UAVs have been studied and manufactured to help create wireless communications networks that are more flexible and cost-effective than a typical wireless network. These UAV networks could help bridge the digital divide in rural America by providing wireless communications service to areas where cell companies find it too expensive to build conventional cell towers. To test different aspects of a UAV based millimeter-wave frequency network, we created a MATLAB simulation. The simulation visualizes a digital twin of a farm in eastern Nebraska where UAVs are tested. The simulation allows for link budgeting and interference management calculations by accommodating changes in transmitter and receiver location, frequency of the network, power of the transmitted signal, weather conditions, and antenna specifications. The simulation is able to calculate critical network values such as signal-to-interference-plus-noise ratio (SINR), path loss, atmospheric loss, and antenna gains under dynamically changing conditions.

Terahertz (THz) band communications are envisioned as a key technology for 6G and Beyond. As a fundamental wireless infrastructure, THz communication can boost abundant promising applications. In 2014, our team published two comprehensive roadmaps for the development and progress of THz communication networks, which helped the research community to start research on this subject afterwards. The topic of THz communications became very important and appealing to the research community due to 6G wireless systems design and development in recent years. Many papers are getting published covering different aspects of wireless systems using the THz band. With this paper, our aim is looking back to the last decade and revisiting the old problems and pointing out what has been achieved in the research community so far. Furthermore, in this paper, open challenges and new research directions still to be investigated for the THz band communication systems are presented, by covering diverse topics ranging from devices, channel behavior, communication and networking, to physical testbeds and demonstration systems. The key aspects presented in this paper will enable THz communications as a pillar of 6G and Beyond wireless systems in the next decade.

Intelligent surfaces at millimeter wave and terahertz band employ large antenna arrays based on advanced metamaterials to realize controllable wireless propagation. Such intelligent surfaces can achieve polarization tuning of electromagnetic waves, which allows polarization diversity under the ultra-massive MIMO communication, in addition to enhancing the beamforming gain and combating the transmission distance limitation. In this work, we analyze the dual-polarized intelligent surface channel and present an efficient precoding solution based on the array-of-subarray architecture that aims to maximize the spectral efficiency. Simulations are performed in comparison to conventional design based on single-polarized channel and obtained results demonstrate an increase in diversity gain with good spectral efficiency.

Small satellites have shorter development cycles and lower costs than the traditional satellites in low-earth orbits (LEOs) and geostationary orbits (GEOs). These advantages and the ultra-wide spectrum availability in the Terahertz (THz) band enable profound potentials for inter-satellite links (ISLs) to achieve ultra-high throughput in both near-Earth and deep-space orbits. In this paper, the ISL channels at THz band are characterized and their link capacity is analyzed. First, the propagation channel effects in both Earth’s upper atmosphere and deep space are investigated to demonstrate the feasibility of THz band for ISLs. Second, an orbital perturbation model for satellite formation flying is proposed and analyzed with actual ephemeris information. Finally, a channel model is developed to investigate the diversity in both polarization and frequency domain for THz band inter-small-satellite communications. Our numerical results demonstrate that: 1) the ionospheric plasma does not affect the THz band signal propagation in near-Earth orbits; 2) the beam misalignment loss can be characterized by orbital perturbation factors; and 3) the ISLs in THz band can utilize the diversity techniques to achieve high throughput in both near-Earth and deep-space links.

Programmable wireless environments (PWEs) utilize electromagnetic metasurfaces to transform wireless propagation into a software-controlled resource. In this work we study the effects of user device mobility on the efficiency of PWEs. An analytical model is proposed, which describes the potential misalignment between user-emitted waves and the active PWE configuration, and can constitute the basis for studying queuing problems in PWEs. Subsequently, a novel, beam steering approach is proposed which can effectively mitigate the misalignment effects. Ray-tracing-based simulations evaluate the proposed scheme.

