Estimation of the carrier frequency error of a π/4 DQPSK transmitter signal using the intersymbol correlation method

This papers discusses an algorithm to estimate the modulation accuracy of transmitters using π/4 DQPSK modulation, one of the digital modulation methods in use. In the past, modulation accuracy has been measured using a estimation method based on linear approximation. However, the need to repeat calculations with this method made it difficult to achieve high speed. Another problem involved with the estimation method using linear approximation was the large estimation error that occurred if the offset between the transmitted carrier frequency and the nominal frequency is large, or if the S/N ratio is small. A new algorithm was developed to estimate the carrier frequency error by calculating the intersymbol correlation. Because it does not require iteration, this intersymbol correlation achieves a two-fold improvement in speed over the linear approximation method, and exhibits only a small error, even when the carrier frequency error is large or the S/N ratio is small