I work with plasma engineering and RF power daily, but it's a very messy field with lots of questions when it comes to plasma and RF interactions. so I will try to translate to ELIundergraduate.

The three channels of S-band, pulse-coded RF signals are heterodyned in microwave stripline mixers with the output of a single crystal-controlled solid-state local oscillator to obtain a pulsed IF signal.

the pulsed RF signal is combined with a fixed reference signal very close to its operating frequency, creating an intermediate frequency which is the beat between the two. for instance, you can subtract an S-band 4 GHz wave from a 4.1 GHz signal to obtain a new signal of VHF-band 100 MHz (.1 GHz), an entirely different wavelength from the original. that new wavelength interacts with the plasma differently.

in general, plasma of whatever species/conditions/what have you fucks with signals in a fixed frequency range. so the solution is to move your signal out of that range until the plasma impedance is not high enough to interfere with your signal. at the receiver you can then add the fixed frequency back in and re-obtain the original signal.