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  • 1.
    Do, Tan Tai
    et al.
    Linköping University, Department of Electrical Engineering. Linköping University, Faculty of Science & Engineering.
    Ngo, Hien
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Duong, Trung Q.
    Queens University of Belfast, North Ireland.
    Oechtering, Tobias J.
    KTH Royal Institute Technology, Sweden.
    Skoglund, Mikael
    KTH Royal Institute Technology, Sweden.
    Massive MIMO Pilot Retransmission Strategies for Robustification Against Jamming2017In: IEEE Wireless Communications Letters, ISSN 2162-2337, E-ISSN 2162-2345, Vol. 6, no 1, p. 58-61Article in journal (Refereed)
    Abstract [en]

    This letter proposes anti-jamming strategies based on pilot retransmission for a single user uplink massive MIMO under jamming attack. A jammer is assumed to attack the system both in the training and data transmission phases. We first derive an achievable rate which enables us to analyze the effect of jamming attacks on the system performance. Counter-attack strategies are then proposed to mitigate this effect under two different scenarios: random and deterministic jamming attacks. Numerical results illustrate our analysis and benefit of the proposed schemes.

  • 2.
    Duc Ho, Chung
    et al.
    Queens Univ Belfast, North Ireland.
    Ngo, Hien
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering. Queens Univ Belfast, North Ireland.
    Matthaiou, Michail
    Queens Univ Belfast, North Ireland.
    Three-Way Massive MIMO Relaying with Successive Cancelation Decoding2018In: INDUSTRIAL NETWORKS AND INTELLIGENT SYSTEMS, INISCOM 2017, Springer, 2018, Vol. 221, p. 79-90Conference paper (Refereed)
    Abstract [en]

    In this paper, we propose a novel transmission scheme for a three-way massive multiple-input multiple-output (MIMO) relay network where three users exchange their data with the help of a decode-and-forward relay station equipped with a very large antenna array. Our proposed scheme needs only two time-slots for the information exchange. More precisely, the three users first send their symbols to the relay. Then, the relay uses the maximum-ratio combining technique to decode all transmitted symbols and simultaneously transmits these symbols to all three users. Each user applies successive cancelation decoding to decode symbols transmitted from other users. We study the sum spectral efficiency of our proposed transmission protocol. We show that the sum spectral efficiency of our proposed scheme increases noticeably compared to the one of the conventional scheme where three time-slots are required to exchange data among the three users, without increasing the system complexity.

  • 3.
    Duc Ho, Chung
    et al.
    Queens University of Belfast, North Ireland.
    Ngo, Hien
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Matthaiou, Michail
    Queens University of Belfast, North Ireland.
    Duong, Trung Q.
    Queens University of Belfast, North Ireland.
    On the Performance of Zero-Forcing Processing in Multi-Way Massive MIMO Relay Networks2017In: IEEE Communications Letters, ISSN 1089-7798, E-ISSN 1558-2558, Vol. 21, no 4, p. 849-852Article in journal (Refereed)
    Abstract [en]

    We consider a multi-way massive multiple-input multiple-output relay network with zero-forcing processing at the relay. By considering the time-division duplex protocol with channel estimation, we derive an analytical approximation of the spectral efficiency. This approximation is very tight and simple, which enables us to analyze the system performance, as well as to compare the spectral efficiency with zero-forcing and maximum-ratio processing. Our results show that by using a very large number of relay antennas and with the zero-forcing technique, we can simultaneously serve many active users in the same time-frequency resource, each with high spectral efficiency.

  • 4.
    Duc Ho, Chung
    et al.
    Queens University of Belfast, North Ireland.
    Ngo, Hien Quoc
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering. Queens University of Belfast, North Ireland.
    Matthaiou, Michail
    Queens University of Belfast, North Ireland.
    Duong, Trung Q.
    Queens University of Belfast, North Ireland.
    Multi-way Massive MIMO Relay Networks with Maximum-Ratio Processing2017In: 2017 INTERNATIONAL CONFERENCE ON RECENT ADVANCES IN SIGNAL PROCESSING, TELECOMMUNICATIONS and COMPUTING (SIGTELCOM), IEEE , 2017, p. 124-128Conference paper (Refereed)
    Abstract [en]

    This paper considers a multi-way massive multiple-input multiple-output (MIMO) relaying system. The bearing-information is exchanged among multiple users with the help of a multiple-antenna relay (the base station). The maximum-ratio (MR) processing is applied at the relay under the assumption of perfect channel state information. The spectral efficiency and the asymptotic results for the signal-to-interference-plus-noise ratio (when the number of relay antennas becomes large) are derived. By using a massive number of antennas, the transmit power at both user side and/or relay can be made inversely proportional to the number of relay antennas without degradation in the system performance.

  • 5.
    Duong, Trung Q.
    et al.
    Blekinge Institute of Technology, Karlskrona, Sweden..
    Ngo, Hien Quoc
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Zepernick, Hans-Jurgen
    Blekinge Institute of Technology, Karlskrona, Sweden..
    Nallanathan, Arumugam
    King’s College London, London, United Kingdom..
    Distributed Space-Time Coding in Two-Way Fixed Gain Relay Networks over Nakagami-m Fading2012In: Proceedings of the IEEE International Conference on Communicatons (ICC), IEEE , 2012, p. 3521-3525Conference paper (Refereed)
    Abstract [en]

    The distributed Alamouti space-time code in two-way fixed gain amplify-and-forward (AF) relay is proposed in this paper. In particular, closed-form expressions for approximated ergodic sum-rate and exact pairwise error probability (PWEP) are derived for Nakagami-m fading channels. To reveal further insights into array and diversity gains, an asymptotic PWEPis also obtained. Finally, numerical results are provided to corroborate the proposed theoretical analysis.

  • 6.
    Ho, Chung Duc
    et al.
    Queens Univ Belfast, North Ireland.
    Ngo, Hien
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering. Queens Univ Belfast, North Ireland.
    Matthaiou, Michail
    Queens Univ Belfast, North Ireland.
    Duong, Trung Q.
    Queens University Belfast, Belfast, Ireland.
    Multi-Way Massive MIMO with Maximum-Ratio Processing and Imperfect CSI2017In: 2017 25TH EUROPEAN SIGNAL PROCESSING CONFERENCE (EUSIPCO), IEEE , 2017, p. 1704-1708Conference paper (Refereed)
    Abstract [en]

    This paper considers a multi-way massive multiple-input multiple-output amplify-and-forward relaying system, where single-antenna users exchange their information-bearing signals with the assistance of one relay station equipped with unconventionally many antennas. The relay first estimates the channels to all users through the pilot signals transmitted from them. Then, the relay uses maximum-ratio processing (i.e. maximum-ratio combining in the multiple-access phase and maximum-ratio transmission in the broadcast phase) to process the signals. A rigorous closed-form expression for the spectral efficiency is derived. We show that by deploying massive antenna arrays at the relay and simple maximum-ratio processing, we can serve many users in the same time-frequency resource, while maintaining a given quality-of-service for each user.

  • 7.
    Interdonato, Giovanni
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering. Ericsson Research.
    Ngo, Hien Quoc
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Larsson, Erik G
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Frenger, Pål
    Ericsson Research.
    On the Performance of Cell-Free Massive MIMO with Short-Term Power Constraints2016In: 2016 IEEE 21ST INTERNATIONAL WORKSHOP ON COMPUTER AIDED MODELLING AND DESIGN OF COMMUNICATION LINKS AND NETWORKS (CAMAD), IEEE , 2016, p. 225-230Conference paper (Refereed)
    Abstract [en]

    In this paper we consider a time-division duplex cell-free massive multiple-input multiple-output (MIMO) system where many distributed access points (APs) simultaneously serve many users. A normalized conjugate beamforming scheme, which satisfies short-term average power constraints at the APs, is proposed and analyzed taking into account the effect of imperfect channel information. We derive an approximate closed-form expression for the per-user achievable downlink rate of this scheme. We also provide, analytically and numerically, a performance comparison between the normalized conjugate beamforming and the conventional conjugate beamforming scheme in [1] (which satisfies long-term average power constraints). Normalized conjugate beamforming scheme reduces the beamforming uncertainty gain, which comes from the users lack of the channel state information knowledge, and hence, it improves the achievable downlink rate compared to the conventional conjugate beamforming scheme.

  • 8.
    Marzetta, Thomas L.
    et al.
    Nokia.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Hong, Yang
    Nokia.
    Ngo, Hien Quoc
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Fundamentals of massive MIMO2016Book (Refereed)
    Abstract [en]

    "Written by the pioneers of the concept, this is the first complete guide to the physical and engineering principles of Massive MIMO. Assuming only a basic background in communications and statistical signal processing, it will guide readers through key topics such as propagation models, channel modeling, and multi-cell performance analyses. The authors' unique capacity-bound approach will enable readers to carry out more effective system performance analysis and develop advanced Massive MIMO techniques and algorithms. Numerous case studies, as well as problem sets and solutions accompanying the book online, will help readers put knowledge into practice and acquire the skillset needed to design and analyze complex wireless communication systems. Whether you are a graduate student, researcher, or industry professional working in the field of wireless communications, this will be an indispensable guide for years to come"

  • 9.
    Matthaiou, Michail
    et al.
    Chalmers University of Technology, Gothenburg, Sweden.
    Alexandropoulos, George C.
    Athens Information Technology (AIT), Greece.
    Ngo, Hien Quoc
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Analytic framework for the effective rate of MISO fading channels2012In: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, Vol. 60, no 6, p. 1741-1751Article in journal (Refereed)
    Abstract [en]

    The delay constraints imposed by future wireless applications require a suitable metricfor assessing their impact on the overall system performance. Since the classical Shannon's ergodic capacityfails to do so, the so-called effective rate was recently established as a rigorous alternative. While prior relevant works have improved our knowledge on the effective rate characterization of communication systems, an analytical framework encompassing several fading models of interest isnot yet available. In this paper, we pursue a detailed effective rate analysis of Nakagami-m, Ricianand generalized-K multiple-input single-output (MISO) fading channels by deriving new, analytical expressions for their exact effective rate. Moreover, we consider the asymptotically low and high signal-to-noise (SNR) regimes, for which tractable, closed-form effective rate expressions are presented. These results enable us to draw useful conclusions about the impact of system parameters on the effective rate of different MISO fading channels. All the theoretical expressions are validated via Monte-Carlo simulations.

