Theodoros Tsiftsis - Academia.edu (original) (raw)

Papers by Theodoros Tsiftsis

ICASSP 2023 - 2023 IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP)

In this paper, a radio-frequency (RF) domain symbol level precoding technique is developed for re... more In this paper, a radio-frequency (RF) domain symbol level precoding technique is developed for reconfigurable intelligent surface (RIS)-assisted downlink multiuser multiple-input single-output (MU-MISO) systems. We study a system with a base station (BS) employing an analog architecture formed by a phase shifting network which serves a number of single antenna users with the help of a RIS. Such an architecture facilitates significant reductions in power consumption and hardware complexity. The objective of this paper is to jointly derive the optimal RF precoder, RIS reflection matrix and receive processing coefficients, subject to constraints on the BS analog architecture, the total transmit power, and the structure of the RIS reflection matrix. To that end, a difficult nonconvex optimization problem is formulated and solved. An efficient algorithmic solution is developed for the considered problem. Numerical results show that the derived solution offers significant energy efficiency gains when compared to non-RIS-assisted approaches.

One of the candidate technologies for sixth-generation (6G) wireless communication systems is cel... more One of the candidate technologies for sixth-generation (6G) wireless communication systems is cell-free massive multiple-input multiple-output (CF-mMIMO) communication, which can control inter-cell interference in MIMO systems. This paper investigates the performance of a full-duplex (FD) CF-mMIMO system with practical limited-capacity fronthaul links. The proposed system employs a large number MMM distributed FD access points (APs), arbitrarily distributed KdK_dKd downlink (DL) and KuK_uKu uplink (UL) half-duplex (HD) single-antenna user equipments (UEs), and a central processing unit (CPU). To exploit energy efficiency (EE) and potential throughput gains of FD systems, each AP is linked to the CPU by a fronthaul link with limited capacity that handles the quantized UL/DL data to and from the CPU. On the same spectrum resource, each AP is expected to support KKK single-antenna HD UEs, where K=(Ku+Kd)K = (K_u + K_d)K=(Ku+Kd). Despite having a minimal signal processing complexity, our approach offers a...

Journal of Communications and Networks

Intelligent reflecting surface (IRS) has been regarded as promising technique to improve system p... more Intelligent reflecting surface (IRS) has been regarded as promising technique to improve system performance for wireless communications. In this paper, we propose a ratesplitting (RS) scheme for an IRS-assisted cognitive radio-inspired non-orthogonal multiple access (CR-NOMA) system, where the primary user's (PU's) quality of service (QoS) requirements must be guaranteed to be same as in orthogonal multiple access. Assisted by IRS, the threshold for the PU's tolerable interference power is improved, which in turn makes it possible to increase the achievable rate for the secondary user (SU). The optimal transmit power allocation, target rate allocation, and successive interference cancellation (SIC) decoding order are jointly designed for the proposed RS scheme. Taking into account the statistics of the direct link and IRS reflecting channels, closedform expression for the PU's and SU's outage probabilities are respectively derived. Various simulation results are presented to clarify the enhanced outage performance achieved by the proposed RS scheme over the existing CR-NOMA and IRSassisted CR-NOMA schemes.

IEEE Open Journal of the Communications Society

As massive ground users access wireless networks with limited available spectrum, coexisting user... more As massive ground users access wireless networks with limited available spectrum, coexisting users that share the same spectrum band severely interfere with each other. Moreover, communication pairs without direct communication links may suffer from severe outage issues. In this paper, unmanned aerial vehicle (UAV) relaying-assisted interference coordination is proposed to enhance data transmission reliability by 1) relaying transmission, and 2) alleviating mutual interference among ground users. To efficiently utilize confined UAV energy such that the UAV relaying service life can be significantly prolonged, the full-duplex (FD) technique is exploited. An exact energy efficiency (EE) maximization problem is formulated, which jointly optimizes throughput and UAV energy consumption. Different from the traditional offline method, online power management and trajectory designs are performed under the information causality and rate constraints. Considering that power management and trajectory are coupled, we design a block coordinate descent method to efficiently solve the optimization problem. Numerical results demonstrate that the proposed scheme outperforms existing schemes, and the EE is about 30% and 46% higher than those of the genetic algorithm and half-duplex scheme, respectively. Moreover, impacts of different positions and the maximum power of ground users on the EE have been demonstrated. INDEX TERMS Energy efficiency (EE), full-duplex relaying, mutual interference coordination, unmanned aerial vehicle (UAV) relay.

