Question

Write a note on equalizer.

Answer 1

In telecommunication, equalization is the reversal of distortion incurred by a signal transmitted through a channel. Equalizers are used to render the frequency response—for instance of a telephone line—flat from end-to-end. When a channel has been equalized the frequency domain attributes of the signal at the input are faithfully reproduced at the output. Telephones, DSL lines and television cables use equalizers to prepare data signals for transmission.

Equalizers are critical to the successful operation of electronic systems such as analog broadcast television. In this application the actual waveform of the transmitted signal must be preserved, not just its frequency content. Equalizing filters must cancel out any group delay and phase delay between different frequency components.

Digital equalizer typesEdit

Linear equalizer: processes the incoming signal with a linear filter

MMSE equalizer: designs the filter to minimize E[|e|2], where e is the error signal, which is the filter output minus the transmitted signal.

Zero forcing equalizer: approximates the inverse of the channel with a linear filter.

Decision feedback equalizer: augments a linear equalizer by adding a filtered version of previous symbol estimates to the original filter output.

Blind equalizer: estimates the transmitted signal without knowledge of the channel statistics, using only knowledge of the transmitted signal's statistics.

Adaptive equalizer: is typically a linear equalizer or a DFE. It updates the equalizer parameters (such as the filter coefficients) as it processes the data. Typically, it uses the MSE cost function; it assumes that it makes the correct symbol decisions, and uses its estimate of the symbols to compute e, which is defined above.

Viterbi equalizer: Finds the maximum likelihood(ML) optimal solution to the equalization problem. Its goal is to minimize the probability of making an error over the entire sequence.

BCJR equalizer: uses the BCJR algorithm (also called the Forward-backward algorithm) to find the maximum a posteriori (MAP) solution. Its goal is to minimize the probability that a given bit was incorrectly estimated.

Turbo equalizer: applies turbo decoding while treating the channel as a convolutional code.