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  • 1.
    Ben Slimane, Slimane
    et al.
    KTH, School of Information and Communication Technology (ICT), Communication Systems, CoS.
    Osseiran, Afif
    Ericsson Research, Ericsson AB, Stockholm, Sweden.
    Relay Communication with Delay Diversity for Future Communication Systems2006In: 2006 IEEE 64TH VEHICULAR TECHNOLOGY CONFERENCE, VOLS 1-6, New York: IEEE , 2006, p. 1-5Conference paper (Refereed)
    Abstract [en]

    In this paper we consider relay communication as a way of increasing the diversity gain in fading wireless environments. The idea is to combine relay communication with delay diversity where each relay introduces a certain time delay to the signal before forwarding. The result is an increased frequency selectivity in the relay channel which can be exploited at the receiver. The obtained results show that, for single carrier signals with frequency domain equalization, considerable performance gain is obtained.

  • 2.
    Logothetis, Andrew
    et al.
    Ericsson Research, Ericsson AB, Stockholm, Sweden.
    Osseiran, Afif
    Ericsson Research, Ericsson AB, Stockholm, Sweden.
    Performance of Speech Services in WCDMA using Fixed-Beams and Transmit Diversity Systems2005In: VTC2005-SPRING: 2005 IEEE 61ST VEHICULAR TECHNOLOGY CONFERENCE, VOLS 1-5, PROCEEDINGS, IEEE , 2005, p. 2890-2894Conference paper (Refereed)
    Abstract [en]

    The system capacity of speech users in WCDMA system is investigated using a number of different downlink transmit modes. A single antenna sectorized system, a two fixed-beam antenna array, a closed-loop (mode I) and an open-loop space time transmit diversity methods are compared in a dynamic radio network simulator. Frequency selective and flat fading channels are considered. The results show that the diversity gain in flat fading channels is substantial. In frequency selective fading, the benefits of fixed-beam systems are encouraging, whereas the performance of transmit diversity methods (especially the open-loop scheme) are rather unsatisfactory.

  • 3.
    Logothetis, Andrew
    et al.
    Ericsson Research, Ericsson AB, Stockholm, Sweden.
    Osseiran, Afif
    Ericsson Research, Ericsson AB, Stockholm, Sweden.
    SINR Estimation and Orthogonality Factor Calculation of DS-CDMA Signals in MIMO Channels Employing Linear Transceiver Filters2007In: Wireless Communications & Mobile Computing, ISSN 1530-8669, E-ISSN 1530-8677, Vol. 7, no 1, p. 103-112Article in journal (Refereed)
    Abstract [en]

    A discrete-time model of DS-CDMA signaling using multiple transmit and receive antennas employing linear transceiver filters is derived. For each link, connecting a base-station to a wireless mobile user, the downlink signal to interference plus noise ratio (SINR) after despreading is derived analytically, and as a by-product an exact closed-form solution of the orthogonality factor is obtained.

    The orthogonality factor is derived for any linear receiver structure that is implemented by a bank of correlators and for any linear combining techniques (such as MRC and MMSE), and for any number of transmit and/or receive antennas that utilize transmit (pre-RAKE) and receive (post-RAKE) filtering.

    The MIMO DS-CDMA model is derived using a filter representation, and is extended to a vector/matrix formulation that permits a systematic and efficient way of computing the SINR in a radio network simulator.

  • 4.
    Logothetis, Andrew
    et al.
    Ericsson Research, Ericsson AB, Stockholm, Sweden.
    Osseiran, Afif
    Ericsson Research, Ericsson AB, Stockholm, Sweden.
    Zetterberg, Per
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Single Carrier Frequency Domain Equalization on a Real-Time DSP-Based MIMO Test-Bed2005Conference paper (Refereed)
    Abstract [en]

    In this paper, single carrier frequency domain equalization with space time transmit diversity was implemented and evaluated in real-time using a DSP-based wireless MIMO test-bed. The implemented schemes supported a large set of modulations, from BPSK up to QAM64. Zero padding is introduced to mitigate inter-block interference. Robust time and frequency synchronization together with channel estimation based on Kalman filtering is used to address the adverse intersymbol interference and the severe carrier frequency offset due to hardware impairments.

