Sebastià Galmés

Nanoscale Communication With Molecular Arrays in Nanonetworks

Molecular communication is a promising nanoscale communication paradigm that enables nanomachines to exchange information by using molecules as communication carrier. Up to now, the molecular communication channel between a transmitter nanomachine (TN) and a receiver nanomachine (RN) has been modeled as either concentration channel or timing channel. However, these channel models necessitate exact time synchronization of the nanomachines and provide a relatively low communication bandwidth. In this paper, the Molecular ARray-based COmmunication (MARCO) scheme is proposed, in which the transmission order of different molecules is used to convey molecular information without any need for time synchronization. The MARCO channel model is first theoretically derived, and the intersymbol interference and error probabilities are obtained. Based on the error probability, achievable communication rates are analytically obtained. Numerical results and performance comparisons reveal that MARCO provides significantly higher communication rate, i.e., on the scale of 100 Kbps, than the previously proposed molecular communication models without any need for synchronization. More specifically, MARCO can provide more than 250 Kbps of molecular communication rate if intersymbol time and internode distance are set to 2 μs and 2 nm, respectively.

Dedicated Radio Utilization for Spectrum Handoff and Efficiency in Cognitive Radio Networks

To perform spectrum handoff, cognitive radio (CR) nodes communicating with each other need to exchange licensed user detection information, i.e., perform spectrum coordination, over a common control channel. The spectrum coordination can be fulfilled either via existing cognitive radio interface with time division or via a separate dedicated radio, i.e., a common control interface (CCI), continuously. CR nodes with CCI can instantly exchange licensed user detection information and cease frame transmission, while spectrum coordination can only be performed after the frame transmission period without CCI. Nevertheless, the impact of CCI incorporation into CR nodes in terms of common performance metrics must be thoroughly assessed to evaluate the worthiness of additional radio cost. In this paper, an analytical framework is presented to assess the impact of CCI incorporation into CR nodes for spectrum handoff. The developed framework enables analyzing potential benefits and disadvantages of employing CCI for spectrum handoff, in terms of achievable delay, energy consumption, spectrum utilization and event estimation performance. Extensive performance evaluations are presented to illustrate the impact of CCI utilization on efficiency of spectrum handoff. The network and communication regimes that would yield having CCI favorable are characterized in terms of spectrum conditions and CR parameters.

Simulated annealing approach to optimizing the lifetime of sparse time-driven sensor networks

Time-driven sensor networks are devoted to the continuous reporting of ambient data to the base station. In many cases, these data are provided by nodes that have been deployed in a structured manner, either by selecting strategic locations or by adopting some regular sampling pattern. In either case, the resulting inter-node distances may not be small, and thus additional supporting nodes may be necessary. This suggests that the problem could be better addressed from a network planning perspective. In this sense, a particular approach is proposed and, as part of it, the paper focuses on optimizing the lifetime that can be predicted from the network topology, which is assumed to be a static data gathering tree. It is shown that this problem requires the exploration of all possible spanning trees, since the energy consumed by a node depends on its workload, which in turn depends on how this node is connected to its neighborhood. Because this is an NP-hard problem, the use of a heuristic approach is required. Then, an algorithm based on simulated annealing is proposed, which converges asymptotically to the global optimum. This algorithm is tested on different scenarios and its computational complexity is proved to be linearly dependent on the number of nodes.

Correlation analysis of a discrete-time flexible arrival process

In this paper, a flexible discrete-time arrival process is introduced and its correlation properties are analyzed. The arrival process is the so-called batch-on/off model, an extension of the original on/off source used in the context of ATM networks. In the batch-on/off model, a group of arrivals may be generated at any given active slot. General distributions are assumed for the three input random variables characterizing the process: busy and idle periods, and batch size. The analysis focuses on two related processes: the process of counts and the sequence of interarrival times. For each process, an exact closed-form expression of its complete autocorrelation function is obtained. Explicit algorithms are provided to compute both autocorrelation functions, which are numerically evaluated for different distributions of the busy and idle periods and the batch size. The results provided in this paper reveal the analytical tractability of these models which, in addition to their flexibility, makes them very suitable for the performance evaluation of discrete-time communication systems and for general research in the area of queuing theory.

A source independent traffic model for ATM networks

At present, most of research on ATM traffic modelling is application dependant. This means that each type of B-ISDN application is described by a set of traffic models, but these models are not capable of capturing the characteristics of other types of applications. On the other hand, from the network performance point of view, just some teletraffic features at cell level are finally of interest, no matter what the sources are. In this paper, we propose the idea of constructing a source independent model, that may capture those relevant features for any type of source. As a first attempt, we focus on the on-off model with general distributions as a possible candidate to reach this objective. For this model, we compute exact-closed form expressions of the burstiness and correlations of the inter-arrival times between cells. Finally, we use these results to set up an algorithm for the evaluation of an equivalent on-off model for any ATM source.

