T-APPM is combined of TCM (trellis-coded modulation) and APPM (Amplitude-Pulse-position modulation) and
has broad application prospects in space optical communication. Set partitioning in standard T-APPM algorithm has
the optimal performance in a multi-carrier system, but whether this method has the optimal performance in APPM
which is a single-carrier system is unknown. To solve this problem, we first research the atmospheric channel model
with weak turbulence; then a modified T-APPM algorithm was proposed, compared to the standard T-APPM
algorithm, modified algorithm uses Gray code mapping instead of set partitioning mapping; finally, simulate the two
algorithms with Monte-Carlo method. Simulation results showed that, when bit error rate at 10-4, the modified
T-APPM algorithm achieved 0.4dB in SNR, effectively improve the system error performance.
Theoretical analysis as well as system simulation of a NRZ/RZ (NRZ and duty cycle 33% 50% 66% RZ) shaped APPM
(2×4 and 4×4) modulated scheme for gamma-gamma distribution turbulent free-space optical communication is done.
Results show lower BER in RZ scheme especially with duty cycle 33%, however the NRZ scheme has higher frequency
efficiency. So both BER performance and frequency efficiency can be synthetically considered to choose a proper
scheme according to simulation results.