Recent techniques utilizing reflectarrays or novel metamaterial-based surfaces to control wireless propagation environments have attracted great attention. While most solutions focus on sub-6 GHz wireless channel to assist in throughput enhancement, these intelligent surfaces have significant potentials in combating the transmission distance limitation and solving the non-line-of-sight transmission problems for the millimeter wave (30–300 GHz) and Terahertz-band (0.3–10 THz) where signal propagation is significantly attenuated in the atmosphere. The Ultra-Massive MIMO (UM MIMO) communication framework has been proposed in such frequency bands, which relies on the plasmonic antenna arrays to realize different operation modes, including anomalous reflection, transmission and reception, among others. In this paper, a joint beamforming scheme is developed based on fractional programming optimization to maximize the spectral efficiency under practical consideration of energy constraints of the UM MIMO communication platform. Numerical results are presented to show the performance comparison with existing solutions.

Intelligent surfaces exert deterministic control over the wireless propagation phenomenon, enabling novel capabilities in performance, security and wireless power transfer. Such surfaces come in the form of rectangular tiles that cascade to cover large surfaces such as walls, ceilings or building facades. Each tile is addressable and can receive software commands from a controller, manipulating an impinging electromagnetic wave upon it by customizing its reflection direction, focus, polarization and phase. A new problem arises concerning the orchestration of a set of tiles towards serving end-to-end communication objectives. Towards that end, we propose a novel intelligent surface networking algorithm based on interpretable neural networks. Tiles are mapped to neural network nodes and any tile line-of-sight connectivity is expressed as a neural network link. Tile wave manipulation functionalities are captured via geometric reflection with virtually rotatable tile surface norm, thus being able to tunable distribute power impinging upon a tile over the corresponding neural network links, with the corresponding power parts acting as the link weights. A feedforward/backpropagate process optimizes these weights to match ideal propagation outcomes (normalized network power outputs) to wireless user emissions (normalized network power inputs). An interpretation process translates these weights to the corresponding tile wave manipulation functionalities.

Millimeter-wave (30-300 GHz) and Terahertz-band communications (0.3-10 THz) are envisioned as key wireless technologies to satisfy the demand for Terabit-per-second (Tbps) links in the 5G and beyond eras. The very large available bandwidth in this ultra-broadband frequency range comes at the cost of a very high propagation loss, which combined with the low power of mm-wave and THz-band transceivers limits the communication distance and data-rates. In this paper, the concept of intelligent communication environments enabled by Ultra-Massive MIMO platforms is proposed to increase the communication distance and data-rates at mm-wave and THz-band frequencies. An end-to-end physical model is developed by taking into account the capabilities of novel intelligent plasmonic antenna arrays which can operate in transmission, reception, reflection and waveguiding, as well as the peculiarities of the mm-wave and THz-band multi-path channel. Based on the developed model, extensive quantitative results for different scenarios are provided to illustrate the performance improvements in terms of both achievable distance and data-rate in Ultra-Massive MIMO environments.

Programmable wireless environments enable the software-defined propagation of waves within them, yielding exceptional performance. Several building-block technologies have been implemented and evaluated at the physical layer in the past. The present work contributes a network-layer solution to configure such environments for multiple users and objectives, and for any underlying physical-layer technology. Supported objectives include any combination of Quality of Service and power transfer optimization, eavesdropping, and Doppler effect mitigation, in multi-cast or uni-cast settings. In addition, a graph-based model of programmable environments is proposed, which incorporates core physical observations and efficiently separates physical and networking concerns. The evaluation takes place in a specially developed simulation tool, and in a variety of environments, validating the model and reaching insights into the user capacity of programmable environments.

Non-orthogonal multiple access (NOMA) scheme has been regarded as one of the promising solutions to realize a flexible and robust multiple access scheme in next generation wireless communication system. However, the inter-carrier interference caused by time-varying fading is not considered in conventional NOMA. In this paper, a novel multi-user access scheme, adaptive subcarrier-spacing multi-carrier non-orthogonal multiple access (ASMC- NOMA), is proposed to provide robust multi-user downlink transmission in time-varying channels and support user velocities up to 500 km/h. Based on the filtered-OFDM waveform, ASMC-NOMA allows flexible power-time- frequency three-dimensional resource allocation and subcarrier spacing adaptation according to the channel state and user velocities. With the user-specific adaption and coexistence of different parameters, ASMC-NOMA is able to repel the impact of high-range Doppler effects and control the signal-to-interference ratio for every downlink user. The pre-grouped resource adaptive allocation scheme is designed with the consideration of reducing the transmission overhead and receiver-end implementation complexity. Transceiver structure and signal model are discussed in detail. Theoretical analysis on the signal properties and numerical system-level simulation results prove that the proposed ASMC-NOMA effectively enhances the transmission robustness, reduces the transmission outage due to the high mobility of users, and provides increased transmission efficiency over conventional OFDM- based NOMA and multi-carrier orthogonal multiple access.

