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Sökning: WFRF:(Tufvesson Fredrik)

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31.
  • Rusek, Fredrik, et al. (författare)
  • Scaling up MIMO : Opportunities and Challenges with Very Large Arrays
  • 2013
  • Ingår i: IEEE signal processing magazine (Print). - : IEEE. - 1053-5888 .- 1558-0792. ; 30:1, s. 40-60
  • Tidskriftsartikel (refereegranskat)abstract
    • Multiple-input multiple-output (MIMO) technology is maturing and is being incorporated into emerging wireless broadband standards like long-term evolution (LTE) [1]. For example, the LTE standard allows for up to eight antenna ports at the base station. Basically, the more antennas the transmitter/receiver is equipped with, and the more degrees of freedom that the propagation channel can provide, the better the performance in terms of data rate or link reliability. More precisely, on a quasi static channel where a code word spans across only one time and frequency coherence interval, the reliability of a point-to-point MIMO link scales according to Prob(link outage) ` SNR-ntnr where nt and nr are the numbers of transmit and receive antennas, respectively, and signal-to-noise ratio is denoted by SNR. On a channel that varies rapidly as a function of time and frequency, and where circumstances permit coding across many channel coherence intervals, the achievable rate scales as min(nt, nr) log(1 + SNR). The gains in multiuser systems are even more impressive, because such systems offer the possibility to transmit simultaneously to several users and the flexibility to select what users to schedule for reception at any given point in time [2].
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32.
  • Sanchez, Juan, et al. (författare)
  • Optimal, Low-Complexity Beamforming for Discrete Phase Reconfigurable Intelligent Surfaces
  • 2021
  • Ingår i: 2021 IEEE Global Communications Conference, GLOBECOM 2021 - Proceedings. - 9781728181042
  • Konferensbidrag (refereegranskat)abstract
    • Reflective reconfigurable intelligent surface (RIS) technology is regarded as an innovative, cost- and power-effective solution that aims at influencing the wireless channel through controlled scattering. The technology can be realized by using metamaterials and/or resonant elements that scatter electromagnetic waves with a configurable phase shift. Most of the previous work on beamforming techniques for RIS assumes ideal hardware and, thus, continuous phase shifts. However, hardware constraints limit the phase shift resolution, manifested into the amount of discrete phase shifts that can be configured into each RIS element. This paper aims to offer a discrete phase shift beamforming algorithm for reflective RISs that targets minimization of the quantization error resulting from discretization of continuous phase shifts. The beamforming solution proves to be optimal under perfect channel knowledge for any discrete set of uniformly distributed phase shifts. The required complexity to find the optimal beamforming vector for our approach is found to be linear with the number of RIS elements, the minimum needed to obtain optimal results. Simulated behavior is validated by measurements, showing robustness against angle misalignments and distance variations.
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33.
  • Vieira, Joao, et al. (författare)
  • A Generalized Method of Moments Detector for Block Fading SIMO Channels
  • 2016
  • Ingår i: IEEE Communications Letters. - 1089-7798. ; 20:7, s. 1477-1480
  • Tidskriftsartikel (refereegranskat)abstract
    • In this letter we apply the Generalized Method of Moments (GMM), widely used in econometrics, to receivers operating with imperfect channel state information (CSI) of singleinput-multiple-output (SIMO) block-fading channels where a single pilot symbol is used. The GMM results in the standard maximum ratio combining (MRC) receiver, but with an improved channel estimate. Although not our goal at the outset, this result reveals an inherent capability of the GMM to improve any channel estimate through filtering of the initial channel estimate with a matrix that is constructed from the received signals. The filtering involves a matrix inverse of size min {T, M}, where M is the number of receive antennas, and T + 1 is the coherence interval of the channel. The gain over an MRC receiver, using a scaled version of the pilot observation as channel estimate, lies in the range 0.1-3 dB depending on the system configuration.A coherence interval of about 5 symbol intervals is sufficient to reach these gains
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34.
  • Vieira, Joao, et al. (författare)
  • A receive/transmit calibration technique based on mutual coupling for massive MIMO base stations
  • 2016
  • Ingår i: 2016 IEEE 27th Annual International Symposium on Personal, Indoor, and Mobile Radio Communications, PIMRC 2016. - 9781509032549
  • Konferensbidrag (refereegranskat)abstract
    • This paper presents a calibration technique for massive MIMO base stations, where the frequency responses of the transmit and/or receive analog front-ends are individually estimated and compensated for. Calibration is achieved by a first-round of channel sounding between base station antennas, followed by post-processing and a compensation stage. The proposed technique is general in the sense that is does not use external sources, nor internal dedicated circuits for calibration purposes. The only requirement of the technique is that mutual coupling between all pairs of sounded base station antennas exists and is known. Our analysis suggests that mutual coupling can be conveniently used for calibration purposes, and that multipath propagation during calibration is the most prominent source for calibration inaccuracies.
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35.
  • Vieira, Joao, et al. (författare)
  • Reciprocity Calibration for Massive MIMO: Proposal, Modeling and Validation
  • 2017
  • Ingår i: IEEE Transactions on Wireless Communications. - 1536-1276. ; 16:5, s. 3042-3056
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper presents a mutual coupling based calibration method for time-division-duplex massive MIMO systems, which enables downlink precoding based on uplink channel estimates. The entire calibration procedure is carried out solely at the base station (BS) side by sounding all BS antenna pairs. An Expectation-Maximization (EM) algorithm is derived, which processes the measured channels in order to estimate calibration coefficients. The EM algorithm outperforms current state-of-the-art narrow-band calibration schemes in a mean squared error (MSE) and sum-rate capacity sense. Like its predecessors, the EM algorithm is general in the sense that it is not only suitable to calibrate a co-located massive MIMO BS, but also very suitable for calibrating multiple BSs in distributed MIMO systems. The proposed method is validated with experimental evidence obtained from a massive MIMO testbed. In addition, we address the estimated narrow-band calibration coefficients as a stochastic process across frequency, and study the subspace of this process based on measurement data. With the insights of this study, we propose an estimator which exploits the structure of the process in order to reduce the calibration error across frequency. A model for the calibration error is also proposed based on the asymptotic properties of the estimator, and is validated with measurement results.
