  The demand for high-speed mobile wireless communications is rapidly growing.
Orthogonal Frequency Division Multiplexing (OFDM) technology promises to be a
key technique for achieving the high data capacity and spectral efficiency
requirements for wireless communication systems in the near future. This paper
investigates the performance of OFDM-based system over static and non-static or
fading channels. In order to investigate this, a simulation model has been
created and implemented using MATLAB. A comparison has also been made between
the performances of coherent and differential modulation scheme over static and
fading channels. In the fading channels, it has been found that OFDM-based
system's performance depends severely on Doppler shift which in turn depends on
the velocity of user. It has been found that performance degrades as Doppler
shift increases, as expected. This paper also performs a comparative study of
OFDM-based system's performance on different fading channels and it has been
found that it performs better over Rician channel, as expected and system
performance improves as the value of Rician factor increases, as expected. As a
last task, a coding technique, Gray Coding, has been used to improve system
performace and it is found that it improves system performance by reducing BER
about 25-32 percent.
