Partially coherent constellations for multi-carrier systems

For wireless communication systems in high mobility environments, the usual assumption of perfect channel state information at the receiver is not very realistic. In the presence of channel estimation errors, the conventional PSK or QAM constellations, which are designed based on maximizing the minimum Euclidean distance between constellation points, are no longer optimal. Multi-carrier systems, which are the main focus of this work, are no exception to this fact. In this work, we first characterize the estimation variance of a pilot-based channel estimator for multi-carrier systems. Then, based on a previously derived KL-based criterion, we design new constellations for the ITU vehicular A channel at different SNR regions. The proposed constellations show significant performance improvement over the conventional QAM constellations. We show that the error floor due to the residual channel taps can be reduced by as much as one order of magnitude, if the proposed constellations are used in a multi-carrier system.