What is an equalizer in communication

Balance and eye pattern in digital communication | their important properties

CONTENT: Balance and eye pattern in digital communication

  • What is compensation?
  • Role of an equalizer
  • Eye pattern
  • What is ISI
  • Zero forcing equalizer

What is balance in communication?

Definition of compensation:

"Equalization is a special process that involves a device called an "equalizer" that is used to reverse the distortion caused by a signal transmitted over a particular channel. "

In communication systems, the main purpose of using equalization is to remove interference between symbols and to restore the lost signals.

What role does an equalizer play?

When a pulse train traverses a transmission medium or channel, the pulse train is attenuated and distorted. The distortion is indicated by “high frequency. Components of the damping of the pulse train ".

The process of correcting such a channel-induced distortion is called Compensation. A filter circuit used to perform such equalization is referred to as Equalizer.

Ideally, an equalizer should have a frequency response that is the reverse of that of the channel. Thus, an equalizer is designed so that the overall amplitude and phase response of the transmission medium and the cascaded equalizer are the same as those for distortion-free transmission.

Let us consider a communication channel with a transfer function Hc(F).

The equalizer has a transfer function Heq(F) and it is cascaded to the communication channel as shown in the figure above.

The total TF of the combination is Hc (f) H.eq (F).

This is necessary for distortion-free transmission

                                     Hc (f) H.eq (f) = K exp (-j2πft0)

Where K = a scaling factor

tO = constant time delay

Thus, H.eq (f) =

What is eye pattern?

Provide a brief glimpse of the eye pattern used in balancing:

The inter symbol in a PCM or data transmission system can be examined experimentally with the aid of a display in the oscilloscope. Now the received distorted waves are functional for the oscilloscope's vertical baffle and the sawtooth waves for a transmitter with a symbol rate of R = 1 / T is is placed on the horizontal plate. The resulting display on the oscilloscope is referred to as Eye pattern or eye diagram.

Eye pattern or eye diagram is named because of its similarity to human eyes. The inner area of ​​the eye pattern is called the eye opening.

In an eye pattern, a digital signal is generated from the digital source. The digital signal is passed through the channel, which creates inter-symbol interference. The digital signal loaded by ISI is applied to the vertical input of the CRO. An external sawtooth time base signal is applied to the horizon input of the oscilloscope. The sawtooth generator is triggered by the symbol clock, which also synchronizes the digital source. As a result, the eye pattern will appear on the oscilloscope screen.

What information do we get from Eye Pattern?

An eye pattern provides the following information about the performance of a digital communication system:

  1. The width of the eye opening indicates the interval time over which a received wave could be scanned by the ISI without errors. The ideal time to take a sample is when the eye is wide open. The point in time is shown in the above eye pattern as the "best sampling time".
  2. The height of the eye opening at the quantified sapling time is the degree of the edge of the channel noise. This is shown as "Edge over Noise" in the diagram above.
  3. The sensitivity of the system to timing errors is calculated by the rate at which the eye closes, since the scan times are long.
  4. The non-linear transmission distortions are represented by asymmetrical or squinted eyes.

How many types of equalizers are there?

Important types of equalizers:

  • Linear Equalizer - Its function is to process the incoming signal with the linear filter.
  • MSME Equalizer - Its function is to minimize the filter and eliminate the error
  • Zero Forcing Equalizer - computes the inverse of a channel with a linear filter type
  • Adaptive Equalizer - Basically this is also a linear equalizer that helps process the data along with some equalizer parameters.
  • Turbo Equalizer - This type of equalizer offers turbo decoding.

What is a zero forcing equalizer?

In a tapped delay linear filter, it is possible to minimize the effect of interference between symbols by selecting {Cn}, ie the tap coefficient, so that the equalizer output is forced to zero at M sample points on either side of the desired pulse.

This means that the sampling tap coefficients are chosen so that the output samples {ZK} of the equalizer are given by:

1 for k = 0

ZK =

0 for k = ± 1, ± 2, ……. ± M.

The required length of the filter, i.e. the No. The tap coefficient is a function of how much smear the channel might know. For such a finite length zero forcing equalizer, the peak ISI is minimized when the eye pattern is mainly opened.

Even so, the eye is regularly closed prior to compensation for high speed transmission. In such a case, the forcing equalizer is not always the best solution, as such an equalizer neglects the effect of noise.

What is Inter Symbol Interference (ISI)?

Define the term ISI in connection with the communication system:

When digital data is transmitted over a band-limited channel, the dispersion in the channel causes a temporal overlap between successive symbols.

This effect is known as Inter-Symbol Interference (ISI).

A baseband communication system can be viewed as a low pass filter. It has a limited bandwidth and a non-linear frequency response. So when digital pulses are transmitted over this channel, the shape of the pulses is distorted. Because of this distortion, one distorted pulse affects another pulse, and the cumulative effect of this distortion causes the decision-making process to be flawed in favor of "one" or "zero".

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