2 research outputs found
Spatial Frequencies and Degrees of Freedom : Their roles in near-field communications
No full textAs wireless technology begins to utilize physically larger arrays and/or higher frequencies, the transmitter and receiver will reside in each other's radiative near field. This fact gives rise to unusual propagation phenomena, such as spherical wavefronts and beam focusing, creating the impression that new spatial dimensions-called degrees of freedom (DOF)-can be exploited in the near field. However, this is a fallacy because the theoretically maximum DOF are already achievable in the far field. This article sheds light on these issues by providing a tutorial on spatial frequencies, which are the fundamental components of wireless channels, and by explaining their role in characterizing the DOF in the near and far fields. In particular, we demonstrate how a single propagation path utilizes one spatial frequency in the far field and an interval of spatial frequencies in the near field. We explain how the array geometry determines the number of distinguishable spatial frequency bins and, thereby, the spatial DOF. We also describe how to model near-field multipath channels and their spatial correlation matrices. Finally, we discuss the research challenges and future directions in this field.</p
Prevalence of Vitamin D Deficiency and Hypervitaminosis D Among Adult Patients Admitted to the Tertiary Care Hospitals in Turkey
No full textObjective: Vitamin D deficiency is a common health problem around the world. This study aimed to evaluate the nationwide prevalence of vitamin D status in tertiary care hospitals in Turkey. Methods: Retrospectively, the data on vitamin D levels from 33 tertiary care hospitals’ clinical biochemistry laboratories around Turkey between January and December were collected. Results: In total, 706 434 serum samples from adult subjects (female/male: 469 028/237 406; 66.4%/33.6%) were included. While vitamin D levels were sufficient in 20.3% (n = 14 222), they were insufficient in 21.9% (n = 154 360) and deficient in 57.8% (n = 408 882). We observed the highest rates of deficiency in those aged between 18 and 29 years (62.9%, n = 70 235) and lowest rates between 60 and 69 years (52.3%, n = 61 121) and between 70 and 79 years (52.3%, n = 32 397). Hypervitaminosis D was detected in 5.5% of adult subjects; highest rates of hypervitaminosis D were observed in those who were over 80 years (6.6%) and 70-79 years (6.5%) and the lowest in 18-29 years (2.8%). Discussion: In this cohort, over half of the subjects admitted to the tertiary care hospitals in Turkey had vitamin D deficiency and required vitamin D supplementation. The elderly population had the lowest prevalence of vitamin D insufficiency and the highest prevalence of hypervitaminosis D. © Author(s)
