An approximate nyquist transmission scheme for QPSK digital mobile communications

This paper presents the design and implementation of an approximate Nyquist transmission scheme that employs a rational‐function receiving filter. A transitional Butterworth‐Thomson (TBT) filter is examined as a rational‐function receiving filter. This filter can easily be realized as a ceramic filter. It can also function as a channel filter to realize selectivity. Therefore, the newly proposed transmission scheme provides not only a simple and economical receiver circuit but also low power consumption. These features are suitable for mobile radios which must be as small as possible and operate with limited power sources. The effects of filter approximation error on waveform transmission characteristics and adjacent channel interference selectivity are evaluated by computer simulation, and the filter characteristics are optimized in terms of both characteristics. A ceramic receiving filter is developed with the optimum TBT characteristic. A π/4‐shifted QPSK transmission experiment performed with this filter confirms that it has such superior transmission characteristics as receiving sensitivity, robustness against co‐channel interference, and robustness against frequency selective fading. Such characteristics are superior to those of the conventional transmission scheme using a maximally flat receiving filter. It is verified that approximate Nyquist transmission provides improved transmission quality and spectrum utilization for digital mobile communication systems.