| 1. |
- Bengtsson, Erik L, et al.
(författare)
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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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Ingår i: IEEE Wireless Communications Letters. - 2162-2337. ; , s. 1-6
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Tidskriftsartikel (refereegranskat)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.
(författare)
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A Simulation Framework for Multiple-Antenna Terminals in 5G Massive MIMO Systems
- 2017
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Ingår i: IEEE Access. - 2169-3536. ; 5, s. 26819-26831
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Analysis of Transmission Schemes for Dual-Antenna Terminals in Massive MIMO Systems
- 2018
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Ingår i: 2018 25th International Conference on Telecommunications, ICT 2018. - 9781538623213 ; , s. 76-82
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Konferensbidrag (refereegranskat)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.
(författare)
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Simulation of Multiple-Antenna Terminal Performance in Massive MIMO Systems based on Indoor Measurements
- 2020
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Ingår i: IEEE Transactions on Vehicular Technology. - 0018-9545. ; 69:1, s. 418-427
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Tidskriftsartikel (refereegranskat)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.
(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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| 6. |
- Dahman, Ghassan, et al.
(författare)
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On the probability of non-shared multipath clusters in cellular networks
- 2015
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Ingår i: IEEE Wireless Communications Letters. - 2162-2345. ; :99
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Exploiting antenna correlation in measured massive MIMO channels
- 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
- 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.
(författare)
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Massive MIMO Performance - TDD Versus FDD : What Do Measurements Say?
- 2018
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Ingår i: IEEE Transactions on Wireless Communications. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 1536-1276 .- 1558-2248. ; 17:4, s. 2247-2261
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Tidskriftsartikel (refereegranskat)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.
(författare)
-
Spatial Separation of Closely-Located Users in Measured Massive MIMO Channels
- 2018
-
Ingår i: IEEE Access. - 2169-3536. ; 6, s. 40253-40266
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Tidskriftsartikel (refereegranskat)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.
(författare)
-
Spatial Separation of Closely-Spaced Users in Measured Massive Multi-User MIMO Channels
- 2015
-
Ingår i: 2015 IEEE International Conference on Communications (ICC). ; , s. 1441-1446
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Konferensbidrag (refereegranskat)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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| 11. |
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| 12. |
- Gao, Xiang, et al.
(författare)
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Channel behavior for very-large MIMO systems - initial characterization
- 2012
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Wireless communication using very-large multiple-input multiple-output (MIMO) antennas is a new research field, where base stations are equipped with a very large number of antennas as compared to previously considered systems. Unlike the conventional small and compact antenna arrays, the very-large arrays may span many wavelengths in space, thus they can experience large-scale fading over the array. This power variation over the antenna array may be critical to algorithm design and performance evaluations for very-large MIMO systems, and it is thus important that it is included in channel models. Based on channel measurements using a 128-antenna linear array in a semi-urban area, we characterize and model the large-scale fading properties. We focus on cluster-based modeling which can be seen as an extension of the current COST 2100 channel model. In the paper, we report our latest channel behavior modeling progress for very-large MIMO systems.
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| 13. |
- Gao, Xiang, et al.
(författare)
-
Extension of the COST 2100 channel model for massive MIMO
- 2015
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Massive MIMO has been shown, both in theory and through experiments, to offer very promising properties. These include the possibility to decrease output power by at least an order of magnitude while still achieving large gains in spectral efficiency, as compared to today’s access schemes. To efficiently design communication algorithms and evaluate massive MIMO schemes, channel models that capture important massive MIMO channel characteristics are needed. We pursue this by extending a well-known cluster-based MIMO channel model - the COST 2100 model. In the paper, we suggest a model extension for massive MIMO, and we discuss main concepts, parameters and implementation issues. The modeling work is based on measurement data from a measurement campaign in the 2.6 GHz frequency range using a physically-large array with 128 elements.
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| 14. |
- Gao, Xiang, et al.
(författare)
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Large antenna array and propagation environment interaction
- 2014
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Ingår i: Proc. 48th Asilomar Conference on Signals, Systems and Computers, ACSSC 2015. - 9781479982974 ; , s. 666-670
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Konferensbidrag (refereegranskat)abstract
- In conventional MIMO, propagation conditions are often considered wide-sense stationary over the entire antenna array. In massive MIMO systems, where arrays can span over large physical dimensions, the situation is quite different. For instance, significant variations in signal strength, due to shadowing, can be experienced across a large array. These effects vary with propagation environment in which the array is placed, and influence achievable sum-rates. We characterize these variations for several measured propagation scenarios in the 2.6 GHz frequency range and illustrate how power variations and correlation properties change along the array.
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| 15. |
- Gao, Xiang, et al.
(författare)
-
Linear pre-coding performance in measured very-large MIMO channels
- 2011
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Ingår i: Proc. of the 74th IEEE Vehicular Technology Conference.