  • 10.
    Matthaiou, Michail
    et al.
    Department of Signals and Systems, Chalmers University of Technology, Gothenburg, Sweden.
    Alexandropoulos, George C.
    Broadband Wireless and Sensor Networks Group, Athens Information Technology, Athens, Greece.
    Ngo, Hien Quoc
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Effective Rate Analysis of MISO Rician Fading Channels2012In: Proccedings of the 2012 IEEE 7th Sensor Array and Multichannel Signal Processing Workshop (SAM), June 17-20, 2012 in Hoboken, NJ, USA, IEEE , 2012, p. 53-56Conference paper (Refereed)
    Abstract [en]

    The delay constraints imposed by future wirelessapplications require a suitable metric for assessing their impacton the overall system performance. Since the classical Shannon’s ergodic capacity fails to do so, the so-called effective rate was recently established as a rigorous alternative. Yet, most priorrelevant works have considered only the typical case of Rayleighfading which allows for tractable manipulations. In this paper,we relax this assumption by considering the more general Rician fading model for multiple-input single-output (MISO) systems. Anew, analytical expression for the exact effective rate is derived, along with tractable expressions for the key parameters dictating the effective rate performance in the high and low signal-to-noise(SNR) regimes.

  • 11.
    Ngo, Hien
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Larsson, Erik G
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Spectral Efficiency of the Multipair Two-Way Relay Channel with Massive Arrays2013In: Asilomar Conference on Signals, Systems and Computers, 2013, IEEE , 2013, p. 275-279Conference paper (Refereed)
    Abstract [en]

    We consider a multipair two-way relay channel where multiple communication pairs share the same time-frequency resource and a common relay node. We assume that all users have a single antenna, while the relay node is equipped with a very large antenna array. We consider two transmission schemes: (I) separate-training zero-forcing (ZF) and (II) a new proposed coupled-training ZF. For both schemes, the channels are estimated at the relay by using training sequences, assuming time-division duplex operation. The relay processes the received signals using ZF. With the separate-training ZF, the channels from all users are estimated separately. By contrast, with the coupled-training ZF, the relay estimates the sum of the channels from two users of a given communication pair. This reduces the amount of resources spent in the training phase. Self-interference reduction is also proposed for these schemes. When the number of relay antennas grows large, the effects of interpair interference and self-interference can be neglected. The transmit power of each user and of the relay can be made inversely proportional to the square root of the number of relay antennas while maintaining a given quality-of-service. We derive a lower bound on the capacity which enables us to evaluate the spectral efficiency. The coupled-training ZF scheme is preferable for the high-mobility environment, while the separate-training ZF scheme is preferable for the low-mobility environment.

  • 12.
    Ngo, Hien Quoc
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Massive MIMO: Fundamentals and System Designs2015Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The last ten years have seen a massive growth in the number of connected wireless devices. Billions of devices are connected and managed by wireless networks. At the same time, each device needs a high throughput to support applications such as voice, real-time video, movies, and games. Demands for wireless throughput and the number of wireless devices will always increase. In addition, there is a growing concern about energy consumption of wireless communication systems. Thus, future wireless systems have to satisfy three main requirements: i) having a high throughput; ii) simultaneously serving many users; and iii) having less energy consumption. Massive multiple-input multiple-output (MIMO) technology, where a base station (BS) equipped with very large number of antennas (collocated or distributed) serves many users in the same time-frequency resource,  can meet the above requirements, and hence, it is a promising candidate technology for next generations of wireless systems. With massive antenna arrays at the BS, for most propagation environments, the channels become favorable, i.e., the channel vectors between the users and the BS are (nearly) pairwisely orthogonal, and hence, linear processing is nearly optimal. A huge throughput and energy efficiency can be achieved due to the multiplexing gain and the array gain. In particular, with a simple power control scheme, Massive MIMO can offer uniformly good service for all users. In this dissertation, we focus on the performance of Massive MIMO. The dissertation consists of two main parts: fundamentals and system designs of Massive MIMO.

    In the first part, we focus on fundamental limits of the system performance under practical constraints such as low complexity processing, limited length of each coherence interval, intercell interference, and finite-dimensional channels. We first study the potential for power savings of the Massive MIMO uplink with maximum-ratio combining (MRC), zero-forcing, and minimum mean-square error receivers, under perfect and imperfect channels. The energy and spectral efficiency tradeoff is investigated. Secondly, we consider a physical channel model where the angular domain is divided into a finite number of distinct directions. A lower bound on the capacity is derived, and the effect of pilot contamination in this finite-dimensional channel model is analyzed. Finally, some aspects of favorable propagation in Massive MIMO under Rayleigh fading and line-of-sight (LoS) channels are investigated. We show that both Rayleigh fading and LoS environments offer favorable propagation.

    In the second part, based on the fundamental analysis in the first part, we propose some system designs for Massive MIMO. The acquisition of channel state information (CSI) is very importantin Massive MIMO. Typically, the channels are estimated at the BS through uplink training. Owing to the limited length of the coherence interval, the system performance is limited by pilot contamination. To reduce the pilot contamination effect, we propose an eigenvalue-decomposition-based scheme to estimate the channel directly from the received data. The proposed scheme results in better performance compared with the conventional training schemes due to the reduced pilot contamination. Another important issue of CSI acquisition in Massive MIMO is how to acquire CSI at the users. To address this issue, we propose two channel estimation schemes at the users: i) a downlink "beamforming training" scheme, and ii) a method for blind estimation of the effective downlink channel gains. In both schemes, the channel estimation overhead is independent of the number of BS antennas. We also derive the optimal pilot and data powers as well as the training duration allocation to maximize the sum spectral efficiency of the Massive MIMO uplink with MRC receivers, for a given total energy budget spent in a coherence interval. Finally, applications of Massive MIMO in relay channels are proposed and analyzed. Specifically, we consider multipair relaying systems where many sources simultaneously communicate with many destinations in the same time-frequency resource with the help of a massive MIMO relay. A massive MIMO relay is equipped with many collocated or distributed antennas. We consider different duplexing modes (full-duplex and half-duplex) and different relaying protocols (amplify-and-forward, decode-and-forward, two-way relaying, and one-way relaying) at the relay. The potential benefits of massive MIMO technology in these relaying systems are explored in terms of spectral efficiency and power efficiency.

    List of papers
    1. Energy and Spectral Efficiency of Very Large Multiuser MIMO Systems
    Open this publication in new window or tab >>Energy and Spectral Efficiency of Very Large Multiuser MIMO Systems
    2013 (English)In: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, Vol. 61, no 4, p. 1436-1449Article in journal (Refereed) Published
    Abstract [en]

    A multiplicity of autonomous terminals simultaneously transmits data streams to a compact array of antennas. The array uses imperfect channel-state information derived from transmitted pilots to extract the individual data streams. The power radiated by the terminals can be made inversely proportional to the square-root of the number of base station antennas with no reduction in performance. In contrast if perfect channel-state information were available the power could be made inversely proportional to the number of antennas. Lower capacity bounds for maximum-ratio combining (MRC), zero-forcing (ZF) and minimum mean-square error (MMSE) detection are derived. An MRC receiver normally performs worse than ZF and MMSE. However as power levels are reduced, the cross-talk introduced by the inferior maximum-ratio receiver eventually falls below the noise level and this simple receiver becomes a viable option. The tradeoff between the energy efficiency (as measured in bits/J) and spectral efficiency (as measured in bits/channel use/terminal) is quantified for a channel model that includes small-scale fading but not large-scale fading. It is shown that the use of moderately large antenna arrays can improve the spectral and energy efficiency with orders of magnitude compared to a single-antenna system.

