Browsing by Author "Lin, David W."

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A Complex Modulo-and-Forward Strategy for Two-Way Relay Systems
(2012-01-01)
Chung, I-Chou A.
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Lu, Shih-Jung
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Lin, David W.
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
Relays have become increasingly important components in modern wireless communication systems. We consider an amplify-and-forward-based two-way relay scenario where the two end terminals employ two-dimensional modulations. We propose a complex modulo-and-forward strategy to improve the energy efficiency of the relay station. This improvement comes about by the ability of the complex modulo operation to reduce the relay transmission power while maintaining the error performance at the terminal nodes. The amount of reduction in relay transmission power is evaluated for QAM modulations. When the two channel coefficients between the two terminal nodes and the relay have equal magnitude, a reduction close to 3 dB can be realized, depending on the modulation order. The transmission performance is verified in terms of QAM symbol error rates.
A Decision-Directed Channel Estimator for OFDM-Based Bursty Vehicular Communication
(2017-06-01)
Wei, Chunfan
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Lin, David W.
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
We consider bursty orthogonal frequency-division multiplexing (OFDM) signal transmission, where each signal burst (or packet) consists of a preamble followed by data symbols which may contain few pilots. Channel estimation in vehicular environments for such signals is a challenging issue, as evidenced by related studies for the IEEE 802.11p standard. Existing channel estimation methods often yield deficient performance or require relatively high latency to achieve better performance, unless additional pilots are introduced. We propose a technique that addresses this problem. The primary aspect of the design is to exploit the temporal correlation in channel responses over close-by OFDM symbols in a decision-oriented manner. A secondary aspect is to take advantage of any available pilots in the data symbols in a frequency-domain filtering built on the temporal processing results. Regarding the performance of the proposed method, we first analyze the proposed temporal processing technique under some simplistic channel conditions both for insight and for verification. Further simulation results based on the IEEE 802.11p specifications under a measurement-based fading multipath vehicle-to-vehicle (V2V) channel model show the superiority of the proposed method.
A group matching pursuit algorithm for sparse channel estimation for OFDM transmission
(2006)
Wu, Chun-Jung
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Lin, David W.
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
Time-domain channel estimation techniques have been proposed for OFDM systems for their ability to yield relatively accurate estimates with only a few pilots. Key information needed in such techniques is the multipath delays of the channel. Prior approaches to estimation of multipath delays require regular pilot structures and may not work in slow fading. We propose a group matching pursuit technique for channel estimation. The technique is an extension of the orthogonal matching pursuit technique. It employs the pilots in several OFDM symbols to estimate the multipath delays in a sequential manner, where the pilots can have an arbitrary structure. Simulation results show that the proposed algorithm has superior performance.
A group matching pursuit algorithm for sparse channel estimation for OFDM transmission
(2006-01-01)
Wu, Chun-Jung
;
Lin, David W.
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
Time-domain channel estimation techniques have been proposed for OFDM systems for their ability to yield relatively accurate estimates with only a few pilots. Key information needed in such techniques is the multipath delays of the channel. Prior approaches to estimation of multipath delays require regular pilot structures and may not work in slow fading. We propose a group matching pursuit technique for channel estimation. The technique is an extension of the orthogonal matching pursuit technique. It employs the pilots in several OFDM symbols to estimate the multipath delays in a sequential manner, where the pilots can have an arbitrary structure. Simulation results show that the proposed algorithm has superior performance.
A group matching pursuit algorithm for sparse channel estimation for OFDM transmission
(2006)
Wu, Chun-Jung
;
Lin, David W.
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
Time-domain channel estimation techniques have been proposed for OFDM systems for their ability to yield relatively accurate estimates with only a few pilots. Key information needed in such techniques is the multipath delays of the channel. Prior approaches to estimation of multipath delays require regular pilot structures and may not work in slow fading. We propose a group matching pursuit technique for channel estimation. The technique is an extension of the orthogonal matching pursuit technique. It employs the pilots in several OFDM symbols to estimate the multipath delays in a sequential manner, where the pilots can have an arbitrary structure. Simulation results show that the proposed algorithm has superior performance.
An Analysis of the Performance of ML Blind OFDM Symbol Timing Estimation
(2018-10-15)
Lin, David W.
