In telecommunication, the term intersymbol interference has the following meanings:
1. In a digital transmission system, distortion of the received signal, which distortion is manifested in the temporal spreading what are interchannel and intersymbol interference and consequent overlap of individual pulses to the degree that the receiver cannot reliably distinguish between changes of state, i.e., between individual signal elements.
At a certain threshold, intersymbol interference will compromise the integrity of the received data.
Intersymbol interference attributable to the statistical nature of quantum mechanisms sets the fundamental limit to receiver sensitivity.
Intersymbol interference may be measured by eye patterns.
2. Extraneous energy from the signal in one or more keying intervals that interferes with the reception of the signal in another keying interval.
3. The disturbance caused by extraneous energy from the signal in one or more keying intervals that interferes with the reception of intersymbol interference the signal in another keying interval.
Source: from Federal Standard 1037C and from MIL-STD-188
There are several techniques in telecommunication and data storage that try to work around the problem of intersymbol interference.
- Some techniques try to reduce the amount of interference between consecutive symbols, such as Gaussian minimum shift keying and certain other phase-shift keying variants
- Other techniques design symbols that are more robust against intersymbol interference. Decreasing the symbol rate (the "baud rate"), and keeping the data bit rate constant (by coding more symbols per bit), reduces intersymbol interference. Perhaps the extreme case is OFDM systems that reduce the symbol rate to one per second or less.
- Other techniques try to compensate for intersymbol interference. For example, when hard drive manufacturers found they could pack much more data on a disk platter when they switched from MFM to Partial Response Maximum Likelihood (PRML). Even though it's impossible to tell the difference between a "1" bit and a "0" bit if you only look at the signal during that bit, you can still tell them apart by looking at a cluster of bits at once and figuring out which binary sequence, when smeared out by the (well-characterized on a particular hard drive) intersymbol interference, most closely matches the observed signal. Trellis modulation is a closely related technique.
Electronics Topics
The field of electronics is the study and use of systems that operate by controlling the flow of electrons or other electrically charged particles in devices such as thermionic valves and semiconductors. The design and construction of electronic circuits to solve practical problems is part of the fields of electronic engineering, and the hardware design side of computer engineering. The study of new semiconductor devices and their technology is sometimes considered as a branch of physics.
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