Branching Random Walks in Time Inhomogeneous Environments

We study the maximal displacement of branching random walks in a class of time inhomogeneous environments. Specifically, binary branching random walks with Gaussian increments will be considered, where the variances of the increments change over time macroscopically. We find the asymptotics of the maximum up to an $O_P(1)$ (stochastically bounded) error, and focus on the following phenomena: the profile of the variance matters, both to the leading (velocity) term and to the logarithmic correction term, and the latter exhibits a phase transition

Beam Management for Millimeter Wave Beamspace MU-MIMO Systems

Millimeter wave (mmWave) communication has attracted increasing attention as a promising technology for 5G networks. One of the key architectural features of mmWave is the use of massive antenna arrays at both the transmitter and the receiver sides. Therefore, by employing directional beamforming (BF), both mmWave base stations (MBSs) and mmWave users (MUEs) are capable of supporting multi-beam simultaneous transmissions. However, most researches have only considered a single beam, which means that they do not make full potential of mmWave. In this context, in order to improve the performance of short-range indoor mmWave networks with multiple reflections, we investigate the challenges and potential solutions of downlink multi-user multi-beam transmission, which can be described as a high-dimensional (i.e., beamspace) multi-user multiple-input multiple-output (MU-MIMO) technique, including multi-user BF training, simultaneous users' grouping, and multi-user multibeam power allocation. Furthermore, we present the theoretical and numerical results to demonstrate that beamspace MU-MIMO compared with single beam transmission can largely improve the rate performance of mmWave systems.Comment: The sixth IEEE/CIC International Conference on Communications in China (ICCC2017

Annihilation of vortex-antivortex pairs in a superconducting shield

A new type of magnetic shielding device has been constructed for use in superconducting microelectronic applications where low ambient magnetic field is desirable. The shield was a specially designed superconducting thin film in the shape of a long cylindri- cal tube with a low pinning material in the central region and high flux pinning regions at the end. It was shown that magnetic flux perpendicular to the axis of a cylinder could be swept out of the interior of the shield by moving vortex-antivortex pairs to one side of the shield where they annihilate. The flux sweeping properties of the shield were found to work well in a small temperature window close to the transition temperature where the critical current for depinning vortices was less than the critical current for the forma- tion of hot spots. Once the vortices are swept out, the shield can be cooled to enhance the Meissner shielding effects against other stray fields. In the course of designing and testing the shield, a systematic;study of flux pinning was made for the low magnetic field regime between 1 mOe and 30 Oe; *DOE Report IS-T-1276. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy