A new power efficient multi-level correlative digital FM system

A novel modulation technique to transmit bandlimited and intersymbol interference (ISI)-free multilevel digital signals is proposed for the use in digital radio-microwave, mobile and satellite radio systems. Emphasis is on the design of a 9.6 kb/s 4-level digital FM modem for satellite communications, especially for VSAT application. Experimental measured-eye diagrams and the power spectrum of this modem are reported. Optimization between the required signal power and bandwidth of the transmission channels could be done by selecting the number of signal levels and modulation index of the modem. A preliminary study shows that the modem technique may give better performance than coherent PSK systems, especially for low-speed data transmission where phase noise and lock-up speed of carrier recovery loop limit the system performance