| 1. |
- Bengtsson, Erik L, et al.
(author)
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A Case Study on the Influence of Multiple Users on the Effective Channel in a Massive MIMO System
- 2019
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In: IEEE Wireless Communications Letters. - 2162-2337. ; , s. 1-6
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Journal article (peer-reviewed)abstract
- We investigate the importance of weak clusters when modeling a wireless massive MIMO channel. We do this by studying the influence of densely spaced terminals and the number of base-station antennas for a zero-forcing precoded massive MIMO system. In particular, we focus on the influence on the correlation and imbalance between the signals at the terminal antennas, the effective channel-gain, the eigenvalue distributions and the number of clusters.The study is based on measured radio-channels from terminal prototypes with integrated antennas connected to a massive MIMO testbed.We further evaluate the advantage of using block-diagonalized zero-forcing compared to conventional zero-forcing in a massive MIMO system. Unexpectedly, terminals with low antenna envelope correlation coefficient may benefit significantly from block-diagonal zero-forcing in a massive MIMO system.The main conclusion is that weaker clusters are important when modeling multi-user scenarios.
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| 2. |
- Bengtsson, Erik L., et al.
(author)
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A Simulation Framework for Multiple-Antenna Terminals in 5G Massive MIMO Systems
- 2017
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In: IEEE Access. - 2169-3536. ; 5, s. 26819-26831
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Journal article (peer-reviewed)abstract
- The recent interest in massive MIMO has spurred intensive work on massive MIMO channel modeling in contemporary literature. However, current models fail to take the characteristics of terminal antennas into account. There is no massive MIMO channel model available that can be used for evaluation of the influence of different antenna characteristics at the terminal side. In this paper, we provide a simulation framework that fills this gap. We evaluate the framework with antennas integrated into Sony Xperia handsets operating at 3.7 GHz as this spectrum is identified for the 5G new radio standard by 3GPP. The simulation results are compared with measured terminal performance when communicating with the Lund University’s massive MIMO testbed under the same loading conditions. Expressions are derived for comparison of the gain obtained from different diversity schemes computed from measured far-field antenna patterns. We conclude that the simulation framework yields results close to the measured ones and that the framework can be used for antenna evaluation for terminals in a practical precoded massive MIMO system.
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| 3. |
- Bengtsson, Erik L., et al.
(author)
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Analysis of Transmission Schemes for Dual-Antenna Terminals in Massive MIMO Systems
- 2018
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In: 2018 25th International Conference on Telecommunications, ICT 2018. - 9781538623213 ; , s. 76-82
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Conference paper (peer-reviewed)abstract
- The overall system performance of massive MIMO is improved by equipping user terminals with multiple antennas. In this paper, we investigate transceiver designs for the case of a single downlink stream and in particular, we study the uplink pilot design. Moreover, we study the consequences of channel estimation errors at the base-station, and to what extent a dual-antenna terminal can get access to relevant channel statistics for optimization of the pilot signal. Gain expressions for comparison of different designs are derived. We verify the analytic results based on antennas integrated into Sony-Xperia handsets measured with the Lund University massive MIMO testbed. The measurements are performed at frequencies <6GHz since this part of the spectrum is a candidate for NR standard according to 3GPP.
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| 4. |
- Bengtsson, Erik L, et al.
(author)
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Simulation of Multiple-Antenna Terminal Performance in Massive MIMO Systems based on Indoor Measurements
- 2020
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In: IEEE Transactions on Vehicular Technology. - 0018-9545. ; 69:1, s. 418-427
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Journal article (peer-reviewed)abstract
- In massive MIMO systems the uplink pilot signalstransmitted by a terminal define the channel seen by the basestation. This gives the terminal some degree of freedom selectingan uplink pilot transmission strategy. In this paper, we investigatethe benefit of different pilot transmission strategies when increasingthe number of antennas in the terminal. Building on previouswork on a simulation framework for Multiple-antenna terminalsin 5G massive MIMO systems, this paper presents simulatedperformance results for various transmission schemes. The resultsare calibrated to reflect a real communication situation in a largeauditorium. Emulating the measurement set-up, we show that theframework can be tuned to generate channel distributions thatmatch measured data. Under generalized conditions, we performsimulations for different terminal transmission-strategies, bothrelated to single stream and multiple streams. All evaluations arebased on terminals with four antennas integrated into real SonyXperia smartphone-chassis, tuned to 3.7 GHz. The measurementsare conducted by using the Lund University Massive MIMOtestbed with its 100 antennas. The results clearly show theadvantage of increasing the antenna-count also at the terminalside in massive MIMO systems.
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| 5. |
- Bengtsson, Erik L, et al.
(author)
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Transmission schemes for Multiple Antenna Terminals in Real Massive MIMO systems
- 2016
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In: 2016 IEEE Global Communications Conference (GLOBECOM). - 9781509013289 - 9781509013296
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Conference paper (peer-reviewed)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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| 6. |
- Dahman, Ghassan, et al.
(author)
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On the probability of non-shared multipath clusters in cellular networks
- 2015
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In: IEEE Wireless Communications Letters. - 2162-2345. ; :99
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Journal article (peer-reviewed)abstract
- In Space Division Multiple Access (SDMA) systems, users are separated based on their positions. The base-station (BS) signals typically reach the users through multipath clusters. The goal of this work is to find the probability that a randomly selected user will receive the signal from the BS via at least one nonshared (by any other user) multipath cluster. This probability, referred to as PSuccess, provides (under some conditions) the limit of the probability that the BS is able to communicate successfully with a randomly selected user using SDMA without causing significant interference to other users. In order to find PSuccess, we first derive a model describing the probability distribution function (pdf) of the non-overlapping area of a set of circles, that have the same radius and are spatially distributed according to a Poisson distribution. Then, the proposed model for this pdf is used to express PSuccess as a function of the user’s intensity and the clusters’ intensity. Simulations, using the COST 2100 model, for outdoor and indoor scenarios, are used to validate the derived expression for PSuccess with a very good agreement, where the maximum error was 0:04.
