Publications (12)6.41 Total impact
- [Show abstract] [Hide abstract] ABSTRACT: As the key technique of smart antenna, a beamformer can enhance the desired signal relative to the directional interference and the background noise. The diagonal loading beamformer is a simple and efficient method that can improve the robustness of the conventional beamformers. However, determining an ideal diagonal loading factor is not a trivial problem, one that still has not been adequately addressed. On the other hand, the channel attenuation estimation is also an important issue in communication for power control, target positioning, tracking and etc. Many adaptive algorithms have been presented to estimate the channel attenuation by using training sequence. The main difference between the proposed method and the conventional ones is that the proposed method considers the beamformer and channel attenuation jointly so that the training sequence for channel attenuation estimation is no longer required. In digital communications, user information is coded into a sequence with the constant modulus (CM). Invoking this property, we propose a diagonal loading beamformer that can determine diagonal loading factor and channel attenuation simultaneously. Firstly, we propose a CM cost function with respect to the diagonal loading factor and the channel attenuation. However, this cost function is complicated and non-convex. For solving the globally optimal solution, we further propose a method to cast the channel attenuation as a function about diagonal loading factor using the KKT condition of optimization. Substituting this function for the channel attenuation, the CM cost function will become simple and only depend on diagonal loading factor. We can employ a one-dimensional search method for the optimization which will be less computationally consuming and efficient, even if the cost function is non-convex. Once the diagonal loading factor has been solved, one can solve the channel attenuation easily. Numerical experiments show the effectiveness of the proposed method.
- [Show abstract] [Hide abstract] ABSTRACT: In the conventional Space-Division Multiple-Access (SDMA) scheme, Direc-tion-of-Arrival (DoA) estimation and beamforming are usually performed sequentially and independently. Such a cascade structure may transmit the error or deviation from the DoA stage to the beamforming stage. Two of the main reasons for such error or deviation are the effect of imprecise array manifold and the effect of finite sample size. The deviation, especially error may subsequently lead a beamformer to produce a beam pointing in wrong DoA that can severely degrade the performance of the beamformer. To address this problem, we propose a robust algorithm both for DoA estimation and for beamforming using the constant modulus (CM) feature. In the algorithm, the DoA esti-mation and the beamforming are no longer independent but work in an inter cooperative way. One can regard them as two parallel outputs of the algorithm. In this way, the ro-bustness of DoA estimation and beamforming can be improved simultaneously. Finally, numerical experiments show the effectiveness of the proposed method.
- [Show abstract] [Hide abstract] ABSTRACT: The optimal multiuser detection (OMD) for binary Multiple-Access (MA) system can be posed as a binary quadratic programming (BQP) which belongs to a nondeterministic polynomial-time hard (NP-hard) problem unless the systems have some special structures. While, the OMD for PAM real-valued MA system can be posed as an integer quadratic programming (IQP) which is a problem much more difficult than BQP. Furthermore, the most generalized MA system is the M-ary complex-valued system and the OMD for this system can be posed as a complex-valued integer quadratic programming (CIQP). To the best of our knowledge, the optimal detector with polynomial complexity has never been presented for any nonorthogonal Mary complex-valued MA system. In this paper, we focus our interests on a certain M-ary complex-valued nonorthogonal MA system with a hierarchical structure of the signature waveform set. The OMD for this system can be posed as a CIQP. By using two properties of the hierarchical signature waveform set, we propose a polynomial complexity detector which can solve this CIQP optimally.
- [Show abstract] [Hide abstract] ABSTRACT: In this paper, we propose an advanced automatic modulation classification (AMC) method for cognitive radio (CR). Conventional AMC algorithms employ some pattern recognition algorithms such as hidden markov model (HMM) and support vector machine (SVM) to recognize the signal modulations through the characters of spectral correlation, e.g., α-profile, f- profile, average value, and etc. However, these methods are one dimensional approaches and might not extract the whole characteristics of modulations completely. In this paper, we exploit a two dimensional property of cyclostationarity: spectral correlation function (SCF). Compared with those of one dimensional spectral correlation, the SCF exhibit more classification information. Moreover, we employ two dimensional principal component analysis (PCA) which minimize the size of original data not losing own features so that we can have better performance than choice of few characteristics.
- [Show abstract] [Hide abstract] ABSTRACT: In this paper, we investigate the cooperative spectrum sensing when primary user data is forwarded to fusion center with the help of amplify-forward (AF) based cognitive relay in cognitive radio networks. We proceed by obtaining the probability density function (PDF) statistics of the received signal via AF relay link at the fusion center. Then we obtain the PDF of combined signal from relay link and direct link at the output of maximum ratio combiner (MRC). Finally, to avoid cumbersome integrations, we use an alternative representation of generalized marcum Q-function to obtain an upper bound for the detection probability when the network is operating over independent and identically distributed (IID) rayleigh fading channels. We also obtain an exact expression for detection probability when no cooperation is taking place among cognitive users. In the end, we present simulation results and compare them with the analytical expressions for the validity of our mathematical derivations.
