135 research outputs found
AoA-Based Low Complexity Beamforming for Aerial RIS Assisted Communications at mmWaves
No full textReconfigurable intelligent surface (RIS) mounted on mobile devices (aerial RIS), such as unmanned aerial vehicles, is envisioned to increase coverage for future millimeter wave (mmWave) communications. In literature, RIS beamformer is designed using channel state information (CSI) or by scanning through gNodeB and user codebooks over different training RIS weight matrices. Both of these methods incur significant training overhead. Also, the traditional subspace-based AoA estimators require several power-intensive radio frequency (RF) chains, which is not practically affordable to users. As a low-power and low-complexity alternative, we propose to design RIS beamforming weights based on angle-of-arrival (AoA) from gNodeB-to-RIS and from RIS-to-user with a single RF chain coupled to an antenna array. The proposed estimator is a combination of maximum likelihood and maximum correlation estimator and uses a single RIS training matrix. The simulation results show that, though a slightly larger number of antenna elements is required for the same spectral efficiency, the proposed approach offers a significantly energy- and computationally-efficient beamforming alternative compared to the existing subspace-based AoA estimator and CSI based techniques
Joint Beamwidth and Number of Concurrent Beams Estimation in Downlink mmWave Communications
No full textThis paper proposes a sectored-cell framework for mmWave communication. It consists of multiple concurrent beams generated from a partially-connected hybrid precoder at an eNodeB (eNB) to serve a dense user population in urban scenarios. Multiple beams sweep the cell in a round-robin fashion to serve the sectors with fair scheduling opportunities. Each beam serves all the users located within a sector using orthogonal frequency division multiple access. We aim to estimate an optimum beamwidth and an optimum number of beams required to maximize the average of long-run user rates with a given power budget for transmission and hardware consumption at the eNB. Simulation results demonstrate that employing higher beams increases the side-lobe interference still, the achievable average long-run user rate improves on account of longer sector sojourn time and higher frequency reuse. On the other hand, employing a very narrow beam is also not optimal
Multi-RF Beamforming-Based Cellular Communication Over Wideband mmWaves
No full textThe existing literature on cellular multi-user mmWave communication focus on joint baseband and RF precoding designs to enable spatial multiple access with minimum interference. These studies either assume the number of users M is less than the number of RF units N(RF )or schedule the users in time domain by dedicating one RF unit to each user if M > N-RF. It is expected that serving multiple users over OFDMA in each analog beam will offer better utilization of the wideband channel. To this end, for the scenario with M >> N-RF we propose a sectored-cell model that is supported by multi-RF chains over the widehand mmWave channel, with each beam serving multiple users within a sector and the sectors being scheduled in round-robin fashion. We also propose a variable time frame structure that conducts sector-wise initial access, with simultaneous access to all users within a sector. It provides improved spectrum efficiency and decreased initial access delay as compared to the initial access using exhaustive beam search method. We then jointly estimate the optimum beamwidth and optimum N-RF that offer maximum average long-run user rate. Further, we introduce a reduced-complexity sector sojourn time optimization for non-homogeneously distributed users, that improves fairness of long-run user rates leveraging the variable time frame structure. The numerical results show that, while a high value of N-RF causes more interference to peak data rate, the average long-run user rate improves. Additionally, using a very narrow beam is also not optimal for providing maximum rate support. The proposed beamforming method offers a higher average long-run user rate over the competitive beamforming schemes while the complexity of user scheduling is independent of M
Design Optimization for UAV Aided Sustainable 3D Wireless Communication at mmWaves
