Performance comparisons among digital radio techniques subjected to multipath fading

The performances of different digital radio techniques in the presence of multipath fading are compared. The measure employed is the expected fraction of time, conditioned on the occurrence of a fading event, that the eye pattern at the receiver detector is closed. This measure is plotted against spectral efficiency (bit rate/channel bandwidth) for various types of modulation and for various forms of carrier recovery, transmit/receive filtering, timing recovery, and adaptive equalization. The types of modulation are QPSK (quaternary phase shift keying), offset QPSK, 8-PSK (eight-level phase shift keying), 16-PSK, 16-QAM (16-level quadrature amplitude modulation), and offset 16-QAM. The computations assume a multipath fading frequency response described by a first-order complex polynomial, the coefficients obeying an empirically derived probability law. The comparisons show the superiority of high-level modulation and the inferiority of offset modulation. They also demonstrate significant improvements for each of the two adaptive equalizers postulated here.