EARTH SATELLITE COMMUNICATION-SYSTEMS WITH LOW ORBITS, AND EFFECTS OFTHE DOPPLER-SHIFT

With the increase in the need for versatility in satellite communicati on, improvements in satellite launching technology and development of small satellites with higher capabilities, there is tremendous researc h activity in satellite communications using nongeostationary satellit es. However, most of these satellites are meant for the polar regions and the application to general communication between widely scattered areas or the marine and airborne in which traditional communication sy stems are difficult to use. This paper shows first the possibility of a low-orbital satellite system for an area such as Japan where populat ion density is high and where a geostationary satellite or a terrestri al communication system may easily be employed, and the performance of such a system is investigated. Next, we investigate the Doppler shift in the low orbit. It is shown that complete correction on the transmi tting station is not possible, which is different from a long elliptic al-orbit satellite. Moreover, phase-locked loop (PLL) and block demodu lation techniques are considered for carrier recovery in the presence of Doppler shift. A comparative study of the Doppler shift on the spec tral efficiencies of multiple access techniques is performed, and the superiority of CDMA over FDMA is shown.