The solution of electromagnetic scattering by a homogeneous prolate (or oblate) spheroidal particle with an arbitrary size and refractive index is obtained for any angle of incidence by solving Maxwell's equations under given boundary conditions. The method used is that of separating the vector wave equations in the spheroidal coordinates and expanding them in terms of the spheroidal wavefunctions. The unknown coefficients for the expansion are determined by a system of equations derived from the boundary conditions regarding the continuity of tangential components of the electric and magnetic vectors across the surface of the spheroid. The solutions both in the prolate and oblate spheroidal coordinate systems result in a same form, and the equations for the oblate spheroidal system can be obtained from those for the prolate one by replacing the prolate spheroidal wavefunctions with the oblate ones and vice versa. For an oblique incidence, the polarized incident wave is resolved into two components, the TM mode for which the magnetic vector vibrates perpendicularly to the incident plane and the TE mode for which the electric vector vibrates perpendicularly to this plane. For the incidence along the rotation axis the resultant equations are given in the form similar to the one for a sphere given by the Mie theory. The physical parameters involved are the following five quantities: the size parameter defined by the product of the semifocal distance of the spheroid and the propagation constant of the incident wave, the eccentricity, the refractive index of the spheroid relative to the surrounding medium, the incident angle between the direction of the incident wave and the rotation axis, and the angles that specify the direction of the scattered wave.

Light Scattering by a Spheroidal Particle

Starting with post World War II studies of fading of radio star sources and continuing with fading of satellite signals of Sputnik, vast quantities of data have built up on the effect of ionospheric irregularities on signals from beyond the F layer. The review attempts to organize the available amplitude and phase scintillation data into equatorial, middle-, and high-latitude morphologies. The effect of magnetic activity, solar sunspot cycle, and time of day is shown for each of these three latitudinal sectors. The effect of the very high levels of solar flux during the past sunspot maximum of 1979-1981 is stressed. During these years unusually high levels of scintillation were noted near the peak of the Appleton equatorial anomaly (∼ ±15° away from the magnetic equator) as well as over polar latitudes. New data on phase fluctuations are summarized for the auroral zone with its sheet-like irregularity structure. One model is now available which will yield amplitude and phase predictions for varying sites and solar conditions. Other models, more limited in their output and use, are also available. The models are outlined with their limitations and data bases noted. New advances in morphology and in understanding the physics of irregularity development in the equatorial and auroral regions have taken place. Questions and unknowns in morphology and in the physics of irregularity development remain. These include the origin of the seeding sources of equatorial irregularities, the physics of development of auroral irregularity patches, and the morphology of F-layer irregularities at middle latitudes.

/pdf/global-morphology-of-ionospheric-scintillations-37hijos7s3.pdf

Global morphology of ionospheric scintillations

An enigma of equatorial research has been the observed seasonal and longitudinal occurrence patterns of equatorial scintillations (and range-type spread F). We resolve this problem by showing that the seasonal maxima in scintillation activity coincide with the times of year when the solar terminator is most nearly aligned with the geomagnetic flux tubes. That is, occurrence of plasma density irregularities responsible for scintillations is most likely when the integrated E-region Pedersen conductivity is changing most rapidly. Hence the hitherto puzzling seasonal pattern of scintillation activity, at a given longitude, becomes a simple deterministic function of the magnetic declination and geographic latitude of the magnetic dip equator. This demonstrated relationship is consistent with equatorial irregularity generation by the collisional Rayleigh-Taylor instability and irregularity growth enhancement by the current convective and (wind-driven) gradient drift instabilities. Some discrepancies in this relationship, however, have been found in scintillation data obtained at lower radio frequencies (below, say, 300 MHz) that suggest the presence of other irregularity-influencing processes. The role of field-aligned currents, associated with the longitudinal gradient in integrated E-region Pedersen conductivity produced at the solar terminator, in equatorial irregularity generation via the current convective instability has not been discussed previously.

