116 research outputs found
Energy Efficient Massive MIMO and Beamforming for 5G Communications
Massive multiple-input multiple-output (MIMO) has been a key technique
in the next generation of wireless communications for its potential to achieve
higher capacity and data rates. However, the exponential growth of data
traffic has led to a significant increase in the power consumption and system
complexity. Therefore, we propose and study wireless technologies to improve the trade-off between system performance and power consumption of wireless communications.
This Thesis firstly proposes a strategy with partial channel state information
(CSI) acquisition to reduce the power consumption and hardware complexity of massive MIMO base stations. In this context, the employment of partial CSI is proposed in correlated communication channels with user mobility. By exploiting both the spatial correlation and temporal correlation of the channel, our analytical results demonstrate significant gains in the energy efficiency of the massive MIMO base station.
Moreover, relay-aided communications have experienced raising interest; especially, two-way relaying systems can improve spectral efficiency with short required operating time. Therefore, this Thesis focuses on an uncorrelated massive MIMO two-way relaying system and studies power
scaling laws to investigate how the transmit powers can be scaled to improve the energy efficiency up to several times the energy efficiency without power scaling while approximately maintaining the system performance.
In a similar line, large antenna arrays deployed at the space-constrained relay would give rise to the spatial correlation. For this reason, this Thesis presents an incomplete CSI scheme to evaluate the trade-off between the spatial correlation and system performance. In addition, the advantages of linear processing methods and the effects of channel aging are investigated to further improve the relay-aided system performance.
Similarly, large antenna arrays are required in millimeter-wave communications to achieve narrow beams with higher power gain. This poses the problem that locating the best beam direction requires high power and complexity consumption. Therefore, this Thesis presents several low-complexity beam alignment methods with respect to the state-of-the-art to evaluate the trade-off between complexity and system performance.
Overall, extensive analytical and numerical results show an improved performance and validate the effectiveness of the proposed techniques
Towards Context Information-based High-Performing Connectivity in Internet of Vehicle Communications
Internet-of-vehicles (IoV) is one of the most important use cases in the fifth generation (5G) of wireless networks and beyond. Here, IoV communications refer to two types of scenarios: serving the in-vehicle users with moving relays (MRs); and supporting vehicle-to-everything (V2X) communications for, e.g., connected vehicle functionalities. Both of them can be achieved by transceivers on top of vehicles with growing demand for quality of service (QoS), such as spectrum efficiency, peak data rate, and coverage probability. However, the performance of MRs and V2X is limited by challenges such as the inaccurate prediction/estimation of the channel state information (CSI), beamforming mismatch, and blockages. Knowing the environment and utilizing such context information to assist communication could alleviate these issues. This thesis investigates various context information-based performance enhancement schemes for IoV networks, with main contributions listed as follows.In order to mitigate the channel aging issue, i.e., the CSI becomes inaccurate soon at high speeds, the first part of the thesis focuses on one way to increase the prediction horizon of CSI in MRs: predictor antennas (PAs). A PA system is designed as a system with two sets of antennas on the roof of a vehicle, where the PAs positioned at the front of the vehicle are used to predict the CSI observed by the receive antennas (RAs) that are aligned behind the PAs. In PA systems, however, the benefit is affected by a variety of factors. For example, 1) spatial mismatch between the point where the PA estimates the channel and the point where the RA reaches several time slots later, 2) antenna utilization efficiency of the PA, 3) temporal evolution, and 4) estimation error of the PA-base station (BS) channel. First, in Paper A, we study the PA system in the presence of the spatial mismatch problem, and propose an analytical channel model which is used for rate adaptation. In