Duo Binary Turbo Coded 256 QAM with Constellation Shaping

Abstract

Bit interleaved coded modulation (BICM) is a widely used technique which associates powerful error correction codes with high order modulations in order to achieve a robust system with high spectral efficiency. The use of non-equiprobable modulations (constellation shaping) makes it possible to approach the capacity limit and a shaping gain is obtained. Contrary to what is used traditionally to improve the performances of BICM systems, in this paper we use only one shaping code to control the two components (I and Q) of the duo binary turbo coded 256 QAM (Quadrature Amplitude Modulation) modulation. Furthermore, no iterative process is used between decoding and demapping. This leads to a reduction in complexity with an interesting gain. Simulation results over Gaussian channel show a shaping gain of 1.2 dB compared to the uniform 256 QAM constellation. Moreover, a gain in terms of computation time of more than 45% is observed. In Rayleigh channel a shaping gain of 0.6 dB is obtained. To improve this gain a $\pi/\mathbf{8}$ rotated 256 QAM constellation is used with iterative processes between decoding and demapping. This gives a gain of 1.1 dB compared to the classical BICM systems.