1-{2-[(2,4-Dichloro­benzyl­idene)amino]eth­yl}-3-methyl­imidazolium hexa­fluoro­phosphate

In the title Schiff base compound, C13H14Cl2N3 +·PF6 −, the dihedral angle between the aromatic ring and imidazole ring in the cation is 6.10 (2)°. Inter­molecular C—H⋯F hydrogen-bonding inter­actions and π–π stacking inter­actions [centoid–centroid distance = 3.7203 (12) Å] help stabilize the crystal packing

Study on the layered dusty plasma structures in the summer polar mesopause

Traditional hydrodynamic equations are adopted to build a one-dimensional theoretical model to study the effect of gravity wave on layered dusty plasma structures formation and evolution near the polar summer mesospause region associated with polar mesosphere summer echoes (PMSE). The proposed mechanism gives consideration to the charged ice particle motion by the gravity wave modulation, making a significant contribution to the vertical transport of heavy ice particles and convergence into thin layers. And numerical results show that the pattern of the multi-layer structure depends on the ration of the initial ice particles density distribution to the vertical wavelength of the gravity waves, the ice particle size and the wind velocity caused by gravity wave. Also, the variation of ion density distribution under the influence of gravity wave has also been examined. Finally, the electron density depletions (bite-outs) layers has been simulated according to the charge conservation laws, and the results are compared to the ECT02 rocket sounding data, which agree well with the measuring

Frequency-Domain Channel Estimation and Equalization for Single Carrier Underwater Acoustic Communications

A new frequency-domain channel estimation and equalization (FDE) scheme is proposed for single carrier (SC) underwater acoustic communications. The proposed SC-FDE employs a small training signal block for initial channel estimation in the frequency domain and converts the estimated transfer function to a desired DFT (discrete Fourier transform) size for channel equalization of the data blocks. The frequency domain equalizer is designed using the linear minimum mean square error criterion. A new phase coherent detection scheme is also proposed and deployed to combat the phase drift due to the instantaneous Doppler in the underwater channels. The channel transfer functions and group-averaged phase drift are re-estimated adaptively in a decision-directed manner for each data block in a packet, which contains M blocks of QPSK data. The proposed SC-FDE method is applied to single input multiple output (SIMO) systems using the experimental data measured off the coast of Panama City, Florida, USA, June 2007. The uncoded bit error rate of the SIMO systems varies between 1.3% to 6.8 x 10^-5 when 4 ~ 8 receive hydrophones are utilized, and the source-receiver range is 5.06 km

Further Results on Frequency-Domain Channel Equalization for Single Carrier Underwater Acoustic Communications

A frequency-domain channel equalization and phase correction method was proposed in a previous work published in Oceans\u2707 conference, in which 10^-4 BER performance was presented for fixed-to-fixed source/receiver channels. The method is improved by using minimum mean square error (MMSE) estimation in channel updates rather than least squares estimation. It is applied to moving-to-fixed channels in the AUVFest\u2707 ocean experiment where single-carrier wideband transmission was employed with quadrature phase shift keying modulation. The moving source channels exhibit higher Doppler shift and larger Doppler drift than the fixed-to-fixed channels. Therefore, the number of symbols required for initial phase estimation is increased from 2 to 8 symbols when the group- wise phase correction algorithm is applied to the moving sources. Thirty-six packets with data block length of 512 symbols have been processed and 34 of them achieved an uncoded Bit Error Rate (BER) lower than 10-2. The overall uncoded BER performance of all 36 packets is 1.81 times 10^-3 which is slightly higher than that of the fixed-to-fixed channels