Study On Coding Method For Code Division Multiple Access Spread-Spectrum Communication Systems

The codes of the spreading-spectrum communication systems are investigated in this thesis. The spreading codes designs are one of the key points of the spreading-spectrum communication systems. Two spreading codes design topics related to the correlation properties of spreading codes are discussed, which are the construction of odd-periodic complementary spreading codes set and the design of spreading codes with the good correlation properties. Error control coding is an other key point of the spreading spectrum communication systems. The low density-parity check (LDPC) codes are researched, which can approach the Shannon bound with lower decoding complexity. The algebraic constructing LDPC codes from the optical orthogonal codes are presented. The researches of the iterative decoding of complex-rotary codes (CRC) whose performance is close to that of LDPC codes is given.Two or more code-sequences are called a set of odd-periodic complementary spreading codes (OPCS) if the sum of their respective odd-periodic autocorrelation function is a delta function. The definition of OPCS is given. The construction and synthesis methods of OPCS are discussed. The relation of the sets of odd-periodic complementary binary sequences with the sets of periodic complementary binary sequences is pointed out. Some new PCS are obtained.A construction method to generate binary extended d-form spreading codes is proposed. By using the TN spreading codes (a special case of d-form spreading codes), the optimal extended TN spreading codes set in the sense of Welch bounds are constructed. Finally, an example of the families of the extended TN spreading codes, which are constructed from Legendre spreading codes is given.Based on the optical orthogonal codes, a method for constructing regular LDPC codes was presented. The new codes are called OOC-LDPC codes. By using row and column decomposition, the extended OOC-LDPC with different rate and length is presented. OOC-LDPC codes and extended OOC-LDPC codes performwell with the iterative decoding based on belief propagation (BP) on an AWGN channel. The resulting codes are quasi-cyclic codes and can be encoded by using the shift registers. The encoding complexity is low.The new irregular OOC-LDOC codes are constructed on the base of the regular OOC-LDOC codes. The irregular codes are compared to the regular codes with similar parameters. Simulations demonstrate that the decoding performance of the new carefully constructed irregular codes achieves a modest gain over that of the new regular codes. The resulting codes are also quasi-cyclic codes. The encoding complexity is low.Both LDPC codes and one-step majority logic decodable codes can be defined by a set of orthogonal parity check sums for each bit. In this thesis, the iterative decoding of Complex Rotary Codes (CRC) that is one-step majority logic decodable codes is investigated. The decoding performance of CRC is as good as the finite geometry codes and the difference set cyclic (DSC) codes and a modest performance gain to be made over LDPC codes with similar parameters. But complex-rotary codes have more choice about code parameters than the finite geometry codes and the difference set cyclic codes.Two new low complexity algorithms for decoding complex-rotary codes are developed. The complexity of the new algorithms is remarkably low in comparison with BP algorithm. The belief-propagation iterative decoding algorithm with the most computational complexity is combined with algebraic decoding algorithm to form hybrid decoding.