Wideband Sequential Spectrum Sensing with Varying Thresholds

In this contribution, time varying threshold sequential detectors are employed for energy detection-based spectrum sensing in low-SNR regimes. Sequential detection is proven to be faster (on average) than any other multi-sample detector for a set of given probabilities of detection and false-alarm. In this report, exact performance of a sequential detector for spectrum sensing is analyzed using the direct method. The theoretical results presented herein are verified with Monte-Carlo simulations. It is shown that for a SNR of −10 dB, among tests with Wald and triangular thresholds with similar probabilities of mis-detection and false-alarm, triangular performs 54% faster in terms of maximum detection time (90 percentile)

Multi-agent Q-Learning of Channel Selection in Multi-user Cognitive Radio Systems: A Two by Two Case

Resource allocation is an important issue in cognitive radio systems. It can be done by carrying out negotiation among secondary users. However, significant overhead may be incurred by the negotiation since the negotiation needs to be done frequently due to the rapid change of primary users' activity. In this paper, a channel selection scheme without negotiation is considered for multi-user and multi-channel cognitive radio systems. To avoid collision incurred by non-coordination, each user secondary learns how to select channels according to its experience. Multi-agent reinforcement leaning (MARL) is applied in the framework of Q-learning by considering the opponent secondary users as a part of the environment. The dynamics of the Q-learning are illustrated using Metrick-Polak plot. A rigorous proof of the convergence of Q-learning is provided via the similarity between the Q-learning and Robinson-Monro algorithm, as well as the analysis of convergence of the corresponding ordinary differential equation (via Lyapunov function). Examples are illustrated and the performance of learning is evaluated by numerical simulations.Comment: submitted to 2009 IEEE International Conference on Systems, Man, and Cybernetics; the results of general n by m case will be published soo

Quick Search for Rare Events

Rare events can potentially occur in many applications. When manifested as opportunities to be exploited, risks to be ameliorated, or certain features to be extracted, such events become of paramount significance. Due to their sporadic nature, the information-bearing signals associated with rare events often lie in a large set of irrelevant signals and are not easily accessible. This paper provides a statistical framework for detecting such events so that an optimal balance between detection reliability and agility, as two opposing performance measures, is established. The core component of this framework is a sampling procedure that adaptively and quickly focuses the information-gathering resources on the segments of the dataset that bear the information pertinent to the rare events. Particular focus is placed on Gaussian signals with the aim of detecting signals with rare mean and variance values

Evolución del protocolo MAC multicanal para redes de radio cognitiva

Este artículo presenta la evolución del protoco­lo MMAC-CR (Multichannel MAC protocol for Cognitive Radio). Para ello, se analizan las venta­jas que presenta y se estudian aquellas funciones de la radio cognitiva en las cuales el control de acceso al medio tiene una alta influencia, como la movilidad espectral, la detección de canal, la asignación de recursos y el acceso al espectro. También se describen las variaciones del proto­colo MMAC-CR de acuerdo con el mecanismo de negociación y reserva que cada una de ellas implementa

Quickest Change Detection of a Markov Process Across a Sensor Array

Recent attention in quickest change detection in the multi-sensor setting has been on the case where the densities of the observations change at the same instant at all the sensors due to the disruption. In this work, a more general scenario is considered where the change propagates across the sensors, and its propagation can be modeled as a Markov process. A centralized, Bayesian version of this problem, with a fusion center that has perfect information about the observations and a priori knowledge of the statistics of the change process, is considered. The problem of minimizing the average detection delay subject to false alarm constraints is formulated as a partially observable Markov decision process (POMDP). Insights into the structure of the optimal stopping rule are presented. In the limiting case of rare disruptions, we show that the structure of the optimal test reduces to thresholding the a posteriori probability of the hypothesis that no change has happened. We establish the asymptotic optimality (in the vanishing false alarm probability regime) of this threshold test under a certain condition on the Kullback-Leibler (K-L) divergence between the post- and the pre-change densities. In the special case of near-instantaneous change propagation across the sensors, this condition reduces to the mild condition that the K-L divergence be positive. Numerical studies show that this low complexity threshold test results in a substantial improvement in performance over naive tests such as a single-sensor test or a test that wrongly assumes that the change propagates instantaneously.Comment: 40 pages, 5 figures, Submitted to IEEE Trans. Inform. Theor

