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In this paper, we consider a dual-hop mixed THz-RF system model for backhaul-fronthaul applications where the link between source and destination is established only through the relay node in which decode-and-forward relaying protocol is used. The THz link suffers from the joint impact of antenna misalignment and stochastic characteristics of wireless channels, including the effect of environmental conditions such as pressure, humidity, and temperature. The envelope of THz link in the first hop follows a generalized $\alpha-\mu$ distribution, and for the RF end, the Nakagami-$m$ distribution is considered. In this context, we obtain new closed-form expressions of the cumulative density function and the moment-generating function of the end-to-end signal-to-noise ratio. Further, we derive the average symbol error rate expressions for coherent rectangular quadrature amplitude modulation (RQAM) and coherent hexagonal QAM (HQAM), as well as the non-coherent modulation scheme. The asymptotic behavior is also discussed to examine the system's diversity. Furthermore, the impact of several parameters, such as fading coefficients of individual links and antenna misalignment, as well as the distance between nodes, are also highlighted in the system's performance. Moreover, Monte Carlo simulations are used to validate the presented analytical framework. Finally, the presented numerical insights aid in the extraction of practical design principles.

High energies emissions observed in X-ray binaries (XRBs), active galactic nuclei (AGNs) are linearly polarized. The prominent mechanism for X-ray is the Comptonization process. We revisit the theory for polarization in Compton scattering with unpolarized electrons, and note that the ($k \times k'$)-coordinate (in which, ($k \times k'$) acts as a $z$-axis, here $k$ and $k'$ are incident and scattered photon momentum respectively) is more convenient to describe it. Interestingly, for a fixed scattering plane the degree of polarization PD after single scattering for random oriented low-energy unpolarized incident photons is $\sim$0.3. At the scattering angle $\theta$ = 0 or $\theta \equiv$ [0,25$^o$], the modulation curve of $k'$ exhibits the same PD and PA (angle of polarization) of $k$, and even the distribution of projection of electric vector of $k'$ ($k'_e$) on perpendicular plane to the $k$ indicates same (so, an essential criteria for detector designing). We compute the polarization state in Comptonization process using Monte Carlo methods with considering a simple spherical corona. We obtain the PD of emergent photons as a function of $\theta$-angle (or alternatively, the disk inclination angle $i$) on a meridian plane (i.e., the laws of darkening, formulated by Chandrasekhar 1946) after single scattering with unpolarized incident photons. To explore the energy dependency we consider a general spectral parameter set corresponding to hard and soft states of XRBs, we find that for average scattering no. $\langle N_{sc}\rangle$ $\sim$1.1 the PD is independent of energy and PA $\sim$90$^o$ ($k'_e$ is parallel to the disk plane), and for $\langle N_{sc}\rangle$ $\sim$5 the PD value is maximum for $i$=45$^o$. We also compare the results qualitatively with observation of IXPE for five sources., Comment: 14 figures, 3 tables (Key words: X-rays: individual: 4U 1630-47, Cyg X-2, GX 9+9, XTE J1701-462, Cyg X-1)

Supplement 1: More details on minimum required reflectivity, observed deposition rates and a comparison of the proposed method with the existing methods

Response function of ammonium perchlorate (AP) has been studied. An unsteady model of AP combustion has been used based on the recent experimental findings reported. The key findings of this work are the good comparisons with experimental results, which until date had not been predicted accurately. Variation of thermal properties as a function of temperature in the condensed phase predicted the experimental data better.

The present study primarily concerns itself to validate the novel technique presented by Vijay and Ramakrishna (2020) to predict the initial temperature sensitivity of non-aluminized composite propellant compositions. The technique is used here to predict the combustion characteristics of non-aluminized propellants with 86% loading. The coarse to fine ratio of 1:1 and 4:1 for the AP particles was used in theses propellants. The burn rate and temperature sensitivity of these propellants were determined experimentally and compared with the computed ones. A reasonable match was found between the predicted and experimentally obtained values.

Ageing is an irreversible process during which, the material undergoes a series of slow chemical degradation/oxidation reactions. The aim of this paper is to study the effect of ageing of ammonium perchlorate-copper chromite (AP-CC) pellets on the burn rate. It also focuses on any change in CC properties due to direct contact with an oxidizer (AP). Accelerated ageing with cyclic temperature profile has been used to replicate the natural ageing. The effect of ageing of individual ingredients on the burn rate was studied. The burn rate of AP-CC pellets was found to reduce when AP-CC were aged together. X-ray diffraction (XRD) and Energy Dispersive Spectroscopy (EDS) analysis were used to identify the change in properties of copper chromite during ageing.

