  In this paper, we propose a study of performance of the channel estimation
using LS, MMSE, LMMSE and Lr-LMMSE algorithms in OFDM (Orthogonal Frequency
Division Multiplexing) system which, as known suffers from the time variation
of the channel under high mobility conditions, using block pilot insertion. The
loss of sub channel orthogonality leads to inter-carrier interference (ICI).
Using many algorithms for channel estimation, we will show that, for a 16- QAM
modulation, the LMMSE algorithm performs well to achieve this estimation but
when the SNR (Signal Noise Rate) is high, the four algorithms (LS, MMSE, LMMSE
and Lr-LMMSE) perform similarly, this is not always the case for another scheme
of modulation. We will improve also the mean squared error for these
algorithms. It will be illustrious in this paper that the LMMSE algorithm
performs well with the block- pilot insertion as well as its low rank version
which behave very good even when the size of FFT is very high.
