@article{rahmati_hosseinalipour_yapici_he_guvenc_dai_bhuyan_2022, title={Dynamic Interference Management for UAV-Assisted Wireless Networks}, volume={21}, ISSN={["1558-2248"]}, url={https://doi.org/10.1109/TWC.2021.3114234}, DOI={10.1109/TWC.2021.3114234}, abstractNote={We investigate a transmission mechanism aiming to improve the data rate between a base station (BS) and a user equipment (UE) through deploying multiple relaying UAVs. We consider the effect of interference incurred by another established communication network, which makes our problem challenging and different from the state of the art. We aim to design the 3D trajectories and power allocation for the UAVs to maximize the data flow of the network while keeping the interference on the existing communication network below a threshold. We utilize the mobility feature of the UAVs to evade the (un)-intended interference caused by (un)-intentional interferers. To this end, we propose an alternating-maximization approach to jointly obtain the 3D trajectories and the UAVs transmission powers. We handle the 3D trajectory design by resorting to spectral graph theory and subsequently address the power allocation through convex optimization techniques. We also approach the problem from the intentional interferer’s perspective where smart jammers chase the UAVs to effectively degrade the data flow of the network. We also extend our work to the case for multiple UEs. Finally, we demonstrate the efficacy of our proposed method through extensive simulations.}, number={4}, journal={IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Rahmati, Ali and Hosseinalipour, Seyyedali and Yapici, Yavuz and He, Xiaofan and Guvenc, Ismail and Dai, Huaiyu and Bhuyan, Arupjyoti}, year={2022}, month={Apr}, pages={2637–2653} }
 @article{maeng_yapici_guvenc_bhuyan_dai_2022, title={Precoder Design for Physical-Layer Security and Authentication in Massive MIMO UAV Communications}, volume={71}, ISSN={["1939-9359"]}, url={https://doi.org/10.1109/TVT.2022.3141055}, DOI={10.1109/TVT.2022.3141055}, abstractNote={Supporting reliable and seamless wireless connectivity for unmanned aerial vehicles (UAVs) has recently become a critical requirement to enable various different use cases of UAVs. Due to their widespread deployment footprint, cellular networks can support beyond visual line of sight (BVLOS) communications for UAVs. In this paper, we consider cellular connected UAVs (C-UAVs) that are served by massive multiple-input-multiple-output (MIMO) links to extend coverage range, while also improving physical layer security and authentication. We consider Rician channel and propose a novel linear precoder design for transmitting data and artificial noise (AN). We derive the closed-form expression of the ergodic secrecy rate of C-UAVs for both conventional and proposed precoder designs. In addition, we obtain the optimal power splitting factor that divides the power between data and AN by asymptotic analysis. Then, we apply the proposed precoder design in the fingerprint embedding authentication framework, where the goal is to minimize the probability of detection of the authentication tag at an eavesdropper. In simulation results, we show the superiority of the proposed precoder in both secrecy rate and the authentication probability considering moderate and large number of antenna massive MIMO scenarios.}, number={3}, journal={IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Maeng, Sung Joon and Yapici, Yavuz and Guvenc, Ismail and Bhuyan, Arupjyoti and Dai, Huaiyu}, year={2022}, month={Mar}, pages={2949–2964} }
 @article{yapici_rupasinghe_guvenc_dai_bhuyan_2021, title={Physical Layer Security for NOMA Transmission in mmWave Drone Networks}, volume={70}, ISSN={["1939-9359"]}, url={https://doi.org/10.1109/TVT.2021.3066350}, DOI={10.1109/TVT.2021.3066350}, abstractNote={The non-orthogonal multiple access (NOMA) and millimeter-wave (mmWave) transmission enable the unmanned aerial vehicle (UAV) assisted wireless networks to provide broadband connectivity over densely packed urban areas. The presence of malicious receivers, however, compromise the security of the UAV-to-ground communications links, thereby degrading secrecy rates. In this work, we consider a NOMA-based transmission strategy in a mmWave UAV-assisted wireless network, and investigate the respective secrecy-rate performance rigorously. In particular, we propose a protected-zone approach to enhance the secrecy-rate performance by preventing the most vulnerable subregion (outside the user region) from the presence of malicious receivers. The respective secrecy rates are then derived analytically as a function of the particular protected zone, which verifies great secrecy rate improvements through optimizing shape of the protected zone in use. Furthermore, we show that the optimal protected zone shape for mmWave links appears as a compromise between protecting the angle versus distance dimension, which would otherwise form to protect solely the distance dimension for sub-6 GHz links. We also numerically evaluate the impact of transmission power, protected-zone size, and UAV altitude on the secrecy-rate performance improvement for the sake of practical deployments.}, number={4}, journal={IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Yapici, Yavuz and Rupasinghe, Nadisanka and Guvenc, Ismail and Dai, Huaiyu and Bhuyan, Arupjyoti}, year={2021}, month={Apr}, pages={3568–3582} }
