Abstract
This paper investigates the ergodic capacity performance of hybrid free space optics (FSO)/ radio frequency (RF) system by employing an adaptive-combining-based switching scheme. In the adaptive combining scheme, the FSO and RF links are combined using the maximal-ratio combining (MRC) at the output, when the instantaneous signal-to-noise ratio (SNR) of the FSO link drops below a predefined threshold SNR value. The FSO link experiences the atmospheric turbulence, which is modeled using the generalized Malaga distribution, non-zero boresight misalignment or pointing errors, and atmospheric attenuation, under two types of detection techniques, i.e. heterodyne detection (HD) and intensity modulation/direct detection (IM/DD). Similarly, the fading of the RF link is characterized using the generalized <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\kappa$</tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> distribution. In particular, we derive the probability density function (PDF) of the instantaneous SNR of the adaptive combining scheme. Capitalizing on the SNR statistics, we obtain the unified closed-form ergodic capacity expressions for the adaptive-combining-based hybrid FSO/RF system. In addition, the asymptotic expressions for the ergodic capacity of the system at the high-SNR region are presented. All the derived analytical expressions are validated by using Monte-Carlo simulations. Numerical results and discussions are provided to compare the ergodic capacity performance of the adaptive-combining-based hybrid FSO/RF system with the existing system models such as single-link FSO, MRC-based hybrid FSO/RF, and hard-switching-based hybrid FSO/RF systems.
Highlights
Free space optics (FSO) is one of the most prominent technology for massively increasing data-driven applications and services for the next-generation wireless communications due to its outstanding features
We present the numerical results for ergodic capacity performance
It is to be noted that the bandwidths of free space optics (FSO) and radio frequency (RF) links are assumed as WR = WF = 1 Hz for normalized ergodic capacity results
Summary
To minimize the effects of atmospheric turbulence-induced fading and pointing errors, spatial diversity techniques were studied in [5]-[8]. In [19], a comprehensive performance analysis of the hard-switching-based hybrid FSO/RF system was carried out using the generalized Malaga and α-η-κ-μ fading distributions with pointing errors. Frequent hardware switching between FSO and RF sub-systems is a major bottleneck in the case of a hard-switching scheme and it requires the channel state information (CSI) at the transmitter To alleviate these issues, the FSO and RF links of the hybrid FSO/RF system were combined using SC and MRC in [24] and [25], where transmission of feedback bits to the transmitter as well as the requirement of CSI at the transmitter are not mandatory. The adaptive combining scheme can be considered as an excellent solution to counteract the limitations of hard-switching and diversity combining schemes for hybrid FSO/RF systems as well as nullifying the effects of atmospheric channel distortions encountered by FSO signal. The main motivations and contributions of our work are given as follows
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