Browsing by Author "Wu, Wen-Rong"

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A low-complexity adaptive antenna array code acquisition
(2008-05-01)
Yang, Hua-Lung
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Wu, Wen-Rong
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電信工程研究所
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Institute of Communications Engineering
Conventionally, code acquisition with antenna array employs a correlator bank and the serial-search technique. Due to the inherent properties of correlators and serial search, mean acquisition time of the correlator-based approach is large, especially in strong interference environments. Recently, an adaptive-filtering approach has been applied to code acquisition. This method simultaneously performs beamforming and code-delay estimation with a spatial and a temporal filter. Its performance is significantly better than that of the correlator-based approach. However, its computational complexity will be high when the code-delay uncertainty is large. In this paper, we propose a low-complexity adaptive-filtering scheme to solve the problem. Incorporating a serial-search technique, we are able to significantly reduce the size of the temporal filter, so does the computational complexity. We also analyze the proposed algorithm and derive closed-form expressions for optimum solutions, mean-squared error, and mean acquisition time. Simulations show that while the proposed system somewhat compromises the performance, the computational complexity is much lower that of the original adaptive-filtering approach. (C) 2007 Elsevier B.V. All rights reserved.
A low-complexity channel-estimation method for IEEE 802.16e systems
(2007)
Wu, Wen-Rong
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Lin, Wan-Yi
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Kao, Shiang-Lun
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電信工程研究所
;
Institute of Communications Engineering
In wireless communication, the channel response may vary rapidly when the user's mobility is high. Since pilot signals are usually limited, channel estimation often involves interpolation. How to perform channel interpolation/estimation in high-mobility scenario is then a challenge problem. This is particularly true for an OFDM system operated in long-delay-profile channels. In this paper, we consider the channel estimation problem in IEEE802.16e systems. Using the special Structure defined in the system, we propose an effective channel estimation algorithm. The main idea is to perform a two-dimensional interpolation with a cascade of two one-dimensional interpolations, one is linear and the other is nonlinear. Simulations show that this method has good performance while its computational complexity is low.
A low-complexity ICI mitigation method for high-speed mobile OFDM systems
(2006)
Hsu, Chao-Yuan
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Wu, Wen-Rong
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電信工程研究所
;
Institute of Communications Engineering
Orthogonal frequency division multiplexing (OFDM) is a promising technique for frequency-selective fading channels. However, in high-speed mobile environments, the channel may be time-variant in one OFDM symbol. This will incur the intercarrier interference (ICI) degrading seriously the performance of an OFDM system. A simple remedy is to apply the zero-forcing (ZF) equalization method. Unfortunately, the direct ZF method requires an inverse of a huge matrix requiring a prohibitively high complexity. In this paper, we propose a low-complexity ZF method for ICI mitigation. The main idea is to explore the special structure of the ICI matrix and apply the Newton's iterative matrix inversion method. With our formulation, fast Fourier transforms (FFTs) can be used to reduce the complexity. The proposed method can reduce the complexity from the order of O(N(c)(3)) to that of O(N(c)log(2)N(c)) where N(c) is the number of subcarriers. Computer simulations show that the proposed method can cancel the ICI effectively and at the same time reduces the computational requirement dramatically.
A Low-Complexity Joint Angle-of-Arrival and Channel Estimation for Single-Input-Multiple-Output OFDM Systems
(2015)
Chen, Chih-Yu
;
Wu, Wen-Rong
;
Gau, Chi-Shiang
;
資訊工程學系
;
電機學院
In future wireless communication system, beam-forming plays an important role in improving link quality. As a result, the angle information is required. In this paper, a low complexity joint angle-of-arrival (AoA) and channel estimation for single-input-multiple-output (SIMO) OFDM system is considered. By exploiting the sparsity of wireless channel, we first propose a compressive-sensing (CS) based channel estimation for pilot-assisted OFDM systems with multiple receive antennas. Then, a low-complexity method is proposed to conduct AoA estimation. Finally, simulations are provided to show the effectiveness of the proposed scheme.
