Gianluca Mazzini

Chaotic complex spreading sequences for asynchronous DS-CDMA. I. System modeling and results

This paper and its companion are devoted to the evaluation of the impact of chaos-based techniques on communications systems with asynchronous code division multiple access. Sequences obtained by repeating a truncated and quantized chaotic time series are compared with classical m-sequences and Gold sequences by means of a performance index taken from communication theory which is here defined and thoroughly discussed. This analysis reveals that, unlike conventional sequences, chaotic spreading codes can be generated for any number of users and allocated bandwidth. Numerical simulations are reported, showing that systems based on chaotic spreading sequences perform generally better than the conventional ones. Some analytical tools easing the comprehension of these advantages are summarized.

Localization in sensor networks with fading and mobility

In sensor networks the device localization is an interesting topic due to its relationship with routing and energy consumption. We propose a scheme to perform localization, based on the estimation of the power received by only two beacons placed in known positions. By starting from the received powers, eventually averaged on a given window to counteract interference and fading, the actual distance between the sensor and the beacons is derived and the position obtained by means of triangulation. The paper shows the effectiveness of this approach in different environments, by including the possible disturbance due to fading channels and sensor mobility.

Chaotic complex spreading sequences for asynchronous DS-CDMA. Part II. Some theoretical performance bounds

For pt.I see G. Mazzini et al., vol.44, pp.937-47 (Oct. 1997). This paper and its companion (Part I) are devoted to the evaluation of the impact of chaos-based techniques on communications systems with asynchronous Code Division Multiple Access. In Part I, a performance index was introduced and exploited to a priori estimate the performance of DS-CDMA communications systems based on chaotic spreading sequences, and to compare it to that of conventional systems. Here, tools from nonlinear dynamical system theory are employed to give a formal ground for those results. Analytical bounds on the expected partial cross correlation between spreading sequences obtained by quantizing and repeating a chaotic time series are derived, ensuring general applicability of such a technique in a real environment. Further analytical arguments guarantee that, when particular chaotic generators are used, expected performance is not worse than that of a well-behaving communications system. This analysis ensures also that, unlike conventional sequences, chaotic spreading codes can be generated for any number of users and allocated bandwidth.

Distributed power control for energy efficient routing in ad hoc networks

In this paper, distributed power control is proposed as a means to improve the energy efficiency of routing algorithms in ad hoc networks. Each node in the network estimates the power necessary to reach its own neighbors, and this power estimate is used both for tuning the transmit power (thereby reducing interference and energy consumption) and as the link cost for minimum energy routing. With reference to classic routing algorithms, such as Dijkstra and Link State, as well as more recently proposed ad hoc routing schemes, such as AODV, we demonstrate by extensive simulations that in many cases of interest our scheme provides substantial transmit energy savings while introducing limited degradation in terms of throughput and delay.

Design and performance of an enhanced IEEE 802.11 MAC protocol for multihop coverage extension

Ad hoc communication is gaining popularity, not only for pure ad hoc communication networks but also as a viable solution for coverage extension in wireless networks. Especially for upcoming WLAN hotspots, this is an interesting option to decrease installation costs. In this article we introduce a new MAC protocol that needs only marginal changes to the standard and enables efficient multihop networking. We advocate the use of multiple IEEE 802.11 channels, where one channel is reserved as a common signalling channel for the task of assigning the others (data channels) among wireless terminals. The proposed MAC protocols are based on a four-way handshake over the common signalling channel, while data transmission occurs on a dedicated channel. We propose a further optimization applying multiple wireless network interface cards. This improvement in performance comes at the price of a slightly more complex hardware. Two different simulation models are implemented to investigate our approach. The first model investigates the MAC protocol and its improvements, while the second model analyzes the multihop performance in terms of delivery ratio and transmission delay. BY means of numerous simulations we present the performance of our MAC approach in comparison with two standard approaches in terms of bandwidth, packet delivery, and transmission delay. For our performance evaluation we apply the IEEE 802.11a technology, but we note that our approach can also be used for IEEE 802.11b.

Performance comparison of routing protocols for ad hoc networks

Routing protocols for ad hoc networks are currently a hot research topic and when a lot of metrics are jointly considered, some nontrivial conclusions can be drawn. In this work, we make a comparison of link state, AODV and DSR protocols for two different traffic classes, in a selected environment. The classic Dijkstra is also reported as comparison term. As performance metric, packet delivery fraction, throughput, average delay and energy are considered. We show that AODV and DSR perform well when the network load is moderate, while, if the traffic load is heavy, simple link state outperforms the reactive protocols and becomes a good candidate to be used.

Chaos-based asynchronous DS-CDMA systems and enhanced Rake receivers: measuring the improvements

Results of recent theoretical investigations highlighted that the use of chaos in direct-sequence code-division multiple access (DS-CDMA) systems may lead to nonnegligible improvements in communication quality for several scenarios. We here briefly review the main steps in this derivation and report the experimental verifications of the corresponding theoretical prediction. With this, we confirm that chaos-based spreading sequences outperform classical pseudo-random sequences in at least two important cases. Over nonselective channels, the ability of chaos-based spreading of minimizing multiple-access interference leads to a measured 60% improvement in P/sub err/ with respect to classical spreading. Over selective channels, the possibility of jointly optimizing chaos-based spreading and Rake receiver profiles leads to improvements of up to 38% in P/sub err/ with respect to systems with either conventional spreading or conventional Rake policies.

A tensor approach to higher order expectations of quantized chaotic trajectories. I. General theory and specialization to piecewise affine Markov systems

The problem of computing any-order expectations of trajectories generated by discrete-time one-dimensional chaotic systems is addressed by means of a suitable generalization of the Perron-Frobenius operator and its quantization. Tools from tensor algebra are introduced and analytical expressions for the special case of piecewise-affine Markov maps are obtained. Results are further specialized for a family of maps with quite general features. As an example application, some cross- and self-interference terms are computed, which are involved in the evaluation of the performance of chaos-based DS-CDMA systems in an asynchronous multipath environment.

Statistical modeling and design of discrete-time chaotic processes: advanced finite-dimensional tools and applications

With the aim of explaining the formal development behind the chaos-based modeling of network traffic and other similar phenomena, we generalize the tools presented in the paper of Setti et al. (see ibid., vol.90, p.662-90, May 2002) to the case of piecewise-affine Markov maps with a possibly infinite, but countable number of Markov intervals. Since, in doing so, we keep the dimensionality of the space of the observables finite, we still obtain a finite tensor-based framework. Nevertheless, the increased complexity of the model forces the use of tensors of functions whose handling is greatly simplified by extensive z transformation. With this, a systematic procedure is devised to write analytical expressions for the tensors that take into account the joint probability assignments needed to compute any-order expectations. As an example of use, this machinery is finally applied to the study of self-similarity of quantized processes both in the analysis of higher order phenomena as well as in the analysis and design of second-order self-similar sources suitable for artificial network traffic generation.

Enhanced rake receivers for chaos-based DS-CDMA

We address here the problem of optimizing the performance of chaos-based dielectric strength-code division multiple access systems over selective channels. A simple rake receiver structure is considered. Some formal arguments show that a nonconventional policy for the adaptation of its coefficients, which exploits the knowledge of the statistical properties of chaos-based spreading sequences, leads to significant improvement in the communication quality.