Hybrid 4DPPM-DQPSK Modulation for Terrestrial Free-Space Optical Links Under Atmospheric Turbulence

Terrestrial free-space laser communication will play a pivotal role in future 6G networks, owing to its high bandwidth and low latency advantages. However, atmospheric turbulence degrades link performance due to intensity scintillation and phase distortion. This paper proposes a hybrid modulation scheme that combines differential pulse position modulation (DPPM) with differential quadrature phase-shift keying (DQPSK). The scheme exploits the power efficiency of DPPM and the spectral efficiency of DQPSK, while differential phase detection eliminates the need for carrier recovery and enhances robustness to phase noise. A theoretical bit error rate (BER) model is derived and validated under Gamma–Gamma and Log-normal turbulence channels. Results show that the 4DPPM-DQPSK configuration achieves BER up to several orders of magnitude lower than standalone DPPM or DQPSK under weak-to-moderate turbulence, and improves spectral efficiency by 50% relative to 4DPPM. Packet error rate analysis further confirms its reliability for high-capacity terrestrial FSO links.