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  • 1.
    Apelfröjd, Rikke
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Channel Estimation and Prediction for 5G Applications2018Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Accurate channel state information (CSI) is important for many candidate techniques of future wireless communication systems. However, acquiring CSI can sometimes be difficult, especially if the user equipment is mobile in which case the future channel realisations must be estimated/predicted. In realistic settings the predictability of radio channels is limited due to measurement noise, limited model orders and since the fading statistics must be modelled based on a set of limited and noisy training data.

    In this thesis, the limits of predictability for the radio channel are investigated. Results show that the predictability is limited primarily due to limitations in the training data, while the model order provides a second order limitation effect and the measurement noise comes in as a third order effect.

    Then, a Kalman-based linear filter is studied for potential 5G technologies:

    Coherent coordinated multipoint joint transmission, where channel predictions and the covariance matrix of the prediction error are used to design a robust linear precoder, evaluated in a three base station system. Results show that prediction improves the CSI for the pedestrian users such that system delays of 10 ms are acceptable. The use of the covariance matrix is important for difficult user groups, but of less importance with a simple user grouping system proposed.

    Massive multiple-input multiple-output (MIMO) in frequency division duplex (FDD) systems were a reduced, suboptimal, Kalman filter is suggested to estimate channels based on non-orthogonal pilots. By introducing a fixed grid of beams, the system generates sparsity in the channel vectors seen by each user, which then estimates its most relevant channels based on unique pilot codes for each beam. Results show that there is a 5 dB loss compared to orthogonal pilots.

    Downlink time division duplex (TDD) channels are estimated based on uplink pilots. By using a predictor antenna, which scouts the channel in advance, the desired downlink channel can be estimated using pilot-based estimates of the channels before and after it (in space). Results indicate that, with the help of Kalman smoothing, predictor antennas can enable accurate CSI for TDD downlinks at vehicular velocities of 80 km/h.

    Delarbeid
    1. Kalman predictions for multipoint OFDM downlink channels
    Åpne denne publikasjonen i ny fane eller vindu >>Kalman predictions for multipoint OFDM downlink channels
    2014 (engelsk)Rapport (Annet vitenskapelig)
    HSV kategori
    Forskningsprogram
    Elektroteknik med inriktning mot signalbehandling
    Identifikatorer
    urn:nbn:se:uu:diva-235010 (URN)
    Tilgjengelig fra: 2014-10-28 Laget: 2014-10-28 Sist oppdatert: 2018-03-07
    2. Design and measurement based evaluations of coherent JT CoMP: A study of precoding, user grouping and resource allocation using predicted CSI
    Åpne denne publikasjonen i ny fane eller vindu >>Design and measurement based evaluations of coherent JT CoMP: A study of precoding, user grouping and resource allocation using predicted CSI
    2014 (engelsk)Inngår i: EURASIP Journal on Wireless Communications and Networking, ISSN 1687-1472, E-ISSN 1687-1499Artikkel i tidsskrift (Fagfellevurdert) Published
    HSV kategori
    Identifikatorer
    urn:nbn:se:uu:diva-235014 (URN)DOI:10.1186/1687-1499-2014-100 (DOI)
    Tilgjengelig fra: 2014-10-28 Laget: 2014-10-28 Sist oppdatert: 2018-11-08
    3. Robust linear precoder for coordinated multipoint joint transmission under limited backhaul with imperfect CSI
    Åpne denne publikasjonen i ny fane eller vindu >>Robust linear precoder for coordinated multipoint joint transmission under limited backhaul with imperfect CSI
    2014 (engelsk)Inngår i: 2014 11TH INTERNATIONAL SYMPOSIUM ON WIRELESS COMMUNICATIONS SYSTEMS (ISWCS), 2014, s. 138-143Konferansepaper, Publicerat paper (Fagfellevurdert)
    Abstract [en]

    Coordinated Multipoint (COMP) transmission provides high theoretic gains in spectral efficiency, in particular with coherent linear Joint Transmission (JT) to multiple users. However, this requires high backhaul capacity. If the backhaul requirement cannot be met by the system, then CoNIP gains decrease as the linear precoder matrix must be adjusted to include zeros. To minimize the loss of CoMP gains, all elements in the precoder should be adjusted as zeros are added to the precoder. We here propose a low complexity method for adjusting a precoder matrix when some elements are required to be zero, with respect to a robust MSE criterion. This is done by introducing penalties on specific precoder matrix elements. This generalized MSE criterion can then be used as a low complexity tool for optimizing e.g. with respect to sum-rate. Results show that this does indeed provide a better solution than if zeros are added separately. It is especially beneficial for cell edge users, i.e. for the same users that can gain the most from JT CoNIP.

