Implementation of a Software-Defined Acoustic Modem for MBPS Wireless Underwater Communication

Wireless communication technologies such as cellular networks, GPS and Wi-Fi are ubiquitous on land, but underwater, they are inadequate. The radio waves that these technologies use to carry information wirelessly over land can only penetrate a few centimeters of saltwater. This paper will discuss the implementation of an acoustic modem that uses sound waves, as whales and dolphins do, for sending information and that can support data rates beyond 1 Mbps, 1000 times faster than existing commercial systems. We developed and implemented a physical layer protocol specifically tailored to the needs of a reliable high-speed acoustic communication link. First a custom hyperbolic chirp waveform is sent for robust receiver synchronization and initialization, then a stream of QAM symbols follows. Some of these QAM symbols are known a priori, such that the receiver algorithms can adapt to the time-varying reverberation and Doppler structure of the acoustic communication channel. The modem is implemented in software on the BeagleBone Black (BBB), a cheap single board computer, and an attached analog frontend (AFE) daughter card is used for signal generation and acquisition. The signal processing software runs in a non-preemptive operating system (Debian) on an ARM processor. The programmable real-time units (PRUs) on the BBB are utilized to interface with the AFE. The PRUs stream signals from and to the AFE in real time and communicate with the ARM processor through fast direct memory access transfers.Ope

The Effect of Credit Guarantees on Survival and Performance of SMEs in Korea

This study evaluates the impact of provision of credit guarantee in Korea at the firm level. The data is assembled from two public funds providing credit guarantees covering the period 2000 to 2003. The sample firms consist of SMEs mainly. To measure the effects of credit guarantee, the relationship between credit guarantees, survival of firms, and their productive performance is analyzed. Since the data is collected as repeated cross sections and firms are not identified over time, the analysis is carried out by using a pseudo panel data approach. The pseudo panel data is created using time invariant firm characteristics. The result from regression analysis conducted indicates that the amounts of credit guarantee and the number of times a firm receives credit guarantees have effects on their survival and growth. The amounts of credit guarantee increase the growth of sales and productivity while frequency of credit guarantees decreases business failure. Size and age play a decisive role in survival of firms and their employment growth as well. Moreover, survival and performance of firms are different across periods, industries, and locations.Credit guarantee; SMEs; Pseudo panel data; survival; performance

Display screens with variable refresh rate

Device displays of a mobile device such as a smartphone or tablet have capabilities to support different refresh rates. For optimal operation, the refresh rate of the screen of a mobile device can be refreshed based on the image or frame of video being displayed and on certain other conditions. This disclosure describes low-complexity, low-latency techniques to seamlessly transition between different screen refresh rates with minimal-to-zero changes in display settings

Analysis of electrical signatures in synchronous generators characterized by bearing faults

Synchronous generators play a vital role in power systems. One of the major mechanical faults in synchronous generators is related to bearings. The popular vibration analysis method has been utilized to detect bearing faults for years. However, bearing health monitoring based on vibration analysis is expensive. One of the reasons is because vibration analysis requires costly vibration sensors and the extra costs associated with its proper installation and maintenance. This limitation prevents continuous bearing condition monitoring, which gives better performance for rolling element bearing fault detection, compared to the periodic monitoring method that is a typical practice for bearing maintenance in industry. Therefore, a cost effective alternative is necessary. In this study, a sensorless bearing fault detection method for synchronous generators is proposed based on the analysis of electrical signatures, and its bearing fault detection capability is demonstrated. Experiments with staged bearing faults are conducted to validate the effectiveness of the proposed fault detection method. First, a generator test bed with an in- situ bearing damage device is designed and built. Next, multiple bearing damage experiments are carried out in two vastly different operating conditions in order to obtain statistically significant results. During each experiment, artificially induced bearing current causes accelerated damage to the front bearing of the generator. This in-situ bearing damage process entirely eliminates the necessity of disassembly and reassembly of the experimental setup that causes armature spectral distortions. The electrical fault indicator is computed based on stator voltage signatures without the knowledge of machine and bearing specific parameters. Experimental results are compared using the electrical indicator and a vibration indicator that is calculated based on measured vibration data. The results indicate that the electrical indicator can be used to analyze health degradation of rolling element bearings in synchronous generators in most instances. Though the vibration indicator enables early bearing fault detection, it is found that the electrical fault indicator is also capable of detecting bearing faults well before catastrophic bearing failure

Relaxation of superfluid turbulence in highly oblate Bose-Einstein condensates

We investigate thermal relaxation of superfluid turbulence in a highly oblate Bose-Einstein condensate. We generate turbulent flow in the condensate by sweeping the center region of the condensate with a repulsive optical potential. The turbulent condensate shows a spatially disordered distribution of quantized vortices and the vortex number of the condensate exhibits nonexponential decay behavior which we attribute to the vortex pair annihilation. The vortex-antivortex collisions in the condensate are identified with crescent-shaped, coalesced vortex cores. We observe that the nonexponential decay of the vortex number is quantitatively well described by a rate equation consisting of one-body and two-body decay terms. In our measurement, we find that the local two-body decay rate is closely proportional to $T^2/\mu$, where $T$ is the temperature and $\mu$ is the chemical potential.Comment: 7 pages, 9 figure

Observation of a Geometric Hall Effect in a Spinor Bose-Einstein Condensate with a Skyrmion Spin Texture

For a spin-carrying particle moving in a spatially varying magnetic field, effective electromagnetic forces can arise due to the geometric phase associated with adiabatic spin rotation of the particle. We report the observation of a geometric Hall effect in a spinor Bose-Einstein condensate with a skyrmion spin texture. Under translational oscillations of the spin texture, the condensate resonantly develops a circular motion in a harmonic trap, demonstrating the existence of an effective Lorentz force. When the condensate circulates, quantized vortices are nucleated in the boundary region of the condensate and the vortex number increases over 100 without significant heating. We attribute the vortex nucleation to the shearing effect of the effective Lorentz force from the inhomogeneous effective magnetic field.Comment: 9 pages, 11 figure

Fabrication of Microstructure Array using the Projection Microstereolithography System

Microstereolithography technology is similar to the conventional stereolithography process and enables to fabricate a complex 3D microstructure. This is divided into scanning and projection type according to aiming at precision and fabrication speed. The scanning MSL fabricates each layer using position control of laser spot on the resin surface, whereas the projection MSL fabricates one layer with one exposure using a mask. In the projection MSL, DMD used to generate dynamic pattern consists of micromirrors which have per side. The fabrication range and resolution are determined by the field of view of the DMD and the magnification of the projection lens. If using the projection lens with high power, very fine microstructures can be fabricated. In this paper, the projection MSL system adapted to a large surface for array-type fabrication is presented. This system covers the meso range, which is defined as the intermediate range between micro and macro, with a resolution of a few . The fabrication of array-type microstructures has been demonstrated to verify the performance of implemented system