Minimax design of factorable Nyquist filters for data transmission systems

The use of Nyquist filters in data transmission systems is important in avoiding intersymbol interference. Moreover, the Nyquist filters should be factorable into lowpass transmitter/received filter pairs. Here, the design problem is formulated so as to generate zero-phase FIR lowpass Nyquist filters that can be split into minimum and maximum phase parts. Two factorable minimax design methods are given. These methods use the McClellan-Parks algorithm as a first step to control the stopband behaviour. The time domain constraints, imposed by solving a linear system of equations, determine the passband response. The final filter exhibits equiripple stopband behaviour. The advantages of these methods are that the minimum and maximum phase parts are obtained without direct factorization and that arbitrary frequency weighting can be easily incorporated to allow for a nonequiripple behaviour. Design examples depict both equiripple and nonequiripple magnitude responses. The new design approach is compared with other methods in terms of both magnitude and group delay behaviour. Finally, a practical design that conforms to a CCITT voice band modem specification is shown.