    HSV kategori
    Forskningsprogram
    Elektroteknik med inriktning mot signalbehandling
    Identifikatorer
    urn:nbn:se:uu:diva-235007 (URN)000363906500027 ()978-1-4799-5863-4 (ISBN)
    Konferanse
    International Symposium on Wireless Communication Systems ISWCS, Barcelona, Spanien, 25-29 Augusti
    Tilgjengelig fra: 2014-10-28 Laget: 2014-10-28 Sist oppdatert: 2018-03-07bibliografisk kontrollert
    4. Joint reference signal design and Kalman/Wiener channel estimation for FDD massive MIMO. Extended Report Version.
    Åpne denne publikasjonen i ny fane eller vindu >>Joint reference signal design and Kalman/Wiener channel estimation for FDD massive MIMO. Extended Report Version.
    2017 (engelsk)Rapport (Annet vitenskapelig)
    sted, utgiver, år, opplag, sider
    Uppsala: Signals and Systems, Uppsala University, 2017
    Serie
    Report r1701
    HSV kategori
    Identifikatorer
    urn:nbn:se:uu:diva-330705 (URN)
    Tilgjengelig fra: 2017-10-03 Laget: 2017-10-03 Sist oppdatert: 2018-03-07
    5. Kalman smoothing for irregular pilot patterns; A case study for predictor antennas in TDD systems
    Åpne denne publikasjonen i ny fane eller vindu >>Kalman smoothing for irregular pilot patterns; A case study for predictor antennas in TDD systems
    (engelsk)Manuskript (preprint) (Annet vitenskapelig)
    Abstract [en]

    For future large-scale multi-antenna systems, channel orthogonal downlink pilots are not feasible due to extensive overhead requirements. Instead, channel reciprocity can be utilized in time division duplex (TDD) systems so that the downlink channel estimates can be based on pilots transmitted during the uplink. User mobility affects the reciprocity and makes the channel state information outdated for high velocities and/or long downlink subframe durations. Channel extrapolation, e.g. through Kalman prediction, can reduce the problem but is also limited by high velocities and long downlink subframes. An alternative solution has been proposed where channel predictions are made with the help of an extra antenna, e.g. on the roof of a car, so called predictor antenna, with the primary objective to measure the channel at a position that is later encountered by the rearward antenna(s). The predictor antenna is not directly limited by high velocities and allows the channel in the downlinks to be interpolated rather than extrapolated. One remaining challenge here is to obtain a good interpolation of the uplink channel estimate, since a sequence of uplink reference signals (pilots) will be interrupted by downlink subframes. We here evaluate a Kalman smoothing estimate of the downlink channels and compare it to a cubic spline interpolation. These results are compared to results where uplink channels are estimated through Kalman filters and predictors. Results are based on measured channels and show that with Kalman smoothing, predictor antennas can enable accurate channel estimates for a longer downlink period at vehicular velocities.

    HSV kategori
    Identifikatorer
    urn:nbn:se:uu:diva-344267 (URN)
    Tilgjengelig fra: 2018-03-06 Laget: 2018-03-06 Sist oppdatert: 2018-03-07
  • 2.
    Apelfröjd, Rikke
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Design Aspects of Coordinated Multipoint Transmission: A Study of Channel Predictions, Resource Allocation, User Grouping and Robust Linear Precoding for Coherent Joint Transmission2014Licentiatavhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Shadowed areas and interference at cell borders pose great challenges for future wireless broadband systems. Coordinated Multipoint (CoMP) coherent joint transmission has shown the potential to overcome these challenges by turning harmful interference into useful signal power. However, there are obstacles to overcome before coherent joint transmission CoMP can be deployed. Some of these are the investigated in this thesis.

    First, coherent joint transmission requires very accurate Channel State Information (CSI), but unfortunately long system latencies cause outdating of the CSI. This can to some extend be counteracted by channel predictions. Two schemes are here investigated for predicting downlink Frequency Division Duplex (FDD) Orthogonal Frequency Division Multiplexing (OFDM) channels; Kalman filters and “predictor antennas”. The first is well suited for slow moving users, e.g. pedestrians or cyclists, as it does not require any special antenna setup. The second, which utilizes an extra antenna, located in front of the main receive antennas, is well suited for vehicular users, such as buses or trams, as these require long spatial prediction horizon.