2022 IEEE International Conference on Communications Workshops (ICC Workshops)

In this paper, we investigate rate splitting multiple access (RSMA) aided mobile edge computing (... more In this paper, we investigate rate splitting multiple access (RSMA) aided mobile edge computing (MEC) in a cognitive radio network. We propose a RSMA scheme that enables the secondary user to offload tasks to the MEC server utilizing dynamic rate splitting without deteriorating the primary user's offloading. The expressions for the optimal rate splitting parameters that maximize the achievable rate for the secondary user and successful computation probability of the proposed RSMA scheme are derived in closed-form. We formulate a problem to maximize successful computation probability by jointly optimizing task offloading ratio and task offloading time and obtain the optimal solutions in closed-form. Simulation results clarify that the proposed RSMA scheme achieves a higher successful computation probability than the existing nonorthogonal multiple access scheme. Index Terms-Rate splitting, cognitive radio, mobile edge computing, task offloading.

Performance analysis - optimization of wireless relaying communications systems

Η έννοια της ψηφιακής αναμετάδοσης έχει μελετηθεί σαν ένα ?εωρητικό πρόβλημα από την πλευρά της ?... more Η έννοια της ψηφιακής αναμετάδοσης έχει μελετηθεί σαν ένα ?εωρητικό πρόβλημα από την πλευρά της ?εωρίας της πληροφορίας στις δεκαετίες του ’70 και ’80 όπου υπολογίστηκε η χωρητικότητα συστημάτων με ένα αναμεταδότη. Το σενάριο στο οποίο η επικοινωνία δύο τερματικών σταθμών πραγματοποείται με τη ?οήθεια ενός τρίτου το οποίο αναλαμβάνει να αναμεταδόσει την πληροφορία, υλοποιήθηκε αρχικά στους δορυφόρους με ανακλαστική επανεκπομπή. Ενδιαφέρον, σύμφωνα με όσα γνωρίζουμε, προκαλεί το γεγονός ότι δεν υπήρχε έως πολύ πρόσφατα καμία εργασία σχετική με την ?εωρία της αναμετάδοσης - πιο πιθανή αιτία η μη ύπαρξη σχετικών εφαρμογών. Με την ωριμότητα των ψηφιακών ασύρματων επικοινωνιών και λόγω των ?αγδαία αυξανόμενων αναγκών για επικοινωνίες υψηλής απόδοσης, ?αίνεται ότι έφτασε η ώρα για την ανάπτυξη ‘‘έξυπνης’’ αναμετάδοσης στο άμεσο μέλλον. Η παρούσα διδακτορική διατριβή (DD) έρχεται να συμβάλλει προς την κατεύθυνση αυ- τή και να δώσει στους μελετητές των ψηφιακών επικοινωνιών νέες τεχνικές κα...

2022 IEEE Wireless Communications and Networking Conference (WCNC)

Fluid antenna systems (FAS) enable mechanically flexible antennas that offer adaptability and fle... more Fluid antenna systems (FAS) enable mechanically flexible antennas that offer adaptability and flexibility for modern communication devices. In this work, we present a conceptual model for a single-antenna N-port (SAN P) FAS over spatially correlated Nakagami-m fading channels and compare it with the traditional diversity schemes in terms of outage probability. The proposed FAS model switches to the best antenna port and resembles the operation of a selection combining (SC) diversity. FAS improves the quality of service (QoS) of the network through antenna port selection. The advantage of FAS is the ability to fit hundreds of antenna ports into a half-wavelength antenna size at the cost of spatial channel correlation. Simulation results demonstrate the superior outage probability performance of FAS at several tens of antenna ports compared to the traditional diversity schemes such as maximum ratio combining, equal gain combining, and SC. Moreover, the novel probability and cumulative density functions for the land mobile correlated Nakagami-m random variates are evaluated in this paper. Index Terms-Fluid antenna system (FAS), Nakagami-m fading, outage probability, single-antenna N-port (SAN P) FAS.

