The spectral/power efficiency and performance of the pi /4-quadrature phase-shift keying (QPSK) modems (American and Japanese digital cellular emerging standards) are studied and briefly compared to GMSK modems (proposed for European Digital European Cordless Telephone System) (DECT) and Group Special Mobile System (GSM) cellular standards). Improved filtering strategies and digital pilot aided (digital channel sounding) techniques are considered for pi /4-QPSK and for other digital modems. More spectrally efficient modem trends and some research results for future cellular/mobile (land mobile) and satellite communication systems applications are also highlighted.
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