52,493 research outputs found
Off-Axis monitoring of the laser beam directed energy deposition process of metals (DED-LB/M)
No full textLa Fabbricazione Additiva (AM) ha rivoluzionato la progettazione e la produzione di componenti metallici
complessi, offrendo flessibilità e personalizzazione senza pari. Tra i vari processi di AM, il processo di
Deposizione Diretta di Metalli mediante Fascio Laser (DED-LB) eccelle nella costruzione, riparazione e
miglioramento di parti metalliche, apportando vantaggi in particolare ai settori ad alte prestazioni come
quello aerospaziale. Tuttavia, la sua complessità e variabilità portano spesso a difetti che compromettono
la qualità finale delle parti, sottolineando la necessità di strategie avanzate di monitoraggio e controllo.
Questa tesi indaga le tecniche di monitoraggio fuori-asse per il processo DED-LB/M focalizzandosi sul
miglioramento della rilevazione dei difetti e sull'ottimizzazione della qualità, efficienza e affidabilità del
processo attraverso nuove soluzioni. Il monitoraggio off-axis ha il vantaggio significativo di catturare dati
spaziali relativi al bagno di fusione e alla dinamica della deposizione che spesso non sono visibili dalle
prospettive dei dispositivi in asse, ed è anche relativamente più facile da implementare. La ricerca esplora
metodi di monitoraggio termico e ottico, insieme ad approcci guidati dall'intelligenza artificiale (IA), per
sviluppare un quadro completo di monitoraggio in-process per il processo DED-LB/M.
Lo studio esamina l'evoluzione, i vantaggi e le applicazioni della AM in vari settori, con un focus specifico
sul DED-LB/M. Vengono analizzati i componenti chiave del processo, come il sistema laser, il meccanismo
di alimentazione della polvere e i sistemi di controllo, evidenziandone le applicazioni, come la produzione
di parti 3D complesse e la riparazione di componenti meccanici, mediante l’uso di materiali comuni nel
settore aerospaziale, come acciai, superleghe a base di nichel e leghe di titanio.
Vengono esaminate le tecnologie di monitoraggio all'avanguardia esistenti, seguite dalla presentazione di
diversi casi studio sul monitoraggio termico e ottico utilizzando telecamere CCD/CMOS. Questi sistemi
analizzano l'evoluzione della pozza fusa, consentendo di rilevare, durante il processo di deposizione,
distorsioni geometriche e anomalie come spegnimenti del laser e interruzioni del flusso di polvere. Viene
inoltre introdotto un sistema di monitoraggio multi-vista, che offre soluzioni flessibili adatte a varie
configurazioni di DED-LB/M, in grado di catturare con maggior accuratezza l'evoluzione spaziale della pozza
fusa.
Viene presentato uno studio comparativo tra nuovi metodi di monitoraggio mediante scansione laser e
quelli basati sull’uso di telecamere CMOS ad alta risoluzione che mette in evidenza i rispettivi punti di forza
e ne suggerisce le relative applicazioni ottimali. Sono stati sviluppati indicatori chiave di prestazione (KPIs)
per la rilevazione di difetti geometrici e sub-superficiali attraverso algoritmi personalizzati di elaborazione
delle immagini, validati da analisi metallografiche. Viene proposto un approccio innovativo per la
valutazione in-process dell'efficienza di deposizione, utilizzando i dati di uno scanner laser prototipale per
identificare deviazioni del volume delle tracce depositate rispetto a quello previsto.
Sono state applicate tecniche di IA, per la prima volta nella letteratura scientifica, per monitorare in modo
innovativo la distanza di standoff e rilevare l'eventuale otturazione dell'ugello. In particolare, un modello
di rilevamento degli oggetti, appositamente addestrato, migliora la sicurezza e il controllo del processo
avvisando gli utenti su condizioni critiche come l'otturazione dell'ugello o la diminuzione della distanza di
standoff. D’altro canto, un modello di machine learning (ML) getta le basi per poter prevedere le modifiche
ottimali da apportare a parametri di processo chiave, come la distanza di standoff o la potenza laser, per
migliorare la precisione dimensionale dei prodotti attraverso un approccio “feed forward”.
