George Karagiannidis

We study the impact of residual radio frequency (RF) impairments and fading on the spectrum sensing performance of a classical energy detector (ED), which is employed in a full-duplex wireless system. Specifically, we present novel closed-form expressions for the false-alarm and detection probabilities, assuming Nakagami-m fading, and the impact of residual RF impairments. The results reveal the importance of taking into account the wireless environment and the ED's capabilities, when evaluating the ED performance and setting its threshold.

The next generation wireless networks are envisioned to deal with the expected thousand-fold increase in total mobile broadband data and the hundred-fold increase in connected devices. In order to provide higher data rates, improved end-to-end performance, low latency, and low energy consumption at low cost per transmission, the fifth generation (5G) systems are required to overcome various handicaps of current cellular networks and wireless links. One of the key handicaps of 5G systems is the performance degradation of the communication link, due to the use of low-cost transceiver in high data rate. Motivated by this in this paper, we discuss the impact of transceiver front-end hardware imperfections on the spectrum sensing performance of cognitive radios.

—We evaluate and quantify the joint effect of fading and multiple interferers on the physical-layer (PHY) security of a system consisted of a base-station (BS), a legitimate user, and an eavesdropper. To this end, we present a novel closed-form expression for the secrecy outage probability, which takes into account the fading characteristics of the wireless environment, the location and the number of interferers, as well as the transmission power of the BS and the interference. The results reveal that the impact of interference should be seriously taken into account in the design and deployment of a wireless system with PHY security.

—We study the error performance of an heterodyne differential phase-shift keying (DPSK) optical wireless (OW) communication system operating under various intensity fluctuations conditions. Specifically, it is assumed that the propagating signal suffers from the combined effects of atmospheric turbulence-induced fading, misalignment fading (i.e., pointing errors) and path-loss. Novel closed-form expressions for the statistics of the random at-tenuation of the propagation channel are derived and the bit-error rate (BER) performance is investigated for all the above fading effects. Numerical results are provided to evaluate the error performance of OW systems with the presence of atmospheric turbulence and/or misalignment. Moreover, nonlinear optimization is also considered to find the optimum beamwidth that achieves the minimum BER for a given signal-to-noise ratio value.

A novel closed-form expression for the achievable average channel capacity of a generalized-selection combining RAKE receiver in Rayleigh fading, is derived. Performance comparison for the capacity achieved with maximal-ratio combining and RAKE receivers is also presented. The expression derived, fully conforms to the upper bound of the Shannon–Hartley theorem. Copyright © 2004 AEI.

Motivated by the suitability of the Weibull distribution to model multipath fading channels, the second order statistics and the spectral efficiency (SE) of L-branch SC receivers are studied. Deriving a novel closed-form expression for the probability density function (pdf) of the SC output SNR, the average level crossing rate (LCR), the average fade duration (AFD) and the Shannon's average SE, at the output of the SC, are derived in closed-forms. Our results are sufficiently general to handle arbitrary fading parameter and dissimilar branch powers.

... Numerical results reveal that, as expected, the diversity gain that such systems offer is, under the weak source–destination channel assumption, on the order of the ... [5] GK Karagiannidis, NC Sagias and TA Tsiftsis, Closed-form statistics for the sum of squared Nakagami-m ...

—In this paper, we systematically study the average rate and outage probability tradeoffs of full-duplex two-way and one-way relaying under residual self-interference. Among various relaying protocols, two common of them are considered: amplify-and-forward (AF) and decode-and-forward (DF). Furthermore, we consider the application of physical-layer network coding (PNC) and analog network coding (ANC) to full-duplex two-way relaying. Novel closed-form expressions for the average rate and outage probability, are presented. The results show that full-duplex two-way relaying can achieve higher rate than one-way relaying in the medium to high signal-to-noise ratio (SNR) region, at the cost of a certain loss in the outage performance. Moreover, DF protocol can achieve better outage performance than the AF one, but it suffers from a certain loss in the rate in the high SNR region. It is also shown that PNC can further improve the rate and outage performance. In addition, the results clearly reveal the effects of time multiplexing, forward protocol, and network coding on relaying systems, which would shed light on designing practical full-duplex relaying schemes.

