Browsing by Author "Chang, MX"

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2D regression channel estimation for equalizing OFDM signals
(2000)
Chang, MX
;
Su, YT
;
電信工程研究所
;
Institute of Communications Engineering
In this paper, we present a channel estimation method for Orthogonal Frequency Division Multiplexing (OFDM) signals. Our method is based on a two-dimensional nonlinear regression method, taking into account the correlations of the fading process in both time and frequency domains. We derive a general bit error rate (BER) expression which can also used to predict the performance of many other OFDM channel estimates. The performance of this new estimate is very close to the theoretical bit error probability lower bound that is obtained by assuming that the-channel response is perfectly known. Unlike linear minimum mean-squared-error (LMMSE) channel estimates, it needs not to know or estimate channel statistics like channel correlation matrix and SNR whence is insensitive to additive white Gaussian noise (AWGN).
Blind and semiblind detections of OFDM signals in fading channels
(2004-05-01)
Chang, MX
;
Su, YT
;
電信工程研究所
;
Institute of Communications Engineering
This paper considers the problem of blind joint channel estimation and data detection for orthogonal frequency-division multiplexing (OFDM) systems in a fading environment. Employing a regression model for a time-varying channel, we convert the problem into one that finds the data sequence x whose associated least-squares'(LS) channel estimate z(x) is closest to the space of some regression curves (surfaces). We apply the branch-and-bound principle to solve the nonlinear integer programming problem associated with finding the curve that fits a subchannel in the LS sense. A recursive formula for fast metric update is obtained by exploiting the intrinsic characteristic of our objective function. The impacts of reordering the data sequence and selective detection are addressed. By employing a preferred order along with a selective detection method, we greatly reduce the detector complexity while giving up little performance loss. Both the complete and the reduced-complexity algorithms can be used for blind and semiblind detections of OFDM signals in a subchannel-by-subchannel manner. To further reduce the complexity and exploit the frequency-domain channel correlation, we suggest a two-stage approach that detects a few selected positions in some subchannels first, and then, treating the detected symbols as pilots, determines the remaining symbols within a properly chosen time-frequency block by a two-dimensional model-based pilot-assisted algorithm. The proposed methods do not require the information of the channel statistics like signal-to-noise ratio or channel correlation function. Performance of differential modulations like differential quaternary phase-shift keying and STAR 16-ary quadrature amplitude modulation are provided. Both blind and semiblind schemes yield satisfactory performance.
Blind detection of OFDM signals in fading channels
(2003)
Su, YT
;
Chang, MX
;
電信工程研究所
;
Institute of Communications Engineering
Blind joint channel and data estimation for OFDM signals in Rayleigh fading
(2001)
Chang, MX
;
Su, YT
;
電信工程研究所
;
Institute of Communications Engineering
In this paper we present several blind model-based joint channel and data estimation methods for Orthogonal Frequency Division Multiplexing (OFDM) signals. We model the time-varying channel response as a polynomial in time. Joint channel estimation and data detection are accomplished by finding the data sequence and regression coefficients that results in the minimum metric between the data-dependent polynomial and the received samples. Our method does not require the information of the channel statistics like signal-to-noise ratio (SNR) or correlation function. Performing exhaustive search among candidate sequences, though optimal, is impractical for long sequences. We develop some suboptimal methods and discuss their pro and con. A two-stage hybrid detection algorithm is proposed and used for detecting differential phase shift keying (DPSK) signals in Rayleigh fading.
Model-based channel estimation for OFDM signals in Rayleigh fading
(2002-04-01)
Chang, MX
;
Su, YT
;
資訊工程學系
;
電信工程研究所
;
Department of Computer Science
This paper proposes a robust pilot-assisted channel estimation method for orthogonal frequency division multiplexing (OFDM) signals in Rayleigh fading. Our estimation method is based on nonlinear regression channel models. Unlike the linear minimum mean-squared error (LMMSE) channel estimate, the method proposed does not have to know or estimate channel statistics like the channel correlation matrix and the average signal-to-noise ratio (SNR) per bit. Numerical results indicate that the performance of the proposed channel estimator is very close to the theoretical bit error propagation lower bound that is obtained by a receiver with perfect channel response information.
Performance analysis of equalized OFDM systems in Rayleigh fading
(2002-10-01)
Chang, MX
;
Su, YT
;
電信工程研究所
;
Institute of Communications Engineering
Channel estimation is usually needed to compensate for the amplitude and phase distortions associated with a received orthogonal frequency-division multiplexing (OFDM) waveform. This paper presents a systematic approach for analyzing the bit-error probability (BEP) of equalized OFDM signals in Rayleigh fading. Closed-form expressions for BEP performance of various signal constellations [phase-shift keying (PSK), differential phase-shift keying (DPSSK), quaternary phase-shift keying (QPSK)] are provided for receivers that use a linear pilot-assisted channel estimate. We. also derive the optimal linear channel estimates that yield the minimum BEP and show that some previous known results are special cases of. our general formulae. The results obtained here can be applied to evaluate the performance of equalized single-carrier narrowband systems as well.