Wireless communication environments are unaware of the ongoing data exchange efforts within them. Moreover, their effect on the communication quality is intractable in all but the simplest cases. The present work proposes a new paradigm, where indoor scattering becomes software-defined and, subsequently, optimizable across wide frequency ranges. Moreover, the controlled scattering can surpass natural behavior, exemplary overriding Snell's law, reflecting waves towards any custom angle (including negative ones). Thus, path loss and multi-path fading effects can be controlled and mitigated. The core technology of this new paradigm are metasurfaces, planar artificial structures whose effect on impinging electromagnetic waves is fully defined by their macro-structure. The present study contributes the software-programmable wireless environment model, consisting of several HyperSurface tiles controlled by a central, environment configuration server. HyperSurfaces are a novel class of metasurfaces whose structure and, hence, electromagnetic behavior can be altered and controlled via a software interface. Multiple networked tiles coat indoor objects, allowing fine-grained, customizable reflection, absorption or polarization overall. A central server calculates and deploys the optimal electromagnetic interaction per tile, to the benefit of communicating devices. Realistic simulations using full 3D ray-tracing demonstrate the groundbreaking potential of the proposed approach in 2.4GHz and 60GHz frequencies.

Terahertz (0.06-10 THz) band serves as a promising technology to accommodate the exponential increase in data rates and number of connected devices. The THz band propagation is greatly affected by channel variations. In this paper, a novel and accurate approach is developed for time-varying channel modeling and tracking for the THz indoor communications. In particular, a three-dimensional time-varying THz channel model that captures the peculiarities in propagation parameters is presented. Moreover, based on the THz channel characteristics, an extended Kalman filtering approach is proposed to accurately track the line-of-sight component along time variation. Additionally, the proposed channel modeling and tracking approach under a realistic access-point-to-user-equipment link scenario in an office environment is evaluated in detail. Based on the simulation results validated from ray-tracing techniques, the proposed time-varying channel model yields satisfying system capacity.

This paper presents a time-varying channel model in three- dimensional environments for the 5G wireless system. Using the continuous-time Markov chain, the proposed model captures the statistical movement of the transceiver, the transition between line-of-sight and non-line-of-sight propagation scenarios, and the dynamic variation of propagation parameters. Moreover, a novel frequency- dependent factor that describes the dynamic path behavior is described in this model. According to the simulation results based on ray-tracing techniques and field measurements from the literature, the proposed time- varying channel model is validated at 5.2, 10, and 60 GHz. Furthermore, under realistic 5G device-to-device communication scenario, the simulation results and time- varying channel characteristics are highlighted, including the number of paths, path gain, delay spread, delay variation, Rician K-factor, and coherence bandwidth.

This paper presents millimeter wave propagation measurements in New York City and an analysis of signal outage at 28 and 73 GHz using similar spread spectrum sliding correlator channel sounders that employed high gain, directional steerable antennas (24.5 dBi gain antennas at 28 GHz and 27 dBi gain antennas at 73 GHz) at both the transmitter and receiver. Three identical transmitter locations were used for both the 28 and 73 GHz campaigns, while the 73 GHz campaign included two new TX locations. The 28 GHz campaign tested 25 receiver locations for each of the three transmitter locations, and the 73 GHz campaign tested 27 receiver locations in various combinations with the five transmitter sites. Overall, 75 TX-RX location combinations were tested at 28 GHz and 74 TX-RX combinations were tested at 73 GHz, with T-R (transmitter-receiver) separation distances up to 425 m. The maximum transmit power was 30 dBm at 28 GHz and 14.6 dBm at 73 GHz. Our analysis shows that the estimated outage probabilities at 28 and 73 GHz for the cellular communication scenario are 14% and 17%, respectively, and is 16% for the 73 GHz backhaul scenario.

