@article{ganesh_khawaja_ozdemir_guvenc_nomoto_ide_2023, title={Propagation Measurements and Coverage Analysis for mmWave and Sub-THz Frequency Bands with Transparent Reflectors}, ISSN={["2577-2465"]}, DOI={10.1109/VTC2023-Spring57618.2023.10200244}, abstractNote={The emerging 5G and future 6G technologies are envisioned to provide higher bandwidths and coverage using millimeter wave (mmWave) and sub-Terahertz (THz) frequency bands. The growing demand for higher data rates using these bands can be addressed by overcoming high path loss, especially for non-line-of-sight (NLOS) scenarios. In this work, we investigate the use of passive transparent reflectors to improve signal coverage in an NLOS indoor scenario. Measurements are conducted to characterize the maximum reflectivity property of the transparent reflector using channel sounder equipment from NI. Flat and curved reflectors, each with a size of 16 inches by 16 inches, are used to study coverage improvements with different reflector shapes and orientations. The measurement results using passive metallic reflectors are also compared with the ray-tracing-based simulations, to further corroborate our inferences. The analysis reveals that the transparent reflector outperforms the metal reflector and increases the radio propagation coverage in all three frequencies of interest: 28 GHz, 39 GHz, and 120 GHz. Using transparent reflectors, there is an increase in peak received power that is greater than 5 dB for certain scenarios compared to metallic reflectors when used in flat mode, and greater than 3 dB when used in curved (convex) mode.}, journal={2023 IEEE 97TH VEHICULAR TECHNOLOGY CONFERENCE, VTC2023-SPRING}, author={Ganesh, Ashwini P. and Khawaja, Wahab and Ozdemir, Ozgur and Guvenc, Ismail and Nomoto, Hiroyuki and Ide, Yasuaki}, year={2023} }
 @article{anjinappa_ganesh_ozdemir_ridenour_khawaja_guvenc_nomoto_ide_2022, title={Indoor Propagation Measurements with Transparent Reflectors at 28/39/120/144 GHz}, ISSN={["2164-7038"]}, DOI={10.1109/ICCWorkshops53468.2022.9814550}, abstractNote={One of the critical challenges of operating with the terahertz or millimeter-wave wireless networks is the necessity of at least a strong non-line-of-sight (NLoS) reflected path to form a stable link. Recent studies have shown that an economical way of enhancing/improving these NLoS links is by using passive metal-lic reflectors that provide strong reflections. However, despite its inherent radio advantage, metals can dramatically influence the landscape's appearance - especially the indoor environment. A conceptual view of escaping this is by using transparent reflectors. In this work, for the very first time, we evaluate the wireless propagation characteristics of passive transparent reflectors in an indoor environment at 28 GHz, 39 GHz, 120 GHz, and 144 GHz bands. In particular, we investigate the penetration loss and the reflection characteristics at different frequencies and compare them against the other common indoor materials such as ceiling tile, clear glass, drywall, plywood, and metal. The measurement results suggest that the transparent reflector, apart from an obvious advantage of transparency, has a higher penetration loss than the common indoor materials (excluding metal) and performs similarly to metal in terms of reflection. Our experimental results directly translate to better reflection performance and preserving the radio waves within the environ-ment than common indoor materials, with potential applications in controlled wireless communication.}, journal={2022 IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS WORKSHOPS (ICC WORKSHOPS)}, author={Anjinappa, Chethan K. and Ganesh, Ashwini P. and Ozdemir, Ozgur and Ridenour, Kris and Khawaja, Wahab and Guvenc, Ismail and Nomoto, Hiroyuki and Ide, Yasuaki}, year={2022}, pages={1118–1123} }
 @article{khawaja_ozdemir_guvenc_2021, title={Channel Prediction for mmWave Ground-to-Air Propagation Under Blockage}, volume={20}, ISSN={["1548-5757"]}, url={https://doi.org/10.1109/LAWP.2021.3078268}, DOI={10.1109/LAWP.2021.3078268}, abstractNote={Ground-to-air (GA) communication using unmanned aerial vehicles (UAVs) has gained popularity in recent years and is expected to be part of 5G networks and beyond. However, the GA links are susceptible to frequent blockages at millimeter-wave (mmWave) frequencies. During a link blockage, the channel information cannot be obtained reliably. In this letter, we provide a novel method of channel prediction during the GA link blockage at 28 GHz. In our approach, the multipath components (MPCs) along a UAV flight trajectory are arranged into independent path bins based on the minimum Euclidean distance among the channel parameters of the MPCs. After the arrangement, the channel parameters of the MPCs in individual path bins are forecasted during the blockage. An autoregressive model is used for forecasting. The results obtained from ray-tracing simulations indicate a close match between the actual and the predicted mmWave channel.}, number={8}, journal={IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Khawaja, Wahab and Ozdemir, Ozgur and Guvenc, Ismail}, year={2021}, month={Aug}, pages={1364–1368} }
