A Robust Sequence Synchronization Unit for Multi-user Chaos Based DS-CDMA Communication Systems

Abstract

This chapter demonstrates two ways of achieving and maintaining sequence synchronization in multi-user chaos based direct sequence code division multiple access (CBDS-CDMA) communication systems. In both cases, synchronization is achieved and maintained through code acquisition and code tracking phases, respectively. The performance of the proposed systems is evaluated in the presence of additive white Gaussian noise and interuser interferences as well as in a Rayleigh fading channel. A pseudo random binary sequence (PRBS) and a logistic chaotic map are used as the synchronizing periodic, and non-periodic, pilot signals within the multi-user chaotic communication system. In addition, the Bernoulli chaotic map is also used as the pilot signal in the investigation of the code acquisition performance. The code acquisition circuit is evaluated in terms of the probability of detection and probability of false alarm. The corresponding results demonstrate an ability to achieve initial synchronization. Furthermore, it is shown that in terms of code acquisition the PRBS outperforms the logistic and Bernoulli chaotic maps when used as pilot signals. The mathematical models of the code tracking loops are then developed and their validity demonstrated by means of a simulation for both PRBS and chaotic pilot based CBDS-CDMA systems. From the models, the control laws for the generation of time offset estimates are derived. The robustness of the synchronization units is then demonstrated in terms of the bit error rate. It has been shown that for the PRBS based system, in an AWGN channel, for the case of 1, 2, 3, 4, and 5 users the bit error rate goes below the maximum acceptable limit it of 10− 3 at the bit energy to noise power spectral density ratio of approximately 8, 9, 9.5, 11 and 12 dB, respectively. The chaotic pilot based CBDS-CDMA systems exhibit marginally better performance for a single user plus a chaotic pilot signal than the corresponding PRBS pilot based CBDSCDMA system at the BER level of 10− 4 and below. In particular, at the BER level of 10− 6 , this improvement in performance is approximately equal to 0.175 dB. Their BER performances match for more than one user in the system. It has also been shown that the periodic and non-periodic chaotic pilot based CBDS-CDMA systems’ BER performances match for any number of users in the system. Furthermore a gradual degradation in performance, above the maximum acceptable bit error rate limit, is demonstrated for the increasing number of users for all systems. Finally, it is shown that although the systems are robust to the influence of AWGN and interuser interferences, they all fail to satisfy the maximum allowable bit error rate limit of 10− 3 in the Rayleigh fading channel. By introducing a chaotic pilot signal in place of a PRBS signal, the CBDS-CDMA system is made fully chaotic. In this way, the CBDS-CDMA systems’ security is significantly improved by eliminating an inherently different PRBS pilot signal.KeywordsCode Division Multiple AccessRayleigh Fading ChannelTracking LoopAdditive White Gaussian Noise ChannelCode Division Multiple Access SystemThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.