P.V. Luferchik1, A.A. Komarov2, A.N. Konev3, R.G. Galeev4, E.V. Bogatyrev5

1-5 JSC SPE Radiosvyaz (Krasnoyarsk, Russia)

It is known that intersymbol interference may occur during data transmission in radio relay and tropospheric communication systems. The presence of multipath propagation and frequency-selective fading in the tropospheric, radio relay and near-field magnetic induction radio channel, in general, significantly reduces the energy efficiency of the communication system. The purpose of the work was to increase the efficiency of the channel usage for radio relay and tropospheric communication by using OFDM (orthogonal frequency-division multiplexing) signals, adaptive coding and modulation, as well as methods to reduce the peak factor of the OFDM signal and to increase the linearity of the transmitting path. OFDM communication system model was developed for operation in multipath channel, including models of an OFDM modulator and demodulator, as well as a model of a radio frequency path using the characteristics of an existing amplifier.

To evaluate the effectiveness of digital pre-distortion algorithms in the Matlab/Simulink environment, a transmission path model was developed with the LMS, NLMS, RLS, and RPE DSP algorithms. Parameters of a real power amplifier: frequency range 4.4 - 5 GHz, maximum radiated power - 100 W, third-order intermodulation distortion - minus 27 dB.

According to the results of algorithms simulation, RLS turned out to be the most effective. In addition, within the framework of this work, a modified version of the adaptation algorithm based on the recursive least squares method (RLSm) was developed, characterized by a decrease in the number of arithmetic operations per iteration (decrease by more than 5 times), an increase in the stability of adaptation algorithms, due to the introduction of regularization methods, reducing the convergence time, due to the introduction of an exponential dependence of the “forgetting” factor.

Various algorithms for reducing the peak factor of an OFDM signal have been studied and investigated. As a result of the modeling, the combination of Tone reservation (TR) and Active constellation extension (ACE) algorithms gave the best results for peak factor reduction. The combination of TR and ACE algorithms reduces the peak factor of OFDM signals by ~5dB for BPSK data streams and ~4.5 dB for 8-PSK, QAM-16, QAM-64, QAM-128 and QAM-256.

When using various coding and modulation in dynamic receiving/transmission conditions, it is possible to achieve optimal use of the frequency and energy resources by creating systems that adapt to signal propagation conditions. To implement the ACM mechanism, a service field was introduced into the transmitted service data, which contains information about the code rate, the modulation type and the interleaving depth used. By adjusting the interleaving depth, it is possible to optimize the SNR threshold or the amount of information delay in the channel, depending on the system requirements.

As a result of the analysis and modeling, methods were developed that made it possible to increase the energy efficiency of the OFDM mode in multipath channels. Simulation of various algorithms for decreasing the peak factor has been carried out. Combination of two best suitable peak factor reduction algorithms are implemented. Active Constellation Extension (ACE) and Tone Reservation (TR) algorithms combination reduces the peak factor of OFDM signals by ~5dB for QPSK data stream and ~4.5 dB for 8-PSK, QAM-16, QAM-64, QAM-128 and QAM-256.

To increase the linearity of the transmission path, the RLS digital pre-distortion algorithm was upgraded. This made it possible to reduce the error vector magnitude (EVM) by 13.5 dB, and also increase the modulation/error ratio (MER) by 13.6 dB. The use of adaptive code rate selection and modulation will result in more efficient resource utilization in non-stationary multipath channels. The obtained results will significantly improve the energy efficiency of the OFDM mode.

Luferchik P.V., Komarov A.A., Konev A.N., Galeev R.G., Bogatyrev E.V. Energy efficient OFDM mode with the ability to adapt to propagation conditions in multipath channels. Radiotekhnika. 2022. V. 86. № 8. P. 122−131. DOI: https://doi.org/10.18127/j00338486-202208-13 (In Russian)

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