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The Influence of the Nonlinearity of Gain Characteristic of the Transmitter on the Main Characteristics of the Radio Link


M. S. Nemirovskiy

This article is dedicated to the studies of the influence of the nonlinearity of gain characteristic (GC) of the transmitter on the main characteristics of the radio link: energy and band-pass performance index. Two cases of using of the transmitter are discussed: signal amplification on one carrier frequency and parallel transmission of signals on several carrier frequencies. In the second case situations, when number of carrier frequencies is high and when it is small, are discussed separately. Five types of GC transmitting sets are reviewed, which differ in the behavior of the derivative of the level of the signal output as a function of signal input. A log-leg characteristic (the derivative is constant over the linear range and is equal to zero in the saturation mode) corresponds to the usage of the optimum linearizer in the set. Other characteristics represent different degrees of approximation to this option and include the most typical GC of the amplifier units, which are used more often. The influence of the nonlinearity of the GC on the energy performance index is estimated by analyzing the noise immunity ability of the signal reception of QAM of different ratios in the case, when on the transmitting side it is amplified by the set with the discussed GC. Herewith added amplitude oscillations of the signal, which are caused by the smoothing in the modulator, is taken in account to decrease the rate of out-of-band emissions. As the model of the case of amplification of the small number of carrier frequencies, transmitted parallelly, the case with bisignal input action is used, and for the case of the high number of carrier frequencies– the case with noise emission. Unified estimation of the influence of the nonlinearity is used – reduction of the of the power of the useful signal output in relation to the power of saturation, required to satisfy the set requirements on permitted reduction of the energy performance index (the rate of beat-moire components). The analyzing is performed by numerical computer simulation using MathLab package. Rather full quantitative results of the solving of formulated problems are received. It can be stated qualitatively, that the reduction of the power of the transmitter required to offset the negative effect of its nonlinearity in every case is rather significant, so the use of linearizers is justified practically always.

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