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Equalization Techniques. By: Nader Mohammed Abdelaziz. Introduction.
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Equalization Techniques By: Nader Mohammed Abdelaziz
Introduction Wireless communication is the most interesting field of communication these days, because it supports mobility (mobile users). However, many applications of wireless comm. now require high-speed communications (high-data-rates).
What is the ISI Inter-symbol-interference, takes place when a given transmitted symbol is distorted by other transmitted symbols. • Cause of ISI ISI is imposed due to band-limiting effect of practical channel, or also due to the multi-path effects (delay spread).
Definition of the Equalizer: the equalizer is a digital filter that provides an approximate inverse of channel frequency response. • Goal of equalization: is to mitigate the effects of ISI to decrease the probability of error that occurs without suppression of ISI, but this reduction of ISI effects has to be balanced with prevention of noise power enhancement.
Components of ISI-free transmission system:BACK> • Pulse shape g(t), used to improve the spectral properties of the transmitted signal. • Matched filter, which is matched to the pulse shape g(t), used to maximize SNR of the received signal. • Sampler, to sample the signal with higher rate than symbol-rate, and equalizer designed for the over-sampled signal (fractionally-spaced-equalization). • Decision device, used to round the estimated symbol (o/p of the equalizer) to the training sequence. • Tap-update algorithm, to update the tap coefficients to improve the performance of equalizer filter.
methods of implementation of equalizers: • Transversal structure which is a digital filter with N-taps that have a tunable complex coefficients, and N-1 delay elements. • Lattice structure which uses a sophisticated recursive structure that has some advantages such as, better stability, flexibility to change length of equalizer.
Types of Equalization techniques • Linear Equalization techniques which are simple to implement, but greatly enhance noise power because they work by inverting channel frequency response. • Non-Linear Equalization techniques which are more complex to implement, but have much less noise enhancement than linear equalizers.
Linear equalizer with N-taps, and (N-1) delay elements. BACK>
Zero-Forcing technique • It cancels all ISI effect by inverting the channel frequency response, and accordingly leads to large noise enhancement. BACK>
Its goal of design is to minimize the expected MSE between transmitted symbol and its estimation.
It consists of a feed-forward filter B(z), and a feedback filter D(z). • It suffers from Error propagation when bits are decoded in error, which leads to poor performance. BACK>
It is the optimal equalization technique, but its complexity increases exponentially with the delay spread.
Difference between Adaptive Equalization, and Blind Equalization.
Adaptive Equalization • Definition. • What is meant by: Training, and Tracking
Blind Equalization • In which the signal recovery is done by prior knowledge concerning channel, or by array calibration information (no need for training sequence).
