Stevan M. Berber

A robust sequence synchronization unit for multi-user DS-CDMA chaos-based communication systems

This paper demonstrates a way of achieving and maintaining sequence synchronization in multi-user direct sequence code division multiple access (DS-CDMA) based chaotic communication systems. Synchronization is achieved and maintained through the code acquisition and the code tracking phase, respectively. The performance of the proposed system is evaluated in the presence of additive white Gaussian noise and interuser interferences. Throughout, a pseudo-random binary sequence (PRBS) is used as the synchronizing pilot signal within the multi-user chaotic communication system. In addition, the Logistic and Bernoulli chaotic maps are also used as the pilot signals 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. A mathematical model of the code tracking loop is then presented. From the model, a control law for the generation of time offset estimates is derived. The robustness of the synchronization unit is then demonstrated in terms of the bit error rate. It has been shown that for the case of 1, 2, 3, 4, and 5 users, the bit error rate goes below the maximum acceptable limit of 10^-^3 at the bit energy to noise power spectral density ratio of approximately 8, 9, 9.5, 11, and 12dB, respectively. Furthermore, a gradual degradation in performance, above the maximum acceptable bit error rate limit, is demonstrated for the increasing number of users.

A Comparison of Accuracy Using a GPS and a Low-Cost DGPS

After the cancellation of selective availability in May 2000, the accuracy of plain uncorrected global positioning systems (GPSs) is significantly increased. Currently, the main cause of error in plain GPS receivers is imperfect compensation of the ionospheric error. Tests performed in New Zealand show that even a simple code range differential GPS (DGPS) provides significant improvement in accuracy over a plain GPS and can be implemented using low-cost off-the-shelf equipment. A simple statistical model of positioning error estimation is developed and applied on data obtained by measurements in order to verify the findings. The method, which needs only position information without satellite constellation, provides sufficient accuracy when simultaneously gathered plain GPS and DGPS data are compared

Effect of Rayleigh fading on non-coherent sequence synchronization for multi-user chaos based DS-CDMA

This paper presents the theoretical analysis and simulation results of the effects of frequency flat fading on non-coherent synchronization of a chaos-based DS-CDMA system. The quality of synchronization, both in acquisition and tracking stages, is evaluated in the presence of noise, inter-user interference and fading. This is the first time that a chaotic pilot is used for a non-coherent synchronization block giving it both security (complete signal masking including pilot) as well as usability in low SNRs. There is close agreement between the derived analytical upper bound results in the presence of fading and the simulation results. It is shown in theoretical analysis and simulation that the error output of the tracking loop can be approximated to a Gaussian distribution. Two chip waveforms have been investigated for non-coherent tracking and noise performance of the tracking phase is simulated. The bit error rate of a DS-CDMA system with a chaotic pilot is then investigated in the presence of noise and fading. The effect of partial (worst case) synchronization in terms of SNR difference is then evaluated to be 1dB for the noise only case, 0.1dB for single user case with uncompensated fading and 1.1dB for the eight user case.

Partner Choice and Power Allocation for Energy Efficient Cooperation in Wireless Sensor Networks

Energy efficient communication is a key requirement of energy-constrained wireless sensor networks. Cooperative diversity can be applied to wireless sensor networks to significantly reduce node energy consumption. However, judicious selection and coordination of cooperation partners is essential to exploiting this energy saving potential of cooperation. In this paper we investigate optimal partner choice and power allocation for energy efficient cooperation in wireless sensor networks. Our insights lead us to formulate power allocation and partner choice heuristics which form a simple and practical cooperation strategy for energy-constrained wireless sensor networks, allowing nodes to autonomously make near-optimal cooperation decisions. We show that the power allocation optimisation problem is non-linear, necessitating the use of a search to find the optimum solution. We present the resulting cooperative energy savings in terms of network geometry for a range of potential partner locations. Our results reveal that the partner-destination and the source-partner channels have roughly equal influence over cooperation decisions for optimal energy efficiency. We use this observation to devise a simple yet near-optimal power allocation heuristic.

