We investigate the transmission performance, with and without digital backpropagation, in wavelength-division-multiplexed- (WDM) transmission systems using polarisation-division-multiplexed (PDM) differentially-encoded quadrature phase shift keying (QPSK) and 16-quadrature amplitude modulation (16QAM). We consider transmission at 56GBd, 28GBd, 14GBd and 7GBd per WDM-channel while varying the channel spacing from 100GHz to 12.5GHz to maintain constant spectral efficiency per modulation format. The symmetrical split-step method based on the Manakov equation was employed to backpropagate the central WDM-channel with the help of receiver based digital signal processing. Performance of digital backpropagation was found to decrease with reduced symbol-rate, owing to the smaller proportion of the overall spectrum which is backpropagated in these cases. At higher symbol-rates digital backpropagation is up to 0.5dB more effective for 16QAM than for the QPSK format, which we attribute to the increased influence of intra-channel nonlinearities for formats with multiple intensity values leading in turn to a higher benefit when intra-channel effects are compensated for. Improvement in launch power @ BER=3x10-3 amounts to 1.9dB for 56GBd QPSK and 2.4dB of 56GBd 16QAM.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.