  • 5.
    Osseiran, Afif
    KTH, School of Information and Communication Technology (ICT), Communication: Services and Infrastucture, Communication Systems, CoS.
    Advanced Antennas in Wireless Communications: co-located & distributed2006Doctoral thesis, monograph (Other scientific)
    Abstract [en]

    In a wireless radio network system, with an evolving standard, there is a need to increase system capacity due to the increase in the traffic demands, the higher data rate usage, and the need to further extend the coverage of the system. One possible solution is to use multiple antennas (co-located or distributed) for the radio links. The aim of this thesis is to estimate the capacity gain for these advanced antenna scenarios in comparison to the ones in current use (e.g. single antenna in a 3-sector site) with special emphasis on transmit diversity and beamforming techniques. The quantification of such gain is mainly performed, evaluated and analyzed in dynamic system simulators with an accurate interference modeling. A generalized Signal to Interference Noise Ratio (SINR) estimation for a MIMO DS-CDMA system is derived, and as a by-product a closed-form solution of the orthogonality factor is obtained. Moreover the effectiveness of an antenna system is evaluated in a MIMO-test bed.

    Transmit diversity (TXDiv) is evaluated in WCDMA systems for flat fading (i.e. Pedestrian A (PedA)) and frequency selective channels (i.e. Typical Urban (TU)). While in flat fading channels, TXDiv schemes such as Space Time Transmit Diversity (STTD) and Closed Loop Mode 1 (CL1) offer a substantial system capacity gain, the gain is negligible in frequency selective channels. In HSDPA systems, TXDiv offers negligible gain in flat fading channels and causes a significant loss in frequency selective channels. The loss is mainly due to random spatial interference patterns (the so called Flashlight Effect), that are present in the HSDPA system. A simple scheme that mitigates this phenomena is presented. The scheme yields a 70% gain for CL1 in a PedA channel, while 10% gain is observed in the TU channel.

    The introduction of beamforming in WCDMA systems leads to a substantial system capacity and coverage gain. Three different implementations are evaluated and analyzed: Higher Order Sectorization (HOS), Fixed Beams (FB) with S-CPICH as a phase reference and finally FB as P-CPICH as a phase reference. Further, the impact of angular spread, the interaction and impact of radio resource management as power tuning of the common channel, scrambling code allocation technique, admission control, handovers and various antenna configurations are analyzed. The 12-sector sites yield the best system capacity gain in comparison to 3-sector sites equipped with a single antenna, slightly more than a 3-sector sites equipped with 4 FB each. In HSDPA systems, FB offers an impressive capacity gain, up to 200% for a 4 FB system.

    Capacity estimations with a dynamic system simulator give a clear indication about the gain of the simulated system, but the robustness of any method have to be verified through test-beds. STTD with receive diversity is implemented and tested in a real-time DSP MIMO test-bed for a single carrier frequency domain equalization system. A new pilot structure for joint Carrier Frequency Offset (CFO) and channel estimation is proposed and evaluated to address the inter-symbol interference and the severe CFO due to hardware impairments. The new pilot scheme results in significant reduction of the required overhead signalling compared to previous schemes.

    Instead of having antennas co-located and connected to the same radio unit (i.e. BS or UE), antennas can be distributed but having the potential to cooperate together. Furthermore, the system provides a macro-diversity gain and also relaxes the hardware complexity at the BS and/or UE. Two novel methods that provide frequency and spatial diversity are proposed. The first one called Relay Cyclic Delay Diversity (RCDD), provides frequency and spatial diversity for a multihop system while requiring a lower overhead than the methods proposed in the literature. RCDD yields a high SINR gain which translates into a substantial cell throughput gain in comparison to a single hop system. The second method called two dimensional cyclic prefix (2D-CP), introduces artificial time diversity and requires only a single transmission phase for each direction in a cooperative relaying wireless communication system. Besides not requiring an antenna specific pilots, the 2D-CP provides a substantial data rate increase.