 @article{maeng_yapici_guvenc_dai_bhuyan_2021, title={Power Allocation for Fingerprint-Based PHY-Layer Authentication with mmWave UAV Networks}, ISSN={["1550-3607"]}, DOI={10.1109/ICC42927.2021.9500273}, abstractNote={Physical layer security (PLS) techniques can help to protect wireless networks from eavesdropper attacks. In this paper, we consider the authentication technique that uses fingerprint embedding to defend 5G cellular networks with unmanned aerial vehicle (UAV) systems from eavesdroppers and intruders. Since the millimeter wave (mmWave) cellular networks use narrow and directional beams, PLS can take further advantage of the 3D spatial dimension for improving the authentication of UAV users. Considering a multi-user mmWave cellular network, we propose a power allocation technique that jointly takes into account splitting of the transmit power between the precoder and the authentication tag, which manages both the secrecy as well as the achievable rate. Our results show that we can obtain optimal achievable rate with expected secrecy.}, journal={IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS (ICC 2021)}, author={Maeng, Sung Joon and Yapici, Yavuz and Guvenc, Ismail and Dai, Huaiyu and Bhuyan, Arupjyoti}, year={2021} }
 @article{khawaja_ozdemir_yapici_erden_guvenc_2020, title={Coverage Enhancement for NLOS mmWave Links Using Passive Reflectors}, volume={1}, ISSN={["2644-125X"]}, url={https://doi.org/10.1109/OJCOMS.2020.2969751}, DOI={10.1109/OJCOMS.2020.2969751}, abstractNote={The future 5G networks are expected to use millimeter wave (mmWave) frequency bands to take advantage of the large unused spectrum. However, due to the high path loss at mmWave frequencies, coverage of mmWave signals can get severely reduced, especially for non-line-of-sight (NLOS) scenarios as mmWave signals are severely attenuated when going through obstructions. In this work, we study the use of passive metallic reflectors of different shapes/sizes to improve 28 GHz mmWave signal coverage for both indoor and outdoor NLOS scenarios. We quantify the gains that can be achieved in the link quality with metallic reflectors using measurements, analytical expressions, and ray tracing simulations. In particular, we provide an analytical model for the end-to-end received power in an NLOS scenario using reflectors of different shapes and sizes. For a given size of the flat metallic sheet reflector approaching to the size of the incident beam, we show that the reflected received power for the NLOS link is the same as line-of-sight (LOS) free space received power of the same link distance. Extensive results are provided to study the impact of environmental features and reflector characteristics on NLOS link quality.}, journal={IEEE OPEN JOURNAL OF THE COMMUNICATIONS SOCIETY}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Khawaja, Wahab and Ozdemir, Ozgur and Yapici, Yavuz and Erden, Fatih and Guvenc, Ismail}, year={2020}, pages={263–281} }
 @article{yapici_guvenc_2020, title={Energy- vs Spectral-Efficiency for Energy-Harvesting Hybrid RF/VLC Networks}, ISSN={["1058-6393"]}, DOI={10.1109/IEEECONF51394.2020.9443343}, abstractNote={We consider a hybrid radio frequency (RF) / visible light communications (VLC) scenario where the RF link is empowered by the energy harvested during the VLC transmission. We take into account the power consumption of the access point (AP), which is composed of light emitting diodes (LEDs), while it is communicating with a far user over a two-hop hybrid RF/VLC link. In particular, we consider the energy efficiency of the end-to-end network in enhancing the energy-harvesting performance by choosing the optimal direct current (DC) bias, which is overlooked in the existing literature. To this end, we model the energy consumption of the commercially available LEDs, and formulate a joint energy- and spectral-efficiency optimization problem. The extreme cases of optimizing energy and spectral efficiency individually underscores the importance of optimizing the DC bias for a given joint (multi-objective) optimization problem.}, journal={2020 54TH ASILOMAR CONFERENCE ON SIGNALS, SYSTEMS, AND COMPUTERS}, author={Yapici, Yavuz and Guvenc, Ismail}, year={2020}, pages={1152–1156} }
