Shaped PSK in a digital modem with direct digital synthesis

Performance data for the MD-1269A Multipurpose Modem modulator are presented with emphasis on spectrally efficient phase shift keying (PSK). Attention is focused on power spectral containment within 5-kHz and 25-kHz communication satellite channels, measured relative interference levels in adjacent channels, vector rotation effects, and spectral performance as a function of shaping factor. The modulator features a direct digital synthesizer controlled by a digital signal processor. This combination allows simultaneous and independent control of the amplitude, frequency, and phase of the modulated waveform, which is needed to prevent co-user satellite communication interference. This flexible, general-purpose modulator significantly increases the satellite user´s flexibility by providing a variety of spectrally efficient PSK modulation waveforms that can be used at data rates up to 38.4 kb/s, while maintaining compatibility with lower-data-rate modems. The model is designed for UHF satellite communication and line-of-sight communication links. The entire modem is contained in a 25-lb, 1/2 ATR airborne package and is able to withstand airborne, shipboard, or tactical environments