Papers by George Karagiannidis
Electronics, 2021
Artificial Intelligence (AI) and especially Machine Learning (ML) can play a very important role ... more Artificial Intelligence (AI) and especially Machine Learning (ML) can play a very important role in realizing and optimizing 6G network applications. In this paper, we present a brief summary of ML methods, as well as an up-to-date review of ML approaches in 6G wireless communication systems. These methods include supervised, unsupervised and reinforcement techniques. Additionally, we discuss open issues in the field of ML for 6G networks and wireless communications in general, as well as some potential future trends to motivate further research into this area.
IEEE Transactions on Communications, 2016
In this paper, we study a two-phase underlay cognitive relay network, where there exists an eaves... more In this paper, we study a two-phase underlay cognitive relay network, where there exists an eavesdropper who can overhear the message. The secure data transmission from the secondary source to secondary destination is assisted by two decode-and-forward (DF) relays. Although the traditional opportunistic relaying technique can choose one relay to provide the best secure performance, it needs to continuously have the channel state information (CSI) of both relays, and may result in a high relay switching rate. To overcome these limitations, a secure switch-and-stay combining (SSSC) protocol is proposed where only one out of the two relays is activated to assist the secure data transmission, and the secure relay switching occurs when the relay cannot support the secure communication any longer. This security switching is assisted by either instantaneous or statistical eavesdropping CSI. For these two cases, we study the system secure performance of SSSC protocol, by deriving the analytical secrecy outage probability as well as an asymptotic expression for the high main-to-eavesdropper ratio (MER) region. We show that SSSC can substantially reduce the system complexity while achieving or approaching the full diversity order of opportunistic relaying in the presence of the instantaneous or statistical eavesdropping CSI.
IEEE Transactions on Wireless Communications
In this paper, non-orthogonal multiple access (NOMA) networks assisted by multiple intelligent re... more In this paper, non-orthogonal multiple access (NOMA) networks assisted by multiple intelligent reflecting surfaces (IRSs) with discrete phase shifts are investigated, in which each user device (UD) is served by an IRS to improve the quality of the received signal. Two scenarios are considered according to whether there is a direct link between the base station (BS) and each UD, and the outage performance is analyzed for each of them. Specifically, the asymptotic expressions for the upper and lower bounds of the outage probability in the high signal-to-noise ratio (SNR) regime are derived. Following that, the diversity order is obtained. It is shown that the use of discrete phase shifts does not degrade diversity order. More importantly, simulation results reveal that a 3-bit resolution for discrete phase shifts is sufficient to achieve near-optimal outage performance. Simulation results also imply the superiority of IRSs over full-duplex decode-and-forward relays.
IEEE Wireless Communications Letters, 2015
ABSTRACT
In this paper, we study the performance of the transdermal optical wireless communications (OWCs)... more In this paper, we study the performance of the transdermal optical wireless communications (OWCs). In order to provide a quantified understanding of the link's parameters, we establish an appropriate theoretical framework that takes into account the channel particularities, the integration area of the internal unit, the transmitter-receiver misalignment, and the characteristics of the optical units. In more detail, we present closed-form expressions for the instantaneous and average signal to noise ratio (SNR) of the OWCs transdermal link. The findings reveal that misalignment fading drastically affects the reliability and effectiveness of the transdermal link.
We study the impact of residual radio frequency (RF) impairments and fading on the spectrum sensi... more 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 incr... more 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-lay... more —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 w... more —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.
European Transactions on Telecommunications, 2004
A novel closed-form expression for the achievable average channel capacity of a generalized-selec... more 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 ... more 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.
Electronics Letters, 2003
A novel approach to the performance analysis of switched-and-stay combining diversity receivers o... more A novel approach to the performance analysis of switched-and-stay combining diversity receivers over independent Weibull fading channels is presented. Closed-form expressions are extracted for important performance parameters, such as the average output signal-tonoise ratio, the amount of fading, the outage probability, and the switching rate.
IEEE Transactions on Communications, 2004
Ascertaining the importance of the dual selection combining (SC) receivers and the suitability of... more Ascertaining the importance of the dual selection combining (SC) receivers and the suitability of the Weibull model to describe mobile fading channels, we study the performance of a dual SC receiver over correlated Weibull fading channels with arbitrary parameters. Exact closed-form expressions are derived for the probability density function, the cumulative distribution function, and the moments of the output signal-to-noise ratio (SNR). Important performance criteria, such as average output SNR, amount of fading, outage probability, and average bit-error probability for several modulation schemes are studied. Furthermore, for these performance criteria, novel closed-form analytical expressions are derived. The proposed analysis is complemented by various performance evaluation results, including the effects of the input SNR's unbalancing, fading severity, and fading correlation on the overall system's performance. Computer simulation results have verified the validity and accuracy of the proposed analysis.
Journal of The Franklin Institute-engineering and Applied Mathematics, 2008
... Numerical results reveal that, as expected, the diversity gain that such systems offer is, un... more ... Numerical results reveal that, as expected, the diversity gain that such systems offer is, under the weak sourcedestination 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 ...
We perform error probability analysis of the uncoded OFDM fixed gain Amplify-and-Forward (AF) rel... more We perform error probability analysis of the uncoded OFDM fixed gain Amplify-and-Forward (AF) relaying system with subcarrier permutation (SCP). Two SCP schemes, named: the best-to-best SCP (BTB SCP) and the best-to-worst SCP (BTW SCP) are considered. Closedform expressions for the bit error rate (BER) of the above SCP methods are derived. Numerical results manifest that these SCP schemes may outperform one another, depending on the average channel conditions of the links involved. That is, BTB SCP is better at low signal-to-noise ratio (SNR) values, while BTW SCP prevails in the medium and high SNR regime. Thus, it could be concluded that OFDM AF relaying systems may switch from the BTB SCP to BTW SCP in order to achieve optimum BER performance. Moreover, using the derived end-to-end SNR probability density functions (PDF), tight upper bounds for the ergodic capacities of both SCP schemes are obtained.
This is a synopsis of two of the most popular diversity techniques, Polarization Diversity (PD) a... more This is a synopsis of two of the most popular diversity techniques, Polarization Diversity (PD) and Space Diversity (SD), employed in modern communication systems in order to mitigate multipath fading. After outlining the basic principles of the two approaches, we provide representative samples from the literature concerning a hybrid Space-Polarization Diversity (SPD) technique, with particular application of a specific spacetime block code (STBC).
—In this paper, we systematically study the average rate and outage probability tradeoffs of full... more —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 legitima... more 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 (N... more 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.
2011 IEEE International Conference on Communications (ICC), 2011
Page 1. Diversity Combining in Hybrid RF/FSO Systems with PSK Modulation Nestor D. Chatzidiamanti... more Page 1. Diversity Combining in Hybrid RF/FSO Systems with PSK Modulation Nestor D. Chatzidiamantis ∗ , George K. Karagiannidis ∗ , Emmanouil E. Kriezis ∗ and Michail Matthaiou † ∗Department of Electrical & Computer ...
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Papers by George Karagiannidis