Research On Application Of Chaotic Spreading Spectrum Technology To CDMA Communication Systems

Chaotic sequence generation, as a new spreading spectrum technique, has been extensively investigated in its application area of wireless spreading spectrum cotnmunication systems for near ten years. Currently the study of advantages of chaotic spreading spectrum (CSS) over conventional direct spreading (DS) technique has become a key topic, which will decide whether the CSS can or not enter the commercial application through being embedded actual spreading spectrum communication systems, such as CDMA. Through comprehensive investigation, this thesis successfully provide a solution on the topic, and the research results are the following: I ) Feasibility of embedding a CSS sequence into existing CDMA system, such as Qualcomm CDMA (Q-CDMA) system; 11) Statistic characteristics comparison between CSS sequences and m sequence used in Q-CDMA systems; III) Simulation analysis of bit error rate(1BER) and frame error rate (PER) of chaotic spreading spectrum CDMA (CSS/CDMA) systems under different cellular structures; 1V) Optimizing design of chaotic spreading spectrum sequences being used in CSS/CDMA system. The main contributions of the thesis can be listed as below: 1) By modifying long address code and signaling message, the Logistic CSS sequence-based CSS/CDMA system complying with the 18-95 proctocol functions well and is compatible the m sequence as well, that implies the commercial CSS/CDMA system is feasible and worthy being practically developed in the near future. 2) Proposed is the concept of aperiodic partial correlation function, which makes the statistic characteristic comparison between CSS sequences and m in sequence possible. Based on Golomb抯 Randomness Axioms, we found that statistic performance of the Logistic CSS (LCSS) sequence is microscopically better than that of in sequence. 3) An LCSS/CDMA communication system is designed and its two key indexes of the overall system performances: bit error rate (BER) and frame error rate (FER) are measured by computer simulation conducted on the platform consisting of Sun Sparc Ultra 60/Solaris 2.6 and Cadence SPW. 4.5 package. Simulation results are provided for three typical wireless environments, i.e., macro-cell, micro-cell and Saleh indoor models. Comparative simulation results between LCSS/CDMA and Q-CDMA systems show that both the BER and FER performances of the LCSS/CDMA system are better than that of the Q-CDMA system. This observation can be interpreted by the theoretical analysis given in 2). 4) Since various CSS sequences behave differently in CSS/CDMA system, we have to conduct an optimization process before embedding them into the practical DSICDMA system. Liapunov index is assigned as one of the objective functions in optimization process aiming at improving randomness of the CSS sequence. Optimization flow chart is also given. It is found that the chart is effective in selecting qualified candidate for an CSS/CDMA system from a number of CSS sequences.