On The Reliable Communication Transmission Technology Based On The Low Rate Turbo Codes Under Low SNR Condition

In deep space communications and other harsh conditions, reliable transmission technology is highly desriable and imposes challenges in the communication system design. Lower rate channel coding scheme with larger coding gain will be one of the critical technologies to ensure reliable communication under low sigal-to-noise ratio (SNR) conditions. How to design the low rate error control code is always a challenge. The separate design of demodulation and decoding may not be suitable for the reliable transmission over harsh channel condition, because the loss of useful information in separate demodulation design may greatly constraint the achieved performance by the subsequent decoding step. In addition, even small synchronization error can result in substantial degradation in the achieved reception performance too. In this thesis, the complete low rate Turbo code based relabile transmission scheme suitable for low SNR communication environment is investigated.Firstly,some commonly utilized low rate error correcting codes are introduced, their cons and pros are addressed in terms of the encoding complexity, the achieved decoding performance. Then the component encoder structure of BCH plus repetition coding based low-rate Turbo codes is introduced in detail. It is shown that the modified low-rate Turbo code can adjust flexibly the achieved code rate through properly selecting the coding memory and the repetition times utilized. Moreover, the modified low-rate Turbo code exhibits excellent error correction performance over low SNR region. And the achieved decoding reliability performance depends only on the code memory and code length, but not on the number of repetition.Secondly, the iterative receiver with joint demolutation and decoding design is focused in this thesis according to the Turbo principle. And the fundamental idea is to transform the modulation mapping to a generalized convolutional code. For example, the input and output of differential modulation can now be equivalently represented by using a trellis diagram. Therefore, at the iterative receiver, a trellis based soft-input soft-output (SISO) demoulator can be devised to be incorporated with the MAP based decoder, and the exchange of extrinsic information in between the SISO demodulator and the MAP based decoder can significantly improve the whole detection performance.Thirdly, it turns out that the accuracy of the data-assisted carrier synchronization estimator is limited by the number of pilot symbols utilized, while the synchronization accuracy requirement within low SNR region may require a huge number of pilots. Obviously, too much pilot insertion will reduce the bandwidth efficiency. In order to reduce the pilot requirement in the low-rate Turbo code based reliable transmission system design, the code-aided carrier synchronization is investigated in this thesis as well. The expectation maximization (EM) algorithm can be employed in the coded-assisted carrier synchronization design. By fully exploiting the log-likelihood information derived in the iterative decoding, the carrier frequency offset (CFO) and phase offset can be derived in an iterative manner too. Moreover, two effective methods to simplify the EM algorithm are proposed by simplifying the involved Q-function calculation and eliminating the complicated CFO search step. Simulation results are presented to validate that both proposed schemes can achieve reasonable carrier synchronization accuracy. In addition, the combined data-aided and code-aided estimator is proposed to extend the estimation range and accelerate the calculation convergence in EM iteration.The presented comprehensive analysis in this thesis provide useful reference for the low-rate Turbo code based reliable transmission scheme design, the joint iterative receiver with soft decision based demodulation and decoding, as well as the carrier synchronization technique under very low SNR conditions.