Attraverso questi casi studio, questa tesi fornisce preziose informazioni sulle tecniche di monitoraggio
fuori-asse, contribuendo a nuove soluzioni di controllo qualità nella fabbricazione additiva dei metalli e al
progresso del processo DED-LB/M.Additive Manufacturing (AM) has revolutionized the design and production of complex metal components,
offering unparalleled flexibility and customization. Among the various AM processes, the Laser Beam
Directed Energy Deposition (DED-LB) process excels in the construction, repair, and enhancement of metal
parts, particularly benefiting high-performance industries such as aerospace. However, its complexity and
variability often result in defects that compromise the quality of the final parts, underscoring the need for
advanced monitoring and control strategies.
This thesis investigates off-axis monitoring techniques for DED-LB/M, focusing on improving defect
detection and optimizing process efficiency and reliability through new solutions. Off-axis monitoring has
the significant advantage of capturing spatial data related to the melt pool and deposition dynamics, which
are often not visible from on-axis perspectives, and is also relatively easier to implement. The research
explores thermal and optical monitoring methods, along with artificial intelligence (AI) driven approaches,
to develop a comprehensive in-process monitoring framework for DED-LB/M.
The study examines the evolution, benefits, and applications of AM across various industries, with a
specific focus on DED-LB/M. Key process components, such as the laser system, powder feeding
mechanism, and control systems, are analyzed, highlighting applications like the production of complex
3D parts and the repair of mechanical components using materials crucial in the aerospace sector, such as
steels, nickel-based superalloys, and titanium alloys.
Existing state-of-the-art monitoring technologies are reviewed, followed by the presentation of several
case studies on thermal and optical monitoring using CCD/CMOS cameras. These systems analyze the
evolution of the melt pool, allowing for in-process detection of geometric distortions and anomalies such
as laser shutdowns and powder flow interruptions. A multi-view monitoring system is also introduced,
offering flexible solutions suitable for various DED-LB/M configurations, capable of more accurately
capturing the spatial evolution of the melt pool.
A comparative study between new laser scanning monitoring methods and those based on high-resolution
CMOS cameras is presented, highlighting their respective strengths and suggesting optimal applications.
Key performance indicators (KPIs) for detecting geometric and subsurface defects were developed through
custom image processing algorithms, validated against metallographic analyses. An innovative approach
for in-process evaluation of deposition efficiency is proposed, utilizing prototype laser scanner data to
identify deviations in the deposited track volume.
AI techniques have been applied for the first time in the scientific literature to monitor standoff distance
and detect potential nozzle clogging. Specifically, a custom-trained object detection model enhances
process safety and control by alerting users to critical conditions such as nozzle clogging or reduced
standoff distance. On the other hand, a machine learning (ML) model lays the groundwork for predicting
optimal adjustments to key process parameters, such as standoff distance or laser power, to improve the
dimensional accuracy of the products through a feed forward approach.
Through these case studies, this thesis provides valuable insights into off-axis monitoring techniques,
contributing to new quality control solutions in metal additive manufacturing and advancing the DED
LB/M process
Active aging and elderly's quality of life: Comparing the impact on literature of projects funded by the European Union and USA
Get PDFBackground: The objective of this research is to verify whether European projects on Active Aging (AA) and Elderly Quality of Life (Qol) funded by the Seventh Framework Programme (FP7) produce an impact on literature similar to projects funded by the National Health Institute (NHI) of the United States on international literature using well-known bibliometric indicators. This effort may be useful in developing standardized and replicable procedures. Methods: Fifteen randomly selected projects on AA and Elderly Qol concluded in August 2017 and funded by FP7 were compared to similar projects funded by the US NHI with reference to papers published (Scopus and Scholar), papers published in Q1 journals, and the number of citations of the papers linked to the projects. Results: In all the indicators considered, the European projects showed no difference with the US NHI projects. Conclusions: The EU-funded AA and Qol Elderly projects have an impact on scientific literature comparable to projects funded in the United States by the NHI Agency. Our results are consistent with the data on general medical research, which indicates that, European research remains at a high level of competitiveness. In this experimental study, our methodology appeared to be convincing and reliable and it could be applied to the extent of the impact of more extensive research areas. Our research did not evaluate the relationship between funding required by research and scientific productivity
The usefulness of calcitonin assay in fine needle aspiration biopsy (FNAB) fluid of neck masses of patients with primitive and metastatic medullary thyroid carcinoma (MTC).
