RSS 1.0Communication Theory Lab
For more information visit: https://cemse.kaust.edu.sa/ctl
Recent Submissions
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Parallel FSO-RF Transmissions for High-Throughput Remote Access with Satellite Communications(IEEE Transactions on Aerospace and Electronic Systems, Institute of Electrical and Electronics Engineers (IEEE), 2023-09-26) [Article]Mixed usage of radio frequency (RF) and free space optical (FSO) transmissions can improve the reliability and throughput of future satellite communication (Satcom) systems. To overcome the rate limitation of conventional hybrid implementations, we propose parallel FSO and RF transmissions to explore their complementary properties in beamwidth and bandwidth. In particular, RF transmissions serve the users over a large geographical area, while the FSO link is employed to increase the throughput to a particular hot-spot area with higher capacity demand through an access point. Independent data streams are adaptively sent over both links to satisfy capacity and availability requirements. To highlight the significant potential of parallel FSO-RF transmissions, we analytically derive the sum capacity outage probability. The obtained expression is validated by Monte Carlo simulations. Furthermore, we derive an asymptotic expression to obtain diversity gain and to provide further insights into the behavior of the proposed system. The presented results demonstrate the potential of parallel FSO and RF Satcom over existing solutions.
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HAP-enabled Communications in Rural Areas: When Diverse Services Meet Inadequate Communication Infrastructures(IEEE Open Journal of the Communications Society, Institute of Electrical and Electronics Engineers (IEEE), 2023-09-25) [Article]The high altitude platform (HAP) network has been regarded as a cost-efficient solution for providing network access to rural or remote areas. Apart from network connectivity, rural areas are predicted to have demands for diverse real-time intelligent communication services, such as smart agriculture and digital forestry. The effectiveness of real-time decision-making applications depends on the timely updating of sensing data measurements used in generating decisions. As a performance metric capable of quantifying the freshness of transmitted information, the age of information (AoI) can evaluate the freshness-aware performance of the process of updating sensory data. However, unlike urban areas, the available communication resources in rural areas may not allow for maintaining dedicated infrastructures for different types of services, e.g., conventional non-freshness-aware services and freshness-aware real-time services, thereby requiring the proper resource allocation among different services. In this article, we first introduce the anticipated services and discuss the advances of rural networks. Next, a case study on the efficient resource allocation across heterogeneous services characterized by AoI and data rate in HAP networks is presented. We also explore the potential of employing the free-space optical (FSO) backhaul framework to enhance the performance of multi-layer HAP networks. To strike a balance between the AoI and data rate, we develop both static and deep reinforcement learning (DRL)-based dynamic resource allocation schemes to allocate the communication resources provided by HAP networks. The simulation results show that the proposed dynamic DRL-based method outperforms the heuristic algorithm and can surpass the performance ceiling achieved by the proposed static allocation scheme. In particular, our presented method can improve performance by nearly 2.5 times more than the ant colony optimization (ACO) method in terms of weighted sum performance improvements. Some insights on system design and promising future research directions are also given.
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Comparing Aerial-RIS- and Aerial-Base-Station-Aided Post-Disaster Cellular Networks(IEEE Open Journal of Vehicular Technology, Institute of Electrical and Electronics Engineers (IEEE), 2023-09-22) [Article]Reconfigurable intelligent surface (RIS) technology and its integration into existing wireless networks have recently attracted much interest. While an important use case of said technology consists in mounting RISs onto unmanned aerial vehicles (UAVs) to support the terrestrial infrastructure in post-disaster scenarios, the current literature lacks an analytical framework that captures the networks' topological aspects. Therefore, our study borrows stochastic geometry tools to estimate both the average and local coverage probability of a wireless network aided by an aerial RIS (ARIS); in particular, the surviving terrestrial base stations (TBSs) are modeled by means of an inhomogeneous Poisson point process, while the UAV is assumed to hover above the disaster epicenter. Our framework captures important aspects such as the TBSs' altitude, the fact that they may be in either line-of-sight or non-line-of-sight condition with a given node, and the Nakagami- m fading conditions of wireless links. By leveraging said aspects we accurately evaluate three possible scenarios, where TBSs are either: (i) not aided, (ii) aided by an ARIS, or (iii) aided by an aerial base station (ABS). Our selected numerical results reflect various situations, depending on parameters such as the environment's urbanization level, disaster radius, and the UAV's altitude.
