Uniformly Projected RCQD QAM: A Low-Complexity Signal Space Diversity Solution over Fading Channels with or Without Erasures

Rotated and cyclic Q-delayed (RCQD) quadrature amplitude modulation provides signal space diversity and thus improves system performance over fading channels. However, previously published RCQD solutions were designed without fully considering the high demodulation complexity which prohibits wider applications. In this paper, a complete solution is proposed to reduce complexity for both the modulator and the demodulator. This solution uses a series of rotation angles {\alpha =} arctan( 1/{\sqrt {M}} ) which bring many interesting properties to the RCQD signals. A simplified sphere demapping algorithm is derived for fading channels with and without erasure events. In contrast to the sphere-decoder used for multiple input multiple output detection, the radius of the proposed sphere-demapper involves an exact amount of constellation points, thereby ensuring to perform the soft demapping operation successfully. Moreover, when either the in-phase ( {I} ) or the quadrature component ( {Q} ) is erased, the proposed demapping algorithm performs as well as the full-complexity max-log algorithm, with a reduced complexity. Compared to the solution currently used in DVB-T2, the proposed method reduces tremendously the computational complexity while still achieving similar performance over fading channels and even better performance over fading erasure channels.