We investigate the physical layer (PHY) security of a system with a base-station (BS), a legitimate user, and an eavesdropper, whose exact location is unknown but within a ring-shaped area around the BS. To this end, we present novel closed-form expressions for the secrecy outage probability, which take into consideration both the impact of fading, as well as the eavesdropper's location uncertainty. The derived expressions are validated through simulations, which reveal that the level of uncertainty should be seriously taken into account in the design and deployment of a wireless system with PHY security.

In this paper, a downlink multiple-input-multiple-output (MIMO) non-orthogonal multiple access (NOMA) scenario is considered. We investigate a dynamic user clustering problem from a fairness perspective. In order to solve this optimization problem, three sub-optimal algorithms, namely top-down A, top-down B, and bottom up, are proposed to realize different tradeoffs of complexity and throughput of the worst user. In addition, for each given user clustering case, we optimize the power allocation coefficients for the users in each cluster by adopting a bisection search based algorithm. Numerical results show that the proposed algorithms can lower the complexity with an acceptable degradation on throughput compared with the exhaustive search method. It is worth noting that top-down B algorithm can achieve a good tradeoff between complexity and throughput among the three proposed algorithms.

Direct-conversion radio (DCR) receivers can offer highly integrated low-cost hardware solutions for spectrum sensing in cognitive radio (CR) systems. However, DCR receivers are susceptible to radio frequency (RF) impairments, such as in-phase and quadrature-phase imbalance, low-noise amplifier nonlinearities and phase noise, which limit the spectrum sensing capabilities. In this paper, we investigate the joint effects of RF impairments on energy detection based spectrum sensing for CR systems in multi-channel environments. In particular, we provide novel closed-form expressions for the evaluation of the detection and false alarm probabilities, assuming Rayleigh fading. Furthermore, we extend the analysis to the case of CR networks with cooperative sensing, where the secondary users suffer from different levels of RF imperfections, considering both scenarios of error free and imperfect reporting channel. Numerical and simulation results demonstrate the accuracy of the analysis as well as the detrimental effects of RF imperfections on the spectrum sensing performance, which bring significant losses in the spectrum utilization.

Radio frequency (RF) front-ends constitute a fundamental part of both conventional and emerging wireless systems. However, in spite of their importance they are often assumed ideal, although they are practically subject to certain detrimental impairments, such as amplifier nonlinearities, phase noise and in phase and quadrature (I/Q) imbalance (IQI). This paper is devoted to the quantification and evaluation of the effects of IQI on the secrecy capacity of a wiretap channel. Novel closed-form expressions are derived for the secrecy outage probability for the case of ideal transmitter and I/Q imbalanced legitimate user's and eavesdropper's receivers. Numerical results and simulations illustrate the detrimental effects of IQI on the physical layer security performance, and reveal that IQI should be seriously taken into consideration in the design of secure wireless systems. Index Terms—I/Q imbalance, Outage secrecy capacity, Physical layer security, Probability of zero secrecy capacity.

We investigate the impact of multiple primary users (PUs) and fading on the spectrum sensing of a classical energy detector (ED). Specifically, we present novel closed-form expressions for the false-alarm and detection probabilities in a multiple PUs environment, assuming Nakagami-m fading and complex Gaussian PUs transmitted signals. The results reveal the importance of taking into consideration the wireless environment, when evaluating the ED spectrum sensing performance and selecting the ED threshold.

Direct-conversion architectures (DCA) can offer highly integrated low-cost hardware solutions to communication transceivers. However, DCA devices are sensitive to radio frequency (RF) imperfections such as amplifier non-linearities, phase noise and in-phase/quadrature-phase imbalances (IQI), which typically lead to a severe degradation of the performance of such systems. Motivated by this, we quantify and evaluate the impact of RF IQI on wireless communications in the context of cascaded fading channels. Novel closed form expressions are derived for the corresponding outage probability for the case of ideal transmitter (TX) and receiver (RX), ideal TX and I/Q imbalanced RX, I/Q imbalanced TX and ideal RX, and joint I/Q imbalanced TX/RX. The offered analytic results have a relatively convenient algebraic representation and their validity is extensively justified through simulations. Based on these, it is shown that cascaded fading leads to considerable degradation in the system performance and that assuming ideal RF front- ends at the TX and RX induces non-negligible errors in the outage probability that can exceed 20% in several communication scenarios. We further demonstrate that the effects by cascaded multipath fading conditions are particularly severe, as they typically result to considerable performance losses of around or over an order of magnitude.

