Woongsup Lee

Enhanced Spectrum Sensing Scheme in Cognitive Radio Systems With MIMO Antennae

Cognitive radio (CR) is a promising technology for overcoming the lack of available communication bands. In CR technology, spectrum sensing is an important issue, which has recently been extensively studied. We provide a solution to the spectrum-sensing problem for multiple cognitive terminals (CTs) that takes into account the difference among CTs with respect to the probabilities of a false detection and a missed detection. We optimize the spectrum-sensing performance by differentiating the number of spectrum-sensing operations that each CT performs. This has not previously been proposed in the literature. Moreover, we use a simultaneous spectrum-sensing and data transmission scheme that utilizes multiple-input-multiple-output (MIMO) antenna technology. As a result, the degradation of quality of service (QoS) that is caused by spectrum sensing can be reduced, and the throughput of CR systems can be increased, while maintaining the accuracy of spectrum sensing. Through performance analysis, we show that our proposed scheme can achieve the desired levels of performance with respect to the probabilities of a false detection and a missed detection and improve the performance of the CR systems with respect to throughput and delay.

Direct Electricity Trading in Smart Grid: A Coalitional Game Analysis

Integration of distributed generation based on renewable energy sources into the power system has gained popularity in recent years. Many small-scale electricity suppliers (SESs) have recently entered the electricity market, which has been traditionally dominated by a few large-scale electricity suppliers. The emergence of SESs enables direct trading (DT) of electricity between SESs and end-users (EUs), without going through retailers, and promotes the possibility of improving the benefits to both parties. In this paper, the cooperation between SESs and EUs in DT is analyzed based on coalitional game theory. In particular, an electricity pricing scheme that achieves a fair division of revenue between SESs and EUs is analytically derived by using the asymptotic Shapley value. The asymptotic Shapley value is shown to be in the core of the coalitional game such that no group of SESs and EUs has an incentive to abandon the coalition, which implies the stable operation of DT for the proposed pricing scheme. Unlike the existing pricing schemes that typically require multiple stages of calculations and real time information about each participant, the electricity price for the proposed scheme can be determined instantaneously based on the number of participants in DT and statistical information about electricity supply and demand. Therefore, the proposed pricing scheme is suitable for practical implementation. Using computer simulations, the price of electricity for the proposed DT scheme is examined in various environments, and the numerical results validate the asymptotic analysis. Moreover, the revenues of the SESs and EUs are evaluated for various types of SESs and different numbers of participants in DT. The optimal ratio of different types of SESs is also investigated.

Electric Vehicle Charging Stations With Renewable Power Generators: A Game Theoretical Analysis

Summary form only given. In this paper, we study the price competition among electric vehicle charging stations (EVCSs) with renewable power generators (RPGs). As electric vehicles (EVs) become more popular, a competition among EVCSs to attract EVs is inevitable. Thereby, each EVCS sets its electricity price to maximize its revenue by taking into account the competition with neighboring EVCSs. We analyze the competitive interactions between EVCSs using game theory, where relevant physical constraints such as the transmission line capacity, the distance between EV and EVCS, and the number of charging outlets at the EVCSs are taken into account. We show that the game played by EVCSs is a supermodular game and there exists a unique pure Nash equilibrium for best response algorithms with arbitrary initial policy. The electricity price and the revenue of EVCSs are evaluated via simulations, which reveal the benefits of having RPGs at the EVCSs.

Effect of bicarbonate ion additives on pitting corrosion of type 316L stainless steel in aqueous 0.5 M sodium chloride solution

Abstract The effect of bicarbonate ions (HCO3−) on pitting corrosion of type 316L stainless steel (SS, UNS S31603) was investigated in aqueous 0.5 M sodium chloride (NaCl) solution using potentiody...

