Application of turbo equalization in PPC underwater acoustic communication

In this paper, an iterative channel equalization and decoding technique are introduced to improve the performance of passive phase conjugation (PPC) based underwater acoustic communication system. The PPC processor can realize pulse compression reducing the computational burden of equalizer. The turbo equalizer is applied to existing PPC underwater communication structure (PPC-TEQ). Moreover, an adaptive combiner based on MMSE criteria (Adap-PPC-TEQ) is introduced to further improve the system performance. Further more, the adaptive combiner can form a new multi-channel equalizer structure with fewer sub-channels (Adap-PPC-Multi-TEQ) which provides a trade-off between performance and complexity. The turbo equalizer is implemented in a direct adaptive manner to track slow envelop variation while a digital phase lock loop (DPLL) is used for tracking fast phase variation. These three different receiver structures are demonstrated by field experiment conducted at Trondheim Fjord, Norway. An error free coded data transmission at 4 kb/s for a range of 7.4 km is achieved.