Control of the seasonal and longitudinal occurrence of equatorial scintillations by the longitudinal gradient in integrated E region Pedersen conductivity

Determining the morphology of F-layer irregularities as a function of longitude in the equatorial region is vital for understanding the physics of the development of these irregularities. We aim to lay the observational basis which then can be used to test theoretical models. Theoretical models have been developed, notably in the papers by Tsunoda (1985) and by T. Maruyama and N. Matuura (1984). The question is whether the models are consistent with the morphology as we see it. According to our criteria, the data used should be confined to observations taken near the magnetic equator during quiet magnetic periods and at times within a few hours after sunset. Anomaly region scintillation data have to be used in a limited manner for studying the generation mechanism. The questions to be answered by proposed mechanisms are (1) why do the equinox months have high levels of occurrence over all longitudes and (2) why are there relatively high levels of occurrence in the Central Pacific Sector in the July–August period and in the 0–75° West Sector in the November-December period and (3) why are there very low levels of occurrence in November and December in the Central Pacific Sector and in July and August in the 0–75° West Sector. In the paper by Maruyama and Matuura, the authors have taken observations of topside soundings of spread-F. With this data set in hand, they conclude: “During the northern winter periods, there is maximum enhancement at the Atlantic longitudes of large westward geomagnetic declination and during the northern summer at the Pacific longitude of large eastward declination”. Tsunoda's conclusions from his use of scintillation data is that “scintillation activity appears to maximize at times of the year when the suset nodes occur”. The emphasis of one paper is on the maximum enhancement during the solstices and in the other paper on variations from the equinox as determined by latitude and declination. Each stresses certain characteristics of the morphology. While the two papers explain relatively different morphologies, each makes contributions. However there remain problems to be resolved before certifying a solution as to the physics explaining the longitudinal pattern of F-region irregularities. Satellitein-situ data, scintillation and spread-F observations will be reviewed. The limitation of each data set will be outlined particularly as relevant to the bias produced by the existence of thin versus extended layers of irregularities. A cartoon as to the occurrence pattern, as we see it, as a function of longitude will be shown.

The longitudinal morphology of equatorial F-layer irregularities relevant to their occurrence

Fifth-generation (5G) cellular networks will almost certainly operate in the high-bandwidth, underutilized millimeter-wave (mmWave) frequency spectrum, which offers the potentiality of high-capacity wireless transmission of multi-gigabit-per-second (Gbps) data rates. Despite the enormous available bandwidth potential, mmWave signal transmissions suffer from fundamental technical challenges like severe path loss, sensitivity to blockage, directivity, and narrow beamwidth, due to its short wavelengths. To effectively support system design and deployment, accurate channel modeling comprising several 5G technologies and scenarios is essential. This survey provides a comprehensive overview of several emerging technologies for 5G systems, such as massive multiple-input multiple-output (MIMO) technologies, multiple access technologies, hybrid analog-digital precoding and combining, non-orthogonal multiple access (NOMA), cell-free massive MIMO, and simultaneous wireless information and power transfer (SWIPT) technologies. These technologies induce distinct propagation characteristics and establish specific requirements on 5G channel modeling. To tackle these challenges, we first provide a survey of existing solutions and standards and discuss the radio-frequency (RF) spectrum and regulatory issues for mmWave communications. Second, we compared existing wireless communication techniques like sub-6-GHz WiFi and sub-6 GHz 4G LTE over mmWave communications which come with benefits comprising narrow beam, high signal quality, large capacity data transmission, and strong detection potential. Third, we describe the fundamental propagation characteristics of the mmWave band and survey the existing channel models for mmWave communications. Fourth, we track evolution and advancements in hybrid beamforming for massive MIMO systems in terms of system models of hybrid precoding architectures, hybrid analog and digital precoding/combining matrices, with the potential antenna configuration scenarios and mmWave channel estimation (CE) techniques. Fifth, we extend the scope of the discussion by including multiple access technologies for mmWave systems such as non-orthogonal multiple access (NOMA) and space-division multiple access (SDMA), with limited RF chains at the base station. Lastly, we explore the integration of SWIPT in mmWave massive MIMO systems, with limited RF chains, to realize spectrally and energy-efficient communications.