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Pointing-and-Acquisition for Optical Wireless in 6G: From Algorithms to Performance Evaluation(arXiv, 2023-09-20) [Preprint]The increasing demand for wireless communication services has led to the development of non-terrestrial networks, which enables various air and space applications. Free-space optical (FSO) communication is considered one of the essential technologies capable of connecting terrestrial and non-terrestrial layers. In this article, we analyze considerations and challenges for FSO communications between gateways and aircraft from a pointing-and-acquisition perspective. Based on the analysis, we first develop a baseline method that utilizes conventional devices and mechanisms. Furthermore, we propose an algorithm that combines angle of arrival (AoA) estimation through supplementary radio frequency (RF) links and beam tracking using retroreflectors. Through extensive simulations, we demonstrate that the proposed method offers superior performance in terms of link acquisition and maintenance.
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Precoding for High Throughput Satellite Communication Systems: A Survey(IEEE Communications Surveys & Tutorials, Institute of Electrical and Electronics Engineers (IEEE), 2023-09-20) [Article]With the expanding demand for high data rates and extensive coverage, high throughput satellite (HTS) communication systems are emerging as a key technology for future communication generations. However, current frequency bands are increasingly congested. Until the maturity of communication systems to operate on higher bands, the solution is to exploit the already existing frequency bands more efficiently. In this context, precoding emerges as one of the prolific approaches to increasing spectral efficiency. This survey presents an overview and a classification of the recent precoding techniques for HTS communication systems from two main perspectives: 1) a problem formulation perspective and 2) a system design perspective. From a problem formulation point of view, precoding techniques are classified according to the precoding optimization problem, group, and level. From a system design standpoint, precoding is categorized based on the system architecture, the precoding implementation, and the type of the provided service. Further, practical system impairments are discussed, and robust precoding techniques are presented. Finally, future trends in precoding for satellites are addressed to spur further research.
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Optimal Photodetector Size for High-Speed Free-Space Optics Receivers(arXiv, 2023-09-16) [Preprint]The selection of an optimal photodetector area is closely linked to the attainment of higher data rates in optical wireless communication receivers. If the photodetector area is too large, the channel capacity degrades due to lower modulation bandwidth of the detector. A smaller photodetector maximizes the bandwidth, but minimizes the captured signal power and the subsequent signal-to-noise ratio. Therein lies an opportunity in this trade-off to maximize the channel rate by choosing the optimal photodetector area. In this study, we have optimized the photodetector area in order to maximize the channel capacity of a free-space optical link for a diverse set of communication scenarios. We believe that the study in this paper in general -- and the closed-form solutions derived in this study in particular -- will be helpful to maximize achievable data rates of a wide gamut of optical wireless communication systems: from long range deep space optical links to short range indoor visible light communication systems.
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Mobile Sub-diffusion Molecular Communication Channel(ACM, 2023-09-12) [Conference Paper]This work considers the sub-diffusion motion of information-carrying molecules (IM) inside a molecular communication (MC) channel. We also explore the impacts of transmitter (Tx) and receiver (Rx) mobility. To do so, we obtain the closed-form expressions of the absorption probability (AP) and the first-passage-time density (FPTD).
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A Dominant Interferer-based Approximation for Uplink SINR Meta Distribution in Cellular Networks(arXiv, 2023-09-10) [Preprint]This work studies the signal-to-interference-plus-noise-ratio (SINR) meta distribution for the uplink transmission of a Poisson network with Rayleigh fading by using the dominant interferer-based approximation. The proposed approach relies on computing the mix of exact and mean-field analysis of interference. In particular, it requires the distance distribution of the nearest interferer and the conditional average of the rest of the interference. Using the widely studied fractional path-loss inversion power control and modeling the spatial locations of base stations (BSs) by a Poisson point process (PPP), we obtain the meta distribution based on the proposed method and compare it with the traditional beta approximation, as well as the exact results obtained via Monte-Carlo simulations. Our numerical results validate that the proposed method shows good matching and is time competitive.
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Electromagnetic Exposure Aware RSMA for Multiuser Massive MIMO Uplink(IEEE, 2023-08-25) [Conference Paper]This paper investigates the rate-splitting multiple access (RSMA) transmission design for multiuser multiple-input multiple-output (MIMO) uplink with EM exposure constraints. Specifically, the transmit covariance matrices and decoding order are optimized at the users and BS, respectively, via utilizing statistical channel state information (CSI) to maximize the energy efficiency (EE). The problem is formulated as non-convex mixed integer program, which is divided into the equivalent two subproblems. We first handle the inner problem by adopting the minorization-maximization (MM) and Dinkelbachs methods. Then, a modified water-filling scheme is proposed to obtain the transmit covariance matrices with fixed decoding permutation. For the outer problem, a greedy approach is proposed to obtain the decoding permutation. Numerical results verify the effectiveness of the proposed EM exposure aware EE maximization scheme for uplink RSMA.
