Paper 1, Section I, G

Coding \& Cryptography
Part II, 2017

Let CC be a binary code of length nn. Define the following decoding rules: (i) ideal observer, (ii) maximum likelihood, (iii) minimum distance.

Let pp denote the probability that a digit is mistransmitted and suppose p<1/2p<1 / 2. Prove that maximum likelihood and minimum distance decoding agree.

Suppose codewords 000 and 111 are sent with probabilities 4/54 / 5 and 1/51 / 5 respectively with error probability p=1/4p=1 / 4. If we receive 110 , how should it be decoded according to the three decoding rules above?