    Second, a user grouping and resource allocation scheme is investigated. This scheme forms CoMP groups by local resource allocations and provides multi-user diversity gains very close to the optimal gains, found through an extensive combinatorial search. It has very low complexity, requires less feedback capacity than other schemes and places no demands on backhaul capacity.

    Finally, a linear precoder, which is robust to errors in the CSI, is investigated. This precoder takes the covariances of the channel errors into account while optimizing a Mean Squared Error (MSE) criterion. The MSE criterion includes design parameters that can be used as flexible tools for low dimensional searches with respect to an arbitrary optimization criterion, e.g. a weighted sum-rate criterion. The precoder design is also extended to handle backhaul constraints.

    Results show that with the combination of these three schemes: channel predictions, the proposed user grouping and resource allocation scheme and the robust linear precoder, then coherent joint transmission will indeed provide large capacity gains.

    Delarbeid
    1. Measurement-based evaluation of robust linear precoding for downlink CoMP
    Åpne denne publikasjonen i ny fane eller vindu >>Measurement-based evaluation of robust linear precoding for downlink CoMP
    2012 (engelsk)Inngår i: IEEE International Conference on Communications, ICC, Ottawa, Canada, 2012Konferansepaper, Publicerat paper (Fagfellevurdert)
    Abstract [en]

    We study the design and evaluation of joint processing coordinated multipoint (CoMP) downlink transmission. Precoders will then be designed based on outdated channel state information (CSI), so interference cannot be eliminated completely as by an ideal zero-forcing (ZF) solution. We here strive to design and evaluate realistic linear transmit schemes. Kalman predictors are used for orthogonal frequency-division multiplexing (OFDM) channels. They provide optimal linear predictions and also estimates of their uncertainty. Robust linear precoders are designed based on these uncertainty estimates. We introduce and use robust linear quadratic optimal feedforward control, with the criterion averaged (marginalized) over the CSI uncertainty. This flexible solution performs minimum mean square error (MSE) minimization. It can also iteratively optimize other criteria, such as sum-rate. The prediction- and transmission performance is evaluated using measured data on 20 MHz OFDM downlinks from three base stations, for users at fast pedestrian velocities. Downlink CoMP is here also compared to cellular transmission, that uses orthogonal resources within cells but allows uncontrolled interference between cells.

    HSV kategori
    Forskningsprogram
    Elektroteknik med inriktning mot signalbehandling
    Identifikatorer
    urn:nbn:se:uu:diva-181051 (URN)
    Konferanse
    IEEE International Conference on Communications, ICC, June 10 - 15, Ottawa, Canada.
    Tilgjengelig fra: 2012-09-17 Laget: 2012-09-17 Sist oppdatert: 2014-06-04bibliografisk kontrollert
    2. Design and measurement based evaluations of coherent JT CoMP: a study of precoding, user grouping and resource allocation using predicted CSI
    Åpne denne publikasjonen i ny fane eller vindu >>Design and measurement based evaluations of coherent JT CoMP: a study of precoding, user grouping and resource allocation using predicted CSI
    2014 (engelsk)Inngår i: EURASIP Journal on Wireless Communications and Networking, ISSN 1687-1472, E-ISSN 1687-1499, s. 100-Artikkel i tidsskrift (Fagfellevurdert) Published
    Abstract [en]

    Coordinated multipoint (CoMP) transmission provides high theoretic gains in spectral efficiency with coherent joint transmission (JT) to multiple users. However, this requires accurate channel state information at the transmitter (CSIT) and also user groups with spatially compatible users. The aim of this paper is to use measured channels to investigate if significant CoMP gains can still be obtained with channel estimation errors. This turns out to be the case, but requires the combination of several techniques. We here focus on coherent downlink JT CoMP to multiple users within a cluster of cooperating base stations. The use of Kalman predictors is investigated to estimate the complex channel gains at the moment of transmission. It is shown that this can provide sufficient CSIT quality for JT CoMP even for long (> 20 ms) system delays at 2.66 GHz at pedestrian velocities or, for lower delays, at 500 MHz, at vehicular velocities. A user grouping and resource allocation scheme that provides appropriate groups for CoMP is also suggested. It provides performance close to that obtained by exhaustive search at very low complexity, low feedback cost and very low backhaul cost. Finally, a robust linear precoder that takes channel uncertainties into account when designing the precoding matrix is considered. We show that, in challenging scenarios, this provides large gains compared with zero-forcing precoding. Evaluations of these design elements are based on measured channels with realistic noise and intercluster interference assumptions. These show that high JT CoMP gains can be expected, on average over large sets of user positions, when the above techniques are combined - especially in severely intracluster interference limited scenarios.

