DEVELOPMENT OF ENERGY EFFICIENT COMMUNICATION SYSTEM IN THE TROPOSPHERE RADIO CHANNEL BASED ON OFDM SIGNALS
Abstract
It is known that inter-symbol interference may occur during data transmission in radio relay and tropospheric communication systems. The presence of multipath propagation and frequencyselective fading in the tropospheric, radio relay channel significantly reduces the energy efficiency of the communication system as a whole. The aim of the work was to increase the efficiency of using the channel for radio relay and tropospheric communication by using OFDM (orthogonal frequency-division multiplexing using the methods of reducing the peak factor of the OFDM signal and increasing the linearity of the transmission path. To evaluate digital predistortion algorithms in the Matlab/Simulink environment, a model was developed for the LMS, NLMS, RLS, RPEM methods and a power amplifier model with real characteristics. Based on the results of modeling algorithms, RLS was chosen. In addition, in this work, a modified version of the adaptation algorithm based on the recursive least squares method (RLSm) was developed. The main result of the modification is: a decrease in the number of arithmetic operations per iteration (more than 5 times), an increase in the stability of adaptation algorithms, due to the introduction of regularization methods, a decrease in the convergence time, due to the introduction of an exponential dependence. Various algorithms for reducing the peak factor of an OFDM signal were investigated, the best result was achieved by combining Tone reservation (TR) and Active constellation extension (ACE). Simulation in the Matlab/Simulink environment showed that the combination of TR and ACE algorithms reduces the peak factor of OFDM signals by ~5dB for BPSK data stream and ~4.5dB for 8-PSK, QAM-16, QAM-64, QAM-128 and QAM- 256. To increase the linearity of the transmission path, the RLSm digital pre-distortion algorithm was selected and upgraded, it made it possible to reduce the magnitude of the error vector modulus (EVM) by 13.5dB, and also increase the modulation/error ratio (MER) by 13.6dB.