    National Category
    Communication Systems Signal Processing
    Identifiers
    urn:nbn:se:liu:diva-85224 (URN)10.1109/TCOMM.2013.020413.110848 (DOI)000318998100022 ()
    Available from: 2012-11-12 Created: 2012-11-12 Last updated: 2017-12-07
    2. The Multicell Multiuser MIMO Uplink with Very Large Antenna Arrays and a Finite-Dimensional Channel
    Open this publication in new window or tab >>The Multicell Multiuser MIMO Uplink with Very Large Antenna Arrays and a Finite-Dimensional Channel
    2013 (English)In: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, Vol. 61, no 6, p. 2350-2361Article in journal (Refereed) Published
    Abstract [en]

    We consider multicell multiuser MIMO systems with a very large number of antennas at the base station (BS). We assume that the channel is estimated by using uplink training. We further consider a physical channel model where the angular domain is separated into a finite number of distinct directions. We analyze the so-called pilot contamination effect discovered in previous work, and show that this effect persists under the finite-dimensional channel model that we consider. In particular, we consider a uniform array at the BS. For this scenario, we show that when the number of BS antennas goes to infinity, the system performance under a finite-dimensional channel model with P angular bins is the same as the performance under an uncorrelated channel model with P antennas. We further derive a lower bound on the achievable rate of uplink data transmission with a linear detector at theBS. We then specialize this lower bound to the cases of maximum-ratio combining (MRC) and zero-forcing (ZF) receivers, for a finite and an infinite number of BS antennas. Numerical results corroborate our analysis and show a comparison between the performances of MRC and ZF in terms of sum-rate.

    Keywords
    Finite-dimensional channel, multicell mutiuser MIMO, pilot contamination, very large MIMO systems
    National Category
    Communication Systems Signal Processing
    Identifiers
    urn:nbn:se:liu:diva-85223 (URN)10.1109/TCOMM.2013.032713.120408 (DOI)000321219100023 ()
    Available from: 2013-04-04 Created: 2012-11-12 Last updated: 2017-12-06Bibliographically approved
    3. Aspects of Favorable Propagation in Massive MIMO
    Open this publication in new window or tab >>Aspects of Favorable Propagation in Massive MIMO
    2014 (English)In: European Signal Processing Conference (EUSIPCO), EURASIP , 2014, p. 76-80Conference paper, Published paper (Refereed)
    Abstract [en]

    Favorable propagation, dened as mutual orthogonality among the vector-valued channels to the terminals, is one of the key properties of the radio channel that is exploited in Massive MIMO. However, there has been little work that studies this topic in detail. In this paper, we first show that favorable propagation offers the most desirable scenario in terms of maximizing the sum-capacity. One useful proxy for whether propagation is favorable or not is the channel condition number. However, this proxy is not good for the case where the norms of the channel vectors are not equal. For this case, to evaluate how favorable the propagation oered by the channel is, we propose a distance from favorable propagation measure, which is the gap between the sum-capacity and the maximum capacity obtained under favorable propagation. Secondly, we examine how favorable the channels can be for two extreme scenarios: i.i.d. Rayleigh fading and uniform random line-of-sight(UR-LoS). Both environments offer (nearly) favorable propagation. Furthermore, to analyze the UR-LoS model, we propose an urns-and-balls model. This model is simple and explains the singular value spread characteristic of the UR-LoS model well.

    Place, publisher, year, edition, pages
    EURASIP, 2014
    Series
    Proceedings of the European Signal Processing Conference, ISSN 2076-1465
    National Category
    Communication Systems Signal Processing
    Identifiers
    urn:nbn:se:liu:diva-112757 (URN)2-s2.0-84911883360 (Scopus ID)978-099286261-9 (ISBN)
    Conference
    22nd European Signal Processing Conference, EUSIPCO 2014
    Available from: 2014-12-15 Created: 2014-12-15 Last updated: 2016-09-14
    4. EVD-based Channel Estimations for Multicell Multiuser MIMO with Very Large Antenna Arrays
    Open this publication in new window or tab >>EVD-based Channel Estimations for Multicell Multiuser MIMO with Very Large Antenna Arrays
    2012 (English)In: Proceedings of the IEEE International Conference on Acoustics, Speed and Signal Processing (ICASSP), IEEE , 2012, p. 3249-3252Conference paper, Published paper (Refereed)
    Abstract [en]

    This paper considers multicell multiuser MIMO systems with verylarge antenna arrays at the base station. We propose an eigenvalue-decomposition-based approach to channel estimation, that estimates the channel blindly from the received data. The approach exploits the asymptotic orthogonality of the channel vectors in very large MIMO systems. We show that the channel to each user can be estimated from the covariance matrix of the received signals, up to a remaining scalar multiplicative ambiguity. A short training sequence is required to resolve this ambiguity. Furthermore, to improve the performance of our approach, we combine it with the iterative least-square with projection (ILSP) algorithm. Numerical results verify the effectiveness of our channel estimation approach.

    Place, publisher, year, edition, pages
    IEEE, 2012
    Series
    Acoustics, Speech and Signal Processing (ICASSP), ISSN 1520-6149
    National Category
    Communication Systems
    Identifiers
    urn:nbn:se:liu:diva-74084 (URN)10.1109/ICASSP.2012.6288608 (DOI)000312381403081 ()978-1-4673-0045-2 (ISBN)978-1-4673-0044-5 (ISBN)
    Conference
    IEEE International Conference on Acoustics, Speed and Signal Processing (ICASSP, March 25-30, Kyoto, Japan
    Available from: 2012-01-18 Created: 2012-01-18 Last updated: 2016-08-31Bibliographically approved
    5. Massive MU-MIMO Downlink TDD Systems with Linear Precodingand Downlink Pilots
    Open this publication in new window or tab >>Massive MU-MIMO Downlink TDD Systems with Linear Precodingand Downlink Pilots
    2013 (English)In: 51st Annual Allerton Conference on Communication, Control, and Computing (Allerton), 2013, IEEE , 2013, p. 293-298Conference paper, Published paper (Refereed)
    Abstract [en]

    We consider a massive MU-MIMO downlink time-division duplex system where a base station (BS) equipped with many antennas serves several single-antenna users in the same time-frequency resource. We assume that the BS uses linear precoding for the transmission. To reliably decode the signals transmitted from the BS, each user should have an estimate of its channel. In this work, we consider an efficient channel estimation scheme to acquire CSI at each user, called beamforming training scheme. With the beamforming training scheme, the BS precodes the pilot sequences and forwards to all users. Then, based on the received pilots, each user uses minimum mean-square error channel estimation to estimate the effective channel gains. The channel estimation overhead of this scheme does not depend on the number of BS antennas, and is only proportional to the number of users. We then derive a lower bound on the capacity for maximum-ratio transmission and zero-forcing precoding techniques which enables us to evaluate the spectral efficiency taking into account the spectral eciency loss associated with the transmission of the downlink pilots. Comparing with previous work where each user uses only the statistical channel properties to decode the transmitted signals, we see that the proposed beamforming training scheme is preferable for moderate and low-mobility environments.

    Place, publisher, year, edition, pages
    IEEE, 2013
    National Category
    Communication Systems Signal Processing
    Identifiers
    urn:nbn:se:liu:diva-112758 (URN)10.1109/Allerton.2013.6736537 (DOI)978-1-4799-3409-6 (ISBN)
    Conference
    51st Annual Allerton Conference on Communication, Control, and Computing (Allerton), October 2-4, Monticello, Illinois, USA
    Available from: 2014-12-15 Created: 2014-12-15 Last updated: 2016-08-31Bibliographically approved
    6. Blind estimation of effective downlink channel gains in massive MIMO
    Open this publication in new window or tab >>Blind estimation of effective downlink channel gains in massive MIMO
    2015 (English)In: 2015 IEEE International Conference on Acoustics, Speech, and Signal Processing, Proceedings, IEEE , 2015, p. 2919-2923Conference paper, Published paper (Refereed)
    Abstract [en]

    We consider the massive MIMO downlink with time-division duplex (TDD) operation and conjugate beamforming transmission. To reliably decode the desired signals, the users need to know the effective channel gain. In this paper, we propose a blind channel estimation method which can be applied at the users and which does not require any downlink pilots. We show that our proposed schemecan substantially outperform the case where each user has only statistical channel knowledge, and that the difference in performance is particularly large in certain types of channel, most notably keyhole channels. Compared to schemes that rely on downlink pilots(e.g., [1]), our proposed scheme yields more accurate channel estimates for a wide range of signal-to-noise ratios and avoid spending time-frequency resources on pilots.

    Place, publisher, year, edition, pages
    IEEE, 2015
    Series
    International Conference on Acoustics Speech and Signal Processing ICASSP, ISSN 1520-6149
    Keywords
    Blind channel estimation, downlink, massive MIMO, time-division duplex
    National Category
    Communication Systems Telecommunications
    Identifiers
    urn:nbn:se:liu:diva-129072 (URN)10.1109/ICASSP.2015.7178505 (DOI)000427402903006 ()978-1-4673-6997-8 (ISBN)
    Conference
    40th IEEE International Conference on Acoustics, Speech, and Signal Processing (ICASSP)
    Available from: 2016-06-10 Created: 2016-06-10 Last updated: 2019-01-04Bibliographically approved
    7. Massive MIMO with Optimal Power and Training Duration Allocation
    Open this publication in new window or tab >>Massive MIMO with Optimal Power and Training Duration Allocation
    2014 (English)In: IEEE Wireless Communications Letters, ISSN 2162-2337, E-ISSN 2162-2345, Vol. 3, no 6, p. 605-608Article in journal (Refereed) Published
    Abstract [en]

    We consider the uplink of massive multicell multiple-input multiple-output systems, where the base stations (BSs), equipped with massive arrays, serve simultaneously several terminals in the same frequency band. We assume that the BS estimates the channel from uplink training, and then uses the maximum ratio combining technique to detect the signals transmitted from all terminals in its own cell. We propose an optimal resource allocation scheme which jointly selects the training duration,training signal power, and data signal power in order to maximize the sum spectral efficiency, for a given total energy budget spent in a coherence interval. Numerical results verify the benets of the optimal resource allocation scheme. Furthermore, we show that more training signal power should be used at low signal-to-noise ratio(SNRs), and vice versa at high SNRs. Interestingly, for the entire SNR regime, the optimal training duration is equal to the number of terminals.

