Evaluation of Channel Coding in OFDM Systems

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