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  • 251.
    Karlsson, Marcus
    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.
    Massive MIMO as a Cyber-Weapon2014In: 48th Asilomar Conference on Signals, Systems and Computers, Institute of Electrical and Electronics Engineers (IEEE), 2014, p. 661-665Conference paper (Refereed)
    Abstract [en]

    Abstract—A scheme for a multi-antenna jammer, jamming a SISO primary link working in TDD mode is presented. The jammer relies on reciprocity to estimate the channel to a terminal in the primary link blindly. The jammer can then beamform noise towards this user to degrade the performance of the primary link. Simulations show that, for certain scenarios, the proposed jammer can do the same damage to the primary link as an omnidirectional jammer, but with close to two orders of magnitude lower output power.

  • 252.
    Karlsson, Marcus
    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.
    On the Operation of Massive MIMO with and without transmitter CSI2014In: Signal Processing Advances in Wireless Communications (SPAWC), 2014 IEEE 15th International Workshop on, IEEE , 2014, p. 1-5Conference paper (Refereed)
    Abstract [en]

    The paper considers the issue of activating inactive terminals by control signaling in the downlink in a massive MIMO system. There are two basic difficulties with this. First, the lack of CSI at the transmitter. Second, the short coherence interval, which limits the number of orthogonal pilots in the case of many antennas. The proposed scheme deals with these issues by repeating the transmission over the antennas. We show that this repetition does not affect the spectral efficiency significantly, while making it possible to estimate the channel in a standard way using MMSE. The paper also sheds some light the uplink-downlink power balance in massive MIMO.

  • 253.
    Karlsson, Mikael
    Linköping University, Department of Electrical Engineering, Communication Systems.
    Beam-Forming-Aware Link-Adaptation for Differential Beam-Forming in an LTE FDD System2016Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The ability for base stations to be able to beam-form their signals, directing the signal energy to specific users, is a topic of research that has been heavily studied during the last decades. The beam-forming technique aims to increase the signal-to-interference-and-noise-ratio of the user and, consequently, increase the capacity and coverage of the communication system. One such method is the Differential Beam-Forming technique, that has been developed at Ericsson Research. In this version of beam-forming, the beams can be dynamically sharpened and widened when tracking a specific terminal, to try to optimize the signal energy sent to that terminal.

    Beam-forming, however, makes the link-adaptation algorithm process substantially harder to perform. The reason for this is that the link-adaptation algorithm now has to take into account not only the changing radio environment, but also the changing transmit signal that is being beam-formed. Fortunately, since the beam-formed signal is known at the point of transmission, there should be a potential to utilize this knowledge to make the link-adaptation more efficient.

    This thesis, investigates how the link-adaptation algorithm could be changed to perform better in beam-forming setups, as well as what information from the beam-forming algorithm that could be included and utilized in the link-adaptation algorithm. This is done by designing and investigating three new link-adaptation algorithms, in the context of Differential Beam-Forming in an LTE FDD system. The algorithms that has been designed are both of a beam-forming-aware and beam-forming-unaware character, meaning if the beam-forming information is utilized within the algorithm, or not. These algorithms have been simulated for different base station antenna array-sizes. Unfortunately, due to simulator restrictions, the terminals have been simulated in a stationary environment, which has proven to be a limiting factor for the results. However, the results still show that smarter beam-forming-aware link-adaptation could possibly be used to increase the performance of the link-adaptation when using beam-forming.

  • 254.
    Kashyap, Salil
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Björnson, Emil
    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.
    Can wireless power transfer benefit from large transmitter arrays?2015In: Proceedings of IEEE Wireless Power Transfer Conference (WPTC), Institute of Electrical and Electronics Engineers (IEEE), 2015, p. 1-3Conference paper (Refereed)
    Abstract [en]

    In this paper, we illustrate the potential benefits of using large transmitter arrays for wireless power transfer. Specifically, we analyze the probability of outage in energy transfer over fading channels when the base station (BS) with multiple antennas beamforms energy to a wireless sensor node. Our analytical and numerical results show that by using large transmitter arrays, the range of wireless power transfer can be increased while maintaining a target outage probability. We also observe and quantify that by using multi-antenna arrays at the BS, a lower downlink energy is required to get the same outage performance

  • 255.
    Kashyap, Salil
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Björnson, Emil
    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.
    On the Feasibility of Wireless Energy Transfer Using Massive Antenna Arrays2016In: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, Vol. 15, no 5, p. 3466-3480Article in journal (Refereed)
    Abstract [en]

    We illustrate potential benefits of using massive antenna arrays for wireless energy transfer (WET). Specifically, we analyze probability of outage in WET over fading channels when a base station (BS) with multiple antennas beamforms energy to a wireless sensor node (WSN). Our analytical results show that by using massive antenna arrays, the range of WET can be increased for a given target outage probability. We prove that by using multiple-antenna arrays at the BS, a lower downlink energy is required to get the same outage performance, resulting in savings of radiated energy. We show that for energy levels used in WET, the outage performance with least-squares or minimum mean-square-error channel estimates is the same as that obtained based on perfect channel estimates. We observe that a strong line-of-sight component between the BS and WSN lowers outage probability. Furthermore, by deploying more antennas at the BS, a larger energy can be transferred reliably to the WSN at a given target outage performance for the sensor to be able to perform its main tasks. In our numerical examples, the RF power received at the input of the sensor is assumed to be on the order of a mW, such that the rectenna operates at an efficiency in the order of 50%.

  • 256.
    Kashyap, Salil
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Björnson, Emil
    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.
    On the feasibility of wireless energy transfer using massive antenna arrays in Rician channels2015In: 16th IEEE International Workshop on Signal Processing Advances in Wireless Communications, SPAWC 2015, Institute of Electrical and Electronics Engineers (IEEE), 2015, p. 46-50Conference paper (Refereed)
    Abstract [en]

    In this paper, we examine the feasibility of wireless energy transfer (WET) using arrays with multiple antennas. Specifically, we compute the probability of outage in energy transfer over a Rician fading channel when the base station (BS) with multiple antennas transfers energy to a wireless sensor node (WSN). Through our analytical and numerical results, we prove that by deploying more antennas at the BS, the range of WET can be increased while maintaining a target outage probability. We observe that the use of massive antenna arrays at the BS results into huge savings of radiated energy. We show that for typical energy levels used in WET, the outage performance with imperfect channel state information (CSI) is essentially the same as that obtained based on perfect CSI. We also observe that a strong line-of-sight component between the BS and the WSN lowers the probability of outage in energy transfer.

