| 11. |
- 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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| 12. |
- Flordelis, Jose, et al.
(författare)
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Spatial Separation of Closely-Located Users in Measured Massive MIMO Channels
- 2018
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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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| 13. |
- Flordelis, Jose, et al.
(författare)
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Spatial Separation of Closely-Spaced Users in Measured Massive Multi-User MIMO Channels
- 2015
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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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| 14. |
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| 15. |
- Gao, Xiang, et al.
(författare)
-
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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| 16. |
- Gao, Xiang, et al.
(författare)
-
Extension of the COST 2100 channel model for massive MIMO
- 2015
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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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| 17. |
- 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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| 18. |
- Gao, Xiang, et al.
(författare)
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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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| 19. |
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| 20. |
- Gao, Xiang, et al.
(författare)
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Massive MIMO performance evaluation based on measured propagation data
- 2015
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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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