Chaos Sequences And It's Applications In Sepread Spectrum Communication

CDMA is a kind of main techniques of the third generation personal wireless communication system. The basic element of CDMA is to construct a kind of spread sequence which has the excellent self-correlation and inter-correlation, at the same time, it must have enough coding sequences to meet the multi-users need. Though, in the existing spread sequences system, the m and Gold sequences are used widely, which are generated by the longest linear shift register, they all have the limitation of length and number in some sense.The chaotic phenomenon is a deterministic and random-like process appearing in the non-linearly dynamic system. This process is non-period, non-convergence but bounded. It has sensitive dependence on the initial values, that is, the little difference of the initial conditions will separate each other according to the Lyapunov exponential rule with time passing by, and at last it will turn into two tracks with absolutely distinct track of motion or feature. This random-like characteristic of chaotic phenomenon is very suitable for disguising modulation as noise in communication. And it is most important that chaotic system can provide signal which is plentiful, uncorrelated, random-like, deterministic and being easy to generate and regenerate because of the sensitive dependence on the initial values. Therefore, chaotic sequence has a good application foreground in communication domain especially in the generation of pseudo random spread spectrum code because of these especial characteristics.In this paper, we will focus on the application of chaotic sequence used as spread spectrum code in CDMA communication system. We propose the two-dimensional chaotic sequence on the base of the combination of time discrete chaotic system and over-sampled theory in communication. This paper mainly research on the various properties and application of the binary chaotic sequence, which includes the following aspects: 1 We generate the chaotic sequence by using Logistic-Map , improved Logistic-Map, Tent-Map, Chebyshev-Map, and analyze their sensitive dependence on the initial values. The numerous simulation results show that when the initial value difference is only 10?6 , the two tracks separate only after four iterations, and at last the two tracks become absolutely different. Analysis shows that chaotic sequence is sensitive to the initial value,easy to generate, rich in sequence code. Therefore, we can make use of the chaotic sequence to implement access.2. We analyze that how each parameter of improved Logistic-Map, improved Logistic-Map, Tent-Map, Chebyshev-Map influences the balance performance of the chaotic sequence. Simulation results show that: The initial value parameter of each affects smally the balance performance, but all have several peak value points, such as: Logistic-map: x0 =-1,-0.5,0,0.5,1, the balance value E%≥0.5;Chebyshev-map: x0 =-1,0,1, the balance value E=1; Tent-map: x0 =0.89,the balance value E=1. So we should avoid these points when we generate chaotic sequence. The balance performance of each chaotic sequence tends to balance gradually along with the increase of the sequence length. Besides, power series k of Chebyshev-map affects smally the balance performance, and we select k=4 this paper. Coefficient a of Tent-map affects smally the balance performance, but the balance value has a high peak value when a is 0.50, Finally, comparing to the simulation results of the original map we can conclude that chaotic sequence has a better balance performance than the original map, and chaotic sequence can improve the secrecy performance of communication system.3.we analyze the self-correlation, the mutual-correlation, of improved Logistic-Map, Tent-Map, and Chebyshev-Map. Simulation results show that the self-correlation of the above the chaotic sequence is perfect, the absolute value of the mutual-correlation of the above chaotic sequence is near in 0.05, Simulation results tell us that chaotic sequence has good anti-mutiaccess-interference and it can increase greatly the capacity of the communication system.4. we analyze the run distribution performance of the above four chaotic sequence, simulation results show that the simulated run distribution curve is basically identical to the ideal run distribution curve, that is, the over sampled chaotic sequence has good run balance, therefore, the over sampled chaotic sequence has good pseudo random performance, it is very appropriate to the disgusting modulate as noise in the spread spectrum communication.5. A new chaotic sequence generation scheme based on improved logistic-map and Chebyshev is proposed. It has better performance of balance and correlated characteristic. Especially in multi-user chaotic spread-spectrum communication system, its error-bit property is better too. The series sequence provides a new idea for the optimizing of the chaotic spread-spectrum sequence.6. On the base of MATLAB software simulated platform, we used the CDMA communications system model and the Monte Carlo simulation method to the chaotic spread-spectrum sequence, the m sequence and the Gold sequence carried on the system error rate simulation computation. Simulation results show that the chaotic spread-spectrum sequence can relatively reduce the bit-error-rate and improve the system transform performance. Meanwhile, when the chaotic spread-spectrum sequence is applied in the CDMA communication system as spread spectrum code, the bit-error-rate dose not go up along with the increase of the number of the parallel users. That is to say, the over sampled chaotic sequence used as spread spectrum code can commendably stand against the muti-access interference, and improve the channel capacity.