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Beyond Boundaries: Evolving Connectivity with Massive MIMO
Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.ORCID iD: 0000-0001-9504-3975
2024 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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. 

Abstract [sv]

Den kommande sjätte generationens (6G) trädlösa nätverk innebär ett evolutionärt språng inom anslutningsteknik, som överträffar konventionella begränsningar genom integrationen av massiv MIMO-teknik (eng: multiple-input multiple-output). Tre primära tillämpningsscenarier som förväntas driva utvecklingen av 6G-nätverk är följande: (i) Förbättrat mobilt bredband (eMBB) utlovar blixtsnabba datatakter och utökad kapacitet för bandbreddsintensiva applikationer så som virtuell verklighet (VR) och högupplöst videoströmning, vilket är en betydande evolution inom anslutningsteknik. (ii) Ultratillförlitlig kommunikation med låg latens (URLLC) säkerställer ultratillförlitlig anslutning med låg latens som är kritisk för applikationer så som autonoma fordon och fjärrkirurgi, som ytterligare understryker utvecklingen mot mer tillförlitlig och responsiv anslutning. (iii) Massiv maskintypkommunikation (mMTC) tillgodoser anslutningsbehoven hos miljarder enheter inom sakernas internet (IoT) inom olika sektorer så som smarta städer och hälsovård, och utvidgar anslutningens gränser för att omfatta en mängd olika enheter som är sammankopplade. Dessutom prioriterar 6G-nätverk landsbygdsanslutning och syftar till att överbrygga den digitala klyftan genom att utöka höghastighetsinternet till underbetjänade områden. Genom integrationen av banbrytande trådlösa teknologier är 6G-nätverk redo att omdefiniera anslutningstekniken, erbjuda ultrasnabba, pålitliga och allmänt tillgängliga kommunikationstjänster för en mängd olika tillämpningsscenarier, och därigenom bana väg för en mer uppkopplad och rättvis framtid.

I denna avhandling utforskas olika aspekter av utvecklingen av distribuerad massiv MIMO-anslutningsteknik, vilket inkluderar: (i) Synkronisering av distribuerade åtkomstpunkter (AP:er); (ii) delvis koherent (PC) drift av AP:er; (iii) Grant-fri slumpmässig åtkomst för mMTC-enheter; (iv) Extrema multiplexegenskaper; och (v) SuperCell massiv MIMO-kommunikationsteknik.

Distribuerade AP:er drivs med oberoende lokala oscillatorer (LO:er), vilket resulterar i frekvens- och tidsasynkronisering mellan dem. I smalbandssystem medför skillnader i frekvens eller tid mellan distribuerade sändande noder relativa fasförskjutningar inom ett symbolintervall. Dessa förskjutningar måste kompenseras för att tillåta koherent signalbildning under överföring till en användare. Artikel A fördjupar sig i synkroniseringen av distribuerade AP:er i massiva MIMO-system. Denna synkronisering är avgörande för att dra nytta av fördelarna med distribuerad massiv MIMO. Artikeln analyserar synkroniseringskrav från ett reciprocitetsperspektiv och betraktar multiplikativa störningar som uppstår på grund av olikheter i radiofrekvenshårdvaror (RF-hårdvaror). Den introducerar BeamSync, ett nytt protokoll för synkronisering över luften, som kalibrerar geografiskt separerade AP:er utan att förlita sig på att skicka mätningsdata via kabel till centralenheten (CPU:n). Det huvudsakliga konceptet innebär att lobforma synkroniseringssignalen i den dominerande riktningen för kanalen mellan två AP:er. Resultaten visar att den föreslagna BeamSync-metoden presterar betydligt bättre än traditionella lobformningstekniker.