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
Symbol timing offset (STO) estimation constitutes an important part in orthogonal frequency-division multiplexing (OFDM) signal synchronization, and many blind methods based on the maximum likelihood (ML) principle have been proposed. A theoretical analysis of the estimation performance, however, is very difficult, due in part to the discrete nature of the STO. As a result, most studies on OFDM STO estimation have relied only on computer simulation for performance evaluation. This paper presents a theoretical performance analysis for ML-based blind OFDM STO estimation. We start with a review of the formulation of ML blind STO estimation problem and its solution. We then conduct the said performance analysis. The analysis turns out to involve a study of the eigenvalues of some matrix functions of the autocorrelation of the received signal. As the general case leads to highly complicated mathematics, we treat the case of full-band OFDM in additive white Gaussian noise in more detail. In this case, we are able to obtain mathematically tractable bounds and approximation to the estimation error probability and the corresponding mean-square error. In high signal-to-noise ratio (SNR), these bounds and approximation show inverse power-law dependence on the SNR. We also compare the bounds with simulation results and with an existing approximation in the literature.
CHANNEL ESTIMATION FOR LTE AND LTE-A MU-MIMO UPLINK WITH A NARROW TRANSMISSION BAND
(2014-01-01)
Chen, Chih-Ying
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Lin, David W.
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
The 3GPP LTE and LTE-A standards provide for multi-user multi-input multi-output (MU-MIMO) transmission in the uplink. But the property of reference signals (RSs) is such that care must be exercised in channel estimation to minimize the interference among different antenna channels. Typical DFT- and DCT-based channel estimation methods have low complexity but may yield relatively high inter-channel interference, especially when user equipments (UEs) are allocated narrow transmission bands. Methods that can attain better channel separation often have much higher computational requirements. In this paper, we propose an MU channel estimation technique that seeks to minimize an L-1 norm of error. It does not need second-order statistics as the linear minimum mean-square error (LMMSE) technique, has good numerical properties, and yields a performance between the least-square (LS), the DFT, and the LMMSE techniques.
CHANNEL ESTIMATION TECHNIQUE FOR MULTI-CARRIER SYSTEM
(2010-10-14)
Hung, Kun-Chien
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Lin, David W.
The present invention relates to a channel estimation technique for multi-carrier system which uses pilot signal to first obtain pilot subcarrier channel responses, then estimate the mean delay and the root-mean-square delay spread of the channel, and then uses interpolation to obtain the channel responses of arbitrary subcarriers. The present invention is able to perform channel estimation and to accurately estimate various channel delay parameters (such as the initial delay, the mean delay, and the root-mean-square delay spread) with very low quantity of pilot signal and with low computational complexity. In addition, the present invention has broad applicability, as it does not require the channel power-delay profile to be of certain specific types and it can operate in systems where pilot subcarriers have an irregular configuration.
Design and DSP software implementation of mobile WiMAX baseband transceiver functions
(2007)
Wang, Hai-wei
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Lin, David W.
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Hungi, Kun-Chien
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Lee, Youn-Tai
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電子工程學系及電子研究所
This paper considers the orthogonal frequency-division multiple access (OFDMA) physical layer of the IEEE 802.16e standard. We discuss the design of some key baseband signal processing functions, including synchronization, channel estimation, and forward-error-correction (FEC) decoding. Further, we describe a fully software implementation of these functions employing Texas Instruments' digital signal processors (DSPs).
Design of Distributed Amplify-and-Forward Relay Network for MIMO Transmission
(2010)
Wu, Chun-Jung
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Lin, David W.
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Sang, Tzu-Hsien
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
We consider the design of a multi-input multi-output (MIMO) transmission system equipped with distributed, single-antenna amplify-and-forward relays. In particular, we consider how to optimize the complex forwarding coefficients of the relays to maximize the system capacity. Existing studies on MIMO relay network designs mostly concentrate on the case of single relay with multiple antennas, whose results are not applicable to distributed networks. We first analyze how the system capacity relates to the relay transmission powers and the channel coefficients. An upper bound on the asymptotic capacity at high relay powers is derived. Aided by the capacity results, we consider the optimization of relay coefficients under two noise conditions: that where the destination noise dominates the total noise and that where the relay noise dominates the total noise. It turns out that no simple analytic solutions can be found for all relay network sizes. Therefore, we propose suboptimal solutions that involve computation of optimal relay coefficients with optimal selection of relays for network sizes that can be solved analytically. The results on diversity order are found to show resemblance to that for single-hop MIMO systems employing antenna selection. Simulation results also verify the superior performance of the proposed technique.
Detection of Cooperative OFDM Signals in Time-Varying Channels with Partial Whitening of Intercarrier Interference
(2013-01-01)
Wang, Hai-wei
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Lin, David W.
