An examination of HF modem platform architectures

This paper discusses aspects of an HF modem platform architecture with particular emphasis on the use of the PC as a component of the modem. The paper begins by tracing the evolution of the HF modem architectures. This follows the path from early, all-hardware designs, through reprogrammable custom processors to current designs which make use of powerful DSP and control processors to provide a tremendous level of flexibility. It is suggested that the use of PC hardware is the logical continuation of this trend toward more general-purpose platforms. The limitations and advantages of using a PC platform to implement an HF modem are explored and the authors' describe their experience in developing such a modem running under the Linux operating system. The paper concludes with a look at how the modem architecture may further evolve as a result of previous developments, both in software radio and in code-combining ARQ protocols.