摘要：To overcome multipath fading and Inter symbol Interference (ISI), in convolutional single carrier systems equalizers are used. But it increases the system complexity. Another approach is to use a multicarrier modulation technique such as OFDM, where the data stream to be transmitted is divided into several lower rate data streams each being modulated on a subcarrier. To avoid ISI, a small interval, known as the guard time interval, is inserted into OFDM symbols. The length of the guard time interval is chosen to exceed the channel delay spread. Therefore, OFDM can combat the multipath fading and eliminate ISI almost completely. The another problem is the reduction of the error rate in transmitting digital data. For that we use error correcting Codes in the design of digital transmission systems. Turbo Codes have been widely considered to be the most powerful error control code of practical importance. Turbo codes can be achieved by serial or parallel concatenation of two (or more) codes called the constituent codes. The constituent codes can be either block codes or convolutional codes. Currently, most of the work on turbo codes have essentially focused on Convolutional Turbo Code (CTC)s and Block Turbo Code (BTC)s have been partially neglected. Yet, the BTC solution is more attractive for a wide range of applications. In this paper simple explanation of OFDM theory is given. The BER performance is evaluated for the Turbo coded BPSK and QPSK OFDM system, under AWGN channel. It also compares the BER performance of Turbo coded OFDM with the uncoded OFDM. It is verified in the present work that the TCOFDM system with 4 iterations is sufficient to provide a good BER performance. Additional number of iterations does not show noticeable difference. The simulation results shows that the TCOFDM system achieves large coding gain with lower BER performance and reduced decoding iterations, therefore offering higher data rate in wireless mobile communications.
