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Coding Method to Minimize Intersymbol Interference

IP.com Disclosure Number: IPCOM000090492D
Original Publication Date: 1969-May-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 3 page(s) / 30K

Publishing Venue

IBM

Related People

Tang, DT: AUTHOR

Abstract

By observing certain run-length constraints in coding binary digit sequences for transmission through a band pass channel, the detrimental effect of intersymbol interference in the channel is kept within a tolerable limit without increasing the number of signal levels. A communication channel with an imperfect characteristic, such as a low-pass channel whose characteristic is sloped at the high-frequency end of the pass band, tends to introduce intersymbol interference which limits the rate of data transmission. A common way to nullify the effect of intersymbol interference is to employ correlative level coding in which each digit to be transmitted, commencing with the third digit in the sequence is added algebraically to the negative of the encoded digit that is transmitted two pulse periods earlier in that sequence.

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Coding Method to Minimize Intersymbol Interference

By observing certain run-length constraints in coding binary digit sequences for transmission through a band pass channel, the detrimental effect of intersymbol interference in the channel is kept within a tolerable limit without increasing the number of signal levels. A communication channel with an imperfect characteristic, such as a low-pass channel whose characteristic is sloped at the high-frequency end of the pass band, tends to introduce intersymbol interference which limits the rate of data transmission. A common way to nullify the effect of intersymbol interference is to employ correlative level coding in which each digit to be transmitted, commencing with the third digit in the sequence is added algebraically to the negative of the encoded digit that is transmitted two pulse periods earlier in that sequence. While correlative level coding greatly reduces or eliminates the harmful effect of intersymbol interference, it increases the number of voltage levels at which signals are transmitted. In some instances this is undesirable. This coding method minimizes intersymbol interference without increasing the number of signal levels.

Intersymbol interference is most likely to occur in the case of certain digit sequences, designated worst sequences, in which the same digit recurs at every second position in the sequence. A worst sequence can comprise a series of 1's, a series of 0's, or a series of alternate 1's and 0's. The greater the length of such a sequence the more pronounced is the effect of intersymbol interference. This method insures that data transmitted through the channel cannot contain a worst sequence exceeding a certain tolerable length.

The sequence to be encoded is first passed through a k-limited encoder. This converts the input sequence to a k-limited sequence in which the number of consecutively positioned 0's cannot exceed a maximum run-length k anywhere in the sequence. This k-limited encoder is a form of a general dk-limited encoder, such as described in IBM Research R...