A Robust HF Modem

This paper describes the preliminary performance of a robust and low-cost HF modem design intended for data transmission at 300, 150, and 75 bps. The modem is designed to overcome the effects of multipath fading and frequency offsets. Error-correction coding is used to reduce the signal-to-noise ratio required to support reliable communications. The 300 bps modem design was first tested using an array processor (the FPS AP-120B) implementation. Performance evaluation using the SIGNATRON HF channel simulator has shown the modem to be capable of acquiring the received signal frequency and timing, and performing channel adaptation in a fraction of a second. Satisfactory performance has been measured even under fast multipath fading conditions.