    Emneord
    Coordinated Multipoint, channel predictions, user grouping, resource allocation, robust precoding
    HSV kategori
    Forskningsprogram
    Elektroteknik med inriktning mot signalbehandling
    Identifikatorer
    urn:nbn:se:uu:diva-224254 (URN)10.1186/1687-1499-2014-100 (DOI)000347399300001 ()
    Tilgjengelig fra: 2014-05-07 Laget: 2014-05-07 Sist oppdatert: 2018-11-08bibliografisk kontrollert
    3. Robust Linear Precoder for Coordinated Multipoint Joint Transmission under Limited Backhaul with Imperfect CSI
    Åpne denne publikasjonen i ny fane eller vindu >>Robust Linear Precoder for Coordinated Multipoint Joint Transmission under Limited Backhaul with Imperfect CSI
    (engelsk)Manuskript (preprint) (Annet vitenskapelig)
    HSV kategori
    Forskningsprogram
    Elektroteknik med inriktning mot signalbehandling
    Identifikatorer
    urn:nbn:se:uu:diva-224255 (URN)
    Tilgjengelig fra: 2014-05-07 Laget: 2014-05-07 Sist oppdatert: 2014-06-04
    4. Analysis and Measurement of Multiple Antenna Systems for Fading Channel Prediction in Moving Relays
    Åpne denne publikasjonen i ny fane eller vindu >>Analysis and Measurement of Multiple Antenna Systems for Fading Channel Prediction in Moving Relays
    Vise andre…
    2014 (engelsk)Inngår i: 2014 8TH EUROPEAN CONFERENCE ON ANTENNAS AND PROPAGATION (EUCAP), 2014, s. 2015-2019Konferansepaper, Publicerat paper (Annet vitenskapelig)
    Abstract [en]

    The performance of wireless data transmission to mobile vehicles is improved if channel state information is available at the transmitter but movement of vehicles causes outdating of channel estimates. The concept of a predictor antenna has recently been proposed, where an antenna is placed in front of other antennas on the roof of the vehicle to sense the radio environment in advance. This can comparatively provide an order-of-magnitude improvement in channel prediction performance. A potential problem with this idea is that closely placed antennas will experience mutual electromagnetic couplings. These may reduce the efficiency of the predictor antenna concept if they are not taken into account. In this paper, we discuss about how to treat the forgoing issue and eventually evaluate a promising candidate on measured channels. We argue that only open-circuit voltage method would be realistic for the present application. The usefulness of the proposed decoupling method is demonstrated on field measurements obtained in downtown Dresden, Germany. We also partly address the sensitivity of the open-circuit decoupling method to the accuracy of the utilized network parameters.

    Serie
    Proceedings of the European Conference on Antennas and Propagation, ISSN 2164-3342
    Emneord
    Multi-element antennas; channel state prediction; moving relays; multipath measurement
    HSV kategori
    Forskningsprogram
    Elektroteknik med inriktning mot signalbehandling
    Identifikatorer
    urn:nbn:se:uu:diva-224256 (URN)978-88-907018-4-9 (ISBN)
    Konferanse
    The 8th European Conference on Antennas and Propagation (EuCAP), to be held at the World Forum in The Hague, The Netherlands, on 6-11 April 2014.
    Merknad

    This paper has been presented as a poster on: the 8th European Conference on Antennas and Propagation (EuCAP), in The Hague, The Netherlands, on 6-11 April 2014, and will appear in the proceedings

    Tilgjengelig fra: 2014-05-08 Laget: 2014-05-07 Sist oppdatert: 2015-11-06bibliografisk kontrollert
    5. Kalman Predictions for Multipoint OFDM Downlink Channels
    Åpne denne publikasjonen i ny fane eller vindu >>Kalman Predictions for Multipoint OFDM Downlink Channels
    (engelsk)Manuskript (preprint) (Annet vitenskapelig)
    Abstract [en]

