In the convolutional coding, the message bits come in serially instead of large blocks. The name convolutional codes are due to the fact that the redundant bits are generated by the use of modulo-2 convolutions in a convolutional encoder. The convolutional encoder can be considered as finite-state machine consisting of an M-stage shift register, modulo-2 adders and multiplexers. The rate of a convolutional encoder with k inputs and n outputs is \(k/n\). Often the manufacturers of convolutional code chips specify the code by parameters \((n,k,L)\). The quantity L is called the constraint length of the code that represents the maximum number of bits in a single output stream that can be affected by any input bit.
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