Age of Information in Downlink Systems: Broadcast or Unicast Transmission?

We analytically decide whether the broadcast transmission scheme or the unicast transmission scheme achieves the optimal age of information (AoI) performance of a multiuser system where a base station (BS) generates and transmits status updates to multiple user equipments (UEs). In the broadcast transmission scheme, the status update for all UEs is jointly encoded into a packet for transmission, while in the unicast transmission scheme, the status update for each UE is encoded individually and transmitted by following the round robin policy. For both transmission schemes, we examine three packet management strategies, namely the non-preemption strategy, the preemption in buffer strategy, and the preemption in serving strategy. We first derive new closed-form expressions for the average AoI achieved by two transmission schemes with three packet management strategies. Based on them, we compare the AoI performance of two transmission schemes in two systems, namely, the remote control system and the dynamic system. Aided by simulation results, we verify our analysis and investigate the impact of system parameters on the average AoI. For example, the unicast transmission scheme is more appropriate for the system with a large number UEs. Otherwise, the broadcast transmission scheme is more appropriate

Age of Information of Multi-user Mobile Edge Computing Systems

In this paper, we analyze the average age of information (AoI) and the average peak AoI (PAoI) of a multiuser mobile edge computing (MEC) system where a base station (BS) generates and transmits computation-intensive packets to user equipments (UEs). In this MEC system, we focus on three computing schemes: (i) The local computing scheme where all computational tasks are computed by the local server at the UE, (ii) The edge computing scheme where all computational tasks are computed by the edge server at the BS, and (iii) The partial computing scheme where computational tasks are partially allocated at the edge server and the rest are computed by the local server. Considering exponentially distributed transmission time and computation time and adopting the first come first serve (FCFS) queuing policy, we derive closed-form expressions for the average AoI and average PAoI. To address the complexity of the average AoI expression, we derive simple upper and lower bounds on the average AoI, which allow us to explicitly examine the dependence of the optimal offloading decision on the MEC system parameters. Aided by simulation results, we verify our analysis and illustrate the impact of system parameters on the AoI performance

Time Reversal Method for Arch Bridge Cables Inspection using Longitudinal Guided Waves

Arch bridge cables consist of anchor heads, steel wires parallel arranged in an equilateral hexagon and hot-extruding PE sheathing layers outside the wires. The complex structure and contact force between wires aggravates the dispersion and attenuation of guided waves in steel wires. In order to reduce the attenuation of acoustical energy, below 80kHz low-frequency longitudinal guided waves is usually adopted. Low-frequency guided waves attenuate more slowly than high-frequency waves, but the received signal packets are wider and less recognizable. In this paper, the process of the time reversal method[1] is presented and the related parameters are calculated. Over a wide frequency range, using narrow-band pulse signals with different center-frequencies to drive comb-like magnetostrictive transducer array round the cable, extract the echo signals, which contains some feature information such as flaws, anchor heads, structural noise caused by contact force between wires. By taking advantage of the time-space compression characteristics of the method, the identification of anchor heads and flaws can be improved effectively and noise can also be decreased by driving the transducers again with the time reversed signal. Verification experiments show that the acoustical energy of guided waves can be focused on the position of flaws and the amplitude of flaws echo waves can be increased. At severe dispersion frequency, time reversal focusing process can improve the signal-noise ratio and suppress dispersion phenomenon caused by structural contact force

Magnetostrictive Helical Array Transducers for Inspecting Spiral Welded Pipes Using Torsional Flexural Waves

Wavefront analysis indicates that a flexural wave propagates at a helix angle with respect to the pipe axis. The expression for calculation of the helix angle for each flexural mode is given, and the helix angle dispersion curves for flexural modes are calculated. According to the new understanding of flexural guided waves, a magnetostrictive helical array transducer (MHAT) is proposed for selectively exciting a single predominant flexural torsional guided wave in a pipe and inspecting spiral welded pipes using flexural waves. A MHAT contains a pre-magnetized magnetostrictive patch that is helically coupled with the outer surface of a pipe, and an array of novel compound comb coils that are wrapped around the helical magnetostrictive patch. The proposed wideband MHAT possesses the direction control ability. A verification experiment indicates that flexural torsional mode T(3,1) at center frequency f=64kHz is effectively actuated by a MHAT with 13-degree helix angle. A 20-degree MHAT is adopted to inspect a spiral welded pipe, an artificial notch with cross section loss CSL=2.7% is effectively detected by using flexural waves

Eigenface recognition using different training data sizes

ABSTRACT This paper explores the relationship between eigenface recognition performance and different training data sets. Using the Multilevel Dominant Eigenvector Estimation (MDEE) method we are able to compute eigenfaces from a large number of training samples. This allows us to compare the recognition performance using different training data sizes. Experimental results show that increasing the number of people benefits the recognition performance more than increasing the number of images per person

Coherent perfect absorber and laser induced by directional emissions in the non-Hermitian photonic crystals

In this study, we propose the application of non-Hermitian photonic crystals (PCs) with anisotropic emissions. Unlike a ring of exceptional points (EPs) in isotropic non-Hermitian PCs, the EPs of anisotropic non-Hermitian PCs appear as lines symmetrical about the $\Gamma$ point. The non-Hermitian Hamiltonian indicates that the formation of EPs is related to the non-Hermitian strength. The real spectrum appears in the $\Gamma$Y direction and has been validated as the complex conjugate medium (CCM) by effective medium theory (EMT). But for the $\Gamma$X direction, EMT indicates that the effective refractive index has a large imaginary part, which forms an evanescent wave inside the PCs. Thence, coherent perfect absorber (CPA) and laser effects can be achieved in the directional emission of the $\Gamma$Y. The outgoing wave in the $\Gamma$X direction is weak, which can significantly reduce the losses and electromagnetic interference caused by the leakage waves. Furthermore, the non-Hermitian PCs enable many fascinating applications such as signal amplification, collimation, and angle sensors.Comment: 11 pages, 11 figure

Toward a High-Efficient Utilization of Solar Radiation by Quad-Band Solar Spectral Splitting

The promising quad-band solar spectral splitter incorporates the properties of the optical filter and the spectrally selective solar thermal absorber can direct PV band to PV modules and absorb thermal band energy for thermal process with low thermal losses. It provides a new strategy for spectral splitting and offers potential ways for hybrid PVT system design.United States. Department of Energy (contract DE-AR0000471)United States. Advanced Research Projects Agency-Energy (contract DE-AR0000471)United States. Department of Energy. Office of Science. Solid-State Solar Thermal Energy Conversion Center (Award # DE-FG02-09ER46577