IEEE Transactions on Vehicular Technology

In uplink non-orthogonal multiple access (NOMA) channels, the existing cooperative successive int... more In uplink non-orthogonal multiple access (NOMA) channels, the existing cooperative successive interference cancellation (SIC) and power control (PC) schemes lack the capability of achieving the full capacity region, which restricts the outage performance of uplink NOMA users. For the uplink cognitive radio inspired NOMA system, we propose a new rate splitting (RS) strategy to maximize the achievable rate of the secondary user without deteriorating the primary user's outage performance. Based on the interference threshold and its own channel gain, the secondary user adaptively conducts RS, transmit power allocation and SIC, which utilizes the transmit power efficiently. The closedform expression for the outage probability is derived for the secondary user. Numerical results show that the proposed RS scheme achieves the best outage performance for the secondary user among the existing cooperative SIC and PC schemes. Index Terms-Rate splitting (RS), cognitive radio (CR), nonorthogonal multiple access (NOMA), outage probability.

2021 International Conference on Computer Communications and Networks (ICCCN), 2021

The majority of systems rely on user authentication on passwords, but passwords have so many weak... more The majority of systems rely on user authentication on passwords, but passwords have so many weaknesses and widespread use that easily raise significant security concerns, regardless of their encrypted form. Users hold the same password for different accounts, administrators never check password files for flaws that might lead to a successful cracking, and the lack of a tight security policy regarding regular password replacement are a few problems that need to be addressed. The proposed research work aims at enhancing this security mechanism, prevent penetrations, password theft, and attempted break-ins towards securing computing systems. The selected solution approach is two-folded; it implements a two-factor authentication scheme to prevent unauthorized access, accompanied by Honeyword principles to detect corrupted or stolen tokens. Both can be integrated into any platform or web application with the use of QR codes and a mobile phone.

Resource Allocation for Energy-Efficient NOMA Network Based on Super-Modular Game

2018 IEEE International Conference on Communications Workshops (ICC Workshops), 2018

In this paper, we consider a single cell downlink non-orthogonal multiple access (NOMA) network a... more In this paper, we consider a single cell downlink non-orthogonal multiple access (NOMA) network and aim at maximizing the energy efficiency. The energy-efficient resource allocation problem is formulated as a non-convex and NP-hard problem. To decrease the computation complexity, we decouple the optimization problem as subchannel matching scheme and power allocation subproblems. We introduce a super-modular game and then design an algorithm to converge to the Nash equilibrium (NE) point. Then, a greedy subchannel matching algorithm with low complexity is given through a two-way choice between users and subchannels. However, for given subchannel matching scheme, power allocation is still a non- convex problem. We then transform the non-convex problem to a convex problem by applying a successive convex approximation method. Afterwards we provide an algorithm to converge to suboptimal solution by solving a convex problem iteratively. Finally, simulation result demonstrates that the energy efficiency performance of NOMA system is better than orthogonal frequency division multiple access (OFDMA) system.

IEEE Wireless Communications Letters, 2021

We consider classification of points on a complex-valued Grassmann manifold of m-dimensional subs... more We consider classification of points on a complex-valued Grassmann manifold of m-dimensional subspaces within the n-dimensional complex Euclidean space. We introduce a trellis-based hierarchical classification network, which is based on an orthogonal product decomposition of the orthogonal basis representing the m-dimensional subspace. Exploiting the similarity of the proposed trellis classifier with a neural network, we propose stochastic gradient-based training techniques. We apply the proposed methods to two important applications in wireless communication, namely Grassmannian channel state information quantization in multiple-input multiple-output communications and non-coherent Grassmannian multi-resolution transmission.