  • 6.
    Osseiran, Afif
    et al.
    Ericsson Research, Sweden.
    Boccardi, F.
    Braun, V.
    Kusume, K.
    Marsch, P.
    Maternia, M.
    Queseth, O.
    Schellmann, M.
    Schotten, H.
    Taoka, H.
    Tullberg, H.
    Uusitalo, M. A.
    Timus, B.
    Fallgren, Mikael
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Scenarios for 5G mobile and wireless communications: The vision of the METIS project2014In: IEEE Communications Magazine, ISSN 0163-6804, E-ISSN 1558-1896, Vol. 52, no 5, p. 26-35Article in journal (Refereed)
    Abstract [en]

    METIS is the EU flagship 5G project with the objective of laying the foundation for 5G systems and building consensus prior to standardization. The METIS overall approach toward 5G builds on the evolution of existing technologies complemented by new radio concepts that are designed to meet the new and challenging requirements of use cases today¿s radio access networks cannot support. The integration of these new radio concepts, such as massive MIMO, ultra dense networks, moving networks, and device-to-device, ultra reliable, and massive machine communications, will allow 5G to support the expected increase in mobile data volume while broadening the range of application domains that mobile communications can support beyond 2020. In this article, we describe the scenarios identified for the purpose of driving the 5G research direction. Furthermore, we give initial directions for the technology components (e.g., link level components, multinode/multiantenna, multi-RAT, and multi-layer networks and spectrum handling) that will allow the fulfillment of the requirements of the identified 5G scenarios.

  • 7.
    Osseiran, Afif
    et al.
    Ericsson Research, Ericsson AB, Stockholm, Sweden.
    Logothetis, Andrew
    Ericsson Research, Ericsson AB, Stockholm, Sweden.
    A Method for Designing Fixed Multi-Beam Antenna Arrays in WCDMA Systems2006In: IEEE Antennas and Wireless Propagation Letters, ISSN 1536-1225, E-ISSN 1548-5757, Vol. 5, no 1, p. 41-44Article in journal (Refereed)
    Abstract [en]

    In this paper, Simulated Annealing is used for designing a fixed multi-beam antenna array (AA) in a multi-user radio WCDMA system. The method can be used dynamically in a real system and/or in a radio planning tool. The proposed method is applied on a linear array by designing the Antenna Element (AE) pattern, the number of fixed beams, the AE spacing, and the AA transmit weights. The proposed method leads to a substantial BS power reduction which can be used to increase the system capacity or data throughput in a WCDMA system.

  • 8.
    Osseiran, Afif
    et al.
    KTH, School of Information and Communication Technology (ICT), Communication Systems, CoS.
    Logothetis, Andrew
    Ericsson Research, Ericsson AB, Stockholm, Sweden.
    Capacity Evaluation of Fixed Beams in a WCDMA System using channel estimation based on P-CPICH2006In: IEEE International Conference on Communications, ISSN 1550-3607, E-ISSN 1938-1883, Vol. 11, p. 4900-4905Article in journal (Refereed)
    Abstract [en]

    A fixed multi-beam system that uses the Primary- Common PIlot CHannel (P-CPICH) as a phase reference for channel estimation and demodulation in WCDMA is evaluated in a dynamic radio network simulator. The impact of the angular spread on the downlink system performance is analyzed. Furthermore, a scrambling code allocation strategy and an adaptive load-dependent power tuning algorithm for the P-CPICH are proposed. Extensive simulation studies are carried out to evaluate the capacity gains of 3 sector sites where each site is equipped with 1, 2 or 4 beams in a typical urban radio channel. Moreover an alternative antenna configuration consisting of 6 sector sites where each sector is equipped with 2 beams is evaluated.

  • 9.
    Osseiran, Afif
    et al.
    Ericsson Research, Ericsson AB, Stockholm, Sweden.
    Logothetis, Andrew
    Ericsson Research, Ericsson AB, Stockholm, Sweden.
    Closed Loop Transmit Diversity in WCDMA HS-DSCH2005In: VTC2005-Spring: 2005 61st IEEE Vehicular Technology Conference, vols 1-5, proceedings, 2005, p. 349-353Conference paper (Refereed)
    Abstract [en]

    The paper summarizes the system performance of the high speed downlink shared channel (HS-DSCH) in WCDMA using closed loop mode 1 transmit diversity. The results show that, scheduling one user at a time in the Pedestrian A channel model, transmit diversity yields similar capacity gains as the sector antenna. On the other hand, in highly dispersive channels, a loss in the system throughput relative to the sector antenna, is observed. This loss is mainly due to random spatial interference patterns (the so called flashlight effect), that are present in the HS-DSCH setting when a single user is scheduled with the maximum available resources (power/codes) at each time instant. In order to mitigate the flashlight effect, a simple scheme is proposed in which multiple users are simultaneously scheduled using different scrambling codes. Scheduling multiple users makes the interference almost spatially white, resulting in a system throughput gain.

