liu.seSearch for publications in DiVA
1 - 4 of 4
rss atomLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • oxford
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
  • Presentation: 2020-02-27 10:15 Systemet, B-Building, Linköping
    Becirovic, Ema
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    On Massive MIMO for Massive Machine-Type Communications2020Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    To cover all the needs and requirements of mobile networks in the future, the predicted usage of the mobile networks has been split into three use-cases: enhanced mobile broadband, ultra-reliable low-latency communication, and massive machine-type communication. In this thesis we focus on the massive machine-type communication use-case which is intended to facilitate the ever increasing number of smart devices and sensors.

    In the massive machine-type communication use-case, the main challenges are to accommodate a huge number of devices while keeping the battery lives of the devices long, and allowing them to be placed in far-away locations. However, these devices are not concerned about other features such as latency, high data rate, or mobility.

    In this thesis we study the application of massive MIMO (multiple-input multiple-output) technology for the massive machine-type communication use-case. Massive MIMO has been on the radar as an enabler for future communication networks in the last decade and is now firmly rooted in both academia and industry. The main idea of massive MIMO is to utilize a base station with a massive number of antennas which gives the ability to spatially direct signals and serve multiple devices in the same time- and frequency resource.

    More specifically, in this thesis we study A) a scenario where the base station takes advantage of a device's low mobility to improve its channel estimate, B) a random access scheme for massive machine-type communication which can accommodate a huge number of devices, and C) a case study where the benefits of massive MIMO for long range devices are quantified. The results are that the base station can significantly improve the channel estimates for a low mobility user such that it can tolerate lower SNR while still achieving the same rate. Additionally, the properties of massive MIMO greatly helps to detect users in random access scenarios and increase link-budgets compared to single-antenna base stations.  

    List of papers
    1. DETECTION OF PILOT-HOPPING SEQUENCES FOR GRANT-FREE RANDOM ACCESS IN MASSIVE MIMO SYSTEMS
    Open this publication in new window or tab >>DETECTION OF PILOT-HOPPING SEQUENCES FOR GRANT-FREE RANDOM ACCESS IN MASSIVE MIMO SYSTEMS
    2019 (English)In: 2019 IEEE INTERNATIONAL CONFERENCE ON ACOUSTICS, SPEECH AND SIGNAL PROCESSING (ICASSP), IEEE , 2019, p. 8380-8384Conference paper, Published paper (Refereed)
    Abstract [en]

    In this paper, we study an active user detection problem for massive machine type communications (mMTC). The users transmit pilot-hopping sequences and detection of active users is performed based on the received energy. We utilize the channel hardening and favorable propagation properties of massive multiple- input multipleoutput (MIMO) to simplify the user detection. We propose and compare a number of different user detection methods and find that using non- negative least squares (NNLS) is well suited for the task at hand as it achieves good results as well as having the benefit of not having to specify further parameters.

    Place, publisher, year, edition, pages
    IEEE, 2019
    Series
    International Conference on Acoustics Speech and Signal Processing ICASSP, ISSN 1520-6149
    Keywords
    massive MIMO; machine type communication; compressed sensing
    National Category
    Signal Processing
    Identifiers
    urn:nbn:se:liu:diva-160640 (URN)000482554008124 ()978-1-4799-8131-1 (ISBN)
    Conference
    44th IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP)
    Note

    Funding Agencies|ELLIIT; Swedish Research Council (VR)Swedish Research Council

    Available from: 2019-10-11 Created: 2019-10-11 Last updated: 2020-02-03
    2. How Much Will Tiny IoT Nodes Profit from Massive Base Station Arrays?
    Open this publication in new window or tab >>How Much Will Tiny IoT Nodes Profit from Massive Base Station Arrays?
    2018 (English)In: 2018 26TH EUROPEAN SIGNAL PROCESSING CONFERENCE (EUSIPCO), IEEE COMPUTER SOC , 2018, p. 832-836Conference paper, Published paper (Refereed)
    Abstract [en]

    In this paper we study the benefits that Internet-of-Things (IoT) devices will have from connecting to a massive multiple-input-multiple-output (MIMO) base station. In particular, we study how many users that could be simultaneously spatially multiplexed and how much the range can be increased by deploying massive base station arrays. We also investigate how the devices can scale down their uplink power as the number of antennas grows with retained rates. We consider the uplink and utilize upper and lower bounds on known achievable rate expressions to study the effects of the massive arrays. We conduct a case study where we use simulations in the settings of existing IoT systems to draw realistic conclusions. We find that the gains which ultra narrowband systems get from utilizing massive MIMO are limited by the bandwidth and therefore those systems will not be able to spatially multiplex any significant number of users. We also conclude that the power scaling is highly dependent on the nominal signal-to-noise ratio (SNR) in the single-antenna case.