    Place, publisher, year, edition, pages
    IEEE, 2014
    Keywords
    Channel estimation, MIMO, Power distribution, Resource management, Signal to noise ratio, Uplink
    National Category
    Communication Systems Signal Processing
    Identifiers
    urn:nbn:se:liu:diva-112760 (URN)10.1109/LWC.2014.2359220 (DOI)000209681700015 ()2-s2.0-84919821841 (Scopus ID)
    Available from: 2014-12-15 Created: 2014-12-15 Last updated: 2017-12-05Bibliographically approved
    8. Large-Scale Multipair Two-Way Relay Networks with Distributed AF Beamforming
    Open this publication in new window or tab >>Large-Scale Multipair Two-Way Relay Networks with Distributed AF Beamforming
    2013 (English)In: IEEE Communications Letters, ISSN 1089-7798, E-ISSN 1558-2558, Vol. 17, no 12, p. 1-4Article in journal (Refereed) Published
    Abstract [en]

    We consider a multipair two-way relay network where multiplecommunication pairs simultaneously exchange information with the helpof multiple relay nodes. All nodes are equipped with a single antennaand channel state information is available only at the relay nodes. Each relay uses very simplesignal processing in a distributed manner, called distributed amplify-and-forward (AF)  relaying. A closed-form expression for the achievable rate is  derived. We show that the distributed AF scheme outperforms conventional orthogonal relaying. When the number of relay nodes is large, the distributed AF relaying scheme can achieve the capacity scaling given by the cut-set upper bound. Furthermore, when the number of relay nodes grows large, the transmit powers of each terminal and of the relay can be made inversely proportional to the number of relay nodes while maintaining a given quality-of-service. If the transmit power of each terminal is kept fixed, the transmit power of each relay node can be scaled down inversely proportional to the square of the number of relays.

    Place, publisher, year, edition, pages
    IEEE Press, 2013
    National Category
    Communication Systems Signal Processing Telecommunications
    Identifiers
    urn:nbn:se:liu:diva-98122 (URN)10.1109/LCOMM.2013.102213.131813 (DOI)000329528200021 ()
    Available from: 2013-09-30 Created: 2013-09-30 Last updated: 2017-12-06
    9. Spectral Efficiency of the Multipair Two-Way Relay Channel with Massive Arrays
    Open this publication in new window or tab >>Spectral Efficiency of the Multipair Two-Way Relay Channel with Massive Arrays
    2013 (English)In: Asilomar Conference on Signals, Systems and Computers, 2013, IEEE , 2013, p. 275-279Conference paper, Published paper (Refereed)
    Abstract [en]

    We consider a multipair two-way relay channel where multiple communication pairs share the same time-frequency resource and a common relay node. We assume that all users have a single antenna, while the relay node is equipped with a very large antenna array. We consider two transmission schemes: (I) separate-training zero-forcing (ZF) and (II) a new proposed coupled-training ZF. For both schemes, the channels are estimated at the relay by using training sequences, assuming time-division duplex operation. The relay processes the received signals using ZF. With the separate-training ZF, the channels from all users are estimated separately. By contrast, with the coupled-training ZF, the relay estimates the sum of the channels from two users of a given communication pair. This reduces the amount of resources spent in the training phase. Self-interference reduction is also proposed for these schemes. When the number of relay antennas grows large, the effects of interpair interference and self-interference can be neglected. The transmit power of each user and of the relay can be made inversely proportional to the square root of the number of relay antennas while maintaining a given quality-of-service. We derive a lower bound on the capacity which enables us to evaluate the spectral efficiency. The coupled-training ZF scheme is preferable for the high-mobility environment, while the separate-training ZF scheme is preferable for the low-mobility environment.

    Place, publisher, year, edition, pages
    IEEE, 2013
    Series
    ASILOMAR CONFERENCE ON SIGNALS, SYSTEMS AND COMPUTERS, ISSN 1058-6393
    National Category
    Electrical Engineering, Electronic Engineering, Information Engineering
    Identifiers
    urn:nbn:se:liu:diva-111472 (URN)000341772900049 ()978-1-4799-2388-5 (ISBN)
    Conference
    47th Asilomar Conference on Signals, Systems and Computers, November 3-6, Pacific Grove, Califronia, USA
    Available from: 2014-10-17 Created: 2014-10-17 Last updated: 2016-08-31Bibliographically approved
    10. Multipair Full-Duplex Relaying with Massive Arrays and Linear Processing
    Open this publication in new window or tab >>Multipair Full-Duplex Relaying with Massive Arrays and Linear Processing
    2014 (English)In: IEEE Journal on Selected Areas in Communications, ISSN 0733-8716, E-ISSN 1558-0008, Vol. 32, no 9, p. 1721-1737Article in journal (Refereed) Published
    Abstract [en]

    We consider a multipair decode-and-forward relay channel, where multiple sources transmit simultaneously their signals to multiple destinations with the help of a full-duplex relay station. We assume that the relay station is equipped with massive arrays, while all sources and destinations have a single antenna. The relay station uses channel estimates obtained from received pilots and zero-forcing (ZF) or maximum-ratio combining/maximum-ratio transmission (MRC/MRT) to process the signals. To signicantly reduce the loop interference effect, we propose two techniques: i) using a massive receive antenna array; or ii) using a massive transmit antenna array together with very low transmit power at the relay station. We derive an exact achievable rate expression in closed-form for MRC/MRT processing and an analytical approximation of the achievable rate for ZF processing. This approximation is very tight, particularly for a large number of relay station antennas. These closed-form expressions enable us to determine the regions where the full-duplex mode outperforms the half-duplex mode, as well as to design an optimal power allocation scheme. This optimal power allocation scheme aims to maximize the energy efficiency for a given sum spectral efficiency and under peak power constraints at the relay station and sources. Numerical results verify the effectiveness of the optimal power allocation scheme. Furthermore, we show that, by doubling the number of transmit/receive antennas at the relay station, the transmit power of each source and of the relay station can be reduced by 1.5 dB if the pilot power is equal to the signal power, and by 3 dB if the pilot power is kept fixed, while maintaining a given quality of service.

    Place, publisher, year, edition, pages
    IEEE: , 2014
    National Category
    Communication Systems Signal Processing
    Identifiers
    urn:nbn:se:liu:diva-112761 (URN)10.1109/JSAC.2014.2330091 (DOI)000346043400009 ()
    Available from: 2014-12-15 Created: 2014-12-15 Last updated: 2017-12-05
  • 13.
    Ngo, Hien Quoc
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Performance Bounds for Very Large Multiuser MIMO Systems2012Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The last ten years have seen significant advances of multiuser MIMO (MU-MIMO) in wireless communication. MU-MIMO is now being introduced in several new generation wireless standards (e.g.,LTE-Advanced, 802.16m). The number of users is increasing with more and more applications. At the same time, high transmission data rate and communication reliability are required. Furthermore, there is a growing concern about green communication which relates to the effects of  the radiation emitted from wireless devices onthe human body. Therefore, the future MU-MIMO systems have to satisfy three main requirements: i) serving many autonomous users in the same time-frequency resource, ii) having high data rate and communication reliability, and iii) less energy consumption/radiation. These are seemingly contradictory requirements since the more users are served, the more interference the systems will suffer, or the more data rate is transmitted, the more power is required. MU-MIMO with very large antenna arrays seems to meet above demands and hence, it can be considered as a promising technology for next generation wireless systems. With very large antenna arrays (we mean arrays comprising say a hundred of antennas), the channel vectors are nearly-orthogonal and hence, multiuser interference can be significantly reduced. As a result, many users can be simultaneously served with high data rate. In particular, with coherent processing, transmit power can be reduced dramatically owing to array gain. In this thesis, we focus on the performance bounds of MU-MIMO with very large antenna arrays. We study the fundamental limits on the system performance when using large antenna arrays under practical constraints such as low complexity processing, imperfect channel state information, intercell interference, and finite-dimensional channels.

    List of papers
    1. Energy and Spectral Efficiency of Very Large Multiuser MIMO Systems
    Open this publication in new window or tab >>Energy and Spectral Efficiency of Very Large Multiuser MIMO Systems
    2013 (English)In: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, Vol. 61, no 4, p. 1436-1449Article in journal (Refereed) Published
    Abstract [en]

    A multiplicity of autonomous terminals simultaneously transmits data streams to a compact array of antennas. The array uses imperfect channel-state information derived from transmitted pilots to extract the individual data streams. The power radiated by the terminals can be made inversely proportional to the square-root of the number of base station antennas with no reduction in performance. In contrast if perfect channel-state information were available the power could be made inversely proportional to the number of antennas. Lower capacity bounds for maximum-ratio combining (MRC), zero-forcing (ZF) and minimum mean-square error (MMSE) detection are derived. An MRC receiver normally performs worse than ZF and MMSE. However as power levels are reduced, the cross-talk introduced by the inferior maximum-ratio receiver eventually falls below the noise level and this simple receiver becomes a viable option. The tradeoff between the energy efficiency (as measured in bits/J) and spectral efficiency (as measured in bits/channel use/terminal) is quantified for a channel model that includes small-scale fading but not large-scale fading. It is shown that the use of moderately large antenna arrays can improve the spectral and energy efficiency with orders of magnitude compared to a single-antenna system.