Direct-conversion architectures can offer highly integrated low-cost hardware solutions to communication transceivers. However, it has been demonstrated that radio frequency (RF) impairments such as amplifier nonlinearities, phase noise and in-phase/quadrature-phase imbalances (IQI) can lead to a severe degradation in the performance of such systems. Motivated by this, the present work is devoted to the quantification and evaluation of the effects of RF IQI on wireless communications in the context of cascaded fading channels for both single-carrier and multi-carrier systems. To this end, closed form expressions are firstly derived for the outage probability (OP) over N*Nakagami−m channels for the cases of ideal transmitter (TX) and receiver (RX), ideal TX and IQI RX, IQI TX and ideal RX, and joint TX/RX IQI. The offered expressions along with several deduced corresponding special cases are subsequently employed in the context of vehicular-to- vehicular (V2V) communications to justify their importance and practical usefulness in the context of emerging communication systems. We demonstrate that considering non-ideal RF front- ends at the TX and/or RX, introduces non-negligible errors in the OP performance that can exceed 20% in several communication scenarios. We further demonstrate that the effects by cascaded multipath fading conditions are particularly detrimental, as they typically result in considerable performance losses of around or over an order of magnitude.

Radio frequency (RF) front-ends constitute a fundamental part of both conventional and emerging wireless communication systems. However, in spite of their importance they are often assumed ideal, although they are practically subject to certain detrimental impairments, such as amplifier nonlinearities, phase noise and in phase and quadrature (I/Q) imbalance (IQI). The present work is devoted to the quantification and evaluation of the RF IQI effects in the context of realistic wireless vehicle-to- vehicle (V2V) communications over double-Nakagami−m fading channels. Novel closed form expressions are derived for the corresponding outage probability for the case of ideal transmitter (TX) and receiver (RX), ideal TX and I/Q imbalanced RX, I/Q imbalanced TX and ideal RX, and joint I/Q imbalanced TX/RX. The offered analytic results have a relatively convenient algebraic representation and their validity is extensively justified through comparisons with respective results from computer simulations. Based on these, it is shown that cascaded fading results to considerable degradations in the system performance and that assuming ideal RF front-ends at the TX and RX induces non- negligible errors in the outage probability evaluation that can exceed 20% in several V2V communication scenarios.

Direct-conversion radio receivers can offer highly integrated low-cost hardware solutions for spectrum sensing in cognitive radio systems. However, these receivers are susceptible to radio frequency (RF) impairments, such as in-phase and quadrature-phase imbalance, low-noise amplifier nonlinearities and phase noise, which limit the spectrum sensing capabilities. In this paper, we study the joint effects of RF impairments on energy detection based spectrum sensing in multi-channel environments. In particular, we provide an analytical framework for the evaluation of the probabilities of detection and false alarm, assuming Rayleigh fading and approximating the joint effects of RF impairments by a Gaussian distribution. Numerical results illustrate the detrimental effects of RF imperfections on the spectrum sensing performance, which bring significant losses in the spectrum utilization.

This paper studies the secrecy rate maximization problem of a secure wireless communication system, in the presence of multiple eavesdroppers. The security of the communication link is enhanced through cooperative jamming, with the help of multiple jammers. First, a feasibility condition is derived to achieve a positive secrecy rate at the destination. Then, we solve the original secrecy rate maximization problem, which is not convex in terms of power allocation at the jammers. To circumvent this non-convexity, the achievable secrecy rate is approximated for a given power allocation at the jammers and the approximated problem is formulated into a geometric programming one. Based on this approximation, an iterative algorithm has been developed to obtain the optimal power allocation at the jammers. Next, we provide a bisection approach, based on one-dimensional search, to validate the optimality of the proposed algorithm. In addition, by assuming Rayleigh fading, the secrecy outage probability (SOP) of the proposed cooperative jamming scheme is analyzed. More specifically, a single-integral form expression for SOP is derived for the most general case as well as a closed-form expression for the special case of two cooperative jammers and one eavesdropper. Simulation results have been provided to validate the convergence and the optimality of the proposed algorithm as well as the theoretical derivations of the presented SOP analysis.