Cipher and convolutional encoder/viterbi decoder accomplish different things. Cipher package deals with encryption/decryption, while convolutional encoder/viterbi decoder deals with error correction. Here is a brief explanation:
A convolutional encoder is a type of error-correcting code used to encode digital data for transmission over noisy channels. It works by taking a sequence of input bits and transforming them into a longer sequence of output bits, which includes redundant bits that can be used to detect and correct errors that may occur during transmission.
The key idea behind a convolutional encoder is to use a linear feedback shift register (LFSR) to generate the output bits. The LFSR is essentially a shift register with feedback connections that allow it to generate a sequence of bits based on the current input and the previous state of the register. By carefully choosing the feedback connections, it's possible to create an encoder that can detect and correct a certain number of errors in the received data.
A Viterbi decoder, on the other hand, is a type of maximum likelihood decoder used to decode the output of a convolutional encoder. It works by using a trellis diagram to represent all possible paths that the encoder could have taken to generate the output sequence. The decoder then uses the Viterbi algorithm to find the most likely path through the trellis, which corresponds to the most likely sequence of input bits.
In space communications, convolutional encoding and Viterbi decoding are commonly used to improve the reliability of data transmission over long distances. Space communications are subject to various sources of noise and interference, including solar radiation, cosmic rays, and other forms of electromagnetic interference. Convolutional encoding and Viterbi decoding can help mitigate these effects by adding redundancy to the transmitted data and using sophisticated decoding algorithms to correct errors that may occur during transmission.