1. Field of the Invention
The present invention relates to method and apparatus for communicating with one or more receiver stations disposed within a blackout region located in the vicinity of and surrounding each spot beam coverage area, the blackout region being caused by interference between concurrently transmitted and overlapping global coverage and spot coverage antenna radiated beams using the same frequency band. More particularly, this invention relates to method and apparatus for communicating with the one or more receiver stations disposed within the above-mentioned blackout region by selectively turning off the associated interfering spot beam during the predetermined interval of time to permit the global beam to communicate with the normally blacked out receivers.
2. Description of the Prior Art
It is most desirable in a domestic satellite communications system to have a coexistence of overlapping spot and global coverage beams. For each high traffic ground station, for example, a separate spot coverage beam can be used for communication therebetween. Alternately, communication between the satellite and a plurality of low traffic ground stations can be accomplished with an overlapping global coverage beam where it might not be desirable to interconnect the individual low traffic ground stations to the nearest high traffic ground station for access to the satellite system. However, signals transmitted in the global beam cause interference in the spot beam signals and, therefore, to acquire an optimum signal in spot beam coverage area, various prior art interference suppression techniques may be employed.
To avoid signal degradation and permit separation of the overlapping spot coverage and global coverage beams, separate bandwidths or polarizations for the spot coverage beams and the global coverage beam can be used. Using separate bandwidths, however, results in inefficient use of the frequency spectrum and different polarizations may not be available where dual polarization is already used by each of the beams of the satellite system. In addition, substantial reduction of the interference between the overlapping global and spot coverage beams can be achieved, for example, through the use of antenna pattern shaping or channel coding techniques. More specifically, both antenna pattern shaping and channel coding techniques substantially remove interference in the spot beam coverage areas and dramatically reduce the area of interference in the vicinity of and surrounding these areas. However, neither prior art technique completely removes the interference in the area in the vicinity of and surrounding each of the spot beam coverage areas and, in fact, an area often termed "a blackout region" is produced within the above-mentioned area where signal separation between the two overlapping beams becomes virtually impossible without the use of highly sophisticated circuitry.
Numerous other techniques have been devised to suppress interference particularly between two beams arriving at a receiver from separate directions. In this regard, see for instance, U.S. Pat. Nos. 2,579,882 issued to L. E. Thompson on Dec. 25, 1951; 3,094,695 issued to D. M. Jahn on June 18, 1963; 3,205,443 issued to D. R. Ludwig on Sept. 7, 1965; and 3,987,444 issued to R. J. Masak et al. on Oct. 19, 1976. Since the global and spot coverage beams transmitted from a satellite arrive at the area in the vicinity of and surrounding each spot beam coverage area from the same direction, techniques for separating signals arriving from different directions are not usable.
The problem still remaining in the prior art is to provide a technique which permits communication with receivers disposed within the blackout region formed in the vicinity of and surrounding each spot beam coverage area by the interference between signals in the concurrently transmitted and overlapping spot coverage and global coverage beams which use the same frequency band.