Low-Complexity Geometrical Shaping for 4D Modulation Formats via Amplitude Coding

Signal shaping is vital to approach Shannon's capacity, yet it is challenging\nto implement at very high speeds. For example, probabilistic shaping often\nrequires arithmetic coding to realize the target distribution. Geometric\nshaping requires look-up tables to store the constellation points. In this\npaper, we propose a four-dimensional amplitude coding (4D-AC) geometrical\nshaper architecture. The proposed architecture can generate in real time\ngeometrically shaped 4D formats via simple logic circuit operations and two\nconventional quadrature amplitude modulation (QAM) modulators. This paper\ndescribes the 4D-AC used in generating approximated versions of two recently\nproposed 4D orthant symmetric modulation formats with spectral efficiencies of\n6 bit/4D-sym and 7 bit/4D-sym, respectively. Numerical results show losses\nbelow 0.05 dB when compared against the baseline formats.\n