A Shared Resource TDMA Approach to Increase the Rain Margin of 12/14GHz Satellite Systems

The current trend in communication satellites appears to be increasingly toward the use of the 12/14-GHz frequency bands and the use of digital modulation formats with time division multiple access (TDMA) techniques. The former provides freedom from existing 4/6-GHz terrestrial interference and also provides higher antenna gain and narrower beams for a given size aperture, while digital transmission in conjunction with TDMA provides for more efficient utilization of the available satellite system resources. A major drawback associated with 12/14-GHz systems is the signal attenuation associated with rainfall.12 In general, attenuation at these 2097 frequencies is an increasing function of rain rate, with the result that, over a large portion of the United States, significant power margin must be provided to prevent excessive outage due to rain fades. Standard techniques that might be employed to provide rain margin include (i) increasing the radiated power of the satellite and earth stations, (ii) improving the noise figure of the receivers, (iii) installing larger ground station antennas, and (ii) providing site diversity. Unfortunately, all these techniques are costly in that permanently dedicated system resources are used only infrequently, i.e., when it rains. Looking at this another way, the system has been tremendously overdesigned for the clear air conditions that might exist more than 99.9 percent of the time at any particular ground station location if, say, a 15- or 20-dB rain margin is required to achieve the desired rain outage.