No full textObjective: To evaluate the usefulness of calcitonin (CT) assay in fine needle aspiration biopsy (CT-FNAB) wash-out fluid alone or combined to cytology in the pre-surgical study of medullary thyroid carcinoma (MTC) patients with thyroid nodules and of suspicious neck MTC recurrences/metastases.
Subjects and methods: Thirthy-six ultrasound (US)-guided FNAB were performed in neck masses from 23 patients with borderline or high basal and pentagastrin (PG) stimulated serum CT. Cytology and CT-FNAB were performed on a total of 18 thyroid nodules (TN) and 3 neck lymph nodes (LN) from 12 patients examined before thyroidectomy and on 6 suspicious local recurrences (LR) and 9 LN from 9 totally thyroidectomized MTC patients. On the basis of CT-FNAB values found in 15 non-MTC lesions, CT-FNAB >36 pg/ml was considered as indicative of MTC.
Results: All the 21 positive CT-FNAB lesions (10 TN, 6 LN and 5 LR), 13 with positive cytology, were confirmed as MTC at histology. Among the 15 negative CT-FNAB suspicious masses (8 TN, 6 LN and 1 LR) 5 displayed a benign lesion at histology. The remaining 10 cases, all with benign cytology, were not operated and no evidence of MTC has been detected at follow-up. CT-FNAB reached 100% of sensitivity and specificity for MTC, while cytology displayed 61.9% of sensitivity and 80% of specificity when calculated in the operated lesions.
Conclusions: US-guided CT-FNAB resulted the best tool to identify primary MTC and local recurrences/node metastases in MTC operated subjects with much more sensitivity and specificity than cytology. This may have important implications in the management of MTC patients
Turismo crocieristico e percezioni della popolazione residente: i risultati di un’analisi cluster
No full textUtilità dell’ecocolor Doppler nella diagnosi differenziale tra gozzo multinodulare tossico e variante multinodulare del morbo di Basedow.
No full textScopo. Valutare l’utilità della ecotomografia tiroidea tradizionale e color Doppler nella diagnosi differenziale tra ipertiroidismo da gozzo multinodulare tossico (non autoimmune) e variante multinodulare del morbo di Basedow (autoimmune).
Materiali e Metodi. Sono stati studiati 50 pazienti ipertiroidei non trattati (6 maschi, 44 femmine; età media 48 anni). Di questi 24 erano affetti da gozzo diffuso (gruppo A) e 26 da gozzo multinodulare (gruppo B). Di questi ultimi, 18 (gruppo B1) presentavano segni clinici (oftalmopatia basedowiana) e/o biochimici (positività per autoanticorpi antitiroide) compatibili con ipertiroidismo autoimmune (variante multinodulare del morbo di Basedow) e 8 (gruppo B2) senza alcun segno clinico né biochimico di ipertiroidismo autoimmune. In tutti i casi è stata eseguita ecografia tiroidea tradizionale e color Doppler e i dati ottenuti sono stati confrontati in ciascun paziente con i parametri biochimici e scintigrafici.
Risultati. Le differenze ecografiche più significative tra il gruppo B1 e B2 erano a carico del parenchima extranodulare, che risultava nel solo gruppo B1 ipoecogeno, con vascolarizzazione parenchimale (VP) quasi esclusivamente di pattern di tipo IV e con elevati picchi sistolici parenchimali (PSP) analoghi a quanto osservato nel gruppo A. Per contro, un aumento della vascolarizzazione nodulare (VN) con pattern tipo IIIb ed elevati picchi sistolici nodulari (PSN) era rilevata solo a livello dei noduli nei pazienti del gruppo B2. Degno di rilievo era che la scintigrafia non consentiva una chiara differenziazione tra i gruppi B1 e B2.
Conclusioni. L’ecografia tiroidea tradizionale, integrata con color Doppler, può rappresentare un semplice ed efficace presidio diagnostico nella diagnosi differenziale tra gozzo multinodulare tossico non autoimmune e la variante multinodulare del morbo di Basedow
Interstitial lung disease in systemic autoimmune rheumatic diseases: a comprehensive review
No full textBackground: Interstitial lung diseases (ILDs) are among the most serious complications associated with
systemic rheumatic diseases, and lead to significant morbidity and mortality; they may also be the first
manifestation of connective tissue diseases (CTDs). The aim of this narrative review is to summarise the
data concerning the pathogenesis of CTD/ILD and its distinguishing features in different rheumatic
diseseas.