  • 257.
    Kashyap, Salil
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology. Indian Institute Science, India.
    Mehta, Neelesh B.
    Indian Institute Science, India.
    Optimal Binary Power Control for Underlay CR With Different Interference Constraints and Impact of Channel Estimation Errors2014In: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, Vol. 62, no 11, p. 3753-3764Article in journal (Refereed)
    Abstract [en]

    Adapting the power of secondary users (SUs) while adhering to constraints on the interference caused to primary receivers (PRxs) is a critical issue in underlay cognitive radio (CR). This adaptation is driven by the interference and transmit power constraints imposed on the secondary transmitter (STx). Its performance also depends on the quality of channel state information (CSI) available at the STx of the links from the STx to the secondary receiver and to the PRxs. For a system in which an STx is subject to an average interference constraint or an interference outage probability constraint at each of the PRxs, we derive novel symbol error probability (SEP)-optimal, practically motivated binary transmit power control policies. As a reference, we also present the corresponding SEP-optimal continuous transmit power control policies for one PRx. We then analyze the robustness of the optimal policies when the STx knows noisy channel estimates of the links between the SU and the PRxs. Altogether, our work develops a holistic understanding of the critical role played by different transmit and interference constraints in driving power control in underlay CR and the impact of CSI on its performance.

  • 258.
    Kashyap, Salil
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Mehta, Neelesh B.
    Indian Institute Science IISc, India.
    Power Gain Estimation and Its Impact on Binary Power Control in Underlay Cognitive Radio2015In: IEEE Wireless Communications Letters, ISSN 2162-2337, E-ISSN 2162-2345, Vol. 4, no 2, p. 193-196Article in journal (Refereed)
    Abstract [en]

    We propose a novel minimum mean square error estimator that estimates the channel power gain of the link from the secondary transmitter to the primary receiver (PRx). It lowers the root mean square error compared to several other estimators used in the underlay cognitive radio literature that first estimate the channel amplitude. We then analyze its system impact for two types of interference constraints. To this end, for the optimal binary transmit power control policy, we derive closed-form expressions for the average interference and the probability that the interference at the PRx violates a peak interference constraint with the proposed estimator. We show that the proposed estimator performs closer to the perfect channel state information scenario compared to the other estimators.

  • 259.
    Kashyap, Salil
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Mollén, Christopher
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Björnson, Emil
    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.
    Frequency-Domain Interpolation of the Zero-Forcing Matrix in Massive MIMO-OFDM2016In: 2016 IEEE 17TH INTERNATIONAL WORKSHOP ON SIGNAL PROCESSING ADVANCES IN WIRELESS COMMUNICATIONS (SPAWC), IEEE , 2016Conference paper (Refereed)
    Abstract [en]

    We consider massive multiple input multiple output (MIMO) systems with orthogonal frequency division multiplexing (OFDM) that use zero-forcing (ZF) to combat interference. To perform ZF, large dimensional pseudo-inverses have to be computed. In this paper, we propose a discrete Fourier transform (DFT)-interpolation-based technique where substantially fewer ZF matrix computations have to be done with very little deterioration in data rate compared to computing an exact ZF matrix for every subcarrier. We claim that it is enough to compute the ZF matrix at L(amp;lt;amp;lt; N) selected subcarriers where L is the number of resolvable multipaths and N is the total number of subcarriers and then interpolate. The proposed technique exploits the fact that in the massive MIMO regime, the ZF impulse response consists of L dominant components. We benchmark the proposed method against full inversion, piecewise constant and linear interpolation methods and show that the proposed method achieves a good tradeoff between performance and complexity.

  • 260.
    Kashyap, Salil
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Mollén, Christopher
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Emil, Björnson
    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.
    Performance Analysis of (TDD) Massive MIMO with Kalman Channel Prediction2017In: Acoustics, Speech and Signal Processing (ICASSP), 2017 IEEE International Conference on, 2017, p. 3554-3558Conference paper (Refereed)
    Abstract [en]

    In massive MIMO systems, which rely on uplink pilots to estimate the channel, the time interval between pilot transmissions constrains the length of the downlink.  Since switching between up- and downlink takes time, longer downlink blocks increase the effective spectral efficiency.  We investigate the use of low-complexity channel models and Kalman filters for channel prediction, to allow for longer intervals between the pilots.  Specifically, we quantify how often uplink pilots have to be sent when the downlink rate is allowed to degrade by a certain percentage.  To this end, we consider a time-correlated channel aging model, whose spectrum is rectangular, and use autoregressive moving average (ARMA) processes to approximate the time-variations of such channels.  We show that ARMA-based predictors can increase the interval between pilots and the spectral efficiency in channels with high Doppler spreads.  We also show that Kalman prediction is robust to mismatches in the channel statistics.

  • 261.
    Khan Mohammed, Saif
    et al.
    Indian Institute Technology, India; Indian Institute Technology, India.
    Larsson, Erik G
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Improving the Performance of the Zero-Forcing Multiuser MISO Downlink Precoder Through User Grouping2016In: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, Vol. 15, no 2, p. 811-826Article in journal (Refereed)
    Abstract [en]

    We consider the multiple input single output (MISO) Gaussian broadcast channel with N-t antennas at the base station (BS) and N-u single-antenna users in the downlink. We propose a novel user grouping precoder which improves the sum rate performance of the zero-forcing (ZF) precoder specially when the channel is ill-conditioned. The proposed precoder partitions all the users into small groups of equal size. Downlink beamforming is then done in such a way that, at each users receiver, the interference from the signal intended for users not in its group is nulled out. Intragroup interference still remains, and is cancelled through successive interference presubtraction at the BS using dirty paper coding (DPC). The proposed user grouping method is different from user selection, since it is a method for precoding of information to the selected (scheduled) users, and not for selecting which users are to be scheduled. The proposed precoder is a generalization of two special cases, one where each group has only one user (ZF precoder) and another where all users are in a single group (ZF-DP precoder). A larger group size helps improve the sum rate performance but at the cost of greater complexity. The proposed generalization, therefore, allows for tradeoff between performance and complexity.

  • 262.
    Khan Mohammed, Saif
    et al.
    Linköping University, Department of Electrical Engineering. Linköping University, The Institute of Technology.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Per-Antenna Constant Envelope Precoding for Large Multi-User MIMO Systems2013In: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, Vol. 61, no 3, p. 1059-1071Article in journal (Refereed)
    Abstract [en]

    We consider the multi-user MIMO broadcast channel with M single-antenna users and N transmit antennas under the constraint that each antenna emits signals having constant envelope (CE). The motivation for this is that CE signals facilitate the use of power-efficient RF power amplifiers. Analytical and numerical results show that, under certain mild conditions on the channel gains, for a fixed M, an array gain is achievable even under the stringent per-antenna CE constraint. Essentially, for a fixed M, at sufficiently large N the total transmitted power can be reduced with increasing N while maintaining a fixed information rate to each user. Simulations for the i.i.d. Rayleigh fading channel show that the total transmit power can be reduced linearly with increasing N (i.e., an O(N) array gain). We also propose a precoding scheme which finds near-optimal CE signals to be transmitted, and has O(MN) complexity. Also, in terms of the total transmit power required to achieve a fixed desired information sum-rate, despite the stringent per-antenna CE constraint, the proposed CE precoding scheme performs close to the sum-capacity achieving scheme for an average-only total transmit power constrained channel.