paper B, we propose different approximation schemes for the analytical investigation of PA systems, and study the effect of different parameters on the network performance. Then, involving PAs into data transmission, Paper C and Paper D analyze the outage- and the delay-limited performance of PA systems using hybrid automatic repeat request (HARQ), respectively. As we show in the analytical and the simulation results in Papers C-D, the combination of PA and HARQ protocols makes it possible to improve spectral efficiency and adapt the transmission parameters to mitigate the effect of spatial mismatch. Finally, a review of PA studies in the literature, the challenges and potentials of PA as well as some to-be-solved issues are presented in Paper E.The second part of the thesis focuses on using advanced technologies to further improve the MR/IoV performance. In Paper F, a cooperative PA scheme in IoV networks is proposed to mitigate both the channel aging effect and blockage sensitivity in millimeter-wave channels by collaborative vehicles and BS handover. Then, in Paper G, we study the potentials and challenges of dynamic blockage pre-avoidance in IoV networks
SWIPT aided Cooperative Communications with Energy Harvesting based Selective-Decode-and-Forward Protocol: Benefiting from Channel Aging Effect
Simultaneous wireless information and power transfer (SWIPT) in radio-frequency (RF) bands enables flexible deployment of battery-powered relays for extending communication coverage. Relays receive downlink RF signals emitted by a source for information decoding and energy harvesting, while the harvested energy is consumed for both information decoding and information forwarding to a destination. An energy harvesting based selective-decode-and-forward (EH-SDF) protocol is proposed, where only the relays having information correctly decoded are activated for information forwarding, while others harvest and store energy for the future use. By considering the channel aging effect, we propose a joint relay selection, power allocation, transmit beamforming and signal splitting design in order to maximise the end-to-end (e2e) throughput of this EH-SDF aided cooperative communication system. Two scenarios with/without direct link between the source and the destination are studied, respectively. The original formulated non-convex optimisation problems with coupled variables are decoupled into three subproblems which are solved by an iterative optimisation algorithm. Numerical results demonstrate that our design with the EH-SDF protocol achieves a higher e2e throughput than the traditional decode-and-forward (DF) counterpart. Moreover, the impact of the channel aging effect on the e2e throughput is also evaluated
Propagation measurement based study on relay networks
Von der nächsten Generation von Mobilfunksystemen erwartet man eine
umfassende Versorgung mit breitbandigen Multimediadiensten. Um die dafĂĽr
erforderliche flächendeckende Versorgung mit hohen Datenraten zu
gewährleisten, können Relay-Netzwerke einen wesentlichen Beitrag liefern.
Hierbei werden Netzwerkstationen mit Relay-Funktionalität in zellulare
Netzwerke integriert.
Diese Dissertation befasst sich mit der Untersuchung Relay-basierter
Netzwerke unter Verwendung von Ausbreitungsmessungen. Die Arbeit deckt
Fragen zur Kanalmodellierung, Systemevaluierung bis hin zur
Systemverifikation ab. - Zunächst wird ein auf Funkkanalmessungen
beruhendes experimentelles Kanalmodell fĂĽr Relay-Netzwerke vorgestellt. Im
Weiteren werden technische Verfahren fĂĽr Mehrfachzugriffs-Relay-Netzwerke
MARN diskutiert. Die erreichbare Systemleistung wurde unter Verwendung von
Rayleigh-Kanälen innerhalb einer Systemsimulation bestimmt und im Anschluss
mit realen Kanälen, die sowohl direkt aus Funkkanalmessungen als auch
indirekt aus dem bereits erwähnten Kanalmodell abgeleitet wurden,
verifiziert.
Bisherige Arbeiten zur Modellierung breitbandiger Multiple-Input
Multiple-Output (MIMO) Kanäle berücksichtigen nicht oder nur sehr stark
vereinfacht die Langzeitkorrelationseigenschaften zwischen den Links und
werden damit der vermaschten und räumlich weit verteilten Topologie von
Relay-Netzwerken gerecht. In der vorliegenden Dissertation erfolgte daher
eine experimentelle Untersuchung zu den Korrelationseigenschaften von
Large-Scale-Parametern LSP, die unter Verwendung von Funkkanalmessdaten aus
urbanen Umgebungen und aus Innenräumen abgeleitet wurden. Die Ergebnisse
hierzu fanden Eingang in das vom WINNER-Projekt entwickelte Kanalmodell.
Sie erlauben damit eine realistischere Simulation von Relay-unterstĂĽtzten
Netzen.
