Temporal Focusing Effect Research Articles

Moving Target Detection Based on Time Reversal in a Multipath Environment

Time reversal can resolve multipath problems and increase the detection probability, but the detection performance decreases when the target is moving. Because of the change in the channel response due to target motion, the focusing effect of a time-reversal-transmitted signal is reduced. In this article, a novel method based on time reversal, which detects a moving target embedded in a multipath environment with channel variation, is proposed. We establish conventional and time-reversal signal models of a moving target in an indoor diffuse multipath environment and derive likelihood ratio tests under these two conditions. The channel parameters and the changes due to target motion are obtained by echo estimation and are exploited as useful information by the detectors. The channel changes are taken into account in the time-reversal detector to improve its robustness. The time-reversal detector proposed in this article possesses the constant-false-alarm-rate property with respect to the unknown background noise. Although Monte Carlo simulation analysis has illustrated the superior performance of both conventional and time-reversal detectors and proved that both methods can improve the detection probability by exploiting the multipath effect, the time-reversal method outperforms the conventional method due to the spatial and temporal focusing effect on the channel.

High Resolution Carrier Frequency Offset Estimation in Time-Reversal Wideband Communications

Time-reversal (TR) wideband communication systems enjoy the spatial–temporal focusing effect in a rich-scattering environment. However, the performance degrades in the presence of carrier frequency offset (CFO). The impact of CFO can be mitigated by compensating the estimated CFO values obtained using CFO estimators. Yet, CFO estimators in literature cannot work well in wideband TR systems due to the fact that the normalized CFO values are very small and thus cannot be estimated accurately using conventional schemes. To address this issue, we propose four CFO estimators which are capable of accurate CFO estimations for wideband TR systems. The theoretical performances of the proposed estimators are analyzed. Additionally, realizing that phase wrapping might introduce severe bias into CFO estimations, we present the conditions on the system parameters so that phase wrapping can be avoided. Extensive simulations and experimental results demonstrate the superiority of the proposed methods.

Waveforming Optimizations for Time-Reversal Cloud Radio Access Networks

Due to the unique spatial and temporal focusing effects, time-reversal (TR) communication can be utilized in the cloud radio access network (C-RAN), where it creates “tunneling effects” such that the traffic load in the front-haul links can be alleviated in both downlink and uplink. Although the basic TR waveforms are simple to use, and they cannot provide the optimal performance in some cases. Since the C-RAN is usually expected to serve massive wireless devices, the severe inter-user interference will limit the performance of the system, especially in the high signal-to-noise ratio region where the interference power dominates the noise power. In this paper, we propose to optimize both downlink and uplink transmissions in the TR-based C-RAN so as to alleviate the interference. In the downlink transmission, an optimal content-aware waveform design is proposed, so that the baseband units (BBUs) are able to combine both the channel information and the content information to suppress the interference. In the uplink transmission, an optimal receiver design algorithm is proposed, such that the BBUs can detect the symbols transmitted by the terminal devices more accurately by leveraging the channel information. We study the bit error rate performance of the proposed algorithms based on extensive measurements of the wireless channel in a real-world environment. Numerical results demonstrate the significant performance improvement over the basic TR transmission techniques and the traditional waveform design techniques.

Vertical multiuser communication using adaptive time reversal

Time reversal is an attractive solution to achieve channel equalization in a rich multipath environment such as underwater acoustic communication (UAC) channel because of its spatial and temporal focusing effect. Recently, demands for multiuser communication have increased in the field of UAC. In Japan Agency Marine-Earth Science and Technology (JAMSTEC), multiple autonomous underwater vehicles (AUVs) operation is planned to widen observation area. Adaptive time reversal is a method for space division multiplexing (SDM) based on its spatial focusing and nulling effect, and a promising solution for multiuser communication. In previous studies, it has been demonstrated that adaptive time reversal is very effective for “horizontal” multiuser communication, in which signals from multiple sources are received vertical receiver array. In this study, adaptive time reversal is applied for “vertical” multiuser communication, which is between a support vessel and multiple vehicles below the vessel. At-sea experiments were carried out in which signals from two sources near the seabed were measured at the receivers on the bottom of the research vessel. As results, adaptive time reversal is also effective for such vertical multiuser communication and has better performance than orthogonal frequency-division multiplexing (OFDM) with SDM combiners.

All-optical sampling and magnification based on XPM-induced focusing

We theoretically and experimentally investigate the design of an all-optical magnification and sampling function free from any active gain medium or additional amplified spontaneous noise emission. The proposed technique is based on the co-propagation of an arbitrary shaped signal together with an orthogonally polarized intense fast sinusoidal beating within a normally dispersive optical fiber. This process allows us to experimentally demonstrate a 40-GHz sampling operation as well as an 8-dB magnification of an arbitrary shaped nanosecond signal around 1550 nm in a 5-km long optical fiber. The experimental observations are in good agreement with numerical and theoretical analysis.

