Seokwon Lee

Energy-efficient opportunistic spectrum access in cognitive radio networks with energy harvesting

In cognitive radio networks, secondary user performs spectrum sensing to detect primary user activities and to access the channel opportunistically. We investigate the problem of developing energy efficient opportunistic spectrum access strategy for a secondary user with energy harvesting capability. We formulate the problem to determine the optimal sensing and access policy as a partially observable Markov decision process (POMDP). Since obtaining the optimal policy is computationally prohibitive, we provide a suboptimal myopic policy that maximizes expected immediate throughput instead of expected total throughput. Simulation results demonstrate that the proposed myopic policy yields improved throughput compared to a random channel selection strategy.

Effects of Deposit Insurance Reform on Moral Hazard in US Banking

In cross sections of US banks before the deposit-insurance system was reformed in the early 1990s, bank risk-taking was positively associated with bank size and negatively associated with the value of bank charters and bank capital. These empirical associations have an easy theoretical interpretation. Bank size is positively related, while charter value and capital are negatively related, to the moral hazard associated with flat insurance premiums and other aspects of a laxly administered system. Hence the observed associations of risk-taking with size, charter value, and capital reflected the expected positive relation between moral hazard and risk-taking. We test the hypothesis that the three associations became weaker after reform. In the case of unsystematic risk, we find no evidence of significant changes for any of the three. In the case of systematic risk, we find that risk-taking associated with lower charter values and larger size is indeed significantly weaker after reform. Risk-taking associated with capital ratios is also weaker after reform, though not significantly so. Since systematic risk is undoubtedly the more appropriate measure, reform seems to have reduced moral hazard. Copyright Blackwell Publishers Ltd 2001.

Goodness-of-Fit-Based Malicious User Detection in Cooperative Spectrum Sensing

Cooperative spectrum sensing improves sensing accuracy in primary user detection, but can be threatened by malicious users. Malicious users may try to falsify the sensing result to indicate that the primary user exists even when there is no primary user in order to monopolize the spectrum usage, thereby depriving other users of their spectrum opportunities. To address this, we propose a malicious user detection scheme where the malicious users are identified and cut off from the cooperative sensing process. The proposed scheme exploits the Anderson-Darling (AD) goodness-of-fit technique which tests whether the empirical distribution of the sensing data from each secondary user fits the expected distribution for a malicious user. In addition, we derive false alarm and detection probabilities for when malicious users are cut off by the malicious user detection scheme. Simulation results show that the proposed goodness-of-fit-based malicious user detection significantly improves sensing performance in comparison with conventional outlier detection-based schemes.

Tag detection algorithm for improving the instability problem of an augmented reality

Detection technology is a requirement for an Augmented Reality system. One of the problems with detection technology is the instability problem, which occurs when an obstacle occludes a tag while detecting the tag, and the augmented object suddenly disappears. We have proposed a corner detection algorithm to solve this instability problem. The key feature is that if the tag can recognize its position using its four corner cells despite the obstacle being present, then it can maintain its augmented object. We defined the corner case for all types of cases where the instability problem occurs in ARToolkit or ARTag. We have adapted our proposed algorithm to the corner case in ARToolkit, ARTag and ColorCode vision systems and have compared their false detection rates.

A new frame structure for asynchronous in-band full-duplex systems

In in-band full-duplex employing conventional frame structure, pilot contamination is inevitable due to asyn-chronism between the nodes. Asynchronism can occur for several reasons such as propagation delay and synchronization error. Pilot contamination increases channel estimation error for both the desired and self-interference channels. We propose a new frame structure that avoids interference during pilot transmission. The proposed frame structure utilizes maximum time offset information in order to cover all the possible time offsets. We then derive the mean square error (MSE) of the channel estimation when the proposed frame structure is employed. Furthermore, based on the MSE analysis, we deliver superior conditions for the proposed frame structure compared to the conventional one. Finally, we show that sacrificing pilot length to avoid interference during pilot transmission guarantees channel estimation performance when the power of the interference is large.

