Simulation and Measurement of High Frequency Ionospheric Channels. Time-and Frequency-Dispersive Properties of HF Sky Wave Propagation Paths can be Modeled for Use in Adapting Signal Format and Signal Processing Algorithms for Communication System Development.

摘要

The report presents literature search of hf channel simulators,describes the Watterson model and its limitations,and discusses distortion of ionospherically propagated signals due to ionospheric refraction. Significant dispersion occurs if the half-bandwidth exceeds pie/(delta TAO/delta f) to the 1/2power,where (delta TAO/delta f) to the 1/2power (in s/Hz) is the ionogram slope at center frequency. For 100kHz bandwidth,dispersion is significant (delta TAO/delta f) to the 1/2power exceeds 3.14 X 10to the -4th ms/kHz,its measurement requiring 62.8microsec. resolution. Recommendations:For bandwidths above 2.5kHz at night and 12kHz during the day,add frequency dispersion correction filters to the Watterson simulator tap delay lines. To achieve required time delay resolution,use a complementary-sequence or modulated-pulse oblique sounder system. For the microprocessor or minicomputer controlled receiver,provide an atomic time-frequency standard and interfaces for terminal,CRT display,magnetic tape recorder,and auxiliary devices. When measuring modem performance capabilities with a channel simulator,take a 'single and multiple distortions'approach. Use the Jones ray-tracing program to obtain the frequency dispersion relationships needed for applying corrective filters to the Watterson model;relate results to channel-caused system distortions. Use a suggested approach if too many delay line taps would be needed. (Author)