Faculty of Electrical Engineering and Information Technologies

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Optimized Scheduling Transmissions for Wireless Powered Federated Learning Networks
(Institute of Electrical and Electronics Engineers (IEEE), 2025-03)
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Poposka, Marija
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Hadzi-Velkov, Zoran
We have developed a resource allocation scheme that minimizes the training process of federated machine models in the wireless powered communication networks. The new resource sharing method allows energy harvesting (EH) clients (EHCs) to train their local models for extended periods that overlap with data transmissions of other EHCs. Training latency minimization leads to mixed integer non-convex problem, which is tackled by exploiting the sensitivity properties of the corresponding Lagrange multipliers. If the local training models at all EHCs use equal size datasets, the optimal transmission order is in the decreasing order of the EHC-base station channels gains. The proposed resource allocations significantly reduce the training latency compared to the state-of-the-art benchmark schemes.
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Full-Duplex Covert Communications Assisted by Intelligent Reflective Surfaces
(Institute of Electrical and Electronics Engineers (IEEE), 2022-12)
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Hadzi-Velkov, Zoran
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Zlatanov, Nikola
The letter proposes resource allocation in uplink covert communication systems, which aim to hide the existence of the covert user from the warden. The wireless system consists of a full-duplex receiver, an intelligent reflective surface (IRS), a public user, a covert user, and a watchful warden. The receiver sets the IRS reflection matrix so as to maximize the downlink throughput to the public user. In doing so, in each transmission block we optimize the covert user’s transmit power and rate, subject to the detection error probability at the warden and predefined interference level at the public user. The numerical results unveil the importance of increasing the number of reflecting elements at the IRS.
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Achieving Near Ideal Covertness in NOMA Systems With Channel Inversion Power Control
(Institute of Electrical and Electronics Engineers (IEEE), 2022-11)
Hadzi-Velkov, Zoran
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Zlatanov, Nikola
In this letter, we propose two resource allocation schemes for covert communication in a downlink NOMA system, where the transmitter can send information simultaneously both to the public and the covert receiver, while maintaining ideal (or near-ideal) covertness with respect to the warden. The covert transmission is concealed by combining NOMA and channel inversion power control, acting as a random source of uncertainty at the warden. The transmission scheme supports zero outage transmission to the public user and non-zero outage transmission to the covert user, while guaranteeing an ideal or near-ideal covertness at the warden, depending on whether the covert user employs single-user decoding or successive interference cancellation (SIC) decoding. In the latter case, we determine the optimal power sharing factor between the public and covert signals that maximizes the average covert throughput, given a predefined fixed transmission rate to the public user. If the channel between the transmitter and the covert receiver is moderate or strong, the SIC decoding, compared to the single-user decoding, leads to much higher covert throughput at the cost of minor degradation of covertness.
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Lyapunov Drift-Plus-Penalty Based Resource Allocation in IRS-Assisted Wireless Networks with RF Energy Harvesting
(Brno University of Technology, 2022-09)
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Hadzi-Velkov, Zoran
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Shuminoski, Tomislav
We propose a resource allocation policy for intelligent reflective surface (IRS)-assisted wireless powered communication network (WPCN) where the energy harvesting (EH) users (EHUs) have finite energy storage and data buffers, for storing the harvested energy and the input (sensory) data, respectively. The IRS reflecting coefficients for uplink and downlink are chosen to focus the beam towards a specific EHU, but have additional constant phase offsets (different for uplink and downlink) in order to account for the direct link between the base station and the IRS targeted EHU, and the influence to the EH process of other EHUs in downlink. The EHUs acquire data from their sensors, receive energy in downlink and send information in uplink. We maximize the overall average amount of sensor information in the WPCN by optimizing the IRS reflecting coefficients for the downlink transmissions, the amount of acquired sensor information and the duration of the information transmission period for each EHU in each epoch using the Lyapunov drift-plus-penalty optimization technique. The simulation results demonstrate the effectiveness of the proposed solution.
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Designing Wireless Powered Networks Assisted by Intelligent Reflecting Surfaces With Mechanical Tilt
(Institute of Electrical and Electronics Engineers (IEEE), 2021-10)
Hadzi-Velkov, Zoran
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Zlatanov, Nikola
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Gacanin, Haris
In this letter, we propose a fairness-aware rate maximization scheme for a wireless powered communications network (WPCN) assisted by an intelligent reflecting surface (IRS). The proposed scheme combines user scheduling based on time division multiple access (TDMA) and (mechanical) angular displacement of the IRS. Each energy harvesting user (EHU) has dedicated time slots with optimized durations for energy harvesting and information transmission whereas, the phase matrix of the IRS is adjusted to focus its beam to a particular EHU. The proposed scheme exploits the fundamental dependence of the IRS channel path-loss on the angle between the IRS and the node’s line-of-sight, which is often overlooked in the literature. Additionally, the network design can be optimized for large number of IRS unit cells, which is not the case with the computationally intensive state-of-the-art schemes. In fact, the EHUs can achieve significant rates at practical distances of several tens of meters to the base station (BS) only if the number of IRS unit cells is at least a few thousand.
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Slotted ALOHA wireless networks with RF energy harvesting in Nakagami-m fading
(Elsevier BV, 2020-10)
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Hadzi-Velkov, Zoran
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Design of two level transmit signal waveform for wireless powered communication networks
(ETAI society of R. N. Macedonia, 2018-09)
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Hadzi-Velkov, Zoran
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Poposka, Marija
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Resource Allocation in Wireless Powered Communication Networks With Imperfect CSI
(ETAI society of R. N. Macedonia, 2018-09)
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Hadzi-Velkov, Zoran
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RF Energy Harvesting ALOHA Networks with Local Channel State Information
(2019-06)
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Hadzi-Velkov, Zoran
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A Practical Protocol for Wireless Powered Communications
(IEEE, 2019-10)
Poposka, Marija
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Hadzi-Velkov, Zoran
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