  To achieve reliable and efficient transmissions in free-space optical (FSO)
communication, this paper designs a new protograph low-density parity-check
(PLDPC) coded generalized spatial multipulse position modulation (GSMPPM)
system over weak turbulence channels. Specifically, we investigate the PLDPC
code, generalized space shift keying (GSSK) modulation, and MPPM constellation.
First, we propose a type of novel GSMPPM constellations that intelligently
integrates the GSSK into MPPM, referred to as asymmetric dual-mode (ADM)
constellations, so as to improve the performance of the PLDPC-coded GSMPPM
system. Furthermore, exploiting a protograph extrinsic information transfer
(PEXIT) algorithm, we construct a type of improved PLDPC code, referred to as
I-PLDPC code, which outperforms the existing PLDPC codes over weak turbulence
channels. Analytical and simulation results show that the proposed ADM
constellations and the proposed I-PLDPC code can obtain noticeable performance
gains over their counterparts. Therefore, the proposed PLDPC-coded GSMPPM
system with ADM constellations is competent to satisfy the high-reliability
requirement for FSO applications.
