Intersymbol interference in DQPSK fibre-optic systems

Artikel i vetenskaplig tidskrift, 2009

The 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 fiber-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 equalization 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 3 dB is gained, at a target bit error rate of 10^(-6), by allowing the transmitted phase levels to deviate from their nominal multiples of pi/2 by up to 7 %.