A Low-complexity Precoder Searching Algorithm for MIMO-OFDM Systems
(2009)
Wu, Wen-Rong
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Hsu, Tzu-Han
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電信工程研究所
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Institute of Communications Engineering
Precoding is an effective technique enhancing the performance of MIMO-OFDM systems. In practical systems, the precoding matrix is computed at the receiver, and then fed back to the transmitter. To reduce the amount of the feedback data, only the index representing a quantized precoding matrix is fed back. The quantized matrix is selected from a set of predetermined matrices called a codebook. Since the number of matrices in the codebook may be large, the search for the optimum precoder requires a high computational complexity. In this paper, we propose a low-complexity precoder searching algorithm to solve the problem. The basic idea is to construct a tree-like search strategy such that the complexity can be reduced from O(L) is O(log(2)L)) where L is the number of the codewords. Compared to the exhaustive search, the proposed searching method can reduce the searching complexity significantly while the performance loss is small.
A Low-Complexity Zero-forcing CFO Compensation Scheme for OFDMA Uplink Systems
(2008-10-01)
Hsu, Chao-Yuan
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Wu, Wen-Rong
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電信工程研究所
;
Institute of Communications Engineering
Similar to the conventional orthogonal frequency-division multiplexing (OFDM) system, an OFDM multiple access (OFDMA) system will have a carrier frequency offset (CFO) problem. Since CFOs of all users are different, CFO compensation in the OFDMA uplink system is much more involved. A simple, yet efficient, method is the zero-forcing (ZF) compensation method. However, it involves an inverse of an N x N CFO-induced ICI matrix, where N is the number of subcarriers. Thus, the complexity can become very high when N is large, a case commonly seen in OFDMA systems. In this work, we propose a low-complexity ZF method to overcome the problem. The main idea is to use Newton's method to solve matrix inversion iteratively. We explore the structure of the CFO-induced ICI matrix and develop a method that can implement Newton's method with fast Fourier transforms (FFTs). As a result, the required computational complexity is significantly reduced from O(N-3) to O(2N log(2) N). Simulations show that, with only three iterations, the proposed method can have similar performance to the direct ZF method.
A MULTI-STAGE IIR TIME DOMAIN EQUALIZER FOR OFDM SYSTEMS WITH ISI
(2010-04-01)
Wu, Wen-Rong
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Lee, Chun-Fang
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電信工程研究所
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Institute of Communications Engineering
In an orthogonal frequency division multiplexing (OFDM) system, it is known that when the delay spread of the channel is larger than the cyclic prefix (CP) size, intersymbol interference will occur. The time-domain equalizer (TEQ), designed to shorten the channel impulse response (CIR), is a common device to solve this problem. Conventionally, the TEQ is treated as a finite-impulse-response (FIR) filter, and many TEQ design methods have been proposed. However, a wireless channel typically has multi-path responses, exhibiting FIR characteristics. Thus, the corresponding TEQ will have an infinite impulse response (IIR), and the FIR modeling of the TEQ is inefficient, i.e., the required order for the TEQ will be high. The conventional approach will then suffer from the high computation complexity problem, both in the derivation of TEQ and in the operation of channel shortening. In this paper, we propose a new scheme to overcome these problems. In the derivation of the TEQ, we propose to use a multistage structure, replacing a high-order TEQ with a cascade of several low-order TEQs. In the shortening operation, we propose to use an IIR TEQ approximating a high-order FIR TEQ. Since the ideal TEQ exhibits low-order IIR characteristics, the order of the IIR TEQ can be much lower than the FIR TEQ. Simulations show that while the proposed method can reduce computational complexity significantly, its performance is almost as good as existing methods.
