| 1. |
- Kaplan, Ahmet, 1994-
(författare)
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Signal Processing Aspects of Bistatic Backscatter Communication
- 2024
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Passive Internet-of-Things (IoT), a new paradigm based on battery-free devices, is a promising technology to enable several use cases that require connectivity with stringent requirements in terms of cost, complexity, and energy efficiency. These use cases span critical sectors, such as healthcare, transportation, and agriculture. Passive IoT relies on the development of technologies such as radio frequency (RF) energy harvesting, low-power computing, and backscatter communication. Particularly, backscatter communication allows devices to modulate its information on external RF signals that are backscattered to the receiver or reader.BC considers the following elements: a carrier emitter (CE), a reader, and a backscatter device (BD). The main BC configurations are monostatic BC (MoBC), ambient BC (AmBC) and bistatic BC (BiBC). In a MoBC setup, the CE and reader are co-located and share parts of the same infrastructure. A monostatic system suffers from round-trip path loss, and requires full-duplex technology if the same antennas are simultaneously used for transmission and reception. In an AmBC setup, CE and reader are in different locations, while the CE is not considered dedicated. AmBC uses ambient sources to transmit information, such as Wi-Fi, Bluetooth, and TV signals. In BiBC, the CE and reader are also spatially separated from each other, but there is a dedicated CE. In addition, BiBC can operate in half duplex mode, thus avoiding the complexity associated to the full-duplex operation.Due to the double path-loss effect on the two-way backscatter link, the received backscattered signal is typically weak compared to the direct link interference (DLI) from a CE. This requires a high dynamic range of the circuitry in the reader. As a result, a high-resolution analog-to-digital converter (ADC) is required to detect the weak backscattered signal under heavy DLI; this represents a great limitation because ADCs are major power consumers. Nonetheless, the benefits provided by multiple-antenna and distributed multiple-input multiple-output (MIMO) technologies can be explored to circumvent the limitations of BiBC, which is the main focus of this thesis.In this context, the contributions of this thesis are two-fold. First, we propose a novel transmission scheme that includes a protocol for channel estimation at the multiple-antenna CE as well as a transmit beamformer design to suppress the DLI between the two ends of a bistatic link (namely CE and reader) and increase the detection performance of the BD symbol. Further, we derive a generalized log-likelihood ratio test (GLRT) to detect the symbol/presence of the BD and provide an iterative algorithm to estimate the unknown parameters in the GLRT. Simulation results show that the required dynamic range of the system is significantly decreased while the detection performance of the BD symbol is increased, by the proposed algorithm compared to a system not using beamforming at the CE.For the second part, we consider BiBC in the context of cell-free MIMO networks by exploring the optimal selection of CE and reader among multiple access points, leveraging prior knowledge about the area where the BD is located. First, a maximum a posteriori probability (MAP) detector to decode the BD information bits is derived. Then, the exact probability of error for this detector is obtained. In addition, an algorithm to select the best CE-reader pair for serving the specified area is proposed. Finally, simulation results show that the error performance of the backscatter communication (BC) is improved by the proposed algorithm compared to the benchmark scenario.
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| 2. |
- Kunnath Ganesan, Unnikrishnan, 1989-
(författare)
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Beyond Boundaries : Evolving Connectivity with Massive MIMO
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The forthcoming sixth generation (6G) wireless networks signify an evolutionary leap in connectivity, surpassing conventional limitations through the integration of massive multiple-input multiple-output (MIMO) technology. Three primary application scenarios that are expected to drive the development of 6G networks are as follows: (i) enhanced mobile broadband (eMBB) promises lightning-fast data rates and expanded capacity for bandwidth-intensive applications such as virtual reality (VR) and high-definition (HD) video streaming, marking a significant evolution in connectivity; (ii)ultra-reliable low latency communication (URLLC) ensures ultra-reliable, low-latency connectivity critical for applications like autonomous vehicles and remote surgery, further emphasizing the evolution towards more reliable and responsive connections; (iii) massive machine type communications (mMTC) cater to the connectivity needs of millions of Internet-of-Things (IoT) devices across various sectors like smart cities and healthcare, expanding the boundaries of connectivity to encompass a vast array of interconnected devices. Moreover, 6G networks prioritize rural connectivity, aiming to bridge the digital divide by extending high-speed internet access to underserved areas. Through the integration of cutting-edge wireless technologies, 6G networks stand ready to redefine connectivity, offering ultra-fast, reliable, and ubiquitous communication services across a wide range of application scenarios, thereby paving the way for a more connected and equitable future. In this thesis, various aspects of the evolution of distributed massive MIMO connectivity are explored, including: (i) synchronization of distributed access points (APs); (ii) partially coherent (PC) operation of APs; (iii) Grant-free random access of mMTC devices; (iv) extreme multiplexing capabilities; and (v) SuperCell massive MIMO communication. Distributed APs operate with independent local oscillators (LOs), resulting in frequency and timing mismatches between them. In narrow-band systems, any discrepancies in frequency or timing among distributed transmit nodes translate into relative phase offsets within a symbol interval. These offsets must be compensated for coherent signal combining during transmission to a user. Paper A delves into the synchronization of distributed APs in massive MIMO systems. This synchronization is pivotal for reaping the benefits of distributed massive MIMO. The paper analyzes the synchronization requirements from a reciprocity perspective, considering multiplicative impairments due to mismatches in radio frequency (RF) hardware. It introduces BeamSync, a novel over-the-air synchronization protocol, which calibrates geographically separated APs without relying on fronthaul for sending measurement data to the central processing unit (CPU). The key concept involves beamforming the synchronization