Optimizing of Iterative Turbo Equalizer for Underwater Sensor Communication

We presented an iterative turbo equalization to cope with intersymbol interference induced by reflection of sea level and sea bottom for underwater sensor communication channel. Iterative turbo equalizer consists of inner codes and outer codes; we employ decision feedback equalizer as an outer code and turbo codes as an inner code. Equalizer and decoder are connected through the interleaving and deinterleaving that update each other's information repeatedly. At the receiver side, we resort to powerful turbo equalization algorithms that iteratively exchange probabilistic information between inner decoder and outer decoder, thereby reducing the error rates significantly. Furthermore, we expand iterative turbo equalizer techniques for single-input-single-output (SISO) system to multiple-input-multiple-output (MIMO) system in order to increase data rates for underwater sensor communication channel. Based on experimental channel response, we confirmed that the performance is improved as iteration number is increased. The performance is improved by 3.5？[dB] compared to noniteration for SISO channel and by 1？[dB] for MIMO channel, respectively. We also decided that optimal iterations are 3. Very important for a successful decoding is the channel estimation, which is also discussed. 1. Introduction The excessive multipath encountered in underwater sensor communication (USC) channel is creating intersymbol interference (ISI), which is limiting factor to achieve a high data rate and bit error rate (BER) performance. Various different methods to cope with multipath situation have been developed. In addition to ISI, cochannel interference (CoI) is also occurred resulting from the use of multiple transmitters in UW communication. Removal of both CoI and ISI is a challenging problem in view of difficult channel conditions. The optimal detector is a maximum likelihood detector (MLD), which can be realized, for example, by the soft Viterbi algorithm. Due to the length of the impulse response in the UW channel, the number of states in the decoder will be increased. One well-proven method to counteract ISI is the decision feedback equalized (DFE), which has been used in many UW communication links [1, 2]. However, the use of DFE has difficulties when a multipath with a number of arrivals has equal strength or low SNR [3]. The other way to cope with ISI, iterative equalizer which constitutes an outer loop is used in the receiver. An inner loop consists of iterative decoder. The assembly utilizes the error correcting power of the iterative codes to get an efficient