This paper applies a frequency offset transmitter diversity method for Machine-to-Machine (M2M) communication systems. Wireless terminals based on ZigBee and Wi-SUN offer long term usage and low cost. Frequency offset transmitter diversity is useful for these M2M applications because all it needs are additional access point antennas and an error correction function. Conventionally, the error correction function must employ hard decision to support various modulation/demodulation schemes. This paper proposes a Viterbi algorithm with soft decision that uses received signal strength metric calculated by an external circuit.