  • 263.
    Khosravi Dehkourdi, Tony
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Simulation Platform for Resource Allocation in Multi-Cellular Wireless Networks2012Independent thesis Advanced level (degree of Master (One Year)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The goal of this Master's thesis was to solve resource allocation problems in wireless networks through the implementation of a lightweight simulation platform. The spectrum and power resources of wireless networks have to be efficiently used to accommodate the growing number of wireless terminals and the massive increase of data transferred by their applications. The major problem that needs to be tackled is interference, which significantly limits the performance of wireless systems. In this thesis, the resource allocation of interest was the joint problem of scheduling and power control with Quality of Service (QoS) constraints. The Signal-to-Interference-plus-Noise Ratio (SINR) was used to quantify QoS. This thesis studied the recently proposed mixed-integer linear programming (MILP) formulation of the problem. Due to the scheduling component, the problem is inherently combinatorial and NP-hard, therefore computationally expensive and difficult to solve in tractable time. A simulation platform was implemented in order to automate and facilitate the solving process.As a starting point, wireless channels and channel modeling issues were studied. Then, the platform was implemented to simulate random instances of multi-cellular wireless networks, with several mobile stations per cell, and generate the corresponding channels. Finally, the platform was extended to use the GNU Linear Programming Kit (GLPK) API in order to optimally solve the aforementioned formulated problem for various inputs of generated channels.Tests of the simulation platform were performed to check the consistency of the results. Indeed, the output results satisfied the initial expectations regarding the SINR constraints and the formulation. Moreover, they were produced in reasonable time. An analysis of the output results was presented.This thesis resulted in a configurable and lightweight simulation platform which is able to solve the MILP-formulated resource allocation problem. The simulation platform is basic and does not cover all the aspects of multi-cellular wireless networks and wireless channels. Due to its modularity, it can be extended in a future project.

  • 264.
    Kong, Chuili
    et al.
    Zhejiang University, Peoples R China.
    Zhong, Caijun
    Zhejiang University, Peoples R China; Southeast University, Peoples R China.
    Matthaiou, Michail
    Queens University, North Ireland.
    Björnson, Emil
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Zhang, Zhaoyang
    Zhejiang University, Peoples R China.
    MULTI-PAIR TWO-WAY AF RELAYING SYSTEMS WITH MASSIVE ARRAYS AND IMPERFECT CSI2016In: 2016 IEEE INTERNATIONAL CONFERENCE ON ACOUSTICS, SPEECH AND SIGNAL PROCESSING PROCEEDINGS, IEEE , 2016, p. 3651-3655Conference paper (Refereed)
    Abstract [en]

    We consider a multi-pair two-way amplify-and-forward relaying system with a massive antenna array at the relay and estimated channel state information, assuming maximum-ratio combining/transmission processing. Closed-form approximations of the sum spectral efficiency are developed and simple analytical power scaling laws are presented, which reveal a fundamental trade-off between the transmit powers of each user/the relay and of each pilot symbol. Finally, the optimal power allocation problem is studied.

  • 265.
    Kong, Chuili
    et al.
    Zhejiang Univ, Peoples R China; Xidian Univ, Peoples R China.
    Zhong, Caijun
    Zhejiang Univ, Peoples R China; Xidian Univ, Peoples R China.
    Matthaiou, Michail
    Queens Univ Belfast, North Ireland.
    Björnson, Emil
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Zhang, Zhaoyang
    Zhejiang Univ, Peoples R China.
    Multipair Two-Way Half-Duplex DF Relaying With Massive Arrays and Imperfect CSI2018In: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, Vol. 17, no 5, p. 3269-3283Article in journal (Refereed)
    Abstract [en]

    This paper considers a two-way half-duplex decode-and-forward relaying system, where multiple pairs of single-antenna users exchange information via a multiple-antenna relay. Assuming that the channel knowledge is nonideal and the relay employs maximum ratio processing, we derive a large-scale approximation of the sum spectral efficiency (SE) that is tight when the number of relay antennas M becomes very large. Furthermore, we study how the transmit power scales with M to maintain a desired SE. In particular, three special power-scaling cases are discussed and the corresponding asymptotic SE is deduced with clear insights. Our elegant power-scaling laws reveal a tradeoff between the transmit powers of the user/relay and pilot symbol. Finally, we formulate a power allocation problem in terms of maximizing the sum SE and obtain a local optimum by solving a sequence of geometric programming problems.

  • 266.
    Kong, Chuili
    et al.
    Zhejiang Univ, Peoples R China; Xidian Univ, Peoples R China.
    Zhong, Caijun
    Zhejiang Univ, Peoples R China; Xidian Univ, Peoples R China.
    Matthaiou, Michail
    Queens Univ Belfast, North Ireland.
    Björnson, Emil
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Zhang, Zhaoyang
    Zhejiang Univ, Peoples R China.
    Spectral Efficiency of Multipair Massive MIMO Two-Way Relaying With Imperfect CSI2019In: IEEE Transactions on Vehicular Technology, ISSN 0018-9545, E-ISSN 1939-9359, Vol. 68, no 7, p. 6593-6607Article in journal (Refereed)
    Abstract [en]

    We consider a two-way half-duplex relaying system where multiple pairs of single-antenna users exchange information assisted by a multiple-antenna relay. Taking into account the practical constraint of imperfect channel knowledge, we study the achievable sum spectral efficiency (SE) of the amplify-and-forward protocol, assuming that the relay employs maximum ratio processing. We derive a closed-form expression for the sum SE for arbitrary system parameters and a large-scale approximation for the sum SE when the number of relay antennas M becomes sufficiently large. In addition, we study how the transmit power reduces with M to maintain a desired SE. Our results show that by using a large number of relay antennas, the transmit powers of the user, relay, and pilot symbol can be scaled down proportionally to 1/M-alpha, 1/M beta, and 1/M-gamma for certain combinations of a, beta, and gamma, respectively. This elegant power scaling law reveals a fundamental tradeoff between the transmit powers of the user/relay and pilot symbol. Finally, capitalizing on the new expressions for the sum SE, novel power allocation schemes are designed to further improve the sum SE.

  • 267.
    Konuskan, Cagatay
    Linköping University, Department of Electrical Engineering, Communication Systems.
    Turbo Equalization for HSPA2010Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    New high quality mobile telecommunication services are offered everyday and the demand for higher data rates is continuously increasing. To maximize the uplink throughput in HSPA when transmission is propagated through a dispersive channel causing self-interference, equalizers are used. One interesting solution, where the equalizer and decoder exchange information in an iterative way, for improving the equalizer performance is Turbo equalization.

    In this thesis a literature survey has been performed on Turbo equalization methods and a chosen method has been implemented for the uplink HSPA standard to evaluate the performance in heavily dispersive channels. The selected algorithm has been adapted for multiple receiving antennas, oversampled processing and HARQ retransmissions. The results derived from the computer based link simulations show that the implemented algorithm provide a gain of approximately 0.5 dB when performing up to 7 Turbo equalization iterations. Gains up to 1 dB have been obtained by disabling power control, not using retransmission combining and utilizing a single receiver antenna. The algorithm has also been evaluated considering alternative dispersive channels, Log-MAP decoding, different code rates, number of Turbo equalization iterations and number of Turbo decoding iterations.

    The simulation results do not motivate a real implementation of the chosen algorithm considering the increased computational complexity and small gain achieved in a full featured receiver system. Further studies are needed before concluding the HSPA uplink Turbo equalization approach.