/pdf/a-comprehensive-survey-on-millimeter-wave-communications-for-2hd6bv4yun.pdf

A Comprehensive Survey on Millimeter Wave Communications for Fifth-Generation Wireless Networks: Feasibility and Challenges

The difference in rain drop size distribution (DSD) characteristics between convective and stratiform rain and in monsoon and premonsoon season have been investigated over a tropical location, Kolkata, using seven years of data collected from a Joss–Waldvogel disdrometer (JWD). During the Indian summer monsoon (ISM) and the premonsoon season, Kolkata gets substantial rain. Interannual variability of DSD has been evident. The normalized drop size distributions of DSD show a double peak at 20–50 mm/h rain rate due to drop break up and coalescence process. For convective rain, the concentration of higher drops (<inline-formula> <tex-math notation="LaTeX">$>$ </tex-math></inline-formula>3 mm) is more significant compared to stratiform rain. In the premonsoon period, a significant contribution of normalized DSD in the afternoon to late evening has been evident due to nor’westers. The contribution of normalized DSD of higher drops (<inline-formula> <tex-math notation="LaTeX">$>$ </tex-math></inline-formula>3 mm) is more in the premonsoon convective rain compared to monsoon convective rain. The larger slope of <inline-formula> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula>–<inline-formula> <tex-math notation="LaTeX">$\Lambda $ </tex-math></inline-formula> relationship has been observed in the premonsoon convective period which corresponds to higher <inline-formula> <tex-math notation="LaTeX">$D_{m}$ </tex-math></inline-formula> values. The relationship between radar reflectivity (<inline-formula> <tex-math notation="LaTeX">$Z$ </tex-math></inline-formula>) and rain rate (<inline-formula> <tex-math notation="LaTeX">$R$ </tex-math></inline-formula>) distinguishes the monsoon and premonsoon seasons. Radiometric measurements have been used to examine the effects of the atmospheric instability parameter convective available potential energy (CAPE) and cloud base height (CBH) on the formation of larger drop sizes. This study will be useful for understanding precipitation microphysics and improving the quantitative precipitation estimation (QPE) algorithm.

Modeling of Rain Drop Size Distribution in Association With Convective and Cloud Parameter Over a Tropical Location

The interseasonal features of precipitation microphysics over Thiruvananthapuram and Kolkata - the two tropical stations of Indian sub-continent

The precipitation radar(PR) onboard Tropical Rain Monitoring Mission( TRMM) estimates rainfall from space covering vast geographical area. It is necessary to compare such estimates with ground truth. In this paper PR estimated rain has been compared with that obtained from India Meteorological Department( IMD) and also disdrometer over Calcutta. PR estimated rainfall statistics have been presented along with IMD derived rainfall statistics. Regression relationships of rainfall have been derived as a function of surface temperature (DBT) and surface dew point temperature (DPT). Also rainfall phenomena have been studied as a signature of EL-NINO and LA-NINA occurrence.

http://ieeexplore.ieee.org/iel5/5398767/5407062/05407104.pdf

Comparison of TRMM estimated rainfall with ground truth over Calcutta

Nowcasting of intense rain is important in various fields of life. In this paper, radiometric brightness temperature measurements at Ka and V bands are utilized to predict rain event associated with impending convective processes. The instability parameters are estimated from radiometric data to point the development of atmospheric instability and an estimation of liquid water content is made from brightness temperature at 31.4 GHz, prior to rain events. The nowcasting technique is, therefore, able to predict both rain occurrence and rain accumulation. The model, when validated, gives a reasonable prediction efficiency of around 80%.

Rain prediction using radiometric observations at a tropical location

Characterization of convective rain at tropical station Kolkata, located near the land-ocean boundary, in the eastern part of India, has been made using observations from multi-frequency radiometer, micro rain radar, disdrometer and electric field monitor. Features of convective rain in respect of liquid water content, rain rate profile, and electric field variation have been identified. Multi-frequency radiometric brightness temperatures are used to nowcast impending rain events and rain fall amount.

Animesh Maitra

Papers

Modeling of Rain Drop Size Distribution in Association With Convective and Cloud Parameter Over a Tropical Location

The interseasonal features of precipitation microphysics over Thiruvananthapuram and Kolkata - the two tropical stations of Indian sub-continent

Comparison of TRMM estimated rainfall with ground truth over Calcutta

Rain prediction using radiometric observations at a tropical location

Convective rain study with radiometer, radar and electric field observations at a tropical location