On the characterization of Aloha in underwater wireless networks

Standard analyses of wireless random access protocols that are available in the literature assume negligible propagation delay between any two nodes. This assumption holds good in reasonably short-range terrestrial RF (radio frequency) wireless networks. On the contrary, in wireless communications involving acoustic wave propagation, as in underwater wireless networks, even short distance propagation has appreciably large propagation delay. This observation has led to several recent simulation and experimental studies on underwater Aloha and slotted-Aloha (S-Aloha) protocols and also a few new proposals on random access protocols for underwater wireless ad hoc networks (UWN). To study the efficiency of more advanced multiaccess communication protocols for UWN, it is important to benchmark their performances with respect to the two basic random access protocols, Aloha and S-Aloha. This paper provides an analytic framework to capture the performance of Aloha and S-Aloha protocols in an underwater environment with high and random internodal signal propagation delay. The performance of underwater Aloha and S-Aloha are contrasted with those in short-range terrestrial RF wireless networks. The analysis shows that random internodal propagation delay has no effect on the underwater Aloha performance. It also sheds light on the throughput degradation of underwater S-Aloha with a slotting concept that achieves RF S-Aloha equivalent one-slot vulnerability. Additionally, a modified slotting concept is introduced where the slot size is judiciously reduced such that even by allowing some collisions the overall system throughput can be increased. Our calculations show that, with the modified slotting approach up to 17% throughput performance gain can be achieved over the naive (RF S-Aloha equivalent) slotting approach in UWN. Our analytic results are supported by discrete event simulations.     

An atom-based explanation for the absence of excimer emission in the fluorescence of dilute solutions of poly(pentafluorostyrene)

The steady-state fluorescence emission spectrum of poly(pentafluorostyrene) in dilute fluid solution shows no excimer emission. An atomic level modeling study explains why this polymer cannot form an excimer. Repulsive Coulombic interactions prohibit the attainment of the extensive overlap of the two rings in the classic face-to-face sandwich conformation of a singlet excimer.     

Higher dimensional global monopole in Brans-Dicke theory

The gravitational field of a higher dimensional global monopole in the context of Brans-Dicke theory of gravity is investigated. The space time metric and the scalar field generated by a global monopole are obtained using the weak field approximation. Finally, the geodesic of a test particle due to the gravitational field of the monopole is studied.     

On the number of Fourier coefficients that determine a Hilbert modular form

We estimate the number of Fourier coefficients that determine a Hilbert modular cusp form of arbitrary weight and level. The method is spectral (Rayleigh quotient) and avoids the use of the maximum principle.

     

Photoionization of some closed shell atoms and ions

Dipole-allowed single photoionization of some closed shell atoms and ions has been investigated in the relativistic random-phase approximation (RRPA). Application of relativisticmultichannel quantum defect theory (RMQDT) is made together with RRPA to analyse autoionizing resonances. Analysis points to the importance of interchannel coupling in high energy photoionization and reveals various degeneracies in relativistic atomic spectra to influence the low energy dynamics. Interesting threshold behavior in photoelectron spin polarization has been seen. Prospective future studies have been indicated.     

Brans-Dicke Cosmology–A Generalization in Anisotropic Space-Time Models

The generalized Brans-Dicke (BD) Cosmology has been studied for Bianchi-I, Bianchi-III and Kantowski-Sachs anisotropic space-time models. Solutions have been obtained for radiation-dominated era, stiff matter epoch and other equations of state.     

Synthesis of Nitrogen Doped Multilayered Graphene Flakes: Selective Non‐enzymatic Electrochemical Determination of Dopamine and Uric Acid in presence of Ascorbic Acid.

Nitrogen doped multilayered graphene (NDMLG) is synthesized by annealing the black fluffy mass obtained by decomposing the complex prepared from Zn(OAc)₂, diethanolamine and triethanolamine. The NDMLG has been characterized by HRTEM, FESEM, XRD, XPS, Raman spectroscopy, BET which indicate formation of nitrogen doped multilayer graphene flakes with high surface area. NDMLG shows better electrochemical oxidation property towards Dopamine, Uric acid and Ascorbic acid. The linear response ranges for determination of DA, UA and AA are 0.5–150 µM, 3–60 µM and 80–2000 µM respectively and the detection limits (S/N=3) are 15 nM, 15 nM, 580 nM, respectively in the mixture.     

Synthesis of Some Novel Bisisoxazolidine Derivatives from Glyoxal‐derived Bisnitrones via Simultaneous Double Cycloaddition Reactions in Water

Simultaneous double 1,3‐dipolar cycloaddition reactions of glyoxal‐derived bisnitrones have been described in water. Significant rate acceleration and improved yields of exclusively diastereoselective and regioselective bisisoxazolidines in water have been observed at room temperature in a short reaction time compared with conventional solvents.     

Two CdII/CoII‐Imidazolate Coordination Polymers: Syntheses,Crystal Structures,Stabilities, and Luminescent/Magnetic Properties

Cadmium(II) based 2D coordination polymer [Cd(L1)₂(DMF)₂] ( 1 ) (L1 = 4,5‐dicyano‐2‐methylimidazolate, DMF = N,N′‐dimethylformamide) and 2D cobalt(II)‐imidazolate framework [Co(L3)₄] ( 2 ) (L3 = 4,5‐diamide‐2‐ethoxyimidazolate) were synthesized under solvothermal reaction conditions. The materials were characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, powder X‐ray diffraction measurement (PXRD) and single‐crystal X‐ray diffraction. Compound 1 has hexacoordinate Cd^II ions and forms a zigzag chain‐like coordination polymer structure, whereas compound 2 exhibits a 2D square grid type structure. The thermal stability analysis reveals that 2 showed an exceptional thermal stability up to 360 °C. Also, 2 maintained its fully crystalline integrity in boiling water as confirmed by PXRD. The solid state luminescent property of 1 was not observed at room temperature. Compound 2 showed an independent high spin central Co^II atom.