25-08-2012, 03:42 PM
Dft Based Channel Estimation Ofdm
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The great increase in the demand for high-speed data services requires the rapid growth
Of mobile communications capacity. Orthogonal frequency division multiplexing (OFDM)
Provides high spectral efficiency, robustness to intersymbol interference (ISI), as well as
Feasibility of low cost transceivers Multiple input multiple output (MIMO) systems offer the potential to obtain a diversity gain and to improve system capacity
Hence the combination of MIMO and
OFDM techniques (MIMO-OFDM) is logically widely considered in the new generation of
Standards for wireless transmission In these MIMO-OFDM systems,
Considering coherent reception, the channel state information (CSI) is required for recovering
Transmitted data and thus channel estimation becomes necessary.
Main goals of DFT based channel estimation:
Noise reduction
DFT-based channel estimation methods allow a reduction of the noise component owing to
operations in the transform domain, and thus achieve higher estimation accuracy (Van de
Beek et al., 1995) (Zhao & Huang, 1997). In fact, after removing the unused subcarriers, the
LS estimates are first converted into the time domain by the IDFT (inverse discrete fourier
transform) algorithm. A smoothing filter is then applied in the time domain assuming that
the maximum multi-path delay is kept within the cyclic prefix (CP) of the OFDM symbol. As
a consequence, the noise power is reduced in the time domain. The DFT is finally applied to
return to the frequency domain.