The performance of multi-beam antenna equal gain combining for improving signal quality in future millimeter-wave cellular systems is evaluated in this article. Employing experimental data obtained from 28 GHz and 73 GHz propagation measurements in the dense urban environment of New York City, we present the impact of coherent bi-beam, tri-beam and quad-beam combining on path loss and shadow factors. The results reveal that a maximum of 24.9 dB improvement in path loss at 28 GHz and 34.8 dB at 73 GHz for 100 m T-R (transmitter-receiver) separation distances can be achieved via combining the strongest four received signals from distinct beams, when compared to the case of signals at the receiver with randomly pointed beams. Comparable path loss values are achieved at both 28 and 73 GHz bands. This paper demonstrates the potential of utilizing spatial filtering and beam combining to significantly improve received signal levels and link margins at millimeter-wave frequencies.

Wireless data traffic is projected to skyrocket 10 000 fold within the next 20 years. To tackle this incredible increase in wireless data traffic, a first approach is to further improve spectrally efficient systems such as 4G LTE in bands below 6 GHz by using more advanced spectral efficiency techniques. However, the required substantial increase in system complexity along with fundamental limits on hardware implementation and channel conditions may limit the viability of this approach. Furthermore, the end result would be an extremely spectrally efficient system with little room for future improvement to meet the ever-growing wireless data usage. The second approach is to move up in frequency, into an unused nontraditional spectrum where enormous bandwidths are available, such as at millimeter wave (mmWave). The mmWave option enables the use of simple air interfaces since large bandwidths can be exploited (e.g., 2 GHz) to achieve high data rates rather than relying on highly complex techniques originally aimed at achieving a high spectral efficiency with smaller bandwidths. In addition, mmWave systems will easily evolve to even higher system capacities, because there will be plenty of margin to improve the spectral efficiency as data demands further increase. In this paper, a case is made for using mmWave for a fifth generation (5G) wireless system for ultradense networks by presenting an overview of enhanced local area (eLA) technology at mmWave with emphasis on 5G requirements, spectrum considerations, propagation and channel modeling, air-interface and multiantenna design, and network architecture solutions.

Measurements for future outdoor cellular systems at 28 GHz and 38 GHz were conducted in urban microcellular environments in New York City and Austin, Texas, respectively. Measurements in both line-of-sight and non-line-of-sight scenarios used multiple combinations of steerable transmit and receive antennas (e.g. 24.5 dBi horn antennas with 10.9° half power beamwidths at 28 GHz, 25 dBi horn antennas with 7.8° half power beamwidths at 38 GHz, and 13.3 dBi horn antennas with 24.7° half power beamwidths at 38 GHz) at different transmit antenna heights. Based on the measured data, we present path loss models suitable for the development of fifth generation (5G) standards that show the distance dependency of received power. In this paper, path loss is expressed in easy-to-use formulas as the sum of a distant dependent path loss factor, a floating intercept, and a shadowing factor that minimizes the mean square error fit to the empirical data. The new models are compared with previous models that were limited to using a close-in free space reference distance. Here, we illustrate the differences of the two modeling approaches, and show that a floating intercept model reduces the shadow factors by several dB and offers smaller path loss exponents while simultaneously providing a better fit to the empirical data. The upshot of these new path loss models is that coverage is actually better than first suggested by work in [1], [7] and [8].

As the mobile cellular carriers are currently facing a spectrum crunch, researchers are concentrating on higher carrier frequency bands, such as E-band (71-76 GHz and 81-86 GHz) for the next generation wireless communication systems. The E-band is promising due to its large available, continuous bandwidth and robust weather resilience. In this paper, we demonstrate a spread spectrum sliding correlator channel sounder operating at a center frequency of 73.5 GHz with an 800 MHz null-to-null bandwidth. The channel sounder provides a multipath time resolution of 2.33 ns. 72 GHz millimeter wave propagation and penetration characteristics in an indoor office environment are investigated using the sliding correlator channel sounding system. Data collected and processed from the measurements shows that strong received power can be achieved from the multipath-rich indoor environment, in the presence of multiple obstructions. The data obtained from this measurement campaign may be utilized for the design of future fifth generation millimeter wave indoor cellular systems.