Research Of Iterative Stopping Criterion And Pilot Optimization Based On Turbo-OFDM

With the development of underwater acoustic communication technology,the use of channel resources has become the focus of attention.Because the available part of underwater acoustic channel resources is less and has been widely used,underwater acoustic channel resources are in a very tense state.How to make full use of the limited channel resources for efficient data transmission has become the focus of research.In many communication systems and coding,OFDM communication system has high spectrum efficiency,and turbo code can greatly reduce the bit error rate of the system.Therefore,digital communication system based on Turbo OFDM coding and modulation technology has become one of the important research directions in underwater acoustic communication due to its high spectrum efficiency and low bit error rate.Based on Turbo OFDM coding and modulation system,this paper optimizes the channel estimation and decoding process in the receiving phase.For the decoding part of the system,this paper improves and optimizes the HDA and SDR criteria in the traditional iterative stop criteria of Turbo codes,establishes auxiliary decision criteria by introducing external variables,and combines with the original criteria to form a new improved criterion,and then compares and simulates the improved criterion with the original criterion.It is verified that the improved criterion can reduce the iterative decoding times of Turbo code while maintaining the decoding performance of the system,thus reducing the computational complexity of the system.For the channel estimation part of the system,this paper uses compressed sensing technology which is more consistent with the underwater acoustic channel model to estimate the underwater acoustic channel,and then improves the pilot design of compressed sensing channel estimation.Firstly,the pilot evaluation criteria is determined,and then the Tabu search in intelligent algorithm is used to improve the pilot search process.Finally,the performances of the pilot generated by Tabu search,random search and tree search in multipath channel and Gaussian white noise environment are compared and simulated.It is verified that under the same number of pilots,the bit error rate of improved Tabu search pilot generation method for compressed sensing channel estimation is lower than the tree search and random search algorithm,under the premise of the same bit error rate,less pilots are needed for Tabu search.On the premise of the same performance,compared with the random search,the number of pilots used by Tabu search can be saved by25%.Finally,the underwater acoustic communication experiment is carried out in Songhua River.After transmitting and collecting LFM signal,the sliding correlation method is used to collect the actual channel impulse response.In this paper,the simulation environment of real channel impulse response with Gaussian white noise and random fluctuation is used to compare the performance of the improved iterative stop criterion and the original criterion of Turbo code,and the pilot generated by three different search algorithms in OFDM compressed sensing channel estimation.The results are compared to verify the performance improvement effect of the improved criterion in the actual communication application.