A novel adaptive antenna array for DS/CDMA code acquisition
(2007-09-01)
Yang, Hua-Lung
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Wu, Wen-Rong
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電信工程研究所
;
Institute of Communications Engineering
Equipped with an adaptive beamformer, existing adaptive array code acquisition still relies on the correlator structure. Due to the inherent property of the associated serial-search scheme, its mean acquisition time is large, especially in strong interference environments. In this paper, we propose a novel adaptive filtering scheme to solve the problem.. The proposed scheme comprises two adaptive filters, an adaptive spatial and an adaptive temporal filter. With a specially designed structure, the spatial filter can act as a beamformer suppressing interference, while the temporal filter can act as a code-delay estimator. A mean squared error (MSE) criterion is proposed such that these filters can be simultaneously adjusted by a stochastic gradient descent method. The performance as well as the convergence behavior of the proposed algorithm are analyzed in detail. Closed-form expressions for optimum filter weights, optimum beamformer signal-to-interference-plus-noise ratio (SINR), steady-state MSE, and mean acquisition time are derived for the additive white Gaussian noise (AWGN) channel. Computer simulations show that the mean acquisition time of the proposed algorithm is much shorter than that of the correlator-based approach, and the derived theoretical expressions are accurate.
Adaptive decision feedback space-time equalization with generalized sidelobe cancellation
(2008-09-01)
Lee, Yinman
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Wu, Wen-Rong
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電信工程研究所
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Institute of Communications Engineering
In wireless communications, cochannel interference (CCI) and intersymbol interference (ISI) are two main factors that limit system performance. Conventionally, a beamformer is used to reduce CCI, whereas an equalizer is used to compensate for ISI. These two devices can be combined into one as space-time equalizer (STE). A training sequence is usually required to train the STE prior to its use. In some applications, however, spatial information corresponding to a desired user is available, but the training sequence is not. In this paper, we propose an adaptive decision feedback STE to cope with this problem. Our scheme consists of an adaptive decision feedback generalized sidelobe canceller (DFGSC), a blind decision feedback equalizer (DFE), and a channel estimator. Due to the feedback operation, the proposed DFGSC is not only superior to the conventional generalized sidelobe canceller but also robust to multipath channel propagation and spatial signature error. Theoretical results are derived for optimum solutions, convergence behavior, and robustness properties. With the special channel-aided architecture, the proposed blind DFE can reduce the error propagation effect and be more stable than the conventional blind DFE. Simulation results show that the proposed STE is effective in mitigating both CCI and ISI, even in severe channel environments.
Adaptive two-stage GSC-based PIC detection for time-varying MIMO channels
(2008-04-01)
Lee, Yinman
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Wu, Wen-Rong
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電信工程研究所
;
Institute of Communications Engineering
Adaptive parallel interference cancellation (PIC) has been recently proposed for the signal detection in multiple-input multiple-output (MIMO) systems. However, it suffers from error propagation when operated in time-varying channels. In this letter, an adaptive two-stage PIC with the minimum variance (MV) criterion is proposed to solve the problem. Adaptation with the MV criterion is realized with a decision feedback generalized sidelobe canceller (DFGSC). In the first-stage cancellation, a special structure involving dual DFGSCs is developed. All adaptation operations are implemented with the least-mean-square (LMS) algorithm. Simulations show that the proposed adaptive PIC detection can significantly outperform the conventional adaptive PIC detection in time-varying MIMO channel environments.
An Enhanced Zero-forcing Equalizer for Combinerless LINC-OFDM Systems
(2014)
Cheng, Sheng-Lung
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Wu, Wen-Rong
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Hsu, Ying-Pei
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電信工程研究所
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Institute of Communications Engineering
The energy efficiency of OFDM systems is known to be low due to the high peak-to-average power ratio problem. The linear-amplification-with-nonlinear-component (LINC) technique can solve the problem by decomposing the input signal into two constant-envelop component signals and applying high-efficient nonlinear amplifiers. However, the power combiner, a key component used to combine the amplified signals, is difficult to implement. Combinerless LINC systems employ two transmit antennas such that two component signals can be naturally combined at the receiver. Unfortunately, the performance of combinerless LINC-OFDM systems is seriously degraded if difference exists, even small, between the two channels that the two component signals propagate. In this paper, we propose an enhanced zero-forcing equalizer to solve this problem. The main idea is to reduce a self-interference induced by the channel difference. A closed-form expression for the optimum parameter of the proposed equalizer is also derived. Simulations show that the proposed equalizer can effectively enhance the performance of combinerless LINC-OFDM systems.
