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Improved Channel Estimation Based on Parametric Channel Approximation Modeling for OFDM Systems IEEE TRANSATIONS ON BROADCASTING , VOL. 54 NO. 2 JUNE 2008 指導老師 : 王瑞騰 老師 學 生 : 李裕銘
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OUTLINE Introduction System model
Fraction Taps Channel Approximation Estimations (FTCA) Simulations and analyses Conclusions Rererence
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INTRODUCTION In this paper, improved channel estimation methods based on the parametric channel approximation model using pilot tones are proposed for the OFDM system . The full-rank MMSE estimator has large computational complexity , and in parametric channel estimator can only be adopted in sparse multi-path fading channels. In order to solve these problems lying in with the channel models , a parametric model is proposed . This model is called “fraction taps channel approximation(FTCA)” channel model .
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Channel Model Channel impulse response of the multi-path fading channel the complex gain of the i-th propagation path the delay of the i-th propagation path Frequency response
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Orthogonal frequency division multiplexing(OFDM) system model
Discrete-time baseband equivalent model of OFDM system
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OFDM channel frequency response
, k=0, 1, … , N-1 N : number of the subcarriers k : subcarrier index T : sampling interval hi : the complex gain of the i-th propagation The observed channel
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Time domain signal , n = 0 , 1, … , N-1
Ng : the number of samples in the guard interval which satisfies Ng x T≧ τmax
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The received baseband signal
, n = 0 , 1 , … , N-1 : additive white Gaussian noise (AWGN) : circular convolution , k = 0 ,1, … ,N-1
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Fraction Taps Channel Approximation (FTCA) Estimations
The FTCA channel model : fraction factor selected from (0,1] : complex gain of the l-th approximation M : number of approximation taps
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In matrix notation , the channel frequency response vector
and then , , k=0~N-1 and l=1~M , k=0~N-1 and i=1~L
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Based on the Least Square(LS) criterion g = (FHF)-1FHH = (FHF)-1FHWNh
He : commonly very small and can be negligible when Ka is properly selected .
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The pilot subcarriers arrangement
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In OFDM system , the S pilot subcarriers are assumed to be evenly inserted into the N transmission subcarriers Let P denote the set that contains the position indexes of the S pilot tones
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The FTCA Estimators where , m=0,…,S-1 and l=1,…,M
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The FTCA-MMSE Estimator(Minimu mean- square error)
where
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The Average Channel Energy Approximation Error (ACEAE) Be
For the channel frequency response is achieve by
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The FTCA-LS Estimator For the channel frequency response is achieve by
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Flowchart M
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Simulations and analyses
In the OFDM system Multi-path slow fading channel Carrier frequency : 1GHz Signal bandwidth (BS) : 2.5MHz Number of subcarriers (N) : 1024 Number of samples in the guard interval (Ng) : 32 Sampling interval (T) : 0.4 us
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Τmax=0.64us
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Τmax=0.64us , L=10
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Τmax=0.64us , L=10 , Ka=0.72
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Τmax,A=1.6us , Τmax,E=6.4us , L=10 , Ka,E=0.72 , Ka,A=0.51
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Conclusions As compared to the observed channel model , its dimension reduced, where the full-rank estimators using pilots tones can be adopted, and consequently, improves the channel estimation performance . It eliminates the problem of multi-path delay estimation and can be adopted in a channel not restricted to a sparse mlti-path fading.
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REFERENCES J.-J. van de Beek , O. Edfors , and M. Sandell , “On channel estimation in OFDM systems,” in Proc. IEEE Vehicular Technology Conf. , Jul , vol. 2 , pp B. Yang , K. B. Letaief , R. S. Cheng , and Z. Cao , “Channel estimation for OFDM transmission in multipath fading channeds based on parametric channel modeling,” IEEE Trans. Commun. , vol. 49 , pp , Mar
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