Areas covered: The pathogenesis, clinical aspects and treatment of ILD associated with rheumatic
systemic diseases and CTDs were reviewed by searching the PubMed, Medline, and Cochrane Library
databases for papers published between 1995 and February 2017 using combinations of words or
terms. Articles not written in English were excluded.
Expert commentary: The management of CTD–ILD is challenging because of the lack of robust data
regarding the treatments used, the heterogeneity of the diseases themselves, and the scarcity of welldefined
outcome measures. Treatment decisions are often made clinically on the basis of functional,
radiographic progression, and exacerbating factors such as age and the burden of comorbidities. Given
the complexities of diagnosis and the paucity of treatment trials, the management of CTD patients with ILD
requires multidisciplinary collaboration between rheumatologists and pulmonologists in CTD-ILD clinics
Beamforming designs for low-power transceiver structures in large-scale MIMO systems
Get PDFAbstract
This dissertation investigates efficient beamforming techniques for low-power transceiver architectures in large-scale multiple-input multiple-output (MIMO) systems, a key technology in millimeter-wave (mmWave) and terahertz (THz) wireless communications. While these systems enable extremely high data rates, they are often associated with significant power consumption. Digital beamforming (DBF) and hybrid beamforming (HBF) approaches, combined with low-resolution analog-to-digital converters (ADCs), offer promising solutions to these challenges.
The study begins by exploring phase shifter-based HBF transceiver architectures, which are vulnerable to beam squint effects that significantly degrade spectral efficiency (SE) in wideband systems. Analyses reveal that beam squint severity depends on the antenna array structure and fractional bandwidth. To mitigate the beam squint, efficient beamforming designs are proposed to maximize the SE. Switch-based HBF architectures are extensively studied, demonstrating greater robustness to beam squint, superior SE and energy efficiency (EE) than phase shifter-based designs.
Next, the focus shifts to low-resolution ADCs, where quantization distortion (QD) complicates performance optimization. An efficient joint transmit-receive beamforming and bit allocation scheme is proposed for DBF architectures, achieving significant SE and EE improvements over traditional beamforming schemes. Additionally, novel digital and hybrid precoding designs are introduced to maximize the SE given low-resolution receivers. The results highlight the potential of the partially connected HBF architecture in achieving comparable SE to the DBF and fully connected HBF schemes while requiring significantly less power consumption.
Extending to wideband systems, the joint effects of oversampling, signal-to-noise ratio (SNR) and ADC resolutions for SE are examined, showing that oversampling can effectively mitigate QD at high SNRs. With sufficiently large oversampling ratios and SNR, QD becomes negligible. Building on these insights, a hybrid receiver design with oversampling is proposed for low-resolution systems. The design demonstrates more resilience to the beam squint than in full-resolution systems. It showcases improved EE over the digital receiver when slightly more quantization bits (≥4) per radio-frequency chain are employed. Original papers Ma, M., Nguyen, N. T., & Juntti, M. (2025). Switch-based hybrid beamforming transceiver design for wideband communications with beam squint. IEEE Transactions on Vehicular Technology, 74(2), 