    Place, publisher, year, edition, pages
    IEEE COMPUTER SOC, 2018
    Series
    European Signal Processing Conference, ISSN 2076-1465
    National Category
    Signal Processing
    Identifiers
    urn:nbn:se:liu:diva-154134 (URN)10.23919/EUSIPCO.2018.8553057 (DOI)000455614900168 ()978-90-827970-1-5 (ISBN)
    Conference
    European Signal Processing Conference (EUSIPCO)
    Note

    Funding Agencies|Swedish Research Council (VR); ELLIIT

    Available from: 2019-01-29 Created: 2019-01-29 Last updated: 2020-02-03
  • Presentation: 2020-03-02 10:15 Transformen, B-house, entrance 27, Linköping
    Özdogan, Özgecan
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Analysis of Cellular and Cell-Free Massive MIMO with Rician Fading2020Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The data traffic in cellular networks has grown at an exponential pace for decades. This trend will most probably continue in the future, driven by new innovative applications. One of the key enablers of future cellular networks is the massive MIMO technology. A massive MIMO base station is equipped with a massive number (e.g., a hundred) of individually steerable antennas, which can be effectively used to serve tens of user equipments simultaneously on the same time-frequency resource. It can provide a notable enhancement of both spectral efficiency and energy efficiency in comparison with conventional MIMO.

    In the literature, the achievable spectral efficiencies of massive MIMO systems with a practical number of antennas have been rigorously characterized and optimized when the channels are subject to either spatially uncorrelated or correlated Rayleigh fading. Typically, in massive MIMO research, i.i.d. Rayleigh fading or less frequently free-space line-of-sight (LoS) channel models are assumed since they simplify the analysis. Massive MIMO technology is able to support both rich scattering and LoS scenarios. However, practical channels can consist of a combination of an LoS path and a correlated small-scale fading component caused by a finite number of scattering clusters that can be modeled by spatially correlated Rician fading. In the first part of this thesis, we consider a multi-cell scenario with spatially correlated Rician fading channels and derive closed-form achievable spectral efficiency expressions for different signal processing techniques.

    Alternatively, a massive number of antennas can be spread over a large geographical area and this concept is called cell-free massive MIMO. In the canonical form of cell-free massive MIMO, the access points cooperate via a fronthaul network to spatially multiplex the users on the same time-frequency resource using network MIMO methods that only require locally obtained channel state information. Cellfree massive MIMO is a densely deployed system. Hence, the probability of having an LoS path between some access points and the users is quite high. In the second part of this thesis, we consider a practical scenario where the channels between the access points and the users are modeled with Rician fading.

    List of papers
    1. Massive MIMO With Spatially Correlated Rician Fading Channels
    Open this publication in new window or tab >>Massive MIMO With Spatially Correlated Rician Fading Channels
    2019 (English)In: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, Vol. 67, no 5, p. 3234-3250Article in journal (Refereed) Published
    Abstract [en]

    This paper considers multi-cell massive multiple-input multiple-output systems, where the channels are spatially correlated Rician fading. The channel model is composed of a deterministic line-of-sight path and a stochastic non-line-of-sight component describing a practical spatially correlated multipath environment. We derive the statistical properties of the minimum mean squared error (MMSE), element-wise MMSE, and least-square channel estimates for this model. Using these estimates for maximum ratio combining and precoding, rigorous closed-form uplink (UL) and downlink (DL) achievable spectral efficiency (SE) expressions are derived and analyzed. The asymptotic SE behavior, when using the different channel estimators, are also analyzed. The numerical results show that the SE is higher when using the MMSE estimator than that of the other estimators, and the performance gap increases with the number of antennas.