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Aerial Base Stations for Global Connectivity: Is It a Feasible and Reliable Solution?(IEEE Vehicular Technology Magazine, Institute of Electrical and Electronics Engineers (IEEE), 2023-08-25) [Article]Even though achieving global connectivity represents one of the main goals of 5G and beyond wireless networks, exurban areas are still suffering frequent outages because of the lack of proper telecom infrastructures, which are often available only in urban areas. Indeed, cellular network design is usually capacity driven, and thus the densities of base stations (BSs) follow mostly population and especially revenue densities. Contextually, we focus on one of the most promising solutions to provide sufficient and reliable coverage in far-flung areas: aerial base stations (ABSs), which consist of unmanned aerial vehicles (UAVs) carrying cellular BS equipment. In this article, we extensively discuss the problem of bridging what is called the urban–rural digital divide (i.e., the connectivity gap between urban and rural areas) from various perspectives. First, we showcase various alternative solutions and compare conventional terrestrial networks with aerial networks from a techno-economic point of view. Then, we highlight the topological aspects of rural environments and explain how they can affect the actual design of cellular networks. In addition, we investigate both the coverage probability and the reliability of the communication links via simulations, proving that the integration of ABSs can be quite promising in a 6G perspective. Finally, we propose two original extensions of our case study as open problems.
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eVTOL Communications and Networking in UAM: Requirements, Key Enablers, and Challenges(IEEE Communications Magazine, Institute of Electrical and Electronics Engineers (IEEE), 2023-08-24) [Article]Electric vertical takeoff and landing (eVTOL) aircraft have attracted great attention during the last years as the long-awaited enabler of urban air mobility (UAM), allowing a more sustainable method of transportation and accelerating the development of smart cities. The operation of eVTOLs over urban areas introduces safety hazards for passengers, pedestrians, and buildings, which prioritises safety considerations. Ensuring the safe operation of eVTOLs requires studying their communication, networking, and computing requirements. In this article, we showcase eVTOL requirements in UAM in terms of coverage, data rate, latency, spectrum efficiency, networking, and computing. Then, we identify potential key technological enablers to address these requirements and their challenges. Finally, we conduct a comparative case study between the terrestrial and aerial communication infrastructures to serve eVTOLs in UAM.
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Coverage Analysis and Trajectory Optimization for Aerial Users with Dedicated Cellular Infrastructure(IEEE Transactions on Wireless Communications, Institute of Electrical and Electronics Engineers (IEEE), 2023-08-21) [Article]In this paper, we consider a novel cellular network for aerial users, which is composed of dedicated base stations (BSs), whose antennas are directed towards aerial users, and traditional terrestrial BSs (TBSs). Besides, the dedicated BSs are deployed on roadside furniture, such as lampposts and traffic lights, to achieve multiple features while occupying less space. Therefore, the locations of dedicated BSs and TBSs are modeled by a Poisson-line-Cox-process (PLCP) and Poisson point process (PPP), respectively. For the proposed network, we first compute the aerial coverage probability and show that the deployment of dedicated BSs improves the coverage probability in both high dense areas and rural areas. We then consider a cellular-connected UAV that has a flying mission and optimize its trajectory to maximize the minimal achievable signal-to-interference-plus-noise ratio (SINR) (Max-Min SINR). To obtain the Max-Min SINR and minimal time trajectory that satisfies the Max-Min SINR, we proposed two algorithms that are practical in large-scale networks. Finally, our results show that the optimal density of dedicated BSs which maximizes Max-Min SINR decreases with the increase of the road densities.
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Beampattern-Invariant MIMO Covariance Matrices: Synthesis and Applications(IEEE Transactions on Wireless Communications, Institute of Electrical and Electronics Engineers (IEEE), 2023-08-20) [Article]The conventional method of designing the desired beampattern in MIMO systems involves the computation of the waveform covariance and/or weight matrices. In this work, through some simple derivations, it is demonstrated that there are infinitely many covariance matrices that yield the same beampattern. Furthermore, when beam is created using weighted sums of orthogonal waveforms, there are infinitely many weight matrices that also generate the same beampattern. Thus, the conditions for the beampattern invariance are formulated with respect to the covariance and the weight matrices. This theoretical foundation allows the transmitted waveform to be altered, without changing the beampattern and the orthogonal radar waveforms. Methods for the computation of beampattern-invariant covariance and weight matrices are proposed. Consequently, it is demonstrated that there is additional degrees of freedom in the design space of many applications, such as the dual-function radar communication (DFRC), in which information can be embedded in radar transmissions while keeping the radar function intact. Other potential applications are peak-to-average power ratio (PAPR) reduction at the radar transmitter and deceptive jamming avoidance.
