| 1. |
- Alegria, Juan Vidal, et al.
(författare)
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Achievable Rate with Correlated Hardware Impairments in Large Intelligent Surfaces
- 2019
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Ingår i: 2019 IEEE 8th International Workshop on Computational Advances in Multi-Sensor Adaptive Processing, CAMSAP 2019 - Proceedings. - 9781728155494 ; , s. 559-563
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Konferensbidrag (refereegranskat)abstract
- Large intelligent surface (LIS) is a new technology yet to be developed. However, this technology is likely to be implemented using cheap components so as to facilitate its deployment. Therefore, we can expect a performance degradation due to hardware impairments. In this paper, we present a model for these hardware impairments in a receiving LIS and we analyze their effect on the achievable rate using a simplified receiver structure. We study the effect of the correlation structure of these impairments by modeling this correlation as a function of the distance between the considered points of the LIS. We present numerical results, as well as closed form formulas, in order to get a general intuition on the problem.
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| 2. |
- Alegría, Juan Vidal, et al.
(författare)
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Cell-Free Massive MIMO: Exploiting The Wax Decomposition
- 2022
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Ingår i: 2022 IEEE International Conference on Acoustics, Speech, and Signal Processing, ICASSP 2022 - Proceedings. - 1520-6149. - 9781665405409 ; 2022-May, s. 5393-5397
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Konferensbidrag (refereegranskat)abstract
- Cell-free massive multiple-input multiple-output (MIMO) consists of a large set of distributed access points (APs) serving a number of users. The APs can be far from each other, and they can also have a big number of antennas. Thus, decentralized architectures have to be considered so as to reduce the interconnection bandwidth to a central processing unit (CPU) and make the system scalable. On the other hand, the APs in a heterogeneous network might have limited processing capabilities and fully-decentralized processing may not be available. In a recent paper, a trade-off between level of decentralization and decentralized processing complexity has been identified. Furthermore, a novel matrix decomposition - the WAX decomposition - which, if applicable to the channel matrix, allows for exploitation of said trade-off without loss of information. The results on WAX decomposition are only available for random channel matrices without specific structures, while in a cell-free massive MIMO scenario the channel can have sparse structures. In this work, we study the applicability of WAX decomposition to cell-free massive MIMO with its implications to the above-mentioned trade-off.
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| 3. |
- Alegria, Juan Vidal, et al.
(författare)
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Channel Orthogonalization with Reconfigurable Surfaces
- 2022
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Ingår i: 2022 IEEE GLOBECOM Workshops, GC Wkshps 2022 - Proceedings. - 9781665459754 ; , s. 37-42
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Konferensbidrag (refereegranskat)abstract
- Orthogonal multi-user multiple-input multiple-output (MU-MIMO) channels allow for optimum performance with simplified precoding/equalization, and they achieve maximum multiplexing gain which is shared fairly among users. Reconfigurable intelligent surface (RIS) constitutes a promising cost-efficient solution to improve the wireless channel, since they consist of passive reflecting elements able to adjust the phases of the incoming waves. However, it is still widely unclear how these surfaces can improve spatial-multiplexing. In fact, the common RIS model cannot achieve perfect orthogonalization of MU-MIMO channels with a reasonable number of elements. Furthermore, efficient channel estimation algorithms for RIS, which are key for taking advantage of its benefits, are still a matter of research. We study two types of reconfigurable surfaces (RSs), namely amplitude-reconfigurable intelligent surface (ARIS) and fully-reconfigurable intelligent surface (FRIS), with extended capabilities over RIS. We show how these RSs allow for perfect channel orthogonalization, and, by minimizing the applied power, we show that they can potentially be implemented without the need of amplification. We also present an efficient channel estimation method for each of them that allows the base station (BS) to select the desired propagation channel.
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| 4. |
- Alegria, Juan Vidal, et al.
(författare)
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Cramér-Rao Lower Bounds for Positioning with Large Intelligent Surfaces using Quantized Amplitude and Phase
- 2020
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Ingår i: 2019 53rd Asilomar Conference on Signals, Systems, and Computers. - 9781728143002 - 9781728143019 ; , s. 10-14
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Konferensbidrag (refereegranskat)abstract
- We envision the use of large intelligent surface (LIS) technology, which is a promising concept that goes beyond massive multiple-input multiple-output (MIMO), for positioning applications due to its ability to focus energy in the 3D space. The Cramér-Rao lower bounds (CBLBs) for positioning using a LIS which can resolve amplitude and phase with full resolution have already been determined in the previous literature. However, in real applications, and specially if we consider cheap hardware components to enable the deployment of LIS at reasonable costs, the phase and amplitude have to be quantized before any information can be extracted from them. Furthermore, the phase information is more difficult to resolve due to phase noise, non-coherence, etc. In this paper we compute the CRLBs for positioning using LIS with quantized phase and amplitude. We also derive analytical bounds for the CRLB for positioning with LIS when all phase information is disregarded and amplitude is measured with full resolution. We present numerical results in the form of tables including the CRLB loss due to the different quantization resolutions, which can serve as a design guideline for hardware developers.
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| 5. |
- Alegria, Juan Vidal, et al.