    Coordinated Multipoint (CoMP) transmission provides high theoreticgains in spectral eciency with coherent Joint Transmission (JT) to mul-tiple users. However, this requires accurate Channel State Information atthe Transmitter (CSIT). Unfortunately, coherent JT CoMP often is accom-panied by long system delays, due to e.g. data sharing over backhaul links.Therefore, the CSIT will be outdated.This report provides a detailed description on how to increase the accu-racy of the CSIT by utilizing Kalman lters to predict Orthogonal FrequencyDivision Multiplexing (OFDM) downlink channels. The small scale fading ofthese channels are modeled by Auto Regressive (AR) models of nite order.The report includes descriptions on how to estimate these models basedon past knowledge of the channel as well as analytical result on the pre-dictability of such models. Dierent technical design aspects for deployingthe Kalman lters in communication, such as pilot patterns, AR model esti-mations and the location of Kalman lters that predict downlink FrequencyDivision Duplex (FDD) channels, are also discussed.The aim of the report is to in detail describe the prediction procedureused in previous work. Some of the results from this previous work arehere presented and extended to provide a complete overview. All simulationresults are based on measured channels.The report also includes a description on how to model block-fading chan-nels with a specied channel accuracy that would have been obtained withKalman predictions. This model can then be used for system simulations.V:

    Emneord
    Linear predictions of OFDM channels, outdated channel state information (CSI), Coordinated Multipoint (CoMP), Predictability of radio channles, AR modelling of radio channels
    HSV kategori
    Forskningsprogram
    Elektroteknik med inriktning mot signalbehandling
    Identifikatorer
    urn:nbn:se:uu:diva-224263 (URN)
    Tilgjengelig fra: 2014-05-08 Laget: 2014-05-08 Sist oppdatert: 2014-06-04
  • 3.
    Apelfröjd, Rikke
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Kalman predictions for multipoint OFDM downlink channels2014Konferansepaper (Fagfellevurdert)
  • 4.
    Apelfröjd, Rikke
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Kalman predictions for multipoint OFDM downlink channels2014Rapport (Annet vitenskapelig)
  • 5.
    Apelfröjd, Rikke
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Björsell, Joachim
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Sternad, Mikael
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Phan Huy, D-T
    Kalman smoothing for irregular pilot patterns: A case study for predictor antennas in TDD systems2018Inngår i: 2018 IEEE 29th Annual International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC), 2018Konferansepaper (Fagfellevurdert)
    Abstract [en]

    For future large-scale multi-antenna systems, channel orthogonal downlink pilots are not feasible due to extensive overhead requirements. Instead, channel reciprocity can be utilized in time division duplex (TDD) systems so that the downlink channel estimates can be based on pilots transmitted during the uplink. User mobility affects the reciprocity and makes the channel state information outdated for high velocities and/or long downlink subframe durations. Channel extrapolation, e.g. through Kalman prediction, can reduce the problem but is also limited by high velocities and long downlink subframes. An alternative solution has been proposed where channel predictions are made with the help of an extra antenna, e.g. on the roof of a car, so called predictor antenna, with the primary objective to measure the channel at a position that is later encountered by the rearward antenna(s). The predictor antenna is not directly limited by high velocities and allows the channel in the downlinks to be interpolated rather than extrapolated. One remaining challenge here is to obtain a good interpolation of the uplink channel estimate, since a sequence of uplink reference signals (pilots) will be interrupted by downlink subframes. We here evaluate a Kalman smoothing estimate of the downlink channels and compare it to a cubic spline interpolation. These results are also compared to results where uplink channels are estimated through Kalman filters and predictors. Results are based on measured channels and show that with Kalman smoothing, predictor antennas can enable accurate channel estimates for a longer downlink period at vehicular velocities. The gaps in the uplink pilot stream, due to downlink subframes, can have durations that correspond to a vehicle movement of up to 0.75 carrier wavelengths in space, for Rayleigh-like non-line-of-sight fading.

  • 6.
    Apelfröjd, Rikke
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Sternad, Mikael
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Design and measurement based evaluations of coherent JT CoMP: a study of precoding, user grouping and resource allocation using predicted CSI2014Inngår i: EURASIP Journal on Wireless Communications and Networking, ISSN 1687-1472, E-ISSN 1687-1499, s. 100-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Coordinated multipoint (CoMP) transmission provides high theoretic gains in spectral efficiency with coherent joint transmission (JT) to multiple users. However, this requires accurate channel state information at the transmitter (CSIT) and also user groups with spatially compatible users. The aim of this paper is to use measured channels to investigate if significant CoMP gains can still be obtained with channel estimation errors. This turns out to be the case, but requires the combination of several techniques. We here focus on coherent downlink JT CoMP to multiple users within a cluster of cooperating base stations. The use of Kalman predictors is investigated to estimate the complex channel gains at the moment of transmission. It is shown that this can provide sufficient CSIT quality for JT CoMP even for long (> 20 ms) system delays at 2.66 GHz at pedestrian velocities or, for lower delays, at 500 MHz, at vehicular velocities. A user grouping and resource allocation scheme that provides appropriate groups for CoMP is also suggested. It provides performance close to that obtained by exhaustive search at very low complexity, low feedback cost and very low backhaul cost. Finally, a robust linear precoder that takes channel uncertainties into account when designing the precoding matrix is considered. We show that, in challenging scenarios, this provides large gains compared with zero-forcing precoding. Evaluations of these design elements are based on measured channels with realistic noise and intercluster interference assumptions. These show that high JT CoMP gains can be expected, on average over large sets of user positions, when the above techniques are combined - especially in severely intracluster interference limited scenarios.