signal in the dominant direction of the channel between APs. Results demonstrate that the proposed BeamSync method outperforms traditional beamforming techniques significantly. While advancements like those detailed in Paper A enable synchronization within localized areas, achieving network-wide AP synchronization remains daunting. Consequently, synchronized clusters exist, yet phase alignment across clusters poses a challenge. Addressing this, Paper B introduces a novel PC framework crucial for realizing the full potential of cell-free massive MIMO technology. Initially, an AP clustering algorithm groups APs into phase-aligned clusters. Subsequently, combining and precoding optimization algorithm maximizes the downlink sum data rates. Additionally, a novel data stream allocation algorithm enhances the sum data rate of PC operation. Results demonstrate that PC operation approaches the sum rate of ideal fully coherent (FC) operation. This underscores the practical deployment potential of PC in cell-free massive MIMO networks. As cellular technology advances, machine type communication (MTC) becomes increasingly significant. The Paper C delves into activity detection within grant-free random access for mMTC in cell-free massive MIMO net-works. It introduces a maximum-likelihood (ML) activity detection algorithm and demonstrates through results that the macro-diversity gain inherent in the cell-free architecture significantly enhances activity detection performance, particularly in scenarios with large coverage areas, outperforming co-located architectures. RadioWeaves technology is a novel wireless infrastructure designed for indoor applications, leveraging the advantages of both massive MIMO and cell-free massive MIMO systems. Paper D explores the extreme multiplexing capabilities of RadioWeaves, demonstrating its ability to deliver high data rates while operating at low power levels. The findings reveal that RadioWeaves deployments excel in spatially separating users compared to conventional co-located setups, thus mitigating losses attributed to grating lobes and substantially reducing transmit power requirements.Rural connectivity is an important aspect to consider in the development of 6G networks, as it addresses the challenge of providing high-quality, reliable internet access to under-served or remote areas. Amidst the surge in data traffic within urban areas and the consequent neglect of rural regions due to lower profitability, Paper E explores the performance of SuperCell massive MIMO systems in rural settings. Addressing the unique challenges and opportunities of rural communication environments, the paper proposes an innovative solution. This solution involves the utilization of elevated base stations (BSs) equipped with sectorized antennas boasting large apertures, coupled with a user scheduling algorithm to ensure comprehensive digital coverage. Through rigorous analysis, the study evaluates the coverage range and high-rate service provision capabilities of this SuperCell system in rural contexts.
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| 3. |
- Abrahamsson, Olle, 1988-
(författare)
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On Aggregation and Dynamics of Opinions in Complex Networks
- 2024
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis studies two problems defined on complex networks, of which the first explores a conceivable extension of structural balance theory and the other concerns convergence issues in opinion dynamics. In the first half of the thesis we discuss possible definitions of structural balance conditions in a network with preference orderings as node attributes. The main result is that for the case with three alternatives (A, B, C) we reduce the (3!)3 = 216 possible configurations of triangles to 10 equivalence classes, and use these as measures of balance of a triangle towards possible extensions of structural balance theory. Moreover, we derive a general formula for the number of equivalent classes for preferences on n alternatives. Finally, we analyze a real-world data set and compare its empirical distribution of triangle equivalence classes to a null hypothesis in which preferences are randomly assigned to the nodes. The second half of the thesis concerns an opinion dynamics model in which each agent takes a random Bernoulli distributed action whose probability is updated at each discrete time step, and we prove that this model converges almost surely to consensus. We also provide a detailed critique of a claimed proof of this result in the literature. We generalize the result by proving that the assumption of irreducibility in the original model is not necessary. Furthermore, we prove as a corollary of the generalized result that the almost sure convergence to consensus holds also in the presence of a fully stubborn agent which never changes its opinion. In addition, we show that the model, in both the original and generalized cases, converges to consensus also in rth moment.
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| 4. |
- Kunnath Ganesan, Unnikrishnan, 1989-, et al.
(författare)
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Cell-Free Massive MIMO With Multi-Antenna Users and Phase Misalignments: A Novel Partially Coherent Transmission Framework
- 2024
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Ingår i: IEEE Open Journal of the Communications Society. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 2644-125X. ; 5, s. 1639-1655
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Tidskriftsartikel (refereegranskat)abstract
- Cell-free massive multiple-input multiple-output (MIMO) is a promising technology for next-generation communication systems. This work proposes a novel partially coherent (PC) transmission framework to cope with the challenge of phase misalignment among the access points (APs), which is important for unlocking the full potential of cell-free massive MIMO technology. With the PC operation, the APs are only required to be phase-aligned within clusters. Each cluster transmits the same data stream towards each user equipment (UE), while different clusters send different data streams. We first propose a novel algorithm to group APs into clusters such that the distance between two APs is always smaller than a reference distance ensuring the phase alignment of these APs. Then, we propose new algorithms that optimize the combining at UEs and precoding at APs to maximize the downlink sum data rates. We also propose a novel algorithm for data stream allocation to further improve the sum data rate of the PC operation. Numerical results show that the PC operation using the proposed framework with a sufficiently small reference distance can offer a sum rate close to the sum rate of the ideal fully coherent (FC) operation that requires network-wide phase alignment. This demonstrates the potential of PC operation in practical deployments of cell-free massive MIMO networks.
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