Optimizing Design Of Collision-Free And Quadratic Permutation Polynomial Interleavers For Turbo Codes

Turbo code, invented in1993, has outstanding performance which close to the Shannon limit, due to the use of interleavers which makes it meets the randomness encoding condition of Shannon channel coding theory. A good interleaver can make the input information series randomness, thus avoiding the series generate the low weight codes after it is interleavered. For an interleaver frame length N, the number of interleavers is N!, and the challenge is how to search an excellent interleaver for Turbo code.To begin with, the principle of Turbo encoding and decoding, the content, main contribution and organizational structure of this thesis are firstly presented. Followed by the analysis of the advantages and disadvantages of the classic interleavers and this provides a reference standard for the optimization design of interleavers.Using iterative decoding scheme is the key to the excellent performance of Turbo code, but it requires a large amount of delay. With the development of3G and4G communication system, low latency and high transmission rate are required in a real-time communication system. For high data rate transmission, the parallel Turbo decoding method can reduce the decoding latency greatly, and this makes it being widely adopted. However, in the parallel decoding method, the random permutation of interleaver may probably induce memory collision issue. A challenge in the design of parallel Turbo decoder is the design of collision-free interleavers in order to avoid the memory access collision. In this thesis, following the way of previous design of parallel collision-free interleavers and avoiding their disadvantages, two new generation algorithm of new collision-free S-random interleavers are proposed. The proposed algorithms can obtain larger spreading factor, S, and better average code distance and bit error rate performance than the existing collision-free interleavers.Compared to S-random interleaver, the quadratic permutation polynomial interleaver proposed by J. Sun and O.Y. Takeshita in2005is also an excellent interleaver. For some short frame lengths, it can obtain a better performance than the S-random interleaver, and for longer frame lengths it can also have the performance which closes to S-random interleaver. There are some other advantages of quadratic permutation polynomial interleaver that make it become the3GPP LTE standard interleaver, such as, collision-free characteristics, litter storage space is need, and it is easy to use mathematical theory to analyze a QPP interleaver due to its polynomial form. Therefore, this thesis focuses on the QPP interleaver research. Firstly, through the QPP interleaver screening criteria given by the inventors of QPP, the interleavers collections with the corresponding maximization D parameter and best Ω’ parameter are get. To avoid the shortcomings of the search method only using D and Ω’, a new method to search excellent QPP interleavers is proposed, in the new method, the minimum distance parameter which has important effect to the BER performance of Turbo code is considered. Computer search results and simulation results show that excellent interleavers can be search through this method, and for some frame lengths, the interleavers searched by the method proposed in this thesis have better BER performances than the3GPP LTE standard interleavers.