  A simple geometric shaping method is proposed for optical wireless
communication systems based on intensity modulation and direct detection
(IM/DD). Constellations consisting of equiprobable levels with exponential-like
distribution are obtained. The method possesses asymptotic optimality in the
sense that the high-SNR channel capacity can be approached by such
constellations with increasing size. By an additional quantization step, all
$2^b$ levels ($b \in \mathbb N$) of the obtained constellation can be
represented by a basic level and $b+2$ bits, thereby reducing the required
resolution of the digital-to-analog converter (DAC) without affecting the
asymptotic optimality. Achievable information rate evaluations verify the
asymptotic optimality of the proposed method. As an example, error performance
results of a simple 16-level LDPC coded modulation scheme show that a shaping
gain of 0.8 dB can be obtained by applying the proposed constellation design.
