Multiplicative Noise-Induced Electrohydrodynamic Pattern Formations by Controlling Electric Conductivity

We report multiplicative noise impacts on electroconvections (ECs) in a nematic liquid crystal. By controlling the intensity and cutoff frequency of a superposed electric noise (on a sinusoidal field for ECs), we investigate the variation in the characteristics of ECs such as thresholds and pattern diagrams in high-conductivity cells (σ ∼ 10−6 Ω−1 m−1), in comparison with that in usual conventional cells (σ ∼ 10−8 − 10−7 Ω−1 m−1). Unpredictable threshold behaviors, unknown pattern formations such as isotropic liquid bubbles and EC-sustained phases, and undesirable dielectric breakdown induced by thermal focusing are found in high-conductivity cells

Achieving "Massive MIMO" Spectral Efficiency with a Not-so-Large Number of Antennas

The main focus and contribution of this paper is a novel network-MIMO TDD architecture that achieves spectral efficiencies comparable with "Massive MIMO", with one order of magnitude fewer antennas per active user per cell. The proposed architecture is based on a family of network-MIMO schemes defined by small clusters of cooperating base stations, zero-forcing multiuser MIMO precoding with suitable inter-cluster interference constraints, uplink pilot signals reuse across cells, and frequency reuse. The key idea consists of partitioning the users population into geographically determined "bins", such that all users in the same bin are statistically equivalent, and use the optimal network-MIMO architecture in the family for each bin. A scheduler takes care of serving the different bins on the time-frequency slots, in order to maximize a desired network utility function that captures some desired notion of fairness. This results in a mixed-mode network-MIMO architecture, where different schemes, each of which is optimized for the served user bin, are multiplexed in time-frequency. In order to carry out the performance analysis and the optimization of the proposed architecture in a clean and computationally efficient way, we consider the large-system regime where the number of users, the number of antennas, and the channel coherence block length go to infinity with fixed ratios. The performance predicted by the large-system asymptotic analysis matches very well the finite-dimensional simulations. Overall, the system spectral efficiency obtained by the proposed architecture is similar to that achieved by "Massive MIMO", with a 10-fold reduction in the number of antennas at the base stations (roughly, from 500 to 50 antennas).Comment: Full version with appendice (proofs of theorems). A shortened version without appendice was submitted to IEEE Trans. on Wireless Commun. Appendix B was revised after submissio

Electroconvection in nematic liquid crystals in Hele-Shaw cells

We report electrohydrodynamic instability in nematic liquid crystals found in Hele-Shaw cells. Due to the present cell geometry, the convective structures could be directly visualized as surface or bulk flows. An unexpected structure is observed, which is completely different from the well-known patterns in the standard cells. By using the voltage-frequency jump method, the stability of a convective structure in Hele-Shaw cells is discussed in terms of the Busse diagram

Colored noise-induced threshold shifts and phase diagrams in electroconvections

We report noise-induced threshold shifts and phase diagrams in electroconvections (ECs) in a nematic liquid crystal by controlling the frequency band (i.e., cutoff frequency fc) of noise. A crucial relationship between the internal characteristic time (i.e., charge relaxation time ¸·) of the EC system and the correlation time [¸N = 1/(2³fc)] of the external colored noise is found, which determines the role of noise in the nonequilibrium EC system. Modified numerical results using the relationship are quantitatively compared with the present experimental results. From the relationship, two types of phase diagrams can be classified in white-like and colored noises

Exchange rate regimes and international business cycle transmission revisited

노트 : A paper prepared for the conference on 'Korea and the World Economy', 21-22 July 2002, Seoul, South Korea

Observation and determination of abnormal rolls and abnormal zigzag rolls in electroconvection in homeotropic liquid crystals

Direct evidence for two different types of normal rolls and of zigzag rolls in homeotropically aligned nematic liquid crystals in a magnetic field is reported. The conventional normal rolls have the reflection symmetry in the xy plane. The instability, however, breaks the reflection symmetry y→-y on the director and then the abnormal rolls are expected to be observed. We have investigated the instability experimentally and discussed it in terms of the recent numerical results by Plaut et al. [Phys. Rev. Lett. 79, 2367 (1997)]. Due to the new instability, the abnormal zigzag rolls are also found below the Lifshitz frequency

Multiuser MISO Transmitter Optimization for Inter-Cell Interference Mitigation

The transmitter optimization (i.e., steering vectors and power allocation) for a MISO Broadcast Channel (MISO-BC) subject to general linear constraints is considered. Such constraints include, as special cases, the sum power, the per-antenna or per-group-of-antennas power, and "forbidden interference direction" constraints. We consider both the optimal dirty-paper coding and the simple suboptimal linear zero-forcing beamforming strategies, and provide numerically efficient algorithms that solve the problem in its most general form. As an application, we consider a multi-cell scenario with partial cell cooperation, where each cell optimizes its precoder by taking into account interference constraints on specific users in adjacent cells. The effectiveness of the proposed methods is evaluated in a simple system scenario including two adjacent cells, under different fairness criteria that emphasize the bottleneck role of users near the cell "boundary". Our results show that "active" Inter-Cell Interference (ICI) mitigation outperforms the conventional "static" ICI mitigation based on fractional frequency reuse.Comment: 30 pages, 10 figures, and 1 table. revised and resubmitted to IEEE Transactions on Signal Processin