Cooperative Radio Source Positioning and Power Map Reconstruction: A Sparse Bayesian Learning Approach
(2015-06-01)
Huang, Din-Hwa
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Wu, Sau-Hsuan
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Wu, Wen-Rong
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Wang, Peng-Hua
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電機資訊學士班
It is known that in addition to spectrum sparsity, spatial sparsity can also be used to further enhance spectral utilization in cognitive radio systems. To achieve that, secondary users (SUs) must know the locations and signal strength distributions (SSDs) of primary users\' base stations (PUBSs). Recently, a group sparse total least squares method was developed to cooperatively sense the PUBSs\' signal strength and estimate their locations. It approximates PUBSs\' power decay with a path loss model (PLM), assumes PUBSs\' locations on some grid points, and then accomplishes the estimation tasks. However, the parameters of the PLM have to be known in advance, and the accuracy of the location estimation is bounded by the resolution of the grid points, which limit its practical applications. In this paper, we propose a sparse Bayesian learning method to solve the problems. We use a Laplacian function to model the power decay of a PUBS and then derive learning rules to estimate corresponding parameters. The distinct features of the proposed method are that most parameters are adaptively estimated, and little prior information is needed. To further enhance the performance, we incorporate source number detection methods in the proposed algorithm such that the number of the PUBSs can be precisely detected, facilitating the estimation of PUBSs\' locations and SSDs. Moreover, the proposed algorithm is modified into a recursive mode to adapt to SUs\' mobility and time-variant observations. Simulations show that the proposed algorithm has good performance, even when the spatial measurement rate is low.
Digital predistortion for concurrent dual-band transmitter using a 2D-LUT based Hammerstein architecture
(2017-01)
Cheng, Sheng-Lung
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Wu, Wen-Rong
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Peng, Chun-Hsien
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Liu, Jen-Yang
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Hsu, Chen-Jui
Digital predistortion has been proved to be an effective remedy for the nonlinear distortion in radio-frequency power amplifier. Unfortunately, conventional digital predistorters (DPDs) developed for single-band transmitters can not be directly applied to concurrent dual-band. To solve the problem, DPDs with two dimensional memory polynomials were then proposed. Although they can provide good performance, its computational complexity is high for the estimation of polynomial coefficients. Look-up-table (LUT) based DPDs, developed for single-band transmitters, can effectively solve the problem. However, there are limited studies about LUT based DPDs for concurrent dual-band transmitters. In this paper, we propose a two-dimensional LUT based Hammerstein DPD to solve the problem. Analyzing the signals on both bands, we propose a new adaptive algorithm with the gradient-descent and recursive-least-squares (RLS) methods to train the DPD iteratively. With the proposed method, the computational complexity in the identification of the DPDs can be significantly reduced. Finally, simulations and experiments are conducted to demonstrate that the performance of the proposed DPD is comparable to that of the polynomial based DPD.