Cognitive networks: a Darwinian approach

In this paper we present a new approach for cognitive radio. In the usual approach the secondary network is in charge of monitoring the channel to determine whether or not the primary network is active in the area. If not, the secondary network is allowed to use the spectrum. In the new scheme we propose, the primary network encompasses the techniques which allow it to capture the bandwidth even if the secondary network is transmitting in the area. The access scheme of the primary network is preemptive towards the secondary network. In this paper we present a scheme which is preemptive over the IEEE 802.11 decentralized access scheme. This protocol is a generalized Carrier Sense Multiple Access scheme using active signaling. Instead of only sensing the carrier, this algorithm also transmits bursts of signal which may be sensed by the other nodes. If so, they give up the selection process. We show that this scheme is preemptive over the IEEE 802.11 decentralized access scheme if the bursts transmitted by the node in the primary network are built with special sequences which alternate bursts of signal and periods of sensing. These sequences called $(d, k)$ sequences~\cite{jasz06} encompass a maximum number of zeros during which the node senses the channel to find other possible concurrent transmissions. In practice we use $d=0$ and $k$ depends on the duration on the IEEE 802.11 interframe and the duration of a signaling burst. We compute the number of $(0, k)$ sequences with respect to the length $n$ of the sequence. We also show that $(d, k)$ sequences (with $2d > k$) can used if by mistake during the the signaling phase one burst is not detected. We evaluate the number of such sequences. In a second part of this paper we propose a simple analytical model to compute the mean delay for the primary users versus the load of the primary and secondary users. We show that we have hierarchical independence between the primary and secondary users.Dans ce papier nous présentons une nouvelle approche pour les réseaux cognitifs radio. Dans l'approche traditionnelle le réseau secondaire est en charge de la surveillance du canal pour déterminer si oui ou non le réseau primaire est actif dans dans la zone. Si ce n'est pas le cas, le réseau secondaire est autorisé á utiliser le spectre. Dans le nouveau mécanisme que nous proposons, le réseau primaire utilise une technique qui lui permet de préempter la bande passante même dans le cas oú le réseau secondaire transmet dans cette zone. La technique d'accés du réseau primaire est préemptive sur le schéma décentralisé de la norme IEEE 802.11. Ce protocole est un systéme á détection de porteuse généralisé qui utilise un signalement actif. Au lieu d'uniquement sonder le canal avant la transmission, la technique d'accés envoie également des busrts de signal qui peuvent être détectés par les autres noeuds qui cherchent á transmettre. Si c'est le cas, ces noeuds abandonnent, pour ce cycle, leur tentative de transmission. Nous montrons que ce mécanisme est préemptif sur l'accés décentralisé de 802.11 si les enchaiments de transmission/écoute sont construits avec des séquences binaires spéciales. Ces séquences appelées $(d,k)$ séquences doivent contenir un nombre un nombre maximal $k$ de $0$ successifs et un nombre minimum $d$. En pratique nous utiliserons $d=0$ et $k$ dépend de la durée des inter-trames dans 802.11 et de la durée des burts de signalement actifs. Nous calculons le nombre de $(0,k)$ séquences en fonction de $k$ et de leur longueur $n$. Nous montrons également que des $(d,k)$ séquences (avec $2d > k$) peuvent être utilisées pour pallier á la non détection d'un burst de signalement. Nous évaluons aussi le nombre de ces séquences. Dans une seconde partie de cet article nous calculons le délai moyen pour les utilisateurs du réseau primaires en fonction de la charge des réseaux primaire et secondaire. Nous montrons que nous avons une indépendance hiérachique des performances entre réseau primaire et secondaire