modulo-2 convolutions
Convolutional encoding with Viterbi decoding is a FEC technique that is particularly suited to a channel in which the transmitted signal is corrupted mainly by additive white gaussian noise (AWGN). You can think of AWGN as noise whose voltage distribution over time has characteristics that can be described using a Gaussian, or normal, statistical distribution, i.e. a bell curve. This voltage distribution has zero mean and a standard deviation that is a function of the signal-to-noise ratio (SNR) of the received signal. Let's assume for the moment that the received signal level is fixed. Then if the SNR is high, the standard deviation of the noise is small, and vice-versa. In digital communications, SNR is usually measured in terms of Eb/N0, which stands for energy per bit divided by the one-sided noise density.
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Ahmed