An adaptive QAM linearizer using data postdistortion

Abstract

An adaptive Volterra linearizer for reducing channel nonlinearities in QAM (quadrature amplitude modulation) data transmission systems is described, and its performance is evaluated. The linearizer is implemented as a modern receiver digital circuit which employs two read-only memories that are programmed to cancel either third- or fifth-order nonlinearities. The memory outputs are scaled by adaptive coefficients which minimize the channel nonlinearity. The linearizer has been shown to be effective for input nonlinear bit error rates up to 10/sup -3/. At input error rates up to 10/sup -6/, the linearizer reduces the nonlinear error rate by five orders of magnitude in a 64-QAM digital radio system, and the linearized system has a selective fading performance approximately equal to that of a linear system. The circuit is entirely digital and can be manufactured with automated processes.<>