Francois T. Assal

Multiple spot-beam systems for satellite communications

On-board satellite switching is performed in three stages, an outer stage of switching on the up-link side for routing the received signals to either demodulators or frequency translators, a first inner stage of switching, preferably a baseband switch and processor, for routing and processing the outputs of the demodulators, a second inner stage of switching, e.g., a microwave switch matrix, for routing the outputs of the frequency translators, and an outer stage on the down-link side for assembling and routing the down-link spot beams. For a multiple hopping-beam system, the outer stages are microwave switch matrices, and for a hybrid scanning-hopping system the outer stages are beam forming matrices. A general purpose modified rearrangeable switch matrix is also disclosed.

A Wide-Band Satellite Microwave Switch Matrix for SS/TDMA Communications

A lightweight, broad-band, 8 × 8 crossbar microwave switch matrix (MSM) has been developed for satellite-switched timedivision multiple-access (SS/TDMA) operation anywhere in the 6 GHz up-link and/or 4 GHz down-link bands. The nearly transparent performance characteristics in the 3.5-6.5 GHz band, for any of the highly flexible interconnect possibilities, have been facilitated by the design of a low-power-consuming resistively matched PIN diode switch. Broad-band push-pull connectors are used to simplify assembly and disassembly of this modular three-dimensional matrix. The switch control electronics (SCE) circuits have been implemented as custom-designed LSI chips for integration within the MSM input distribution networks. Design considerations and performance characteristics of the satellite switch matrix are presented.

Miniaturized reverse modulation loop of a CQPSK 120 Mbit/s modem for spacecraft applications

The design and performance of a miniaturized reverse modulation loop (RML) for a 120 Mbit/s coherent quadrature phase shift keying (CQPSK) modem for onboard satellite applications are presented. Analysis of time delays within the RML circuit indicates that any differential time-delay errors can adversely affect the associated BER and should be minimized. The RML circuit, consisting of a modulator, demodulator, and comparator circuit, has been fabricated using quasi-monolithic techniques with dimensions of 1.65*4 cm. The relative phase for all four states of the modulator is in close agreement with design values of 90 degrees +or-1 degrees over a 200 MHz bandwidth at 3.95 GHz. The demodulator and comparator circuits of the RML have successfully recovered a 120 Mbit/s bit stream. The RML circuit is capable of recovering higher bit rates because of relatively uniform amplitude and phase performance over the 3.7- to 4.2 GHz communications satellite band. >

MMIC: a key technology for future communications satellite antennas

The results of ongoing development programs in the area of advanced satellite antenna technology currently being pursued at COMSAT Laboratories, under the sponsorship of the World Systems Division of COMSAT Corporation, are described. These programs are exploiting the promise of monolithic microwave integrated circuits (MMICs) as they apply to active phased arrays. Performance data on two phased-array programs are presented. The first program involves the development of a 64-element array in which MMICs are used to optimize and reconfigure the radiation pattern performance. The array is capable of generating a single beam and accurately synthesizing a specified radiation performance. The second array program, building on knowledge gained in the first program, addresses the problems of generating multiple beams, as well as the effect of power amplifiers on multicarrier performance. MMICs are employed in the beam-forming matrix and in the distributed 2-W solid-state power amplifiers. This design is capable of forming four simultaneous, independently steerable beams while allowing for flexible power sharing among the beams.