Introduction to Digital Mobile Communication, 2nd Edition

7OTHER TOPICS IN DIGITAL MOBILE RADIO TRANSMISSION

This chapter introduces other important technologies. Diversity transmission system, multi-input multi-output (MIMO) systems, adaptive automatic equalizers, error control techniques, trellis modulation, adaptive interference cancellation, and voice coding are described.

7.1 DIVERSITY TRANSMISSION SYSTEM

Diversity techniques mitigate fading by using multiple received signals that experience different fading conditions. This system is based on the fact that it is unlikely that all signals experience simultaneously bad fading conditions. A fading channel causes three kinds of transmission errors: (1) thermal noise error due to level fading, (2) irreducible error due to random FM effects, and (3) errors due to waveform distortion caused by frequency-selective fading. The term irreducible errors means that the error rates cannot be reduced even if average signal to noise power ratio (SNR) is arbitrarily increased. A diversity system is effective for reducing the three kinds of errors. The effect of a diversity system on the thermal noise error is easily understood since the signal level dropping probability is diminished with a diversity system. The random FM effect and the waveform distortion become significant when the signal level drops. Thus, we can understand that the random FM effect and the waveform distortion caused by the fading channel are mitigated in the diversity system by preventing the signal level from dropping ...