A particle method for the homogeneous Landau equation

We propose a novel deterministic particle method to numerically approximate the Landau equation for plasmas. Based on a new variational formulation in terms of gradient flows of the Landau equation, we regularize the collision operator to make sense of the particle solutions. These particle solutions solve a large coupled ODE system that retains all the important properties of the Landau operator, namely the conservation of mass, momentum and energy, and the decay of entropy. We illustrate our new method by showing its performance in several test cases including the physically relevant case of the Coulomb interaction. The comparison to the exact solution and the spectral method is strikingly good maintaining 2nd order accuracy. Moreover, an efficient implementation of the method via the treecode is explored. This gives a proof of concept for the practical use of our method when coupled with the classical PIC method for the Vlasov equation.Comment: 27 pages, 14 figures, debloated some figures, improved explanations in sections 2, 3, and

RF system for mmWave massive MIMO

Due to rapid developments in communication technology, it is likely that 5G networks will be rolled out in 2019. To adapt to 5G, hardware will have to develop to meet the requirements of this new technology. The mmWave communication is one of the main elements of 5G technology. The mmWave frequency bandwidth is used to carry the data links and can achieve a higher transmission data rate than the current LTE system. There are few continuous frequency resources under 3GHz that can be allocated. As such, the International Telecommunication Union (ITU) and the 3GPP organization mutually agree that the mmWave is the most suitable option for exploring new frequency resources. However, the mmWave has the one key weakness: high path loss for short transmission range. To compensate for this negative effect, a massive MIMO system can be used to have spatial multiplexing gains and array an-tenna gains. This article seeks a method that can acknowledge the funda-mental concepts and requirements of the mmWave massive MIMO system, from both theoretical and practical perspectives. In order to find proof of the concepts, the practical limitations, and the guild of the real design, a prototype of the system has been built. The current industry standard when creating a prototype is to use PCB. We will develop our system proposals from the prototype. To do so we use the evaluation boards to test system level performances such as link budget and identifying the most suitable components etc. Then in the PCB design, we integrate the radio frequency of the mmWave system. This has the scalability to collaborate with massive MIMO system test-bed to observe the system level performance. Finally, to verify our methods, we carry out experiments on both component level and system level in order to identify the feasibility of the prototype system. The performance of each individual component is tested using an evaluation board. Separate tests are performed for both transmitting (Tx) and receiving (Rx) chains. Finally, over-air-tests are conducted at the sys-tem level to evaluate the performance of our design5G communication system is the next milestone that will soon approach our lives. The first specification of 5G is called Release 15 and the system struc-tures and requirements of it have been identified. Compared to the LTE system, it can deliver an even higher data rate and adapt more transmission situations in the future. 5G consists of five essential technologies, mmWave, massive MIMO, the advanced channel coding, scalable OFDM and self-contained slot structure. The first two technologies, the mmWave and massive MIMO, are indispensable in this thesis. As the fre-quency resources scarcely go below 3GHz, they are retrieving the mmWave spectrum to allocate more accessible bandwidth. Nevertheless, the mmWave has high free space path loss, and the signal will be harshly weakened before it reaches the receiver. The massive MIMO is an extension of the MIMO system with massive antenna elements in the antenna array This sound solution can contradict the high free space path loss, achieve high throughput and serve tens of users simultaneously. This thesis concentrates on building a radio frequency system PCB prototype based on mmWave and massive MIMO. Now, the existing hardware devices will not placate the 5G system necessi-ties. The new system structure must be developed, and the performance will be evaluated. Prototype is a realization method to transition from theory to practice which can help the engineers comprehend the theoretical and prac-tical perspectives. In electronic industry, PCB is the correct way of building the radio frequency prototype, in which the system schematic is construct-ed in a dense area with mass circuit distribution. All electronic components are surfaced and then mounted on it and connected by conductive wires through the various layers. A flawless PCB needs to be shaped with full operational functions before the final products. To do this, the perfor-mance will be evaluated when we are building the PCB prototype and im-provements will be analysed, and additional developments will be encom-passed in the future version of PCB. In this thesis, the PCB performance is evaluated individually for different transmission chains, and then the over air test is assessed

Fighting against fast speckle decorrelation for light focusing inside live tissue by photon frequency shifting

Light focusing inside live tissue by digital optical phase conjugation (DOPC) has drawn increasing interest due to its potential biomedical applications in optogenetics, microsurgery, phototherapy, and deep-tissue imaging. However, fast physiological motions in a live animal, including blood flow and respiratory motions, produce undesired photon perturbation and thus inevitably deteriorate the performance of light focusing. Here, we develop a photon-frequency-shifting DOPC method to fight against fast physiological motions by switching the states of a guide star at a distinctive frequency. Therefore, the photons tagged by the guide star are well detected at the specific frequency, separating them from the photons perturbed by fast motions. Light focusing was demonstrated in both phantoms in vitro and mice in vivo with substantially improved focusing contrast. This work puts a new perspective on light focusing inside live tissue and promises wide biomedical applications

Government Control, Uncertainty, and Investment Decisions in China's Listed Companies

AbstractThis paper empirically investigates the relation between uncertainty and investment among China's listed companies, and analyzes the influence of government control on the investment-uncertainty relation. We find that there is a negative relation between total firm uncertainty and investment in China's listed companies. However, this holds only for privately controlled firms. Among privately controlled listed firms, investment is negatively related to firm-specific uncertainty, whereas among government-controlled ones, investment is positively related to market uncertainty. We also find that the risk-taking preference of government-controlled listed companies is greater among those firms with fewer investment opportunities. Finally, among financially distressed firms, the negative relation between investment and uncertainty becomes nonsignificant because of risk shifting, which is more serious among government-controlled listed companies. We conclude that government control leads to state interference and weak corporate governance, which, in turn, distorts investment decision making among listed companies