Abstract
Previous studies show that, compared to on-off keying (OOK) signaling, pulse-position modulation (PPM) is favorable in FSO/CDMA systems thanks to its energy efficiency and simple detection. Nevertheless, when the system bit rate increases and the transmission distance is far, the FSO/CDMA systems using PPM signaling critically suffer from the impact of pulse broadening caused by dispersion, especially when the modulation level is high. In this paper, we therefore propose to use multi-wavelength PPM (MWPPM) signaling to overcome the limitation of PPM. To further improve the system performance, avalanche photodiode (APD) is also used. The performance of the proposed system is theoretically analyzed using a realistic model of Gaussian pulse propagation. To model the impact of intensity fluctuation caused by the atmospheric turbulence, the log-normal channel is used. We find that, by using MWPPM, the effects of both intensity fluctuation and pulse broadening are mitigated, the BER is therefore significantly improved. Additionally, we quantitatively show that the system performance is further improved by using APD, especially when the average APD gain is chosen properly.
Highlights
Over past few years, free-space optical communication (FSO) has attracted considerable attention for a variety of applications thanks to its cost-effectiveness, license-free, quick deployment and flexibility [1]
To avoid using short pulse for M-pulse-position modulation (PPM) with high-level modulation (i.e., M is large), we propose to use multi-wavelength PPM (MWPPM), which is a combination of wavelength shift keying (WSK) [12] and PPM
A L-M-MWPPM is a combination of L-WSK and M-ary pulse-position modulation (M-PPM), where L is the number of transmitted wavelengths and M is the modulation level of PPM
Summary
Free-space optical communication (FSO) has attracted considerable attention for a variety of applications thanks to its cost-effectiveness, license-free, quick deployment and flexibility [1]. A major impairment over FSO links is the atmospheric turbulence, a phenomenon occurs as a result of the variations in the refractive index due to inhomogeneties in temperature and pressure changes [9] These index inhomogeneities can deteriorate the quality of the received signal and can cause fluctuations in both the intensity and the phase of the received signal. We propose to use a realistic model of Gaussian pulse propagation in order to comprehensively analyze the effects of atmospheric turbulence on the performance of FSO/CDMA systems. This model should be able to analyze all effects of atmospheric turbulence, including intensity fluctuation, pulse broadening, and attenuation.
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