Method employed in ldpc decoder and the decoder

An LDPC decoder, comprising:

• a barrel shifter, configured for shifting and rotating a first codeword by a specified number of bits;
• a syndrome calculator, coupled to the barrel shifter, configured for receiving an output of the barrel shifter to calculate a syndrome of a specific data portion of the first codeword based on checking equations of a parity check matrix;
• an inverse barrel shifter, coupled to the syndrome calculator, configured for performing an inverse operation of the barrel shifter upon an output of the syndrome calculator to generate a checking value of the specific data portion for a checking equation at a particular row of the parity check matrix;
• a flipping function value calculator, coupled to the inverse barrel shifter, configured for calculating a flipping function value of the specific data portion according to the specific data portion by using checking values which are generated from checking equations of the parity check matrix;
• and
• a determining block, coupled to the flipping function value calculator, configured for determining whether to flip the specific data portion of the first codeword by comparing the flipping function value with a flipping threshold which has been calculated based on a plurality of flipping function values of a plurality of previous data portions earlier than the specific data portion.