Wai M. Tam

Chaos-based digital communication systems

Publisher Summary This chapter reviews the common chaos-based digital modulation schemes. It also explains the corresponding coherent and noncoherent demodulation schemes. The two multiple-access schemes, based on the differential chaos-shift-keying (DCSK) or the frequency-modulated DCSK modulation (FM-DCSK), and the one multiple-access scheme based on the chaotic frequency modulation are also discussed. In the chaos-based communication systems, the detection schemes are broadly classified into the coherent and noncoherent types. The coherent systems require an exact replica of the chaotic carrier used to carry the information to be reproduced at the receiver, while the noncoherent systems have no such requirement. The chaos-based digital modulation schemes occupy a much wider bandwidth than is required to transmit a stream of binary symbols. Allowing multiple users to share the same bandwidth ensures an efficient use of the spectrum. When the chaos-shift-keying (CSK) signals are decoded based on the estimation of the bit energy, the threshold of the detector is expected to shift with the noise level. This leads to a large number of errors. The DCSK modulation scheme can be useful in overcoming such a problem.

Generalized correlation-delay-shift-keying scheme for noncoherent chaos-based communication systems

In this paper, we propose a generalized correlation-delay-shift-keying (GCDSK) scheme for noncoherent chaos-based communications. In the proposed scheme, several delayed versions of a chaotic signal are first produced. Some of them will be modulated by the binary data to be transmitted. The delayed signals will then be added to the original chaotic signal and transmitted. At the receiver, a simple correlator-type detector is employed to decode the binary symbols. The approximate bit error rate (BER) of the GCDSK scheme is derived analytically based on Gaussian approximation. Simulations are performed and compared with the noncoherent correlation-delay-shift-keying (CDSK) and differential chaos-shift-keying (DCSK) modulation schemes. The effects of the spreading factor, length of delay, and the number of delay units on the BER are fully studied. It is found that GCDSK can achieve better BER performance than DCSK under reasonable bit-energy-to-noise-power-spectral-density ratios

An approach to calculating the bit-error rate of a coherent chaos-shift-keying digital communication system under a noisy multiuser environment

Assuming ideal synchronization at the receivers, an approach to calculating the approximate theoretical bit-error rate (BER) of a coherent chaos-shift-keying (CSK) digital communication system under an additive white Gaussian noise environment is presented. The operation of a single-user coherent CSK system is reviewed and the BER is derived. Using a simple cubic map as the chaos generator, it is demonstrated that the calculated BERs are consistent with those found from simulations. A multiuser coherent CSK system is then defined and the BER is derived in terms of the noise intensity and the number of users. Finally, the computed BERs under a multiuser environment are compared with the simulation results.

Analysis of bit error rates for multiple access CSK and DCSK communication systems

In this paper, multiple-access techniques for use with chaos shift keying (CSK) and differential CSK under noisy conditions are analyzed using a discrete-time approach. A mixed analysis-simulation technique is developed to calculate the bit error rates (BERs). When certain assumptions are made, closed-form analytical expressions of the BERs are found.

Modeling the Telephone Call Network

It is natural to envisage that in a telephone network, some telephone numbers originate or receive more calls than others. Indeed, real-life data have verified the conjecture that the number of calls originated from or received by a telephone number in a network has a power-law property. Further, the number of calls made or received by the same telephone number can be very different. In this paper, we construct a self-growing complex network for modeling the aforementioned telephone call network. The complex network obtained is a directed and weighted network. Moreover, the nodes (telephone numbers) of the network exhibit power-law properties in the following aspects: total-degree, out-degree, in-degree, total-strength, out-strength and in-strength.

Capacity analysis of a CDMA cellular system with power control schemes

We develop a general model to study the forward link capacity of a CDMA cellular system with power control. Using this model, two forward link power control schemes (nth-power-of-distance power control scheme and optimum power control scheme) are examined. The increase in capacity by using power control has been studied. The capacity of the forward link is also compared with that of the reverse link with perfect power control.

An improved multiple access scheme for chaos-based digital communications using adaptive receivers

In this paper, a multiple access technique is proposed for chaos-based communication systems in which chaotic reference signals are transmitted together with the information-bearing signals. Chaotic reference signals modulated by a binary training sequence are sent periodically. The same chaotic signals are then modulated by binary data and transmitted. To achieve multiple access, different chaotic signals and training sequences are assigned to different users. At each receiver, an adaptive filter is employed to perform the demodulation based on the users pre-assigned training sequence. The bit error rates of the proposed scheme are simulated and compared.

Multiuser detection techniques for multiple-access coherent antipodal chaos-shift-keying systems

This chapter discusses the use of linear and nonlinear multiuser detectors in a multiple-access antipodal chaos-shift-keying (CSK) system (MA- antipodal chaos shift keying (ACSK)). It reviews the development of some analytical techniques for evaluation of the bit error rates (BERs) of the system. The multiuser detection is effective to combat the multiple-access interference (MAI) in conventional direct-sequence code-division-multiple-access (DS-CDMA) systems. Based on the available spreading codes, the operating principle of a multiuser detector is to cancel the interference between the users, and to jointly decode the transmitted symbols. The suboptimum multiuser detectors are broadly classified into: the linear detectors and the subtractive interference cancellation detectors. For the linear detectors, a linear transformation is performed at the receiver to mitigate the MAI. For the decorrelating detector, the interference can be completely eliminated. The parallel interference cancellation (PIC) detector is a nonlinear multiuser detector that cancels the total interference for all the users simultaneously. The detector requires accurate knowledge of the spreading sequences of all the users as well as an estimation of the transmitted symbols.

Performance analysis of multiple access chaotic-sequence spread-spectrum communication systems employing parallel interference cancellation detectors

In this paper, parallel interference cancellation (PIC) detectors are applied to jointly decode symbols in a multiple access chaotic-sequence spread-spectrum communication system. In particular, three types of linear detectors, namely, single-user detector, decorrelating detector and minimum mean-square-error detector, are used to estimate the transmitted symbols for the first stage of the PIC detector. The technique for deriving the approximate bit error rate (BER) is described and computer simulations are performed to verify the analytical BERs.

Reducing the bit-mapping search space of a bit-interleaved polar-coded modulation system

The bit mapping pattern has a crucial effect on the error performance of a bit-interleaved polar-coded modulation (BIPCM) system. In this paper, we have shown that a large proportion of the mapping patterns are producing the same error performances and are hence redundant as far as mapping optimization is concerned. An effective method is then proposed to eliminate the redundant mapping patterns and the optimal pattern is found within the substantially reduced search space. Results show that the mapping optimized by the proposed method can further improve the block error performance compared to existing mapping schemes.