Coherent frequency exchange keying helps to overcome the `near-far´ effect in mobile satellite communications

Echoing the trend in terrestrial mobile (cellular) communications, frequency-hopping is anticipated as a preferred spectral spreading technique, and one which additionally copes better with the `near-far´ effect. It demands robust modulation standards, usually some kind of frequency-shift keying. A new form of binary digital modulation is described, related to frequency-shift keying but with superior sidelobe suppression. Coherent frequency exchange keying (CFEK) is generated entirely by linear processes and its spectral purity excels even GMSK at the second adjacent channel. The spectral efficiency is one half that of conventional angle-modulation standards and its usefulness would be restricted to those applications where EMC takes an overriding consideration, such as in co-located simplex transmitters and receivers. The actual performance of a data transmission system using an engineered CFEK modem is presented with measurements which confirm the theoretical predictions