Einen weiteren Schwerpunkt dieser Arbeit stellen technische Verfahren dar,
die eine Erhöhung der Systemleistung in MARN mit unbekannter Interferenz
UKIF versprechen. Im Einzelnen handelt es sich um die
Mehrfachzugriffs-Kodierung MAC - die eine verbesserte Signaltrennung auf
der Empfängerseite und eine Erhöhung des Datendurchsatzes erlaubt, den
Entwurf eines Relay-Protokolls zur Erhöhung der Systemeffizienz, einen
Minimum Mean Square Error (MMSE) Algorithmus zur UnterdrĂĽckung unbekannter
Interferenzen bei Erhaltung der MAC-Signalstruktur mehrerer Mobilstationen
MS, und ein fehlererkennungsbasiertes Signalauswahlverfahren zur
Diversitätserhöhung.
Die vorgenannten Verfahren werden in einer Systemsimulation zunächst mit
Rayleigh-Kanälen evaluiert und demonstrieren die erzielbare theoretische
Leistungssteigerung. Die Berücksichtigung realer Funkkanäle innerhalb der
Systemsimulation zeigt allerdings, dass die theoretische Systemleistung so
in der Realität nicht erreichbar ist. Die Ursache hierfür ist in den
idealisierten Annahmen theoretischer Kanäle zu suchen.
FĂĽr die Entwicklung kĂĽnftiger Relay-Netzwerke bieten die in dieser Arbeit
aufbereiteten Erkenntnisse hinsichtlich der
Langzeitkorrelationseigenschaften zwischen den Links einen wertvollen
Beitrag für die Abschätzung ihrer Systemleistung auf der Basis eines
verbesserten Kanalmodells.Considering technological bases of next generation wireless systems, it is
expected that systems can provide a variety of coverage requirements to
support ubiquitous communications. To satisfy the requirements, an
innovative idea, integrating network elements with a relaying capability
into cellular networks, is one of the most promising solutions.
The main topic of this dissertation is a propagation measurement based
study on relay networks. The study includes three parts: channel modeling,
performance evaluation, and verification. First of all, an empirical
channel model for relay networks is proposed based on statistical analyses
of measurement data. Then, advanced techniques for the throughput
improvement and interference cancellation are proposed for Multiple Access
Relay Networks (MARN) which are used as an example of relay networks. The
performance of the considered MARN is evaluated for Rayleigh channels, and
then verified for realistic channels, obtained from measurement data and
from the experimental relay channel model as well.
For relay channel modeling, the long-term correlation properties between
links are of crucial importance due to the meshed-network topology.
Although, there is a wide variety of research results for Multiple-Input
Multiple-Output (MIMO) channel modeling available, the characterization of
correlation properties has been significantly simplified or even completely
ignored which motivates this research to be performed. In this
dissertation, the experimental results of the correlation properties of
Large Scale Parameters (LSP) are presented through the analysis on the
real-field measurement data for both the urban and indoor scenarios.
furthermore, the correlation properties have been fully introduced into the
WINNER channel Model (WIM) for realistic relay channel simulations.
As a further contribution of this dissertation, various advanced techniques
are proposed for MARN in the presence of Unknown Interference (UKIF).
Multiple Access Coding (MAC) is introduced as a multiple access technique.
The use of MAC provides the signal separability at the receiver and
improves throughput. Thereafter, high system resource efficiency can be
achieved through relay protocol design. At the receiver, Minimum Mean
Square Error (MMSE)-based spatial filtering is used to suppress UKIF while
preserving multiple Mobile Station (MS)s’ MAC-encoded signal structure.
Furthermore, an error detection aided signal selection technique is
proposed for diversity increasing.
The theoretical system performance with aforementioned techniques is
simulated for Rayleigh channels. Thereafter, realistic channels are
exploited for the performance verification. The gap between the theoretical
performance and the realistic performance indicates that the assumptions
made to the simplified Rayleigh-channels do not fully hold in reality.
For the future relay system design, this work provides valuable information
about the performance evaluation of relay networks in consideration of the
correlation properties between links
Joint Design of Wireless Fronthaul and Access Links in Massive MIMO CRANs
Cloud radio access network (CRAN) has emerged as a promising mobile network architecture for the current 5th generation (5G) and beyond networks. This thesis focuses on novel architectures and optimization approaches for CRAN systems with massive multiple-input multiple-output (MIMO) enabled in the wireless fronthaul link. In particular, we propose a joint design of wireless fronthaul and access links for CRANs and aim to maximize the network spectral efficiency (SE) and energy efficiency (EE).