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Coexistence of Terrestrial and Satellite Networks in the 28 GHz Band(IEEE Transactions on Aerospace and Electronic Systems, Institute of Electrical and Electronics Engineers (IEEE), 2023-08-16) [Article]We present a practical framework for maximizing the average data rates of terrestrial networks operating in the 28-GHz band while considering their coexistence with satellite networks. The 28-GHz mmWave band is licensed to fixed satellite services for Earth-to-space uplink transmissions, which are also used by the terrestrial cellular and backhaul networks for downlink operation. Our approach focuses on finding the optimal radii for exclusion zones, which are the areas where certain network elements are restricted from operating in the 28-GHz band. Through stochastic geometry, we derive the average data rate expressions for the terrestrial networks as functions of the exclusion zone radii of the Earth stations and the backhaul points. We then convert the discrete problem of frequency allocation into a continuous problem through Poisson point process approximation of the transmitters' locations. We perform logistic regression on the integral-form coverage probability expression to obtain closed-form approximation of the data rate expressions. This facilitates faster optimization, making our framework viable for deployment in frequency allocation systems, offering near-optimal results with lower complexity compared with combinatorial techniques. We improve the data rate of cellular users by up to ∼30 at the expense of the data rate of the backhaul points degrading by ∼2 .
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On the Downlink SINR Meta Distribution of UAV-assisted Wireless Networks(IEEE Transactions on Communications, Institute of Electrical and Electronics Engineers (IEEE), 2023-08-15) [Article]The meta distribution of the signal-to-interference-plus-noise ratio (SINR) provides fine-grained information about each link’s performance in a wireless system and the reliability of the whole network. While the UAV-enabled network has been studied extensively, most of the works focus on the spatial average performance, such as coverage probability, while SINR meta distribution has received less attention. In this paper, we use the SINR meta distribution to systematically analyze the improvement and the influence of deploying UAVs on the reliability of a wireless network. We first derive the b -th moments of the conditional success probability of the UAV-enabled network and give the approximated expressions derived by Gil-Pelaez theorem and the beta approximation of the meta distribution. Our numerical results show that deploying UAVs in wireless networks in most cases can greatly improve the system reliability, which denotes the fraction of users achieving cellular coverage, especially for the spatially-clustered users. In addition, establishing LoS links is not always beneficial since it also increases the interference. For instance, with the increase of the SINR threshold, the system reliability of a high LoS probability environment decreases dramatically and it is even lower than a low LoS probability environment. We also show that in highrise urban areas, UAVs can help in establishing extremely reliable (very high SINR) links.
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Beam wander prediction with recurrent neural networks(Optics Express, Optica Publishing Group, 2023-08-14) [Article]Among the problems that prevent free-space optical communication systems from becoming a truly mainstream technology is beam wander, which is especially important for structured light beams since beam misalignment introduces additional crosstalk at the receiver. The paper suggests a recurrent neural network-based (RNN) solution to predict beam wander in free space optics (FSO). The approach uses past beam center of mass positions to predict future movement, significantly outperforming various prediction types. The proposed approach is demonstrated using under-sampled experimental data over a 260 m link as a worst-case and over-sampled simulated data as a best-case scenario. In addition to conventional Gaussian beams, Hermite- and Laguerre-Gaussian beam wander is also investigated. With a 20 to 40% improvement in error over naive and linear predictions, while predicting multiple samples ahead in typical situations and overall matching or outperforming considered predictions across all studied scenarios, this method could help mitigate turbulence-induced fading and has potential applications in intelligent re-transmits, quality of service, optimized error correction, maximum likelihood-type algorithms, and predictive adaptive optics.
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An Interdisciplinary Approach to Optimal Communication and Flight Operation of High-Altitude Long-Endurance Platforms(IEEE Transactions on Aerospace and Electronic Systems, Institute of Electrical and Electronics Engineers (IEEE), 2023-08-14) [Article]Aerial communication platforms, stratospheric high-altitude platform stations (HAPS), have the potential to provide/catalyze advanced mobile wireless communication services with its ubiquitous connectivity and ultra-wide coverage radius. Recently, HAPS has gained immense popularity - achieved primarily through self-sufficient energy systems - to render long-endurance characteristics. The photo-voltaic cells mounted on the aircraft harvest solar energy during the day, which can be partially used for communication and station-keeping, whereas, the excess is stored in the rechargeable batteries for the night time operation. We carry out an adroit power budgeting to ascertain if the available solar power can simultaneously and efficiently self-sustain the requisite propulsion and communication power expense. We further propose an energy optimum trajectory for station-keeping flight and non-orthogonal multiple access (NOMA) for multi-cell users, which are served by the directional beams from HAPS communication systems. We design optimal power allocation for downlink (DL) NOMA users along with the ideal position and speed of flight with the aim to maximize sum data rate during the day and minimize power expenditure during the night, while ensuring quality of service. Our findings reveal the significance of joint design of communication and aerodynamic parameters for optimum energy utilization and resource allocation.