  • 7.
    Apelfröjd, Rikke
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Sternad, Mikael
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Robust linear precoder for coordinated multipoint joint transmission under limited backhaul with imperfect CSI2014Inngår i: 2014 11TH INTERNATIONAL SYMPOSIUM ON WIRELESS COMMUNICATIONS SYSTEMS (ISWCS), 2014, s. 138-143Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Coordinated Multipoint (COMP) transmission provides high theoretic gains in spectral efficiency, in particular with coherent linear Joint Transmission (JT) to multiple users. However, this requires high backhaul capacity. If the backhaul requirement cannot be met by the system, then CoNIP gains decrease as the linear precoder matrix must be adjusted to include zeros. To minimize the loss of CoMP gains, all elements in the precoder should be adjusted as zeros are added to the precoder. We here propose a low complexity method for adjusting a precoder matrix when some elements are required to be zero, with respect to a robust MSE criterion. This is done by introducing penalties on specific precoder matrix elements. This generalized MSE criterion can then be used as a low complexity tool for optimizing e.g. with respect to sum-rate. Results show that this does indeed provide a better solution than if zeros are added separately. It is especially beneficial for cell edge users, i.e. for the same users that can gain the most from JT CoNIP.

  • 8.
    Apelfröjd, Rikke
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Sternad, Mikael
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Aronsson, Daniel
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Measurement-based evaluation of robust linear precoding for downlink CoMP2012Inngår i: IEEE International Conference on Communications, ICC, Ottawa, Canada, 2012Konferansepaper (Fagfellevurdert)
    Abstract [en]

    We study the design and evaluation of joint processing coordinated multipoint (CoMP) downlink transmission. Precoders will then be designed based on outdated channel state information (CSI), so interference cannot be eliminated completely as by an ideal zero-forcing (ZF) solution. We here strive to design and evaluate realistic linear transmit schemes. Kalman predictors are used for orthogonal frequency-division multiplexing (OFDM) channels. They provide optimal linear predictions and also estimates of their uncertainty. Robust linear precoders are designed based on these uncertainty estimates. We introduce and use robust linear quadratic optimal feedforward control, with the criterion averaged (marginalized) over the CSI uncertainty. This flexible solution performs minimum mean square error (MSE) minimization. It can also iteratively optimize other criteria, such as sum-rate. The prediction- and transmission performance is evaluated using measured data on 20 MHz OFDM downlinks from three base stations, for users at fast pedestrian velocities. Downlink CoMP is here also compared to cellular transmission, that uses orthogonal resources within cells but allows uncontrolled interference between cells.

  • 9.
    Apelfröjd, Rikke
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Zirwas, Wolfgang
    Nokia Bell Labs, D-81541 Munich, Germany.
    Sternad, Mikael
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Low-Overhead Cyclic Reference Signals for Channel Estimation in FDD Massive MIMO2019Inngår i: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, Vol. 67, nr 5, s. 3279-3291Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Massive multiple input multiple output (MIMO) transmission and coordinated multipoint transmission are candidate technologies for increasing data throughput in evolving 5G standards. Frequency division duplex (FDD) is likely to remain predominant in large parts of the spectrum below 6 GHz for future 5G systems. Therefore, it is important to estimate the downlink FDD channels from a very large number of antennas, while avoiding an excessive downlink reference signal overhead. We here propose and investigate a three part solution. First, massive MIMO downlinks use a fixed grid of beams. For each user, only a subset of beams will then be relevant, and require estimation. Second, sets of coded reference signal sequences, with cyclic patterns over time, are used. Third, each terminal estimates its most relevant channels. We here propose and compare a linear mean square estimation and a Kalman estimation. Both utilize frequency and antenna correlation, and the later also utilizes temporal correlation. In extensive simulations, this scheme provides channel estimates that lead to an insignificant beamforming performance degradation as compared to full channel knowledge. The cyclic pattern of coded reference signals is found to be important for reliable channel estimation, without having to adjust the reference signals to specific users.