 @article{rahmati_hosseinalipour_yapici_guvenc_dai_bhuyan_2020, title={Energy-Efficient Beamforming and Power Control for Uplink NOMA in mmWave UAV Networks}, ISSN={["2576-6813"]}, DOI={10.1109/GLOBECOM42002.2020.9348114}, abstractNote={The integration of unmanned aerial vehicles (UAVs) into the terrestrial communications networks with a variety of tasks is viewed as a key technology for 5G and beyond. In this work, we consider the uplink millimeter-wave (mmWave) transmission between a set of UAVs and a base station (BS), where the UAVs deploy uplink non-orthogonal multiple access (NOMA) in multiple clusters. Furthermore, the BS also serves its own desired ground user equipment (UE) in the presence of many other ground UEs associated with other cells, which share the same frequency band. Considering the limited energy budget of UAVs, we formulate an energy efficiency (EE) problem, and propose a solution aided by the Dinkelbach's algorithm and successive convex approximation (SCA). Using realistic air-to-ground (A2G) and terrestrial channel models, we assess the performance of the proposed algorithm under various circumstances (maximum transmit power for UAVs, quality-of-service (QoS) constraint for the desired UE, etc.), and identify the best use cases.}, journal={2020 IEEE GLOBAL COMMUNICATIONS CONFERENCE (GLOBECOM)}, author={Rahmati, Ali and Hosseinalipour, Seyyedali and Yapici, Yavuz and Guvenc, Ismail and Dai, Huaiyu and Bhuyan, Arupjyoti}, year={2020} }
 @article{yalcin_yapici_2020, title={Multiuser Precoding for Sum-Rate Maximization in Relay-Aided mmWave Communications}, volume={69}, ISSN={["1939-9359"]}, DOI={10.1109/TVT.2020.2987473}, abstractNote={Relay-aided transmission is envisioned as a key strategy to combat severe path loss and link blockages emerging as unique challenges in millimeter-wave (mmWave) communications. This work considers a relay-aided multiuser mmWave communications scenario aiming at maximizing the sum rate through optimal transmit and relay precoder design. We propose a novel joint precoder design strategy, which exploits weighted minimum mean-square error (WMMSE) using its equivalency to sum-rate maximization. We obtain closed form expressions of transmit and relay precoders, and propose to compute them through alternating-optimization iterations without having to resort to complicated numerical optimization techniques. Numerical results verify the superiority of the proposed precoding strategy as compared to conventional precoding schemes.}, number={6}, journal={IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY}, author={Yalcin, Ahmet Zahid and Yapici, Yavuz}, year={2020}, month={Jun}, pages={6808–6812} }
 @article{eroglu_yapici_guvenc_2019, title={Impact of Random Receiver Orientation on Visible Light Communications Channel}, volume={67}, ISSN={["1558-0857"]}, url={https://doi.org/10.1109/TCOMM.2018.2879093}, DOI={10.1109/TCOMM.2018.2879093}, abstractNote={Visible light communications (VLC) has been studied thoroughly in recent years as an alternative or complementary technology to radio frequency communications. The reliability of VLC channels highly depends on the availability and alignment of line of sight links. In this paper, we study the effect of random receiver orientation for mobile users over VLC downlink channels, which affects the existence and quality of line of sight links. Based on the statistics of the receiver location and relative orientation with respect to the transmitter LED, we develop an analytical framework to characterize the statistical distribution of VLC downlink channels, which is then utilized to obtain the outage probability and the bit error rate. Our analysis is generalized for arbitrary distributions of relative orientation and location for a single transmitter, and extended to multiple transmitter case for some certain scenarios. Extensive Monte Carlo simulations show a perfect match between the analytical and the simulation data in terms of both the statistical channel distribution and the resulting bit error rate. Our results also characterize the channel attenuation due to random receiver orientation and location for various scenarios of interest.}, number={2}, journal={IEEE TRANSACTIONS ON COMMUNICATIONS}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Eroglu, Yusuf Said and Yapici, Yavuz and Guvenc, Ismail}, year={2019}, month={Feb}, pages={1313–1325} }