  • 268.
    Kron, Johannes
    et al.
    School of Electrical Engineering, KTH, Stockholm.
    Persson, Daniel
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Skoglund, Mikael
    School of Electrical Engineering, KTH, Stockholm.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Closed-Form Sum-MSE Minimization for the Two-User Gaussian MIMO Broadcast Channel2011In: IEEE Communications Letters, ISSN 1089-7798, E-ISSN 1558-2558, Vol. 15, no 9, p. 950-952Article in journal (Refereed)
    Abstract [en]

    We study the Gaussian multiple-input, multipleoutput broadcast channel, where a base station with 𝑁𝑇 antennas transmits 𝐾 independent messages to 𝐾 users, each having a single receive antenna. The messages consist of independent, identically distributed Gaussian random variables and we study linear transmission with an end-to-end distortion criterion. By using an already established uplink/downlink duality and a recently discovered special relation between beamforming vectors and channel vectors, we present a closed-form expression for the optimal power allocation in the two-user case. We also outline an iterative algorithm that finds the optimal power allocation for an arbitrary number of users.

  • 269.
    Kumar, Sidharth
    et al.
    IIT Delhi, India.
    De, Swades
    IIT Delhi, India.
    Mishra, Deepak
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    RF Energy Transfer Channel Models for Sustainable IoT2018In: IEEE Internet of Things Journal, ISSN 2327-4662, Vol. 5, no 4, p. 2817-2828Article in journal (Refereed)
    Abstract [en]

    Self-sustainability of wireless nodes in Internet-of-Things applications can be realized with the help of controlled radio frequency energy transfer (RF-ET). However, due to significant energy loss in wireless dissipation, there is a need for novel schemes to improve the end-to-end RF-ET efficiency. In this paper, first we propose a new channel model for accurately characterizing the harvested dc power at the receiver. This model incorporates the effects of nonline of sight (NLOS) component along with the other factors, such as radiation pattern of transmit and receive antennas, losses associated with different polarization of transmitting field, and efficiency of power harvester circuit. Accuracy of the model is verified via experimental studies in an anechoic chamber (a controlled environment). Supported by experiments in controlled environment, we also formulate an optimization problem by accounting for the effect of NLOS component to maximize the RF-ET efficiency, which cannot be captured by the Friis formula. To solve this nonconvex problem, we present a computationally efficient golden section-based iterative algorithm. Finally, through extensive RF-ET measurements in different practical field environments we obtain the statistical parameters for Rician fading as well as path loss factor associated with shadow fading model, which also asserts the fact that Rayleigh fading is not well suited for RF-ET due to presence of a strong line of sight component.

  • 270.
    Larsson, Erik G
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Joint beamforming and broadcasting in massive MIMO2015In: IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC, Institute of Electrical and Electronics Engineers (IEEE), 2015, p. 266-270Conference paper (Refereed)
    Abstract [en]

    A technique is developed, by which a massive antenna array simultaneously can: (i) coherently beamform to a set of terminals, for which estimated channel state information is available; and (ii) broadcast public information to another set of terminals, for which no channel state information is available. The broadcast information does not interfere with the beamforming as it is placed in nullspace of the channel matrix collectively seen by the terminals targeted by the beamforming. 

  • 271.
    Larsson, Erik G
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Massive MIMO for 5G: Overview and the Road Ahead2017In: 2017 51ST ANNUAL CONFERENCE ON INFORMATION SCIENCES AND SYSTEMS (CISS), IEEE , 2017Conference paper (Refereed)
    Abstract [en]

    n/a

  • 272.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    MIMO Detection Methods: How They Work2009In: IEEE signal processing magazine (Print), ISSN 1053-5888, E-ISSN 1558-0792, Vol. 26, no 3, p. 91-95Article in journal (Refereed)
    Abstract [en]

    The goal of this lecture has been to provide an overview of approaches, in the communications receiver context. Which method is the best in practice? This depends much on the purpose of solving : what error rate can be tolerated, what is the ultimate measure of performance (e.g., frame-error-rate, worst-case complexity, or average complexity), and what computational platform is used. Additionally, the bits in s may be part of a larger code word and different s vectors in that code word may either see the same H (slow fading) or many different realizations of H (fast fading). This complicates the picture, because notions that are important in slow fading (such as spatial diversity) are less important in fast fading, where diversity is provided anyway by time variations. Detection for MIMO has been an active field for more than ten years, and this research will probably continue for some time.

  • 273.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Optimal OFDMA Downlink Scheduling Under a Control Signaling Cost Constraint2010In: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, Vol. 58, no 10, p. 2776-2781Article in journal (Refereed)
    Abstract [en]

    This paper proposes a new algorithm for downlink scheduling in OFDMA systems. The method maximizes the throughput, taking into account the amount of signaling needed to transmit scheduling maps to the users. A combinatorial problem is formulated and solved via a dynamic programming approach reminiscent of the Viterbi algorithm. The total computational complexity of the algorithm is upper boundedby O(K^4N) where K is the number of users that are being considered for scheduling in a frame and N is the number of resource blocks per frame.

  • 274.
    Larsson, Erik G.
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Danev, Danyo
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Accuracy Comparison of LS and Squared-Range LS for Source Localization2010In: IEEE Transactions on Signal Processing, ISSN 1053-587X, E-ISSN 1941-0476, Vol. 58, no 2, p. 916-923Article in journal (Refereed)
    Abstract [en]

    In this correspondence we compute a closed-form expression for theasymptotic (large-sample) accuracy of the recently proposedsquared-range least-squares (SR-LS) method for source localization. We compare its accuracy to that of the classicalleast-squares (LS) method and show that LS and SR-LS performdifferently in general. We identify geometries where theperformances of the methods are identical but also geometries when thedifference in performance is unbounded.

  • 275.
    Larsson, Erik G.
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Danev, Danyo
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Jorswieck, Eduard A.
    Dresden University of Technology.
    Asymptotically Optimal Transmit Strategies for the Multiple Antenna Interference Channel2008In: Proceedings of the 46th Annual Allerton Conference on Communication, Control, and Computing (Allerton'08), 2008, p. 708-714Conference paper (Refereed)
    Abstract [en]

    We consider the interference channel with multiple antennas at the transmitter. We prove that at high signal-to-noise ratio (SNR), the zero-forcing transmit scheme is optimal in the sum-rate sense. Furthermore we prove that at low SNR, maximum-ratio transmission is optimal in the sum-rate sense. We also provide a discussion of the connection to classical results on spectral efficiency in the wideband regime. Finally, we propose a non-convex optimization approach based on monotonic optimization to solve the sum rate maximization problem.

