This paper proposes an underwater communication receive model based on the spread spectrum technique in order to provide the characteristic of a low probability of interception. To do this, turbo equalization techniques employing Bahl-Cocke-Jelinek-Raviv (BCJR) decoding to improve performance through repetition are applied to offer excellent performance even at a low signal to noise ratio (SNR) of transmitted signals due to the spread spectrum technique. A turbo equalization model based on RAKE which increase signal power by summing the received signal through the multipath is proposed to compensate distorted data due to multipath channel and the performance improvements were proven by applying the threshold and weighted coefficient in the RAKE receiver model. The model was applied to covert underwater communication in a multi-sensor environment, and the efficiency of the proposed method was proven through underwater experiments.

Faster-than-Nyquist (FTN) signal processing, which transmits signals faster than the Nyquist rate, is a representative method for improving throughput efficiency sacrificed performance degradation due to inter-symbol interference. To overcome this problem, this paper proposed FTN signal processing based on the unequal error probability to improve performance. The unequal error probability method divides encoded bits into groups according to priority, and FTN interference rates are differently applied to each group. A lower FTN interference ratio is allocated to the group to which high-priority encoded bits belong and a higher FTN interference ratio is allocated to the group to which low-priority encoded bits belong, thus performance improvement can be obtained compared to the conventional FTN method, with the same interference ratio. In addition, we applied the proposed FTN signal processing, based on the unequal error probability method, to the OFDM (orthogonal frequency division multiplexing) system in multipath channel environments. In the simulations, the performance of the proposed method was better than that of the conventional FTN method by about 0.2 dB to 0.3 dB, with an interference ratio of 20%, 30%, and 40%. In addition, in multipath channels, we confirmed that by applying the proposed unequal error probability, the OFDM-FTN method improves performance to a larger extent than the conventional OFDM-FTN method.

Acoustic channels are characterized by long multipath spreads that cause intersymbol interference. The way in which this fact influences the design of the receiver structure is considered in this paper. To satisfy performance and throughput requirements, we propose a consecutive iterative BCJR equalization scheme. To achieve a low error performance, we resort to the powerful BCJR equalization algorithms to iteratively update probabilistic information between inner decoder and outer decoder. Also, to achieve a high throughput, we divide a long packet into a group of small consecutive packets, estimate channel information of the current packets, and use it to decode the next packets. Based on an experimental channel response, we confirm that the performance is indeed improved for long packet size.

Next-generation wireless and/or satellite communications require high transmission efficiency and high reliability to provide various services with subscribers. To satisfied these requirements, incorporated MIMO (multipleinput-multiple-output) system with FTN (faster-than-Nyquist) techniques are considered in the paper. To improve performance and throughput, two kinds of MIMO turbo equalization techniques such as STTC (space-time trellis codes) and W-ZF(Weighted-Zero Forcing) are employed. They can yield significantly increased data rates and improved link reliability without additional bandwidth. In receiver side, BCJR algorithm is used for eliminating interferences induced by FTN transmission. Through the simulation results, based on MIMO-FTN transmission method, we compared the performance of layered space time codes with weighted zero forcing according to interference rate of FTN.

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