This letter presents the carrier phase recovery for 16-ary quadrature amplitude modulation (16-QAM) optical coherent systems using the quaternary phase-shift keying (QPSK) partitioning with sliding window averaging and differential decoding. We assess the increase in linewidth tolerance achievable with sliding window averaging as opposed to block averaging. Simulation results demonstrate that combined linewidth symbol duration product, Δv · T _s , 10 ^-4 is tolerable at the target bit error ratio (BER) of 10 ^-2 and 10 ^-3 for a penalty of 0.6 and 0.8 dB, respectively, compared with the theoretical limit with differential decoding. The impact of analog-to-digital converter (ADC) resolution on the performance of the QPSK partitioning algorithm is also investigated. Finally, the performance of the algorithm using the measured phase noise for a distributed feedback (DFB) laser is presented for different values of Δv · T _s . We show that for Δv · T _s > 10 ^-4 , the penalty of block averaging is > 0.5 dB with respect to sliding window averaging at the target BER of 10 ^-3 with the measured phase noise. The degradation associated with block averaging at the target BER of 10 ^-2 is shown to be less significant compared to sliding window averaging.