Throughput maximization in linear multiuser MIMO-OFDM downlink systems

In this paper, we study the problem of maximizing the throughput of a multiuser multiple-input-multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) system in the downlink with a total power constraint using a beamforming approach. An iterative algorithm that takes turns to optimize, jointly among users, the power allocation in the downlink, the transmit and the receive beamforming antenna vectors, and the power allocation in the virtual uplink is proposed. The algorithm is proved to converge, and the throughput increases from one iteration to the next. In addition to the total power constraint, the proposed algorithm is also capable of handling individual users' rate constraints. To reduce complexity, a geometric-programming-based power control in the high signal-to-interference-plus-noise ratio (SINR) region and an orthogonal frequency-division multiple-access scheme in the low SINR region are proposed. Numerical results illustrate that the proposed algorithm significantly outperforms the generalized zero-forcing (GZF) approach. © 2008 IEEE.published_or_final_versio

Energy-efficient multiuser SIMO: Achieving probabilistic robustness with Gaussian channel uncertainty

This paper addresses the joint optimization of power control and receive beamforming vectors for a multiuser singleinput multiple-output (SIMO) antenna system in the uplink in which mobile users are single-antenna transmitters and the base station receiver has multiple antennas. Channel state information at the receiver (CSIR) is exploited but the CSIR is imperfect with its uncertainty being modeled as a random Gaussian matrix. Our objective is to devise an energy-efficient solution to minimize the individual users' transmit power while meeting the users' signal-to-interference plus noise ratio (SINR) constraints, under the consideration of CSIR and its error characteristics. This is achieved by solving a sum-power minimization problem, subject to a collection of users' outage probability constraints on their target SINRs. Regarding the signal power minus the sum of inter-user interferences (SMI) power as Gaussian, an iterative and convergent algorithm which is proved to reach the global optimum for the joint power allocation and receive beamforming solution, is proposed, though the optimization problem is indeed non-convex. A systematic scheme to detect feasibility and find a feasible initial solution, if there exists any, is also devised. Simulation results verify the use of Gaussian approximation and robustness of the proposed algorithm in terms of users' probability constraints, and indicate a significant performance gain as compared to the zero-forcing (ZF) and minimum meansquare-error (MMSE) beamforming systems. © 2009 IEEE.published_or_final_versio

Robust precoder design in MISO downlink based on quadratic channel estimation

In [1], it has been proposed that channel estimates in quadratic form can be obtained at the base station by sending training sequences to the mobiles where the received signals are forwarded back to the base for channel estimation. In this paper, we first examine the optimal training sequence design for such quadratic channel estimation and then analyze the error bound and statistics of the channel estimates in quadratic form. With the analytical results, two problems for a multiple-input single-output (MISO) antenna system in the downlink are constructed and optimally solved: Power minimization with individual users' 1) worst-case signal-to-interference plus noise ratio (SINR) and 2) average mean-square-error (MSE) constraints, through optimal multiuser MISO beamforming and power allocation. ©2008 IEEE.published_or_final_versio

Robust beamforming in cognitive radio

In cognitive radio, it is crucial to control the interference from secondary users (SUs) to primary users (PUs). This paper studies the use of transmit beamforming in the cognitive secondary network for enhancing the performance of a SU while controlling the interference to the PUs. In particular, we propose to maximize the service probability of the SU with a number of probability constraints on the interference level at the PUs with the aid of imperfect channel state information (CSI). Modeling the CSI uncertainty as an additive Gaussian noise, it is shown that the optimum can be realized by second-order cone-programming (SOCP) in tandem with a one-dimensional search. Results reveal that the proposed approach provides a technique to tradeoff the performance between the PUs and the SU, making an analytical connection between non-robust and worst-case systems. © 2009 IEEE.published_or_final_versionThe 69th IEEE Vehicular Technology Conference (VTC Spring 2009), Barcelona, Spain, 26-29 April 2009. In Proceedings of the 69th IEEE - VTS, Spring 2009, p. 1-

Robust beamforming in cognitive radio

This letter considers the multi-antenna cognitive radio (CR) network, which has a single secondary user (SU) and coexists with a primary network of multiple users. Our objective is to maximize the service probability of the SU, subject to the interference constraints on the primary users (PUs) in the form of probability. Exploiting imperfect channel state information (CSI), with its error modeled by added Gaussian noise, we address the optimization for the beamforming weights at the secondary transmitter. In particular, this letter devises an iterative algorithm that can efficiently obtain the robust optimal beamforming solution. For the case with one PU, we show that a much simpler algorithm based on a closed-form solution for the antenna weights of a given power can be presented. Numerical results reveal that the optimal solution for the constructed problem provides an effective means to tradeoff the performance between the PUs and the SU, bridging the non-robust and worstcase based systems. © 2010 IEEE.published_or_final_versio

The Mechanical Behavior of a 25Cr Super Duplex Stainless Steel at Elevated Temperature

Peer reviewedPostprin

The Minimal Solution to the mu/B_mu Problem in Gauge Mediation

We provide a minimal solution to the mu/B_mu problem in the gauge mediated supersymmetry breaking by introducing a Standard Model singlet filed S with a mass around the messenger scale which couples to the Higgs and messenger fields. This singlet is nearly supersymmetric and acquires a relatively small Vacuum Expectation Value (VEV) from its radiatively generated tadpole term. Consequently, both mu and B_mu parameters receive the tree-level and one-loop contributions, which are comparable due to the small S VEV. Because there exists a proper cancellation in such two kinds of contributions to B_mu, we can have a viable Higgs sector for electroweak symmetry breaking.Comment: 15 pages, 2 figures, version published on JHE

Three-Dimensional Microscopy Characterization of Death Receptor 5 Expression by Over-Activated Human Primary CD4+ T Cells and Apoptosis

Activation-induced cell death is a natural process that prevents tissue damages from over-activated immune cells. TNF-Related apoptosis ligand (TRAIL), a TNF family member, induces apoptosis of infected and tumor cells by binding to one of its two death receptors, DR4 or DR5. TRAIL was reported to be secreted by phytohemagglutinin (PHA)-stimulated CD4+ T cells in microvesicles