Enhanced Group Handover Scheme in Multiaccess Networks

We propose a group handover scheme for multiaccess networks that uses adjusted delay to reduce the number of handover blockings that occur during group handover. In a group handover, many users simultaneously start the handover process, which causes network congestion and increases the number of handover blockings. In the proposed scheme, each user in the group handover selects a base station (BS) on the basis of the remaining bandwidth of the BS at that time and initiates the handover process after adjusted delay. To find a strategy for selecting BSs, an optimization problem whose objective is to minimize the probability that handover blocking will occur is formulated, and a solution is derived by using the Karush-Kuhn-Tucker condition. Through performance analysis and simulation results, we show that the probability that handover blocking will occur during group handover is less in our proposed scheme than in a conventional scheme.

CQI Feedback Reduction Based on Spatial Correlation in OFDMA System

In OFDMA systems, channel quality information (CQI) feedback is used to determine the modulation and coding level of a mobile station (MS). When the MS uses diversity, it transmits the CQI feedback information which contains channel condition which is averaged over all the sub-bands allocated to itself. In conventional CQI feedback scheme, every MS should send its own CQI feedback. So, as the number of MSs in the system increases, the amount of CQI feedbacks increases too, and it results in the reduction of system throughput. To solve this problem, we have proposed a new CQI feedback scheme. In our CQI feedback scheme, MSs which are close each other make a CQI feedback group. And only one representative CQI feedback per one CQI feedback group is transmitted because MSs which are in the same CQI feedback group have similar channel condition. In this paper, we also have derived the PER equation of MSs when the MSs use a representative CQI feedback. And also, through simulations and numerical results, we have shown that by using our proposed scheme, CQI feedbacks can be reduced.

A New Neighbor Discovery Scheme Based on Spatial Correlation of Wireless Channel

We propose a new neighbor discovery scheme for cellular communication systems that utilizes the azimuth spread (AS), delay spread (DS) and shadow fading of a mobile station (MS). Given that the AS, DS and shadow fading are spatially correlated, MSs that are close to each other will have similar AS, DS and shadow fading values. In our proposed scheme, we use the AS, DS and shadow fading of an MS to find its neighbor MSs. The proposed scheme filters out MSs that are unlikely to be neighbor MSs and uses the Kolmogorov-Smirnov (K-S) test for more refined neighbor discovery. Through analysis and simulation, it is shown that we can effectively find neighbor MSs.

New D2D Peer Discovery Scheme Based on Spatial Correlation of Wireless Channel

Due to the increasing popularity of device-to-device (D2D) services, it is becoming increasingly important to find efficient means of identifying nearby users, i.e., by peer discovery. Herein, we propose a low-power peer discovery scheme, in which we exploit the tradeoff between the power consumption and the accuracy/scope of peer discovery. In our proposed scheme, the transmission of a proximity beacon is scheduled based on channel values, such that users in close proximity are likely to transmit beacons at similar time instants due to the spatially correlated wireless channel. As a result, users can find nearby peers accurately with a lower power consumption by shortening the reception period for the beacons. The performance of the proposed scheme, in terms of accuracy and power consumption, is derived. By means of simulation results, we show that nearby users can be found with lower power consumption than in conventional schemes, while achieving a high accuracy of peer discovery.

Resource Allocation for Vehicle-to-Infrastructure Communication Using Directional Transmission

We herein consider directional transmission based on adaptive beamforming in which multiple vehicles are served at the same time using multiple directional beams; this represents one promising technology that could improve the capacity of vehicle-to-infrastructure (V2I) communication. We propose an iterative resource-allocation method to maximize the capacity of V2I communication by taking into proper account the interference between directional beams, which reduces capacity. We also consider a heuristic resource allocation scheme, which has significantly less computational complexity than the iterative scheme and negligible performance degradation. We make use of computer simulations to show that the capacity of V2I communication can be improved using our proposed schemes.

Mean velocity estimation of mobile stations by spatial correlation of channels in cellular systems

We propose a velocity estimation scheme for mobile stations (MSs) in cellular communication systems that uses the azimuth spread (AS), delay spread (DS), and shadow fading of MSs. Due to the fact that the AS, DS, and shadow fading are correlated in the spatial domain, the variation of the AS, DS, and shadow fading of an MS is related to its velocity. And, we also consider the variation of velocity which was not considered in previous velocity estimation studies. Through analysis and numerical results, we show that we can estimate the velocity of an MS with lower estimation error compared to previous estimation schemes, despite of measurement error.