No full textUnmanned aerial vehicles (UAVs) operating at different altitudes will be integral to the 5th generation and beyond (5G+) communication network to provide ubiquitous coverage. Though 5G+ communications target to operate in sub-6 GHz as well as millimeter wave range (mmWaves), sub-6 GHz being already congested, mmWaves are seen as a viable technology that can support high data rates. To this end, in this paper, we study the feasibility of using UAVs at mmWaves deployed at low altitudes to serve a user population higher than the number of RF chains available at the UAV. Since the UAVs are energy constrained devices, solar harvesting is considered for the UAVs to act as access nodes for an extended period of time. Practical 3-dimensional antenna array radiation pattern and the resulting inter-beam interference from the sidelobes are taken into account in the analysis. We devise a sub-array hybrid precoder that provides minimum rate support to all users across the wideband mmWave channel. The RF precoder is designed according to the users' locations. In addition, we propose a low complexity iterative joint subcarrier allocation and baseband precoder optimization algorithm with faster convergence. In the proposed algorithm, we employ weighted minimum mean squared error (WMMSE) to design baseband precoder to reduce inter-beam interference while jointly optimizing subcarrier allocation to maintain minimum user rate constraint. Next, we determine the optimal number of beams at UAV to optimize throughput while guaranteeing minimum user rate support for energy sustainable UAV operation. Finally, we compare the performance gain achieved by low-altitude UAV deployed at mmWaves over the UAV deployed at sub-6 GHz frequency range
Optimum Downlink Beamwidth Estimation in mmWave Communications
No full textWith increasing density of data-hungry devices per unit area, allocating single highly-directed beam per user in millimeter-wave communications is not practical. Therefore, the requirement is to serve multiple users over a single beam. Considering a single-cell scenario with a fixed number of users, this paper addresses the problem of selection of optimal beamwidth depending on user density and distribution. First, by considering fixed beam service time in each sector, optimal beamwidth is estimated using exhaustive search for average long-run user rate and base station energy efficiency maximization. Based on the results of the average long-run user rate maximization using an exhaustive search, another method of reduced complexity is proposed to find sub-optimal beamwidth. Subsequently, optimum beamwidth is estimated with user density dependent variable time scheduling in a sector, that offers improved performances over fixed time scheduling. An efficient algorithm on variable time scheduling is also provided. Finally, the effect of localization error on optimal beamwidth estimation is investigated. The numerical results show that using the narrowest beam does not necessarily result in achieving a better average long-run user rate. Further, localization error does not affect the selection of optimal beamwidth, however, user Quality-of-Service degrades
RF Beamforming and Subcarrier Allocation Using Beam Squint in mmWave Systems
No full textIn a multi-user wideband millimeter wave (mmWave) communication system, the existing works optimize the hybrid precoder by assuming a priori subcarrier allocation. However, the beam squint effect in mmWave affects the gain over the subcarriers differently for different beam steer directions. Thus, the design of radio frequency (RF) precoder and subcarrier allocation are intertwined. Based on this observation, in this letter we propose a sub-array hybrid precoder design wherein we jointly estimate the RF precoder and subcarrier allocation and then design the baseband precoder. Our numerical results demonstrate the benefits of the proposed strategy compared to the existing approach wherein subcarrier allocation precedes the RF and baseband precoder design
Optimal Beamforming Strategies At Mmwaves For 5G+ Networks
No full textBeyond fifth generation (5G+) communication aims to provide a 3-dimensional ubiquitous network to support high data rates caused by device proliferation. To aid 5G+ communications, three spectrum bands are being investigated: sub-6 GHz, millimeterwave (mmWave), and terahertz frequencies. Higher frequency range has the advantage of increased bandwidth availability, but it also has its own set of challenges such as increased signal attenuation, reduced cell range, increased computational and hardware complexities, and increased hardware cost. However, substantial research has been conducted to demonstrate the feasibility of communication at higher frequencies. This dissertation focuses specifically on
mWave-enabled communications for serving users under different 2-dimensional as well as 3-dimensional network. The number of radio frequency (RF) chains that can be deployed at a device is a bottleneck in the mmWave range due to high hardware power consumption and hardware complexity. This, in turn, presents challenges in designing precoders and combiners, parameter estimation, signal processing,
and user scheduling. Therefore, the objective of this dissertation is to investigate and design energy and spectrally efficient reduced complexity terrestrial and aerial mmWave communications frameworks/architectures with a limited number of RF chains.