- [Show abstract] [Hide abstract] ABSTRACT: Secondary users (SUs) should have power constraints because primary users (PUs) should not be interfered by SUs in cognitive radio (CR) network. Therefore it is important for SUs to maximize throughput and ensure quality of service (QoS) on the premise of generating no interference to PUs. In this paper, SUs are equipped with multiple antennas. Multiple-input multiple-output (MIMO) wireless communication systems can offer high data rate or reliability through spatial multiplexing or diversity. Therefore we present a method which user satisfaction-based on switching between spatial multiplexing and transmit diversity to maximize total user satisfaction. When we use spatial multiplexing, user satisfaction is formulated as a function with data rate and power consumed. In case we use transmit diversity, user satisfaction is formulated as a function with reliability and power consumed. For estimating reliability, we provide a novel bit error rate (BER) equation. And we use different power allocation method in each case. We apply capped water-filling method for spatial multiplexing and Lagrangian method for transmit diversity. Simulation results show that proposed switching scheme increase user satisfaction. And we study the effect of interference of PU on the performance.
- [Show abstract] [Hide abstract] ABSTRACT: By applying smart beamforming technologies, it is effective to protect primary user (PU) while guarantee that cognitive user (CU) has better spectrum sharing with PU in a rarely conflictual manner. In this paper, firstly, we implement direction of arrival (DOA) estimation for obtaining the location information of signals, then exploit uniform circular array antenna applying adaptive algorithm to achieve desired transmit beamforming gains in the required directions. The simulation results demonstrate that the proposed beamforming algorithm achieves desired beamforming pattern and higher power efficiency.
- [Show abstract] [Hide abstract] ABSTRACT: Cognitive radio is an advanced technology for efficient utilization of under-utilized spectrum since it is able to sense the available spectrum and use empty frequency bands if there is no use of primary user. These sensing observations are forwarded a cognitive radio base-station through fading channels, the sensing performance will be severely degraded. In order to mitigate this effect, weighted-cooperative sensing was considered to be a promising way in our previous work, which combines the sensing result of each CR user to improve the performance. However, in practical, the probability of detection is unavailable to calculate in each user. In this paper, we propose a weighted-cooperative sensing scheme using clustering to improve the performance and to overcome the critical problem. Numerical results show that the sensing performance is improved obviously as opposed to conventional spectrum scheme.
Conference Paper: Weighted-Collaborative Spectrum Sensing in Cognitive Radio[Show abstract] [Hide abstract] ABSTRACT: Cognitive radio is an advanced enabling technology for efficient utilization of under-utilized spectrum since it is able to sense the spectrum and use the frequency when the primary users are absent. Recent investigation suggests that spectrum sensing is compromised when a user experiences fading or shadowing. In order to combat such effects, collaborative sensing is presented. However, the conventional collaborative sensing is not effective when users suffer from different fading environments. In this paper, we propose a weighted-collaborative scheme to improve the spectrum sensing performance under fading environment. The analysis of the simulation results proves that the weighted-collaborative scheme improves sensing performance obviously.
- [Show abstract] [Hide abstract] ABSTRACT: A novel and compact ultra-wideband (UWB) microstrip-fed monopole antenna having frequency band notch function is presented. To increase the impedance bandwidth of an antenna, a narrow slit is used. By inserting a modified inverted U-slot on the proposed antenna, the frequency band notch characteristic is obtained. The designed antenna satisfies the voltage standing wave ratio requirement of less than 2.0 in the frequency band between 3 and 11 GHz while showing the band rejection performance in the frequency band of 5.0 to 5.9 GHz.
- [Show abstract] [Hide abstract] ABSTRACT: This letter presents an ultra wide-band (UWB) antenna with a printed monopole structure fed by a coplanar waveguide (CPW) line. The antenna is analyzed by using the finite-difference time-domain (FDTD) method and genetic algorithm (GA) to achieve an ultra wide bandwidth characteristic. The measured frequency response shows an impedance bandwidth of 7.25GHz or 104.7% over 3.3 to 10.55GHz for VSWR less than 2.
Conference Paper: Reduced order RLS polynomial predistortion[Show abstract] [Hide abstract] ABSTRACT: In this paper, a modified recursive least square (RLS) based polynomial predistortion algorithm is proposed by reducing the order of the polynomials used in the RLS algorithm. The basic idea of the reduced order RLS based polynomial predistortion algorithm is to approximate the target function with two simpler functions (with a smaller number of coefficients) instead of a single function. A complex function is decomposed into two simpler functions so that we can compute the RLS algorithm with much less complexity. The proposed reduced order RLS polynomial predistortion shows superior performance compared to the conventional RLS polynomial predistortion with the same number of coefficients or with the same computational complexity.
Dalian University of TechnologyLü-ta-shih, Liaoning, China
Incheon, Incheon, South Korea
- Department of Information and Communication Engineering
Electronics and Telecommunications Research InstituteSŏul, Seoul, South Korea