Time-Reversal Tunneling Effects for Cloud Radio Access Network

The explosion of today’s wireless traffic requires operators to deploy more access points (APs) and design efficient collaboration mechanism to alleviate the interference among them. However, the collaborative techniques cannot work efficiently due to the high latency and low bandwidth interface between the APs in traditional networks. To address this challenge, cloud radio access network (C-RAN) is proposed, where a pool of base band units (BBUs) are connected to the distributed remote radio heads (RRHs) via high bandwidth and low latency links (i.e., the front-haul) and are responsible for all the baseband processing. But the limited front-haul link capacity may prevent the C-RAN from fully utilizing the benefits made possible by the centralized baseband processing. As a result, the front-haul link capacity becomes a bottleneck. To address this challenge, in this work, we propose using the time-reversal (TR)-based communication as the air interface in C-RAN. Due to the unique spatial and temporal focusing effects of TR-based communications, multiple terminal devices (TDs) are naturally separated by their location-specific signatures. Such a property allows signals to be combined to deliver without demanding more bandwidth. Therefore, the TR-based communication in essence creates a “tunneling” effect such that the baseband signals for all the TDs can be efficiently combined and transmitted in the front-haul. We study the performance of the proposed C-RAN architecture in terms of spectral efficiency and front-haul rate, based on extensive measurements of the wireless channel in a real-world environment. It is shown that with nearly the same amount of traffic load in the front-haul, more information can be transmitted when there are more TDs. The proposed TR tunneling effect can help deliver more information in the C-RAN and alleviate the burden of the front-haul caused by network densification.

A time reversal damage imaging method for structure health monitoring using Lamb waves

This paper investigates the Lamb wave imaging method combining time reversal for health monitoring of a metallic plate structure. The temporal focusing effect of the time reversal Lamb waves is investigated theoretically. It demonstrates that the focusing effect is related to the frequency dependency of the time reversal operation. Numerical simulations are conducted to study the time reversal behaviour of Lamb wave modes under broadband and narrowband excitations. The results show that the reconstructed time reversed wave exhibits close similarity to the reversed narrowband tone burst signal validating the theoretical model. To enhance the similarity, the cycle number of the excited signal should be increased. Experiments combining finite element model are then conducted to study the imaging method in the presence of damage like hole in the plate structure. In this work, the time reversal technique is used for the recompression of Lamb wave signals. Damage imaging results with time reversal using broadband and narrowband excitations are compared to those without time reversal. It suggests that the narrowband excitation combined time reversal can locate and determine the size of structural damage more precisely, but the cycle number of the excited signal should be chosen reasonably.

Time reversal study of ultrasonic waves for anisotropic solids using a Gaussian beam model

Time reversal (TR) of ultrasonic bulk waves in fluids and isotropic solids has been used in many applications including ultrasonic NDE. However, the study of the TR method for anisotropic materials is not well established. In this paper, the full reconstruction of the input signal is investigated for anisotropic media using an analytical formulation, called a modular Gaussian beam (MGB) model. The time reversal operation of this model in the frequency domain is performed by taking the complex conjugate of the Gaussian amplitude and phase received at the TR mirror position. A narrowband reference signal having a particular frequency and number of cycles is then multiplied and the whole signal is inverse Fourier transformed to obtain the time domain signal. The original input signal is seen to be fully restored by the TR process of MGB model and this model can be more generalized to simulate the spatial and temporal focusing effects due to TR process in anisotropic materials.

Simultaneous spatial and temporal focusing in nonlinear microscopy

Simultaneous spatial and temporal focusing (SSTF), when combined with nonlinear microscopy, can improve the axial excitation confinement of wide-field and line-scanning imaging. Because two-photon excited fluorescence depends inversely on the pulse width of the excitation beam, SSTF decreases the background excitation of the sample outside of the focal volume by broadening the pulse width everywhere but at the geometric focus of the objective lens. This review theoretically describes the beam propagation within the sample using Fresnel diffraction in the frequency domain, deriving an analytical expression for the pulse evolution. SSTF can scan the temporal focal plane axially by adjusting the GVD in the excitation beam path. We theoretically define the axial confinement for line-scanning SSTF imaging using a time-domain understanding and conclude that line-scanning SSTF is similar to the temporally-decorrelated multifocal multiphoton imaging technique. Recent experiments on the temporal focusing effect and its axial confinement, as well as the axial scanning of the temporal focus by tuning the GVD, are presented. We further discuss this technique for axial-scanning multiphoton fluorescence fiber probes without any moving parts at the distal end. The temporal focusing effect in SSTF essentially replaces the focusing of one spatial dimension in conventional wide-field and line-scanning imaging. Although the best axial confinement achieved by SSTF cannot surpass that of a regular point-scanning system, this trade-off between spatial and temporal focusing can provide significant advantages in applications such as high-speed imaging and remote axial scanning in an endoscopic fiber probe.