  • 276.
    Larsson, Erik G.
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Danev, Danyo
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering. University of Sofia, Bulgaria.
    Olofsson, Mikael
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Sörman, Simon
    Ericsson Res, Linkoping, Sweden.
    Teaching the Principles of Massive MIMO: Exploring reciprocity-based multiuser MIMO beamforming using acoustic waves2017In: IEEE signal processing magazine (Print), ISSN 1053-5888, E-ISSN 1558-0792, Vol. 34, no 1, p. 40-47Article in journal (Refereed)
    Abstract [en]

    Massive multiple-input, multiple-output (MIMO) is currently the most compelling wireless physical layer technology and a key component of fifth-generation (5G) systems. The understanding of its core principles has emerged during the last five years, and material is becoming available that is rigorously refined to focus on timeless fundamentals [1], facilitating the instruction of the topic to both master- and doctoral-level students [2]. Meaningful laboratory work that exposes the operational principles of massive MIMO is more difficult to accomplish. At Linköping University, Sweden, this was achieved through a project course, based on the conceive-design-implement-operate (CDIO) concept [3], and through the creation of a specially designed experimental setup using acoustic signals.

  • 277.
    Larsson, Erik G
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Edfors, Ove
    Lund University, Sweden .
    Tufvesson, Fredrik
    Lund University, Sweden .
    Marzetta, Thomas L.
    Bell Labs, Alcatel-Lucent, NJ, USA .
    Massive MIMO for Next Generation Wireless Systems2014In: IEEE Communications Magazine, ISSN 0163-6804, E-ISSN 1558-1896, Vol. 52, no 2, p. 186-195Article in journal (Refereed)
    Abstract [en]

    Multi-user MIMO offers big advantages over conventional point-to-point MIMO: it works with cheap single-antenna terminals, a rich scattering environment is not required, and resource allocation is simplified because every active terminal utilizes all of the time-frequency bins. However, multi-user MIMO, as originally envisioned, with roughly equal numbers of service antennas and terminals and frequency-division duplex operation, is not a scalable technology. Massive MIMO (also known as large-scale antenna systems, very large MIMO, hyper MIMO, full-dimension MIMO, and ARGOS) makes a clean break with current practice through the use of a large excess of service antennas over active terminals and time-division duplex operation. Extra antennas help by focusing energy into ever smaller regions of space to bring huge improvements in throughput and radiated energy efficiency. Other benefits of massive MIMO include extensive use of inexpensive low-power components, reduced latency, simplification of the MAC layer, and robustness against intentional jamming. The anticipated throughput depends on the propagation environment providing asymptotically orthogonal channels to the terminals, but so far experiments have not disclosed any limitations in this regard. While massive MIMO renders many traditional research problems irrelevant, it uncovers entirely new problems that urgently need attention: the challenge of making many low-cost low-precision components that work effectively together, acquisition and synchronization for newly joined terminals, the exploitation of extra degrees of freedom provided by the excess of service antennas, reducing internal power consumption to achieve total energy efficiency reductions, and finding new deployment scenarios. This article presents an overview of the massive MIMO concept and contemporary research on the topic.

  • 278.
    Larsson, Erik G
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Electrical Engineering, Communication Systems.
    Gustafsson, Oscar
    Linköping University, Department of Electrical Engineering, Electronics System. Linköping University, The Institute of Technology.
    The Impact of Dynamic Voltage and Frequency Scaling on Multicore DSP Algorithm Design2011In: IEEE SIGNAL PROCESSING MAGAZINE, ISSN 1053-5888, Vol. 28, no 3Article in journal (Refereed)
    Abstract [en]

    n/a

  • 279.
    Larsson, Erik G.
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Jaldén, Joakim
    Vienna University of Technology.
    Fixed-Complexity Soft MIMO Detection via Partial Marginalization2008In: IEEE Transactions on Signal Processing, ISSN 1053-587X, E-ISSN 1941-0476, Vol. 56, no 8(1), p. 3397-3407Article in journal (Refereed)
    Abstract [en]

    This paper presents a new approach to soft demodulationfor MIMO channels. The proposed method is an approximationto the exact a posteriori probability-per-bit computer. Themain idea is to marginalize the posterior density for the receiveddata exactly over the subset of the transmitted bits that are receivedwith the lower signal-to-noise-ratio (SNR), and marginalize thisdensity approximately over the remaining bits. Unlike the exact demodulator,whose complexity is huge due to the need for enumeratingall possible combinations of transmitted constellation points,the proposed method has very low complexity. The algorithm hasa fully parallel structure, suitable for implementation in parallelhardware. Additionally, its complexity is fixed, which makes it suitablefor pipelined implementation. We also show how the methodcan be extended to the situation when the receiver has only partialchannel state information, and how it can be modified to takesoft-input into account. Numerical examples illustrate its performanceon slowly fading 4x4 and 6x6 complex MIMO channels.

  • 280.
    Larsson, Erik G.
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Jorswieck, Eduard
    Dresden University of Technology.
    Competition Versus Cooperation on the MISO Interference Channel2008In: IEEE Journal on Selected Areas in Communications, ISSN 0733-8716, E-ISSN 1558-0008, Vol. 26, no 7, p. 1059-1069Article in journal (Refereed)
    Abstract [en]

    We consider the problem of coordinating two competingmultiple-antenna wireless systems (operators) that operatein the same spectral band. We formulate a rate region whichis achievable by scalar coding followed by power allocationand beamforming. We show that all interesting points on thePareto boundary correspond to transmit strategies where bothsystems use the maximum available power. We then argue thatthere is a fundamental need for base station cooperation whenperforming spectrum sharing with multiple transmit antennas.More precisely, we show that if the systems do not cooperate,there is a unique Nash equilibrium which is inefficient in the sensethat the achievable rate is bounded by a constant, regardless ofthe available transmit power. An extension of this result to thecase where the receivers use successive interference cancellation(SIC) is also provided.Next we model the problem of agreeing on beamformingvectors as a non-transferable utility (NTU) cooperative gametheoreticproblem, with the two operators as players. Specificallywe compute numerically the Nash bargaining solution, which isa likely resolution of the resource conflict assuming that theplayers are rational. Numerical experiments indicate that selfishbut cooperating operators may achieve a performance which isclose to the maximum-sum-rate bound.

  • 281.
    Larsson, Erik G.
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Jorswieck, Eduard
    Dresden University of Technology, Germany.
    Competition Versus Cooperation on the MISO Interference Channel2008Data set
    Abstract [en]

    We consider the problem of coordinating two competing multiple-antenna wireless systems (operators) that operate in the same spectral band. We formulate a rate region which is achievable by scalar coding followed by power allocation and beamforming. We show that all interesting points on the Pareto boundary correspond to transmit strategies where both systems use the maximum available power. We then argue that there is a fundamental need for base station cooperation when performing spectrum sharing with multiple transmit antennas. More precisely, we show that if the systems do not cooperate, there is a unique Nash equilibrium which is inefficient in the sense that the achievable rate is bounded by a constant, regardless of the available transmit power. An extension of this result to the case where the receivers use successive interference cancellation (SIC) is also provided. Next we model the problem of agreeing on beamforming vectors as a non-transferable utility (NTU) cooperative gametheoretic problem, with the two operators as players. Specifically we compute numerically the Nash bargaining solution, which is a likely resolution of the resource conflict assuming that the players are rational. Numerical experiments indicate that selfish but cooperating operators may achieve a performance which is close to the maximum-sum-rate bound.

