Mobile Radio Diversity Reception

D. 0. Reudink, in an unpublished work, has suggested a diversity system especially suited for mobile radio. In his system the transmitter sends a pilot wave along with the modulated signal. The receiver's mixer stage beats the signal against the received pilot (instead of against a locally generated tone). Doppler distortion, which affects the signal and pilot in nearly the same way, cancels out during mixing. The diversity system with N antennas adds the outputs of N such mixers and demodulates the sum by means of an ordinary AM or FM detector. The receiver obtains a signal-to-noise advantage by adding signal components from the N mixers in phase while adding most interference terms powerwise. To obtain this advantage under multipath propagation conditions, the receiver's IF (that is, the difference / between the signal and pilot frequencies) must be chosen small enough. It suffices to make / so small that the propagation times along the different paths all agree to within a small fraction of 1// (see Section 2473