Animesh Maitra

Bio: Animesh Maitra is an academic researcher from University of Calcutta. The author has contributed to research in topics: Disdrometer & Precipitation. The author has an hindex of 20, co-authored 134 publications receiving 1211 citations.

Seasonal Variation of Rain and Cloud Microstructure at a Tropical Location

Abstract: Precipitation is a key element of the Earth’s hydrology cycle and needs to be carefully monitored due to the rapid growth of satellite and terrestrial link based telecommunication services, using higher frequency band particularly above 10 GHz. The presence of raindrops which absorbs and scatters radio wave energy can produce degradation of the reliability and performance of the communication links. Thus, rain drop size distribution (DSD) is one of the most extensively used parameters for improved an accurate description of any rain event. Several DSD models, namely Lognormal, Gamma, Weibull, Marshall and Palmer are employed to characterize the DSD. Our present work employs three parameter Gamma distributions utilizing Method of Moments (MOM) technique to investigate the physical characteristics and microstructure of rain DSD in a tropical region based on three year long disdrometer observations. Characteristics of precipitation and clouds have been analysed in the present study over the tropical station Kolkata $(22.57^{\circ}\mathrm{N}, 88.36^{\circ}\mathrm{E})$, during the pre-monsoon (March, April and May), monsoon (June, July, August and September) and post-monsoon (October and November) months. Three year long measurements of the rain DSD have been made using a ground based impact type JW (Joss and Waldvogel) disdrometer located at the Institute of Radio Physics and Electronics of University of Calcutta during 2014-2016. Cloud microphysics is also observed during the same period as to investigate its role in determining rain microstructure. To depict the cloud microphysics of the above mentioned location, $1^{\circ} \mathrm{X}1^{\circ}$ cloud optical depth (COD) and cloud effective radius (CER) data are collected from Moderate Resolution Imaging Spectroradiometer (MODIS). Rain DSD analysis reveals that larger drops are more dominant in the pre-monsoon season compared to any other seasons (Monsoon and Post-monsoon) whereas comparatively smaller drops are more dominant in the monsoon season than the other seasons (pre-monsoon and post-monsoon) at same rain rate bins. This phenomenon occurs because of the pre-monsoon rain droplets are associated with well-built vertical updraft. Distinct variation of cloud effective radius (CER) value is noticed during different seasons of the year. A combined analysis of cloud drop size and rain drop size indicates that in the pre-monsoon months low CER values are associated with the dominance of large raindrops, and during the monsoon period large cloud droplets effect smaller rain drops to dominate the precipitation at the present location.

Using meteorological data for clear sky and cloud attenuation in Belgium and India

Abstract: Future High Throughput Satellite Communication Systems and Earth Observation Satellite Data Download foresee the use of frequencies in Ka band and above because those frequency bands offer advantages in terms of bandwidth and capacity. In that frequency range, the degradation due to the troposphere becomes important and the designers need a better estimate of the degradations and more specifically the attenuation due to gases and clouds. Even if the specific attenuation is small, except in the frequency band where resonance is present (such as 60 GHz for oxygen), it is always present. The meteorological data available worldwide are now currently used for the estimation of the attenuation due to oxygen, water vapour and clouds [1] [2]. Satellite propagation campaigns are ongoing for the accurate determination of attenuation models in Ka and Q band, using for example the Alphasat satellite from the European Space Agency [3], at 19.7 GHz and 39.4 GHz. The beacon receivers measure directly the power of the beacon received from the satellite but they suffer from various types of instabilities due to tracking inaccuracies, temperature effects, etc and cannot measure the total attenuation of the beacon, in the absence of an external reference (excess attenuation measurement). A concurrent radiometer measures the brightness temperature, enables the accurate estimation of the total attenuation along the path in the absence of scattering, and produce the Integrated Liquid Water (ILW) and Integrated Water Vapour (IWV) content. The cost of the equipment is however high and only a few experimenters use a radiometer [4].

Pre-rain scintillation of Ku-band satellite signal at a tropical location

Abstract: Tropospheric scintillation has relation with rain rate and signal attenuation. The present study indicates strong connection of the pre-rain scintillations with the turbulence associated with convective systems. The role of atmospheric structure constant in enhancing the scintillation effect is also observed.

Light Scattering by a Spheroidal Particle

Abstract: 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.

Global morphology of ionospheric scintillations

Abstract: 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.

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

Abstract: 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.

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

Abstract: 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.

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

Abstract: 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.