High-Resolution AoA Estimation for Hybrid Antenna Arrays
(2015-07-01)
Chuang, Sheng-Fu
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Wu, Wen-Rong
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Liu, Yen-Ting
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傳播研究所
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Institute of Communication Studies
Communication with unlicenced 60-GHz band is attractive for wireless networks since a free-of-charge bandwidth of 7 GHz is available. The antenna array, conducting transmitter/receiver beamforming, is often used to enhance the transmission range. However, to conduct beamforming, the transceivers have to know the angles of signal arrival. Many angle-of-arrival (AoA) estimation algorithms are well known. Unfortunately, most of them are developed for digital arrays, requiring an analog-to-digital-converter (ADC) and a digital-to-analog converter (DAC) for each antenna element. For an array with a large number of antennas, the implementation cost will be very high. Recently, hybrid antenna array, in which multiple antennas share an ADC/DAC, was proposed as a low-cost solution. However, the existing AoA estimation algorithms for hybrid arrays can only support one AoA; their applications are then limited in multipath indoor environments. In this paper, we propose new AoA estimation algorithms to solve the problem. Using an augmented beam-space approach, we extend the use of the well-known AoA estimation algorithms, MUSIC and ESPRIT developed for digital arrays, to the scenario of hybrid antenna arrays. Simulations show that the proposed algorithms, although developed for hybrid arrays, can have similar performance as MUSIC and ESPRIT in digital arrays.
ICI Mitigation for OFDMA Uplink Systems
(2008)
Hsu, Chao-Yuan
;
Wu, Wen-Rong
;
電信工程研究所
;
Institute of Communications Engineering
Orthogonal frequency-division multiplexing multiple access (OFDMA) has been considered as a promising multiple access technique in OFDM-based communication systems. In an ideal OFDMA system, no intercarrier interference (ICI) occurs. However, in high-speed mobile environments, the channel may be time-variant during one OFDMA symbol period. In such a condition, the mobility-induced ICI will degrade the system performance seriously. To remove the ICI effect, the zero-forcing (ZF) method is one of the simple techniques. Unfortunately, the direct ZF method needs to invert an N x N ICI matrix, requiring a prohibitively high complexity when N is large. In this paper, we first derive a structured OFDMA uplink signal model consisting of FFTs/IFFTs. Exploring this structure and using Newton's iteration for matrix inversion, we develop a low-complexity ZF method. With our formulation, fast Fourier transforms (FFTs) can be used to reduce the complexity. Thus, the proposed method can reduce the complexity from O(N(3)) to O(N log(2) N). Simulations show that the proposed method can have the similar performance to the direct ZF method while requires much lower complexity.
ICI mitigation method for high-speed mobile OFDM systems
(2008-05-01)
Wu, Wen-Rong
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Hsu, Chao-Yuan
In high-speed mobile environments, the channel is time-varying within an OFDM symbol. This time-varying characteristic will destroy the orthogonality among subcarriers. Thus, the intercarrier interference (ICI) will occur and the system performance will be degraded. An ICI mitigation method for high-speed mobile OFDM systems is proposed, which explores the special structure of the ICI channel matrix and applies the Newton's iterative matrix inversion method. With our formulation, fast Fourier transform (FFT) can be used to reduce the computational complexity. The object of canceling the ICI can be accomplished without the need of any extra circuit.
Joint AoA and Channel Estimation for Pulse-shpaed SIMO-OFDM systems
(2018-01-01)
Horng, Cian-Ya
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Wu, Wen-Rong
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Huang, Din-Hwa
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電信工程研究所
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Institute of Communications Engineering
Beamforming has been considered a crucial technique in 5G era. To perform received beamforming, one must know the information of angle-of-arrival (AoA). This paper considers AoA estimation in single-input-multiple-output (SIMO) pilot-assisted OFDM systems. Exploiting the sparsity of the wireless channel, we can first efficiently estimate channel impulse response with a compressive-sensing based technique. However, existing works often do not take the pulse-shaping effect into account, resulting in poor performance in real-world systems. We propose a basis adaptive block sparse Bayesian learning framework to solve the problem. Once the CIRs corresponding to received antennas are obtained, AuAs can then be estimated accordingly.
