30/20 GHz Demonstration System for Improving Orbit Utilization

Abstract

The NASA LeRC 30/20 GHz Satellite Communications Program is developing a number of technologies to reduce satellite orbit/spectrum crowding and prevent saturation of our domestic United States Communications capabilities in the 1990 to 2000 decade. Developing the basic hardware technology to operate at 30 and 20 GHz provides 2.5 GHz of new communications bandwidth. This 2.5 GHz additional communications bandwidth is not the primary benefit of the program, however. Rain losses are severe at 30 and 20 GHz, and innovative techniques are required for systems which are both reliable and economic. Techniques being developed include large satellite antennas with simultaneous multiple fixed and multiple scanning beam capabilities. These provide high antenna gain to increase communications margin and frequency reuse capability through beam isolation, while providing complete coverage of the United States. Effective communication bandwidths from a single satellite location can reach ten's of gigahertz, with the communication capacity tailored to match the very nonuniform geographic demand pattern. Satellite onboard processing consisting of demodulation, adaptive forward-error-correction (FEC) decoding and coding, routing of hundreds of thousands of channels to thousands of terminals, and remodulation with independently optimized uplink and downlink modulation structures is being developed. The onboard processing reduces the scanning antenna requirements, allows more effective frequency reuse, and increases the rain margins by adoptively using system margins to support terminals currently experiencing rain. All of the functions described can be performed with reasonable satellite weight, thermal, and power impacts by using large scale integration (LSI) to implement the digital data processor. By designing the onboard processor with parallel internal structure, the hardware can be made extremely reliable (high level redundancy) and the number of LSI chip types required is relatively small. The antenna and onboard processing techniques are readily adaptable to C-band and Kuband, as well as Ka-band. Deployable antennas may be required at the lower bands, but precision deployable antenna designs are available and the feed structures scale directly. Frequency reuse of all three commercial communication bands should greatly ease the orbit crowding problems now being experienced in C-band, and should allow United States domestic communications to accommodate any desired expansion in the next two decades.