadruple integrals appearing in the variational expression are evaluated analytically for saving the CPU time. Resonant frequency, Q value and the magnitude of magnetic field distributions for various radii of the shields are obtained to show the effects of the shield. Some measured date are compared with the theoretical results to confirm the validity of the present analysis.

This letter describes the usefulness of a homogeneous spherical model of the isolated human head in SAR calculation for UHF plane-wave exposure. Comparison is made between this SAR and several results that were computed and measured for the homogeneous but realistic whole-body model of the human by other researchers.