• Journal of Communications and Networks
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• 제14권4호
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• pp.374-384
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• 2012

Utilizing artificial noise (AN) is a good means to guarantee security against eavesdropping in a multi-inputmulti-output system, where the AN is designed to lie in the null space of the legitimate receiver's channel direction information (CDI). However, imperfect CDI will lead to noise leakage at the legitimate receiver and cause significant loss in the achievable secrecy rate. In this paper, we consider a delayed feedback system, and investigate the impact of delayed CDI on security by using a transmit beamforming and AN scheme. By exploiting the Gauss-Markov fading spectrum to model the feedback delay, we derive a closed-form expression of the upper bound on the secrecy rate loss, where $N_t$ = 2. For a moderate number of antennas where $N_t$ > 2, two special cases, based on the first-order statistics of the noise leakage and large number theory, are explored to approximate the respective upper bounds. In addition, to maintain a constant signal-to-interferenceplus-noise ratio degradation, we analyze the corresponding delay constraint. Furthermore, based on the obtained closed-form expression of the lower bound on the achievable secrecy rate, we investigate an optimal power allocation strategy between the information signal and the AN. The analytical and numerical results obtained based on first-order statistics can be regarded as a good approximation of the capacity that can be achieved at the legitimate receiver with a certain number of antennas, $N_t$. In addition, for a given delay, we show that optimal power allocation is not sensitive to the number of antennas in a high signal-to-noise ratio regime. The simulation results further indicate that the achievable secrecy rate with optimal power allocation can be improved significantly as compared to that with fixed power allocation. In addition, as the delay increases, the ratio of power allocated to the AN should be decreased to reduce the secrecy rate degradation.

• The Korean Journal of Pain
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• 제1권1호
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• pp.53-58
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• 1988

암으로 인하여 상하복부 및 하지에 심한 통증을 호소하는 환자에게 지주막하강 및 경막외강에 각각 미량의 몰핀을 지속적으로 주입하여 진통효과를 얻은 바 진통효과 발현시간은 지주막 하강 투여군이 빨랐고 지속시간 별로는 큰 차이가 없었고 제통효과 면에서는 지주막하강 투여 군이 더욱 확실하였고, 지속적 몰핀 주입에 의한 내성발현 정도는 경악의장 투여군이 심하였고 합병증 발현정도로는 경막외강 투여군이 경미하였음을 봐서 내성발생문제를 앞으로 해결한 다면 장기간 몰핀 등 마약성 약물을 투여해야되는 암성통증환자에서는 경막외강 투여법이 지주막하강 투여법보다 안전한 제통방법으로 사료된다.