Även om framsteg som de som beskrivs i Artikel A möjliggör synkronisering inom specifika områden, förblir uppnåendet av nätverksomfattande AP-synkronisering skrämmande. Följaktligen går det att uppnå synkroniserade kluster, men fassynkronisering mellan olika kluster är en utmaning. För att ta itu med detta introducerar Artikel B ett originellt ramverk för PC som är avgörande för att förverkliga hela potentialen för cellfri massiv MIMO-teknik. Först grupperar en AP-klustringsalgoritm alla AP:er i fasjusterade kluster. Därefter maximeras de totala datatakterna i nedlänk och upplänk med hjälp av optimeringsalgoritmer för användarkombinering och AP-förkodning. Dessutom förbättrar en ny algoritm för datatilldelning de totala datatakterna för PC-drift. Resultaten visar att PC-drift närmar sig den totala datatakten för idealisk fullständigt koherent (FC) drift, vilket kräver nätverksomfattande fassynkronisering. Detta understryker potentialen för praktisk implementering av PC i cellfria massiva MIMO-nätverk. När den cellulär anslutningstekniken utvecklas blir maskintypkommunikation (MTC) allt viktigare. Artikel C går in på aktivitetsdetektion inom grant-fri slumpmässig åtkomst för mMTC i cellfria massiva MIMO-nätverk. Den introducerar en maximum likelihood-algoritm för aktivitetsdetektion och demonstrerar genom resultat att den makrodiversitet som är inneboende i cell-fri arkitektur avsevärt förbättrar prestandan för aktivitetsdetektion, särskilt i scenarier med stora täckningsområden, och överträffar samlokaliserade arkitekturer.

RadioWeaves-teknik är en ny trådlös infrastruktur designad för inomhusapplikationer som utnyttjar fördelarna med både massiv MIMO och cellfri massiv MIMO. Artikel D utforskar de extrema multiplexegenskaperna hos RadioWeaves och visar dess förmåga att leverera höga datatakter även vid låg effektförbrukning. Resultaten visar att RadioWeaves-nätverk utmärker sig genom förmågan att spatialt separera användare jämfört med konventionella samlokaliserade nätverk, vilket mildrar förluster som kan tillskrivas sidolober och väsentligt minskar kraven på sändareffekt.

Landsbygdsanslutning är en viktig aspekt att beakta vid utvecklingen av 6G-nätverk, eftersom det adresserar utmaningen att tillhandahålla högkvalitativ, pålitlig internetåtkomst till underbetjänade eller avlägsna områden. Mitt i ökningen av datatrafik inom urbana områden och det följande försummandet av landsbygdsområden på grund av lägre lönsamhet, utforskar Artikel E prestandan hos SuperCell massiv MIMO-system i landsbygdsmiljöer. Genom att ta itu med de unika utmaningarna och möjligheterna i kommunikationsmiljöer på landsbygden föreslår artikeln en innovativ lösning. Denna lösning innebär användning av upphöjda basstationer (BS:er) utrustade med sektoriserade antenner med stora aperturer, tillsammans med en algoritm för användarschemaläggning som säkerställer omfattande digital täckning. Genom rigorös analys utvärderar studien täckningsavståndet och tillhandahållandet av höga tjänstekapaciteter hos detta SuperCell-system i landsbygdskontexter.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2024. , p. 223
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 2395
National Category
Communication Systems
Identifiers
URN: urn:nbn:se:liu:diva-204492DOI: 10.3384/9789180756952ISBN: 9789180756945 (print)ISBN: 9789180756952 (electronic)OAI: oai:DiVA.org:liu-204492DiVA, id: diva2:1868584
Public defence
2024-09-24, Ada Lovelace, B-building, Campus Valla, Linköping, 09:15 (English)
Opponent
Supervisors
Available from: 2024-06-12 Created: 2024-06-12 Last updated: 2024-06-12Bibliographically approved
List of papers
1. BeamSync: Over-The-Air Synchronization for Distributed Massive MIMO Systems
Open this publication in new window or tab >>BeamSync: Over-The-Air Synchronization for Distributed Massive MIMO Systems
2024 (English)In: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, p. 1-1Article in journal (Refereed) Published
Abstract [en]

