Performance of Frequency Shift Keying for Low SEP Angle Communication

Communication during low Solar-Earth-Probe (SEP) angles generally suffers due to high scintillation effects. These effects are generally broken down into two categories: (1) Amplitude scintillation, and (2) Phase Scintillation. The amplitude scintillation effects have been studied and shown to be effectively modeled by wave interference. For scintillation indices under 1.0, it is usually modeled as a Rician random variable (approaching Rayleigh as the scintillation index approaches 1). The majority of the time, this results in destructive interference, effectively attenuating the signal amplitude. This can become a challenge for power constrained missions. The effect on the phase scintillation, however, is the introduction of random phase fluctuations on the intended received signal. This results in a bandwidth expansion of the desired signal and it generally (when in large quantities) renders phase-modulation schemes ineffective. In addition, these channel distorting effects generally refresh at a predictable period, which we denote as the coherence time of the channel. The spacecraft receiver must be able to cope with these effects in order to establish a reliable command link. In this paper, we evaluate the use of frequency shift keying (FSK) for communication during low SEP angles periods. The FSK waveform proposed is semi-coherent, which is important to the receiver operating through the scintillation effects (both amplitude and phase). The symbol-error-rate and bit error rates for the resulting communication scheme will be discussed as well. A GNU Radio implementation of an emulator for solar scintillation effects will be discussed. The parameters of the simulation, including scintillation index, coherence time, and broadened bandwidth models were derived from previous analysis of downlink signals during low SEP angle periods for other spacecraft, such as Cassini.