  • 10.
    Jamaly, Nima
    et al.
    Inst för signaler och system, CTH, Göteborg.
    Apelfröjd, Rikke
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Martinez, Ana Belén
    Tech Univ Dresden, Vodafone Chair Mobile Commun Syst, Dresden, Germany.
    Grieger, Michael
    TU Dresden.
    Svensson, Tommy
    Inst för signaler och system, CTH, Göteborg.
    Sternad, Mikael
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Fettweis, Gerhard
    TU Dresden.
    Analysis and Measurement of Multiple Antenna Systems for Fading Channel Prediction in Moving Relays2014Inngår i: 2014 8TH EUROPEAN CONFERENCE ON ANTENNAS AND PROPAGATION (EUCAP), 2014, s. 2015-2019Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    The performance of wireless data transmission to mobile vehicles is improved if channel state information is available at the transmitter but movement of vehicles causes outdating of channel estimates. The concept of a predictor antenna has recently been proposed, where an antenna is placed in front of other antennas on the roof of the vehicle to sense the radio environment in advance. This can comparatively provide an order-of-magnitude improvement in channel prediction performance. A potential problem with this idea is that closely placed antennas will experience mutual electromagnetic couplings. These may reduce the efficiency of the predictor antenna concept if they are not taken into account. In this paper, we discuss about how to treat the forgoing issue and eventually evaluate a promising candidate on measured channels. We argue that only open-circuit voltage method would be realistic for the present application. The usefulness of the proposed decoupling method is demonstrated on field measurements obtained in downtown Dresden, Germany. We also partly address the sensitivity of the open-circuit decoupling method to the accuracy of the utilized network parameters.

  • 11. Jungnickel, Volker
    et al.
    Manolakis, Konstantinos
    Zirwas, Wolfgang
    Panzner, Berthold
    Braun, Volker
    Lossow, Moritz
    Sternad, Mikael
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Apelfröjd, Rikke
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Svensson, Tommy
    Inst för signaler och system, CTH, Göteborg.
    The role of small cells, coordinated multi-point and massive MIMO in 5G2014Inngår i: IEEE Communications Magazine, ISSN 0163-6804, E-ISSN 1558-1896, Vol. 52, nr 5, s. 44-51Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    5G will have to support a multitude of new applications with a wide variety of requirements, including higher peak and user data rates, reduced latency, enhanced indoor coverage, increased number of devices, and so on. The expected traffic growth in 10 or more years from now can be satisfied by the combined use of more spectrum, higher spectral efficiency, and densification of cells. The focus of the present article is on advanced techniques for higher spectral efficiency and improved coverage for cell edge users. We propose a smart combination of small cells, joint transmission coordinated multipoint (JT CoMP), and massive MIMO to enhance the spectral efficiency with affordable complexity. We review recent achievements in the transition from theoretical to practical concepts and note future research directions. We show in measurements with macro-plus-small-cell scenarios that spectral efficiency can be improved by flexible clustering and efficient user selection, and that adaptive feedback compression is beneficial to reduce the overhead significantly. Moreover, we show in measurements that fast feedback reporting combined with advanced channel prediction are able to mitigate the impairment effects of JT CoMP.

  • 12.
    Klockar, Annika
    et al.
    Karlstad Univ, Comp Sci, Karlstad, Sweden.
    Sternad, Mikael
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Brunström, Anna
    Karlstad universitet.
    Apelfröjd, Rikke
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    User-centric pre-selection and scheduling for coordinated multipoint systems2014Inngår i: 2014 11TH INTERNATIONAL SYMPOSIUM ON WIRELESS COMMUNICATIONS SYSTEMS (ISWCS), 2014, s. 888-894Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The data traffic volumes are constantly increasing in cellular networks. Furthermore, a larger part of the traffic is generated by applications that require high data rates. Techniques including Coordinated Multipoint transmission (CoMP) can increase the data rates, but at the cost of a high overhead. The overhead can be reduced if only a subset of the users is served with CoMP. In this paper, we propose a user selection approach, including pre-selection of CoMP users and short term scheduling, that takes user requirements into account. Users that require a high data rate to reach an acceptable level of service satisfaction are selected to use coherent joint processing CoMP in some of their downlink transmission bandwidth. Simulation results show that both the number of satisfied users and fairness are improved with the proposed user selection as compared to user selection that does not consider individual user requirements.

