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Haewoon Nam,Saeed, Nasir,Ben-Ghorbel, Mand,Alouini, Mohamed Slim IET 2015 IET COMMUNICATIONS Vol.9 No.8
<P>In cognitive radio networks, secondary users (SUs) can share spectrum with primary users (PUs) under the condition that no interference is caused to the PUs. To evaluate the interference imposed to the PUs, the cognitive systems discussed in the literature usually assume that the channel state information (CSI) of the link from a secondary transmitter to a primary receiver (interference link) is known at the secondary transmitter. However, this assumption may often be impractical in cognitive radio systems, since the PUs need to be oblivious to the presence of the SUs. The authors first discuss PU localisation and then introduce an uplink resource allocation algorithm for orthogonal frequency division multiple access-based cognitive radio systems, where relative location information between primary and SUs is used instead of CSI of the interference link to estimate the interference. Numerical and simulation results show that it is indeed effective to use location information as a part of resource allocation and thus a near-optimal capacity is achieved.</P>
Performance Analysis of OT-MRC Over I.I.D. Nakagami and Non-I.I.D. Rayleigh Fading Channels
Nam, Haewoon,Ko, Young-Chai,Womack, Baxter F. IEEE 2006 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY Vol.55 No.6
<P>This paper presents the performance analysis of a conditional diversity combining scheme, which is called output-threshold maximum ratio combining (OT-MRC), over the independent identically distributed Nakagami-m with integer m and independent but not necessarily identically distributed Rayleigh fading channels. Based on the closed-form expressions for the statistics, the authors analyze the performance of the OT-MRC systems and compare it with that of the conventional MRC systems. They exploit the tradeoff between complexity and performance in this scheme over the Nakagami fading channels and see the effect of unbalance of the fading gains over multiple diversity branches on the performance of OT-MRC. They show, from some numerical examples, that the OT-MRC scheme, when compared with the MRC scheme, can reduce the complexity significantly without losing the required performance</P>
Performance analysis of joint switched diversity and adaptive modulation
Haewoon Nam,Young-chai Ko,Alouini, Mohamed-slim IEEE 2008 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS Vol.7 No.10
<P>In this paper, a simple and practical system based on a switched diversity scheme with adaptive modulation is presented. This system provides a reduced number of channel estimation while offering the optimum spectral efficiency given by a selection diversity system. In addition, the switching threshold is easily manipulated so as to make an efficient use of the tradeoff between spectral efficiency and channel estimation overhead. An extension of switched diversity into a multiuser scheduling is later also considered. This switch-based multiuser access scheme results in a lower average feedback load than that for the optimal selection-based multiuser scheme. Numerical results show that we can obtain a trade-off between spectral efficiency and the feedback load by choosing the switching threshold appropriately.</P>
Energy Detection Scheme in the Presence of Burst Signals
Oh, Hyungkook,Nam, Haewoon IEEE Signal Processing Society 2019 IEEE signal processing letters Vol.26 No.4
<P>This letter proposes an energy detection scheme based on short windows to detect the presence of a burst signal that spans over a short period of time. For practical use, the proposed scheme provides a simple computational method for the threshold, with which the measured energy is compared, that satisfies a target false alarm probability and also the optimal length of the short window that minimizes miss detection probability. To evaluate the performance of the proposed scheme, miss detection probability is analyzed as a function of the threshold. Simulation results demonstrate that the proposed scheme provides a lower miss detection probability than the conventional scheme when a burst signal is present.</P>
Ali, Mohsin,Nam, Haewoon IET 2017 IET COMMUNICATIONS Vol.11 No.16
<P>This study presents the trade-off between probability of spectrum hole utilisation (PSHU) and sensing time duration of secondary user (SU) in cognitive radio networks. On the basis of the analytical formulations of PSHU, it is shown that there exist an optimal sensing time duration and the optimal length of time frame for a given mean idle time of primary user (PU). Unlike the existing works, a frame structure of SUs with a fixed length of time frames is considered. The effect of both sensing and transmission time of SUs over PSHU is considered simultaneously. As increasing sensing time duration to improve the probability of detection for PU's presence or absence leads to a reduction in transmission time, which may further decrease PSHU. The numerical and simulation results show that using the optimal sensing time duration (which leads toward the optimal reduction in transmission time) and the optimal length of time frame provides the maximum PSHU.</P>
Design and Performance Analysis of Nonlinearity Preprocessors in an Impulsive Noise Environment
Oh, Hyungkook,Nam, Haewoon IEEE 2017 IEEE Transactions on Vehicular Technology VT Vol.66 No.1
<P>This paper discusses a practical design of nonlinearity preprocessors to be used in a receiver for mitigating performance degradation in an impulsive noise environment. A simple method is proposed for a blanker and a soft limiter to calculate a blanking and a clipping threshold, respectively. In addition, to evaluate the bit error performance of a receiver with the proposed nonlinearity preprocessors, this paper introduces an approach using a periodic pulse train function and Fourier series. When a nonlinearity preprocessor is used, since the output samples show a truncated probability density function, it is generally difficult to evaluate the bit error performance of a receiver. Analytical and simulation results show that the thresholds computed by the proposed method are near optimal in terms of efficacy function and that the error performance of the proposed nonlinearity preprocessors matches well with that of the ideal design of nonlinearity preprocessors with optimal thresholds.</P>
Energy Efficient Localization Algorithm With Improved Accuracy in Cognitive Radio Networks
Saeed, Nasir,Haewoon Nam IEEE 2017 IEEE communications letters Vol.21 No.9
<P>An energy efficient localization algorithm with high accuracy for the cognitive radio networks (CRNs) is proposed in this letter. The key idea underlying this proposed algorithm is to provide the optimal transmission range for secondary users (SUs) to minimize the power consumption of the overall CRNs. Since energy efficiency is a key factor to consider in designing a node in the CRNs, location information of primary users (PUs) and SUs is valuable to calculate the optimal transmission range so that a spectrally efficient CRN can opportunistically take advantage of the spectrum with no or little interference to the PUs. In addition, the optimal positions of the SUs with known location are determined to improve the final localization accuracy for the whole CRN, which enables the optimal transmission range to be obtained. Thereby, the users in the CRN are localized with minimum energy consumption for a given target root mean square error. Simulation results validate the effectiveness of the proposed algorithm by showing the optimal transmission range for various number of users in the network.</P>