Intersymbol interference and error probability for bandlimited QPSK transmission systems

The effects of IF filtering on the error rate performance of QPSK transmission systems utilizing integrate-and-dump detectors are studied. It is assumed that (1) the QPSK demodulator reference signals are noise-free, (2) timing for the integrate-and-dump detectors is perfect, and (3) the channel noise is additive, white, and Gaussian, with zero mean. Two different filter types are considered: the ideal rectangular filter and a practical single-pole filter. It is found that the bit error probability for bandlimited QPSK systems is affected by (1) a reduction in amplitude of the bit under detection, (2) intersymbol interference from adjacent bits in the same channel, and (3) crosstalk from the data stream in the quadrature channel. Computations of error probability are made for each filter type, using a series approximation method which can provide any desired degree of accuracy. For the cases considered, it was sufficient to assume that the effects of intersymbol interference and crosstalk were limited to 5 bits on either side of the bit under detection. It is observed that the ideal filter provides superior performance in the noise-limited (low signal-to-noise ratio) region of operation. However, better performance is generally provided by the single-pole filter in the region of high signal-to-noise ratio where intersymbol interference and crosstalk become significant.