The combustion characteristics of Ammonium Perchlorate (AP) monopropellant have been revisited. The use of silica grease to prevent heat loss from the sides of the pellet and the ignition methodology opened up new area; one of them being the low pressure deflagration limit (LPDL) of AP itself and the other being the burn rate of AP at different pressures. The new burn rate law for pure AP was obtained for conditions close to adiabatic. Using slow depressurization method, wherein the LPDL was independent of ignition transients, the LPDL of pure AP was found to be 14 bar The burn rate of pure AP at 70 bar was found to be 10.66 mm/s, which was much higher than the ones reported in literature. The pressure index for this case was found to be 0.71. This new result completely changes the understanding of AP combustion and called for a new set of parameters to be developed to predict the results obtained with silica grease on the sides. Using the same set of parameters, along with a heat loss model the predictions for the case when there was no silica grease applied was made. These results were in reasonable agreement with the experimental data reported in literature without the silica grease on the sides of the AP pellet. This brings out the critical role of heat loss in AP combustion.

This paper deals with the low pressure deflagration limit (LPDL), which is the minimum pressure below which burning of a propellant ceases without the assistance of an external energy source. The LPDL of AP monopropellant reported in literature is around 20 bar and it is reported to shift to different higher pressures with the addition of small fraction of impurities or additives. Most of these additives (such as iron oxide and copper chromite) are known to catalyze AP combustion, but yet their reported LPDL is significantly higher than the LPDL of AP itself. In present study, reasons for this behavior have been explored experimentally. Three additives (each of 1% by mass) namely iron oxide (IO), copper chromite (CC) and activated charcoal (AC) were used. The LPDL of AP with these additives was determined using three methods. Two of these methods (method I and method II) include the effect of ignition dynamics on LPDL while in method III (slow depressurization) ignition dynamics does not play a role. In method I, hot nichrome wire was used for ignition, while in method II, nichrome wire ignition in conjunction with an in-situ propellant was used to ignite the pellets. The effect of convective heat loss on LPDL was studied with the use of silica grease whose role through carefully thought out experimentation has been identified as an insulation on sides of pellet. The LPDL of AP with additives was found to be higher for method I compared to method II. Lastly, method III (slow depressurization) was found to result in lowest LPDL for these additives. The LPDL of pure AP, which has been reported as around 20 bar, was revisited and method III along with the use of silica grease has lowered the LPDL of pure AP to 14 bar.

The present study reports the development of a novel technique to quantify binder melt on the surface of the propellant. Non-aluminized AP-HTPB propellants of 86% particulate loading are used to illustrate the technique. Elemental maps of unburnt and extinguished propellant surface are obtained using EDS (Energy Dispersive Spectroscopy). Overlap between the elements is identified and the elemental maps are processed to calculate AP and binder area exposed in unburnt and extinguished samples. The AP area exposed is found to be around 72.3% and 63.3% for unburnt and extinguished samples, respectively, indicating a reduction in AP exposed area with extinguished samples. This has been attributed to the binder melt discussed in literature but never quantified. Simulations have been carried out to analyze and understand the effects of this binder melt. A random packing algorithm is used to simulate propellant packs. Also, a methodology to account for binder melt layer is introduced and is used to capture AP exposed areas. Effect of binder melt in propellants with different solid loading and varying particle size is discussed. It is shown that fine AP particles are more prone to being covered by binder melt than the coarse AP particles. A possible explanation to the behavior of plateau burning propellants observed in literature has been provided through this analysis.

Blazars host the most powerful persistent relativistic conical jet -- a highly collimated anisotropic flow of material/plasma. Motivated by this, we explore the blazar's broadband spectral energy distribution (SED) in an anisotropic flow of plasma which emits via synchrotron and inverse Compton (IC) mechanism. The flow is conical with two velocity components: a highly relativistic flow component along the jet axis and a random perpendicular component with average random Lorentz factor $\langle \gamma^{ran} \rangle$ $<, Comment: 22 pages, 11 figures

As the research on beyond 5G heats up, we survey and explore power and bandwidth efficient modulation schemes in details. In the existing publications and in various communication standards, initially square quadrature amplitude modulation (SQAM) constellations (even power of 2) were considered. However, only the square constellations are not efficient for varying channel conditions and rate requirements, and hence, odd power of 2 constellations were introduced. For odd power of 2 constellations, rectangular QAM (RQAM) is commonly used. However, RQAM is not a good choice due to its lower power efficiency, and a modified cross QAM (XQAM) constellation is preferred as it provides improved power efficiency over RQAM due to its energy efficient two dimensional (2D) structure. The increasing demand for high data-rates has further encouraged the research towards more compact 2D constellations which lead to hexagonal lattice structure based constellations, referred to as hexagonal QAM (HQAM). In this work, various QAM constellations are discussed and detailed study of star QAM, XQAM, and HQAM constellations is presented. Generation, peak and average energies, peak-to-average-power ratio, symbol-error-rate, decision boundaries, bit mapping, Gray code penalty, and bit-error-rate of star QAM, XQAM, and HQAM constellations with different constellation orders are presented. Finally, a comparative study of various QAM constellations is presented which justifies the supremacy of HQAM over other QAM constellations. With this, it can be claimed that the use of the HQAM in various wireless communication systems and standards can further improve the performance targeted for beyond 5G wireless communication systems.