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Sang, Tzu-Hsien
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電機工程學系
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Department of Electrical and Computer Engineering
Cooperative communication can yield spatial diversity gains, but its performance can also suffer severely from the multiple carrier frequency offsets (MCFOs) induced by high-speed relative motion, in addition to Doppler spread. In orthogonal frequency-division multiplexing (OFDM) systems, such performance degradation arises as a result of intercarrier interference (ICI). We have shown previously that, in conventional non-cooperative communication, certain ICI components (called residual ICI) display high normalized autocorrelation and a partial whitening of the residual ICI can significantly benefit OFDM signal detection. In this work, we show that similar whitening can also be employed in cooperative communication to address the MCFO issue mentioned above. As an illustration, we simulate a distributed spatial-frequency block coding (SFBC) OFDM system subject to highly disparate MCFOs, where in the receiver we perform partial whitening of the residual ICI followed by maximum-likelihood sequence estimation (MLSE). The results show that the proposed method can comfortably accommodate a maximum MCFO span well exceeding one subcarrier spacing.
Interpolative Reconstruction of Millimeter Wave Mobile Channel Responses from Randomly Scattered Snapshots
(2017-01-01)
Chang, Chih-Hao
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Lin, David W.
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
There is strong recent interest in exploiting the millimeter wave (mmWave) frequencies for cellular communication. For signal transmission studies, one needs a channel model that can simulate the spatial variation of mmWave channel impulse responses (CIRs) in treated operating environments. We consider reconstruction of spatially varying CIRs by interpolation over a set of existing CIR snapshots for some randomly scattered geographical locations. Such interpolation, however, is not trivial work, as we need to handle drifting of path delays over different spatial locations as well as deal with appearing and disappearing of paths. We propose a way to address these issues, and we present some numerical results for a typical urban area.
Joint detection of integral carrier frequency offset and preamble index in OFDMA WiMAX downlink synchronization
(2007)
Hung, Kun-Chien
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Lin, David W.
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
Initial downlink synchronization in orthogonal frequency-division multiple access (OFDMA)-based WiMAX transmission involves carrier frequency synchronization, timing synchronization, and preamble index identification. Due to the structure of the preamble signal, it is reasonable to take a twostage approach in which one first estimates and compensates the timing offset and the so-called fractional carrier frequency offset (CFO) and then deals with the integral CFO and the preamble index. This paper considers joint detection of integral CFO and preamble index, under the assumption that timing and fractional CFO have been acquired to reasonable accuracy. Based on an optimization formulation, we derive a number of detection methods of different complexity and suboptimality. The methods exploit the quasi-orthogonality among the OFDMA WiMAX preamble sequences as well as the organization of the nonzero subcarriers in the preambles. Simulation results are presented to illustrate the performance of the methods.
Jointly iterative channel estimation and data detection for cyclic-prefixed block transmission over time-varying channels
(2007)
Hung, Kun-Chien
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Lin, David W.
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
Various forms of cyclic-prefixed block transmission have been considered for signal transmission over multipath fading channels. Among them the most well-known is orthogonal frequency-division multiplexing, or OFDM. Whatever the form, such systems usually transmit many known pilots to assist channel estimation and data detection. We consider receiver techniques that can help improve the bandwidth efficiency by reducing the needed amount of pilots. In particular, we consider using only a block of pilots as the preamble. Then the receiver tracks the channel variation via jointly iterative channel estimation and data detection. It uses a frequency-domain equalizer to reduce the complexity. Simulations show that the tracking ability of the proposed method is good.
LMMSE Channel Estimation for OFDM Systems with Low Number of Distributed Pilots
(2009)
Hung, Kun-Chien
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Lin, David W.
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
In linear minimum mean-square error (LMMSE) channel estimation for multicarrier transmission, how to estimate the needed channel correlation function remains a problem for systems with small number of pilots operating in time-varying channels. We consider approximating the channel power-delay profile by a shape characterized fully by the mean delay and the root-mean-square (RMS) delay spread. We present a novel, simple way to estimate these parameters in systems with distributed pilots. The ensuing channel estimation method is also applicable to multi-access communication. As a practical example, we also specialize the proposed technique to Mobile WiMAX signals and investigate the resulting performance.
LMMSE-Based Channel Estimation for LTE-Advanced MIMO Downlink Employing UE-Specific Reference Signals
(2015)
Hsieh, Chia-Yu
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Lin, David W.
