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