Reduced state mlse decoding

An apparatus comprising:

• at least one processor;
• and
• a memory to store instructions that, when executed by the at least one processor, enable the at least one processor to:
• initialize four states corresponding to four possible symbol levels in PAM4, where a respective initial state starts with an initial score and an empty survivor path;
• for respective possible transitions between the four initial states and four possible current states, determine expected PAM4 symbols;
• determine error associated with respective transitions based on differences between a received PAM4 symbol and the expected PAM4 symbols;
• discard transitions where the error indicates a difference greater than a single signal level, and keep the other transitions;
• and
• for respective current state groups of a two state trellis, determine one of the incoming transitions that was not discarded having the highest likelihood of being associated with a transmitted symbol.