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Seoul, South Korea

Kim J.,Korea Polytechnic University | Shin Y.,Soongsil University | Ban T.W.,Korea Telecom | Schober R.,University of British Columbia
IEEE Transactions on Vehicular Technology

In this paper, we investigate the performance of opportunistic scheduling in uplink cognitive radio (CR) systems by taking into account the CR users' spectrum sensing reliability. In uplink CR systems, each secondary transmitter (CR user) has a spectrum sensing responsibility to protect the primary system, and the secondary receiver (base station) schedules the transmission opportunities for the CR users. We propose novel optimal and suboptimal scheduling schemes by simultaneously taking into account the spectrum sensing reliability and the data channel quality. Analytical performance results for the proposed suboptimal scheduling schemes show that the spectrum sensing reliability should be considered for scheduling to maximize the capacity of the secondary system. Moreover, we also analyze the achievable multiuser diversity (MUD) gain of one of the proposed suboptimal scheduling schemes, which is based on the linear combination of the sensing channel and the data channel qualities. We show that the MUD gain of uplink CR systems grows significantly slower than that of conventional multiuser systems, particularly if the quality of the sensing channel is poor. Analytical and simulation results confirm that the proposed optimal and suboptimal scheduling schemes taking into account the spectrum sensing reliability and the data channel quality yield significant performance gains compared with conventional opportunistic scheduling. © 2011 IEEE. Source

Lee S.,Seoul National University | Jeong J.,Samsung | Park T.,Korea Telecom | Lee J.-R.,Kangwon National University | Kim C.-K.,Seoul National University
Wireless Personal Communications

Wi-Fi has gained tremendous attention from the research community, yielding successful technological advancements. However, the data throughput efficiency (the ratio of application throughput to the maximum achievable physical data rate) degrades rapidly as the PHY data rate increases when using the current 802.11 medium access control (MAC) protocol. To address this MAC inefficiency, many protocols have been introduced and standardized. This paper describes and examines these state-of-the-art enhancements to MAC efficiency for the 802.11 standard, and proposes a CLACK (Cross-Layer ACK) method that tackles this issue in totally different manner to those previous schemes. The main idea is simple: When a receiver sends an ACK, it transmits the data using the ACK transmission opportunity, and avoids channel contention necessary for data transmissions. The receiver's short signature is piggybacked in the PHY instead of the MAC to acknowledge the packet reception. We have implemented CLACK using USRP toolkits and GNU Software Define Radio. Our implementation demonstrates the feasibility of our key techniques for both PHY andMAC design. Further, we use detailed simulation to evaluate CLACK in general wireless environments under different traffic loads and varying channel conditions. Our results show that CLACK gains up to 52% in terms of throughput, when compared to the basic 802.11 scheme, and up to 18% when compared to existing advanced 802.11e/n schemes. © Springer Science+Business Media New York 2013. Source

Kim S.M.,Korea Advanced Institute of Science and Technology | Choi W.,Korea Advanced Institute of Science and Technology | Ban T.W.,Korea Telecom | Sung D.K.,Korea Advanced Institute of Science and Technology
IEEE Transactions on Wireless Communications

Most analytical studies on hybrid automatic repeat request (HARQ) have been carried out in either slow fading or fast fading assumptions. However, since practical wireless channels typically exhibit a time-correlated property, we need to take into account the effects of time correlation in the design of HARQ. In this paper, we analyze the outage probability and delay-limited throughput of HARQ with rate adaptation in a time-correlated channel model. Then, we propose a rate adaptation scheme to maximize the delay-limited throughput and evaluate the performance of the proposed scheme under a delay constraint. Our analytical and numerical results show that the proposed rate adaptation scheme outperforms the conventional rate adaptation schemes. The proposed rate adaptation scheme is further simplified by using a Gaussian approximation on the effective channel power gain. The rate adaptation based on the proposed Gaussian approximation reduces the complexity and achieves a throughput gain comparable to that with the exact channel distributions. © 2011 IEEE. Source

Shin G.,Korea Telecom | Ahn J.,Seoul National University | Kim T.,Chung - Ang University
WEBIST 2011 - Proceedings of the 7th International Conference on Web Information Systems and Technologies

Television is currently in the throes of a paradigm shift. IPTV, which has already secured a subscriber base of 1.5 million individuals in South Korea, is evidencing rapid growth in the global market. The development of IPTV is also increasing the interactivity between users and the IT media. The existing interactivity between users and IT from the cognitive perspective is insufficient to describe the usage behavior and continuous use intentions of consumers, who evidence a broad diversity of sentiment. This paper begins by addressing the effects of IPTV users' trust and emotions on the basis of the perception of interactivity. According to the results of empirical research into the users of IPTV services, the perception of interactivity was directly related with trust. Users' trust bolsters users' emotion in a positive/negative aspect, and such emotion was verified to affect the intention continuously use IPTV. The results of the following research indicate that the trust and emotion of users should be considered from the MIS perspective in an attempt to build the intention to use IPTV. Additionally, from the executive perspective, cost-effectiveness should be considered to enhance positive emotions for user retention and eventually bolster intentions to continue to use IPTV. Source

Lee S.J.,Kangwon National University | Tcha Y.,Korea Telecom | Seo S.-Y.,Korea Telecom | Lee S.-C.,Korea Telecom
IEEE Transactions on Communications

To overcome the problem that the transmission rate of a multicast channel is limited by the user with the worst channel quality, which causes the total system throughput to be low, the use of both multicast and unicast channels for multicast service is considered in this letter. Unicast channels, in addition to a multicast channel, are allocated to users with signal-to-noise ratios (SNRs) lower than a predefined threshold in order to enable the increase of the transmission rate of the multicast channel. The used unicast channels consume additional radio resource. It is examined whether use of such unicast channels can raise the spectral efficiency defined as the total system throughput normalized by the total consumed bandwidth. The optimal value of the threshold SNR is derived by maximizing the spectral efficiency. When a unicast channel is additionally allocated for multicast service, two decoding schemes are considered: one is to use only the signal of a unicast channel, and the other is to use and combine both the signals of multicast and unicast channels. Numerical examples are presented to show the performance gains obtained by using unicast channels in addition to a multicast channel. © 2011 IEEE. Source

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