Intersymbol interference

related topics
{system, computer, user}
{math, energy, light}
{specie, animal, plant}
{style, bgcolor, rowspan}
{war, force, army}
{@card@, make, design}

In telecommunication, intersymbol interference (ISI) is a form of distortion of a signal in which one symbol interferes with subsequent symbols. This is an unwanted phenomenon as the previous symbols have similar effect as noise, thus making the communication less reliable. ISI is usually caused by multipath propagation or the inherent non-linear frequency response of a channel causing successive symbols to "blur" together. The presence of ISI in the system introduces errors in the decision device at the receiver output. Therefore, in the design of the transmitting and receiving filters, the objective is to minimize the effects of ISI, and thereby deliver the digital data to its destination with the smallest error rate possible. Ways to fight intersymbol interference include adaptive equalization and error correcting codes.

[1]

Contents

Causes

Multipath propagation

One of the causes of intersymbol interference is what is known as multipath propagation in which a wireless signal from a transmitter reaches the receiver via many different paths. The causes of this include reflection (for instance, the signal may bounce off buildings), refraction (such as through the foliage of a tree) and atmospheric effects such as atmospheric ducting and ionospheric reflection. Since all of these paths are different lengths - plus some of these effects will also slow the signal down - this results in the different versions of the signal arriving at different times. This delay means that part or all of a given symbol will be spread into the subsequent symbols, thereby interfering with the correct detection of those symbols. Additionally, the various paths often distort the amplitude and/or phase of the signal thereby causing further interference with the received signal.

Bandlimited channels

Another cause of intersymbol interference is the transmission of a signal through a bandlimited channel, i.e., one where the frequency response is zero above a certain frequency (the cutoff frequency). Passing a signal through such a channel results in the removal of frequency components above this cutoff frequency; in addition, the amplitude of the frequency components below the cutoff frequency may also be attenuated by the channel.

Full article ▸

related documents
Dynamic range
Conformance testing
Chrominance
Capacitive coupling
Frequency-shift keying
RAM disk
Composite video
Internetworking
IRIX
Picture archiving and communication system
Duron
Real-time operating system
Tagged Image File Format
Heterodyning
Noise
Direct-sequence spread spectrum
Companding
COMSEC
Apple IIe Card
Very-large-scale integration
Chaffing and winnowing
Open Systems Interconnection
Amiga 1000
Motorola 68020
Sega 32X
Broadcast domain
Break key
Packet analyzer
Medium wave
Digital image processing