Multiple spot-beam systems for satellite communications

#The present invention relates to satellite communications systems and more particularly to a system using distributed routing and switching systems which minimize the quantity of satellite hardware while achieving the desired objectives of high capacity highly flexible access and high reliability for long life.

#Still more particularly, the present invention is directed to multiple pencil-beam, high capacity satellite systems which provide highly flexible access using a satellite-switched, time-division multiple-access (SS-TDMA) mode of operation with on-board regeneration and processing.

#In satellite communications systems, service costs can be reduced by providing distributed access via low cost earth stations. This is likely to require a proliferation of inexpensive and unattended earth stations that have relatively small antennas and are preferably located at the customer's premises. To compensate for the lower earth station antenna gain, satellite-radiated energy density needs to be increased significantly by concentrating the useful signal energy into a large number of pencil beams having high G/T and high e.i.r.p., rather than spread-out coverage areas such as a hemisphere or the entire globe. The beams may use orthogonal polarization isolation and spatial isolation if necessary. As a result, the overall satellite capacity can be multiplied by reusing the allocated frequency bands at C- and K.sub.u - bands, provided the beam-to-beam co-channel interference can be minimized. This can be achieved by on-board regeneration of all the digital signals, since demodulation and remodulation effectively separate performance degradations of the up- and down-links and only result in the algebraic additions of bit-error rates.

#The invention will be more clearly understood from the following description in conjunction with the accompanying drawings, in which:

#FIG. 1 is a brief schematic diagram of a satellite payload with 8 active channels and 12 power transmitters:

#FIG. 2 is an alternative diagram of the double-ring redundancy arrangement in the subsystem of FIG. 1;

#FIG. 6 is a conceptual block diagram of a multiple scanning beam system with outer routing. As shown on the right-hand side of the figure, the power transmit subsystem consists of a beam-forming matrix (BFM), solid-state power amplifiers (SSPAs), and a cluster of feeds for the down-links (DLFC). The BFM contains N-way equal-power divider and M-way equal-power combiner circuits (illustrated schematically as horizontal and vertical lines, respectively) and a matrix of active modules which connect these lines at their intersections. Each active module contains a multibit phase shifter and amplifiers, and may or may not include a digital attenuator for gain control. The transmit BFM contains (N.times.M) active modules where N=200 for the 1.0.degree. and 700 for the 0.5.degree. beams. A table within FIG. 6 displays the total number of modules per satellite for various numbers of active beams. Each horizontal row defines a beam whose shape and direction of transmission are controlled by combined settings of all the active modules in that row. Since each beam is independently controlled several beams can be co-located and used together with frequency discrimination in regions of high communications capacity. Therefore, this subsystem can transmit up to M=(i+j+k) independently and simultaneously scanning beams. The number of active beams M, is independent of the number of radiating elements N, in the array.

#In this multibeam system each SSPA simultaneously amplifies signal components contributed by all the active beams. As a result the SSPA input drive levels must be set to ensure nearly linear operation with an acceptable level of intermodulation. Also every beam is generated by exciting all elements in the array and by using all of the SSPAs feeding those elements. This offers a graceful degradation in case of amplifier failures.

#By sending a signal through one row, a pencil beam is produced "optically" by the addition in free space of all the phase-controlled outputs emanating from the feed cluster. The superposition of these outputs produces a plane wave, and the beam direction is selected by controlling the relative phase of all the active modules in the row. Multiple pencil beams are obtained by transmitting signals through any number of rows in the BFM. The feed cluster can be used either as a direct radiating array or as an active source for a dual-reflector system such as a near-field Gregorian, as described by R. M. Sorbello A. I. Zaghloul, B. S. Lee, S. Siddiqi and B. D. Geller "20-GHz Phased-Array-Fed Antennas Utilizing Distributed MMIC Modules," COMSAT Technical Review, Vol. 16, No. 2, Fall 1986 pp. 339-373. Operationally most of the beam positions may change in a prescribed way, several times during the 2 ms SS-TDMA frame duration as described in the above cited paper by T. Inukai.