• KSII Transactions on Internet and Information Systems (TIIS)
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• v.6 no.11
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• pp.3008-3025
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• 2012

In this paper, the downlink resource allocation of OFDMA system with decode-and-forward (DF) relaying is investigated. A non-convex optimization problem maximizing system throughput with users' satisfaction constraints is formulated with joint relay selection, subcarrier assignment and power allocation. We first transform it to a standard convex problem and then solve it by dual decomposition. In particular, an Optimal resource allocation scheme With Time-sharing (OWT) is proposed with combination of relay selection, subcarrier allocation and power control. Due to its poor adaption to the fast-varying environment, an improved version with subcarrier Monopolization (OWM) is put forward, whose performance promotes about 20% compared with that of OWT in the fast-varying vehicular environment. In fact, OWM is the special case of OWT with binary time-sharing factor and OWT can be seen as the tight upper bound of the OWM. To the best of our knowledge, such algorithms and their relation have not been accurately investigated in cooperative OFDMA networks in the literature. Simulation results show that both the system throughput and the users' satisfaction of the proposed algorithms outperform the traditional ones.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.31 no.5A
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• pp.451-458
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• 2006

In this paper, we propose a novel scheduling algorithm for downlink transmission which utilizes scarce wireless resource efficiently in an Orthogonal Frequency Division Multiple Access/Time Division Duplex system. Scheduling schemes which exploit channel information between a Base Station and terminals have been proposed recently for improved performance. Time series analysis is used to estimate the channel state of mobile terminals. The predicted information is then used for prioritized scheduling of downlink transmissions for improved throughput, delay and jitter performance. Through simulation, we show that the total throughput and mean delay of the proposed scheduling algorithm are improved compared with those of the Proportional Fairness and Maximum Carrier to Interference Ratio schemes.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.31 no.12A
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• pp.1214-1222
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• 2006

In this paper, an efficient direction-r)f-arrival based adaptive beamforming algorithm for orthogonal frequency-division multiple-access smart antenna systems is proposed. The proposed algorithm provides a high performance by steering main beams to the directions of a desired signal, whereas steering nulls to the directions of the interference, using the estimated directions. The beamforming outputs obtained by steering the main beams to the distinct directions of resolvable multipath signals are combined in a maximal ratio manner to exploit angular diversity gain. The performance elf the proposed algorithm is finally evaluated in cellular mobile environments to verify its efficiency and is compared with that of least-squares beamforming algorithm, by taking the WiBro system as a target system.

• Journal of Korea Multimedia Society
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• v.16 no.5
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• pp.601-609
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• 2013

One of the most serious factors constraining the next generation cellular mobile consumer communication systems will be the severe co-channel interference experienced at the cell edge. Such a capacity-degrading impairment combined with the limited available spectrum resource makes it essential to develop more spectrally efficient solutions to enhance the system performance and enrich the mobile user's application services. This paper proposes a unique hybrid method of frequency hopping (FH) and subcarrier-reuse-partitioning that can maximize the system capacity by efficiently utilizing the available spectrum while at the same time reduce the co-channel interference effect. The main feature of the proposed method is that it applies an optimal combination of different frequency reuse factors (FRF) and FH-subcarrier allocation patterns into the partitioned cell regions. From the simulation results, it is shown that the proposed method can achieve the optimum number of subcarrier subsets according to the frequency-reuse distance and results in better performance than the fixed FRF methods, for a given partitioning arrangement. The results are presented in the context of both blocking probability and BER performances. It will also be shown how the proposed scheme is well suited to FH-OFDMA based cellular systems aiming at low co-channel interference performance and optimized number of subcarriers.