    Place, publisher, year, edition, pages
    Institute of Electrical and Electronics Engineers (IEEE), 2019
    Keywords
    Massive MIMO; spatially correlated Rician fading; channel estimation; spectral efficiency
    National Category
    Signal Processing
    Identifiers
    urn:nbn:se:liu:diva-158364 (URN)10.1109/TCOMM.2019.2893221 (DOI)000468228900011 ()2-s2.0-85059952523 (Scopus ID)
    Conference
    19th IEEE International Workshop on Signal Processing Advances in Wireless Communications (SPAWC)
    Note

    Funding Agencies|ELLIIT; Swedish Research Council

    Available from: 2019-07-02 Created: 2019-07-02 Last updated: 2020-02-18Bibliographically approved
    2. Performance of Cell-Free Massive MIMO With Rician Fading and Phase Shifts
    Open this publication in new window or tab >>Performance of Cell-Free Massive MIMO With Rician Fading and Phase Shifts
    2019 (English)In: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, Vol. 18, no 11, p. 5299-5315Article in journal (Refereed) Published
    Abstract [en]

    In this paper, we study the uplink (UL) and downlink (DL) spectral efficiency (SE) of a cell-free massive multiple-input-multiple-output (MIMO) system over Rician fading channels. The phase of the line-of-sight (LoS) path is modeled as a uniformly distributed random variable to take the phase-shifts due to mobility and phase noise into account. Considering the availability of prior information at the access points (APs), the phase-aware minimum mean square error (MMSE), non-aware linear MMSE (LMMSE), and least-square (LS) estimators are derived. The MMSE estimator requires perfectly estimated phase knowledge whereas the LMMSE and LS are derived without it. In the UL, a two-layer decoding method is investigated in order to mitigate both coherent and non-coherent interference. Closed-form UL SE expressions with phase-aware MMSE, LMMSE, and LS estimators are derived for maximum-ratio (MR) combining in the first layer and optimal large-scale fading decoding (LSFD) in the second layer. In the DL, two different transmission modes are studied: coherent and non-coherent. Closed-form DL SE expressions for both transmission modes with MR precoding are derived for the three estimators. Numerical results show that the LSFD improves the UL SE performance and coherent transmission mode performs much better than non-coherent transmission in the DL. Besides, the performance loss due to the lack of phase information depends on the pilot length and it is small when the pilot contamination is low.

    Place, publisher, year, edition, pages
    IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2019
    Keywords
    Channel estimation; Fading channels; Rician channels; Coherence; Random variables; Decoding; Cell-free massive MIMO; Rician fading; phase shift; performance analysis
    National Category
    Telecommunications
    Identifiers
    urn:nbn:se:liu:diva-162767 (URN)10.1109/TWC.2019.2935434 (DOI)000496947800020 ()
    Note

    Funding Agencies|ELLIIT; Swedish Research CouncilSwedish Research Council; National Natural Science Foundation of ChinaNational Natural Science Foundation of China [61601020, U1834210]; Beijing Natural Science FoundationBeijing Natural Science Foundation [4182049, L171005]

    Available from: 2019-12-17 Created: 2019-12-17 Last updated: 2020-02-18
  • Presentation: 2020-03-05 10:15 ACAS, A-building, Linköping
    Nilsson, Emil
    Linköping University, Department of Management and Engineering, Energy Systems. Linköping University, Faculty of Science & Engineering.
    Borehole Thermal Energy Storage Systems for Storage of Industrial Excess Heat: Performance Evaluation and Modelling2020Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Improving industrial energy efficiency is considered an important factor in reducing carbon dioxide emissions and counteract climate change. For many industrial companies in cold climates, heat generated at the site in summer will not be needed to fulfil the site heat demand during this time, and is thus removed to the outdoor air. Although a mismatch between heat generation and heat demand primarily being seasonal, a mismatch may also exist at times in the winter, e.g. during milder winter days or high production hours. If this excess heat instead of being sent to the outdoors was stored for later use when it is needed, purchased energy for the site could be decreased. One way to do this is by the use of a borehole thermal energy storage (BTES) system.

    A BTES system stores energy directly in the ground by using an array of closely drilled boreholes through which a heat carrier, often water, is circulated. So far, BTES systems used for heating purposes have mainly been used for storage of solar thermal energy. The BTES system has then been part of smaller district solar heating systems to reduce the seasonal mismatch between incoming solar radiation and heat demand, thus increasing system solar fraction. For this application of BTES systems, energy for storage can be controlled by the sizing of the solar collector area. At an industrial site, however, the energy that can be stored will be limited to the excess heat at the site, and the possible presence of several time-varying processes generating heat at different temperatures gives options as to which processes to include in the heat recovery process and how to design the BTES system. Moreover, to determine the available heat for storage at an industrial site, individual measurements of the heat streams to be included are required. Thus, this must be made more site-specific as compared to that of the traditional usage of BTES systems where solar thermal energy is stored, in which case long-time historic solar radiation data to do this is readily accessible for most locations. Furthermore, for performance predictions of industrial BTES systems to be used for both seasonal and short-term storage of energy, models that can treat the short-term effects are needed, as traditional models for predicting BTES performance do not consider this.

