Rain Margin Improvement Using Resource Sharing in 12-GHz Satellite Downlinks

In an earlier paper, 1 a shared-resource concept was described for increasing the rain fade margin of a digital satellite system by as much as 10 dB above the design fade margin. With this approach, unused time slots of the Time-Division-Multiple-Access (TDMA) frame are made available to ground stations experiencing rain fading above the RAIN MARGIN IMPROVEMENT 167 built-in fade margin, and are relinquished when the fade event has ended. Error-correcting coding is introduced to occupy the extra time slots, thereby reducing the carrier-to-noise ratio (CNR) required to maintain the threshold bit-error-rate ( B E R ) . L O W rain outage is therefore achieved without radiating excessive down-link power. Not only does this conserve satellite power, but, also, interference into the systems of other users of the geosynchronous orbit is minimized. In such an application, the operating speed of the decoder is much lower than the transponder data rate by virture of the low T D M A duty cycle associated with each ground station. Because of the infrequency of simultaneous deep fading at multiple sites, a small pool of reserved time slots can often protect a large number of ground stations. The degree of protection so provided is the subject of this current work; we shall evaluate the reduction in rain margin required to achieve a given outage objective when all ground stations in the network are competing for a limited number of shared resources. We restrict our attention to the power-limited down-link since up-link fading can usually be overcome by means of up-link power control.1 A convolutional code yielding a maximum power saving of 10 dB is assumed throughout.