AbstractThe limited bandwidth of high‐speed transmitter and receiver hardware gives rise to intersymbol interference (ISI) in digital communication systems. The influence of such ISI is investigated in a low‐complexity fibre‐optical setting, using differential quaternary phase‐shift keying (DQPSK) modulation. The ISI is approximated by a memoryless, stochastic model, for use in applications where neither equalisation nor sequence detection can be afforded. The channel capacity of this model is calculated and shown to depend strongly on the transmitter calibration. More than 3dB is gained, at a target bit error rate (BER) of 10−6, by allowing the transmitted phase levels to deviate from their nominal multiples of π/2 by up to 7%. Copyright © 2009 John Wiley & Sons, Ltd.