    Although large-scale BTES systems have been around since the 1970’s, little data is to be found in the literature on how design parameters such as borehole spacing and borehole depth affect storage performance, especially for industrial BTES applications. Most studies that can be found with regard to the designing of ground heat exchanger systems are for traditional ground source heat pumps, working at the natural temperature of the ground and being limited to only one or a few boreholes.

    In this work, the performance of the first and largest industrial BTES system in Sweden was first presented and evaluated with regard to the storage’s first seven years in operation. The BTES system, which has been used for both long- and short-term storage of energy, was then modelled in the IDA ICE 4.8 environment with the aim to model actual storage performance. Finally, the model was used to conduct a parametric study on the BTES system, where e.g. the impact on storage performance from borehole spacing and characteristics of the storage supply flow at heat injection were investigated. From the performance evaluation it could be concluded that lower than estimated quantities and/or quality of the excess heat at the site, resulting in lower storage supply flow temperatures at heat injection, has hindered the storage from reaching temperatures necessary for significant amounts of energy to be extracted. Based on the repeating annual storage behavior seen for the last years of the evaluation period, a long-term annual heat extraction and ratio of energy extracted to energy injected of approximately 400 MWh/year and 20% respectively are likely.

    For the comparison of predicted and measured storage performance, which considered a period of three years, predicted values for total injected and extracted energy deviated from measured values by less than 1 and 3% respectively, and predicted and measured values for injected and extracted energy followed the same pattern throughout the period. Furthermore, the mean relative difference for the storage temperatures was 4%. A time-step analysis confirmed that the intermittent heat injection and extraction, occurring at intervals down to half a day, had been captured in the three-year validation. This as predictions would become erroneous when the time step exceeded the time at which these changes in storage operation occur.

    Main findings from the parametric study include that 1) for investigated supply flows at heat injection, a high temperature was more important than a high flow rate in order to achieve high annual heat extractions and that 2) annual heat extraction would rapidly reduce as the borehole spacing was decreased from the one yielding the highest annual heat extraction, whereas the reduction in annual heat extraction was quite slow when the spacing was increased from this point. Another conclusion that came from the performance evaluation and the parametric study, as a consequence of the Emmaboda storage being designed as a high-temperature BTES system, intended working temperatures being 40–55 °C, was that the possibility of designing the BTES system for low working temperatures should be considered in the designing of a BTES system. Lower storage operation temperatures allow for more energy to be injected and in turn for more energy to be extracted and reduces storage heat losses to the surroundings.  

    List of papers
    1. Performance evaluation of an industrial borehole thermal energy storage (BTES) project - Experiences from the first seven years of operation
    Open this publication in new window or tab >>Performance evaluation of an industrial borehole thermal energy storage (BTES) project - Experiences from the first seven years of operation
    2019 (English)In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 143, p. 1022-1034Article in journal (Refereed) Published
    Abstract [en]

    Borehole thermal energy storage (BTES) is a technology which allows for both seasonal and short-to-medium-term storage of thermal energy and which can be used for both heating and cooling. This makes BTES of special interest to many industries. However, post-implementation evaluations of largescale industrial BTES are scarce. The BTES at Xylems production plant in Emmaboda, Sweden is one of the worlds largest BTES systems for storage of industrial excess heat. In this paper, the BTES at Emmaboda was evaluated with respect to how it was integrated and how it has performed during its first seven years of operation. The BTES consists of 140 boreholes, 150 m deep, and heat for storage is mainly recovered from two high-temperature ovens and the foundry ventilation air. So far, the highest heat extraction and BTES efficiency (19%) took place in the storage systems sixth full year of operation, when roughly 2200 MWh and 400 MWh were injected into and extracted from the storage respectively. One main reason extraction is not higher is that the quantities and/or the quality of the excess heat for storage are lower than estimated, thus hindering the storage from reaching the necessary temperatures for heat extraction. (C) 2019 Elsevier Ltd. All rights reserved.