A general rate K/N convolutional decoder based on neural networks with stopping criterion

A novel algorithm for decoding a general rate K/Nconvolutional code based on recurrent neural network (RNN) is described and analysed. The algorithm is introduced by outlining the mathematical models of the encoder and decoder. A number of strategies for optimising the iterative decoding process are proposed, and a simulator was also designed in order to compare the Bit Error Rate (BER) performance of the RNN decoder with the conventional decoder that is based on Viterbi Algorithm (VA). The simulation results show that this novel algorithm can achieve the same bit error rate and has a lower decoding complexity. Most importantly this algorithm allows parallel signal processing, which increases the decoding speed and accommodates higher data rate transmission. These characteristics are inherited from a neural network structure of the decoder and the iterative nature of the algorithm, that outperform the conventional VA algorithm.

Probability of Error Derivatives for Binary and Chaos-Based CDMA Systems in Wide-Band Channels

This paper presents detailed mathematical models of DS-CDMA systems based on binary and chaotic spreading sequences. These systems have been well investigated in the case of flat fading and noise presence in the channel. However, a comprehensive analysis of the systems operating in a wide-band channel does not exist. Therefore, in this paper, the systems are analyzed in detail, and expressions for the probability of error are derived in closed form assuming a wide-band transmission channel. Using both the signal representation in pure discrete time domain and the theory of discrete time stochastic processes, which have not been practically used before, the probability of error expressions are derived in closed form for single- and multi-user binary and chaos based DS-CDMA systems. The improvements in the probability of bit error due to multipath channel nature are quantified in dependence of the random delay and the number of users in the system. The wide-band channel is based on the Saleh-Valenzuela model proposed for the modern wireless networks.

An automated method for BER characteristics measurement

Traditionally, the bit error rate (BER) characteristics were measured for a certain number of bit-energy-to-noise ratio ( Eb/No). For each point, a sample of r bits is used, which depends on the expected BER value. The smaller BER value, the bigger the sample r. Thus, to obtain the same accuracy at each point, a method of trial and error should be used. Therefore, this method extends the time of measurement and measures BER values at all points with different accuracy. This paper presents a method which allows the automated and accurate measurement of the BER characteristics at a number of points with equal accuracy at each point by using the minimum sample size r. The method is tested by measuring BER characteristics of an FFSK modem.

Investigation on operations of a secure communication system based on the chaotic phase shift keying scheme

This paper presents the results of the investigation of a chaotic multi-user modulation method, called chaotic phase shift keying. Only one chaotic sequence generator exists in each transmitter and receiver. We present the theory behind their operations, as well as the bit error rate derivation. It was found to have better performances, confirmed by analytical and simulation results, than the chaotic shift keying (CSK) system, which requires two chaotic sequence generators in each transmitter and receiver.

Blind Multiuser Detector for Chaos-Based CDMA Using Support Vector Machine

The algorithm and the results of a blind multiuser detector using a machine learning technique called support vector machine (SVM) on a chaos-based code division multiple access system is presented in this paper. Simulation results showed that the performance achieved by using SVM is comparable to existing minimum mean square error (MMSE) detector under both additive white Gaussian noise (AWGN) and Rayleigh fading conditions. However, unlike the MMSE detector, the SVM detector does not require the knowledge of spreading codes of other users in the system or the estimate of the channel noise variance. The optimization of this algorithm is considered in this paper and its complexity is compared with the MMSE detector. This detector is much more suitable to work in the forward link than MMSE. In addition, original theoretical bit-error rate expressions for the SVM detector under both AWGN and Rayleigh fading are derived to verify the simulation results.

Energy-Efficiency of Cooperative Diversity Techniques in Wireless Sensor Networks

This paper presents an investigation of the energy-efficiency of two major cooperative diversity techniques in short-range wireless sensor networks: virtual-MISO (multiple-input-single-output) and decode-and-forward. The total energy consumption of a cooperative system consists of the transmission energy, the transceiver circuit energy and the local communication energy cost. In virtual-MISO explicit local communication precedes the long-haul cooperative transmission, whereas in decode-and-forward nodes cooperate by overhearing and repeating each others long-haul transmissions. Our energy analysis shows that decode-and-forward is overall the more energy-efficient cooperative scheme. We also show that cooperation can yield large energy savings compared to traditional SISO (single-input-single-output) communication, even when the local distance between cooperating nodes is increased and becomes comparable to the long-haul distance. Given specific local and long-haul distance ranges, we formulate the recommended cooperation strategy in terms of the number of cooperating partners to employ for optimal energy-efficiency.