6.3.1 Equalizers
Technological advances in equalization techniques have been very effective in minimizing the effects of anomalous propagation on digital radio performance. As the number of modulation states increases, the radio system becomes more vulnerable to multipath fading. Various types of adaptive
equalizers are currently in use for digital fixed wireless systems and are implemented at intermediate and/or at baseband frequencies. Among these equalizers, decision-feedback equalizers and linear transversal equalizers are in common use. A powerful solution is represented by the so-called fractionally-spaced linear transversal equalizer (FSLTE), in which the signal is sampled several (at least two) times within one symbol period. Such a solution shows a better performance/complexity ratio, together with a lower sensitivity to timing phase (when the central tap is not fixed). Moreover, FSLTEs, implemented with a two samples/symbol sampling, combine “naturally” with receiver over-sampling digital filtering (shaping filters), and with simplified minimum mean square error (MMSE) algorithms for timing recovery.
For optimum performance, the dynamic convergence of FSLTEs should be accomplished by means of “blind” algorithms (as with synchronous LTE), combined with the recursive updating of tap coefficients, reference-tap position and timing phase, for both initial convergence and channel tracking.
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