  • 282.
    Larsson, Erik G.
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Jorswieck, Eduard
    Technical University of Dresden, Germany.
    Game theory2011In: Mathematical Foundations for Signal Processing, Communications and Networking / [ed] T. Chen, D. Rajan and E. Serpedin, CRC Press, 2011, p. 691-732Chapter in book (Refereed)
    Abstract [en]

    Mathematical Foundations for Signal Processing, Communications, and Networking describes mathematical concepts and results important in the design, analysis, and optimization of signal processing algorithms, modern communication systems, and networks. Helping readers master key techniques and comprehend the current research literature, the book offers a comprehensive overview of methods and applications from linear algebra, numerical analysis, statistics, probability, stochastic processes, and optimization.

  • 283.
    Larsson, Erik G.
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Jorswieck, Eduard A.
    Dresden University of Technology, Germany.
    The MISO Interference Channel: Competition versus Collaboration2007In: Proceedings of the 45th Allerton Conference on Communication, Control, and Computing (Allerton'07), 2007Conference paper (Refereed)
  • 284.
    Larsson, Erik G.
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Jorswieck, Eduard A.
    Dresden Unoversity of Technology.
    Lindblom, Johannes
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Mochaourab, Rami
    Dresden University of Technology.
    Game Theory and the Flat-Fading Gaussian Interference Channel: Analyzing Resource Conflicts in Wireless Networks2009In: IEEE signal processing magazine (Print), ISSN 1053-5888, E-ISSN 1558-0792, Vol. 26, no 5, p. 18-27Article in journal (Refereed)
    Abstract [en]

    In this article, we described some basic concepts from noncooperative and cooperative game theory and illustrated them by three examples using the interference channel model, namely, the power allocation game for SISO IFC, the beamforming game for MISO IFC, and the transmit covariance game for MIMO IFC. In noncooperative game theory, we restricted ourselves to discuss the NE and PoA and their interpretations in the context of our application. Extensions to other noncooperative approaches include Stackelberg equilibria and the corresponding question "Who will go first?" We also correlated equilibria where a certain type of common randomness can be exploited to increase the utility region. We leave the large area of coalitional game theory open.

  • 285.
    Larsson, Erik G
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Marzetta, Thomas L.
    NYU, NY 10003 USA.
    Ngo, Hien Quoc
    Queens Univ Belfast, North Ireland.
    Yang, Hong
    Nokia Bell Labs, NJ USA.
    Antenna Count for Massive MIMO: 1.9 GHz vs. 60 GHz2018In: IEEE Communications Magazine, ISSN 0163-6804, E-ISSN 1558-1896, Vol. 56, no 9, p. 132-137Article in journal (Refereed)
    Abstract [en]

    If we assume line-of-sight propagation and perfect channel state information at the base station - consistent with slow moving terminals - then a direct performance comparison between Massive MIMO at PCS and mmWave frequency bands is straightforward and highly illuminating. Line-of-sight propagation is considered favorable for mmWave because of minimal attenuation and its facilitation of hybrid beamforming to reduce the required number of active transceivers. We quantify the number of mmWave (60 GHz) service antennas that are needed to duplicate the performance of a specified number of PCS (1.9 GHz) service antennas. As a baseline we consider a modest PCS deployment of 128 antennas serving 18 terminals. At one extreme, we find that, to achieve the same per-terminal maxmin 95 percent-likely downlink throughput in a single-cell system, 20,000 mmWave antennas are needed. To match the total antenna area of the PCS array would require 128,000 half-wavelength mmWave antennas, but a much reduced number is adequate because the large number of antennas also confers greater channel orthogonality. At the other extreme, in a highly interference-limited multi-cell environment, only 215 mmWave antennas are needed; in this case, increasing the transmitted power yields little improvement in service quality.

  • 286.
    Larsson, Erik G.
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Moosavi, Reza
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Piggybacking an Additional Lonely Bit on Linearly Coded Payload Data2012In: IEEE Wireless Communications Letters, ISSN 2162-2337, Vol. 1, no 4, p. 292-295Article in journal (Other academic)
    Abstract [en]

    We provide a coding scheme, by which an additional lonely bit can be piggybacked on a payload data packet encoded with a linear channel code, at no essential extra cost in power or bandwidth. The underlying principle is to use the additional bit to select which of two linear codes that should be used for encoding the payload packet, this way effectively creating a nonlinear code. We give a fast algorithm for detecting the additional bit, without decoding the data packet. Applications include control signaling, for example, transmission of ACK/NACK bits

  • 287.
    Larsson, Erik G.
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Regnoli, Giorgio
    Royal Institute of Technology.
    Primary System Detection for Cognitive Radio: Does Small-Scale Fading Help?2007In: IEEE Communications Letters, ISSN 1089-7798, E-ISSN 1558-2558, Vol. 11, no 10, p. 799-801Article in journal (Refereed)
    Abstract [en]

    We consider the effect of small-scale fading on thedetection of weak signals in cognitive radio systems.We formulatea model for the detection problem taking fading into account,and give the associated likelihood ratio tests. Additionally we givean expression for the asymptotic detection performance and usethis to consider the tradeoff between the detection performanceand the coherence time and bandwidth of the channel.

  • 288.
    Larsson, Erik G.
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Skoglund, Mikael
    Royal Institute of Technology.
    Cognitive Radio in a Frequency Planned Environment: Can it work?2007In: Proceedings of the 50th IEEE Global Telecommunications Conference (GLOBECOM'07), 2007, p. 3548-3552Conference paper (Refereed)
    Abstract [en]

    The objective of this work is to assess some fundamental limits of operation for cognitive radios in a frequency- planned environment. We present a first-order analysis of the carrier-to-noise-and-interference situation in a cellular wireless network, and analyze the impact of cognitive users starting to transmit. The main conclusion is that introducing cognitive transmitters in a frequency-planned cellular network without causing substantial interference is very challenging.

  • 289.
    Larsson, Erik G.
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Skoglund, Mikael
    Royal Institute of Technology (KTH).
    Cognitive Radio in a Frequency-Planned Environment: Some Basic Limits2008In: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, Vol. 7, no 12, p. 4800-4806Article in journal (Refereed)
    Abstract [en]

    The objective of this work is to assess some fundamental limits for opportunistic spectrum reuse via cognitive radio in a frequency-planned environment. We present a first-order analysis of the signal-to-noise-and-interference situation in a wireless cellular network, and analyze the impact of cognitive users starting to transmit. Two main conclusions emerge from our study. First, obtaining any substantial benefits from opportunistic spatial spectrum reuse in a frequency-planned network without causing substantial interference is going to be very challenging. Second, the cognitive users need to be more sensitive, by orders of magnitude, than the receivers in the primary system, especially if there is significant shadow fading. This latter problem can be alleviated by having cognitive users cooperate, but only if they are separated far apart so that they experience independent shadowing.