-
Konferensbidrag (refereegranskat)abstract
- Wireless communication using very-large multiple-input multiple-output (MIMO) antennas is a new research field, where base stations are equipped with a very large number of antennas as compared to previously considered systems. In theory, as the number of antennas increases, propagation properties that were random before start to become deterministic. Theoretical investigations with independent identically distributed (i.i.d.)\ complex Gaussian (Rayleigh fading) channels and unlimited number of antennas have been done, but in practice we need to know what benefits we can get from very large, but limited, number of antenna elements in realistic propagation environments. In this study we evaluate properties of measured residential-area channels, where the base station is equipped with 128 antenna ports. An important property to consider is the orthogonality between channels to different users, since this property tells us how advanced multi-user MIMO (MU-MIMO) pre-coding schemes we need in the downlink. We show that orthogonality improves with increasing number of antennas, but for two single-antenna users there is very little improvement beyond 20 antennas. We also evaluate sum-rate performance for two linear pre-coding schemes, zero-forcing (ZF) and minimum mean squared-error (MMSE), as a function of the number of base station antennas. Already at 20 base station antennas these linear pre-coding schemes reach 98\% of the optimal dirty-paper coding (DPC) capacity for the measured channels.
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| 16. |
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| 17. |
- Gao, Xiang, et al.
(författare)
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Massive MIMO performance evaluation based on measured propagation data
- 2015
-
Ingår i: IEEE Transactions on Wireless Communications. - 1536-1276. ; 14:7, s. 3899-3911
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Tidskriftsartikel (refereegranskat)abstract
- Massive MIMO, also known as very-large MIMO or large-scale antenna systems, is a new technique that potentially can offer large network capacities in multi-user scenarios. With a massive MIMO system, we consider the case where a base station equipped with a large number of antenna elements simultaneously serves multiple single-antenna users in the same time-frequency resource. So far, investigations are mostly based on theoretical channels with independent and identically distributed (i.i.d.) complex Gaussian coefficients, i.e., i.i.d. Rayleigh channels. Here, we investigate how massive MIMO performs in channels measured in real propagation environments. Channel measurements were performed at 2.6 GHz using a virtual uniform linear array (ULA) which has a physically large aperture, and a practical uniform cylindrical array (UCA) which is more compact in size, both having 128 antenna ports. Based on measurement data, we illustrate channel behavior of massive MIMO in three representative propagation conditions, and evaluate the corresponding performance. The investigation shows that the measured channels, for both array types, allow us to achieve performance close to that in i.i.d. Rayleigh channels. It is concluded that in real propagation environments we have characteristics that can allow for efficient use of massive MIMO, i.e., the theoretical advantages of this new technology can also be harvested in real channels.
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| 18. |
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| 19. |
- Imtiaz, Sahar, et al.
(författare)
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On the Directional Reciprocity of Uplink and Downlink Channels in Frequency Division Duplex Systems
- 2014
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Ingår i: Proc. 2014 25th IEEE Annual International Symposium on Personal, Indoor, and Mobile Radio Communication, IEEE PIMRC 2014. - 9781479949120 ; , s. 172-176
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Konferensbidrag (refereegranskat)abstract
- The possibility of using the channel reciprocity between the uplink and downlink channels in Frequency Division Duplex (FDD) systems to improve the efficiency has been deeply investigated. Previous studies have come to different conclusions for the characterization of the dissimilarity in uplink and downlink channel properties. This paper analyzes the mismatch in directional properties of the uplink and downlink channels of FDD systems based on the power of their multipath clusters. At a system level, due to the limited directional resolution, the multipath components arriving at the base station are seen as clusters, rather than individual signal paths. This fact is used to describe the mismatch of directional properties between uplink and downlink. The contribution of this paper is the use of a spectral dissimilarity metric as a measure to characterize this mismatch; a detailed study of this dissimilarity metric is also presented. It is found that under favorable propagation conditions, for both actual channel measurement data and ray-tracing simulations, the directional and power properties of the downlink multipath clusters can be estimated from the uplink channel with high reliability. Therefore, directional-based beamforming transmission techniques for FDD systems will be able to benefit from such similarity in order to improve the system performance.
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| 20. |
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| 21. |
- Rusek, Fredrik, et al.
(författare)
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Scaling up MIMO : Opportunities and Challenges with Very Large Arrays
- 2013
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Ingår i: IEEE signal processing magazine (Print). - : IEEE. - 1053-5888 .- 1558-0792. ; 30:1, s. 40-60
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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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| 22. |
- Sanchez, Juan, et al.
(författare)
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Optimal, Low-Complexity Beamforming for Discrete Phase Reconfigurable Intelligent Surfaces
- 2021
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Ingår i: 2021 IEEE Global Communications Conference, GLOBECOM 2021 - Proceedings. - 9781728181042
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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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| 23. |
- 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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| 24. |
- Vieira, Joao, et al.
(författare)
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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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| 25. |
- 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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