What Is Intersymbol Interference and How to Fix It

Intersymbol interference is a common problem in digital communication systems. It occurs when the signal from one symbol overlaps with the signal from another symbol, causing errors in data transmission.

This interference can be caused by a variety of factors, including the physical properties of the transmission medium and the characteristics of the signal itself. The farther apart the symbols are, the less likely they are to interfere with each other.

To fix intersymbol interference, you can use techniques such as equalization, which involves adjusting the signal to compensate for the interference. Equalizers can be implemented in hardware or software, and are commonly used in high-speed communication systems.

In some cases, the interference can be mitigated by increasing the distance between the symbols, or by using a different modulation scheme that is less susceptible to interference.

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Intersymbol interference (ISI) is a real challenge in digital communication systems, and understanding its causes and effects is crucial for designing effective solutions. In the frequency domain, ISI can be seen as a "splatter" of energy resulting from band-limiting and distortion, while in the time domain, it's revealed by the eye diagram.

The eye diagram is a powerful tool for visualizing ISI, showing the effects of overlapping one bit's energy into adjacent bit periods. A wide-open eye means little or no ISI, while a closed eye indicates significant overlap. Additive noise, on the other hand, typically manifests as jitter around the zero-crossing points of the eye.

ISI is an inevitable problem in bandlimited channels, and every electrical channel is subject to it up to a certain high frequency. It occurs when one signal in the bit stream interferes with later signals when read out by the receiver.

There are several reasons why ISI happens, including:

The eye diagram can also reveal the effects of frequency-dependent frequency response in the channel, even if the receiver is designed to compensate for it. And, surprisingly, ISI can occur even in an ISI-free system if the decision is not made exactly in the center of the eye, but at a detection time different from zero.

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Intersymbol interference (ISI) is a major problem in digital communication systems. It occurs when the energy of one symbol overlaps with the energy of adjacent symbols, causing errors in signal detection.

In the frequency domain, a power spectral density (PSD) graph illustrates the "splatter" of energy resulting from band-limiting and distortion. This graph is a valuable tool for understanding ISI.

A well-known diagram in the time domain, called the eye diagram, is more revealing when it comes to ISI. This diagram shows the effects of overlapping one bit's energy into adjacent bit periods. A wide-open eye means that there is little or no ISI, while a closed eye means that there is a bit of overlap.

The eye diagram also shows the effects of additive noise, which is a non-ISI problem. Noise usually manifests as jitter around the zero-crossing points of the eye, rather than closing of the eye.

ISI can be caused by pulse broadening, which is the impairment of a symbol decision due to pulse broadening. This can be seen in the time domain plot of a coaxial cable impulse response, where the impulse response extends over more than 100 symbol durations.

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The effects of filtering a rectangular pulse not only change the shape of the pulse within the first symbol period, but it is also spread out over the subsequent symbol periods. This causes the spread pulse of each individual symbol to interfere with following symbols.

Here's a breakdown of the effects of different characteristic cable attenuations on the impulse response:

The integral over the impulse response from zero to infinity is the same for all curves, since the frequency response is flat and the shaping filter has a finite bandwidth. This means that the amplitude of the pulse at the cable end decreases with increasing attenuation.

For example, with a characteristic attenuation of 40 dB, the pulse amplitude at the cable end is only less than 7% of the input amplitude. At 60 dB and 100 dB, this value drops to 3% and 2%, respectively.

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Countering ISI is crucial to minimize its effects. To achieve this, system designers can focus on making the impulse response short, ensuring that minimal energy from one symbol bleeds into the next.

One effective technique is to separate symbols in time with guard periods. This allows the receiver to accurately detect each symbol without interference.

Designing systems with short impulse responses is key. This can be achieved by carefully selecting the system's parameters.

Another method is to apply an equalizer at the receiver. This equalizer attempts to undo the effect of the channel by applying an inverse filter.

Applying a sequence detector at the receiver is also effective. This detector uses the Viterbi algorithm to estimate the sequence of transmitted symbols.

Here are some techniques used to counter ISI:

ISI can manifest in different ways, but a key indicator is the shape of the eye diagram in the time domain. This diagram shows the effects of overlapping one bit's energy into adjacent bit periods.

A wide-open eye means that there is little or no ISI, while a closed eye means that there is a bit of overlap. The eye diagram also shows the effects of additive noise, which is a non-ISI problem.

In most cases, the optimum sampling point is at the middle of the eye, where the option is at its widest. The power spectral density (PSD) graph illustrates the "splatter" of energy resulting from band-limiting and distortion.

A closed eye can be caused by a bit of overlap, which is a characteristic of ISI.

Intersymbol interference can be seen in an eye diagram, which is a visual representation of the signal behavior.

Most high-bandwidth oscilloscopes or digital signal analyzers can be used to gather this measurement as long as test fixtures are placed on the PCB.

A perfect eye diagram will have totally overlapping signals without any jitter or noise.

In the time domain, the eye diagram is more revealing, showing the effects of overlapping one bit's energy into adjacent bit periods.

The optimum sampling point is usually at the middle of the eye, where the signal is at its widest.

A wide-open eye indicates little or no ISI, while a closed eye means there is a bit of overlap.

Noise usually manifests as jitter around the zero-crossing points of the eye, rather than closing of the eye.

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Designing systems with short impulse responses can greatly reduce intersymbol interference. This can be achieved by ensuring that the energy from one symbol doesn't smear into the next symbol.

Separating symbols in time with guard periods can also help minimize the effects of ISI. This can be done by inserting a period of time between symbols where no data is transmitted.

Equalization is another technique that can be used to recover signals and remove intersymbol interference. This involves applying an inverse filter at the receiver to undo the effects of the channel.

To reduce intersymbol interference, focus on three areas: impedance mismatch, dispersion, and slow impulse response.

Here are some specific solutions to these problems:

Programming delays into your bit stream can reduce bit error rates, but it will also reduce the total data rate. This can be a trade-off worth considering in high-speed interconnect design.