Adaptive antenna design considerations for satellite communication antennas

Satellite communication systems must often be designed to operate in the presence of interfering signal sources. Conventionally, bandspreading via signal modulation is often used to combat such unwanted interference. Recently, interest has been focused on supplementing the conventional interference reduction achieved via bandspreading by using antenna radiation pattern shaping achievable with adaptive nulling antennas. Because of the well defined field of view of the earth seen by a satellite at geosynchronous altitude (i.e. a ± 9° cone angle) and the relatively `clean¿ multipath environment seen by the high-gain `earth-looking¿antennas which must be used (as compared with low-gain adaptive antennas on aircraft, or antennas located in a terrestrial environment), it is possible to determine several useful trends unique to this class of adaptive antenna systems. The paper addresses the general considerations inherent in choosing the fundamental antenna and adaptive processor design parameters relative to geosynchronous satellite adaptive systems. The following topics are developed in detail: (a) Choice of antenna type: Phased array against the multiple beam antenna (b) Nulling resolution: Minimum toletable separation between interference and signal (user) sources (c) Nulling bandwidth: Frequency-independent weighting against frequency-dependent weighting (for example tapped delay lines) (d) Factors affecting the performance of adaptive antenna systems: Channel tracking errors, weighting circuit imperfections etc.