Shigeru Shimamoto

A global message network employing low Earth-orbiting satellites

A global message communication network for low-density traffic, using satellites at low altitudes, is described. This network affords around-the-clock service to any part of the globe, including the polar regions. Such a network can accommodate static and mobile user terminals simultaneously. The oblate globe is modeled as a regular polyhedron with 12 facets for setting up the orbits. Satellites are positioned uniformly in low-altitude symmetrical orbits. The symmetrical orbits are those whose axes are symmetrical in the three-dimensional space. The authors study the coverage aspects of the 6-orbit scheme and the 10-orbit scheme, each with satellites deployed at an altitude of 5000 km. At this altitude, the terrestrial user terminals can access satellites at a grazing angle of 45 degrees . The method of access over the crosslinks is the slotted ALOHA scheme. For low-density traffic, the downlink and uplink throughput rates are estimated. Simulation results agree with these analytical estimates for low values of network offered load. >

Effect of propagation delays on ALOHA systems

Abstract Effect of propagation delays on the steady state behavior in ALOHA systems is investigated. The basic model of ALOHA systems with propagation delays is introduced and analyzed in line with the catastrophe theory. The bifurcation set, which shows the necessary and sufficient condition that the system is either monostable or bistable, is given explicitly in terms of a few system and user parameters including propagation delays. The global behavior of the system is determined by the cusp catastrophe. Analysis of bifurcation sets shows that propagation delays stabilize the system.

On the structural uniqueness of alternate routing schemes in non-hierarchical networks

The authors examine the structural relationship between load and blocking probability for a fully connected homogeneous nonhierarchical network when blocked messages cause retrial and when different alternate routing strategies are employed to overcome congestion. The model employed for analysis is based on the flow approximation. It is found that retrials push the stable equilibrium state away from the zero blocking state, thereby aggravating congestion. The alternate routing strategies accommodate the stable equilibrium state close to the zero blocking state. However, both the retrials and the alternate routing strategies do not alter the fundamental structure of the system, namely, the fold catastrophe. By solving the degenerate equation, the value of appropriate control parameters can be obtained. According to the present model, it is only under schemes with no alternate routing that the system attains stability, in the mathematical sense of the term.<<ETX>>