A blind adaptation algorithm for decision feedback equalization for dual-mode CAP-QAM reception

This paper introduces an algorithm for blind adaptation of decision feedback equalizers (DFE), which is applied for equalization in the passband domain. The transmission scheme, which is assumed to be unknown at the receiver side, can either be carrierless amplitude/phase (CAP) modulation or quadrature amplitude modulation (QAM). As the input signal of the passband equalizer is real-valued, the tap weights of the complex-valued feedback filter cannot be identified by a standard whitening approach. We propose therefore an algorithm, which determines a starting coefficient set by means of an infinite impulse response (IIR) equalizer updated by constant modulus algorithm (CMA). For stability reasons, the equalizer's feedback filter is implemented in lattice structure during acquisition. In case of QAM transmission, its tap weights have to be rotated before switching to the DFE. The feedback filter is then fed by the soft decision in order to minimize the probability of wrong solutions.