Performance enhancement of atmospheric turbulence channels in DWDM-FSO PON communication systems using M-ary hybrid DPPM-M-PAPM modulation schemes under pointing errors, ASE noise and interchannel crosstalk

This paper focuses on improving the efficiency of free-space optical (FSO) communication by employing a combination of hybrid on–off keying (OOK) modulation, M-ary digital pulse position modulation (M-ary DPPM), and M-pulse amplitude and position modulation (M-PAPM). The study aims to analyze and enhance the bit-error-rate (BER) performance using techniques such as the moment generating function, modified Chernoff bound, and Gaussian approximation, while taking into account challenges like amplified spontaneous emission (ASE) noise, atmospheric turbulence (AT), pointing errors (PEs), and interchannel crosstalk (ICC). The proposed system model revolves around a passive optical network (PON) that utilizes wavelength division multiplexing (WDM) for dense WDM (DWDM), with a focus on the hybrid fiber FSO (HFFSO) link. By utilizing eight wavelength channels transmitting at a rate of 2.5 Gbps over a turbulent HFFSO-DWDM system and PON-FSO optical fiber, starting at a 1550 nm channel spacing in the C-band of 100 GHz, the research demonstrates successful 20 Gbit/s–4000 m transmission with promising results. To enhance performance, the modulation technique of M-ary DPPM-M-PAPM is used to provide additional information bits. The integration of adaptive optics is also suggested to mitigate the effects of atmospheric turbulence and improve modulation efficiency. The study reveals that the proposed M-ary hybrid DPPM-M-PAPM solution increases receiver sensitivity compared to OOK, ensuring reliability and achieving a lower power penalty of 0.2–3.0 dB at a low coding level (M) of 2 in WDM-FSO systems under weak turbulence conditions. The hybrid OOK/M-ary DPPM-M-PAPM modulation scheme offers an optical signal-to-noise ratio ranging from 4 to 8 dB in the DWDM-HFFSO link under conditions of strong turbulence, aiming for a target BER of \({10}^{-{12}}\) ${10}^{-12}$ . Numerical findings suggest that this approach can be further optimized by combining hybrid OOK/M-DPPM and M-PAPM for DWDM-HFFSO systems. Calculations indicate that PAPM-DPPM outperforms OOK non-return-to-zero (NRZ) by approximately 10 dB to 11 dB at a BER of \({10}^{-{12}}\) ${10}^{-12}$ . Incorporating an aperture diameter of 10 cm and an average transmitted optical power of 13 dBm, along with aperture averaging, M-ary hybrid DPPM-M-PAPM, and spatial diversity, results in the average spectral efficiency performance at 53 bit/s/Hz and 38 bit/s/Hz without and with PEs, respectively. Utilizing a coding level of 5 and aiming for a BER of \({10}^{-{12}}\) ${10}^{-12}$ , M-ary DPPM-M-PAPM offers about a 9 dB sensitivity improvement over the OOK-NRZ FSO system. Achieving a BER target of \({10}^{-{12}}\) ${10}^{-12}$ , DPPM enhances sensitivity by approximately 10–11 dB compared to DWDM-FSO OOK-NRZ, Simulation results affirm the viability of the proposed hybrid optical modulation technique for DWDM-FSO hybrid links in both optical-wireless and fiber-optic communication systems, significantly enhancing their overall efficiency. To further improve performance, the paper suggests the integration of adaptive optics to mitigate the effects of atmospheric turbulence and enhance modulation efficiency. The proposed M-ary hybrid DPPM-M-PAPM solution contributes to increased sensitivity and lower power penalty, particularly at low coding levels, demonstrating its potential for DWDM-FSO systems. Numerical findings suggest that this approach can be further optimized for enhanced spectral efficiency and sensitivity over traditional modulation schemes. By leveraging the M-ary DPPM-M-PAPM modulation technique and adaptive optics, the study demonstrates improved receiver sensitivity compared to traditional OOK modulation, ensuring robust performance under varying atmospheric conditions. Numerical findings indicate significant enhancements in system performance metrics, offering a promising solution for optimizing DWDM-FSO communication systems in challenging environments. Finally, the adoption of hybrid OOK/M-ary DPPM-M-PAPM modulation schemes represents a novel strategy for mitigating effects of AT, PEs, ASE, and ICC in DWDM-FSO optical fiber communication systems.