Noise immunity of reception of signals with quadrature amplitude modulation in the presence of interference with phase-shift keying
Signals with multi-position quadrature amplitude modulation, due to their spectral and energy characteristics, have been used long and successfully in the high-capacity digital data transmission systems, e.g., in the present-day satellite communication lines. The spectral efficiency of such signals increases with increasing number of the positions, but at the same time the noise immunity of their reception decreases. In real data transmission radio systems the receiver input along with the noise may have other kinds of interference, both casual and intended. The interference with phase-shift keying is one of the most commonly known, e.g. an interfering narrow-band signal from an adjacent radio station. The article analyzes the noise immunity of receiving signals with multi-position quadrature amplitude modulation in the presence of noise and interference with phase-shift keying by methods of statistical radio engineering. For this purpose, statistical characteristics of the processes distributions at the outputs of correlation receiver’s integrators were calculated. The distributions of all mentioned random processes accounting for a white Gaussian noise present at the demodulator’s input were taken as normal. Then, by averaging over the interference’s random initial phase the dependencies of a bit error probability from the signal-to-noise ratio, interference intensity and its shift relative to the signal’s central frequency were obtained. It is shown that with increase of positions number of the signals, the noise immunity of the reception against the background of interference with phase-shift keying is greatly reduced and even low-level interference cause a significant energy loss. With an increase in the relative transmission rate of interference, its damaging effect decreases, but remains very noticeable in a wide frequency range.