Regarding downlink transmission in massive MIMO CRANs, the precoding designs of the access link are optimized by accounting for both perfect instantaneous channel state information (CSI) and stochastic CSI of the access link separately. The system design adopts a decompress-and-forward (DCF) scheme at the remote radio heads (RRHs), with optimization of the multivariate compression covariance noise. Constrained by the maximum power budgets set for the central unit (CU) and RRHs, we aim to maximize the network sum-rate and minimize the total transmit power for all user equipments (UEs). Moreover, we present a separate optimization design and compare its performance, feasibility, and computational efficiency with the proposed joint design. Considering the uplink transmission, we utilize a compress-and-forward (CF) scheme at the RRHs. Assuming that perfect CSI is available at the CU, our objective is to optimize the precoding matrix of the access link while adopting conventional precoding methods for the fronthaul link. This thesis also proposes an unmanned aerial vehicle (UAV)-enabled CRAN architecture with a massive MIMO CU as a supplement system to the terrestrial communication networks. The locations of UAVs are optimized along with compression noise, precoding matrices, and transmit power. To tackle the non-convex optimization problems described above, we employ efficient iterative algorithms and conduct a thorough exploration of practical simulations, yielding promising results that outperform benchmark schemes.
In summary, this thesis explores future wireless CRAN architectures, leveraging promising technologies including massive MIMO and UAV-enabled communications. Furthermore, this work presents comprehensive optimization designs aimed at further enhancing the network efficiency
Spectral Efficiency Analysis of Hybrid Relay-Reflecting Intelligent Surface-Assisted Cell-Free Massive MIMO Systems
A cell-free (CF) massive multiple-input-multiple-output (mMIMO) system can provide uniform spectral efficiency (SE) with simple signal processing. On the other hand, a recently introduced technology called hybrid relay-reflecting intelligent surface (HR-RIS) can customize the physical propagation environment by simultaneously reflecting and amplifying radio waves in preferred directions. Thus, it is natural that incorporating HR-RIS into CF mMIMO can be a symbiotic convergence of these two technologies for future wireless communications. This motivates us to consider an HR-RIS-aided CF mMIMO system to utilize their combined benefits. We first model the uplink/downlink channels and derive the minimum-mean-square-error estimate of the effective channels. We then present a comprehensive analysis of SE performance of the considered system. Specifically, we derive closed-form expressions for the uplink and downlink SE. The results reveal important observations on the performance gains achieved by HR-RISs compared to conventional systems. The presented analytical results are also valid for conventional CF mMIMO systems and those aided by passive reconfigurable intelligent surfaces. Such results play an important role in designing new transmission strategies and optimizing HR-RIS-aided CF mMIMO systems. Finally, we provide extensive numerical results to verify the analytical derivations and the effectiveness of the proposed system design under various settings
Securing internet of medical things with friendly-jamming schemes
The Internet of Medical Things (IoMT)-enabled e-healthcare can complement traditional medical treatments in a flexible and convenient manner. However, security and privacy become the main concerns of IoMT due to the limited computational capability, memory space and energy constraint of medical sensors, leading to the in-feasibility for conventional cryptographic approaches, which are often computationally-complicated. In contrast to cryptographic approaches, friendly jamming (Fri-jam) schemes will not cause extra computing cost to medical sensors, thereby becoming potential countermeasures to ensure security of IoMT. In this paper, we present a study on using Fri-jam schemes in IoMT. We first analyze the data security in IoMT and discuss the challenges. We then propose using Fri-jam schemes to protect the confidential medical data of patients collected by medical sensors from being eavesdropped. We also discuss the integration of Fri-jam schemes with various communication technologies, including beamforming, Simultaneous Wireless Information and Power Transfer (SWIPT) and full duplexity. Moreover, we present two case studies of Fri-jam schemes in IoMT. The results of these two case studies indicate that the Fri-jam method will significantly decrease the eavesdropping risk while leading to no significant influence on legitimate transmission
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