  • 10.
    Osseiran, Afif
    et al.
    Ericsson Research, Ericsson AB, Stockholm, Sweden.
    Logothetis, Andrew
    Ericsson Research, Ericsson AB, Stockholm, Sweden.
    Impact of Angular Spread on Higher Order Sectorization in WCDMA Systems2005In: IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, PIMRC, IEEE , 2005, p. 301-305Conference paper (Refereed)
    Abstract [en]

    This paper analyzes higher order sectorization (HOS) in WCDMA. Sectorization is achieved by splitting the sites into smaller sectors using highly directional antennas. The impact of angular spread on the system throughput is evaluated using a dynamic radio network simulator. Increasing the number of sectors per site from 3 to 6 and 12 in a typical urban radio channel yields a downlink system capacity gain of 80% and 200% respectively. Simulations have shown that the increase in the ratio of users in soft and softer handover per sector is negligible and the impact of angular spread on the system performance of a 6 and 12 sector sites is minor and negligible for the 3 sector sites.

  • 11.
    Osseiran, Afif
    et al.
    Ericsson Research, Ericsson AB, Stockholm, Sweden.
    Logothetis, Andrew
    Ericsson Research, Ericsson AB, Stockholm, Sweden.
    Smart antennas in a WCDMA Radio Network System Revisited: Modeling and Evaluations2006In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 54, no 11, p. 3302-3316Article in journal (Refereed)
    Abstract [en]

    A comprehensive study is given of smart antennas for wide-band code-division multiple access (WCDMA). It analyzes, evaluates, and presents methods and ideas for the deployment of smart antennas in WCDMA third-generation wireless systems and for future generations. Three major implementations are evaluated and analyzed in a dynamic system simulator: higher order sectorization (HOS), fixed beams (FBs) with secondary common pilot channel (S-CPICH), and FBs with primary common pilot channel (P-CPICH) as a phase reference. The system modeling integrates antenna and wave propagation modeling to evaluate and design smart antennas in wireless communications. Furthermore, this paper analyzes the impact of angular spread of various antenna configurations, the interaction and impact of radio resource management as power tuning of the common channel, scrambling code (SCO) allocation technique, and soft and softer handover algorithm. Moreover, an SCO allocation strategy and an adaptive load-dependent power-tuning algorithm of the P-CPICH are proposed. Furthermore, a signal-to-(interference plus noise) ratio derivation is given. Extensive simulation studies are carried out to evaluate the capacity gains of HOS and FB. The 12-sector sites yield the best system downlink capacity gain compared to three-sector sites equipped with a single antenna, slightly more than three-sector sites equipped with four FBs each. The six-sector sites equipped with two FBs each reduce the gap between HOS and FB systems to 10%.

  • 12.
    Osseiran, Afif
    et al.
    Ericsson Research, Ericsson AB, Stockholm, Sweden.
    Logothetis, Andrew
    Ericsson Research, Ericsson AB, Stockholm, Sweden.
    System Performance of Transmit Diversity Methods and a Two Fixed-Beam System in WCDMA2004In: Wireless personal communications, ISSN 0929-6212, E-ISSN 1572-834X, Vol. 31, no 1-2, p. 33-50Article in journal (Refereed)
    Abstract [en]

    Downlink transmit diversity modes for WCDMA together with a two fixed-beam antenna array system are compared relative to the single antenna sectorized system in a radio network simulator. The transmit diversity methods investigated are: space-time transmit diversity and closed-loop mode I transmit diversity. Frequency selective (COST 259) and flat fading channels are considered and their impact to speech-only and data-only services is evaluated. A third service, which highlights the system performance of the various advanced antennas, is also investigated.

    The results in this investigation point out that the diversity gain in flat fading channels is substantial. In frequency-selective fading, the benefits of fixed beam systems is encouraging, whereas transmit diversity methods (especially Space-Time Transmit Diversity) is unsatisfactory.