  • 290.
    Larsson, Erik G.
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Thobaben, Ragnar
    Royal Institute of Technology (KTH).
    Wang, Gang
    Royal Institute of Technology (KTH).
    On Diversity Combining with Unknown Channel State Information and Unknown Noise Variance2010In: Proceedings of the IEEE Wireless Communications and Networking Conference (WCNC), IEEE , 2010Conference paper (Refereed)
    Abstract [en]

    We derive detection metrics for soft-output diversity combining for  the case of imperfect channel state information at the receiver.  We  treat in particular the case when the noise variance at the receiver  is unknown.  We contrast conventional training-based methods to a  detector based on the generalized likelihood-ratio (GLR) test  paradigm.  We study the performance of the detectors via EXIT chart  analysis and via simulations of LDPC coded transmission over a fast  Rayleigh fading channel.  The results show that the GLR receivers  can significantly outperform the conventional detectors.

  • 291.
    Larsson, Erik G.
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Van der Perre, Liesbet
    Katholieke Univ Leuven, Belgium.
    Out-of-Band Radiation From Antenna Arrays Clarified2018In: IEEE Wireless Communications Letters, ISSN 2162-2337, E-ISSN 2162-2345, Vol. 7, no 4, p. 610-613Article in journal (Refereed)
    Abstract [en]

    Non-linearities in radio-frequency transceiver hardware, particularly in power amplifiers, cause distortion in-band and out-of-band. Contrary to claims made in recent literature, in a multiple-antenna system this distortion is correlated across the antennas in the array. A significant implication of this fact is that out-of-band emissions caused by non-linearities are beamformed, in some cases into the same direction as the useful signal.

  • 292.
    Larsson, Erik G
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Vincent Poor, H.
    Princeton University, NJ 08544 USA.
    Joint Beamforming and Broadcasting in Massive MIMO2016In: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, Vol. 15, no 4, p. 3058-3070Article in journal (Refereed)
    Abstract [en]

    The downlink of a massive MIMO system is considered for the case in which the base station must concurrently serve two categories of terminals: one group to which imperfect instantaneous channel state information (CSI) is available and one group to which no CSI is available. Motivating applications include broadcasting of public channels and control information in wireless networks. A new technique is developed and analyzed: joint beamforming and broadcasting (JBB), by which the base station beamforms to the group of terminals to which CSI is available, and broadcasts to the other group of terminals, to which no CSI is available. The broadcast information does not interfere with the beamforming as it is placed in the nullspace of the channel matrix collectively seen by the terminals targeted by the beamforming. JBB is compared to orthogonal access (OA) by which the base station partitions the time-frequency resources into two disjunct parts, one for each group of terminals. It is shown that JBB can substantially outperform OA in terms of required total radiated power for given rate targets.

  • 293.
    Le, Tuan Anh
    et al.
    Department of Design Engineering & Mathematics, Middlesex University, London.
    Van Chien, Trinh
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Nakhai, Mohammad Reza
    Department of Informatics, King’s College London, London.
    A Power Efficient Pilot Design for Multi-cell Massive MIMO Systems2018Conference paper (Refereed)
    Abstract [en]

    In this paper, we address the pilot contamination problem in multi-cell massive MIMO systems. Particularly, we propose a pilot design scheme that simultaneously minimizes the channel estimation errors of all base stations (BSs) and the total pilot power consumption of all users subject to the transmit power constraint for every user in the network. We decompose the proposed non-convex problem into distributed optimization problems to be solved at each BS, assuming the knowledge of pilot signals of the other BSs. Then, we introduce a successive optimization approach to cast each distributed optimization problem into a convex linear matrix inequality form. Simulation results confirm that the proposed approach significantly reduces pilot power while maintain the same level of channel estimation error as a recent work in [1].

  • 294.
    Li, Guoxin
    et al.
    Univ Alberta, Canada.
    Mishra, Deepak
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Jiang, Hai
    Univ Alberta, Canada.
    Cooperative NOMA With Incremental Relaying: Performance Analysis and Optimization2018In: IEEE Transactions on Vehicular Technology, ISSN 0018-9545, E-ISSN 1939-9359, Vol. 67, no 11, p. 11291-11295Article in journal (Refereed)
    Abstract [en]

    In conventional cooperative non-orthogonal multiple access (NOMA) networks, spectral efficiency loss occurs due to a half-duplex constraint. To address this issue, we propose an incremental cooperative NOMA (ICN) protocol for a two-user downlink network. In particular, this protocol allows the source to adaptively switch between a direct NOMA transmission mode and a cooperative NOMA transmission mode according to a 1-bit feedback from the far user. We analytically prove that the proposed ICN protocol outperforms the conventional cooperative NOMA protocol. In addition, an optimal power allocation strategy at the source is studied to minimize the asymptotic system outage probability. Finally, numerical results validate our theoretical analysis. present insights, and quantify the enhancement achieved over the benchmark scheme.

  • 295.
    Li, Jingya
    et al.
    Chalmers, Sweden; Ericsson AB, Sweden.
    Björnson, Emil
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Svensson, Tommy
    Chalmers, Sweden.
    Eriksson, Thomas
    Chalmers, Sweden.
    Debbah, Merouane
    Supelec, France.
    Optimal Design of Energy-Efficient HetNets: Joint Precoding and Load Balancing2015In: 2015 IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS (ICC), IEEE , 2015, p. 4664-4669Conference paper (Refereed)
    Abstract [en]

    This paper considers the downlink of a heterogeneous network, where multiple base stations (BSs) can serve the users by non-coherent multiflow beamforming. We assume imperfect channel state information at both BSs and users. The objective is to jointly optimize the precoding, load balancing, and BS operation mode (active or sleep) for improving the energy efficiency of the network. The considered problem is to minimize the weighted total power consumption (both circuit power and dynamic transmit power), while satisfying per-user quality of service constraints and per-BS transmit power constraints. This problem is non-convex, but we prove that for each combination of BS modes, the considered problem has a hidden convexity structure. Thus, the global optimal solution is obtained by an exhaustive search over all possible BS mode combinations. Furthermore, by iterative convex approximations of the non-convex power consumption functions, a heuristic algorithm is proposed to obtain a local optimal solution with low complexity. Simulation results illustrate that our proposed algorithms significantly reduce the total power consumption, compared to the scheme where all BSs are continuously active. This implies that putting a BS into sleep mode by proper load balancing is an important solution for energy savings in heterogeneous networks.

  • 296.
    Li, Xueru
    et al.
    Tsinghua University, Beijing, China.
    Björnson, Emil
    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.
    Zhou, Shidong
    Tsinghua University, Beijing, China.
    Wang, Jing
    Tsinghua University, Beijing, China.
    A Multi-cell MMSE Detector for Massive MIMO Systems and New Large System Analysis2015In: 2015 IEEE Global Communications Conference, GLOBECOM 2015, Institute of Electrical and Electronics Engineers (IEEE), 2015, p. 1-6Conference paper (Refereed)
    Abstract [en]

    In this paper, a new multi-cell MMSE detector is proposed for massive MIMO systems. Let K and B denote the number of users in each cell and the number of available pilot sequences in the network, respectively, with B = βK, where β ≥ 1 is called the pilot reuse factor. The novelty of the multi-cell MMSE detector is that it utilizes all B channel directions that can be estimated locally at a base station, so that intra-cell interference, parts of the inter-cell interference and the noise can all be actively suppressed, while conventional detectors only use the K intra-cell channels. Furthermore, in the large- system limit, a deterministic equivalent expression of the uplink SINR for the proposed multi-cell MMSE is derived. The expression is easy to compute and accounts for power control for the pilot and payload, imperfect channel estimation and arbitrary pilot allocation. Numerical results show that significant sum spectral efficiency gains can be obtained by the multi-cell MMSE over the conventional single-cell MMSE and the recent multi-cell ZF, and the gains become more significant as β and/or K increases. Furthermore, the deterministic equivalent is shown to be very accurate even for relatively small system dimensions.