2840–2855. https://doi.org/10.1109/TVT.2024.3470530 https://doi.org/10.1109/TVT.2024.3470530 Self-archived version Ma, M., Nguyen, N. T., & Juntti, M. (2023). Beam squint analysis and mitigation via hybrid beamforming design in THz communications. ICC 2023 - IEEE International Conference on Communications, 6486–6491. https://doi.org/10.1109/ICC45041.2023.10279725 https://doi.org/10.1109/ICC45041.2023.10279725 Self-archived version Ma, M., Nguyen, N. T., Atzeni, I., & Juntti, M. (2025). Joint beamforming design and bit allocation in massive MIMO with resolution-adaptive ADCs. IEEE Transactions on Wireless Communications. Advance online publication. https://doi.org/10.48550/ARXIV.2407.03796 https://doi.org/10.48550/ARXIV.2407.03796 Ma, M., Nguyen, N. T., Atzeni, I., Swindlehurst, A. L., & Juntti, M. (2024). Digital and hybrid precoding designs in massive MIMO with low-resolution ADC. Manuscript submitted for publication. Ma, M., Nguyen, N. T., Atzeni, I., & Juntti, M. (2023). Analysis of oversampling in uplink massive MIMO-OFDM with low-resolution ADCs. 2023 IEEE 24th International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), 626–630. https://doi.org/10.1109/SPAWC53906.2023.10304436 https://doi.org/10.1109/SPAWC53906.2023.10304436 Self-archived version Ma, M., Nguyen, N. T., Atzeni, I., & Juntti, M. (in press). Hybrid receiver design for massive MIMO-OFDM with low-resolution ADCs and oversampling. IEEE Wireless Communications and Networking Conference (WCNC). Tiivistelmä
Tässä väitöskirjassa tutkitaan tehokkaita keilanmuodostustekniikoita matalatehoisille lähetin-vastaanotinarkkitehtuureille suurissa monitulo-monilähtöjärjestelmissä (multiple-input multiple-output, MIMO), joiden teknologia on keskeisessä asemassa millimetriaaltojen (mmWave) ja terahertsitaajuuksien hyödyntämisessä langattomassa tietoliikenteessä. Vaikka nämä järjestelmät mahdollistavat erittäin suuret tiedonsiirtonopeudet, niihin liittyy usein merkittävää virrankulutusta. Digitaaliset keilanmuodostus- (digital beamforming, DBF) ja hybridikeilanmuodostusratkaisut (hybrid beamforming, HBF) yhdistettynä matalaresoluutioisiin analogi-digitaalimuuntimiin (ADC) tarjoavat lupaavia ratkaisuja näihin haasteisiin.
Työ alkaa tarkastelemalla vaiheensiirtimiin perustuvien HBF-lähetin-vastaanottimien arkkitehtuureja, jotka ovat laajakaistajärjestelmissä alttiita spektrin käytön tehokkuutta (spectral efficiency, SE) merkittävästi heikentävälle keilan vinoutumalle (beam squint). Analyysit osoittavat, että keilan vinoutuman vakavuus riippuu antenniryhmän rakenteesta ja murtokaistanleveydestä. Tämän ilmiön lieventämiseksi ehdotetaan tehokkaita keilanmuodostusmenetelmiä SE:n maksimoimiseksi. Kytkimiin perustuvia HBF-arkkitehtuureja on tutkittu laajasti, ja ne osoittavat parempaa sietokykyä keilan vinoutumaa vastaan sekä parempaa SE:tä ja energiatehokkuutta (energy efficiency, EE) verrattuna vaiheensiirtimiin perustuviin ratkaisuihin.
Seuraavaksi keskitytään matalaresoluutioisiin analogi-digitaalimuuntimiin (ADC), joissa kvantisointisärö (quantization distortion, QD) vaikeuttaa suorituskyvyn optimointia. Työssä ehdotetaan DBF-arkkitehtuureille tehokasta lähetin-vastaanottimen keilanmuodostus- ja bittijaon yhteissuunnittelua, jolla saavutetaan merkittäviä SE- ja EE-parannuksia perinteisiin menetelmiin verrattuna. Lisäksi uusia digitaalisia ja hybridiesikoodausmenetelmiä kehitetään SE:n maksimoimiseksi matalaresoluutioisilla vastaanottimilla. Tulokset korostavat osittain kytketyn HBF-arkkitehtuurin potentiaalia saavuttaa DBF-ratkaisuihin ja täysin kytkettyihin HBF-ratkaisuihin verrattavissa oleva SE, mutta huomattavasti pienemmällä virrankulutuksella.
