Demodulation of amplitude modulated signals in the presence of multipath

Signals modulated by M-ary pulse amplitude modulation (PAM) or M-ary quadrature amplitude modulation (QAM) have a certain structured constellation. When the communication channel introduces intersymbol interference (ISI) at the receiver end, demodulation of such signals can be performed by constant modulus algorithm (CMA) based equalizers to cancel the interference. However, characteristics of modulated signals are only partially considered in the CMA cost function. In this paper, more constraints are imposed on the equalized signal to fully capture the property of the modulated signal both in its phase and amplitude. Observing that PAM signals are uniformly spaced on the x-axis and QAM signals in two-dimensional signal space, the property of transmitted signals from each category can be included in an equivalent deterministic mathematical description, similar to the constant modulus. This description is absorbed in our modified cost function, resulting in a simultaneous minimization of dispersion relevant to signal phase and amplitude. The performance of the equalizers based on these new algorithms are compared with the CMA equalizer.