Viterbi decoder and a Viterbi decoding method

PURPOSE:To allow the decoder to track automatically a modulation system of a transmitter side and a phase of a carrier. CONSTITUTION:A phase shift circuit 61 sets a phase of a recovered carrier of received and demodulated I and Q signals to a prescribed phase corresponding to a phase shift control signal. A square Euclid distance calculation circuit 51 calculates a square Euclid distance between a signal point corresponding to a modulation system control signal to be received and a signal point of a received signal received from the phase shift circuit 61. A control circuit 63 uses a selection circuit 72 to select a minimum accumulated value for each phase accumulated by a path metric accumulation circuit 62 and provides an output of a phase shift control signal setting a phase corresponding to the minimum value. A comparator circuit 73 compares the minimum value with a preset reference value and provides an output of a modulation system control signal to set the modulation system corresponding to the comparison result.