Simulation and implementation of a modulation system for overcoming ionospheric scintillation fading

Ionospheric scintillation has been recognized as a major problem in VHF/UHF satellite communication systems. An extensive computer simulation was done in an attempt to find an economical coding-interleaving combination which could be implemented for an airborne VHF/SHF SATCOM system. A variety of coding techniques were investigated. An actual ionospheric scintillation signal was recorded and digitized for the use in the simulation. The various coding interleaving techniques were played through this simulated channel; the results of each combination were tabulated. It was decided to implement a complete 75 bit-per-second teletype modem utilizing the results of the simulation. The modem being built utilizes frequency shift keyed modulation. In order to operate the encoder and interleaver, a received data clock is needed. It is derived by sampling the input data and incrementally shifting a reference clock until the best fit is achieved between the reference clock and a number of bits of the incoming data. This data clock is then used for timing the interleaver and encoder.