Pulse position encoded RZ-OOK transmission for free-space optical communications

This paper proposes a pulse position encoded return-to-zero on-off keying (PP-RZ-OOK) transmission for free space optical (FSO) communications. At the transmitter end, PP-RZ-OOK signal is modulated by encoding the pulse position of RZ-OOK in the symbol duration. At the receiver end, PP-RZ-OOK signal is demodulated by symbol-by-symbol signal estimation. First, the existence of pulse is distinguished by adaptive threshold decision using the channel state information (CSI). Then, with respect to the symbol with the pulse, the knowledge of pulse position is estimated by slot-by-slot pulse power comparison. Finally, PP-RZ-OOK signal is effectively demodulated by both pulse existence and pulse position estimation. Therefore, the spectral efficiency of RZ-OOK is increased by the proposed pulse position encoding technique. The performance of the proposed PP-RZ-OOK modulation was verified through simulations under different turbulence channels and pulse durations, and it was compared with non-return-to-zero OOK (NRZ-OOK) and RZ-OOK modulations. The simulation results demonstrated that the capability of scintillation effect compensation was significantly enhanced compared with NRZ-OOK due to the pulse power increasing. Besides, the spectral efficiency was improved compared with RZ-OOK due to the additional pulse position encoding.