Tutkimus siirtyy lopuksi laajakaistajärjestelmiin, joissa tarkastellaan ylinäytteistyksen, signaali-kohinasuhteen (signal-to-noise ratio, SNR) ja ADC-resoluutioiden yhteisvaikutuksia SE:hen. Tulokset osoittavat, että ylinäytteistyksellä voidaan tehokkaasti vähentää QD:tä korkeilla SNR-arvoilla. Riittävän suurilla ylinäytteistyssuhteilla ja SNR-arvoilla QD muuttuu merkityksettömäksi. Näiden havaintojen pohjalta matalaresoluutioisille järjestelmille ehdotetaan hybridivastaanotinarkkitehtuuria ylinäytteistyksellä. Ehdotettu malli osoittaa parempaa sietokykyä keilan vinoutumaa vastaan kuin täyden resoluution järjestelmissä. Lisäksi malli tarjoaa parannetun EE:n digitaaliseen vastaanottimeen verrattuna, kun radiotaajuusketjun kvantisointitarkkuus ylittää nimenomaisen kynnyksen (≥4 bittiä). Osajulkaisut Ma, M., Nguyen, N. T., & Juntti, M. (2025). Switch-based hybrid beamforming transceiver design for wideband communications with beam squint. IEEE Transactions on Vehicular Technology, 74(2), 2840–2855. https://doi.org/10.1109/TVT.2024.3470530 https://doi.org/10.1109/TVT.2024.3470530 Rinnakkaistallennettu versio Ma, M., Nguyen, N. T., & Juntti, M. (2023). Beam squint analysis and mitigation via hybrid beamforming design in THz communications. ICC 2023 - IEEE International Conference on Communications, 6486–6491. https://doi.org/10.1109/ICC45041.2023.10279725 https://doi.org/10.1109/ICC45041.2023.10279725 Rinnakkaistallennettu versio Ma, M., Nguyen, N. T., Atzeni, I., & Juntti, M. (2025). Joint beamforming design and bit allocation in massive MIMO with resolution-adaptive ADCs. IEEE Transactions on Wireless Communications. Advance online publication. https://doi.org/10.48550/ARXIV.2407.03796 https://doi.org/10.48550/ARXIV.2407.03796 Ma, M., Nguyen, N. T., Atzeni, I., Swindlehurst, A. L., & Juntti, M. (2024). Digital and hybrid precoding designs in massive MIMO with low-resolution ADC. Manuscript submitted for publication. Ma, M., Nguyen, N. T., Atzeni, I., & Juntti, M. (2023). Analysis of oversampling in uplink massive MIMO-OFDM with low-resolution ADCs. 2023 IEEE 24th International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), 626–630. https://doi.org/10.1109/SPAWC53906.2023.10304436 https://doi.org/10.1109/SPAWC53906.2023.10304436 Rinnakkaistallennettu versio Ma, M., Nguyen, N. T., Atzeni, I., & Juntti, M. (in press). Hybrid receiver design for massive MIMO-OFDM with low-resolution ADCs and oversampling. IEEE Wireless Communications and Networking Conference (WCNC). Academic dissertation to be presented with the assent of the Doctoral Programme Committee of Information Technology and Electrical Engineering of the University of Oulu for public defence in the OP auditorium (L10), Linnanmaa, on 18 June 2025, at 12 noonAbstract
This dissertation investigates efficient beamforming techniques for low-power transceiver architectures in large-scale multiple-input multiple-output (MIMO) systems, a key technology in millimeter-wave (mmWave) and terahertz (THz) wireless communications. While these systems enable extremely high data rates, they are often associated with significant power consumption. Digital beamforming (DBF) and hybrid beamforming (HBF) approaches, combined with low-resolution analog-to-digital converters (ADCs), offer promising solutions to these challenges.
The study begins by exploring phase shifter-based HBF transceiver architectures, which are vulnerable to beam squint effects that significantly degrade spectral efficiency (SE) in wideband systems. Analyses reveal that beam squint severity depends on the antenna array structure and fractional bandwidth. To mitigate the beam squint, efficient beamforming designs are proposed to maximize the SE. Switch-based HBF architectures are extensively studied, demonstrating greater robustness to beam squint, superior SE and energy efficiency (EE) than phase shifter-based designs.
Next, the focus shifts to low-resolution ADCs, where quantization distortion (QD) complicates performance optimization. An efficient joint transmit-receive beamforming and bit allocation scheme is proposed for DBF architectures, achieving significant SE and EE improvements over traditional beamforming schemes. Additionally, novel digital and hybrid precoding designs are introduced to maximize the SE given low-resolution receivers. The results highlight the potential of the partially connected HBF architecture in achieving comparable SE to the DBF and fully connected HBF schemes while requiring significantly less power consumption.