    National Category
    Communication Systems Signal Processing
    Identifiers
    urn:nbn:se:liu:diva-85224 (URN)10.1109/TCOMM.2013.020413.110848 (DOI)000318998100022 ()
    Available from: 2012-11-12 Created: 2012-11-12 Last updated: 2017-12-07
    2. The Multicell Multiuser MIMO Uplink with Very Large Antenna Arrays and a Finite-Dimensional Channel
    Open this publication in new window or tab >>The Multicell Multiuser MIMO Uplink with Very Large Antenna Arrays and a Finite-Dimensional Channel
    2013 (English)In: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, Vol. 61, no 6, p. 2350-2361Article in journal (Refereed) Published
    Abstract [en]

    We consider multicell multiuser MIMO systems with a very large number of antennas at the base station (BS). We assume that the channel is estimated by using uplink training. We further consider a physical channel model where the angular domain is separated into a finite number of distinct directions. We analyze the so-called pilot contamination effect discovered in previous work, and show that this effect persists under the finite-dimensional channel model that we consider. In particular, we consider a uniform array at the BS. For this scenario, we show that when the number of BS antennas goes to infinity, the system performance under a finite-dimensional channel model with P angular bins is the same as the performance under an uncorrelated channel model with P antennas. We further derive a lower bound on the achievable rate of uplink data transmission with a linear detector at theBS. We then specialize this lower bound to the cases of maximum-ratio combining (MRC) and zero-forcing (ZF) receivers, for a finite and an infinite number of BS antennas. Numerical results corroborate our analysis and show a comparison between the performances of MRC and ZF in terms of sum-rate.

    Keywords
    Finite-dimensional channel, multicell mutiuser MIMO, pilot contamination, very large MIMO systems
    National Category
    Communication Systems Signal Processing
    Identifiers
    urn:nbn:se:liu:diva-85223 (URN)10.1109/TCOMM.2013.032713.120408 (DOI)000321219100023 ()
    Available from: 2013-04-04 Created: 2012-11-12 Last updated: 2017-12-06Bibliographically approved
    3. EVD-based Channel Estimations for Multicell Multiuser MIMO with Very Large Antenna Arrays
    Open this publication in new window or tab >>EVD-based Channel Estimations for Multicell Multiuser MIMO with Very Large Antenna Arrays
    2012 (English)In: Proceedings of the IEEE International Conference on Acoustics, Speed and Signal Processing (ICASSP), IEEE , 2012, p. 3249-3252Conference paper, Published paper (Refereed)
    Abstract [en]

    This paper considers multicell multiuser MIMO systems with verylarge antenna arrays at the base station. We propose an eigenvalue-decomposition-based approach to channel estimation, that estimates the channel blindly from the received data. The approach exploits the asymptotic orthogonality of the channel vectors in very large MIMO systems. We show that the channel to each user can be estimated from the covariance matrix of the received signals, up to a remaining scalar multiplicative ambiguity. A short training sequence is required to resolve this ambiguity. Furthermore, to improve the performance of our approach, we combine it with the iterative least-square with projection (ILSP) algorithm. Numerical results verify the effectiveness of our channel estimation approach.

    Place, publisher, year, edition, pages
    IEEE, 2012
    Series
    Acoustics, Speech and Signal Processing (ICASSP), ISSN 1520-6149
    National Category
    Communication Systems
    Identifiers
    urn:nbn:se:liu:diva-74084 (URN)10.1109/ICASSP.2012.6288608 (DOI)000312381403081 ()978-1-4673-0045-2 (ISBN)978-1-4673-0044-5 (ISBN)
    Conference
    IEEE International Conference on Acoustics, Speed and Signal Processing (ICASSP, March 25-30, Kyoto, Japan
    Available from: 2012-01-18 Created: 2012-01-18 Last updated: 2016-08-31Bibliographically approved
  • 14.
    Ngo, Hien Quoc
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering. Queens Univ Belfast, Sch Elect Elect Engn & Comp Sci, Belfast BT3 9DT, Antrim, North Ireland.
    Ashikhmin, Alexei
    Nokia Bell Labs, Murray Hill, NJ 07974 USA.
    Yang, Hong
    Nokia Bell Labs, NJ 07974 USA.
    Larsson, Erik G
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Marzetta, Thomas L.
    Nokia Bell Labs, NJ 07974 USA.
    Cell-Free Massive MIMO Versus Small Cells2017In: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, Vol. 16, no 3, p. 1834-1850Article in journal (Refereed)
    Abstract [en]

    A Cell-Free Massive MIMO (multiple-input multiple-output) system comprises a very large number of distributed access points (APs), which simultaneously serve a much smaller number of users over the same time/frequency resources based on directly measured channel characteristics. The APs and users have only one antenna each. The APs acquire channel state information through time-division duplex operation and the reception of uplink pilot signals transmitted by the users. The APs perform multiplexing/de-multiplexing through conjugate beamforming on the downlink and matched filtering on the uplink. Closed-form expressions for individual user uplink and downlink throughputs lead to max-min power control algorithms. Max-min power control ensures uniformly good service throughout the area of coverage. A pilot assignment algorithm helps to mitigate the effects of pilot contamination, but power control is far more important in that regard. Cell-Free Massive MIMO has considerably improved performance with respect to a conventional small-cell scheme, whereby each user is served by a dedicated AP, in terms of both 95%-likely per-user throughput and immunity to shadow fading spatial correlation. Under uncorrelated shadow fading conditions, the cell-free scheme provides nearly fivefold improvement in 95%-likely per-user throughput over the small-cell scheme, and tenfold improvement when shadow fading is correlated.

  • 15.
    Ngo, Hien Quoc
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Ashikhmin, Alexei
    Bell Laboratories, Alcatel-Lucent, Murray Hill, USA.
    Yang, Hong
    Bell Laboratories, Alcatel-Lucent, Murray Hill, USA.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Marzetta, Thomas L.
    Bell Laboratories, Alcatel-Lucent, Murray Hill, USA.
    Cell-free massive MIMO: Uniformly great service for everyone2015In: SPAWC 2015T. he 16th IEEE International Workshop on Signal Processing Advances in Wireless Communications, June 28 – July 1, 2015,  Stockholm, Sweden, IEEE , 2015, p. 201-205Conference paper (Refereed)
    Abstract [en]

    We consider the downlink of Cell-Free Massive MIMO systems, where a very large number of distributed access points (APs) simultaneously serve a much smaller number of users. Each AP uses local channel estimates obtained from received uplink pilots and applies conjugate beamforming to transmit data to the users. We derive a closed-form expression for the achievable rate. This expression enables us to design an optimal max-min power control scheme that gives equal quality of service to all users.

    We further compare the performance of the Cell-Free MassiveMIMO system to that of a conventional small-cell network and show that the throughput of the Cell-Free system is much more concentrated around its median compared to that of the small cell system. The Cell-Free Massive MIMO system can provide an almost 20-fold increase in 95%-likely per-user throughput, compared with the small-cell system. Furthermore, Cell-Free systems are more robust to shadow fading correlation than small cell systems.

  • 16.
    Ngo, Hien Quoc
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Duong, Trung Q.
    Blekinge Institute of Technology, Sweden.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Uplink Performance Analysis of Multicell MU-MIMO with Zero-Forcing Receivers and Perfect CSI2011In: Proceedings of the IEEE Swedish Communication Technologies Workshop (Swe-CTW), 2011, p. 40-45Conference paper (Refereed)
    Abstract [en]

    We consider the uplink of a multicell multiuser MIMO system. The data detection is done by using the zero-forcing (ZF) technique, assuming the base station has perfect channel state information. We derive an exact closed-form expression for the uplink rate per user. We further study the asymptotic performance of the system. We show that, at high signal-to-noise ratio, the system is interference-limited and hence, we cannot improve the system performance by increasing the transmit power at each user. Instead, by increasing the number of base station antennas, the effects of interference and noise can be reduced, thereby improving the system performance. In particular, we show that, with very large antenna arrays at the base station, the transmit power of each user can be made inversely proportional to the number of base station antennas while maintaining a desired quality-of-service. Numerical results verify our analysis.

  • 17.
    Ngo, Hien Quoc
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Blind estimation of effective downlink channel gains in massive MIMO2015In: 2015 IEEE International Conference on Acoustics, Speech, and Signal Processing, Proceedings, IEEE , 2015, p. 2919-2923Conference paper (Refereed)
    Abstract [en]

    We consider the massive MIMO downlink with time-division duplex (TDD) operation and conjugate beamforming transmission. To reliably decode the desired signals, the users need to know the effective channel gain. In this paper, we propose a blind channel estimation method which can be applied at the users and which does not require any downlink pilots. We show that our proposed schemecan substantially outperform the case where each user has only statistical channel knowledge, and that the difference in performance is particularly large in certain types of channel, most notably keyhole channels. Compared to schemes that rely on downlink pilots(e.g., [1]), our proposed scheme yields more accurate channel estimates for a wide range of signal-to-noise ratios and avoid spending time-frequency resources on pilots.

