A theoretical model is proposed to analyze the time-domain spectral phase en/decoding and evaluate the unconditional security of the optical code-based secure optical communication systems with DPSK modulation. The confidentiality of the systems with and without bit-by-bit code scrambling technique is investigated. It mathematically proves that the system without code scrambling lacks confidentiality and the system employing code scrambling can realize unconditionally secure transmission. Furthermore, encoding only within the central band of the spectrum is sufficient for perfect secrecy, and the secrecy rate can be potentially improved. It allows the system to operate in the burst mode for code generation and encoding and thus have an idle time for rearm.
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<sup>-4</sup>, 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.