| 1. |
- Bengtsson, Erik L, et al.
(författare)
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Transmission schemes for Multiple Antenna Terminals in Real Massive MIMO systems
- 2016
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Ingår i: 2016 IEEE Global Communications Conference (GLOBECOM). - 9781509013289 - 9781509013296
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Konferensbidrag (refereegranskat)abstract
- In massive MIMO performance evaluations it is often assumed that the terminal has a single antenna. The combination of multiple antennas in a terminal and massive MIMO precoding at the base station side can further improve overall system performance. We present measurement results for multi antenna terminals operating in different transmission schemes and how they perform under varying loading conditions. Gain expressions are derived that enable easy comparison between the transmission schemes. The evaluation is performed on realistic antennas integrated into Sony Xperia handsets tuned to 3.7~GHz and operated together with the Lund University massive MIMO (LuMaMi) test bed. It is concluded that the approach used in today's mobile systems, where up link and down link are addressed independently, will not provide the best performance. The performance can be improved by the selection of transmission schemes optimized for massive MIMO.
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| 2. |
- Vieira, Joao, et al.
(författare)
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A Generalized Method of Moments Detector for Block Fading SIMO Channels
- 2016
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Ingår i: IEEE Communications Letters. - 1089-7798. ; 20:7, s. 1477-1480
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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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| 3. |
- Vieira, Joao, et al.
(författare)
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A receive/transmit calibration technique based on mutual coupling for massive MIMO base stations
- 2016
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Ingår i: 2016 IEEE 27th Annual International Symposium on Personal, Indoor, and Mobile Radio Communications, PIMRC 2016. - 9781509032549
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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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| 4. |
- Vieira, Joao, et al.
(författare)
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Reciprocity Calibration for Massive MIMO: Proposal, Modeling and Validation
- 2017
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Ingår i: IEEE Transactions on Wireless Communications. - 1536-1276. ; 16:5, s. 3042-3056
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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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| 5. |
- Vieira, Joao, et al.
(författare)
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Reciprocity calibration methods for Massive MIMO based on antenna coupling
- 2014
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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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| 6. |
- Harris, Paul, et al.
(författare)
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Performance Characterization of a Real-Time Massive MIMO System with LOS Mobile Channels
- 2017
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Ingår i: IEEE Journal on Selected Areas in Communications. - 0733-8716. ; , s. 1244-1253
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Tidskriftsartikel (refereegranskat)abstract
- The first measured results for massive multiple input, multiple-output (MIMO) performance in a line-of-sight (LOS) scenario with moderate mobility are presented, with 8 users served in real-time using a 100 antenna base Station (BS) at 3.7 GHz. When such a large number of channels dynamically change, the inherent propagation and processing delay has a critical relationship with the rate of change, as the use of outdated channel information can result in severe detection and precoding inaccuracies. For the downlink (DL) in particular, a time division duplex (TDD) configuration synonymous with massive MIMO deployments could mean only the uplink (UL) is usable in extreme cases. Therefore, it is of great interest to investigate the impact of mobility on massive MIMO performance and consider ways to combat the potential limitations. In a mobile scenario with moving cars and pedestrians, the massive MIMO channel is sampled across many points in space to build a picture of the overall user orthogonality, and the impact of both azimuth and elevation array configurations are considered. Temporal analysis is also conducted for vehicles moving up to 29km�h and real-time bit error rates (BERs) for both the UL and DL without power control are presented. For a 100 antenna system, it is found that the channel state information (CSI) update rate requirement may increase by 7 times when compared to an 8 antenna system, whilst the power control update rate could be decreased by at least 5 times relative to a single antenna system.
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| 7. |
- Harris, Paul, et al.
(författare)
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Temporal Analysis of Measured LOS Massive MIMO Channels with Mobility
- 2017
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Ingår i: 2017 IEEE 85th Vehicular Technology Conference, VTC Spring 2017 - Proceedings. - 9781509059324
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Konferensbidrag (refereegranskat)abstract
- The first measured results for massive multiple-input, multiple-output (MIMO) performance in a line-of-sight (LOS) scenario with moderate mobility are presented, with 8 users served by a 100 antenna base Station (BS) at 3.7 GHz. When such a large number of channels dynamically change, the inherent propagation and processing delay has a critical relationship with the rate of change, as the use of outdated channel information can result in severe detection and precoding inaccuracies. For the downlink (DL) in particular, a time division duplex (TDD) configuration synonymous with massive MIMO deployments could mean only the uplink (UL) is usable in extreme cases. Therefore, it is of great interest to investigate the impact of mobility on massive MIMO performance and consider ways to combat the potential limitations. In a mobile scenario with moving cars and pedestrians, the correlation of the MIMO channel vector over time is inspected for vehicles moving up to 29km/h. For a 100 antenna system, it is found that the channel state information (CSI) update rate requirement may increase by 7 times when compared to an 8 antenna system, whilst the power control update rate could be decreased by at least 5 times relative to a single antenna system.