An algorithm for computing the mean response time of a single server queue with generalized on/off traffic arrivals

In this paper, an exact solution for the response time distribution of a single server, infinite capacity, discrete-time queue is presented. This queue is fed by a flexible discrete-time arrival process, which follows an on/off evolution. A workload variable is associated with each arrival instant, which may correspond to the service demand generated by a single arrival, or represent the number of simultaneous arrivals (bulk arrivals). Accordingly, the analysis focuses on two types of queues: (On/Off)/G/1 and (Batch-On/Off)/D/1. For both cases, a decomposition approach is carried out, which divides the problem into two contributions: the response time experienced by single bursts in isolation, and the increase on the response time caused by the unfinished work that propagates from burst to burst. Particularly, the solution for the unfinished work is derived from a Wiener-Hopf factorization of random walks, which was already used in the analysis of discrete GI/G/1 queues. Compared to other related works, the procedure proposed in this paper is exact, valid for any traffic intensity and has no constraints on the distributions of the input random variables characterizing the process: duration of on and off periods, and workload. From the general solution, an efficient and robust iterative algorithm for computing the expected response time of both queues is developed, which can provide results at any desired precision. This algorithm is numerically evaluated for different types of input distributions and proved against simulation.

On the capabilities of on-off models to capture arbitrary ATM sources

B-ISDN is conceived to support all types of existing and new emerging applications. In the context of B-ISDN and the new age of information, the user terminal will generate data, voice, video and, in general, multimedia traffic. ATM is the selected mechanism for the efficient transfer of these information across the future worldwide network. The proper design of this network requires cost-effective solutions that may satisfy the quality of service constraints of all the connections. To achieve this goal, an accurate characterization of the traffic generated by the sources is needed. This characterization should capture the statistical properties of the ATM traffic that are relevant to network performance. A review of literature shows that, from a general point of view, this objective has been reached, but with two drawbacks: traffic models are application dependant and, moreover, for some applications, traffic models are not analytically tractable. The final objective of this paper is to construct a general analytical B-ISDN model. Particularly, the paper focuses on the on-off models with general distributions, for which a complete exact statistical characterization at cell level was already obtained in a previous paper. On the basis of this characterization, an algorithm devoted to capture the statistical behaviour of any source is formulated and developed. Particularly, the algorithm captures the average rate and burstiness of any source in an exact way, and the autocorrelation function in an optimal way. Finally, numerical results for different types of video sources are provided.

Multi-objective Simulated Annealing Approach for Optimal Routing in Time-Driven Sensor Networks

In this work we propose multi-objective simulated annealing as a heuristic technique for optimal routing in time-driven sensor networks. Unlike previous algorithms and methods, this technique is intended to tackle multiple performance-related and other design objectives in a computationally feasible way. Since these objectives are usually in conflict, the general solution is formulated as the so-called Pareto set, which is the set of non-dominated design vectors representing different tradeoffs.

The response time distribution of a discrete-time queue under a generalized batch arrival process

In this paper we obtain the response time distribution of a single server, discrete-time queue, fed by an aggregate extension of the well-known on/off source. Essentially, at each active slot, we allow for a variable number of units of workload (packets, cells, messages, etc.) to enter the queue. So, the resulting input traffic is a batch-on/off process. The server is deterministic, with a service time corresponding to the transmission of a workload unit. We follow an exact analytical procedure, and we provide an effective algorithm to compute the solution. Moreover, this work has been performed under very general conditions, since no particular distributions are assumed for the random variables characterizing the batch-on/off process: on and off periods, and batch sizes. Also, the fact of obtaining the complete response time distribution, allows for the calculation of other parameters than the mean, such as the jitter, for instance, or higher order moments. This becomes essential when analyzing the performance of finite capacity resources in the context of digital communication systems, especially when real time services are supported. Finally, numerical results are provided.

Statistical Analysis of Array Gain for Cooperative MISO Transmitters without CSI

Virtual Multiple-Input Single-Output (MISO) is recently proposed to extend the benefits of transmitter space diversity to networks in which the deployment of antenna arrays on individual nodes is infeasible from a practical point of view. Ad-hoc and sensor networks are examples of these type of networks. In these scenarios, nodes equipped with single antenna can cooperatively transmit to emulate an antenna array. However, cooperative transmissions require knowledge of the channel state either at the transmitter side or the receiver side in order to achieve full performance gains. Several solutions are proposed in the literature under these assumptions, but at the expense of increased overhead and energy consumption. In this paper, the array gain at the receiver from the non-coherent combining of the signals from multiple transmitters is analyzed in statistical sense, under the assumption that the channel knowledge is unavailable. The transmitters are assumed to be randomly spread over a circular region. More specifically, exact or very accurate closed-form expressions for the expectation and variance of the array gain are obtained, and then a complete statistical distribution is postulated and validated by means of heuristic procedures, goodness-of-fit tests and specialized software. The results obtained in this paper can be especially useful for the implementation of two-tiered wide area sensor networks.