| In chaos communication system the bit error rate and the transmission speed are two very important factors. In this article we try to study a multi-ary modulation method to improve the speed of the system. Chaotic signal has a broad band spectrum which is nonperiodical and continuous, and it looks like noise, so chaotic signal has the characteristic of security in nature. It is also highly sensitive to original conditions. The orbits of two chaotic systems will become totally different even though they evolve from the same original conditions. Thus, the chaotic signal can't be predicted, intercepted or captured. The chaotic signal is very complicated because it is a hyperploid chaotic system that has several positive Liyapunov exponents and the complicated orbit. But the chaotic system itself is determinate. It is determined by the nonlinear functions, parameters and the original conditions, so it is easy to produce and copy. The fast development of PLC (Programmed Logic Circuit) and VLSI (Very Large Scale Integration) makes it possible that chaotic signal can be applied to communication. Researches on chaotic communication mainly focus on three aspects: (1) Use the chaotic signal which has the characteristic of the gauss white noise statistic to conceal the information signal, which produces the chaotic concealing signal. Then we get rid of the chaotic signal through synchronization in the receiver and reproduce the information signal. (2) Use the good correlative chaotic sequence, large amount and nonperiodical codes to substitute stochastic sequence in spread-spectrum communication. It is called the chaotic digital spread-spectrum communication. (3) Use the chaotic signal with brand band character to substitute the periodical baseband signal of digital communication system. This kind of communication system includes chaos shift keying(CSK), chaos on-off keying(COOK), differential chaos shift keying(DCSK), quadrature chaos shift keying(QCSK), and frequency-modulation DCSK(FM-DCSK). Because this kind of communication system has good correlative character and continuous brand band energy spectrum character, it can restrain the multipath decline. So it has a bright future. In digital chaos shift keying communication system there are two ways to demodulate signals: correlative demodulation and noncorrelative demodulation. The former demodulates signals by detecting correlation of synchronization rebirth base function and received signal. However, the latter depends on estimating the energy per bit, and we use a threshold to detect the code of received chaotic carrier wave which includes noise and information signal. Comparatively speaking, the noise performance of DCSK and FM-DCSK is the best. In this paper we mostly study the noncorrelative demodulation. It uses a simulative coding to deal with code "0"and "1"in the transmitter. If code "0"is sent in (t i ,ti+ T), the chaos signal is produced by a shift generator. In the receiver signal ri (t) which includes noise is delayed half bit duration, and the correlation value of received signal and delayed signal is just what we want. Noncorrelative demodulation chaos shift keying is not sensitive to channel distortion because reference signal and information signal are in the same channel. And its noise performance is very good. There is a disadvantage in chaos shift keying communication system. If the system works normally, the bit rate can't be too fast and the bit duration should be limited in millisecond. So it is only used in the low speed chaos communication system. Although according to some articles, high frequency chaotic oscillator is used in chaos shift keying system, but in the receiver chaotic signal exists signal estimating. So the frequency of oscillator can't be too fast. There are several ways to improve the speed of chaos communication: (1) the chaotic signal is nonperiodical and its energy per bit is varied in bit duration. So there exists the... |
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