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Deployment Optimization of Tethered Drone-assisted Integrated Access and Backhaul Networks(IEEE Transactions on Wireless Communications, Institute of Electrical and Electronics Engineers (IEEE), 2023-08-10) [Article]Millimeter-wave (mmWave) integrated access and backhaul (IAB) has recently received considerable interest for its advantage in reducing the expenses related to the deployment of fiber optics, such as the Terragraph proposed by Meta’s Connectivity Lab. Terragraph networks aim to provide high-speed internet access to dense urban environments. However, due to the vulnerability to blockages and high path loss associated with mmWave frequencies, the proper deployment planning of mmWave networks is required to achieve the desired service quality. By obtaining a stable power supply through its tether connected to the ground, tethered unmanned aerial vehicle (UAV)-mounted base station (BS) can provide reliable communication service with the sacrifice of limited mobility. In this paper, we investigate the potential of incorporating tethered UAVs into Terragraph-like networks. To this end, we propose a novel deep reinforcement learning (DRL) framework that aims to minimize the overall deployment cost by optimizing the number of required UAVs and terrestrial BSs (TBSs), the hovering positions of deployed UAVs, and the multi-hop backhauling topology. Unlike the conventional DRL frameworks that focus on maximizing the expected cumulative or average reward, we formulate the proposed framework based on the max-Bellman optimality equation in order to maximize the maximum reward. Numerical results reveal that the proposed algorithm is able to yield significant reduction in terms of deployment cost. We also use case studies from cities in Asia, Europe, and North America to verify the practical applicability of the proposed framework.
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Coverage Analysis of Tethered UAV-Assisted Large Scale Cellular Networks(IEEE Transactions on Aerospace and Electronic Systems, Institute of Electrical and Electronics Engineers (IEEE), 2023-08-01) [Article]One of the major challenges slowing down the use of unmanned aerial vehicles (UAVs) as aerial base stations (ABSs) is the limited on-board power supply which reduces the UAV's flight time. Using a tether to provide UAVs with power can be considered a reasonable compromise that will enhance the flight time while limiting the UAV's mobility. In this work, we propose a system where ABSs are deployed at the centers of user hotspots to offload the traffic and assist terrestrial base stations (TBSs). Firstly, given the location of the ground station in the user hotspot (user cluster) and the users spatial distribution, we compute the optimal inclination angle and length of the tether. Using these results, we compute the densities of the tethered UAVs deployed at different altitudes, which enables tractable analysis of the interference in the considered setup. Next, using tools from stochastic geometry and an approach of dividing user clusters into finite frames, we analyze the coverage probability as a function of the maximum tether length, the density of accessible rooftops for UAV ground station deployment, and the density of clusters. We verify our findings using Monte-Carlo simulations and draw multiple useful insights. For instance, we show that it is actually better to deploy UAVs at a fraction of the clusters, not all of them as it is usually assumed in literature.
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Intelligent Reflecting Surfaces Assisted Hyperloop Wireless Communication Network(IEEE Transactions on Mobile Computing, Institute of Electrical and Electronics Engineers (IEEE), 2023-07-31) [Article]Hyperloop or evacuated-tube transportation is a groundbreaking technology that can reach aircraft-like speeds. Its uncommon configuration of a steel-made tube isolates the moving pod from the outside wireless world. In this work, we propose an inner tube network architecture that can provide the moving pod with a seamless and reliable connection. The proposed network consists of successive access points (APs) and intelligent reflecting surfaces (IRS) strategically positioned along the tube and connected to a control station (CS) through wired links to improve the wireless cell coverage. The subsequent entities of the proposed design are intelligently placed along the movement path, steering the transmitted beam towards the receiver, while a soft handover is achieved between consecutive cells. First, we optimize each IRS's positioning and phase shifts to maximize cell coverage thanks to the IRS scanning abilities while keeping a minimum quality of service. Afterward, we exploit the centralized operation at the CS and design a soft handover scheme for the inner-tube wireless network. The numerical results show that the proposed approach provides good cell coverage and spectral efficiency with different IRS scanning ranges.
