Free-space optical (FSO) links are considered as cost-effective, noninvasive alternative to fiber optic cables for 5G cellular backhaul networking. For FSO-based backhaul networks, we propose an additional millimeter-wavelength (MMW) radio-frequency (RF)-FSO link, used as a backup. Uninterrupted and reliable network connection is possible by switching between primary FSO link and the secondary RF-FSO link; when the primary link is under atmospheric turbulence, the secondary link maintains connectivity as the MMW RF link exhibits complementary characteristics to atmospheric effects. In order to analytically assess the improvement, we also derive concise mathematical expressions for different performance metrics, such as outage probability, average bit error rate (BER), and capacity. Our results demonstrate that the FSO/RF-FSO topology performs better than a single FSO link in terms of outage probability and BER. The dual-hop mixed RF-FSO link is realized with an amplify and forward (AF) relay that adapts an average power scaling strategy. The irradiance fluctuations in the FSO links are modeled by gamma-gamma distribution, assuming strong atmospheric turbulence while it is assumed that the RF link experiences multipath Rayleigh fading. For switching between links, a single FSO threshold is considered first, followed by a dual FSO threshold to prevent unnecessary switching.
The binding energy of shallow hydrogenic impurities in spherical quantum dots (QDs) with parabolic confinement is calculated, using a variational approach within the effective mass approximation. The binding energy is computed for GaAs QD as a function of the dot size for different impurity positions, and also as a function of the impurity position for different dot sizes. The results show that the impurity binding energy increases with the reduction in the dot dimension. The binding energy is also found to depend on the location of the impurity, and the same is the maximum for the on-center impurity.
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