  • 18.
    Ngo, Hien Quoc
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    EVD-based Channel Estimations for Multicell Multiuser MIMO with Very Large Antenna Arrays2012In: Proceedings of the IEEE International Conference on Acoustics, Speed and Signal Processing (ICASSP), IEEE , 2012, p. 3249-3252Conference paper (Refereed)
    Abstract [en]

    This paper considers multicell multiuser MIMO systems with verylarge antenna arrays at the base station. We propose an eigenvalue-decomposition-based approach to channel estimation, that estimates the channel blindly from the received data. The approach exploits the asymptotic orthogonality of the channel vectors in very large MIMO systems. We show that the channel to each user can be estimated from the covariance matrix of the received signals, up to a remaining scalar multiplicative ambiguity. A short training sequence is required to resolve this ambiguity. Furthermore, to improve the performance of our approach, we combine it with the iterative least-square with projection (ILSP) algorithm. Numerical results verify the effectiveness of our channel estimation approach.

  • 19.
    Ngo, Hien Quoc
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Large-Scale Multipair Two-Way Relay Networks with Distributed AF Beamforming2013In: IEEE Communications Letters, ISSN 1089-7798, E-ISSN 1558-2558, Vol. 17, no 12, p. 1-4Article in journal (Refereed)
    Abstract [en]

    We consider a multipair two-way relay network where multiplecommunication pairs simultaneously exchange information with the helpof multiple relay nodes. All nodes are equipped with a single antennaand channel state information is available only at the relay nodes. Each relay uses very simplesignal processing in a distributed manner, called distributed amplify-and-forward (AF)  relaying. A closed-form expression for the achievable rate is  derived. We show that the distributed AF scheme outperforms conventional orthogonal relaying. When the number of relay nodes is large, the distributed AF relaying scheme can achieve the capacity scaling given by the cut-set upper bound. Furthermore, when the number of relay nodes grows large, the transmit powers of each terminal and of the relay can be made inversely proportional to the number of relay nodes while maintaining a given quality-of-service. If the transmit power of each terminal is kept fixed, the transmit power of each relay node can be scaled down inversely proportional to the square of the number of relays.

  • 20.
    Ngo, Hien Quoc
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Linear Multihop Amplify-and-Forward Relay Channels: Error Exponent and Optimal Number of Hops2011In: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, Vol. 10, no 11, p. 3834-3842Article in journal (Refereed)
    Abstract [en]

    We compute the random coding error exponent for linear multihop amplify-and-forward (AF) relay channels. Instead of considering only the achievable rate or the error probability as a performance measure separately, the error exponent results can give us insight into the fundamental tradeoff between the information rate and communication reliability in these channels. This measure enables us to determine what codeword length that is required to achieve a given level of communication reliability at a rate below the channel capacity. We first derive a general formula for the random coding exponent of general multihop AF relay channels. Then we present a closed-form expression of a tight upper bound on the random coding error exponent for the case of Rayleigh fading. From the exponent expression, the capacity of these channels is also deduced. The effect of the number of hops on the performance of linear multihop AF relay channels from theerror exponent point of view is studied. As an application of the random coding error exponent analysis, we then find the optimal number of hops which maximizes the communication reliability (i.e., the random coding error exponent) for a given data rate. Numerical results verify our analysis, and show the tightness of the proposed bound.

  • 21.
    Ngo, Hien Quoc
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Larsson, Erik G
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    No Downlink Pilots Are Needed in TDD Massive MIMO2017In: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, Vol. 16, no 5, p. 2921-2935Article in journal (Refereed)
    Abstract [en]

    We consider the Massive Multiple-Input MultipleOutput downlink with maximum-ratio and zero-forcing processing and time-division duplex operation. To decode, the users must know their instantaneous effective channel gain. Conventionally, it is assumed that by virtue of channel hardening, this instantaneous gain is close to its average and hence that users can rely on knowledge of that average (also known as statistical channel information). However, in some propagation environments, such as keyhole channels, channel hardening does not hold. We propose a blind algorithm to estimate the effective channel gain at each user, that does not require any downlink pilots. We derive a capacity lower bound of each user for our proposed scheme, applicable to any propagation channel. Compared with the case of no downlink pilots (relying on channel hardening), and compared with training-based estimation using downlink pilots, our blind algorithm performs significantly better. The difference is especially pronounced in environments that do not offer channel hardening.

  • 22.
    Ngo, Hien Quoc
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Marzetta, Thomas L.
    Bell Laboratories, Alcatel-Lucent Murray Hill, NJ 07974, USA.
    Aspects of Favorable Propagation in Massive MIMO2014In: European Signal Processing Conference (EUSIPCO), EURASIP , 2014, p. 76-80Conference paper (Refereed)
    Abstract [en]

    Favorable propagation, dened as mutual orthogonality among the vector-valued channels to the terminals, is one of the key properties of the radio channel that is exploited in Massive MIMO. However, there has been little work that studies this topic in detail. In this paper, we first show that favorable propagation offers the most desirable scenario in terms of maximizing the sum-capacity. One useful proxy for whether propagation is favorable or not is the channel condition number. However, this proxy is not good for the case where the norms of the channel vectors are not equal. For this case, to evaluate how favorable the propagation oered by the channel is, we propose a distance from favorable propagation measure, which is the gap between the sum-capacity and the maximum capacity obtained under favorable propagation. Secondly, we examine how favorable the channels can be for two extreme scenarios: i.i.d. Rayleigh fading and uniform random line-of-sight(UR-LoS). Both environments offer (nearly) favorable propagation. Furthermore, to analyze the UR-LoS model, we propose an urns-and-balls model. This model is simple and explains the singular value spread characteristic of the UR-LoS model well.

  • 23.
    Ngo, Hien Quoc
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Marzetta, Thomas L.
    Bell Laboratories, Alcatel-Lucent, NJ, USA.
    Energy and Spectral Efficiency of Very Large Multiuser MIMO Systems2013In: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, Vol. 61, no 4, p. 1436-1449Article in journal (Refereed)
    Abstract [en]

    A multiplicity of autonomous terminals simultaneously transmits data streams to a compact array of antennas. The array uses imperfect channel-state information derived from transmitted pilots to extract the individual data streams. The power radiated by the terminals can be made inversely proportional to the square-root of the number of base station antennas with no reduction in performance. In contrast if perfect channel-state information were available the power could be made inversely proportional to the number of antennas. Lower capacity bounds for maximum-ratio combining (MRC), zero-forcing (ZF) and minimum mean-square error (MMSE) detection are derived. An MRC receiver normally performs worse than ZF and MMSE. However as power levels are reduced, the cross-talk introduced by the inferior maximum-ratio receiver eventually falls below the noise level and this simple receiver becomes a viable option. The tradeoff between the energy efficiency (as measured in bits/J) and spectral efficiency (as measured in bits/channel use/terminal) is quantified for a channel model that includes small-scale fading but not large-scale fading. It is shown that the use of moderately large antenna arrays can improve the spectral and energy efficiency with orders of magnitude compared to a single-antenna system.

  • 24.
    Ngo, Hien Quoc
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Marzetta, Thomas L.
    Bell Laboratories, Alcatel-Lucent, Murray Hill, NJ 07974, USA.
    Massive MU-MIMO Downlink TDD Systems with Linear Precodingand Downlink Pilots2013In: 51st Annual Allerton Conference on Communication, Control, and Computing (Allerton), 2013, IEEE , 2013, p. 293-298Conference paper (Refereed)
    Abstract [en]

    We consider a massive MU-MIMO downlink time-division duplex system where a base station (BS) equipped with many antennas serves several single-antenna users in the same time-frequency resource. We assume that the BS uses linear precoding for the transmission. To reliably decode the signals transmitted from the BS, each user should have an estimate of its channel. In this work, we consider an efficient channel estimation scheme to acquire CSI at each user, called beamforming training scheme. With the beamforming training scheme, the BS precodes the pilot sequences and forwards to all users. Then, based on the received pilots, each user uses minimum mean-square error channel estimation to estimate the effective channel gains. The channel estimation overhead of this scheme does not depend on the number of BS antennas, and is only proportional to the number of users. We then derive a lower bound on the capacity for maximum-ratio transmission and zero-forcing precoding techniques which enables us to evaluate the spectral efficiency taking into account the spectral eciency loss associated with the transmission of the downlink pilots. Comparing with previous work where each user uses only the statistical channel properties to decode the transmitted signals, we see that the proposed beamforming training scheme is preferable for moderate and low-mobility environments.

  • 25.
    Ngo, Hien Quoc
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Marzetta, Thomas L.
    Bell Laboratories, Alcatel-Lucent, NJ, USA .
    The Multicell Multiuser MIMO Uplink with Very Large Antenna Arrays and a Finite-Dimensional Channel2013In: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, Vol. 61, no 6, p. 2350-2361Article in journal (Refereed)
    Abstract [en]

    We consider multicell multiuser MIMO systems with a very large number of antennas at the base station (BS). We assume that the channel is estimated by using uplink training. We further consider a physical channel model where the angular domain is separated into a finite number of distinct directions. We analyze the so-called pilot contamination effect discovered in previous work, and show that this effect persists under the finite-dimensional channel model that we consider. In particular, we consider a uniform array at the BS. For this scenario, we show that when the number of BS antennas goes to infinity, the system performance under a finite-dimensional channel model with P angular bins is the same as the performance under an uncorrelated channel model with P antennas. We further derive a lower bound on the achievable rate of uplink data transmission with a linear detector at theBS. We then specialize this lower bound to the cases of maximum-ratio combining (MRC) and zero-forcing (ZF) receivers, for a finite and an infinite number of BS antennas. Numerical results corroborate our analysis and show a comparison between the performances of MRC and ZF in terms of sum-rate.