 @article{erden_ozdemir_khawaja_guvenc_2020, title={Correction of Channel Sounding Clock Drift and Antenna Rotation Effects for mmWave Angular Profile Measurements}, volume={1}, ISSN={["2637-6431"]}, url={https://doi.org/10.1109/OJAP.2020.2979243}, DOI={10.1109/OJAP.2020.2979243}, abstractNote={Proper characterization of the millimeter-wave (mmWave) propagation channel requires measuring the power angular-delay profile of the channel which includes angle-of-departure and angle-of-arrival of the multipath components (MPCs). In this paper, we first describe in detail our rotating directional antennas-based 28 GHz channel sounder. Then, for this specific sounder class, we describe and address the following two problems in extracting the MPCs from the measurements: 1) For long-distance channel measurements, triggering signal cannot be generated for the TX and the RX using a single clock (SICL). This necessitates the use of separate clocks (SECLs) which introduces a random timing drift between the clocks. 2) As positions of the antennas change during scanning, total distance traveled by the same MPC differs at each measurement. These problems together cause missing some of the MPCs and detecting MPCs that do not exist in reality. We propose an algorithm to correct the clock drift and MPC delay errors due to the rotation of the antennas. We compare the MPCs from the SICL measurement and the corrected SECL measurements using a Hungarian algorithm based MPC matching method. We show that the percentage of the matched MPCs increases from 28.36% to 74.13% after the correction process.}, journal={IEEE OPEN JOURNAL OF ANTENNAS AND PROPAGATION}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Erden, Fatih and Ozdemir, Ozgur and Khawaja, Wahab and Guvenc, Ismail}, year={2020}, pages={71–87} }
 @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{gentile_molisch_chuang_michelson_bodi_bhardwaj_ozdemir_khawaja_guvenc_cheng_et al._2020, title={Methodology for Benchmarking Radio-Frequency Channel Sounders Through a System Model}, volume={19}, ISSN={["1558-2248"]}, url={https://doi.org/10.1109/TWC.2020.3003617}, DOI={10.1109/TWC.2020.3003617}, abstractNote={Development of a comprehensive channel propagation model for high-fidelity design and deployment of wireless communication networks necessitates an exhaustive measurement campaign in a variety of operating environments and with different configuration settings. As the campaign is time-consuming and expensive, the effort is typically shared by multiple organizations, inevitably with their own channel-sounder architectures and processing methods. Without proper benchmarking, it cannot be discerned whether observed differences in the measurements are actually due to the varying environments or to discrepancies between the channel sounders themselves. The simplest approach for benchmarking is to transport participant channel sounders to a common environment, collect data, and compare results. Because this is rarely feasible, this paper proposes an alternative methodology – which is both practical and reliable – based on a mathematical system model to represent the channel sounder. The model parameters correspond to the hardware features specific to each system, characterized through precision, in situ calibration to ensure accurate representation; to ensure fair comparison, the model is applied to a ground-truth channel response that is identical for all systems. Five worldwide organizations participated in the cross-validation of their systems through the proposed methodology. Channel sounder descriptions, calibration procedures, and processing methods are provided for each organization as well as results and comparisons for 20 ground-truth channel responses.}, number={10}, journal={IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Gentile, Camillo and Molisch, Andreas F. and Chuang, Jack and Michelson, David G. and Bodi, Anuraag and Bhardwaj, Anmol and Ozdemir, Ozgur and Khawaja, Wahab Ali Gulzar and Guvenc, Ismail and Cheng, Zihang and et al.}, year={2020}, month={Oct}, pages={6504–6519} }
 @article{khawaja_ozdemir_erden_ozturk_guvenc_2020, title={Multiple ray received power modelling for mmWave indoor and outdoor scenarios}, volume={14}, ISSN={["1751-8733"]}, url={https://doi.org/10.1049/iet-map.2020.0046}, DOI={10.1049/iet-map.2020.0046}, abstractNote={Millimetre-wave (mmWave) frequency bands are expected to be used for future fifth generation networks due to the availability of a large unused spectrum. However, the attenuation at mmWave frequencies is high. To resolve this issue, the utilisation of high gain antennas and beamforming mechanisms are widely investigated in the literature. In this work, the authors considered mmWave end-to-end propagation modelled by individual ray sources and explored the effects of the number of rays in the model and radiation patterns of the deployed antennas on the received power. It is shown that taking the dominant two rays is sufficient to model the channel for outdoor open areas as opposed to the indoor corridor which needs five dominant rays to have a good fit for the measurement and simulation results. It is observed that the radiation pattern of the antenna affects the slope of the path loss. Multi-path components increase the received power, thus, for indoor corridor scenarios, path loss according to the link distance is smaller for lower gain antennas due to increased reception of reflected components. For an outdoor open area, the slope of the path loss is found to be very close to that of the free space.}, number={14}, journal={IET MICROWAVES ANTENNAS & PROPAGATION}, publisher={Institution of Engineering and Technology (IET)}, author={Khawaja, Wahab and Ozdemir, Ozgur and Erden, Fatih and Ozturk, Ender and Guvenc, Ismail}, year={2020}, month={Nov}, pages={1825–1836} }
