Decoding method and decoder

A decoder, comprising:

• a receiving unit, configured to receive a to-be-decoded signal;
• an acquiring unit, configured to:
• perform, according to a region decision rule formed by S region decision formulas, region decision on the to-be-decoded signal that is received by the receiving unit, to acquire a region decision result, wherein S is a positive integer less than the total number of constellation points comprised in N subsets that are used as a basis for generating the to-be-decoded signal at an encoding end, N is a positive integer greater than 1, every two constellation points in each subset are separately located in two different regions that are divided by using at least one decision formula in the S region decision formulas, and constellation points in the constellation diagram are divided into N subsets,
• acquire N constellation points according to the decision result, wherein the N constellation points are separately constellation points that are in the N subsets and closest to the to-be-decoded signal, and
• acquire N non-encoded bits corresponding to the N constellation points, and branch metrics between the to-be-decoded signal and the N constellation points;
• and
• a decoding unit, configured to perform Viterbi decoding based on the branch metrics and the N non-encoded bits that are acquired by the acquiring unit, and output a decoding result corresponding to the to-be-decoded signal.