  • 13.
    Li, Jingya
    et al.
    Dept of Signals and Systems, Chalmers Tekniska Universitet, Göteborg.
    Papadogiannis, Agisilaos
    Dept of Signals and Systems, Chalmers Tekniska Universitet, Göteborg.
    Apelfröjd, Rikke
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Svensson, Tommy
    Dept of Signals and Systems, Chalmers Tekniska Universitet, Göteborg.
    Sternad, Mikael
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Performance evaluation of coordinated multi-point transmission schemes with predicted CSI2012Inngår i: IEEE Conference on Personal, Indoor and Mobile Radio Communications, PIMRC, Sydney, Australien, 2012, s. 1055-1060Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Coordinated multi-point (CoMP) transmission is considered as an efficient technique to improve cell-edge performance as well as system spectrum efficiency. In CoMP-enabled systems, a cluster of coordinated base stations (BSs) are typically assumed to be connected to a control unit (CU) via backhaul links, and the provided performance gain relies heavily on the quality of the channel state information (CSI) available at the CU side. In this paper, we consider the downlink of a CoMP cluster and compare three different CoMP transmission schemes: zero-forcing coherent joint transmission, non-coherent joint transmission and coordinated scheduling. Moreover, for each of the analyzed schemes, the performance in terms of average sum rate of the CoMP cluster is studied with predicted CSI, considering the effects of the feedback and backhaul latency, as well as the user mobility. Compared to zero-forcing coherent joint transmission, we show that non-coherent joint transmission and coordinated scheduling are more robust to channel uncertainty. In addition, depending on the latency, user mobility and user locations, different schemes would achieve the highest average sum rate performance. Hence, a system could switch between the transmission schemes to improve the sum rate.

  • 14. Rajesh Lakshmana, T
    et al.
    Apelfröjd, Rikke
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Svensson, Tommy
    Chalmers, Göteborg.
    Sternad, Mikael
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Particle swarm optimization based precoder in CoMP with measurement data2014Konferansepaper (Fagfellevurdert)
  • 15.
    Sternad, Mikael
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Grieger, Michael
    Vodafone Chair Mobile Communications Systems, Technische Universität, Dresden, Tyskland.
    Apelfröjd, Rikke
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Svensson, Tommy
    Inst f signaler o system, Communication Systems Group, Chalmers Tekniska Universitet, Göteborg .
    Aronsson, Daniel
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Belén Martinez, Ana
    Vodafone Chair Mobile Communications Systems, Technische Universität, Dresden, Tyskland.
    Using "Predictor Antennas" for long-range prediction of fast fading for moving relays.2012Inngår i: IEEE Wireless Communications and Networking Conference, ICC, Ottawa, Canada., 2012Konferansepaper (Fagfellevurdert)
  • 16.
    Zirwas, Wolfgang
    et al.
    Nokia Bell Labs, Munich, Germany.
    Sternad, Mikael
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Apelfröjd, Rikke
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Signaler och System.
    Key solutions for a massive MIMO FDD system2017Inngår i: 2017 IEEE 28TH ANNUAL INTERNATIONAL SYMPOSIUM ON PERSONAL, INDOOR, AND MOBILE RADIO COMMUNICATIONS (PIMRC), IEEE, 2017Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The ongoing standardization within 3GPP for the so called new radio (NR) system has identified massive multiple-input multiple output (MIMO) transmission, also called full dimension MIMO, as one of the main contributors to higher spectral efficiency for the mobile broadband case. In particular for radio frequencies below 6 GHz, channel estimation has to be supported in frequency division duplex (FDD) as well as time division duplex (TDD) operation. In TDD we may obtain downlink channels by estimating uplink channels, assuming reciprocity. For FDD, codebook based design as well as some type of explicit feedback is under discussion. Separately, there are also ongoing discussions of the question if massive MIMO in combination with FDD is a reasonable choice at all. Here we highlight some of our recent results obtained within several 5G research projects. To our understanding they overcome some of the inherent limitations of massive MIMO for FDD. As indicated by simulations, the resulting concept enables a grid of beam (GoB) and reference signal design with a reasonable downlink reference signal overhead of around 10 percent, together with reasonable feedback overhead of several hundred kbit/s per UE. Such a design attains around 90 percent of the massive MIMO system performance with ideal channel state information.

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