High-Frequency Band Modem Based on OFDM Technology for Long-Range Communication

This paper presents the development and investigation results of a high-frequency (HF) modem based on OFDM technology for long-range communication. It is shown that the main challenge in HF communication lies in the significant variability and instability of the transmission channel. A promising approach is the use of OFDM signals, which can efficiently adapt to the changing conditions of the HF channel. In the course of the modem development, the following tasks were addressed: the design of a unique OFDM frame structure tailored to the specifics of the HF band; implementation of an autocorrelation method for signal reception synchronization; and modem modeling in the MATLAB environment. Research results demonstrate that the proposed frame structure, featuring one Zadoff-Chu-based synchronization symbol and four informationbearing OFDM symbols, provides reliable time-frequency alignment even under severe channel selectivity. The practical value of this work is reflected in a significant increase in throughput: with QPSK modulation alone, data rates of up to 8 Mbps are achieved within a 6 MHz bandwidth, while a future transition to 16-QAM - supported by the modem's architecture could raise the rate to 20 Mbps without changing the hardware base.