Equalized Megahertz-Bandwidth HF Channels for Spread Spectrum Communications

In this paper we discuss the use of skywave propagation paths in the HF band for spread spectrum communications involving coherent bandwidths of about one megahertz. We present experimental data describing wide-bandwidth one-hop F-layer links, discuss real-time adaptive equalization procedures for wideband spread spectrum signaling on these links, and conclude with some recent measurements over equalized paths of up to 2000 km. Two significant attributes of these equalized wideband channels are (1) time invariance over many seconds (typically ten), and (2) freedom from fading caused by the presence of multiple propagation modes. Both are dependent on achieving mode isolation in the receiver through the use of wideband waveforms. The measurement data was gathered in 1981 using MITRE's wideband HF test facility which consists of a mobile transmit terminal and a fixed receive terminal in Bedford, Massachusetts. The transmit terminal is capable of generating signals for channel measurement and data communications testing. The receive terminal is capable of real-time adaptive equalization over a 1.024 MHz bandwidth, signal processing, and data recording. The data presented here describes unequalized and equalized channel measurements.