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Advanced Wireless Communications for Future Technologies-6G and beyond 6G
Published in Anuj Singal, Sandeep Kumar, Sajjan Singh, Ashish Kr. Luhach, Wireless Communication with Artificial Intelligence, 2023
S. S. Kiran, M. Rajan Babu, B. Kiranmai, K. Gurucharan
Sensors are required to work in various conditions and have long battery life in the IoT world. Backscatter technology is used in Radio Frequency Identification (RFID) solutions to moderate and reflect Radio Frequency signals rather than generate them, saving much energy. Due to signal attenuation over long distances, current modulated backscatter systems impose strict restrictions on the closeness of the backscatter transmitter to the RF source. Furthermore, backscatter transmitters have modulated backscatter receivers. Thus, they are passive and can transmit data only when backscatter receivers [69] make requests. As a result, new solutions in the 6G IoT network are needed to improve energy efficiency while increasing flexibility and scalability.
Remote Sensing Technologies for Multiscale Hydrological Studies: Advances and Perspectives
Published in Prasad S. Thenkabail, Remote Sensing Handbook, 2015
Sadiq I. Khan, Ni-Bin Chang, Yang Hong, Xianwu Xue, Yu Zhang
Table 1.2 shows the most common bands, frequencies, and associated wavelengths that correspond to radars that have hydrologic applications. Note that the typical values for radar microwave frequencies are on the order of 107–1011 Hz; thus, it is convenient to use mega (106) and giga (109) prefixes, or MHz and GHz. The corresponding radar wavelengths span a few millimeters (mm) up to m. The radar wavelength and diameter (d) of the parabolic dish dictate the angular width of the radar beam, or beamwidth (θ), as follows in Equation 1.3: () θ=73λd where λ and d are both in the same distance units and θ is in °. In the case of the Weather Surveillance Radar-1988 Doppler (WSR-88D) radar, it operates at an approximate 10.7 cm wavelength and has an 8.5 m diameter dish. This corresponds to a beamwidth of approximately 0.92° (in both azimuth and elevation directions). Targets with horizontal cross sections (for a horizontally polarized wave) less than λ/16, or approximately 7 mm for the WSR-88D, are Rayleigh scatters and thus have predictable radar signatures for different-sized raindrops. The targets are assumed to produce scattering equal in all directions called isotropic scattering. The radar detects the component of scattering that comes back to the radar (backscatter). Shorter wavelength radars at X-band and shorter have a lower upper limit on the diameter of targets that cause Rayleigh scattering. But, these smaller wavelength radars do not require such large dishes to maintain a small beamwidth desirable for high-resolution precipitation measurements and thus are more amenable to spaceborne, transportable, and mobile radar platforms. Table 1.2 lists the most common radar bands, frequencies, and associated wavelengths with their hydrologic applications.
Spatial search and a three level model based water layer extraction from C-band SAR image
Published in Annals of GIS, 2021
C. Bipin, C. V. Rao, P. V. Sridevi
Unlike optical sensors, a SAR sensor always looks at the scene in the off-nadir direction. This imaging geometry results in geometric and radiometric distortions like foreshortening, layover, shadow etc caused by the topographical variations in a scene. SAR being an active mode of remote sensing, the sensitivity of the image formed varies across the scene due to the gain variation of the antenna system (Loew and Mauser 2007). Radar backscatter from the target is a function of its dielectric constant, orientation (incidence angle), shape and distribution. For a distributed target like the surface of the earth, it is the coherent summation of backscatter from each target added with noise. For a target like water body, the backscattered power can be considered as the sum of noise and low backscatter due to specular reflection over the calm area and coherent summation of scattering from waves on the surface of the water. Speckle noise is characteristic of SAR data due to the limited bandwidth and coherent imaging technique. Speckle by its nature is random and uniformly distributed in space. A detailed study of speckle and its properties was carried out using laser speckle (Goodman 1975).