    Place, publisher, year, edition, pages
    PERGAMON-ELSEVIER SCIENCE LTD, 2019
    Keywords
    Borehole thermal energy storage; Industrial excess heat; BTES performance; Temperature distribution; Geothermal technology; Thermal conductivity
    National Category
    Energy Engineering
    Identifiers
    urn:nbn:se:liu:diva-160376 (URN)10.1016/j.renene.2019.05.020 (DOI)000482686100004 ()
    Note

    Funding Agencies|Swedish Energy Agency

    Available from: 2019-09-23 Created: 2019-09-23 Last updated: 2020-02-20
    2. Empirical Validation and Numerical Predictions of an Industrial Borehole Thermal Energy Storage System
    Open this publication in new window or tab >>Empirical Validation and Numerical Predictions of an Industrial Borehole Thermal Energy Storage System
    2019 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 12, no 12, article id 2263Article in journal (Refereed) Published
    Abstract [en]

    To generate performance predictions of borehole thermal energy storage (BTES) systems for both seasonal and short-term storage of industrial excess heat, e.g., from high to low production hours, models are needed that can handle the short-term effects. In this study, the first and largest industrial BTES in Sweden, applying intermittent heat injection and extraction down to half-day intervals, was modelled in the IDA ICE 4.8 environment and compared to three years of measured storage performance. The model was then used in a parametric study to investigate the change in performance of the storage from e.g., borehole spacing and storage supply flow characteristics at heat injection. For the three-year comparison, predicted and measured values for total injected and extracted energy differed by less than 1% and 3%, respectively and the mean relative difference for the storage temperatures was 4%, showing that the performance of large-scale BTES with intermittent heat injection and extraction can be predicted with high accuracy. At the actual temperature of the supply flow during heat injection, 40 degrees C, heat extraction would not exceed approximately 100 MWh/year for any investigated borehole spacing, 1-8 m. However, when the temperature of the supply flow was increased to 60-80 degrees C, 1400-3100 MWh/year, also dependent on the flow rate, could be extracted at the spacing yielding the highest heat extraction, which in all cases was 3-4 m.

    Place, publisher, year, edition, pages
    MDPI, 2019
    Keywords
    borehole thermal energy storage; industrial excess heat; model validation; performance predictions; IDA ICE
    National Category
    Energy Engineering
    Identifiers
    urn:nbn:se:liu:diva-159071 (URN)10.3390/en12122263 (DOI)000473821400036 ()
    Note

    Funding Agencies|Swedish Energy Agency [40531-1]; Linkoping University

    Available from: 2019-07-22 Created: 2019-07-22 Last updated: 2020-02-20
  • Presentation: 2020-03-24 13:15 Alan Turing, E-building, Linköping
    Toczé, Klervie
    Linköping University, Department of Computer and Information Science, Software and Systems. Linköping University, Faculty of Science & Engineering.
    Latency-aware Resource Management at the Edge2020Licentiate thesis, monograph (Other academic)
    Abstract [en]

    The increasing diversity of connected devices leads to new application domains being envisioned. Some of these need ultra low latency or have privacy requirements that cannot be satisfied by the current cloud. By bringing resources closer to the end user, the recent edge computing paradigm aims to enable such applications.

    One critical aspect to ensure the successful deployment of the edge computing paradigm is efficient resource management. Indeed, obtaining the needed resources is crucial for the applications using the edge, but the resource picture of this paradigm is complex. First, as opposed to the nearly infinite resources provided by the cloud, the edge devices have finite resources. Moreover, different resource types are required depending on the applications and the devices supplying those resources are very heterogeneous. This thesis studies several challenges towards enabling efficient resource management for edge computing.

    The thesis begins by a review of the state-of-the-art research focusing on resource management in the edge computing context. A taxonomy is proposed for providing an overview of the current research and identify areas in need of further work.

    One of the identified challenges is studying the resource supply organization in the case where a mix of mobile and stationary devices is used to provide the edge resources. The ORCH framework is proposed as a means to orchestrate this edge device mix. The evaluation performed in a simulator shows that this combination of devices enables higher quality of service for latency-critical tasks.

    Another area is understanding the resource demand side. The thesis presents a study of the workload of a killer application for edge computing: mixed reality. The MR-Leo prototype is designed and used as a vehicle to understand the end-to-end latency, the throughput, and the characteristics of the workload for this type of application. A method for modeling the workload of an application is devised and applied to MR-Leo in order to obtain a synthetic workload exhibiting the same characteristics, which can be used in further studies.