Browsing by Author "Hung, Kun-Chien"

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CHANNEL ESTIMATION TECHNIQUE FOR MULTI-CARRIER SYSTEM
(2010-10-14)
Hung, Kun-Chien
;
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.
Joint detection of integral carrier frequency offset and preamble index in OFDMA WiMAX downlink synchronization
(2007)
Hung, Kun-Chien
;
Lin, David W.
;
電子工程學系及電子研究所
;
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
;
Lin, David W.
;
電子工程學系及電子研究所
;
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
;
Lin, David W.
;
電子工程學系及電子研究所
;
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.
Optimal delay estimation for phase-rotated linearly interpolative channel estimation in OFDM and OFDMA systems
(2008)
Hung, Kun-Chien
;
Lin, David W.
;
電子工程學系及電子研究所
;
電信研究中心
;
Department of Electronics Engineering and Institute of Electronics
We consider the phase-rotated linearly interpolative channel estimation technique for multicarrier transmission. The technique models the channel frequency response between two nearby subcarriers as the product of a linear function and a linear-phase factor, where the linear-phase factor may be equivalently modeled in the time domain as a reference delay dubbed the anchor delay in this work. We show that the performance of the technique is a fourth-order function of the channel path delays and the anchor delay. We derive a method to estimate the optimal anchor delay. Analysis and simulation in a context of Mobile WiMAX downlink transmission show that, with the proposed anchor delay estimate, we can attain better performance in channel estimation than conventional linear interpolation and a previously proposed method of phase-compensated linear interpolation.
Pilot-Aided Multicarrier Channel Estimation via MMSE Linear Phase-Shifted Polynomial Interpolation
(2010-08-01)
Hung, Kun-Chien
;
Lin, David W.
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
Current wireless multicarrier systems commonly adopt pilot-aided channel estimation, for which a simplest and least restrictive technique is polynomial interpolation in the frequency domain. For channels with large delay spreads, however, the performance of low-order polynomial interpolation suffers from modeling error. The problem may be remedied by adding a linear phase shift to the interpolator, or equivalently, a "window shift" in the time domain. We derive a method to estimate the optimal window shift, in the minimum mean-square error (MMSE) sense, for polynomial-interpolative channel estimation of arbitrary order. As a practical application, we show how to apply the resultant technique to Mobile WiMAX downlink channel estimation. In addition, we propose a method to automatically select the interpolation order based on some estimated MSE.
Pilot-Aided Multicarrier Wireless Channel Estimation via MMSE Polynomial Interpolation
(2008)
Hung, Kun-Chien
;
Lin, David W.
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
Orthogonal frequency division multiplexing (OFDM) and multiple access (OFDMA) signal receivers often employ pilot-aided channel estimation. For it, an often considered technique is frequency-domain polynomial interpolation, due to its simplicity. However, the performance of polynomial interpolators suffers in channels with large delay spreads due to modeling error. The problem can be remedied by including a linear phase to the interpolator. In this paper, we derive a method to estimate the optimal phase shift that minimizes the mean-square channel estimation error. We further consider adaptive selection of the interpolation order for best performance. As a practical application, we adapt the proposed channel estimation technique to Mobile WiMAX downlink transmission and examine the resulting performance.
Pilot-Based LMMSE Channel Estimation for OFDM Systems With Power-Delay Profile Approximation
(2010-01-01)
Hung, Kun-Chien
;
Lin, David W.
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
In linear-minimum-mean-square-error (LMMSE) channel estimation for multicarrier systems, one needs to know the channel correlation function. This poses a problem for systems with a small number of pilots operating in time-varying channels. We propose to approximate the channel power-delay profile (PDP) with a shape that can completely be described in two parameters, namely, the mean delay and the root-mean-square (RMS) delay spread. Furthermore, we develop a simple technique to estimate these delay parameters. The approximate PDP is then used to generate the LMMSE filter coefficients for data-subcarrier channel estimation. Mathematical expressions are derived that can be used to predict the accuracy of the various estimates, and they are verified by simulation. The proposed technique is applicable to both point-to-point communication and multiaccess communication where different users may experience different channel conditions. As a practical application, we also specialize the proposed technique to Mobile WiMAX signals and investigate the resulting performance.
Preamble sequence detection and integral carrier frequency offset estimation method for OFDM/OFDMA wireless communication system
(2009-03-12)
Hung, Kun-Chien
;
Lin, David W.
In multi-cell OFDM/OFDMA wireless communication systems, any subscriber station (SS) or mobile station (MS) that intends to enter the system needs to establish time and frequency synchronization with the base station (BS) and obtain the identification code of the BS, where in frequency synchronization one usually needs to estimate the fractional carrier frequency offset (CFO) and the integral CFO. (“Fractional” and “integral” refer to, respectively, the fractional and the integral parts of the ratio of the CFO to subcarrier spacing.) The present invention assumes that the SS or MS first does timing and fractional CFO synchronization and then conducts integral CFO estimation and BS identity detection. The present invention considers integral CFO estimation and BS identity detection jointly, i.e., it proposes solutions that address these topics jointly. The present invention formulates the problem as a signal detection problem in multi-channel interference and obtains the theoretically optimal solution first, and then derives simplified, approximately optimal solutions, in which the present invention employs frequency-domain filtering to calculate the required correlation values which can drastically reduce the high computational complexity of the original theoretically optimal solution but results in little impact on precision. In addition, the present invention proposes several further simplified algorithms, some of which can even eliminate the use of multipliers. The above proposition of frequency-domain filtering has high extensibility in application to related signal sequence detection problems.
Theory and design of near-optimal MIMO OFDM transmission system for correlated multipath Rayleigh fading channels
(2007-06-01)
Hung, Kun-Chien
;
Lin, David W.
;
電子工程學系及電子研究所
;
電信研究中心
;
Department of Electronics Engineering and Institute of Electronics
We consider channel-coded multi-input multi-output (MIMO) orthogonal frequency-division multiplexing (OFDM) transmission and obtain a condition on its signal for it to attain the maximum diversity and coding gain. As this condition may not be realizable, we propose a suboptimal design that employs an orthogonal transform and a space-frequency interleayer between the channel coder and the multi-antenna OFDM transmitter. We propose a corresponding receiving method based on block turbo equalization. Attention is paid to some detailed design of the transmitter and the receiver to curtail the computational complexity and yet deliver good performance. Simulation results demonstrate that the proposed transmission technique can outperform the conventional coded MIMO OFDM and the MIMO block single-carrier transmission with cyclic prefixing.