Generalized Signal Alignment For MIMO Two-Way X Relay Channels

We study the degrees of freedom (DoF) of MIMO two-way X relay channels. Previous work studied the case $N < 2M$, where $N$ and $M$ denote the number of antennas at the relay and each source, respectively, and showed that the maximum DoF of $2N$ is achievable when $N \leq \lfloor\frac{8M}{5}\rfloor$ by applying signal alignment (SA) for network coding and interference cancelation. This work considers the case $N>2M$ where the performance is limited by the number of antennas at each source node and conventional SA is not feasible. We propose a \textit{generalized signal alignment} (GSA) based transmission scheme. The key is to let the signals to be exchanged between every source node align in a transformed subspace, rather than the direct subspace, at the relay so as to form network-coded signals. This is realized by jointly designing the precoding matrices at all source nodes and the processing matrix at the relay. Moreover, the aligned subspaces are orthogonal to each other. By applying the GSA, we show that the DoF upper bound $4M$ is achievable when $M \leq \lfloor\frac{2N}{5}\rfloor$ ($M$ is even) or $M \leq \lfloor\frac{2N-1}{5}\rfloor$ ($M$ is odd). Numerical results also demonstrate that our proposed transmission scheme is feasible and effective.Comment: 6 pages, 6 figures, to appear in IEEE ICC 201

Numerical Study On Reinforcement Mechanism Of Copper/carbon Nanotubes Composite

Because of its high stiffness, carbon nanotubes (CNTs) are considered as one of the widely used reinforcement materials in the metal matrix composites. In this thesis, finite element (FE) models were built in Ls-Dyna3D to simulate Copper/CNTs composite deformation and fracture, and to explore CNTs reinforcement mechanisms. Several possible mechanisms were discussed. Deformation and failure of Cu/CNT composites were studied numerically using unit cell FE models, which consist of both metal matrix and CNTs. The simulation results have been verified by existing experiment data reported by Chen’s group. The matrix material was modeled as elasto-plastic ductile solids. The CNTs material properties were taken from literature results using molecular dynamics simulation. FE simulations have showed that CNTs deformation exceeds material elastic limit, which means that CNTs plasticity should be taken into account as well. 2D unit cell models were developed using axial symmetric elements with suitable boundary conditions. Several mechanisms are found to affect CNTs reinforcement prediction. The first one is the boundary condition imposed in the models. The CNTs significantly affect the plastic flow of copper during plastic deformation, which is one important reinforcement mechanism. The second reinforcement mechanism is found to be the hardening zone of Cu matrix around CNTs, which is introduced by mismatch of coefficient of thermal expansion (CTE). A round of parametric studies was performed to investigate the effects of several modeling parameters in the FE simulations; these parameters include the volume fraction of CNTs, aspect ratio of CNTs, the size of hardening zone, and the residual plastic strain in the zone. iv A tool combining Matlab and Ls-Dyna was developed to automatically build 2D unit cell models and automatically post-process simulation results. Picking up suitable parameters, 2D unit cell model results well predict the experimental results from Chen’s group. It should be noted that the interface between Cu and CNTs was assumed to be perfect in FE simulations since no CNTs debonding was observed in the experiments. Also, a 3D unit cell model using tetrahedral elements (with element numbers up to one million) was tentatively developed to obtain more accurate results. The purpose was to explore the interface properties of Cu/CNTs, the effect of CNTs orientation distribution, and the other reinforcement mechanism coming from geometry necessary dislocation (GND) since the size of Cu matrix is divided into nano scales by CNTs. 3D unit models are also used to verify the 2D unit cell one, which is a simplified and effective approach. Very interesting results was observed in this part of study. Further works are needed to overcome the difficulties in 3D modeling and the limitation of current CPU spee

Two Essays on CEO Overconfidence in Relation to Speed of Adjustment of Firm Financial Policy and CEO Inside Debt

This dissertation is a thorough examination of CEO overconfidence, and consists of two essays. The first essay focuses on the relationship between CEO overconfidence and the adjustment speed of firm financial policy. No research has examined the relationship between CEO overconfidence and firm financial policies adjustment speed. Previous studies focus solely on the adjustment speed of leverage, we are motivated to examine the adjustment of firm leverage and the adjustment of cash holdings together because there is evidence that firm leverage and firm liquidity are related. We find that CEO overconfidence places an important role in adjusting firm leverage and cash. Specifically, overconfident CEOs speed up (slow down) the adjustment of firm leverage when it is above (below) target leverage. In addition, overconfident CEOs speed up (slow down) the adjustment of firm cash holding when it is below (above) the estimated target. Consistent with the prediction of Acharya et al. (2007), our analysis suggests that cash and reduced leverage serve different purposes in the eyes of overconfident CEOs. Specifically, we find evidence that overconfident CEOs of financially constrained firms that have high (low) hedging needs hoard cash (reduce debt). Although extensive research has been carried out to find the optimal inside debt ratio, no study has examined the association between deviations from the optimal inside debt ratio and the firm’s risk taking behavior. The second essay is to fill out the gap, try to find out what is the effect of Inside debt deviation on risk-taking activities of firms with overconfident CEOs. The second essay is the first paper to examine the effects of deviations of inside debt and CEO overconfidence on firm value. Our results show that positive deviations of inside debt mitigate the risk taking behaviors of firms with overconfident CEOs. We find that CEO overconfidence is negatively related to firm leverage. This result indicates that overconfident CEOs will decrease firm leverage under both positive and negative deviations in inside debt. However, we do find that the amount of the decrease is smaller when the inside debt deviation is negative. we find that overconfident CEOs tend to lower firm cash levels. We also find that if a CEO’s inside debt is above the target level, the amount of the decrease is smaller. However, the results are nonsignificant. We confirm the positive relation between deviations from the target inside debt level and firm value. Our results also show that CEO overconfidence has a positive effect on firm value. This positive effect is more significant for positive deviations of inside debt than for negative deviations