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| 7. |
- Flordelis, Jose, et al.
(author)
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Exploiting antenna correlation in measured massive MIMO channels
- 2016
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In: 2016 IEEE 27th Annual International Symposium on Personal, Indoor, and Mobile Radio Communications, PIMRC 2016. - 9781509032549
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Conference paper (peer-reviewed)abstract
- We investigate antenna correlation of an M-antenna massive multiple-input multiple-output (MIMO) setup with the purpose of obtaining a low-rank representation of the instantaneous massive MIMO channel. Low-rank representation bases using short-term and long-term antenna correlation statistics are defined, and their performance is evaluated with data sets obtained from channel measurements in both indoor and outdoor environments at 2.6 GHz. Our results indicate that the short-term bases can capture a larger amount of the channel energy compared to the long-term ones, but they have a limited timespan, one coherence time or less. On the other hand, the long-term bases are stable over time-spans of a few seconds. Hence, they can be obtained relatively easily. We also investigate a rank-p vector-scalar LMMSE channel estimator that exploits antenna correlation. Our results show that the investigated estimator can achieve a performance similar to that of full-rank LMMSE at a (2p + 1)/M times lower cost. The investigated estimator may be used in conjunction with estimators that exploit correlation in the frequency and time domains or, alternatively, in situations in which these estimators cannot be used, e.g., when pilot separation is larger than the channel coherence bandwidth or time.
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| 8. |
- Flordelis, Jose, et al.
(author)
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Massive MIMO Performance - TDD Versus FDD : What Do Measurements Say?
- 2018
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In: IEEE Transactions on Wireless Communications. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 1536-1276 .- 1558-2248. ; 17:4, s. 2247-2261
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Journal article (peer-reviewed)abstract
- Downlink beamforming in Massive MIMO either relies on uplink pilot measurements--exploiting reciprocity and time-division duplexing (TDD) operation, or on the use of a predetermined grid of beams with user equipments reporting their preferred beams, mostly in frequency-division duplexing (FDD) operation. Massive MIMO in its originally conceived form uses the first strategy, with uplink pilots, whereas there is currently significant commercial interest in the second, grid-of- beams. It has been analytically shown that with isotropic scattering (independent Rayleigh fading) the first approach outperforms the second. Nevertheless, there remains controversy regarding their relative performance in practical channels. In this contribution, the performances of these two strategies are compared using measured channel data at 2.6 GHz.
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| 9. |
- Flordelis, Jose, et al.
(author)
-
Spatial Separation of Closely-Located Users in Measured Massive MIMO Channels
- 2018
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In: IEEE Access. - 2169-3536. ; 6, s. 40253-40266
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Journal article (peer-reviewed)abstract
- We investigate the ability of Massive multiple-input multiple-output (MIMO) systems to spatially separate up to eighteen users located close to one another in line-of-sight (LOS) propagation conditions, in both indoor and outdoor environments. For that, we use fully-synchronous measured channels at 2.6 GHz of single-antenna users moving within a small area and concurrently communicating with a base station (BS) equipped with a compact 128-port array. To quantify the degree of spatial user separability, we use three scalar metrics, namely, the achievable sum-rates, the condition number of the channel matrix, and the angle to interference factor. Our results show that Massive MIMO with zero-forcing (ZF) or regularized ZF (RZF) can spatially separate nine, even eighteen, concurrent users at practical SNR values even in the challenging case of dominant LOS propagation. In particular, signal-to-noise ratio losses relative to ideal (non-interfering and equally strong) channels can be reduced dramatically compared with standard multiuser MIMO systems, which typically have the same number of users as BS antennas. Our findings suggest that with RZF or ZF the ratio of BS antennas to number of served users should be at least three to four, to harvest most of the available spatial gains that the environment can offer. Although orthogonality and array gains complement each other, for the suggested ratios of antennas to users, the main contribution to improving system performance, measured in sum-rates, comes from the orthogonality gain.
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| 10. |
- Flordelis, Jose, et al.
(author)
-
Spatial Separation of Closely-Spaced Users in Measured Massive Multi-User MIMO Channels
- 2015
-
In: 2015 IEEE International Conference on Communications (ICC). ; , s. 1441-1446
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Conference paper (peer-reviewed)abstract
- Fully-synchronous measurements of a massive multi-user multiple-input multiple-output (MU-MIMO) radio propagation channel are presented. We evaluate the ability of a massive MIMO system to spatially separate users located close to each other in line-of-sight (LOS) propagation conditions. The system consists of a base-station (BS) antenna array equipped with 64 dual-polarized antenna elements (128 ports) arranged in a cylindrical configuration, and eight single-antenna users. The users are confined to a five-meter diameter circle and move randomly at pedestrian speeds. The BS antenna array is located on top of a 20 m tall building and has LOS to the users. We examine user separability by studying singular value spread of the MU-MIMO channel matrix for several subsets of BS antenna array ports, along with sum-rate capacity and achievable sum-rates with both zero-forcing and matched-filtering linear precoders. We also analyze the performance of the user with the lowest rate. Finally, a comparison between the performance offered by the massive MIMO system and that of a conventional MU-MIMO system is provided. To the best of our knowledge, this is the first report of fully-synchronous dynamic measurements of a massive MIMO system. Our investigation shows that even users located close to each other in LOS propagation conditions can be spatially separated in a massive MIMO system.
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