A cellular communication system can include a number of roaming, automobile-mounted or handheld telephone sets which are served by either fixed, ground-based stations or by orbiting satellites or by a combination of both. The capacity of such systems to serve a large number of subscribers depends on how much of the radio spectrum is allocated for the service and how efficiently the radio spectrum is used. Efficiency of spectral utilization is measured in units of simultaneous conversations (erlangs) per megahertz per square kilometer. In general, spectral efficiency can be improved more by finding ways to re-use the available bandwidth many times over than by attempting to pack more conversations into the same bandwidth, since narrowing the bandwidth generally results in the need to increase spatial separation between conversations thus negating the gain in capacity. Therefore, it is generally better to increase the bandwidth used for each conversation so that closer frequency re-use is possible.
One method to increase the capacity is to use a phased array communications satellite to relay signals from ground stations to a plurality of mobile stations. A ground-controlled orbiting phased array system is disclosed in allowed U.S. patent application Ser. No. 08/179,953 and allowed U.S. patent application Ser. No. 08/179,947, both of which are expressly incorporated herein by reference.
In phased array antennas, feeder links transmit signals to and from a satellite relay station. However, the bandwidth of the feeder links is limited. allowed U.S. patent application Ser. No. 08/225,399 entitled "Multiple Beamwidth Phased Array", which is expressly incorporated herein by reference, discloses a method for efficiently using the limited bandwidth for conveying composite element signals from the ground station to the phase-array satellite transponder using the feeder links.
The prior art does not disclose the use of coherent time-multiplexed feeder links with distributed demultiplexing for reducing the cabling to a deployable phased array. Some prior art systems have separate coaxial cables or waveguides connecting each element of the array to a centrally located transponder. This arrangement was adequate for prior art systems in which the number of antenna elements or beams to be created was small, for example, 6, 19, or 37, but becomes impractical for the number of elements or beams envisioned in the present invention. The present invention eliminates the multiplicity of feeder cables reducing the weight of the system considerably.
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