Rotated Constellations for Improved Time and Frequency Diversity in DVB-NGH

In this paper, we investigate the potential gains that can be obtained with rotated constellations in DVB-NGH, the next-generation mobile broadcasting standard. Rotated constellations exploit the concept of signal-space diversity (SSD) to increase the diversity order of bit-interleaved coded modulation (BICM) at the expense of higher demodulation complexity without the need of additional transmission power or bandwidth. Two-dimensional rotated constellations (2DRC) were originally included in DVB-T2 (terrestrial second generation) to improve the reception robustness in fading channels. DVB-NGH inherits the same 2DRC from DVB-T2 and includes four-dimensional rotated constellations (4DRC) for certain configurations. Moreover, the standard has adopted a new component interleaver optimized for the utilization of rotated constellations with long time interleaving (TI) and time-frequency slicing (TFS). In this context, the additional robustness of rotated constellations is very interesting to counter the presence of signal outages in the time and frequency domains. To investigate the potential gains of 2DRC and 4DRC, we employ an information-theoretic approach based on mutual information, as well as physical layer simulations in DVB-NGH systems. The results reveal that rotated constellations are important to increase the diversity gains of long TI and TFS, and also to reduce the zapping time perceived by the users.