Demodulation of Double Differential PSK in Presence of Large Frequency Offset and Wide Filter

The autocorrelation demodulator (ACD) for DDPSK is an offset tolerant demodulator which has been introduced for applications where the signal experiences large Doppler shift. Moreover, emerging ultra-narrowband solutions for Internet of Things and Wireless Sensor Networks can exploit DDPSK to avoid the use of costly crystal or power hungry thermal compensators. However, to tolerate frequency offset the bandwidth of the lowpass or bandpass filter before the demodulator must increase which leads to a larger noise bandwidth and degrades BER performance. This work proposes a new method to overcome this problem. Instead of one path of ACD, samples at the output of the filter go through multiple paths with adjusted delay and interval for correlation in the ACD. The sum of the outputs of these paths provide the input to the detector with an increased SNR compared to conventional structure. Using the proposed method, the SNR per bit required for a certain BER remains independent of filter bandwidth if the target BER is less than 0.01.