Earth satellite communication systems with low orbits, and effects of the doppler shift

With the increase in the need for versatility in satellite communication, improvements in satellite launching technology and development of small satellites with higher capabilities, there is tremendous research activity in satellite communications using nongeostationary satellites. 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 systems are difficult to use. This paper shows first the possibility of a low‐orbital satellite system for an area such as Japan where population density is high and where a geostationary satellite or a terrestrial 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 transmitting station is not possible, which is different from a long elliptical‐orbit satellite. Moreover, phase‐locked loop (PLL) and block demodulation techniques are considered for carrier recovery in the presence of Doppler shift. A comparative study of the Doppler shift on the spectral efficiencies of multiple access techniques is performed, and the superiority of CDMA over FDMA is shown.