  Coded modulation with probabilistic amplitude shaping (PAS) is considered for
intensity modulation/direct detection channels with a transmitter peak-power
constraint. PAS is used to map bits to a uniform PAM-6 distribution and
outperforms PAM-8 for rates up to around 2.3 bits per channel use. PAM-6 with
PAS also outperforms a cross-shaped QAM-32 constellation by up to 1 dB and 0.65
dB after bit-metric soft- and hard decoding, respectively. An alternative PAM-6
scheme based on a framed cross-shaped QAM-32 constellation is proposed that
shows similar gains.
