A Scanning Spot-Beam Satellite System

The current approaches to domestic-satellite systems divide along the lines of area-coverage and spot-beam concepts. Each system has its merits as well as disadvantages. A spot-beam satellite system1-2 allows high antenna gain and several reuses of the allocated frequency spectrum. In Ref. 1, a 12/14-GHz system with 11 frequency reuses was described which could provide reliable service at digital rates of 600 Mb/s with 30 watts peak transmitter power, employing a satellite antenna having 47-dB gain in each spot-beam. The disadvantage of such a system stems from the fact that each spot-beam covers only a small area. To avoid cochannel interference, a dead space between any two adjacent beams much larger than the beam coverage area (e.g., 3-dB contour) is 1549 required. 3 ' 4 Also, there are regions needing service which do not have enough traffic to justify a dedicated spot-beam. Area coverage satellites, such as used by AT&T/GTE, Western Union, or RCA use broad antenna beams covering the whole United States. They are capable of providing service everywhere within the continental U.S.A. but lack channel capacity because the alloted spectrum can be reused at most once by polarization reuse. A more significant disadvantage, however, is the power penalty associated with the gain of an area-coverage antenna. The 3-dB contour gain of a U.S. coverage antenna is only 27 dB, and there is little that can be done to improve it further. To obtain the same SNR as the previously mentioned spot-beam antenna system, the required RF power to transmit at a 600-Mb/s data rate would be 3 kW.