Results of systematic analysis of propagation directions and horizontal velocities of gravity waves (GWs) and spread F structures in low-latitude ionosphere (magnetic inclination~27°) in Tucumán region, Argentina, are presented. Measurements were carried out by multipoint continuous Doppler system during 1 year from December 2012 to November 2013. It was found that meridian propagation of GWs dominated and that southward propagation prevailed in the local summer. Oblique spread structures observed in Doppler shift spectrograms and associated with spread F propagated roughly eastward at velocities from 70 to~180 m/s and were observed at night from~September to~March. The velocities were computed for 182 events and the azimuths for 64 events. Continuous Doppler sounding makes it possible to analyze more events compared to optical observations often used for propagation studies since the measurements do not depend on weather.
Solar flares cause a rapid increase in ionization in the ionosphere owing to significant enhancement of ionizing solar radiation in the X-ray and extreme ultraviolet (EUV) spectral ranges. The change of electron densities in the ionosphere influences the propagation of radio waves. The ionospheric response to solar flares is investigated for three selected examples recorded during the maximum and decreasing phase of the solar cycle 24 with time resolution of several seconds by continuous Doppler sounding systems installed in the Czech Republic (50N, 14E), Taiwan (24N, 121E) and Northern Argentina (27S, 65W). The reflection heights of sounding signals are derived from nearby ionospheric sounders. The measured Doppler shifts are compared with EUV and X-ray data from the GOES-15 satellite. It is shown that the largest Doppler shifts are observed at times when the time derivatives of EUV fluxes are maximal, while the Doppler shifts are around zero at times when the EUV fluxes reach maxima. This means that loss processes balance the ionization when the EUV fluxes maximize. The attenuation of Doppler signal caused by enhanced electron density in the D and E layer was well correlated with the cosmic noise absorption measured by riometer. For large ionizing fluxes, the attenuation leads to very low signal-to-noise ratio, loss of the received signal, and inability to process both Doppler shift spectrograms and ionograms.
Device‐to‐device (D2D) communication underlaying cellular networks is considered a promising technology to enhance network throughput, spectral efficiency, and performance of next generation networks. However, these potential gains hinge on the exploiting mechanism for resource sharing between cellular users (CUs) and D2D pairs. In this paper, we analytically formulate the problem of resource sharing as an optimization problem to maximize network throughput while guaranteeing the required quality‐of‐service (QoS) for both cellular and D2D users. We propose a low‐complexity four‐step resource allocation algorithm to address the optimization problem. We exploit a distance‐based method to derive a resource reuse candidacy graph (RCG) and three exclusive regions (ERs) to evaluate the suitability of resource sharing between each CU and D2D pair. Then, we use a paring mechanism to find the optimal set of D2D pairs for spectrum sharing with each CU to maximize network throughput. The performance of the proposed algorithm is investigated in terms of network throughput, outage probability, and computational complexity. Numerical results show that the proposed algorithm provides high throughput and spectrum utilization with low complexity while efficiently guaranteeing QoS for CUs and D2D pairs.
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