Tone interference effects on the performance of QPSK modulation in communication

In contemporary communication satellite systems, a digital processing unit on board the satellite can be employed to filter, channelize, and switch the uplink communication signals. This processing is performed subsequent to the radio frequency (RF) to intermediate frequency (IF) downconversion of the uplinked signals. Then, an analog-to-digital converter (ADC) is employed to convert the analog signals to digital signals. This conversion process can lead to spurious responses, which can act like tone interference to the uplinked modulated signals, including commonly employed quadrature phase-shift keying (QPSK) modulated signals. Additionally, there are a wide variety of other spurs that can be created by the satellite communication payload. All of these spurs can adversely affect bit error rate (BER) performance. This paper discusses the adverse effect such spurious responses have on QPSK modulated signals, and the spurs are modeled as tone interference. Results are presented, including BER curves, for the following: (1) the spur offset in frequency from the center frequency of the QPSK signal, (2) false lock of the receiver onto the spur frequency and phase, and (3) the addition of convolutional coding to the receiver and the improvement in BER that results.