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| 8. |
- Malkowsky, Steffen, et al.
(författare)
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Implementation of Low-latency Signal Processing and Data Shuffling for TDD massive MIMO Systems
- 2017
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Ingår i: ; , s. 260-265
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Konferensbidrag (refereegranskat)abstract
- Low latency signal processing and high throughput implementations are required in order to realize real-time TDD massive MIMO communications, especially in high mobility scenarios. One of the main challenges is that the up-link and down-link turnaround time has to be within the coherence time of the wireless channel to enable efficient use of reciprocity. This paper presents a hardware architecture and implementation of this critical signal processing path, including channel estimation, QRD-based MMSE decoder/precoder and distributed reciprocity calibration. Furthermore, we detail a switch-based router implementation to tackle the stringent throughput and latency requirements on the data shuffling network. The proposed architecture was verified on the LuMaMi testbed, based on the NI SDR platform. The implementation supports real-time TDD transmission in a 128x12 massive MIMO setup using 20 MHz channel bandwidth. The processing latency in the critical path is less than 0.15 ms, enabling reciprocity-based TDD massive MIMO for high-mobility scenarios.
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| 9. |
- Malkowsky, Steffen, et al.
(författare)
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The World's First Real-Time Testbed for Massive MIMO: Design, Implementation, and Validation
- 2017
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Ingår i: IEEE Access. - 2169-3536. ; , s. 9073-9088
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Tidskriftsartikel (refereegranskat)abstract
- This paper sets up a framework for designing a massive multiple-input multiple-output (MIMO) testbed by investigating hardware and system-level requirements such as processing complexity, duplexing mode and frame structure. Taking these into account, a generic system and processing partitioning is proposed which allows flexible scaling and processing distribution onto a multitude of physically separated devices. Based on the given hardware constraints such as maximum number of links and maximum throughput for peer-to-peer interconnections combined with processing capabilities, the framework allows to evaluate available off-the-shelf hardware components. To verify our design approach, we present the Lund University Massive MIMO (LuMaMi) testbed which constitutes the first reconfigurable real-time hardware platform for prototyping massive MIMO. Utilizing up to 100 base station antennas and more than 50 field-programmable gate arrays (FPGAs), up to 12 user equipments are served on the same time/frequency resource using an LTE-like OFDM TDD-based transmission scheme. Field trials with this system show that massive MIMO can spatially separate a multitude of users in a static indoor and static/dynamic outdoor environment.
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| 10. |
- Vieira, Joao, et al.
(författare)
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A flexible 100-antenna testbed for Massive MIMO
- 2014
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Konferensbidrag (refereegranskat)abstract
- Massive multiple-input multiple-output (MIMO) is one of the main candidates to be included in the fifth generation (5G) cellular systems. For further system development it is desirable to have real-time testbeds showing possibilities and limitations of the technology. In this paper we describe the Lund University Massive MIMO testbed – LuMaMi. It is a flexible testbed where the base station operates with up to 100 coherent radio-frequency transceiver chains based on software radio technology. Orthogonal Frequency Division Multiplex (OFDM) based signaling is used for each of the 10 simultaneous users served in the 20 MHz bandwidth. Real time MIMO precoding and decoding is distributed across 50 Xilinx Kintex-7 FPGAs with PCI-Express interconnects. The unique features of this system are: (i) high throughput processing of 384 Gbps of real time baseband data in both the transmit and receive directions, (ii) low-latency architecture with channel estimate to precoder turnaround of less than 500 micro seconds, and (iii) a flexible extension up to 128 antennas. We detail the design goals of the testbed, discuss the signaling and system architecture, and show initial measured results for a uplink Massive MIMO over-the-air transmission from four single-antenna UEs to 100 BS antennas.
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