ERROR CONTROL CODING

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W hen a digital signal is transmitted between two systems(such as computers),the signal gets distorted because of addition of noise to the transmitted signal. The noise can introduce an error in the binary train of bit travelling from one system to the other .this means that a 0 may change to 1 or a 1 may change to 0.These error can become a serious problem to the accuracy and performance of the digital system. Therefore it is essential to detect and correct the errors .In digital communication, errors are introduced during the transmission of data from the transmitter to receiver due to noise or some other reasons. The reliability of data transmission gets severely affected because of these errors.To improve the reliability of data transmission the designer has to increase the signal power or reduce the noise spectral density N0 so as to maximize the ratio Eb/N0 .But in practice there is a limitation on the maximum value of the ratio Eb/N0 .we cannot increase the ratio beyond this limit .Hence for a fixed value of Eb/N0 .We have to use some kind of coding in order to improve the quality of the transmitted signal.


CODE PARAMETER AND THE STRUCTURE OF THE CONVOLUTIONAL CODE
The convolutional code structure is easy to draw from its parameters.First draw m boxes representing the m memory registers.Then draw n-modulo-2 adders to represent the n- output bits.


HOW POLYNOMIALS ARE SELECTED
There are many choices for polynomials for any m order code,They do not all result in output sequence that have good error protection properties