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  • 1.
    Björnson, Emil
    et al.
    Linköping Univ, Dept Elect Engn ISY, Linköping, Sweden..
    de Carvalho, Elisabeth
    Aalborg Univ, Dept Elect Syst, Aalborg, Denmark..
    Larsson, Erik G.
    Linköping Univ, Dept Elect Engn ISY, Linköping, Sweden..
    Popovski, Petar
    Aalborg Univ, Dept Elect Syst, Aalborg, Denmark..
    Random Access Protocol for Massive MIMO: Strongest-User Collision Resolution (SUCR)2016In: 2016 IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS (ICC), IEEE , 2016, p. 820-825Conference paper (Refereed)
    Abstract [en]

    Wireless networks with many antennas at the base stations and multiplexing of many users, known as Massive MIMO systems, are key to handle the rapid growth of data traffic. As the number of users increases, the random access in contemporary networks will be flooded by user collisions. In this paper, we propose a reengineered random access protocol, coined strongest-user collision resolution (SUCR). It exploits the channel hardening feature of Massive MIMO channels to enable each user to detect collisions, determine how strong the contenders' channels are, and only keep transmitting if it has the strongest channel gain. The proposed SUCR protocol can quickly and distributively resolve the vast majority of all pilot collisions.

  • 2.
    Björnson, Emil
    et al.
    Linköping Univ, Dept Elect Engn, SE-58183 Linköping, Sweden..
    de Carvalho, Elisabeth
    Aalborg Univ, Dept Elect Syst, DK-9220 Aalborg, Denmark..
    Sorensen, Jesper H.
    Aalborg Univ, Dept Elect Syst, DK-9220 Aalborg, Denmark..
    Larsson, Erik G.
    Linköping Univ, Dept Elect Engn, SE-58183 Linköping, Sweden..
    Popovski, Petar
    Aalborg Univ, Dept Elect Syst, DK-9220 Aalborg, Denmark..
    A Random Access Protocol for Pilot Allocation in Crowded Massive MIMO Systems2017In: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, Vol. 16, no 4, p. 2220-2234Article in journal (Refereed)
    Abstract [en]

    The massive multiple-input multiple-output (MIMO) technology has great potential to manage the rapid growth of wireless data traffic. Massive MIMO achieves tremendous spectral efficiency by spatial multiplexing many tens of user equipments (UEs). These gains are only achieved in practice if many more UEs can connect efficiently to the network than today. As the number of UEs increases, while each UE intermittently accesses the network, the random access functionality becomes essential to share the limited number of pilots among the UEs. In this paper, we revisit the random access problem in the Massive MIMO context and develop a reengineered protocol, termed strongest-user collision resolution (SUCRe). An accessing UE asks for a dedicated pilot by sending an uncoordinated random access pilot, with a risk that other UEs send the same pilot. The favorable propagation of massive MIMO channels is utilized to enable distributed collision detection at each UE, thereby determining the strength of the contenders' signals and deciding to repeat the pilot if the UE judges that its signal at the receiver is the strongest. The SUCRe protocol resolves the vast majority of all pilot collisions in crowded urban scenarios and continues to admit UEs efficiently in overloaded networks.

  • 3.
    de Carvalho, Elisabeth
    et al.
    Aalborg University.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Leinkeit, Florian
    University of Bremen.
    Bockelmann, Carsten
    University of Bremen.
    Popovski, Petar
    Aalborg University.
    Relaying and Wireless Network Coding2016In: 5g Mobile and Wireless Communications Technology / [ed] Afif Osseiran, Jose F. Monserrat, Patrick Marsch, Cambridge University Press, 2016, p. 277-302Chapter in book (Other academic)
    Abstract [en]

    Relaying and network coding are powerful techniques that improve the performance of a cellular network, for example by extending the network coverage, by increasing the system capacity or by enhancing the wireless link reliability. This chapter focuses on relaying and wireless network coding in 5G. After reviewing the history of relaying, the key envisioned scenarios for relaying in 5G are highlighted, namely the provisioning of wireless backhaul in Ultra-Dense Networks (UDNs), for nomadic cells or for data aggregation in the context of massive machine-type communications. While full-duplex technology is slowly gaining maturity, it is expected that due to complexity reasons most relaying scenarios in 5G will be based on half-duplex devices. Therefore, finding solutions to overcome the half-duplex limitation remains critical. The chapter describes the following three key innovations for efficient half-duplex relaying:

    • By applying the principles of wireless network coding to distributed multi-way traffic, in-band relaying becomes a spectrally efficient solution for wireless backhaul in ultra-dense networks of small cells, despite conventional views.

