Robust Resource Allocation for OFDM-based Cognitive Radio in the Presence of Primary User Emulation Attack

Cognitive radio (CR) is a promising solution to improve the spectrum efficiency in which some unlicensed users are allowed to exploit frequency bands which are not used by licensed network. However, CR technology imposes some threats to the network. One of these threats is primary user emulation attack where some malicious users try to send fake signals similar to the primary user (PU) and prevent secondary users from accessing vacant bands. Moreover, the presence of a primary user emulation attacker (PUEA) leads to additional interference to the CR and consequently, the efficiency of conventional power loading algorithms will be degraded. In this paper, we propose a power allocation scheme in an orthogonal frequency-division multiplexing (OFDM) based CR in the presence of PUEA. Power allocation is performed with the aim of maximizing the downlink transmission capacity achieved by the cognitive user, while keeping the interference level at the PU below a predefined threshold. Simulation results confirm the efficiency of our proposed power loading scheme, compared to classical loading algorithms that do not consider the activity of malicious users in the radio environment

An Adaptive Turbo Coded-OFDM Scheme for Visible Light Communications

In visible light communications (VLC) the main bottleneck is the limited bandwidth of the light emitting diodes, which limits the maximum transmission data rates. Multicarrier modulation schemes are one possible option to increase the data rate as well as improve the link performance. In this paper, we present the turbo coded orthogonal frequency-division multiplexing (OFDM) scheme to mitigate interference and noise as well as to remove multipath induced inter symbol interference in a VLC system. Simulation results are used to compare the performance of the proposed system compared to classical optical OFDM schemes of DC-biased optical and asymmetrically clipped optical-OFDM. The results presented may be used to investigate the interaction between VLC system parameters (e.g., receiver field- of-view limitation, incident angle, light emitting diode power)and the bit error rate performance of the soft turbo decoder

Systèmes OFDM ultra large bande :<br />Estimation de canal et détection améliorée prenant en compte les imprécisions d'estimation

The aim of this thesis is to study the problem of iterative data detection in a realistic wireless communication system, where the receiver disposes only of an imperfect (and possibly poor) estimate of the unknown channel parameters. The application scenarios on which we focus are single- and multi-antenna OFDM systems working over ultra wideband (UWB) channels. First, we propose an efficient receiver jointly estimating the channel and the transmitted symbols in an iterative manner. This receiver is based on a wavelet representation of the unknown channel and exploits the sparseness property of UWB channels in the wavelet domain to reduce the receiver's computational complexity. Second, we rely on the statistics characterizing the quality of the channel estimation as a mean to integrate the imperfect channel knowledge into the design of iterative receivers. In this way, we formulate an improved maximum likelihood (ML) detection metric taking into account the presence of channel estimation errors. We propose a modified iterative detector based on maximum a posteriori (MAP) which mitigates the effect of channel uncertainty on the detector performance, by an appropriate use of this metric. The results are compared to those obtained by using a classical detector based on a mismatched ML metric, which uses the channel estimate as if it was the perfect channel. The influence of the constellation labeling is also experimentally studied. Furthermore, we calculate the achieved throughputs associated to both improved and mismatched ML detectors, in terms of maximal achievable outage rates. Our results may serve to evaluate the trade-off between the requiredquality of service (in terms of BER and achieved throughputs) and the system parameters (e.g., power allocated to pilot and data symbols, number of pilots per frame, number of decoding iterations, outage probability) in the presence of channel estimation errors. Finally, we propose an improved low-complexity iterative detector based on soft parallel interference cancellation and linear minimum mean-square error (MMSE) filtering. This receiver takes into account the presence of channel estimation errors in the formulation of the linear MMSE filter, as well as in the interference cancellation part. The important point is that the performance improvements reported in this thesis are obtained while imposing practically no additional complexity to the receiver.Les travaux présentés dans cette thèse se situent dans le cadre de la transmission OFDM appliqué au contexte Ultra Large Bande. L'objectif principal va être l'estimation du canal de propagation et la conception de récepteurs en prenant en compte la connaissance non idéale du canal. On propose d'abord une approche semi-aveugle d'estimation du canal basée sur l'algorithme EM. Le canal est décrit au moyen de sa décomposition dans une base d'ondelettes, ce qui fournit une représentation parcimonieuse. La complexité de l'estimation est réduite grâce au fait que les coefficients d'ondelettes jugés non significatif sont écartés. Ensuite, on propose des structures de réception où la conception du détecteur prend en compte la présence des erreurs d'estimation de canal. La détection au sens maximum de vraisemblance (MV) est améliorée en modifiant sa métrique de détection pour le cas où le canal est estimé de manière imparfaite. La métrique MV améliorée ainsi obtenue nous permet de remettre en question le schéma de détection de type turbo maximum a posteriori (MAP) et l'adapter à la connaissance imparfaite du canal. En outre, le choix de l'étiquetage lors de l'attribution des bits aux symboles est également discuté. De plus, on dérive les débits de coupure atteignables (achievable outage rates) avec le détecteur MV améliorée ou un détecteur MV désadapté (utilisant les estimés de canal comme s'il s'agissait des vraies) qu'on comparera avec un détecteur théorique défini comme étant le meilleur récepteur possible en présence d'erreurs d'estimation de canal. Enfin, un récepteur itératif à complexité réduite basé sur un filtrage MMSE et l'annulation parallèle d'interférence (turbo-PIC) est introduit. L'idée de notre approche consiste à prendre en compte la connaissance imparfaite du canal dans la conception du filtre MMSE ainsi que dans la partie annulation d'interférence. Il est important de souligner que les améliorations proposées dans cette thèse ne résultent pas en une augmentation sensible de la complexité algorithmique au niveau du récepteur

Cooperative spectrum sensing for cognitive radio networks in the presence of smart malicious users

Cognitive radio (CR) signaling imposes some threats to the network. One of these common threats is commonly referred to as primary user emulation attack, where some malicious users try to mimic the primary signal and deceive secondary users to prevent them from accessing vacant frequency bands. In this paper, we introduce a smart primary user emulation attacker (PUEA) that sends fake signals similar to the primary signal. We assume a smart attacker, in the sense that it is aware of its radio environment and may choose different transmission strategies and then, we compare it to an always present attacker. In the proposed smart attacker strategy, the occurrence of fake signal is adjusted according to the primary user activity. First, we investigate the received signal at the CR users under such attackers. Then, we formulate and derive cooperative spectrum sensing (CSS) rules for a cognitive network operating in the presence of a PUEA and propose a new spectrum sensing scheme based on energy detection. Simulation results show that our proposed method can mitigate the destructive effect of PUEA in spectrum sensing, compared to conventional energy detection spectrum sensing.</p

Cooperative spectrum sensing in cognitive radio networks under primary user emulation attacks

Cognitive radio as a solution of spectrum scarcity, proposes a dynamic spectrum access which imposes some threats to the network. One of these common threats is primary user emulation attack, where some malicious users try to mimic the primary signal and deceive secondary users to prevent them from accessing vacant frequency bands. In this work, we consider the true definition of a primary user emulation attacker (PUEA) in which fake signals are sent when the primary user signal is not present in the radio environment. We formulate and derive cooperative spectrum sensing (CSS) rules for a cognitive network in the presence of a PUEA and propose a new spectrum sensing based on energy detection with fusion centers employing OR/AND logic rules. Simulation results show that our proposed method can mitigate the destructive effect of PUEA in spectrum sensing, compared to conventional energy detection spectrum sensing.</p