Physical Layer Security of a Dual-Hop Regenerative Mixed RF/UOW System

Ensuring physical layer security is a crucial task in conventional and emerging communication systems, which are typically characterized by stringent quality of service and security requirements. This also accounts for wireless technologies in the context of the Internet of Things paradigm, which are expected to exhibit considerably increased computational complexity. Based on this, the present contribution investigates the secrecy outage performance of a dual-hop decode-and-forward (DF) mixed radio-frequency/underwater optical wireless communication (RF/UOWC) system. Such wireless network configurations are particularly useful in efficient and demanding scenarios, such as military communications. Therefore, our analysis considers one single-antenna source node $(S)$ communicating with one legitimate destination node $(D)$ via a DF relay node $(R)$ equipped with multiple antennas for reception. Particularly, the relay receives the incoming signal from S via an RF link, applies selection-combining (SC) technique, fully decodes it, re-encodes it and then forwards it to the destination via a UOWC link. The communication is performed under the eavesdropper's attempt to intercept the $S-R$ hop (RF side). In this context, a closed-form expression for the secrecy outage probability is derived along with a thorough asymptotic analysis in the high SNR regime, based on which the achievable diversity order is provided. The offered results provide useful insights on the impact of some key system and channel parameters on the secrecy outage performance, such as the number of eavesdroppers, the number of relay antennas, fading severity parameters of RF links, and water turbulence severity of the UOWC link. The conducted analysis shows that the secrecy outage probability is dominated only by the $R-D$ link in the high SNR regime, regardless of the $S-R$ parameters, such as the number of relay antennas and the average SNR at the relay branches. The offered analytic results are corroborated with respective results from computer simulations. Since these parameters are closely related with the computational complexity at the involved terminals, the offered insights are useful for the design and deployment of such systems.acceptedVersionPeer reviewe

Novel Approach for Modeling Wireless Fading Channels using a Finite State Markov Chain

yesEmpirical modeling of wireless fading channels using common schemes such as autoregression and thefinitestate Markov chain (FSMC) is investigated. The conceptual background of both channel structures and the establishment of their mutual dependence in a confined manner are presented. The novel contribution lies in the proposal of a new approach for deriving the state transition probabilities borrowed from economic disciplines, which has not been studied so far with respect to the modeling of FSMC wireless fading channels. The proposed approach is based on equal portioning of the received signal-to-noise ratio, realized by using an alternative probability construction that was initially highlighted by Tauchen. The associated statistical procedure shows that afirst-order FSMC with a limited number of channel states can satisfactorily approximate fading. The computational overheads of the proposed technique are analyzed andproven to be less demanding compared to the conventional FSMC approach based on the levelcrossing rate. Simulations confirm the analytical results and promising performance of the new channel modelbased on the Tauchen approach without extracomplexity costs

Spectrum sensing in cognitive radio using multitaper method based on MIMO-OFDM techniques

The current inefficient utilization of frequency spectrum has alerted regulatory bodies to streamline improvements. Cognitive radio (CR) has recently received considerable attention and is widely perceived as a promising improvement tool in estimating, or equivalently sensing, the frequency spectrum for wireless communication systems. The cognitive cycle in CR systems is capable of recognizing and processing better spectrum estimation (SE) and hence promotes the efficiency of spectrum utilization. Among different SE methods, the multi-taper method (MTM) shows encouraging results. Further performance improvement in the SE for CR can be achieved by applying multiple antennas and combining techniques. This paper proposes a constructive development of SE using MTM, abbreviated as MTSE, and by employing multiple-input multiple-output (MIMO), parsed into separate parallel channels using singular value decomposition (SVD), and maximum ratio combining (MRC) configurations. Deviating from these improvements, however, multicarrier systems such as orthogonal frequency division multiplexing (OFDM) show inferior sensing performances due to the noise multiplicity generated and combined from all subcarrier channels. By means of the quadrature matrix form, the probabilities for such integrated settings of SE have been derived to reach at their approximate asymptotes. Numerical simulations revealed specific better performances stemmed from coupling the fashionable MTSE and MIMO technologies

Adaptive threshold and optimal frame duration for multi-taper spectrum sensing in cognitive radio

This paper delivers an accurate approximation for adaptive threshold and optimal frame detection algorithms based on the robust multitaper method aiming at an efficient spectrum sensing in cognitive radio systems. An appropriate adaptive thresholding allows for seamless vacation of unlicensed secondary users from certain bands upon primary users’ requests, while arbitrary optimal frame detection contributes to the computational and throughput demands. Simulation exercises corroborate the given analysis over Rayleigh channel and multiple-input multiple-output configuration and emphasize the critical role of adopting applicable adaptive threshold and optimal frame detection policies

Superior selective reporting-based spectrum sensing in energy harvesting-aided HCRNs

In the present contribution we investigate the performance of superior selective reporting (SSR) for cooperative spectrum sensing in an energy harvesting-enabled multi-channel heterogeneous cognitive radio network (HCRN). To this end, we first analyze the throughput of the SSR and the optimal conventional cooperative sensing (CCS). Then, we formulate a nonlinear integer programming problem to find a throughput-optimal set of spectrum sensors scheduled to sense a particular channel, under primary user (PU) interference and energy harvesting constraints. In this context, we derive a solution based on the cross entropy (CE) method, and compare its performance with the exhaustive-search method counterpart. Furthermore, we study the tradeoff between the channel available time and detection accuracy of the SSR and CCS schemes. It is shown that this inherent tradeoff is between the channel available time and the detection accuracy. Furthermore, it is shown that as the number of spectrum sensors increases, the channel available time turns out to be the system's limiting factor in HCRNs.acceptedVersionPeer reviewe