 @article{rupasinghe_yapici_guvenc_kakishima_2019, title={Non-Orthogonal Multiple Access for mmWave Drone Networks With Limited Feedback}, volume={67}, ISSN={["1558-0857"]}, url={https://doi.org/10.1109/TCOMM.2018.2867465}, DOI={10.1109/TCOMM.2018.2867465}, abstractNote={Unmanned aerial vehicle (UAV) base stations (BSs) can be a promising solution to provide connectivity and quality-of-service guarantees during temporary events and after disasters. In this paper, we consider a scenario where UAV-BSs are serving a large number of mobile users in a hot spot area (e.g., in a stadium). We introduce non-orthogonal multiple-access (NOMA) transmission at UAV-BSs to serve more users simultaneously considering user distances as the available feedback for a user ordering during NOMA formulation. With millimeter-wave transmission and multi-antenna techniques, we assume that UAV-BS generates directional beams and multiple users are served simultaneously within the same beam. However, due to the limitations of physical vertical beamwidth of the UAV-BS beam, it may not be possible to cover the entire user region at UAV altitudes of practical relevance. During such situations, a beam scanning approach is proposed to maximize the achievable sum rates. We develop a comprehensive framework over which outage probabilities and respective sum rates are derived rigorously and we investigate the optimal operational altitude of UAV-BS to maximize the sum rates using our analytical framework. Our analysis shows that NOMA with distance feedback can provide better outage sum rates than orthogonal multiple access.}, number={1}, journal={IEEE TRANSACTIONS ON COMMUNICATIONS}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Rupasinghe, Nadisanka and Yapici, Yavuz and Guvenc, Ismail and Kakishima, Yuichi}, year={2019}, month={Jan}, pages={762–777} }
 @article{rupasinghe_yapici_iscar_guvenc_2018, title={Impact of Angular Spread in Moderately Large MIMO Systems Under Pilot Contamination}, volume={67}, ISSN={["1939-9359"]}, url={https://doi.org/10.1109/TVT.2018.2866799}, DOI={10.1109/TVT.2018.2866799}, abstractNote={Pilot contamination is known to be one of the main bottlenecks for massive multi-input multioutput (MIMO) networks. For moderately large antenna arrays (of importance to recent/emerging deployments) and correlated MIMO, pilot contamination may not be the dominant limiting factor in certain scenarios. To the best of our knowledge, a rigorous characterization of the achievable rates and their explicit dependence on the angular spread (AS) is not available in the existing literature for moderately large antenna array regime. In this paper, considering eigen-beamforming (EBF) precoding, we derive an exact analytical expression for achievable rates in multicell MIMO systems under pilot contamination, and characterize the relation between the AS, array size, and respective user rates. Our analytical and simulation results reveal that the achievable rates for both the EBF and the regularized zero-forcing precoders follow a nonmonotonic behavior for increasing AS when the antenna array size is moderate. We argue that knowledge of this nonmonotonic behavior can be exploited to develop effective user-cell pairing techniques.}, number={11}, journal={IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Rupasinghe, Nadisanka and Yapici, Yavuz and Iscar, Jorge and Guvenc, Ismail}, year={2018}, month={Nov}, pages={10631–10644} }
 @article{eroglu_guvenc_sahin_yapici_pala_yuksel_2018, title={Multi-Element VLC Networks: LED Assignment, Power Control, and Optimum Combining}, volume={36}, ISSN={["1558-0008"]}, DOI={10.1109/jsac.2017.2774378}, abstractNote={Visible light communications (VLCs) are a promising technology to address the spectrum crunch problem in radio frequency networks. A major advantage of VLC networks is that they can use the existing lighting infrastructure in indoor environments, which may have large number of LEDs for illumination. While LEDs used for lighting typically have limited bandwidth, presence of many LEDs can be exploited for indoor VLC networks, to serve each user by multiple LEDs for improving link quality and throughput. In this paper, LEDs are grouped and assigned to the users based on received signal strength from each LED, for which different solutions are proposed to achieve maximum throughput, proportional fairness, and quality of service. Additionally, power optimization of LEDs for a given assignment is investigated, and the Jacobian and Hessian matrices of the corresponding optimization problem are derived. Moreover, for multi-element receivers with LED grouping at the transmitter, an improved optimal combining method is proposed. This method suppresses interference caused by simultaneous data transfer of LEDs and improves the overall signal-to-interference-plus-noise-ratio by 2–5 dB. Lastly, an efficient calculation of channel response is presented to simulate multipath VLC channel with low computational complexity.}, number={1}, journal={IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS}, author={Eroglu, Yusuf Said and Guvenc, Ismail and Sahin, Alphan and Yapici, Yavuz and Pala, Nezih and Yuksel, Murat}, year={2018}, month={Jan}, pages={121–135} }