Note on Coherent Quadrature Modulation

Coherent quadrature modulation has long been identified with the bandwidth-efficient transmission of low-pass signals over bandpass media. A significant recent contribution to the theory of parallel transmission has been that of O'Neill and Saltzberg; they showed that if there was proper synchronization between the modulating carrier and the data clock, then the two parallel channels in coherent quadrature transmission could be equalized by a single transversal filter directly in tandem with the transmission facility, and the same automatic equalization algorithms would be applicable as have previously been associated with strictly serial transmission employing vestigial sideband modulation. It is seen here that the analytic technique of O'Neill and Saltzberg can be used to show that the convenient transversal filter equalization is also possible in the more general coherent quadrature systems in which the synchronization condition imposed by those authors may not hold. Thus it is implied that parallel transmission with coherent quadrature modulation can be adopted in many applications heretofore reserved for serial transmission.