A Geometrical Approach for Highly Efficient Soft Demodulation of Rotated Constellations

Rotated constellations have shown promising potentials for harsh channel conditions and it becomes an crucial feature of DVB-T2. Several other standards, such as new generations of ATSC and DTMB, are also considering it. How-ever, the superiority of performance comes at the cost of orders of magnitudes higher complexity for demodulation, which is a crucial part of the receiver. Constellations of conventional QAM modulations stay on completely regular integer points, so that the demodulation can be as simple as quantization and table lookup. However, the rotation and independent transmissions of I/Q branches break the above properties, so that soft demodulation becomes much more complex. Previous efficient solutions are mostly based on reduced search instead of performing a full search. We take a totally different approach that derives closest constellation points with geometrical transformations. Search is completely avoided. This results in orders of magnitudes of complexity reduction.