DSP modem/codec implementation for long-range radio applications

The paper describes the implementation of a real-time integrated modem/codec system for long-range radio communications, e.g. HF. The system has been implemented using a proprietary DSP-based system which uses the AT&T DSP32C digital signal processor and the initial format comprises MFSK modulation combined with RS coding. It outlines the main features of the system and describes some detailed aspects of the design. It also describes the minimum weight decoding algorithm that is used in the decoder in conjunction with channel measurement information to provide an efficient decoding algorithm that makes use of soft-decision information. The system is to be tested in an aeromobile environment and so the problem of the detection and correction of motion induced time-varying frequency offsets has been addressed. A technique is presented for obtaining correct frequency alignment between the transmitter and receiver terminals in the presence of (relatively) large scale initial frequency offsets