nptelhrd
51 minutes
Wireless Communications: Equalization and Diversity Techniques in Fading Channels
Fading wireless channels degrade modulation performance through inter-symbol interference (ISI) and deep, prolonged signal fades, and are countered by three complementary techniques: equalization (an adaptive inverse filter that compensates ISI in frequency-selective channels), diversity (combining independent, uncorrelated signal paths to reduce the depth and duration of fades), and channel coding (adding structured redundancy so errors can be detected and corrected). Which technique applies is determined by whether the channel is flat or frequency selective (signal bandwidth versus coherence bandwidth, set by RMS delay spread) and slow or fast fading (symbol period versus coherence time, set by Doppler spread). Adaptive equalization requires the equalizer transfer function to invert the channel response, implemented as a time-varying transversal filter whose weights are iteratively adjusted (for example by LMS) to minimize mean square error between desired and actual output.