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.