Research On CDSK In Chaos Communication

Spread spectrum technology has been widely applied in wireless communication due to its advantages of low probability of interception, anti-jamming and multiple access capability etc. Since the beginning of 1970 s, chaotic system and chaotic signal have attracted researchers' wide attention. Chaotic signal's broad bandwidth, pulse-like autocorrelation and low cross-correlation features are considered to be feasible in digital spread spectrum communication, so chaotic signal has a good development prospect.The research of chaos in digital communication system currently is mainly based on Correlation Delay Shift Keying(CDSK) and Differential Chaos Shift Keying(DCSK). Both of them inherit the advantages of traditional spread spectrum communication, while additionally, compared to each other, each of them has advantages and disadvantages mainly in following aspects: CDSK has high bandwidth utilization, high security, and is more flexible in system operation, but Bit Error Rate(BER) is worse than DCSK, and transmission rate is not high. In addition to the study of chaos shift keying in single user environment, the multiple access capability has also become a research focus in recent years, which is one of the key characteristics of spread spectrum communication. For existing problems of CDSK and the improvement of chaotic shift keying performance in multiple access, the main work of this thesis includes the following aspects.(1) An improved CDSK system, Correlation Delay-Frequency Modulated-Correlation Delay Shift Keying(CD-FM-CDSK), is studied in this thesis. In this new system, the information transmission rate of CDSK is mainly improved. To raise the data transmission rate, during a symbol period, the useful information is converted by serial-to-parallel, and modulated by two reference signal respectively. Due to the non-periodic characteristic of chaotic signal, the bit energy of correlation output in CDSK is not constant, which leads to misjudgment and affects the performance of the system. So by adding FM modulation in the generator of CD-FM-CDSK, the power spectral density of output reference signal is uniform. The receiver of CD-FM-CDSK is similar with CDSK. Because of the correlation features of chaotic signal, each branch signal is correlated with its corresponding delayed signal to demodulate and restore the signal. Simulation has been made to analyze the relationship between CD-FM-CDSK system's BER, chaotic sequence length and the signal to noise ratio(SNR). The system BER comparisons of CD-FM-CDSK, CDSK and FM-CDSK under the same conditions verify the improvement effectiveness of CD-FM-CDSK, in addition to increasing the system complexity.(2) A single user Differential-Correlation Delay Shift Keying(D-CDSK) system is studied. The system has the advantages of high security in CDSK and well BER performance in DCSK. In the transmission time slot, the mixed reference signal and modulated signals are transmitted respectively. At the receiving end, the signal is demodulated by correlation with delayed signal. So the single user D-CDSK system's transmission rate is 4/3 times of that in DCSK. Simulation results show that the performance of D-CDSK system is always better than CDSK and is also improved compared with DCSK in low SNR condition, while the system complexity is increased. The performance comparison simulation of D-CDSK and CD-FM-CDSK shows that, by using T-R transmission mode of DCSK, the interference of the output at the receiving end is reduced and the system performance is enhanced.(3) A multi user CDSK-DCSK system based on Walsh codes is studied, which is based on the single user D-CDSK system. The differences lies in: firstly, by introducing Walsh code, the cross-user interference is reduced and the performance of multi user system is improved; secondly, instead of using FM, the sign function mapping method is used to process chaotic signal to eliminate chaos bit energy diffusion problem. Simulation has been made to analyze the performance of multi user CDSK-DCSK system under different user's number and length of chaotic sequences. Compared with variant delay multiple access DCSK(VDMA-DCSK), under the same conditions, the performance of multi user CDSK-DCSK is better than VDMA-DCSK, and with SNR increasing, performance improvement effect getting more obvious.