Energy-efficient spectram access for ultra low power sensor networks

This paper investigates a dynamic spectrum access (DSA) scheme for ultra low power sensor networks (ULPSN) in an open spectrum where multiple systems coexist and interfere with each other. Low transmission power and simple communication protocol are significant obstacles for ULPSN to operate in the open spectrum. The DSA has been researched to solve this problem by assuming that coexisting systems can perfectly detect each other. However, this assumption is limited for ULPSN because detection of ULPSN in coexisting system is difficult due to sensitivity limitation of spectrum sensing. As a result, available spectrum is more scarce for ULP system than other coexisting systems. Thus, we derive a new Markov chain model of open spectrum access. Based on the Markov chain mode, we design a DSA scheme to maximize the energy efficiency and investigate the channel access and switch policy of the proposed DSA within the framework of partially observable Markov decision process (POMDP). The simulation results demonstrate that the proposed DSA scheme yields improved energy efficiency and lifetime compared to a random channel selection for ULPSN.

Group-Wise Distributed Space-Time-Frequency Coded OFDM over Asymmetric Frequency-Selective AF Relay Channels

Distributed space-time-frequency coding (DSTFC) is a method to achieve diversity from both multipaths and user cooperations. The DSTFC has been researched by assuming that the multipath profiles of relays are the same (symmetric). However, the symmetric frequency- selective channel is limited in the wireless relay communications. Thus, in this paper, we propose a new DSTFC for relay-based amplify-and-forward (AF) cooperative scheme over the asymmetric frequency selective channels. The proposed scheme is based on the sub-carrier grouping in OFDM transmission. By analyzing pairwise error probability (PEP), diversity order of the proposed DSTFC is derived. In addition, a necessary condition of sub-carrier grouping to achieve the maximum diversity and minimize the decoding complexity is investigated. Simulation results verify that the proposed scheme can achieve the maximum diversity gain for a given asymmetric multipath profile.

Non-handover Based Mobility Management in Hierarchically Structured Cellular Networks

Abstract—Hierarchical cell structure (HCS) is considered as a possible solution to support users in shadowing area or hot zone area. In HCS systems, both co-channel interference and mobility management are the most important problems. In this paper, we propose a new mobility management method that efficiently preserves the data rate of the mobile user passing small overlaid cell area. In the proposed method, handover is not used when the mobile user passes the overlaid cell area. Therefore, it is not affected by ping-pong and handover overhead. Instead, specific channel utilization is used to prevent co-channel interference and to guarantee quality of service in common channel. We show the procedure of the proposed mobility management method, and channel utilization in this method. We also evaluate the rate regions where the proposed method can support higher data rate than handover based method. System level simulation based on IEEE 802.16m system shows that the proposed method is more efficient in HCS system than handover-based method.

Capacity analysis of outdated relay selection in opportunistic relaying system using an adaptive transmission technique

In this paper, we investigate the impact of outdated channel state information (CSI) on the channel capacity of an opportunistic relaying system in conjunction with adaptive transmission techniques. We derive the closed-form expressions of the channel capacity and its associated outage probability for four adaptive transmission techniques: (1) optimal rate adaptation with constant transmit power, (2) optimal transmit power and rate adaptation, (3) channel inversion with fixed rate, and (4) truncated channel inversion with fixed rate. Analytical and numerical results show the effects of outdated CSI on the capacity of the optimal power and rate adaptation and the sub-optimal channel inversion techniques. In addition, we compare the channel capacity among various adaptive transmission techniques in the presence of outdated CSI, thereby illustrating the intuitive tradeoff between the achievable capacity and implementation complexity.

Spectrum sharing in cognitive radio networks: impacts of interference power constraints

Spectrum sharing is a promising technique to enhance spectrum utilization in cognitive radio environments. Cognitive sharing the spectra of licensed users provides unlicensed users with the opportunity to access rigid spectrum resources without degrading the performance of the licensed users. The result of this simultaneous sharing of limited spectrum resources by licensed and unlicensed users is more efficient spectrum utilization. Spectrum sharing with licensed users is achieved by allowing unlicensed users to transmit when the interference power constraint at the licensed users is being satisfied. The interference power constraint protects the performance of the licensed users while at the same time determining the performance of the unlicensed users sharing the licensed spectrum. Therefore, the interference power constraint plays an important role in the spectrum sharing environment. This article provides a guideline for spectrum sharing in the cognitive radio environment. A part of our effort focuses on examining the impact of the interference power constraint and the performance of the unlicensed users under each constraint. We first investigate the representative interference power constraints. Next, we propose a power allocation scheme to satisfy the outage probability of a primary user. The performance comparison is then used to confirm that the proposed scheme not only satisfy the outage probability of primary user, but also is robust to the outdated channel environments.