Underwater acoustic communication module based on Orthogonal Frequency Division Multiplexing (OFDM) uses rate ½ convolutional encoders and Turbo decoders for error control coding. Turbo decoders are designed with two Viterbi decoders that operate in sequence to improve Log Likelihood Ratio (LLR). The computation complexity of Viterbi decoders limits their use for high speed decoding as the decoders are iterative. In this paper, adaptive Viterbi decoder is designed that is based on novel message decoding logic, branch metric unit and survivor path metric unit. The computation complexity is reduced by replacing the arithmetic and logic unit with Look Up Table (LUT) thus increasing the access time. The adaptive decoder architecture eliminates the iteration process by predicting the state as well as the data and hence is faster in decoding message.

Keywords : Underwater communication, Turbo Codes, OFDM, Viterbi Decoder, FPGA, High Speed