Blind carrier phase acquisition for QAM constellations

A constant need for ever-increasing throughputs through fixed bandwidths, fueled by several high-speed applications (such as digital TV), has pushed system designers toward more throughput-efficient modulation schemes. Because of their relatively good performance, large quadrature amplitude modulation (QAM) constellations are being used in many of these applications. One of the problems associated with the use of large QAM constellations is that of carrier acquisition, which, for efficiency reasons, must often be done without the use of a preamble. The problem is further complicated for cross constellations, for which the high signal-to-noise ratio (SNR) corner points used by some simple carrier phase estimators are not available. We derive simple algorithms for carrier phase acquisition that can be used for both square and cross constellations, and compare their performance to those of the maximum-likelihood (ML), the fourth-power estimator, and a modified fourth-power estimator, obtained by considering a reduced constellation. The introduced algorithms convert the problem of carrier phase estimation into one of estimating the mode of an underlying distribution. An expression for this underlying distribution is also obtained. The results obtained indicate that the introduced algorithms significantly outperform the fourth-power estimator for moderate to high SNRs, especially when cross constellations are used.