  • 26.
    Ngo, Hien Quoc
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Marzetta, Thomas L.
    Bell Laboratories, Alcatel-Lucent, Murray Hill, NJ 07974, USA.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Analysis of the Pilot Contamination Effect in Very Large Multicell Multiuser MIMO Systems for Physical Channel Models2011In: Proceedings of the IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP), IEEE conference proceedings, 2011, p. 3464-3467Conference paper (Refereed)
    Abstract [en]

    We consider multicell multiuser MIMO systems with a very large numberof antennas at the base station. We assume that the channel is estimated by using uplink training sequences, and we consider a physical channel model where the angular domain is separated into a finite number of directions. We analyze the so-called pilot contamination effect discovered in previous work, and show that this effect persists under the finite-dimensional channel model that we consider.  We further derive closed-form bounds on the achievable rate of uplink data transmission with maximum-ratio combining, for a finite and an infinite number of base station antennas.

  • 27.
    Ngo, Hien Quoc
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Matthaiou, Michail
    Chalmers University of Technology, Gothenburg, Sweden.
    Duong, Trung Q.
    Blekinge Institute of Technology, Karlskrona, Sweden.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Uplink Performance Analysis of Multicell MU-SIMO Systems with ZF Receivers2013In: IEEE Transactions on Vehicular Technology, ISSN 0018-9545, E-ISSN 1939-9359Article in journal (Refereed)
    Abstract [en]

    We consider the uplink of a multicell multiuser single-input multiple-output system where the channel experiences both small and large-scale fading. The data detection is done by using the linear zero-forcing technique, assuming the base station (BS) has perfect channel state information of all users in its cell. We derive  new, exact analytical expressions for the uplink rate, symbol error rate, and outage probability per user, as well as alower bound on the achievable rate. This bound is very tight and becomes exact in the large-number-of-antennas limit. We further study the asymptotic system performance in the regimes of high signal-to-noise ratio (SNR), large number of antennas, and large number of users per cell. We show that at high SNRs, the system is interference-limited and hence, we cannot improve the system performance by increasing the transmit power of each user. Instead, by increasing the number of BS antennas, the effects of interference and noise can be reduced, thereby improving the system performance. We demonstrate that, with very large antenna arrays at the BS, the transmit power of each user can be made inversely proportional to the number of BS antennas while maintaining a desired quality-of-service. Numerical results are presented to verify our analysis.

  • 28.
    Ngo, Hien Quoc
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Matthaiou, Michail
    Queen's University Belfast, Belfast, BT3 9DT, U.K..
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Massive MIMO with Optimal Power and Training Duration Allocation2014In: IEEE Wireless Communications Letters, ISSN 2162-2337, E-ISSN 2162-2345, Vol. 3, no 6, p. 605-608Article in journal (Refereed)
    Abstract [en]

    We consider the uplink of massive multicell multiple-input multiple-output systems, where the base stations (BSs), equipped with massive arrays, serve simultaneously several terminals in the same frequency band. We assume that the BS estimates the channel from uplink training, and then uses the maximum ratio combining technique to detect the signals transmitted from all terminals in its own cell. We propose an optimal resource allocation scheme which jointly selects the training duration,training signal power, and data signal power in order to maximize the sum spectral efficiency, for a given total energy budget spent in a coherence interval. Numerical results verify the benets of the optimal resource allocation scheme. Furthermore, we show that more training signal power should be used at low signal-to-noise ratio(SNRs), and vice versa at high SNRs. Interestingly, for the entire SNR regime, the optimal training duration is equal to the number of terminals.

  • 29.
    Ngo, Hien Quoc
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Matthaiou, Michail
    Department of Signals and Systems, Chalmers University of Technology, Gothenburg, Sweden.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Performance Analysis of Large Scale MU-MIMO with Optimal Linear Receiver2012In: Proceedings of the IEEE Swedish Communication Technologies Workshop (Swe-CTW), IEEE , 2012, p. 59-64Conference paper (Refereed)
    Abstract [en]

    We consider the uplink of multicell multiuser MIMO(MU-MIMO) systems with very large antenna arrays at the base station (BS). We assume that the BS estimates the channel through uplink training, and then uses this channel estimate to detect the signals transmitted from a multiplicity of autonomous users in its cell. By taking the correlation between the channel estimate and the interference from other cells into account, we propose an optimal linear receiver (OLR) which maximizes the received signal-to-interference-plus-noise (SINR). Analytical approximations of the exact and lower bound on the achievable rate are then derived. The bound is very tight, especially at large number of BS antennas. We show that at low SINR, maximal-ratio combing (MRC) receiver performs as well as OLR, however at high SINR, OLR outperforms MRC. Compared with the typical minimum mean-square error receiver, our proposed OLR improves systematically the system performance, especially when the interference is large.

  • 30.
    Ngo, Hien Quoc
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Suraweera, Himal A.
    University of Peradeniya, Peradeniya 20400, Sri Lanka .
    Matthaiou, Michail
    Queen's University Belfast, Belfast, BT3 9DT, U.K..
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Multipair Full-Duplex Relaying with Massive Arrays and Linear Processing2014In: IEEE Journal on Selected Areas in Communications, ISSN 0733-8716, E-ISSN 1558-0008, Vol. 32, no 9, p. 1721-1737Article in journal (Refereed)
    Abstract [en]

    We consider a multipair decode-and-forward relay channel, where multiple sources transmit simultaneously their signals to multiple destinations with the help of a full-duplex relay station. We assume that the relay station is equipped with massive arrays, while all sources and destinations have a single antenna. The relay station uses channel estimates obtained from received pilots and zero-forcing (ZF) or maximum-ratio combining/maximum-ratio transmission (MRC/MRT) to process the signals. To signicantly reduce the loop interference effect, we propose two techniques: i) using a massive receive antenna array; or ii) using a massive transmit antenna array together with very low transmit power at the relay station. We derive an exact achievable rate expression in closed-form for MRC/MRT processing and an analytical approximation of the achievable rate for ZF processing. This approximation is very tight, particularly for a large number of relay station antennas. These closed-form expressions enable us to determine the regions where the full-duplex mode outperforms the half-duplex mode, as well as to design an optimal power allocation scheme. This optimal power allocation scheme aims to maximize the energy efficiency for a given sum spectral efficiency and under peak power constraints at the relay station and sources. Numerical results verify the effectiveness of the optimal power allocation scheme. Furthermore, we show that, by doubling the number of transmit/receive antennas at the relay station, the transmit power of each source and of the relay station can be reduced by 1.5 dB if the pilot power is equal to the signal power, and by 3 dB if the pilot power is kept fixed, while maintaining a given quality of service.

  • 31.
    Ngo, Hien Quoc
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering. Queens Univ Belfast, Sch Elect Elect Engn & Comp Sci, Belfast BT3 9DT, Antrim, North Ireland.
    Tran, Le-Nam
    Maynooth Univ, Dept Elect Engn, Maynooth, Kildare, Ireland.
    Duong, Trung Q.
    Queens Univ Belfast, Sch Elect Elect Engn & Comp Sci, Belfast BT3 9DT, Antrim, North Ireland.
    Matthaiou, Michail
    Queens Univ Belfast, Sch Elect Elect Engn & Comp Sci, Belfast BT3 9DT, Antrim, North Ireland.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Energy Efficiency Optimization for Cell-Free Massive MIMO2017In: 2017 IEEE 18TH INTERNATIONAL WORKSHOP ON SIGNAL PROCESSING ADVANCES IN WIRELESS COMMUNICATIONS (SPAWC), 2017Conference paper (Refereed)
    Abstract [en]

    This paper considers a time-division duplex cell-free massive multiple-input multiple-output downlink with imperfect channel station information and conjugate beamforming scheme. The total energy efficiency is investigated taking into account the hardware power consumption and the backhaul power consumption. We propose an optimal power allocation algorithm which aims to maximize the total energy efficiency, under a per-access point power constraint and a per-user spectral efficiency constraint. This optimization problem can be approximately solved via a sequence of second-order cone programs. Compared with the case of without power control, our proposed power allocation scheme can double the total energy efficiency. Furthermore, we show that, when the number of access points is large, the backhaul power consumption affects significantly the total energy efficiency.

  • 32.
    Ngo, Hien
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Suraweerat, H.A.
    Department of Electrical and Electronics Engineering, University of of Peradeniya, Peradeniya, Sri Lanka.
    Matthaiou, M.
    School of Electronics, Electrical Engineering and Computer Science, Queens University of Belfast, Belfast, United Kingdom; Department of Signals and Systems, Chalmers University of of Technology, Gothenburg, Sweden.
    Larsson, Erik G
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Multipair massive MIMO full-duplex relaying with MRC/MRT processing2014In: 2014 IEEE International Conference on Communications, ICC 2014, IEEE Computer Society , 2014, no 6884081, p. 4807-4813Conference paper (Refereed)
    Abstract [en]

    We consider a multipair relay channel, where multiple sources communicate with multiple destinations with the help of a full-duplex (FD) relay station (RS). All sources and destinations have a single antenna, while the RS is equipped with massive arrays. We assume that the RS estimates the channels by using training sequences transmitted from sources and destinations. Then, it uses maximum-ratio combining/maximum-ratio transmission (MRC/MRT) to process the signals. To significantly reduce the loop interference (LI) effect, we propose two massive MIMO processing techniques: i) using a massive receive antenna array; or ii) using a massive transmit antenna array together with very low transmit power at the RS. We derive an exact achievable rate in closed-form and evaluate the system spectral efficiency. We show that, by doubling the number of antennas at the RS, the transmit power of each source and of the RS can be reduced by 1.5 dB if the pilot power is equal to the signal power and by 3 dB if the pilot power is kept fixed, while maintaining a given quality-of-service. Furthermore, we compare FD and half-duplex (HD) modes and show that FD improves significantly the performance when the LI level is low. © 2014 IEEE.

