Block Turbo Code and its Application to OFDM for Wireless Local Area Network

Abstract

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, Block Turbo Codes or Turbo Product Codes are used which is similar to the IEEE 802.11a WLAN standard.

In this thesis work simple explanation of BTCOFDM theory is given. The BER performance is evaluated for the Block Turbo coded BPSK and QPSK OFDM system, under both AWGN channel and Rayleigh fading channel. It also compares the BER performance of Block Turbo coded OFDM with the uncoded OFDM. It is verified in the present work that the BTCOFDM 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 BTCOFDM system achieves large coding gain with lower BER performance and reduced decoding iterations, therefore offering higher data rate in wireless mobile
communications.