Mitigation of rain-induced fading: route diversity vs route-time coding

Rain-induced fading is the main source of low reliability in links operating in the higher microwave and millimeter wave bands. Links passing through different routes are capable of yielding a diversity effect against rain-induced fading. While uneconomic for long-haul radio relay systems, route diversity becomes viable for fixed access systems where the architecture is cellular. In this case, a user terminal must be linked to two base stations rather than to one. Due to the angular dispersion of the rain rate, important diversity gain can be achieved. In the conventional application of diversity, the same bit stream is transmitted over the two (or n) routes. On the other hand, the classical diversity concept (especially the concept of space diversity) has been generalized to what is called space-time coding (STC). In this concept, appropriate coding is applied and this coded signal stream - regarded as one entity - is transmitted over the diversity paths. STC is applied in multiple input-multiple output (MIMO) channels, and the route diversity system forms a special MIMO channel. We term the application of STC to the route diversity system route-time coding (RTC). It is found that, by applying RTC, the performance of route diversity can be much improved.