A new differential adaptive modem for multipath propagation

The presence of intersymbol interference due to multipath propagation in a digital radio transmission tends to degrade considerably the performance of the receiver. In order to compensate for the unknown channel and time varying linear distorsion, adaptive equalizers are frequently used in efficient digital communication systems. Differentially coherent reception does not require sophisticated carrier recovery circuits and it is tolerant to the multipath propagation phenomena. In addition, it is particularly insensitive to local oscillator microphony thus enabling it to work in a severe environment. Finally, it gives satisfactory performance with a very simple receiver. The author presents a modem, based on a new adaptive equalizer for differentially coherent receiver proposed by Sehier and Kawas Kaleh (1990). Descriptions of original diversity combiner and clock signal phase estimation are given