Abstract:
In wireless communication system, Doppler induced inter-carrier interference (ICI)
degrades the system performances severely. In this thesis, a theoretical analysis is
presented to find the performance of an orthogonal frequency division multiplexing
(OFDM) system, considering the channel characterized by frequency selective and
Rayleigh fading environment. The analysis includes the effect of ICI due to frequency
offset caused by Doppler frequency shift. Analysis is carried out to find the expression of
the signal to noise plus interference ratio (SNIR) conditioned on a given value of ICI
which is considered to be Gaussian in a PSK OFDM system. We extend the analysis for
a single input multiple output (SIMO) OFDM and multiple input multiple output
(MIMO) OFDM. Data is encoded using a quasi-orthogonal space–time block code
(QOSTBC), and the encoded data is split into Ns streams which are simultaneously
transmitted using Nt transmit antennas. The received signal at each receive antenna is a
linear superposition of the Nt transmitted signals perturbed by noise. Maximal ratio
combining (MRC) reception is a simple way for decoupling of the signals transmitted
from different antennas by multiple receive antenna.
The result are presented in terms of bit error rate (BER), power penalty, BER-floor for
different values of fading, number of sub-carriers, symbol duration and different number
of antennas for QPSK modulation schemes. It is noticed that the system suffers
significantly due to ICI and it has been shown that quality of service can be improved by
employing diversity scheme. Analytical results show that performance of a MIMOOFDM
system can be significantly improved by increasing the number of antennas.
Improvement around 34 to 37 dB is found while using four transmitter and four receiver
MIMO with respect to SISO OFDM measured BER at 10-5.