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36.
  • Vieira, Joao, et al. (författare)
  • Reciprocity calibration methods for Massive MIMO based on antenna coupling
  • 2014
  • Konferensbidrag (refereegranskat)abstract
    • In this paper we consider time-division-duplex (TDD) reciprocity calibration of a massive MIMO system. The calibration of a massive MIMO system can be done entirely at the base station (BS) side by sounding the BS antennas one-by-one while receiving with the other BS antennas. With an M antenna BS, this generates M(M - 1) signals that can be used for calibration purposes. In this paper we study several least-squares (LS) based estimators, differing in the number of received signals that are being used. We compare the performance of the estimators, and we conclude that is possible to accurately calibrate an entire BS antenna array using the mutual coupling between antennas as the main propagation mechanism.
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37.
  • Abbas, Taimoor, et al. (författare)
  • A Measurement Based Shadow Fading Model for Vehicle-to-Vehicle Network Simulations
  • 2015
  • Ingår i: International Journal of Antennas and Propagation. - : Hindawi Publishing Corporation. - 1687-5869 .- 1687-5877.
  • Tidskriftsartikel (refereegranskat)abstract
    • The vehicle-to-vehicle (V2V) propagation channel has significant implications on the design and performance of novel communication protocols for vehicular ad hoc networks (VANETs). Extensive research efforts have been made to develop V2V channel models to be implemented in advanced VANET system simulators for performance evaluation. The impact of shadowing caused by other vehicles has, however, largely been neglected in most of the models, as well as in the system simulations. In this paper we present a shadow fading model targeting system simulations based on real measurements performed in urban and highway scenarios. The measurement data is separated into three categories, line-of-sight (LOS), obstructed line-of-sight (OLOS) by vehicles, and non-line-of-sight due to buildings, with the help of video information recorded during the measurements. It is observed that vehicles obstructing the LOS induce an additional average attenuation of about 10 dB in the received signal power. An approach to incorporate the LOS/OLOS model into existing VANET simulators is also provided. Finally, system level VANET simulation results are presented, showing the difference between the LOS/OLOS model and a channel model based on Nakagami-m fading.
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38.
  • Abbas, Taimoor, et al. (författare)
  • Directional Analysis of Vehicle-to-Vehicle Propagation Channels
  • 2011
  • Ingår i: IEEE Vehicular Technology Conference.
  • Konferensbidrag (refereegranskat)abstract
    • This paper presents a double directional analysis of vehicle-to-vehicle channel measurements conducted in three different traffic scenarios. Using a high-resolution algorithm, we derive channel parameters like Angle-of-Arrival (AOA), Angle-of-Departure (AOD), propagation delay and Doppler shift and identify underlying propagation mechanisms by combining these estimates with maps of the measurement sites. The results show that first-order reflections from a small number of interacting objects can account for a large part of the received signal in the absence of line-of-sight (LOS). This effect is especially pronounced in the two traffic scenarios where the road is not lined with buildings. We also found that the direction spread is low (and conversely that the antenna correlation is high) in such scenarios, which suggests that beam forming rather than diversity-based methods should be used if multiple antenna elements are used. The situation is reversed, however, in the third scenario, a narrow urban intersection, where a larger number of higher-order reflections is found to result in a higher direction spread.
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39.
  • Abbas, Taimoor, et al. (författare)
  • Directional Analysis of Vehicle-to-Vehicle Propagation in Different Traffic Environments
  • 2010
  • Ingår i: [Host publication title missing].
  • Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
    • This paper presents a double directional analysis of vehicle-to-vehicle channel measurements conducted in two different traffic environments. Using a high-resolution algorithm, we derive channel parameters like angle-of- arrival, angle-of-departure, propagation delay and Doppler’s spread and identify underlying propagation mechanisms by combining these estimates with maps of the measurement sites. The results show that single-bounce interaction with static objects is the dominating effect in the absence of line-of-sight, and that the contribution from other vehicles is small. We also find that in the absence of line-of-sight the direction spread at both sides of link is large which indicates possible diversity gain using multi-antenna arrangements.
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40.
  • Abbas, Taimoor, et al. (författare)
  • Initial Investigation to study the Effect of Antenna Placement in Vehicle-to-Vehicle Communications
  • 2011
  • Ingår i: [Host publication title missing].
  • Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
    • In this paper we present an analysis of the effects of antenna placement on the radio channel properties for Vehicle-to-Vehicle communications based on measurements performed in three different propagation scenarios; highway, urban and rural. Four omni-directional antennas were mounted at four different positions: roof, bumper, left-side mirror and inside windscreen of two standard station wagons (Volvo V70). The channel transfer functions were measured for all antenna combinations with a wideband MIMO channel sounder. The results from this initial analysis show that the antenna mounted on the left-side mirror outperforms other antennas when vehicles are moving in convoy and are aligned well to each other. Whereas, if the TX and RX are moving in opposite direction, i.e., towards each other, bumper antenna shows good performance as long as LOS is available in between the antenna elements. In total some kind of diversity arrangement with complementary antennas seems to be the preferred solution, to decrease the effect of shadowing.
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