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Ma, Chingwo
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
The LTE-Advanced wireless communication standard specifies UE-specific reference signals (short-named DMRS) to facilitate multi-input multi-output (MIMO) downlink channel estimation. This paper presents a linear minimum mean-square error (LMMSE)-based channel estimation technique in this context. Key to LMMSE channel estimation is the acquisition of some second-order channel statistics, for which the present work adopts a previously proposed approach that employs a parametric model for the channel autocorrelation function and estimates the model parameters from the received DM-RS. While such MIMO transmission is usually considered applicable to slow motion, our results show that the channel estimation technique itself can accommodate much faster motion.
LMMSE-Based Sidelink Channel Estimation for LTE Communication Systems
(2019-01-01)
Huang, Yu-Ching
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Lin, David W.
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
The 3GPP LTE series of standards have made provisions for device-to-device (D2D) and vehicle-to-everything (V2X) communication in recent releases. The interface accommodating such communication is termed the sidelink (SL), which reuses some physical layer specifications originally defined for the uplink, such as single-carrier frequency-division multiple access (SC-FDMA), with some modifications. One modification is the use of doubly dense demodulation reference signals (DMRS) in the V2X setting to assist channel estimation. An SC-FDMA channel estimator typically starts with a least-squares (LS) estimation of the channel frequency response (CFR) at the DMRS. It then obtains CFR estimates for the data subcarriers by some time-frequency filtering. This work considers SL channel estimation techniques. The proposed method employs a novel linear minimum mean-square error (LMMSE) filtering mechanism in the frequency domain to reduce the noise in the LS CFR estimates and then, depending on the DMRS density, uses particularly designed LMMSE filtering or polynomial interpolation in the time domain to obtain the desired CFR estimates for the data subcarriers. Simulation results show that the method can yield superior performance over a wide range of channel conditions.
LTE-Advanced Downlink Channel Estimation Under Minimum Resource Allocation With DM-RS
(2018-01-01)
Lin, Yi-Hsiang
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Lin, David W.
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
The LTE-Advanced defines a way of downlink multinput multi-output (MIMO) transmission that supports non-codebook-based precoding with minimum resource allocation consisting of two temporally consecutive physical resource blocks (PRBs). The required channel estimation is facilitated by the specifically provided "demodulation reference signal" (DM-RS). However, such channel estimation is challenging in doubly (time and frequency) selective fading channels under minimum resource allocation due to the fewness of DM-RS subcarriers. We propose a decision-directed channel estimation method that addresses the issue. It conducts a sequence of decision-aided averaging of various tentative channel estimates in both the frequency and time domains to arrive at the resulting channel estimate. The method has relatvely low complexity. Simulation results on multipath fading channels demonstrate the performance of the proposed channel estimation method.
Maximum-Likelihood Blind Synchronization for GFDM Systems
(2016-06)
Wang, Po-Sen
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Lin, David W.
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
Generalized frequency-division multiplexing (GFDM) inherits the cyclic-prefixed (CP) block signaling structure from orthogonal frequency-division multiplexing (OFDM). The feasibility of CP-based maximum-likelihood (ML) blind synchronization for GFDM systems, however, has not been assessed in the literature. We find that such synchronization is complicated by the covariance structure of the signal which, due to nonorthogonal pulse shaping, possesses a time-varying signal power and some weak self-induced interference. Neglecting the latter and approximating the former by the average power enable us to obtain an approximate-ML solution based on a recent result on OFDM synchronization in wide-sense stationary uncorrelated scattering (WSSUS) channels. Simulations show that the resulting method for GFDM yields comparable performance, under commonly considered pulse-shaping filters, to its kin for OFDM.
METHOD FOR OFDM AND OFDMA CHANNEL ESTIMATION
(2009-09-17)
HUNG, Kun-Chien
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Lin, David W.
This invention discloses a method for OFDM and OFDMA channel estimation via phase-rotated polynomial interpolation and extrapolation (inter/extra-polation). For complexity reason, polynomial inter/extra-polation is an often considered method for channel estimation in orthogonal frequency-division multiplexing (OFDM) and orthogonal frequency-division multiple access (OFDMA) systems, in which the simplest choice is linear inter/extra-polation. But the performance of this method depends on the accuracy of symbol timing estimation as well as the channel delay spread. The invention mitigates the problem by adding a linear phase factor to polynomial inter/extra-polation, which corresponds to adding a delay (also called delay shift) in the time domain.