  • 297.
    Li, Xueru
    et al.
    Tsinghua University, Beijing, China.
    Björnson, Emil
    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.
    Zhou, Shidong
    Tsinghua University, Beijing, China.
    Wang, Jing
    Tsinghua University, Beijing, China.
    A Multi-cell MMSE Precoder for Massive MIMO Systems and New Large System Analysis2015In: 2015 IEEE Global Communications Conference, GLOBECOM 2015, Institute of Electrical and Electronics Engineers (IEEE), 2015, p. 1-6Conference paper (Refereed)
    Abstract [en]

    In this paper, a new multi-cell MMSE precoder is proposed for massive MIMO systems. We consider a multi-cell network where each cell has K users and B orthogonal pilot sequences are available, with B = βK and β ≥ 1 being the pilot reuse factor over the network. In comparison with conventional single-cell precoding which only uses the K intra-cell channel estimates, the proposed multi-cell MMSE precoder utilizes all B channel directions that can be estimated locally at a base station, so that the transmission is designed spatially to suppress both parts of the inter-cell and intra-cell interference. To evaluate the performance, a large-scale approximation of the downlink SINR for the proposed multi-cell MMSE precoder is derived and the approximation is tight in the large-system limit. Power control for the pilot and payload, imperfect channel estimation and arbitrary pilot allocation are accounted for in our precoder. Numerical results show that the proposed multi-cell MMSE precoder achieves a significant sum spectral efficiency gain over the classical single-cell MMSE precoder and the gain increases as K or β grows. Compared with the recent M-ZF precoder, whose performance degrades drastically for a large K, our M-MMSE can always guarantee a high and stable performance. Moreover, the large-scale approximation is easy to compute and shown to be accurate even for small system dimensions. 

  • 298.
    Li, Xueru
    et al.
    Tsinghua University, Peoples R China.
    Björnson, Emil
    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.
    Zhou, Shidong
    Tsinghua University, Peoples R China.
    Wang, Jing
    Tsinghua University, Peoples R China.
    Massive MIMO with multi-cell MMSE processing: exploiting all pilots for interference suppression2017In: EURASIP Journal on Wireless Communications and Networking, ISSN 1687-1472, E-ISSN 1687-1499, article id 117Article in journal (Refereed)
    Abstract [en]

    A new state-of-the-art multi-cell minimum mean square error (M-MMSE) scheme is proposed for massive multiple-input-multiple-output (MIMO) networks, which includes an uplink MMSE detector and a downlink MMSE precoder. Contrary to conventional single-cell schemes that suppress interference using only channel estimates for intra-cell users, our scheme shows the optimal way to suppress both intra-cell and inter-cell interference instantaneously by fully utilizing the available pilot resources. Specifically, let K and B denote the number of users per cell and the number of orthogonal pilot sequences in the network, respectively, where beta = B/K is the pilot reuse factor. Our scheme utilizes all B channel directions that can be estimated locally at each base station, to actively suppress both intra-cell and inter-cell interference. Our scheme is practical and general, since power control, imperfect channel estimation, and arbitrary pilot allocation are all accounted for. Simulations show that significant spectral efficiency (SE) gains are obtained over the conventional single-cell MMSE scheme and the multi-cell zero-forcing (ZF) scheme. Furthermore, large-scale approximations of the uplink and downlink signal-to-interference-and-noise ratios (SINRs) are derived, which are tight in the large-system limit. These approximations are easy to compute and very accurate even for small system dimensions. Using these SINR approximations, a low-complexity power control algorithm is further proposed to maximize the sum SE.

  • 299.
    Li, Xueru
    et al.
    Tsinghua University, Peoples R China.
    Björnson, Emil
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Zhou, Shidong
    Tsinghua University, Peoples R China.
    Wang, Jing
    Tsinghua University, Peoples R China.
    Massive MIMO with Multi-Antenna Users: When are Additional User Antennas Beneficial?2016In: 2016 23RD INTERNATIONAL CONFERENCE ON TELECOMMUNICATIONS (ICT), IEEE , 2016Conference paper (Refereed)
    Abstract [en]

    We analyze the performance of massive MIMO systems with N-antenna users, The benefit is that N streams can be multiplexed per user, at the price of increasing the channel estimation overhead linearly with N. Uplink and downlink spectral efficiency (SE) expressions are derived for any AT, and these are achievable using estimated channels and per-user-basis MMSE-SIC detectors. Large-system approximations of the SEs are obtained. This analysis shows that MMSE-SIC has similar asymptotic SE as linear MMSE detectors, indicating that the SE increase from having multi-antenna users can be harvested using linear detectors. We generalize the power scaling laws for massive MIMO to handle arbitrary N, and show that one can reduce the multiplication of the pilot power and payload power as 1/M where M is the number of BS antennas, and still notably increase the SE with M before reaching a non-zero asymptotic limit. Simulations testify our analysis and show that the SE increases with N. We also note that the same improvement can be achieved by serving N times more single-antenna users instead, thus the additional user antennas are particular beneficial for SE enhancement when there are few active users in the system.

  • 300.
    Li, Xueru
    et al.
    Tsinghua University, Peoples R China.
    Zhou, Shidong
    Tsinghua University, Peoples R China.
    Björnson, Emil
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, Faculty of Science & Engineering.
    Wang, Jing
    Tsinghua University, Peoples R China.
    Capacity Analysis for Spatially Non-Wide Sense Stationary Uplink Massive MIMO Systems2015In: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, Vol. 14, no 12, p. 7044-7056Article in journal (Refereed)
    Abstract [en]

    Channel measurements show that significant spatially non-wide-sense-stationary characteristics rise in massive MIMO channels. Notable parameter variations are experienced along the base station array, such as the average received energy at each antenna, and the directions of arrival of signals impinging on different parts of the array. In this paper, a new channel model is proposed to describe this spatial non-stationarity in massive MIMO channels by incorporating the concepts of partially visible clusters and wholly visible clusters. Furthermore, a closed-form expression of an upper bound on the ergodic sum capacity is derived for the new model, and the influence of the spatial non-stationarity on the sum capacity is analyzed. Analysis shows that for non-identically-and-independent-distributed (i.i.d.) Rayleigh fading channels, the non-stationarity benefits the sum capacity by bringing a more even spread of channel eigenvalues. Specifically, more partially visible clusters, smaller cluster visibility regions, and a larger antenna array can all help to yield a well-conditioned channel, and benefit the sum capacity. This shows the advantage of using a large antenna array in a non-i.i.d. channel: the sum capacity benefits not only from a higher array gain, but also from a more spatially non-stationary channel. Numerical results demonstrate our analysis and the tightness of the upper bound.

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