Least-squares carrier frequency offset estimation for coherent optical QPSK receivers

We investigate a carrier frequency offset (CFO) estimation algorithm for coherent optical quaternary phase-shift keying (QPSK) receivers. The algorithm utilizes the phase difference between the samples with different time-domain separations, so that the estimation accuracy can be significantly enhanced, compared with the algorithm that only compares the phase difference between adjacent samples. In a 28-GBaud coherent QPSK system, simulation results show that when 16 time spans and 512 samples are utilized, a uniform standard deviation of less than 3 MHz can be achieved for the CFO estimation error, when the optical signal-to-noise ratio is 11.5 dB.