  • 13.
    Osseiran, Afif
    et al.
    Ericsson Research, Ericsson AB, Stockholm, Sweden.
    Logothetis, Andrew
    Ericsson Research, Ericsson AB, Stockholm, Sweden.
    Ben Slimane, Slimane
    KTH, School of Information and Communication Technology (ICT), Communication Systems, CoS.
    Distributed Relay Diversity Systems for OFDM-Based Networks2008In: International Journal of Communications, Network and System Sciences, ISSN 1913-3715, E-ISSN 1913-3723, Vol. 1, no 3, p. 215-227Article in journal (Refereed)
    Abstract [en]

    In this paper, distributed relay diversity systems are analyzed, modeled and evaluated in an Orthogonal Frequency Division Multiplexing (OFDM) based networks. The investigated distributed relay diversity schemes extend the ideas of a single hop transmit antenna schemes such as Cyclic Delay Diversity (CDD), Space Time Transmit Diversity (STTD), transmit Coherent Combining (CC) and Selection Diversity (SD) to distributed diversity systems. In contrast to the classical single hop system, the antennas in the distributed systems belongs to distributed relays instead of being co-located at the transmitter. The distributed relay diversity methods considered in this paper: Relay CDD (RCDD), Relay Alamouti (i.e.STTD), Relay CC (RCC) and Relay SD (RSD) are compared to the traditional 1-hop system. Analytical expressions for the received Signal to Interference Noise Ratio (SINR) are derived and used in a dynamic multi-cell multi-user simulator. Results show considerable SINR gains for both Round Robin and Max-SINR schedulers. The SINR gains translate into substantial cell throughput gains, up to 200%, compared to 1-hop systems. Despite its low complexity, the RCDD scheme has similar performance to that of other more sophisticated 2-hop schemes such as Relay Alamouti and Relay Coherent Combining. Marginally better results are observed for the Relay Selection Diversity scheme.

  • 14.
    Osseiran, Afif
    et al.
    KTH, School of Information and Communication Technology (ICT), Communication Systems, CoS.
    Logothetis, Andrew
    Ericsson Research, Ericsson AB, Stockholm, Sweden.
    Ben Slimane, Slimane
    KTH, School of Information and Communication Technology (ICT), Communication Systems, CoS.
    Pilot Design Criteria for Single Carrier Frequency Domain Equalization on a Real-Time DSP-Based MIMO Test-Bed2006In: 2006 IEEE 64TH VEHICULAR TECHNOLOGY CONFERENCE, VOLS 1-6, IEEE , 2006, p. 261-264Conference paper (Refereed)
    Abstract [en]

    In this paper, single carrier frequency domain equalization with space time transmit diversity was implemented and evaluated in real-time using a DSP-based wireless MIMO test-bed. A new pilot structure for joint carrier frequency offset (CFO) and channel estimation is proposed and evaluated, to address the adverse inter-symbol interference and the severe CFO due to hardware impairments. The channel estimation method is done in the frequency domain using a non-parametric interpolation method. The new pilot scheme results in significant reduction of the required overhead signalling compared to previous schemes.

  • 15.
    Osseiran, Afif
    et al.
    Ericsson Research, Ericsson AB, Stockholm, Sweden.
    Logothetis, Andrew
    Ericsson Research, Ericsson AB, Stockholm, Sweden.
    Ben Slimane, Slimane
    KTH, School of Information and Communication Technology (ICT), Communication Systems, CoS.
    Larsson, Peter
    Ericsson Research, Stockholm, Sweden.
    Relay Cyclic Delay Diversity: Modeling & System Performance2007In: ICSPC: 2007 IEEE International Conference On Signal Processing And Communications, Vols 1-3, Proceedings, 2007, p. 25-28Conference paper (Refereed)
    Abstract [en]

    A cooperative communication scheme referred to as relay cyclic delay diversity (RCDD) is presented in this paper. The scheme is based on a set of distributed relay nodes (RNs) with each node associated with a certain cyclic shift in order to generate artificially multi-path diversity and equivalently frequency diversity. RCDD is an open loop scheme and the user terminal (UT) only needs to estimate an effective channel from all the RNs and not the channel from each RN. The effective channel induced by RCDD is frequency selective and can be exploited by a maximum signal to interference noise ratio (SINR) scheduler which improves substantially the system throughput. The SINR of the RCDD is derived analytically and the performance of the method is evaluated in a dynamic multi-cell multi-user simulator. The results show considerable SINR gains that translate into substantial cell throughput gains compared to 1-hop systems.

