Kyung Sup Kwak

ABSTRACT A joint Tomlinson-Harashima (TH) source and multi-relay beamforming design in decode-and-forward (DF) relaying systems with relay selection is proposed, which exploits the relationship between QR decomposition (QRD) and Gram- Schmidt orthogonalization (GSO) to jointly optimize the source precoder and relay beamforming vectors of multiple relays. Then we propose a correlation-based approach which leads to a significant reduction of forward overhead information from the source to the selected relays. Finally a “Best-Last” based relay selection approach is proposed which has a performance close to the optimal global relay selection approach with a significantly reduced computational complexity. Simulation results are provided to support the advantages of the proposed schemes.

O. Gervasi et al. (Eds.): ICCSA 2005, LNCS 3481, pp. 373–380, 2005. © Springer-Verlag Berlin Heidelberg 2005 ... Flexible ZCD-UWB with High QoS or High Capacity ... Jaesang Cha*, Kyungsup Kwak**, Changdae Yoon***, and Chonghyun Lee**** ... *Dept. of Information and ...

This paper investigates energy detection based wideband spectrum sensing for dynamic spectrum access (DSA) systems. The licensed frequency band of interest is resolved into a series of contiguous frequency bins via the FFT engine of the detector. By employing forward consecutive mean excision (FCME) algorithm and exploiting primary user (PU) signal's contiguity in frequency domain, computationally simple pre-processing of raw

Wideband primary user (PU) signal identification approaches based on enhanced forward consecutive mean excision (FCME) algorithms are proposed for cognitive MB-OFDM UWB systems. Each single subband is resolved into a series of contiguous frequency bins via the FFT engine. The identification of PU signals can thus be performed in frequency domain with our proposed methods, namely the localization algorithm based on double thresholding with prior one-dimensional median filtering (MF+LAD) and the FCME algorithm with binary integration (FCME+BI). In both strategies, two thresholds are employed in the detection decision making stage to perform the frequency bin cluster based PU signal identification. It is verified through numerical experiments that the two strategies can significantly improve wideband PU signals' bandwidth and signal-to-noise ratio (SNR) estimation accuracies.

ABSTRACT Cognitive radios may operate in practice under various adverse environments. For typical mobile and short-range scenarios, wireless links may tend to be time and frequency selective, i.e., the multipath propagations with time-varying fading coefficients will be inevitable. To cope with the encountered doubly-selective channels, in this paper we present a new spectrum sensing algorithm for distributed applications. First, a dynamic discrete state-space model is established to characterize sensing process, where the occupancy state of primary band and the time-varying multipath channel are treated as two hidden states, while the summed energy is adopted as the observed output. With this new paradigm, spectrum sensing is realized by acquiring primary states and time-dependent multipath channel jointly. For the formulated problem, unfortunately, Bayesian statistical inference may be impractical due to the absence of likelihoods and involved non-stationary distributions. To remedy this problem, an iterative algorithm is further designed by resorting to sequential importance sampling techniques; thus, the dynamic non-Gaussian multipath channel and primary states are estimated recursively. Another critical challenge, e.g., the noise uncertainty, is also considered, which may be incorporated conveniently into this sensing diagram and, furthermore, addressed effectively by the designed algorithm. Simulations validate the proposed algorithm. While classical schemes fail to deal with doubly selective channels, the new sensing scheme can exploit the underlying channel memory and operate well, which provides a great promise to realistic applications.

In this paper, we present the space time block codes (STBC) based M-ary code-selected modulation scheme of direct sequence bipolar pulse amplitude modulation (DSBPAM) ultra wideband (UWB) system with selective Rake (SRake) receiver and maximum ratio combining (MRC) over multipath channels in order to improve the system performance and transmission rate of the conventional DS UWB system. The system bit

Cooperative multi-cell transmit strategies typically require full channel state information (CSI) at the base station. Most previous works in this area have assumed that base stations have common knowledge of both data dedicated to all terminals and full or partial CSI of all links. In this paper, a more practical limited-feedback scenario is studied for multi-cell joint beamforming. The proposed algorithm provides higher sum rates over the virtual SINR one.

This paper addresses pilot-aided multiuser least square (LS) channel estimation for the uplink of MIMO-OFDM systems. The systems under consideration allow all users use all available subcarriers independently and thus involve multiuser interference in the frequency domain. We develop the decentralized and centralized channel estimation algorithms according to different multiuser scenarios. Optimal multiuser pilots are proposed specially for centralized estimation methods with respect to the mean square error (MSE) of LS channel estimate

r Abstract-In this paper, a novel method using bipartite graph matching is proposed to allocate the spectrum in cognitive networks, where users cooperatively optimize spectrum allocation to get the maximal system utility while avoiding interference with its neighbors. A feasible method for distributed cognitive networks is presented; experimental results show that proposed algorithm dramatically maximizes spectrum utilization together with reducing allocation conflictions among users, which minimize the number of starved users during the allocation.

Multicarrier Modulation (MCM) is an attractive transmission technique that provides improved performance in various communication systems. A fundamental issue of multicarrier communication systems is power allocation, which is addressed as a constrained multivariable nonlinear optimization problem. In this paper, we develop a computationally efficient solution for the loading of multicarrier systems that present constraints both on overall available power and maximum power per carrier. The proposed algorithm offers significantly low computational complexity over the well-known water-filling methods.

