  This work investigates protograph-based LDPC codes for the AWGN channel with
OOK modulation. A non-uniform distribution of the OOK modulation symbols is
considered to improve the power efficiency especially for low SNRs. To this
end, a specific transmitter architecture based on time sharing is proposed that
allows probabilistic shaping of (some) OOK modulation symbols. Tailored
protograph-based LDPC code designs outperform standard schemes with uniform
signaling and off-the-shelf codes by 1.1 dB for a transmission rate of 0.25
bits/channel use.
