Investigation of FSO communication system based on various modulation schemes over coastal region of South Africa

Abstract

The recent advancement in intelligent technology, where the highest percentage of the population is connected through smart devices, requires robust wireless communications infrastructure. Free Space Optical (FSO) communication, a system that uses light beams to transmit data wirelessly, has recently emerged to enhance radio frequency (RF). The advantages include a large bandwidth over long distances, low power consumption, an unfettered spectrum, and sizeable high-speed data transfers. However, FSO communication has challenges, particularly the significant impact of weather conditions such as fog visibility, temperature, wind, and atmospheric turbulence on signals' performance and link availability. Furthermore, the scintillations directly impact the FSO communication performance, fading the signal and increasing the bit error rate. The primary objective of this work is to elucidate the functioning of the FSO system, a crucial step in examining various modulation strategies that rely on transmitting FSO signals through the Gamma-Gamma atmospheric channel. For this study, the influence of selected meteorological parameters (visibility range, wind speed, maximum temperature, and relative humidity) on FSO signal attenuation over Cape Town and Port Elizabeth in South Africa, is evaluated and demonstrated for the feasible performance, reliability, and resilience of the FSO communication link. This study utilized two years (2018-2019) meteorological data from the South African Weather Service (SAWS). The average attenuation at the selected locations was calculated by quantifying the impact of aerosol scattering on signal strength, which is attributed to the visibility conditions in foggy environments. Wavelength-dependent parameters (about aerosol scattering) were computed using four distinct operating wavelengths, specifically 650 nm, 850 nm, 1200 nm, and 1550 nm, within the maximum transmission link of 5 km. The result demonstrates that the specific average attenuation at an average visibility of 25.96 km in Cape Town throughout the two years is 2.634 dB/km at 650 nm and 0.851 dB/km at 1550 nm. Port Elizabeth recorded a higher mean visibility of 28.34 km, surpassing Cape Town's value by 2.38 km, and the observed specific average attenuation is 2.412 dB/km at 650 nm and 0.779 dB/km at 1550 nm, respectively. Furthermore, three FSO transmission schemes have been evaluated and analyzed under atmospheric turbulences and weather conditions at 1550 nm wavelength and three different propagation links. The power transmission was constant, only considering varying the required bandwidth for each scheme. The system was modelled and simulated, thus achieving an average channel capacity of 4.559 bps/Hz in Cape Town at the highest transmission range of 2.5 km at a 35 dB signalnoise ratio. This capacity indicates the amount of data reliably transmitted per unit bandwidth. At a medium transmission range of 2.5 km, the achieved channel capacity is 44.928 bps/Hz in Port Elizabeth. However, Port Elizabeth has the highest transmission range of 3.5 km; the achieved channel capacity decreases drastically to 2.12 bps/Hz. This significant decrease in capacity at more extended ranges highlights the practical limitations of FSO communication under certain conditions, emphasizing the need for further research and development in this area to overcome these challenges. These findings underscore the significant influence of visibility variation between Cape Town and Port Elizabeth on transmission range classification, ultimately affecting channel capacity. Port Elizabeth's higher visibility enables a more precise signal path, reducing attenuation and increasing capacity. In contrast, Cape Town's lower visibility can lead to increased signal degradation, reducing capacity. This implies that high visibility and optimized transmission range can enhance channel capacity, while lower visibility and longer ranges can decrease it. This underscores the importance of considering meteorological conditions in designing and deploying FSO systems, as they can significantly impact their performance and reliability. This research provides crucial insights that can inform and prepare the field for the challenges and opportunities in FSO communication. Furthermore, this study contributes to the field of FSO communication by providing insights into the effectiveness of three modulation schemes within the South African coastal regions: binary phase shift, differential phase shift, and on-off keying. This research significantly enhances the research's applicability and potential impact on FSO communication by delving into the influence of meteorological parameters on the transmission path and the resulting changes in the refractive index.