On the secrecy analysis of dual-hop underlay multi-source CRNs with multi-eavesdroppers and a multi-antenna destination

In this paper, the physical layer security in cognitive radio networks (CRNs) is investigated. We consider an underlay relay assisted CRN consisting of multiple secondary sources, multiple-antenna destination, a single relay, and multiple eavesdroppers. The destination node performs the maximal-ratio combining under uncorrelated Rayleigh fading channels. In more details, we consider, a secondary source is randomly selected to transmit its data, and a jammer is chosen from the remaining source nodes to send a jamming signal to the eavesdroppers. The closed-form expression of the secrecy outage probability is derived, based on the statistical characteristics of the communication channels, under the primary user's quality of service constraint. The derived analysis gives deep insights into the impact of network parameters on the performance of on the secrecy outage performance. Analytic results are corroborated through Monte Carlo simulation.acceptedVersionPeer reviewe

Intercept Probability of Underlay Uplink CRNs with Multi-Eavesdroppers

The present contribution investigates the physical layer security in a cognitive radio network (CRN). To this end, we consider an underlay uplink CRN consisting of multiple secondary sources, a single-antenna secondary base station, and multiple eavesdroppers. In addition, we assume that the secondary sources transmit their data sequentially and that a jammer is randomly chosen from the remaining source nodes to send a jamming signal to the eavesdroppers. However, in an uplink underlay CRN, a friendly jammer is not always allowed to use its maximal transmit power as the secondary users are required to continuously adapt their power in order to avoid causing interference to the primary users. As a consequence, enhancing the system security using a jammer with low transmit power in the presence of numerous eavesdroppers turns out to be questionable. In this regard, we derive novel analytic expressions that assist in quantifying the achievable security levels and the corresponding limitations. This leads to the development of useful insights on the impact of network parameters on the performance of the system's security. The offered analytic results are corroborated through Monte Carlo simulation. It is shown, that for a low transmit power of the friendly jammer, the system's security can only be enhanced for a small number of eavesdroppers.acceptedVersionPeer reviewe

Cooperative Energy Harvesting Cognitive Radio Networks with Spectrum Sharing and Security Constraints

Physical layer security is an important and timely topic in the research of future wireless systems and it constitutes a part of the Internet of Things (IoT) notion. IoT oriented systems are largely characterized by a stringent quality of service and enhanced security requirements, which comes at a cost of increased computational complexity that needs to be maintained within sustainable levels. In the present contribution, we investigate the physical-layer security of a dual-hop energy RF-Powered cognitive radio network over realistic multipath fading conditions. Assuming a spectrum sharing scenario, our analysis assumes that a source S communicates with a destination D with the aid of a multi-antenna relay R and in the presence of an eavesdropper E who is attempting to overhear the communication of both S-R and R-D links. The involved relay is powered by the renewable energy harvested from the signal sent by the source based on the power-splitting energy harvesting strategy. Furthermore, the relay uses a maximum ratio combining technique to process effectively the received signals. In addition, owing to the underlying strategy, both S and R adjust their respective transmit powers in order to avoid causing interference to the primary network. By considering both the independent identically distributed and the independent but not necessarily identically distributed flat Rayleigh fading channels, closed-form expressions for the secrecy outage probability are derived, based on which an asymptotic analysis is carried out. Our results quantify the impact of the main key system parameters and point out the optimal values ensuring a high-security performance of such a communication system. The validity of the derived results is verified extensively through comparisons with respective Monte Carlo simulation results and useful theoretical and technical insights are developed which are expected to be useful in the design of future cooperative CRNs.publishedVersionPeer reviewe

Physical layer security for dual-hop SWIPT-enabled CR networks

We investigate the physical layer security of a relayassisted underlay cognitive radio network with simultaneous wireless information and power transfer (SWIPT). To this end, we consider a secondary network comprising a secondary source S, one secondary user (SU) relay R, one SU destination D, one primary user (PU) transmitter, and one PU receiver. In addition, we consider an eavesdropper E which can overhear both communications of the S→R and R→D links whereas power constraints are imposed on the secondary network in order to maintain a tolerable interference level at the primary network. Under these constraints, we derive a closed-form expression for the secrecy outage probability assuming uncorrelated Rayleigh fading channels. Numerical and simulation results are presented to corroborate the corresponding analysis. It is shown that the harvested energy, energy conversion efficiency, and maximum tolerable interference level imposed on the primary receiver impact considerably the overall system's security.acceptedVersionPeer reviewe

Achievable ergodic capacity under f composite fading conditions

The F composite fading model was recently proposed as an accurate and tractable statistical model for the characterization of the composite fading conditions encountered in realistic wireless communication scenarios. In the present contribution we capitalize on the distinct properties of this composite model to evaluate the achievable ergodic capacity over F composite fading channels. To this end, we derive an exact closed-form expression for the ergodic capacity, which is subsequently used as a benchmark for the derivation of a tight approximation and a particularly accurate asymptotic representation for large average signal-to-noise ratio values. The derived analytic expressions are provided in closed-form and benefit from their analytical and numerical tractability. This enables the development of meaningful insights on the effect of fading conditions of different severity levels on the overall system performance. Also, it allows the accurate quantification of the signal to noise ratio required in target quality of service requirements under different composite fading conditions.acceptedVersionPeer reviewe