• Proceedings of the Korean Nuclear Society Conference
• /
• 2004.10a
• /
• pp.255-256
• /
• 2004

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.18 no.11
• /
• pp.2986-2998
• /
• 1994

• Proceedings of the SAREK Conference
• /
• 2004.11a
• /
• pp.12-12
• /
• 2004

• Proceedings of the SAREK Conference
• /
• 2003.07a
• /
• pp.9-9
• /
• 2003

• Proceedings of the Korean Nuclear Society Conference
• /
• 2005.05a
• /
• pp.687-688
• /
• 2005

• Proceedings of the KSME Conference
• /
• 2007.05b
• /
• pp.2873-2878
• /
• 2007

The mixing vanes attached to the spacer grid of rod bundles are used to improve the heat transfer in heat exchanger devices by controlling the characteristics of the flow structures and turbulence. In this study, velocity patterns induced by two types of mixing vane(split and swirl vane) are measured by the PIV technique to better understand how to effect on the cross and secondary vortex flow patterns in $5{\times}$ rod bundle simulating the fuel assembly of the nuclear reactor. A successful measurement of the lateral velocity patterns was conducted using a specially designed beam sheet generator and experimental loop at KAERI. As the result, we found that for the cross flow between subchannels, the split vane is more effective than the swirl vane, while for the secondary vortex flow in each subchannel, the swirl vane's one is larger and longer than split vane's one.

• KSII Transactions on Internet and Information Systems (TIIS)
• /
• v.6 no.7
• /
• pp.1777-1791
• /
• 2012

This paper considers a downlink cognitive radio (CR) network where one secondary user (SU) and one primary user (PU) share the same base station (BS). The spectrum of interest is divided into a set of independent, orthogonal subchannels. The communication of the PU is of high priority and the quality of service (QoS) is guaranteed by the minimum rate constraint. On the other hand, the communication of the SU is of low priority and the SU opportunistically accesses the subchannels that were previously discarded by the PU during power allocation. The BS assigns fractions ?? and 1 ?? of the total available transmit power to the PU and the SU respectively. Two power allocation schemes with opportunistic subchannel access are proposed, in which the optimal values of ??'s are also obtained. The objective of one scheme is to maximize the rate of the SU, and the objective of the other scheme is to maximize the sum rate of the SU and the PU, both under the PU minimum rate constraint and the total transmit power constraint. Extensive simulation results are obtained to verify the effectiveness of the proposed schemes.

• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.21 no.12
• /
• pp.3174-3182
• /
• 1996

In this paper, a new receiver is proposed for the detection of the OFDM(orthogonal frequency division multiplexing) signals in the time-selective multipath fading channel. For the optimal detection, we estimate the transmitted symbols from OFDM demultiplexing signal using the LMLE(linear masimum likelihold estimation) algorithm. Also, in this paper, the lowerbound for BER(bit error rate) using Taylor series approximation is provided. If the matched filter is used for the OFDM receiver in the time-selectivemultipath fading channel, it is known that the SER(symbol error rate) is always greater than $10^{-1}$, due to the cross-talk between adjacent channels. But, the proposed receiver provides of SER with 15dB SNR. Also, it is found that for the receiver implemented using the LMLE algorithm, the performance is shown to be not affected by the increase of th enumber of subchannel and channel path.

• KSII Transactions on Internet and Information Systems (TIIS)
• /
• v.9 no.1
• /
• pp.149-168
• /
• 2015

In cognitive radio networks (CRNs), hybrid overlay and underlay sharing transmission mode is an effective technique for improving the efficiency of radio spectrum. Unlike existing works in the literature, where only one secondary user (SU) uses overlay and underlay modes, the different transmission modes should be allocated to different SUs, according to their different quality of services (QoS), to achieve the maximal efficiency of radio spectrum. However, hybrid sharing mode allocation for heterogeneous services is still a challenge in CRNs. In this paper, we propose a new resource allocation method for hybrid sharing transmission mode of overlay and underlay (HySOU), to achieve more potential resources for SUs to access the spectrum without interfering with the primary users. We formulate the HySOU resource allocation as a mixed-integer programming problem to optimize the total system throughput, satisfying heterogeneous QoS. To decrease the algorithm complexity, we divide the problem into two sub-problems: subchannel allocation and power allocation. Cutset is used to achieve the optimal subchannel allocation, and the optimal power allocation is obtained by Lagrangian dual function decomposition and subgradient algorithm. Simulation results show that the proposed algorithm further improves spectrum utilization with a simultaneous fairness guarantee, and the achieved HySOU diversity gain is a satisfactory improvement.

• Journal of Communications and Networks
• /
• v.15 no.6
• /
• pp.589-600
• /
• 2013

In this paper, we propose a new feedback scheme called mode selection-based feedback by scheduling probability prediction (SPP-MF) for channel state feedback in OFDMA downlink system. We design the scheme such that it determines the more desirable feedback mode among selective feedback by scheduling probability prediction (SPP-SF) mode and bitmap feedback by scheduling probability prediction (SPP-BF) mode, by calculating and comparing the throughputs of the two modes. In both feedback modes, each user first calculates the scheduling probability of each subchannel (i.e., the probability that a user wins the scheduling competition for a subchannel) and then forms a feedback message based on the scheduling probability. Specifically, in the SPP-SF mode, each user reports the modulation and coding scheme (MCS) levels and indices of its best S subchannels in terms of the scheduling probability. In the SPP-BF mode, each user determines its scheduling probability threshold. Then, it forms a bitmap for the subchannels according to the scheduling probability threshold and sends the bitmap along with the threshold. Numerical results reveal that the proposed SPP-MF scheme achieves significant performance gain over the existing feedback schemes.