How do you optimize the link budget and availability of SATCOM links under rain fading scenarios?

2 Rain attenuation and fading models

Rain attenuation and fading are the main sources of signal degradation in SATCOM links operating at frequencies above 10 GHz. Rain attenuation is the reduction of signal power due to absorption and scattering by raindrops along the propagation path. Rain fading is the variation of signal power due to fluctuations in the raindrop size and distribution. Rain attenuation and fading can be modeled using empirical or statistical methods, such as the ITU-R model, the Crane model, or the lognormal model. These models use inputs such as frequency, polarization, elevation angle, rain rate, and path length to estimate the attenuation and fading statistics.

3 Link margin and availability

The link margin is the difference between the SNR at the receiver and the minimum SNR required for a given BER and modulation scheme. The link margin indicates how much excess power is available to overcome the effects of rain attenuation and fading. A higher link margin means a higher availability of the SATCOM link. The availability is the percentage of time that the SATCOM link meets the minimum SNR requirement. The availability depends on the link margin and the rain attenuation and fading statistics.

4 Link budget optimization strategies

To optimize the link budget and availability of SATCOM links under rain fading scenarios, you can consider several strategies. For example, using a lower frequency band such as C-band or Ku-band is less affected by rain attenuation and fading. Choosing a circular polarization is more resilient to rain depolarization than linear polarization, while a higher elevation angle reduces the path length through the rain layer and the attenuation. Additionally, increasing the antenna gain increases the transmitted and received power and SNR, while reducing the data rate or using a more robust modulation scheme reduces the minimum SNR requirement and BER. Moreover, a higher transmit power or larger amplifier increases the link margin and availability. Lastly, diversity techniques such as frequency diversity, polarization diversity, or spatial diversity combine multiple signals from different sources or paths to mitigate the effects of rain fading.

5 Link budget simulation tools

To evaluate the performance of different link budget optimization strategies, you can use various simulation tools, such as the Satellite Link Budget Calculator. This web-based tool allows you to calculate the link budget and availability of a single SATCOM link using the ITU-R model. The Satellite Link Budget Tool (SLBT) is another software tool that enables you to calculate the link budget and availability of multiple SATCOM links with different models and diversity techniques. Additionally, the MATLAB toolbox for satellite communications provides a collection of functions and scripts that allow for advanced analysis and optimization of SATCOM links with various models and techniques.