In this paper, we mainly analyze the performance of the classical time hopping bipolar pulse amplitude modulation (TH-BPAM) UWB system and the coded TH-BPAM system in the multi-user case. The performance of TH-BPAM system is analyzed in detail over an ideal AWGN channel and correlation receiver. For the proposed coded scheme, the Viterbi algorithm (VA) and soft output Viterbi algorithm (SOVA) are employed in the decoding process and the upper bound and lower bound of bit error probability is presented. The analysis shows that TH-BPAM suffers performance degradation with respect to the coded TH-BPAM greatly. Furthermore, we give the computer simulation and our simulation results also prove our theoretical analysis

ABSTRACT This paper proposes an energy detection based time of arrival (TOA) estimation scheme for impulse radio ultra-wideband (IR-UWB) systems. The proposed scheme is composed of two stages in performing the TOA estimation: initial signal acquisition (ISA) and fine timing estimation (FTE). In the ISA stage, a linear quadrature optimization based weighting scheme is proposed to coarsely capture the arrival of the IR-UWB signals. Capitalizing on the acquisition of the IR-UWB signal in a relatively wide time range, this paper develops in the FTE stage a double-threshold based search back method tailored for locating the leading edge of the IR-UWB signal. Simulation results illustrate that the linear quadrature optimization based algorithm can effectively seize the arrival of the UWB signals and the double-threshold strategy significantly improves the TOA estimation accuracy in terms of mean absolute error (MAE) performance.

ABSTRACT Optimal power allocation (OPA) schemes for amplify-and-forward (AAF) relay networks have been widely in- vestigated in recent years, usually under the assumption of readily known channel state information (CSI). This paper proposes a semi-blind iterative relay node selection (SBIRS) scheme to carry out the OPA procedure when the CSI are entirely unknown at the destination. We first develop a semi-blindly weighted multiple-eigenvector (SBWMEV) solution for optimally allocating power among all the relay nodes in the network. Aided by the intrinsic salient feature of the SBWMEV eigenvector structure, we further propose an iterative relay selection algorithm to simplify the SBWMEV solution with enhanced performance. In an iterative relay node selection manner, the proposed SBIRS scheme is capable of potentially allocating more power to the relay nodes with better channel conditions. Simulations verify that the SBIRS schemes outperform the SBWMEV scheme in terms of throughput and outage probability. Index Terms—Semi-blind, optimal power allocation, amplify- and-forward, relay network.

ABSTRACT Cooperative spectrum sensing is investigated for the purpose of detecting the primary user accurately in cognitive radio systems. As the report channel between cognitive users and fusion center experiences fading or shadowing, a cluster based cooperative sensing method is proposed to improve the sensing performance. However, when the number of the cognitive users in each cluster increases, the bandwidth for reporting the sensing results to the cluster head will be very enormous. In this paper, we assume that the cognitive users with enough information in each cluster will send their sensing results to the cluster head so as to decrease the number of sensing bits when performing spectrum sensing. The spectrum efficiency is also studied. Simulation results show that, by using this fusion method in spectrum sensing, the average number of sensing bits decreases greatly at the cost of a little loss in the performance of spectrum sensing.

A new pulse for cognitive radio-ultra wideband (CR-UWB) using multiple modified transform domain communication system (M-TDCS) is presented in this paper. Utilizing multiple M-TDCS to sense the electromagnetic environment and UWB as a versatile PHY layer to adapt various wireless channel conditions, this pulse can avoid interference to/from the existing wireless systems or noise. This pulse can be implemented using surface acoustic wave (SAW) devices and some other accessorial devices. The bit error ratio (BER) performance of system using this pulse can be observably improved for UWB systems.

... The MAC layer utilizes the Dynamic Channel scheme for SNs [4], however in this paper we want to Coding (DCC) MAC protocol [8]. DCC-MAC fully ... within a certain range ofD, the route with less H tends layers and at the same time consume as less as possible ...

1 Dept. of Information and Communication Eng. Seokyeong Univ. Seoul, Korea chajs@skuniv.ac.kr 2 UWB Wireless Communications Research Center(UWB-ITRC), Inha Univ. Incheon, Korea 3 Dept.of Electronic Engineering , Induk Institute of Technology, ...

ABSTRACT For the increasing cost of energy and infrastructures of wireless communication systems, energy-efficient communication technologies become an important concern for the standardization authorities and cellular operators. In this paper, we propose an energy-efficient relay selection and power allocation strategy subject to a total power constraint. The proposed scheme first selects the transmission mode according to the energy efficiency of the different transmission link. Then, we analyze the energy efficiency of the proposed system and present the optimal number of the relay nodes and the best power distribution factors in broadcasting phase and the cooperative transmission phase. Closed-form expressions of the energy efficiency of the proposed scheme are derived. Simulation results show that the proposed scheme achieves good energy efficiency in both low and high signal-to-noise ratio region.

In this paper, a communication scheme based on Gaussian minimum shift keying (GMSK) signal with trellis-coded modulation is presented. The coded/modulation scheme is an attractive technique for transmission of digital information over a bandwidth and power-limited channel. The coding gain of trellis codes provides power efficiency, and GMSK provides bandwidth efficiency due to the constant-envelope properties and good spectral features like all the CPM signals. In the receiver we propose an optimum detection Viterbi algorithm. Optimal trellis codes for the scheme can be found with computer simulation