Adaptive TDMA for fade countermeasures

One of the major drawbacks associated with higher frequency satellite systems is the large signal attenuation caused by precipitation. In particular, rain attenuation dominates the power margin for high reliability systems operating at 30/20 GHz. The normal design procedure to overcome excessive outage associated with rainfall at lower frequency bands has been to incorporate a significant power margin into the system. Clearly this approach is not very acceptable in the higher frequency bands since for most of the time the attenuation is less than the built-in margin. A more efficient technique to achieve the desired availability would be to reduce the rain margin to some nominally low level and provide a common pool of resources to be shared among all earth stations and allocated, as needed, to increase the rain margin only at those stations that are experiencing a fade depth greater than the built-in margin. Examples of this approach include adaptive-TDMA, where the shared resource comprises reserved time within the TDMA frame, variable chip-rate spread-spectrum systems, where the shared resource amounts essentially to reserved RF bandwidth, and frequency diversity, where the shared resource is in the form of a relatively small reserve of capacity at a frequency band less affected by rain attenuation