    • Non-orthogonal multiple access techniques, as required by physical-layer network coding, are essential for increased spectral efficiency when simultaneous multi-flows are exchanged through a same relay. Here, Interleave-Division Multiple-Access (IDMA) is put forward for its ability to support flexible rate requirements.

    • Buffer-aided relaying is featured where different ways to exploit buffering are described for improved diversity and increased rates. This technique targets delay tolerant applications having high data rate requirements.

  • 4.
    de Carvalho, Elisabeth
    et al.
    Aalborg Univ, Dept Elect Syst, Aalborg, Denmark..
    Björnson, Emil
    Linköping Univ, Dept Elect Engn ISY, S-58183 Linköping, Sweden..
    Larsson, Erik G.
    Linköping Univ, Dept Elect Engn ISY, S-58183 Linköping, Sweden..
    Popovski, Petar
    Aalborg Univ, Dept Elect Syst, Aalborg, Denmark..
    Random Access For Massive MIMO Systems With Intra-Cell Pilot Contamination2016In: 2016 IEEE INTERNATIONAL CONFERENCE ON ACOUSTICS, SPEECH AND SIGNAL PROCESSING PROCEEDINGS, IEEE , 2016, p. 3361-3365Conference paper (Refereed)
    Abstract [en]

    Massive MIMO systems, where the base stations are equipped with hundreds of antenna elements, are an attractive way to attain unprecedented spectral efficiency in future wireless networks. In the "classical" massive MIMO setting, the terminals are assumed fully loaded and a main impairment to the performance comes from the inter-cell pilot contamination, i.e., interference from terminals in neighboring cells using the same pilots as in the home cell. However, when the terminals are active intermittently, it is viable to avoid inter-cell contamination by pre-allocation of pilots, while same-cell terminals use random access to select the allocated pilot sequences. This leads to the problem of intra-cell pilot contamination. We propose a framework for random access in massive MIMO networks and derive new uplink sum rate expressions that take intra-cell pilot collisions, intermittent terminal activity, and interference into account. We use these expressions to optimize the terminal activation probability and pilot length.

  • 5.
    de Carvalho, Elisabeth
    et al.
    Aalborg Univ, Dept Elect Syst, DK-9400 Aalborg, Denmark..
    Björnson, Emil
    Linköping Univ, Dept Elect Engn ISY, S-58183 Linköping, Sweden..
    Sorensen, Jesper H.
    Aalborg Univ, Dept Elect Syst, DK-9400 Aalborg, Denmark..
    Larsson, Erik G.
    Linköping Univ, Dept Elect Engn ISY, S-58183 Linköping, Sweden..
    Popovski, Petar
    Aalborg Univ, Dept Elect Syst, DK-9400 Aalborg, Denmark..
    Random Pilot and Data Access in Massive MIMO for Machine-Type Communications2017In: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, Vol. 16, no 12, p. 7703-7717Article in journal (Refereed)
    Abstract [en]

    A massive MIMO system, represented by a base station with hundreds of antennas, is capable of spatially multiplexing many devices and thus naturally suited to serve dense crowds of wireless devices in emerging applications, such as machine-type communications. Crowd scenarios pose new challenges in the pilot-based acquisition of channel state information and call for pilot access protocols that match the intermittent pattern of device activity. A joint pilot assignment and data transmission protocol based on random access is proposed in this paper for the uplink of a massive MIMO system. The protocol relies on the averaging across multiple transmission slots of the pilot collision events that result from the random access process. We derive new uplink sum rate expressions that take pilot collisions, intermittent device activity, and interference into account. Simplified bounds are obtained and used to optimize the device activation probability and pilot length. A performance analysis indicates how performance scales as a function of the number of antennas and the transmission slot duration.