2020 International Conference on Computing, Networking and Communications (ICNC), 2020

Non-orthogonal multiple access (NOMA) is recognized as a promising multiple access technique for ... more Non-orthogonal multiple access (NOMA) is recognized as a promising multiple access technique for upcoming fifth generation networks and is known for being able to accommodate high system throughput, massive connectivity and low latency. This paper investigates an underlay cognitive radio NOMA network by adopting an amplify-and-forward (AF) relaying method. The end-to-end outage probability (OP) is studied to evaluate the performance of secondary NOMA users. Furthermore, the OP performance for NOMA is compared with conventional orthogonal multiple access to show the supremacy of the former. Moreover, the proposed AF system is compared to the detectand-forward one. Finally, presented simulation results validate the derived analytical expressions. Index Terms-Amplify-and-forward (AF), cognitive radio (CR), non-orthogonal multiple access (NOMA), outage probability (OP).

Joint Spectrum and Energy Optimization of NOMA-Enabled Small-Cell Networks With QoS Guarantee

IEEE Transactions on Vehicular Technology, 2021

In recent years, wireless communication has experienced a massive shift from a single service (i.... more In recent years, wireless communication has experienced a massive shift from a single service (i.e., voice) to an interconnected web of networks. Although many techniques have been developed improving the offered services to mobile users, still the demand for high-quality services cannot be reached. Therefore, this paper proposes a joint non-orthogonal multiple access (NOMA)-enabled optimization framework for small-cell network (SCNet) by utilizing the concepts of multi-objective problem. In particular, the transmit power of base station (BS) in each small-cell simultaneously optimizes to maximize the sum-capacity and total energy efficiency (EE) of SCNet. The multi-objective optimization problem is formulated as non-convex subject to several practical constraints, i.e., individual quality of service requirement, maximum power budget of small-cell BS, and efficient decoding of superimposed signal using successive interference cancellation. Based on the nature of the problem, the optimal solutions are provided using sequential quadratic programming, and Karush-Kuhn-Tucker approaches. The obtained results show significant performance gains over conventional orthogonal multiple access technique in terms of sum-capacity and total EE.

IEEE Communications Letters, 2020

This letter considers integrating a backscatter link with a reconfigurable intelligent surface to... more This letter considers integrating a backscatter link with a reconfigurable intelligent surface to enhance backscatter communication while assisting the direct communication. We derive the probability that the backscatter channel dominates in the composite channel. This probability is a useful performance measure to determine the number of reflectors. Since the exact probability lacks a closed-form solution, we develop two approximations by modeling the gain of the backscatter link with a Gaussian or Gamma distribution. We found that these approximations match well with the exact value. Importantly, with a well-designed number of reflectors, the channel gain of the backscatter link may be always stronger than that of the direct one. Index Terms-Backscatter communication, reconfigurable intelligent surface (RIS), parameter design. I. INTRODUCTION Reconfigurable intelligent surface (RIS) technology [1] enables wireless charging, remote sensing and unprecedented data transmission in line with the vision of future wireless communications that are low-cost, energy-efficient and connectivity-ubiquitous. RIS consists of many nearly passive reflectors, each of which can sensibly interact with the impinging signal in a software-controlled way. Specifically, each reflector can vary the phase of the incident signal with the perceived channel state information (CSI) without requiring special energy source, in order to realize a high-performance communication link. Moreover, RIS can be easily embedded into objects such as clothing, furniture, walls, and building facades.