The combustion characteristics of ammonium perchlorate (AP) monopropellant have been discussed. Both experimental and computational tools were utilized to explore AP combustion. Three different methods were used to determine the low-pressure deflagration limit (LPDL) of AP monopropellant. Method I and Method II are found to introduce the ignition dynamics and affect the LPDL. Method III (slow depressurization) is independent of ignition and pressure dynamics. The LPDL of AP is found to be 14 bar using method III. Silica grease is found to act as an insulator. The use of silica grease on the sides of pellet reduces the convective heat loss and both the burning rate and the temperature sensitivity of AP are found to increase. The burning rate and temperature sensitivity of AP at 70 bar are 10.66 mm/s and 0.0038 K−1, respectively. LPDL of AP with 1% of iron oxide (IO) and copper chromite (CC) is significantly lower due to reduced convective heat loss and the prominence of the catalytic effect, which was overlooked in literature. A two-dimensional, unsteady combustion model is used to simulate AP. The combustion parameters of the model are suitably updated and a good match is obtained with the experimentally observed burn rate, pressure index, and temperature sensitivity.

Nowadays online social networks are used extensively for personal and commercial purposes. This widespread popularity makes them an ideal platform for advertisements. Social media can be used for both direct and word-of-mouth (WoM) marketing. Although WoM marketing is considered more effective and it requires less advertisement cost, it is currently being under-utilized. To do WoM marketing, we need to identify a set of people who can use their authoritative position in social network to promote a given product. In this paper, we show how to do WoM marketing in Facebook group, which is a question answer type of social network. We also present concept of reinforced WoM marketing, where multiple authorities can together promote a product to increase the effectiveness of marketing. We perform our experiments on Facebook group dataset consisting of 0.3 million messages and 10 million user reactions.

In recent years, the creation of block-structured dictionary has attracted a lot of interest. Learning such dictionaries involve two step process: block formation and dictionary update. Both these steps are important in producing an effective dictionary. The existing works mostly assume that the block structure is known a priori while learning the dictionary. For finding the unknown block structure given a dictionary commonly sparse agglomerative clustering (SAC) is used. It groups atoms based on their consistency in sparse coding with respect to the unstructured dictionary. This paper explores two innovations towards improving the reconstruction as well as the classification ability achieved with the block-structured dictionary. First, we propose a novel block structuring approach that makes use of the correlation among dictionary atoms. Unlike the SAC approach, which groups diverse atoms, in the proposed approach the blocks are formed by grouping the top most correlated atoms in the dictionary. The proposed block clustering approach is noted to yield significant reductions in redundancy as well as provides a direct control on the block size when compared with the existing SAC-based block structuring. Later, motivated by works using supervised \emph{a priori} known block structure, we also explore the incorporation of class information in the proposed block formation approach to further enhance the classification ability of the block dictionary. For assessment of the reconstruction ability with proposed innovations is done on synthetic data while the classification ability has been evaluated in large variability speaker verification task., Comment: 9 pages, Submitted to IEEE Transactions on Signal Processing

X-ray binaries (XRBs) often exhibit a high energy power-law tail (X-ray) and these tails can be generated by the bulk Comptonization (BMC) process with a free-fall bulk region onto the compact object. We frequently observe a radio emission (which is generated by a synchrotron-emitting outflowing electrons) from the XRBs. We aim to study the high energy power-law tail in BMC process by an outflowing medium. We mainly consider a collimated and conical (of opening angle $\theta_b$ with axis perpendicular to the accretion disk) outflow for a BMC process. We simulate the bulk Comptonized spectra by using a Monte Carlo scheme. Out of two possible conical outflow directions i) along the surface of the cone, ii) inside the conic region, we find that a) the randomness of outflow directions increases with $\theta_b$, b) the emergent spectra have power-law tail (of photon index $\Gamma$ $>$ 2 and with high energy cut-off $E_{c}$ $>$ 200 keV) only for $\theta_b$ greater than $\sim$30 degree, while for a collimated or a conical outflow ($\theta_b$ $, Comment: v1: 4 figures; v2: 5 figures, comparison with observations added, revised version, references removed/ added, comments welcome