The first topic investigates sectored-cell framework for a 2-dimensional multi-user terrestrial mmWave communications when the user population exceeds the number of RF chains available at the gNodeB. This framework is analyzed in the context of a single RF chain that serves all sectors of a cell in a round-robin manner with equal sector sojourn time. The idea is to partition the cell into identical sectors and serve multiple users simultaneously that fall within each sector at a given epoch using a single steerable beam generated from a uniform linear antenna array coupled to an RF chain at the gNodeB. Consequently, a large number of users are served with a limited number of RF chains. An optimization problem is formulated for combined resource allocation of orthogonal frequency division multiple (OFDM) symbols to users in a sector and sector beamwidth optimization in order to maximize average long-term user rate and system energy efficiency. Through simulation results, it is verified that serving multiple users over orthogonal frequency division multiple access with a single RF chain a sectored cell system model achieves better performance than serving single user per RF chain at a time. Additionally, the effect of localization error on the optimum beamwidth, which results from position estimation, is also studied.
The second part of the dissertation extends the study of the sectored-cell framework
with a single RF chain to the case of multiple RF chains at gNodeB. Because of its low complexity, the sub-array based or partially connected hybrid precoder is considered, in which each RF chain, connected to a separate antenna array, generates one steerable beam. Notably, the presence of sidelobes causes inter-beam interference when concurrent beams are generated from multiple RF chains. Therefore, the optimal beamwidth is estimated while accounting for inter-beam interference and beam squint. Furthermore, an optimal number of RF chains at the gNodeB is estimated by accounting for power waste in RF units. A variable time frame structure for the sectored-cell framework is also proposed for a standalone mmWave communications system with variable transmission time interval units as short as one OFDM symbol long. The frame structure for enhanced mobile broadband (eMBB) services is typically made up of slots with a fixed number of OFDM symbols, and the smallest transmission time interval unit is equivalent to one slot duration, as defined by the 3rd Generation Partnership Project (3GPP) New Radio (NR) in Release 15. Furthermore, 3GPP guidelines have suggested a separate beam management phase for the initial gNodeB-user beam pair to establish the best gNodeB-user beam pair to be used for subsequent data transfer in data transmission mode. During the beam management phase, narrow beams must scan the area multiple times before granting users channel access. As a result, using a fixed frame structure along with separate beam management and data transmission phase causes significant initial access delays. Furthermore, because beam training overhead is often low for beam search operations, the wideband mmWave channel is underutilized during the beam training phase. Therefore, a modified sectorwise initial access procedure is proposed, which offers decreased initial access delay and increased bandwidth utilization. The proposed variable time frame structure, along with the modified initial access procedure, offers an improved average and the geometric mean of long-run user rates, especially in the case of non-homogeneous user distribution in the area.
The third part of the dissertation studies in detail the effect of beam squint in wideband mmWave communication. The beam squint effect is caused by the use of a large number of antenna elements connected per RF chain to generate a narrow beam in order to overcome high attenuation at mmWaves. The direction of maximum beam gain in beam squint varies with frequency. Based on this observation, a new reduced complexity joint OFDM resource allocation and beamforming strategy is proposed, which employs beam squint to maximize system performance using sub-array hybrid precoder at the transmitter to serve a clustered user population greater than the number of RF chains. Numerical results demonstrate that the proposed sequence for designing RF precoder, baseband precoder, and subcarrier allocation using beam squint provides a greater increase in spectral efficiency at mmWaves than at sub-6 GHz.
The fourth part of the dissertation explores the feasibility of using an unmanned aerial vehicle (UAV) as a fronthaul unit at mmWaves and contrasts its performance with that at the sub-6 GHz range for ad hoc communication. The ideal user grouping method in a 3-dimensional environment with UAV-assisted mmWave communications is investigated for a system with a user population much higher than the number of RF chains available at the UAV. Similar to the analysis of multiple RF chains in terrestrial communication, multiple users per beam are served employing OFDMA. The optimal number of RF chains at the UAV is estimated to maximize the sum rate while satisfying minimum user rate specifications with a given UAV power constraint. Since UAV performance is constrained by battery size, so it is assumed that the UAV is equipped with a solar panel to harvest solar energy in order to increase operational hours. The effect of additional weight because of the solar panel on the performance of the UAV is also analyzed.
