Abstract. Quadrature
amplitude modulation (M-QAM) signals, due to their spectral and energy
characteristics, have long and successfully been used in the high-capacity
digital data transmission systems, e.g., in the present-day satellite communication
lines. Their use is also envisaged in various communications standards, namely,
DVB-S and DVB-S2/S2X standards. Noise immunity of received M-QAM signals
against the backdrop of white Gaussian noise are well characterized. In real
data transmission radio systems the receiver input along with the noise may
have other kinds of interference, both, casual and intended. Harmonic
interference is one of the most commonly known, e.g. an interfering narrow-band
signal from an adjacent radio station. Harmful effect of such interference on
the quality of received data was proved by some studies.
The article gives an example where the
receiver input is characterized as having an additive mix of M-QAM wanted
signal, white Gaussian noise and harmonic interference which is intensive and
mistuned relative to the wanted signal central frequency and random phase.
Noise immunity of received signals was analyzed using statistical
radio-engineering methods. For this purpose, statistical characteristics of the
processes distributions at the outputs of correlation receiver’s integrators,
conditional noise in terms of initial random phase, 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 mistuning relative to the wanted signal’s central frequency were
obtained.
It was demonstrated that a signal receiver
with a quadrature amplitude modulation of small sizeÌ≤16
is quite stable to the action of harmonic interference, and the higher is the
signal size the lower is the noise immunity of received signals against
harmonic interference, even with the low levels of noise.
The harmful influence of interference is most
noticeable, when enters the main beam of the signal spectrum. The most
dangerous is a targeted harmonic interference the frequency of which coincides
with the central frequency from the wanted signal’s spectrum. Noise
compensation or rejection algorithms shall be used to improve the noise
immunity of received M-QAM signals against the backdrop of harmonic interference.
Keywords: bit error probability, harmonic interference, quadrature amplitude
shift keying, interference immunity.
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