  • 16.
    Osseiran, Afif
    et al.
    KTH, School of Information and Communication Technology (ICT), Communication Systems, CoS.
    Logothetis, Andrew
    Ericsson Research, Ericsson AB, Stockholm, Sweden.
    Molteni, Maurizio
    Ericsson, Milan, Italy.
    System Performance of Fixed Beams with S-CPICH as a Phase Reference in WCDMA2006In: 2006 IEEE 63RD VEHICULAR TECHNOLOGY CONFERENCE, VOLS 1-6, IEEE , 2006, p. 2962-2966Conference paper (Refereed)
    Abstract [en]

    A Fixed Beam (FB) system that uses a beam specific Secondary-Common PIlot CHannel (S-CPICH)  as a phase reference for channel estimation and demodulation in WCDMA is evaluated in a dynamic radio network simulator.  The impact of the angular spread on the DownLink (DL) system performance is analyzed. Furthermore, various scrambling code allocation strategies  and an adaptive load-dependent power tuning algorithm of the common channels are proposed and evaluated. Extensive simulation results have verified that increasing the number of FB from 1 to 2 or 4 beams per sector in a typical urban radio channel yields significant DL capacity gains.

  • 17.
    Osseiran, Afif
    et al.
    Ericsson Research, Sweden.
    Monserrat, J. F.
    Mohr, W.
    Future Directions2011In: Mobile and Wireless Communications for IMT-Advanced and Beyond, Chichester: John Wiley & Sons, 2011, p. 251-257Chapter in book (Other academic)
  • 18.
    Osseiran, Afif
    et al.
    KTH.
    Monserrat, José F.Mohr, Werner
    Mobile and Wireless Communications for IMT-Advanced and Beyond2011Collection (editor) (Refereed)
    Abstract [en]

    A timely addition to the understanding of IMT-Advanced, this book places particular emphasis on the new areas which IMT-Advanced technologies rely on compared with their predecessors. These latest areas include Radio Resource Management, Carrier Aggregation, improved MIMO support and Relaying. Each technique is thoroughly described and illustrated before being surveyed in context of the LTE-Advanced standards. The book also presents state-of-the-art information on the different aspects of the work of standardization bodies (such as 3GPP and IEEE), making global links between them. Explores the latest research innovations to assess the future of the LTE standard. Covers the latest research techniques for beyond IMT-Advanced such as Coordinated multi-point systems (CoMP), Network Coding, Device-to-Device and Spectrum Sharing. Contains key information for researchers from academia and industry, engineers, regulators and decision makers working on LTE-Advanced and beyond.

  • 19.
    Osseiran, Afif
    et al.
    Ericsson Research, Sweden.
    Saadani, A.
    Rost, P.
    Gouraud, A.
    Relaying for Imt-advanced2011In: Mobile and Wireless Communications for IMT-Advanced and Beyond, Chichester: John Wiley & Sons, 2011, p. 157-179Chapter in book (Other academic)
  • 20.
    Tölli, Antti
    et al.
    University of Oulu.
    Komulainen, Petri
    University of Oulu.
    Boccardi, Federico
    Alcatel-Lucent.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Osseiran, Afif
    Ericsson.
    Multiuser MIMO Systems2011In: Mobile and Wireless Communications for IMT-Advanced and Beyond / [ed] A. Osseiran, J. Montserrat, W. Mohr, John Wiley & Sons, 2011, p. 89-120Chapter in book (Refereed)
    Abstract [en]

    In Multiple Input Multiple Output (MIMO) communications, multiple antenna elements are employed both in the transmitter and the receiver, in order to obtain increased data rates or improved reliability compared to single-antenna transmission. In cellular systems, the Base Stations (BS) may operate in Single User (SU)-MIMO mode, i.e. to employ point-to-point transmission of data just for one multi-antenna User Equipment at a time. Alternatively, the BS can perform Multiuser (MU)-MIMO transmission, i.e. spatially multiplex data streams intended for different UEs that may be equipped with arbitrary numbers of antenna elements. The use of MU-MIMO mode offers potential system capacity gains especially when the BS employs a large antenna array. This chapter discusses various aspects of MIMO communications, with a special focus on the downlink direction of cellular systems. First, an overview of the fundamentals and existing techniques is given. Then, the MIMO related standards and standardization activities are described. Finally, some recent advances concerning MU-MIMO communications are addressed.

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