• ETRI Journal
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• v.27 no.6
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• pp.777-787
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• 2005

In this paper, we propose an urgency- and efficiency-based wireless packet scheduling (UEPS) algorithm that is able to schedule real-time (RT) and non-real-time (NRT) traffics at the same time while supporting multiple users simultaneously at any given scheduling time instant. The UEPS algorithm is designed to support wireless downlink packet scheduling in an orthogonal frequency division multiple access (OFDMA) system, which is a strong candidate as a wireless access method for the next generation of wireless communications. The UEPS algorithm uses the time-utility function as a scheduling urgency factor and the relative status of the current channel to the average channel status as an efficiency indicator of radio resource usage. The design goal of the UEPS algorithm is to maximize throughput of NRT traffics while satisfying quality-of-service (QoS) requirements of RT traffics. The simulation study shows that the UEPS algorithm is able to give better throughput performance than existing wireless packet scheduling algorithms such as proportional fair (PF) and modified-largest weighted delay first (M-LWDF), while satisfying the QoS requirements of RT traffics such as average delay and packet loss rate under various traffic loads.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.31 no.11A
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• pp.1068-1075
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• 2006

An adaptive ranging technique for Orthogonal Frequency Division Multiple Access(OFDMA) uplink transmission is proposed for timing synchronization of multiple mobile stations located different distances from a base station. By combining the Timing Phase Compensated Frequency Domain Cross-correlation(TPCFDC) and Frequency Domain Differential Cross-correlation(FDDC), the proposed scheme reduces the number of correlators used in ordinary TPCFDC. Repeated initial ranging attempt with the FDDC in the proposed scheme greatly reduces the hardware implementation complexity. Simulation results for ranging success probability and average ranging attempts count show that the proposed algorithm performs similarly with the ordinary TPCFDC even with the 10 times reduced complexity.

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

• Journal of Communications and Networks
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• v.15 no.1
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• pp.102-110
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• 2013

This paper considers a cooperative orthogonal frequency division multiple access (OFDMA)-based cognitive radio network where the primary system leases some of its subchannels to the secondary system for a fraction of time in exchange for the secondary users (SUs) assisting the transmission of primary users (PUs) as relays. Our aim is to determine the cooperation strategies among the primary and secondary systems so as to maximize the sum-rate of SUs while maintaining quality-of-service (QoS) requirements of PUs. We formulate a joint optimization problem of PU transmission mode selection, SU (or relay) selection, subcarrier assignment, power control, and time allocation. By applying dual method, this mixed integer programming problem is decomposed into parallel per-subcarrier subproblems, with each determining the cooperation strategy between one PU and one SU. We show that, on each leased subcarrier, the optimal strategy is to let a SU exclusively act as a relay or transmit for itself. This result is fundamentally different from the conventional spectrum leasing in single-channel systems where a SU must transmit a fraction of time for itself if it helps the PU's transmission. We then propose a subgradient-based algorithm to find the asymptotically optimal solution to the primal problem in polynomial time. Simulation results demonstrate that the proposed algorithm can significantly enhance the network performance.

• Journal of Satellite, Information and Communications
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• v.5 no.1
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• pp.27-31
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• 2010

Recently, spectrum requirements are rapidly increasing in accordance with wireless communication development. For this reason, FCC(Federal communications commission) is considering cognitive radio system to increase spectral efficiency. In this paper, we present the performance analysis of signal detection by using RS(Reference signal) for LTE environments. Especially, we analyze the performance of detection probability in case of downlink LTE system. In the simulation, we generate OFDMA signal format which is specified in the LTE system. We assume additive white Gausssian noise channel environment. We estimate the performance by setting the threshold value of 5 % and 10 % based on CFAR(Constant false alarm rate) and false alarm rate, respectively. Finally, we discuss a future study plan on the applicability of CR to the LTE system.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.12 no.9
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• pp.1638-1645
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• 2008

WiBro(Wireless Broadband Internet) is the standard of high-speed portable internet based on OFDMA/TDD techniques, and the subset of consolidated version of IEEE802.16e Wireless MAN standard. In this paper, we propose the design method of WiBro system using the SOFM Blind Equalization. Proposed design method used SOFM neural network blind equalization with Bussgang algorithms in the Broadband 16 QAM WiBro system receiver. To verificate the proposed design method usability, the MSE and the BER are simulated. The simulation results shown that is improved the equalization performances of the proposed WiBro system using the SOFM Blind equalization than the existing WiBro system.