Nyquist QPSK transmission using rational-function filter for mobile radio

The design and implementation of a Nyquist QPSK (quadrature phase-shift keying) transmission scheme using a rational-function receiving filter are presented. The scheme is suitable for mobile radios which must be as small as possible and operate with limited power sources. A transitional Butterworth-Thomson (TBT) filter is examined as a rational-function filter, and a ceramic receiving filter is developed with optimum TBT characteristics. π/4-shifted QPSK transmission experiments using this filter are conducted. The experimental results show that the required input level and cochannel interference are superior by 1 dB to 0.9 dB, respectively, to those of the conventional transmission scheme. The maximum permissive delay spread in frequency-selective fading channels is improved by around 30%