수중통신에서 계층적 시공간 부호화 기반의 반복 복호 기법 연구

Abstract

The transmission of acoustic waves is limited because of various factors such as water temperature, salinity, depth in the underwater acoustic communication with a multi-path channel environment. And the performance is limited because the underwater acoustic communication uses low frequency band relative to wireless communication. In the underwater acoustic communication, the performance greatly depends on the characteristics of multi-path channel and the distortion of the received signal is increased due to reverberation and multi-path. Such channel characteristics cause ISI. It is necessary to study on equalizer technique and channel coding to correct error by multi-path and ISI because the optimal communication method has to be designed based on characteristics of multi-path channel. Therefore, in this paper, the performance according to various channel codes is analyzed by simulation. As a result, the turbo encoder based on iteration is suitable for underwater acoustic communication.

We analyze a turbo pi code by permutation pattern, CRSC code, Puncturing pattern to improve BER performance and to solve a problem of error floor at high SNR. In the result of BER performance analysis, error floor phenomenon that occurs in the turbo encoder is removed and the performance is enhanced by 0.3 [dB]. And we proposed a low-power iterative decoding algorithm for turbo decoder basThe transmission of acoustic waves is limited because of various factors such as water temperature, salinity, and water depth in the underwater communications with a multi-path channel environments. The performance is also limited due to low frequency bands relative to wireless communications. The excessive multipath encountered in underwater communications channel is creating inter symbol interference (ISI), which is limiting factor to achieve a high data rate and bit error rate performance. Various different methods to cope with multipath situation have been developed. In addition to ISI. Removal of ISI is a challenging problem in view of difficult channel conditions. The optimal detector is a maximum likelihood detector, which can be realized for example by a soft Viterbi algorithm. Due to the length of the impulse response in the underwater communication 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 underwater communication links. The other way to cope with ISI, iterative equalizer is used which constitutes an outer loop in the receiver. An inner loop consists of iterative decoder. This thesis proposed iterative coding based equalization for single carrier underwater communication channels. Among the iterative coding schemes, turbo pi coding scheme is optimal for underwater communications in aspect to performance, packet size, and underwater environments. As an outer code, DFE is used in the thesis. As an inner code, the turbo pi codes are used. In simulation results, this thesis confirms that the performance is the better as iteration numbers are increased. In the range of iteration number is three or four, we can achieve BER performance enhancement by 3.5 [dB] compared to non-iteration. However, performance gain can’t be achieved after third iterations, and we conclude the optimal iteration numbers are three. Based on the results of single input single output (SISO) systems, our focus expands to multiple input multiple output (MIMO) system. MIMO technique is being studied in underwater communications because of increasing the data rates. MIMO communication systems employ multiple sensors at the transmitter and receiver sides. They can yield significantly increased data rates and improved link reliability without additional bandwidth. Representative method is space-time trellis codes (STTCs). Normally, the candidate of outer codes are space-time block codes (STBCs) and STTCs. Representative method of STBCs is V-BLAST (Vertical-Bell Labs lAyered Space-Time). This system is obtained diversity or spatial multiplexing effect. However, STBCs for MIMO turbo equalization can’t obtain coding gain even if increasing number of iteration. This is the reason that the outputs of STBCs are not soft type symbols. The types of input symbols and output symbols must be soft symbols in order to improve performance by increasing number of iterations. This thesis proposes an efficient iterative layered STTC based on soft decision information with DFE equalizer for MIMO system in the underwater communication channel. As same as SISO turbo equalization models, two codes are concatenated in the serial fashion in MIMO system. The inner codes are turbo pi codes with 16 states and outer codes are STTCs with optimal generator polynomial. The symbols of outer decoder are then subtracted from the input and interleaved. The interleaved symbols are canceled a posteriori from the proceeding received symbol. Interleaving helps receiver convergence. There are two kinds of iterative models. The first model is outer iteration which is done between DFE and STTC decoder. The second one is inner iteration which is done between STTC and turbo pi decoder. Outer iteration type is conventional model and inner iteration model is proposed model in the thesis. To confirm the performance improvement of proposed inner iteration model for MIMO system, the simulation was conducted. The performance of inner iteration model is better than that of outer iteration model. Finally, based on proposed decoder model, this thesis confirms that the coding gain of 1 [dB] can be achieved compared to conventional decoder model in the MIMO system.