  • 33.
    Ngo, Quoc Hien
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Marzetta, Thomas L.
    Bell Laboratories, Alcatel-Lucent, Murray Hill, NJ 07974, USA.
    Uplink Power Efficiency of Multiuser MIMO with Very Large Antenna Arrays2011In: Proceedings of Allerton Conference on Communication, Control, and Computing, IEEE , 2011, p. 1272-1279Conference paper (Refereed)
    Abstract [en]

    A multiplicity of autonomous terminals simultaneously transmits data streams to a compact array of antennas. The array uses imperfect channel-state information derived from transmitted pilots to extract the individual data streams. The power radiated by the terminals can be made inversely proportional to the square-root of the number of base station antennas with no reduction in performance. In contrast if perfect channel-state information were available the power could be made inversely proportional to the number of antennas. A maximum-ratio combining receiver normally performs worse than a zero-forcing receiver. However as power levels are reduced, the cross-talk introduced by the inferior maximum-ratio receiver eventually falls below the noise level and this simple receiver becomes a viable option.

  • 34.
    Nguyen, Nam-Phong
    et al.
    Queens University of Belfast, North Ireland.
    Duong, Trung Q
    Queens University of Belfast, North Ireland.
    Ngo, Hien Quoc
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering. Queens University of Belfast, North Ireland.
    Hadzi-Velkov, Zoran
    St Cyril & Methodius Univ, Skopje, Macedonia.
    Shu, Lei
    Guangdong University of Petrochem Technology, Peoples R China.
    Secure 5G Wireless Communications: A Joint Relay Selection and Wireless Power Transfer Approach2016In: IEEE Access, E-ISSN 2169-3536, Vol. 4, p. 3349-3359Article in journal (Refereed)
    Abstract [en]

    In this paper, we investigate the secrecy performance of an energy harvesting relay system, where a legitimate source communicates with a legitimate destination via the assistance of multiple trusted relays. In the considered system, the source and relays deploy the time-switching-based radio frequency energy harvesting technique to harvest energy from a multi-antenna beacon. Different antenna selection and relay selection schemes are applied to enhance the security of the system. Specifically, two relay selection schemes based on the partial and full knowledge of channel state information, i. e., optimal relay selection and partial relay selection, and two antenna selection schemes for harvesting energy at source and relays, i. e., maximizing energy harvesting channel for the source and maximizing energy harvesting channel for the selected relay, are proposed. The exact and asymptotic expressions of secrecy outage probability in these schemes are derived. We demonstrate that applying relay selection approaches in the considered energy harvesting system can enhance the security performance. In particular, optimal relay selection scheme outperforms partial relay selection scheme and achieves full secrecy diversity order, regardless of energy harvesting scenarios.

  • 35.
    Papazafeiropoulos, Anastasios K.
    et al.
    Imperial Coll London, England; University of Edinburgh, Scotland.
    Ngo, Hien Quoc
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering. Queen’s University Belfast, Belfast, Northern Ireland.
    Ratnarajah, Tharmalingam
    University of Edinburgh, UK.
    Performance of Massive MIMO Uplink With Zero-Forcing Receivers Under Delayed Channels2017In: IEEE Transactions on Vehicular Technology, ISSN 0018-9545, E-ISSN 1939-9359, Vol. 66, no 4, p. 3158-3169Article in journal (Refereed)
    Abstract [en]

    In this paper, we analyze the performance of the up-link communication of massive multicell multiple-input multiple-output (MIMO) systems under the effects of pilot contamination and delayed channels because of terminal mobility. The base stations (BSs) estimate the channels through the uplink training and then use zero-forcing (ZF) processing to decode the transmit signals from the users. The probability density function (pdf) of the signal-to-interference-plus-noise ratio (SINR) is derived for any finite number of antennas. From this pdf, we derive an achievable ergodic rate with a finite number of BS antennas in closed form. Insights into the impact of the Doppler shift (due to terminal mobility) at the low signal-to-noise ratio (SNR) regimes are exposed. In addition, the effects on the outage probability are investigated. Furthermore, the power scaling law and the asymptotic performance result by infinitely increasing the numbers of antennas and terminals (while their ratio is fixed) are provided. The numerical results demonstrate the performance loss for various Doppler shifts. Among the interesting observations revealed is that massive MIMO is favorable even under channel aging conditions.

  • 36.
    Phan, H.
    et al.
    Blekinge Institute of Technology Karlskrona, Sweden; University of of Reading, United Kingdom.
    Chu, T.M.C.
    Blekinge Institute of Technology Karlskrona, Sweden.
    Zepernick, H.-J.
    Blekinge Institute of Technology Karlskrona, Sweden.
    Ngo, Hien
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Performance of cognitive radio networks with finite buffer using multiple vacations and exhaustive service2014In: 2014, 8th International Conference on Signal Processing and Communication Systems, ICSPCS 2014 - Proceedings, Institute of Electrical and Electronics Engineers Inc. , 2014, no 7021133Conference paper (Refereed)
    Abstract [en]

    In this paper, we analyze the performance of a cognitive radio network where the secondary transmitter, besides its own transmission, occasionally relays the primary signal. It is assumed that the secondary transmitter employs the exhaustive service mode to transmit the secondary signal and multiple vacations to relay the primary signal. When assisting the primary transmitter, we assume that the secondary transmitter utilizes the decode-and-forward protocol to process the primary signal and forwards it to the primary receiver. Furthermore, the secondary transmitter has a finite buffer, the arriving packets of the secondary network are modeled as a Poisson process, and all channels are subject to Nakagami-m fading. Modeling the system as an M/G/1/K queueing system with exhaustive service and multiple vacations, using an embedded Markov chain approach to analyze the system, we obtain several key queueing performance indicators, i.e., the channel utilization, blocking probability, mean number of packets, and mean serving time of a packet in the system. The derived formulas are then utilized to evaluate the performance of the considered system.

  • 37.
    Suraweera, Himal A.
    et al.
    Singapore University of Technology and Design.
    Ngo, Hien Quoc
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Duong, Trung Q.
    Blekinge Institute of Technology, Karlskrona, Sweden .
    Yuen, Chau
    Singapore University of Technology and Design.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Multi-Pair Amplify-and-Forward Relaying with Very Large Antenna Arrays2013In: Proceedings of the IEEE International Conference on Communicatons (ICC), IEEE , 2013, p. 4635-4640Conference paper (Refereed)
    Abstract [en]

    We consider a multi-pair relay channel where multiple sources simultaneously communicate with destinations using a relay. Each source or destination has only a single antenna, while the relay is equipped with a very large antenna array. We investigate the power efficiency of this system when maximum ratio combining/maximal ratio transmission (MRC/MRT) or zero-forcing (ZF) processing is used at the relay. Using a very large array, the transmit power of each source or relay (or both) can be made inversely proportional to the number of relay antennas while maintaining a given quality-of-service. At the same time, the achievable sum rate can be increased by a factor of the number of source-destination pairs. We show that when the number of antennas grows to infinity, the asymptotic achievable rates of MRC/MRT and ZF are the same if we scale the power at the sources. Depending on the large scale fading effect, MRC/MRT can outperform ZF or vice versa if we scale the power at the relay.

  • 38.
    Wang, Lifeng
    et al.
    UCL, England.
    Ngo, Hien
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Elkashlan, Maged
    Queen Mary University of London, England.
    Duong, Trung Q.
    Queens University of Belfast, North Ireland.
    Wong, Kai-Kit
    UCL, England.
    Massive MIMO in Spectrum Sharing Networks: Achievable Rate and Power Efficiency2017In: IEEE Systems Journal, ISSN 1932-8184, E-ISSN 1937-9234, Vol. 11, no 1, p. 20-31Article in journal (Refereed)
    Abstract [en]

    Massive multiple input multiple output (MIMO) is one of the key technologies for fifth generation and can substantially improve energy and spectrum efficiencies. This paper explores the potential benefits of massive MIMO in spectrum sharing networks. We consider a multiuser MIMO primary network, with N-P-antenna primary base station (PBS) andK singleantenna primary users (PUs), and a multiple-input-single-output secondary network, with N-S-antenna secondary base station and a single-antenna secondary user. Using the proposed model, we derive a tight closed-form expression for the lower bound on the average achievable rate, which is applicable to arbitrary system parameters. By performing large-system analysis, we examine the impact of large number of PBS antennas and large number of PUs on the secondary network. It is shown that, when N-P and K grow large, N-S must be proportional to lnK or larger, to enable successful secondary transmission. In addition, we examine the impact of imperfect channel state information on the secondary network. It is shown that the detrimental effect of channel estimation errors is significantly mitigated as N-S grows large.

1 - 38 of 38
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