In a typical multipath propagation environment, there exists a strong direct path signal accompanying with several weak multipath signals. Due to the strong direct path interference and other masking effects, the Direction-of-Arrival (DOA) of a weak multipath signal is hard to be estimated. In this paper, a novel method is proposed to estimate the DOA of multipath signals with ultralow signal-to-noise ratio (SNR). The main idea is to increase the SNR and signal-to-interference ratio (SIR) of the desired multipath signal in time-delay domain before DOA estimation processing. Firstly, the cross-correlation functions of the direct path signal and the received array signal are calculated. Then, they are combined and constructed to an enhanced array signal. Under certain conditions, the SNR and SIR of the desired signal can be significantly increased. Finally, the DOAs of multipath signals can be estimated by conventional technologies, and the associated time delays can be measured on the DOA-time-shift map. The SNR and SIR gains of the desired signal are analyzed theoretically, and theoretical analysis also indicates that the Cramer–Rao bound can be reduced. Simulation examples are presented to verify the advantages of the proposed method.
The domain images of ferroelectric capacitors with different top electrode thicknesses in the as-grown and poled states were obtained by piezoresponse force microscopy (PFM). In poled capacitors, a uniform piezoresponse with the same contrast was obtained for top electrodes of different thicknesses. For capacitors in the as-grown state, a lower piezoresponse was observed with thicker top electrodes, due to the different domain orientations, suppression effect of the ferroelectric domains and the damping effect of the electrode. It is shown that the piezoresponse decreases nonlinearly with the increase in electrode thickness. When the top electrode thickness is above 75 nm, the piezoresponse value is almost zero. When the top electrode thickness is increased from less than 10 to 75 nm, the piezoresponse is reduced rapidly to a very small value. Details of the domain wall, grain boundary and defects gradually become indistinguishable. Our investigation suggests that to obtain clear domain information by PFM based on global excitation, the top electrode thickness should be less than 10 nm.
Photon correlation technology is an important technology in the field of nanoparticle measurement at present. This paper expounds the research significance of photon correlation technology, introduces the research background and basic concepts of this technology, and studies and analyzes the research status of the correlation algorithm of the new photon correlator. Finally, prospect and application development trend of this technology are analyzed, from the aspects of practicability, application environment and hardware conditions, the difficulties faced by the photon correlation technology are put forward. There are some difficulties when the photon correlator needs to meet the requirements of accuracy, small sampling interval and large dynamic range, this article presents some possible solutions.
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