This document illustrates existing architectures for High-Frequency (HF) communication systems. Since HF transmissions are mainly used for long distance tactical communications, the discussion will be focused on several recent military standards developed from the early 1990’s by the North Atlantic Treaty Organization (NATO) and the United States Department of Defense (US-DoD). The attention is mainly concentrated on the physical layer (PHY) specifications, which provides details about the baseband signal processing leading from the information bits stream to the waveform that is launched over the air.
After discussing the propagation mechanisms in the HF band, the main parameters of the HF channel are provided in terms of delay spread and Doppler rate for either good, moderate and poor channel conditions. The component parts of an HF waveform are subsequently illustrated, including the adopted modulation formats, the synchronization methods as well as coding and interleaving schemes. The concept of serial-tone and parallel-tone waveforms is also introduced, illustrating their relative advantages or disadvantages under several operating conditions. A comprehensive overview of serial-tone HF standards is presented for both second generation (2G) and third generation (3G) waveforms. Finally, a couple of parallel-tone modems for HF communications are described with some detail.