Extending to wideband systems, the joint effects of oversampling, signal-to-noise ratio (SNR) and ADC resolutions for SE are examined, showing that oversampling can effectively mitigate QD at high SNRs. With sufficiently large oversampling ratios and SNR, QD becomes negligible. Building on these insights, a hybrid receiver design with oversampling is proposed for low-resolution systems. The design demonstrates more resilience to the beam squint than in full-resolution systems. It showcases improved EE over the digital receiver when slightly more quantization bits (≥4) per radio-frequency chain are employed.Tiivistelmä
Tässä väitöskirjassa tutkitaan tehokkaita keilanmuodostustekniikoita matalatehoisille lähetin-vastaanotinarkkitehtuureille suurissa monitulo-monilähtöjärjestelmissä (multiple-input multiple-output, MIMO), joiden teknologia on keskeisessä asemassa millimetriaaltojen (mmWave) ja terahertsitaajuuksien hyödyntämisessä langattomassa tietoliikenteessä. Vaikka nämä järjestelmät mahdollistavat erittäin suuret tiedonsiirtonopeudet, niihin liittyy usein merkittävää virrankulutusta. Digitaaliset keilanmuodostus- (digital beamforming, DBF) ja hybridikeilanmuodostusratkaisut (hybrid beamforming, HBF) yhdistettynä matalaresoluutioisiin analogi-digitaalimuuntimiin (ADC) tarjoavat lupaavia ratkaisuja näihin haasteisiin.
Työ alkaa tarkastelemalla vaiheensiirtimiin perustuvien HBF-lähetin-vastaanottimien arkkitehtuureja, jotka ovat laajakaistajärjestelmissä alttiita spektrin käytön tehokkuutta (spectral efficiency, SE) merkittävästi heikentävälle keilan vinoutumalle (beam squint). Analyysit osoittavat, että keilan vinoutuman vakavuus riippuu antenniryhmän rakenteesta ja murtokaistanleveydestä. Tämän ilmiön lieventämiseksi ehdotetaan tehokkaita keilanmuodostusmenetelmiä SE:n maksimoimiseksi. Kytkimiin perustuvia HBF-arkkitehtuureja on tutkittu laajasti, ja ne osoittavat parempaa sietokykyä keilan vinoutumaa vastaan sekä parempaa SE:tä ja energiatehokkuutta (energy efficiency, EE) verrattuna vaiheensiirtimiin perustuviin ratkaisuihin.
Seuraavaksi keskitytään matalaresoluutioisiin analogi-digitaalimuuntimiin (ADC), joissa kvantisointisärö (quantization distortion, QD) vaikeuttaa suorituskyvyn optimointia. Työssä ehdotetaan DBF-arkkitehtuureille tehokasta lähetin-vastaanottimen keilanmuodostus- ja bittijaon yhteissuunnittelua, jolla saavutetaan merkittäviä SE- ja EE-parannuksia perinteisiin menetelmiin verrattuna. Lisäksi uusia digitaalisia ja hybridiesikoodausmenetelmiä kehitetään SE:n maksimoimiseksi matalaresoluutioisilla vastaanottimilla. Tulokset korostavat osittain kytketyn HBF-arkkitehtuurin potentiaalia saavuttaa DBF-ratkaisuihin ja täysin kytkettyihin HBF-ratkaisuihin verrattavissa oleva SE, mutta huomattavasti pienemmällä virrankulutuksella.
Tutkimus siirtyy lopuksi laajakaistajärjestelmiin, joissa tarkastellaan ylinäytteistyksen, signaali-kohinasuhteen (signal-to-noise ratio, SNR) ja ADC-resoluutioiden yhteisvaikutuksia SE:hen. Tulokset osoittavat, että ylinäytteistyksellä voidaan tehokkaasti vähentää QD:tä korkeilla SNR-arvoilla. Riittävän suurilla ylinäytteistyssuhteilla ja SNR-arvoilla QD muuttuu merkityksettömäksi. Näiden havaintojen pohjalta matalaresoluutioisille järjestelmille ehdotetaan hybridivastaanotinarkkitehtuuria ylinäytteistyksellä. Ehdotettu malli osoittaa parempaa sietokykyä keilan vinoutumaa vastaan kuin täyden resoluution järjestelmissä. Lisäksi malli tarjoaa parannetun EE:n digitaaliseen vastaanottimeen verrattuna, kun radiotaajuusketjun kvantisointitarkkuus ylittää nimenomaisen kynnyksen (≥4 bittiä)
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