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

OUTLINE Introduction System model Fraction Taps Channel Approximation Estimations (FTCA) Simulations and analyses Conclusions Rererence

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 .

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

Orthogonal frequency division multiplexing(OFDM) system model Discrete-time baseband equivalent model of OFDM system

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

Time domain signal , n = 0 , 1, … , N-1 Ng : the number of samples in the guard interval which satisfies Ng x T≧ τmax

The received baseband signal , n = 0 , 1 , … , N-1 : additive white Gaussian noise (AWGN) : circular convolution , k = 0 ,1, … ,N-1

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

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

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 .

The pilot subcarriers arrangement

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

The FTCA Estimators where , m=0,…,S-1 and l=1,…,M

The FTCA-MMSE Estimator(Minimu mean- square error) where

The Average Channel Energy Approximation Error (ACEAE) Be For the channel frequency response is achieve by

The FTCA-LS Estimator For the channel frequency response is achieve by

Flowchart M

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

Τmax=0.64us

Τmax=0.64us , L=10

Τmax=0.64us , L=10 , Ka=0.72

Τmax,A=1.6us , Τmax,E=6.4us , L=10 , Ka,E=0.72 , Ka,A=0.51

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.

REFERENCES J.-J. van de Beek , O. Edfors , and M. Sandell , “On channel estimation in OFDM systems,” in Proc. IEEE Vehicular Technology Conf. , Jul. 1995 , vol. 2 , pp. 815-819 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.467-479 , Mar. 2001.