Limiter Intermodulation Improvement Due to Selective Carrier Spacing

In many space communications applications, the intermodulation between carriers, due to limiting in a common amplifier, reduces the overall signal-to-noise ratio of the signal received at the ground station. By selectively spacing the carriers, one is often able to avoid the strong or low-order intermodulation. The question arises how much improvement is gained in carrier-to-interference ratio and signal-to-interference ratio by selectively spacing carriers such that low-order cross products are avoided or their impact is effectively reduced. The following analysis investigates interference improvement due to selective carrier spacing for several spacing sequences relative to equal carrier spacing. It is shown that the intermodulation noise reduction depends on the carrier packing density of the selected spacing sequence. The carrier-to-intermodulation noise ratio increases with the number of participating carriers, while the bandwidth efficiency decreases. The signal-to-intermodulation noise ratio improves not only because of the improved carrier-to-intermodulation noise ratio, but also because the remaining interference components have spectra with larger frequency spreading.