 @article{khawaja_ozdemir_erden_guvenc_matolak_2020, title={Ultra-Wideband Air-to-Ground Propagation Channel Characterization in an Open Area}, volume={56}, ISSN={["1557-9603"]}, url={https://doi.org/10.1109/TAES.2020.3003104}, DOI={10.1109/TAES.2020.3003104}, abstractNote={This article studies the air-to-ground ultra-wideband channel through propagation measurements between 3.1 to 4.8 GHz using unmanned-aerial-vehicles (UAVs). Different line-of-sight (LOS) and obstructed-LOS scenarios and two antenna orientations were used in the experiments. Multipath channel statistics for different propagation scenarios were obtained, and the Saleh–Valenzuela model was found to provide a good fit for the statistical channel model. An analytical path loss model based on antenna gains in the elevation plane is provided for unobstructed UAV hovering and moving (in a circular path) propagation scenarios.}, number={6}, journal={IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Khawaja, Wahab and Ozdemir, Ozgur and Erden, Fatih and Guvenc, Ismail and Matolak, David W.}, year={2020}, month={Dec}, pages={4533–4555} }
 @article{khawaja_guvenc_chowdhury_2018, title={UWB channel measurements and modelling for hurricanes}, volume={12}, ISSN={["1751-8733"]}, DOI={10.1049/iet-map.2018.0003}, abstractNote={Maintaining communications during major hurricanes is critically important for public safety operations by first responders. This requires accurate knowledge of the propagation channel during hurricane conditions. In this work, the authors have carried out ultra-wideband (UWB) channel measurements during hurricane conditions ranging from Category-1 to Category-4, generated at the wall of wind facility of Florida International University. Time-domain P410 radios are used for channel measurements. From the empirical data analysis in time domain, they developed an UWB statistical broadband channel model for hurricanes. In particular, they characterise the effects of rain and wind speed on large-scale and small-scale UWB propagation parameters considering both line-of-sight (LOS) and non-LOS (NLOS) links. Their experimental results show that number of multipath components reduce considerably with wind driven rain (WDR) and with link obstruction (NLOS scenario). Several remarks are also provided that can help explain the behaviour of UWB propagation characteristics under WDR.}, number={10}, journal={IET MICROWAVES ANTENNAS & PROPAGATION}, author={Khawaja, Wahab and Guvenc, Ismail and Chowdhury, Arindam}, year={2018}, month={Aug}, pages={1691–1699} }
 @inproceedings{khawaja_ozdemir_guvenc_2017, title={UAV air-to-ground channel characterization for mmWave systems}, DOI={10.1109/vtcfall.2017.8288376}, abstractNote={Communication at mmWave bands carries critical importance for 5G wireless networks. In this paper, we study the characterization of mmWave air-to-ground (AG) channels for unmanned aerial vehicle (UAV) communications. In particular, we use ray tracing simulations using Remcom Wireless InSite software to study the behavior of AG mmWave bands at two different frequencies: 28 GHz and 60 GHz. Received signal strength (RSS) and root mean square delay spread (RMS-DS) of multipath components (MPCs) are analyzed for different UAV heights considering four different environments: urban, suburban, rural, and over sea. It is observed that the RSS mostly follows the two ray propagation model along the UAV flight path for higher altitudes. This two ray propagation model is affected by the presence of high rise scatterers in urban scenario. Moreover, we present details of a universal serial radio peripheral (USRP) based channel sounder that can be used for AG channel measurements for mmWave (60 GHz) UAV communications.}, booktitle={2017 ieee 86th vehicular technology conference (vtc-fall)}, author={Khawaja, Wahab Ali Gulzar and Ozdemir, O. and Guvenc, Ismail}, year={2017} }
 @inproceedings{khawaja_koohifar_guvenc_2017, title={UWB radar based beyond wall sensing and tracking for ambient assisted living}, booktitle={2017 14th ieee annual consumer communications & networking conference (ccnc)}, author={Khawaja, W. and Koohifar, F. and Guvenc, I.}, year={2017}, pages={142–147} }
 @inproceedings{khawaja_guvenc_chowdhury_2017, title={Ultra-wideband channel modeling for hurricanes}, DOI={10.1109/vtcfall.2017.8287903}, abstractNote={Maintaining communications during major hurricanes is critically important for public safety operations by first responders. This requires accurate knowledge of the propagation channel during hurricane conditions. In this work, we have carried out ultra-wideband (UWB) channel measurements during hurricane conditions ranging from Category-1 to Category- 4, generated at the Wall of Wind (WoW) facility of Florida International University (FIU). Time Domain P410 radios are used for channel measurements. From the empirical data analysis in time domain, we developed a UWB statistical broadband channel model for hurricanes. In particular, we characterize the effects of rain and wind speed on large scale and small scale UWB propagation parameters.}, booktitle={2017 ieee 86th vehicular technology conference (vtc-fall)}, author={Khawaja, Wahab Ali Gulzar and Guvenc, Ismail and Chowdhury, A.}, year={2017} }