Devising a method for increasing the noise immunity of systems with orthogonal frequency division multiplexing under the conditions of inter-channel interference

Abstract

Modern communication systems are based on orthogonal frequency division multiplexing (OFDM) technology, which allows reliable transmission of information under multipath conditions. The need to preserve the orthogonal properties of subcarriers leads to high sensitivity of these systems to frequency shifts of the signal. The method of signal formation for the OFDM system has been improved in this work. The use of spectrum-selective shaping pulses after the inverse fast Fourier transform (IFFT) stage at the transmitter side to reduce the level of inter-channel interference during carrier frequency shift was investigated. New pulse shapes were synthesized, obtained by using optimized multiparameter functions with a selective spectrum. The effectiveness of the application of synthesized pulses with a selective spectrum in reducing the influence of the frequency shift of the signal on the interference immunity of the OFDM system was analyzed. A comparison of the probability of a bit error with already existing forms of Nyquist pulses was carried out. In the MATLAB environment, a model of the transmitter and receiver of the OFDM system was developed for the experimental assessment of the influence of the proposed forming pulses on the immunity of the system under the conditions of inter-channel interference with different types of modulation. It was established that the lowest level of bit error probability under the conditions of inter-channel interference was observed for a two-parameter pulse with a selective spectrum and a piecewise linear approximation of the transition region. So, for a signal-to-noise ratio of 15 dB, BPSK modulation and a normalized frequency shift of 0.2, the probability of a bit error for a given pulse is 3∙10-4; for QPSK modulation and a normalized frequency shift of 0.1, 10-6; for QAM-16 modulation and a normalized frequency shift of 0.03, 2∙10-4