Evaluation of Channel Coding in OFDM
Systems

Abstract

Channel coding plays a very important role in OFDM systems performance.
The structure of OFDM systems makes channel coding more effective in confronting
fading channels. Sometimes Coded OFDM is known as COFDM. The role of channel
coding in conjunction with frequency and time interleaving is to provide a link between
bits transmitted on separated carriers of the signal spectrum, in such a way that the
information conveyed by faded carriers can be reconstructed in the receiver. Frequency
selectivity, currently known to be a disadvantage, is then turned into an advantage that
can be called frequency diversity. Using Channel State Information (CSI), channel
coding can yield some additional gain. Channel state information is frequency response
of the channel or signal to noise ratio in each carrier.
This thesis analyzes OFDM system and the effect of channel coding in reducing BER.
Along with this soft decoding and decoding with CSI is also studied. Besides,
performance of convolutional codes Turbo codes in OFDM systems is compared and
compared. Besides, we compare the performance of convolution and turbo codes in
OFDM systems. The results have been validated through simulations.
This thesis also presents Space-Frequency Coded OFDM system consisting of two
transmitters and a single receiver. Simple Alamouti space time code is used. An Mary
PSK modulation is used to modulate the symbols across an OFDM channel. We also
proposed a variation of the scheme which tries to spread additional symbols across timefrequency
attempting to increase the rate of transmission without changing the type of
modulation employed or increasing the bandwidth. A Rayleigh frequency selective slow
fading channel is assumed through out the analysis. SER performance of the above
systems is carried out with emphasis on the modulation scheme and number of carriers.