Joint AoA and Channel Estimation for SIMO- OFDM Systems: A Compressive-Sensing Approach
(2016-05)
Chen, Chih-Yu
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Wu, Wen-Rong
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Gau, Chi-Shiang
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資訊工程學系
;
電機學院
Millimeter-wave (mmWave) transmission has been considered in future fifth generation (5G) communication systems. Since the pathloss in mmWave is severe, beamforming with antenna arrays, an enabling technology in the 5G era, will become a must. To conduct receive beamforming, however, we need to know the information about the angle-of-arrival (AoA). In this paper, we consider joint AoA and channel estimation for single-input-multiple-output (SIMO) OFDM systems. As known, wireless channels are sparse, and this is particularly true for mmWave environments. Conventional compressive-sensing (CS) based channel estimation methods only consider single-input-single-output systems. We propose new matching-pursuit-based CS methods for channel estimation in SIMO-OFDM systems, using frequency-domain pilots. With the estimated channels, AoA\'s are then estimated by the maximum-likelihood method. Since a hidden parameter is involved in the problem, an expectation-maximization (EM) algorithm is then employed. The Cramer-Rao lower bound (CRLB) is also derived for the AoA estimation. Simulation results show that the proposed channel estimation can significantly outperform existing methods while the proposed AoA estimation attains the CRLB.
Joint AoD, AoA, and Channel Estimation for MIMO-OFDM Systems
(2018-07-01)
Chen, Chih-Yu
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Wu, Wen-Rong
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電機工程學系
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Department of Electrical and Computer Engineering
Beamforming with antenna arrays has been considered as an enabling technology in future wireless communication systems. To conduct beamforming, one has to know the angle-of-departure (AoD) or angle-of-arrival (AoA). For data detection, the receiver also has to know channel response. In this paper, we propose a new joint. AoD, AoA, and channel estimation scheme for pilot-assisted multiple-input-multiple-output orthogonal-frequency-division-multiplexing (MIMO-OFDM) systems. First, a compressive-sensing technique is employed to estimate the channel impulse response, exploiting the sparsity property of wireless channels. Then, AoA and AoD are jointly estimated for each detected path by the maximum likelihood method. The Cramer-Rao lower hound (CRLB) is also derived and a transmit beamforming scheme is proposed accordingly. In the scenario of available prior information, a maximum a posteriori estimation is proposed. The Bayesian CRLB (BCRLB) for the problem is also derived and a transmit beamforming scheme is further proposed. It turns out that only two training OFDM symbols are required for the estimation. Simulation results show that the proposed methods can approach the CRLB/BCRLB in both scenarios and achieve the same spectral efficiency as that obtained with the ideal channel in millimeter-wave communications.
Joint MMSE Transceiver Design in Amplify-and-Forward MIMO Relay Systems with Tomlinson-Harashima Source Precoding
(2010)
Tseng, Fan-Shuo
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Chang, Min-Yao
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Wu, Wen-Rong
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傳播研究所
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Institute of Communication Studies
Existing precoding schemes in amplify-and-forward (AF) multiple-input-multiple-output (MIMO) relay systems use linear precoders. In this paper, we consider a precoding scheme in which a Tomlinson-Harashima (TH) precoder (THP) is used at the source and a linear precoder at the relay. With a minimummean- squared-error (MMSE) receiver at destination, we propose a new joint precoders design method. Since two precoders are involved, the transceiver design, formulated as an optimization problem, is difficult to solve. To overcome the problem, we propose cascading an additional unitary precoder with the TH precoder. The unitary precoder can not only simplify the optimization problem but also improve the MMSE performance. With the specially designed unitary precoder at the source, we can then adopt the primal decomposition method to solve this problem. With the method, the original optimization problem can first be decomposed into a master and a subproblem optimization problems, and then transferred to a relay precoder optimization problem. However, the optimization is not a convex problem and the solution is not obtainable. We then propose a method being able to transfer it to a convex optimization problem. A closedform solution can then be obtained by the Karuch-Kuhn-Tucker (KKT) conditions. Simulations show that the proposed transceiver can significantly outperform existing linear transceivers.