Journal of Optics, 2019

Light can be used for wireless information transmission apart from illumination; that is the key ... more Light can be used for wireless information transmission apart from illumination; that is the key idea behind visiblelight communication (VLC), one of the disruptive technologies of our days. It combines remarkably high data rates due to ultrashort wavelengths with huge reliability and security due to small distances; nevertheless, it substantially suffers from interference of neighboring light sources in multiple-link configurations. In this manuscript, we investigate a pair of light emitting diodes (LEDs) interfering each other and propose a simple nanoslit metasurface that radically increases the directivity of the transmitting beams. As a result, enhancement of the signalto-interference ratio by several orders of magnitude is reported. The considered generic setup retains its beneficial features in the presence of realistic design defects and, accordingly, may inspire standardization efforts towards the adoption of VLC in next-generation heterogeneous communication networks.

IEEE Transactions on Vehicular Technology, 2019

This paper considers a dual-hop underlay cognitive radio non-orthogonal multiple access (NOMA) ne... more This paper considers a dual-hop underlay cognitive radio non-orthogonal multiple access (NOMA) network. The endto-end outage probability (OP) is evaluated as the performance metric for secondary NOMA users. Furthermore, the OP results of NOMA is compared with conventional orthogonal multiple access to show the supremacy of the former. Representative numerical and simulation results corroborate the effectiveness of the proposed scheme and presented analysis. Index Terms-Amplify-and-forward (AF), cognitive radio (CR), non-orthogonal multiple access (NOMA), outage probability (OP).

IEEE Transactions on Circuits and Systems I: Regular Papers, 2019

A computationally efficient data pre-distorter suitable for the 16-APSK constellation transmitted... more A computationally efficient data pre-distorter suitable for the 16-APSK constellation transmitted through a memoryless non-linear power amplifier is introduced. The proposed method is based on estimating the optimum value of the ring ratio that enables compensation for the power amplifier (PA) distortion occurring at each input back-off (IBO) level of a memoryless behavioural model. Previous results for data predistortion rely on iterative numerical optimization techniques which require increased computational resources. Instead, the proposed approach is numerically efficient and further provides insight on the generation of non-linear distortion. Computed and measured bit-error rate (BER) results for 16-APSK in additive white Gaussian noise (AWGN) channel and the presence of a PA are presented to prove the validity of the method. The performance evaluation of the pre-distorter based on a measured non-linear characteristic of a commercial power amplifier showed a BER reduction of up to four orders of magnitude at low IBO levels.

IEEE Internet of Things Journal, 2019

IEEE Transactions on Wireless Communications, 2019

In this paper, a cooperative single carrier system comprising multiple cooperating remote radio h... more In this paper, a cooperative single carrier system comprising multiple cooperating remote radio heads (RRHs) and spatially distributed interferers is investigated. Due to the random location of the interferers within the communication range, a mixture of line-of-sight (LoS) and non-line-of-sight (nLoS) paths is considered in the channel model. Under a frequency selective fading channel with a mixture of the LoS and nLoS paths, the distributed cyclic delay diversity (dCDD) is employed to achieve the maximum transmit diversity gain without the exact knowledge of the channel state information at the transmitter side. It is shown that the operating signalto-noise (SNR) regions are divided in two regions, i.e., noiselimited and interference-limited. In this paper, the main focus is on the interference-limited region, in which diversity gain is not achieved due to performance limits determined by system and channel parameters. The existence of these limits on performance metrics, such as the outage probability and ergodic capacity is derived analytically, and then verified by link-level simulations. Index Terms-Distributed CDD, LoS and nLoS paths, cyclicprefixed single carrier transmission, interferers, outage probability, ergodic capacity.

IET Communications, 2018

In this study, the authors analyse the outage performance of the indoor relay-assisted hybrid rad... more In this study, the authors analyse the outage performance of the indoor relay-assisted hybrid radio frequency (RF)/ visible light communication (VLC) systems. They derive closed-form expressions for the outage probability of the end-to-end signal-to-noise ratio. Then they obtain the optimal approximation parameter with optimising the outage probability using the differential evolution algorithm. Moreover, they analysed the average bit error rate (BER) performance of the hybrid RF/optical system using pulse position modulation method while assuming the timing error in synchronisation. Finally, they present some numerical results utilising the newly derived exact closed-form expressions.