A common challenge in designing a wireless system is to overcome
the effects of the wireless channel, such as multi-path propagation,
and Inter-symbol Interference. and the ability of OFDM to combat
the effects of multi-path propagation made it th
e modulation of
choice for third and fourth generation networks.
In OFDM system we transmit data symbols on a large number of
subcarriers, and these data symbols exposed to changes in
amplitude and phase, caused of noise and frequency selective
fading and Doppler shift .
To get the transmitted data at the receiver we must cope with these
changes, so we need to know the channel response at every
subcarrier and every symbol, and for that reason we use PILOTS
and we use many methods to estimate the channel response depend
on pilots, and the most important channel is LMMSE Method.
We will discuss in these issue the wiener filtering method
(LMMSE) which give best performance but it is very complicated,
so we will use Discrete Fourier Transform (DFT) to reduce the
complexity of these method and we will study the BER and MSE in
both case when we use QPSK or 16QAM Modulation .