Finally, the dissertation's last part studies the feasibility of using an existing backscatter device infrastructure in an indoor environment to provide data support to an obstructed user at mmWaves. When in idle mode, the backscatter device is used to reflect the incoming signal to desired direction without modulating the data stream. This analogous to a distributed reconfigurable intelligent surface (RIS) system in which the tags acts as distributed RIS in indoor environment. Both users and backscatter devices in the mmWave range will have multiple antenna elements, increasing the system’s complexity. The challenge, however, is to estimate the angle’s direction using a single RF chain at the user. Therefore, a link establishment procedure is proposed that includes estimation of angle of arrival using the retro-reflective property of the antenna array, beamforming design at the user, and beamforming at the backscatter devices
Efficient Charging and Data Collection in UAV-Aided Backscatter Sensor Networks
No full textBackscatter communication based wireless charging of the sensor nodes and data collection from them is a promising solution due to ultra-low power consumption. However, challenges of short transmission range requirement, high self-interference, and simultaneous operation with multiple backscatter nodes (BSNs) need to be addressed. To this end, this paper presents a novel framework for joint field data collection and wireless charging in an unmanned aerial vehicle (UAV)-aided wireless sensor network via monostatic backscatter communication at millimeter waves. The framework is divided into three tasks, namely, energy-optimized UAV transceiver design, UAV constraints aware BSN clustering, and optimized resource allocation per cluster. To strike a balance between serving efficiency and self-interference, optimum BSN cluster size is estimated offline, which in turn governs BSN clustering optimization. With UAV communication energy and clustering information, a joint sum energy transfer and sum data collection maximization problem is formulated by considering the minimum required charging and data collection constraints. To handle non-convexity, an alternating optimization approach is devised, estimating optimal backscatter reflection coefficients, data collection time, and power distribution among the BSNs using successive convex approximation. Finally, via Monte-Carlo simulations, performance of the proposed system is compared with the current state-of-the-art
BackScatter-Assisted Indoor mmWave Communications with Directional Beam at User
No full textOwing to large spectrum availability, millimeter wave (mmWave) communications are being proposed for 5th generation and beyond networks. However, the mmWaves are easily obstructed by objects, resulting in complete link blockage, which is more common in indoor communication. In this study, we propose to use the existing infrastructure of passive backscatter devices to establish links between the access point and a blocked legacy mobile user, with uncertain coordinates, in an indoor mmWave communication system. We set up backscatter devices in retro-reflective mode for the user to estimate the direction of arrival from them. To estimate the angles, we propose an estimator and derive its Cramer-Rao lower bound. Furthermore, while accounting for the antenna array configuration at both the user and the backscatter devices, we maximize the rate support at the user by optimizing the backscatter device reflection coefficient and the user's steering angle. The simulation results show that, by exploiting the backscatter devices already present in the environment with a sufficient number of antenna elements at the user, higher capacity is achieved as compared to that achieved by using a fixed re-configurable intelligent surface
XAI4C: XAI for Conflict Detection and Mitigation in O-RAN Near-RT RIC
No full textThe Open Radio Access Network (O-RAN) architecture introduces a modular approach to network design, enabling flexibility by decoupling hardware and software while fostering innovation in a multi-vendor ecosystem. However, its distributed framework creates challenges in managing network control conflicts across various components. In this work, we provide a comprehensive overview of conflict management in O-RAN focusing on Near-Real-Time RAN Intelligent Controller (Near-RT RIC) conflicts, and state-of-the-art strategies for addressing these challenges. We analyze the limitations of current approaches through the lens of Explainable AI (XAI). Thereafter, we propose XAI4C, a novel framework for conflict detection and mitigation among xApps in Near-RT RIC in O-RAN that leverages XAI to provide more transparent and explainable actions. Our solution improves detection accuracy by 30% and reduces the detection latency by 41% compared to a state-of- the-art benchmark, while also effectively mitigating conflicts to enhance network performance. Finally, we discuss future research directions and highlight key challenges in XAI-driven O- RAN conflict management, critical for adapting to the increasing complexity of next-generation network systems
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