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Lee S.-R.,Korea University | Kim J.-S.,Bell Labs Seoul Ltd. | Moon S.-H.,Electronics and Telecommunications Research Institute | Kong H.-B.,Korea University | Lee I.,Korea University
IEEE Transactions on Wireless Communications | Year: 2013

In this paper, we analyze the average sum rate of downlink multi-antenna systems with zero-forcing beamforming (ZFBF). In practical implementations, each antenna is equipped with its own power amplifier and is limited individually by linearity of the amplifier. Thus, this paper adopts a more realistic per-antenna power constraint instead of conventional sum-power constraint on transmit antennas. To this end, we first show that a distribution of the received signal-to-noise ratio (SNR) of the ZFBF scheme with per-antenna power constraint and equal-rate metric can be approximated as a minimum of chi-square random variables. Based on this result, we present an accurate formula of the average sum rate in a closed form. Furthermore, employing extreme value theory, an expression of the asymptotic average sum rate with large numbers of transmit antennas and users is derived from the limiting distribution of the received SNR. Simulation results verify the validity of our analysis even with not so large numbers of transmit antennas and users. © 2013 IEEE.

Kim J.-S.,Bell Labs Seoul Ltd. | Moon S.-H.,Electronics and Telecommunications Research Institute | Lee I.,Korea University
IEEE Transactions on Wireless Communications | Year: 2012

In this paper, we propose a new coordinated spatial multiplexing algorithm for multiuser multiple-input multiple-output (MIMO) downlink systems. Unlike other conventional coordination methods, our scheme exploits orthogonalized effective channels to yield the transmit precoding and receive combining matrices without requiring an iterative operation. In order to determine each row of the effective channels, a greedy type successive process is utilized. Simulation results show that the performance of our proposed scheme is very close to the conventional iterative scheme with significantly reduced complexity. Also, our method achieves a 3 dB performance gain over the conventional non-iterative scheme. © 2012 IEEE.

Kim J.-S.,Bell Labs Seoul Ltd | Lee K.-J.,University of Texas at Austin | Park H.,Korea University | Lee I.,Korea University
IEEE Transactions on Signal Processing | Year: 2012

In this paper, we study $K$ -user multiple-input multiple-output (MIMO) interference channels (IC) in the presence of antenna correlations. Although interference alignment (IA) achieves the maximum multiplexing gain, i.e., degrees of freedom (DOF), the actual performance is degraded due to ill-conditioned channels caused by the antenna correlations. To enhance the overall system performance, we thus consider a transmission mode selection scheme which adaptively determines the number of data streams at each transmitter-receiver pair in a distributed fashion using local channel state information (CSI). A filter update process is exploited in order to estimate the performance regarding each transmission mode successively by considering the actual channel conditions as well as signal-to-noise ratio (SNR). Simulation results show that our mode selection scheme enhances the sum rate performance compared to the conventional full data stream transmissions in spatially correlated MIMO IC. © 2012 IEEE.

Park Y.M.,Hanyang University | Nam K.W.,Hanyang University | Ahn J.H.,Bell Labs Seoul Ltd. | Jang D.P.,Hanyang University | Kim I.Y.,Hanyang University
Journal of Mechanics in Medicine and Biology | Year: 2015

When an injury due to blast overpressure (BOP) is generated, it is important to estimate the severity of the injury using information about the blast conditions and to supply proper treatments according to the degree of the damage. However, there have been no investigations that have tried to verify the relationship between the blast-related objective indices and the degree of blast-induced injury. In this study, the correlations between the survival rate of the subjects with BOP-induced lung damage and each of four blast-induced indices, first principal strain, first principal strain rate, first principal stress and pulmonary inner pressure, were investigated using a simplified thorax model by introducing the concept of the VACC-VLUNG ratio graph which represents the volume ratio between the seriously-damaged meshes and the overall meshes of the thorax model in respect to each index. Experimental results demonstrated that the decay parameters of the sigmoidal curve-fitted graphs of the first principal stress are the most effective of the analyzed indices for the estimation of the survival rate in patients with blast-induced lung damage. The results have a potential clinical application to improve the efficacy of treatment for blast injury patients. © 2015 World Scientific Publishing Company.

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