Techniques for the real-time identification of radio channels

This paper discusses the use of several different types of aperiodic test signals to identify the characteristics of time-varying and dispersive radio channels. Such characterisation is required to initiate adaptation of radio system parameters to counteract transmitted signal distortions imposed by the channel. Identification of high frequency 2-30 MHz ionospheric skywave channels is considered. Operationally. this propagation mechanism allows communication to take place over ranges from line-of-sight to world-wide. Ionospheric propagation is affected by many physical factors, and this, coupled with the perturbations experienced by a radio signal, gives rise to both time and frequency variability of the received signal. Typical radio system architectures and protocols are described, consideration is then given to preferred forms of channel identification, or real-time channel evaluation (RTCE), for HF paths. Test signal options and trends in identification procedures are then reviewed.