In distributed massive multiple-input multiple-output (MIMO) systems, multiple geographically separated access points (APs) communicate simultaneously with a user, leveraging the benefits of multi-antenna coherent MIMO processing and macro-diversity gains from the distributed setups. However, time and frequency synchronization of the multiple APs is crucial to achieve good performance and enable joint precoding. In this paper, we analyze the synchronization requirement among multiple APs from a reciprocity perspective, taking into account the multiplicative impairments caused by mismatches in radio frequency (RF) hardware. We demonstrate that a phase calibration of reciprocity-calibrated APs is sufficient for the joint coherent transmission of data to the user. To achieve synchronization, we propose a novel over-the-air synchronization protocol, named BeamSync, to calibrate the geographically separated APs without sending any measurements to the central processing unit (CPU) through fronthaul. We show that sending the synchronization signal in the dominant direction of the channel between APs is optimal. Additionally, we derive the optimal phase and frequency offset estimators. Simulation results indicate that the proposed BeamSync method enhances performance by 3 dB when the number of antennas at the APs is doubled. Moreover, the method performs well compared to traditional beamforming techniques.

National Category
Telecommunications
Identifiers
urn:nbn:se:liu:diva-201117 (URN)10.1109/twc.2023.3335089 (DOI)
Funder
EU, Horizon 2020, 101013425
Available from: 2024-02-22 Created: 2024-02-22 Last updated: 2024-06-12
2. Cell-Free Massive MIMO With Multi-Antenna Users and Phase Misalignments: A Novel Partially Coherent Transmission Framework
Open this publication in new window or tab >>Cell-Free Massive MIMO With Multi-Antenna Users and Phase Misalignments: A Novel Partially Coherent Transmission Framework
2024 (English)In: IEEE Open Journal of the Communications Society, E-ISSN 2644-125X, E-ISSN 2644-125X, Vol. 5, p. 1639-1655Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2024
Keywords
Cell-free massive MIMO; downlink; coherent transmission; non-coherent transmission; partially coherent transmission; precoding; combining; data stream allocation
National Category
Telecommunications
Identifiers
urn:nbn:se:liu:diva-201386 (URN)10.1109/OJCOMS.2024.3373170 (DOI)001192352200002 ()
Funder
EU, Horizon 2020, 101013425
Note

Funding Agencies|ELLIIT

Available from: 2024-03-06 Created: 2024-03-06 Last updated: 2024-06-12
3. Clustering-Based Activity Detection Algorithms for Grant-Free Random Access in Cell-Free Massive MIMO
Open this publication in new window or tab >>Clustering-Based Activity Detection Algorithms for Grant-Free Random Access in Cell-Free Massive MIMO
2021 (English)In: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, Vol. 69, no 11, p. 7520-7530Article in journal (Refereed) Published
Abstract [en]

Future wireless networks need to support massive machine type communication (mMTC) where a massive number of devices accesses the network and massive MIMO is a promising enabling technology. Massive access schemes have been studied for co-located massive MIMO arrays. In this paper, we investigate the activity detection in grant-free random access for mMTC in cell-free massive MIMO networks using distributed arrays. Each active device transmits a non-orthogonal pilot sequence to the access points (APs) and the APs send the received signals to a central processing unit (CPU) for joint activity detection. The maximum likelihood device activity detection problem is formulated and algorithms for activity detection in cell-free massive MIMO are provided to solve it. The simulation results show that the macro diversity gain provided by the cell-free architecture improves the activity detection performance compared to co-located architecture when the coverage area is large.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2021
Keywords
Activity Detection, Grant-Free Random Access, Cell-Free massive MIMO, massive machine-type communications (mMTC), Internet-of-Things (IoT)
National Category
Communication Systems
Identifiers
urn:nbn:se:liu:diva-179552 (URN)10.1109/TCOMM.2021.3102635 (DOI)000719563500032 ()2-s2.0-85112198086 (Scopus ID)
Funder
Swedish Research Council
Note

Funding agencies: Unnikrishnan Kunnath Ganesan and Erik G. Larsson were supported in part by ELLIIT and in part by Swedish Research Council (VR). Emil Bjornson was supported by the Grant 2019-05068 from the Swedish Research Council. This article was presented at 21st IEEE International Workshop on Signal Processing Advances in Wireless Communications (SPAWC 2020) [1].