  • 6.
    de Carvalho, Elisabeth
    et al.
    Aalborg Univ, Aalborg, Denmark..
    Björnson, Emil
    Linköping Univ, Linköping, Sweden..
    Sorensen, Jesper H.
    Aalborg Univ, Aalborg, Denmark..
    Popovski, Petar
    Aalborg Univ, Aalborg, Denmark..
    Larsson, Erik G.
    Linköping Univ, Linköping, Sweden..
    Random Access Protocols for Massive MIMO2017In: IEEE Communications Magazine, ISSN 0163-6804, E-ISSN 1558-1896, Vol. 55, no 5, p. 216-222Article in journal (Refereed)
    Abstract [en]

    5G wireless networks are expected to support new services with stringent requirements on data rates, latency and reliability. One novel feature is the ability to serve a dense crowd of devices, calling for radically new ways of accessing the network. This is the case in machine-type communications, but also in urban environments and hotspots. In those use cases, the high number of devices and the relatively short channel coherence interval do not allow per-device allocation of orthogonal pilot sequences. This article addresses the need for random access by the devices to pilot sequences used for channel estimation, and shows that Massive MIMO is a main enabler to achieve fast access with high data rates, and delay-tolerant access with different data rate levels. Three pilot access protocols along with data transmission protocols are described, fulfilling different requirements of 5G services.

  • 7.
    Shokri-Ghadikolaei, Hossein
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Fischione, Carlo
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Fodor, Gabor
    KTH, School of Electrical Engineering (EES), Automatic Control. Ericsson Res, Sweden.
    Popovski, Petar
    Aalborg Univ, Denmark.
    Zorzi, Michele
    Univ Padua, Italy.
    Millimeter Wave Cellular Networks: A MAC Layer Perspective2015In: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, Vol. 63, no 10, p. 3437-3458Article in journal (Refereed)
    Abstract [en]

    The millimeter wave (mmWave) frequency band is seen as a key enabler of multi-gigabit wireless access in future cellular networks. In order to overcome the propagation challenges, mmWave systems use a large number of antenna elements both at the base station and at the user equipment, which lead to high directivity gains, fully-directional communications, and possible noise-limited operations. The fundamental differences between mmWave networks and traditional ones challenge the classical design constraints, objectives, and available degrees of freedom. This paper addresses the implications that highly directional communication has on the design of an efficient medium access control (MAC) layer. The paper discusses key MAC layer issues, such as synchronization, random access, handover, channelization, interference management, scheduling, and association. The paper provides an integrated view on MAC layer issues for cellular networks, identifies new challenges and tradeoffs, and provides novel insights and solution approaches.

  • 8.
    Shokri-Ghadikolaei, Hossein
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Fischione, Carlo
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Popovski, Petar
    Aalborg University, Denmark.
    Zorzi, Michele
    Design aspects of short range millimeter wave networks: A MAC layer perspective2016In: IEEE Network, ISSN 0890-8044, E-ISSN 1558-156X, Vol. 30, no 3, p. 88-96Article in journal (Refereed)
    Abstract [en]

    Increased density of wireless devices, ever growing demands for extremely high data rate, and spectrum scarcity at microwave bands make the millimeter wave (mmWave) frequencies an important player in future wireless networks. However, mmWave communication systems exhibit severe attenuation, blockage, deafness, and may need microwave networks for coordination and fall-back support. To compensate for high attenuation, mmWave systems exploit highly directional operation, which in turn substantially reduces the interference footprint. The significant differences between mmWave networks and legacy communication technologies challenge the classical design approaches, especially at the medium access control (MAC) layer, which has received comparatively less attention than PHY and propagation issues in the literature so far. In this paper, the MAC layer design aspects of shortrange mmWave networks are discussed. In particular, we explain why current mmWave standards fail to fully exploitthe potential advantages of short range mmWave technology, and argue for the necessity of new collision-awarehybrid resource allocation frameworks with on-demand control messages, the advantages of a collision notification message, and the potential of multihop communication to provide reliable mmWave connections.

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