Available from: 2021-09-24 Created: 2021-09-24 Last updated: 2024-06-12Bibliographically approved
4. RadioWeaves for Extreme Spatial Multiplexing in Indoor Environments
Open this publication in new window or tab >>RadioWeaves for Extreme Spatial Multiplexing in Indoor Environments
2020 (English)In: 2020 54th Asilomar Conference on Signals, Systems, and Computers, Pacific Grove, CA, USA, 2020, p. ¨1007-1011Conference paper, Published paper (Refereed)
Abstract [en]

With the advances in virtual and augmented reality, gaming applications, and entertainment, certain indoor scenarios will require vastly higher capacity than what can be delivered by 5G. In this paper, we focus on massive MIMO for indoor environments. We provide a case study of the distributed deployment of the antenna elements over the walls of a room and not restricting the antenna separation to be half the wavelength. This is a new paradigm of massive MIMO antenna deployment, introduced in [1] under the name RadioWeaves. We investigate different antenna deployment scenarios in line of sight communication. We observe that the RadioWeaves deployment can spatially separate users much better than a conventional co-located deployment, which outweighs the losses caused by grating lobes and thus saves a lot on transmit power. Through simulations, we show that the RadioWeaves technology can provide high rates to multiple users by spending very little power at the transmitter compared to a co-located deployment.

Place, publisher, year, edition, pages
Pacific Grove, CA, USA: , 2020
Series
IEEE, ISSN 1058-6393, E-ISSN 2576-2303
National Category
Communication Systems
Identifiers
urn:nbn:se:liu:diva-178180 (URN)10.1109/IEEECONF51394.2020.9443342 (DOI)000681731800194 ()978-0-7381-3126-9 (ISBN)978-1-6654-4707-2 (ISBN)
Conference
2020 54th Asilomar Conference on Signals, Systems, and Computers, Pacific Grove, CA, USA, 1-4 Nov. 2020
Funder
ELLIIT - The Linköping‐Lund Initiative on IT and Mobile Communications
Note

Funding: ELLIIT; Swedish Research Council (VR)Swedish Research Council

Available from: 2021-08-11 Created: 2021-08-11 Last updated: 2024-06-12Bibliographically approved
5. Bridging the Digital Divide Using SuperCell Massive MIMO
Open this publication in new window or tab >>Bridging the Digital Divide Using SuperCell Massive MIMO
2022 (English)In: 2022 IEEE 96th Vehicular Technology Conference (VTC2022-Fall), London, United Kingdom: IEEE, 2022, , p. 6Conference paper, Published paper (Refereed)
Abstract [en]

Massive multiple input multiple output (MIMO)emerged as the leading technology for supporting fifth generation(5G) and beyond 5G cellular communication systems. Due to thetremendous increase in data traffic in urban areas and to meetsuch a significant demand, most studies consider macro/micro celldeployments in urban environments. Internet service providers(ISPs) are less interested in providing communication services inrural areas considering the relatively low profits compared to thedeployment and maintenance costs. In this paper, we investigatethe massive MIMO performance in rural scenarios. In particular,we investigate different aspects to consider while designing along-range communication system. We propose to use elevatedbase station (BS) with sectorized antennas with unusually largeaperture and implement a user scheduling algorithm at theBS to provide full digital coverage. We analyze the coveragerange of a massive MIMO system to provide high-rate services.Furthermore, we also analyze the link budget requirements andthe rates users can achieve in such a SuperCell massive MIMOnetwork.

Place, publisher, year, edition, pages
London, United Kingdom: IEEE, 2022. p. 6
Series
IEEE Conference on Vehicular Technology (VTC), ISSN 2577-2465, E-ISSN 1090-3038
Keywords
Massive MIMO; SuperCell; digital divide; scalability; coverage
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:liu:diva-191098 (URN)10.1109/VTC2022-Fall57202.2022.10012724 (DOI)000927580600032 ()9781665454681 (ISBN)9781665454698 (ISBN)
Conference
2022 IEEE 96th Vehicular Technology Conference (VTC2022-Fall), London, United Kingdom, 26-29 September, 2022
Note

Funding: ELLIIT; Swedish Research Council (VR); KAW foundation

Available from: 2023-01-18 Created: 2023-01-18 Last updated: 2024-06-12Bibliographically approved

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