(författare)
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Decentralized equalizer construction for large intelligent surfaces
- 2019
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Ingår i: 2019 IEEE 90th Vehicular Technology Conference, VTC 2019 Fall - Proceedings. - 1550-2252. - 9781728112206 ; 2019-September
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Konferensbidrag (refereegranskat)abstract
- In this paper we present fully decentralized methods for calculating an approximate zero- forzing (ZF) equalizer in a large intelligent surface (LIS). A LIS is intended for wireless communication and facilitates unprecedented MU- MIMO performance, far superior to that of Massive MIMO. Antenna modules in the grid connect to their neighbors to exchange messages of information needed for interference cancellation in a fully- decentralized fashion, making the system scalable. By a careful design of how the messages are routed, we show that the proposed method is able to cancel inter-user interference sufficiently well without any centralized coordination, opening the door for the realization of this type of structures.
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| 6. |
- Alegria, Juan Vidal, et al.
(författare)
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Enabling Decentralized Computation of the WAX Decomposition
- 2022
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Ingår i: ICC 2022 - IEEE International Conference on Communications. - 1550-3607. ; 2022
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Konferensbidrag (refereegranskat)abstract
- As the number of antennas scales up, research efforts have been directed towards finding decentralized receiver solutions for multi-antenna architectures. In a recent paper, a novel matrix decomposition - the WAX decomposition - was proposed, which has direct bearing on characterizing the level of decentralization that is possible if the complexity of the decentralized processing is limited. However, the available work on WAX decomposition has left some gaps in determining how one of its basis matrices has to be defined. In this paper we present a number of constructions for effectively defining said basis matrix, and we show how they facilitate decentralized calculation of the WAX decomposition itself for applying information-lossless processing in architectures with an arbitrary level of decentralization. Thereby, taking a major step towards realizing decentralized receiver solution for large scale antenna systems.
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| 7. |
- Alegría, Juan Vidal, et al.
(författare)
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Impact of Quantization in Decentralized Processing for Large Multi-Antenna Architectures
- 2022
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Ingår i: 2022 56th Asilomar Conference on Signals, Systems, and Computers. - 9781665459068 ; , s. 1351-1356
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Konferensbidrag (refereegranskat)abstract
- The demand for an increased number of antennas at base stations is driving research on decentralized processing schemes aimed at reducing the information volume that has to be transferred to, and processed at, a central processing unit (CPU). Some of these schemes can reduce the dimensions of the data while achieving information-lossless processing with respect to centralized architectures. However, little is known about the impact of quantization in these decentralized schemes. Moreover, it is unclear if an information-lossless reduction of dimensions directly corresponds to a reduction in the bit-rate that has to be transmitted to the CPU after quantization. This paper studies how quantization affects the performance of decentralized processing. Bit rates after quantization of a received vector (in a centralized scheme) are contrasted with bit rates after quantization of post-processed vectors using various information-lossless dimension reductions that can potentially be applied in decentralized schemes.
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| 8. |
- Alegría, Juan Vidal, et al.
(författare)
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Increased Multiplexing Gain with Reconfigurable Surfaces : Simultaneous Channel Orthogonalization and Information Embedding
- 2023
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Ingår i: GLOBECOM 2023 - 2023 IEEE Global Communications Conference. - 9798350310900 ; , s. 5714-5719
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Konferensbidrag (refereegranskat)abstract
- Reconfigurable surface (RS) has been shown to be an effective solution for improving wireless communication links in general multi-user multiple-input multiple-output (MU-MIMO) setting. Current research efforts have been largely directed towards the study of reconfigurable intelligent surface (RIS), which corresponds to an RS made of passive reconfigurable elements with only phase shifting capabilities. RIS constitutes a cost- and energy- efficient solution for increased beamforming gain since it allows to generate constructive interference towards desired directions, e.g., towards a base station (BS). However, in many situations, multiplexing gain may have greater impact on the achievable transmission rates and number of simultaneously connected devices, while RIS has only been able to achieve minor improvements in this aspect. Recent work has proposed the use of alternative RS technologies, namely amplitude-reconfigurable intelligent surface (ARIS) and fully-reconfigurable intelligent surface (FRIS), to achieve perfect orthogonalization of MU-MIMO channels, thus allowing for maximum multiplexing gain at reduced complexity. In this work we consider the use of ARIS and FRIS for simultaneously orthogonalizing a MU-MIMO channel, while embedding extra information in the orthogonalized channel. We show that the resulting achievable rates allow for full exploitation of the degrees of freedom in a MU-MIMO system with excess of BS antennas.
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| 9. |
- Alegría, Juan Vidal, et al.
(författare)
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Modular binary tree architecture for distributed large intelligent surface
- 2021
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Ingår i: International Conference on Acoustics, Speech, and Signal Processing (ICASSP). - 1520-6149. - 9781728176062 - 9781728176055 ; 2021-June, s. 4865-4869
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Konferensbidrag (refereegranskat)abstract
- Large intelligent surface (LIS) is a technology that extends massive MIMO by considering an even greater number of antennas distributed throughout vast areas. In order to be able to implement this technology, it is crucial to consider decentralized architectures so as to make the whole system scalable. We consider a LIS divided into several LIS panels of smaller size, which can be located far away from each other. We present a modular architecture that allows combining different LIS panels using a binary tree. This architecture is also valid in a cell-free massive MIMO scenario. We make use of a newly defined matrix decomposition, the WAX decomposition, to define the modules that are used within our architecture. We also study the lossless dimensionality reduction in the data to be processed, which can be achieved using our proposed architecture.
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| 10. |
- Alegria, Juan Vidal, et al.
(författare)
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Trade-Offs in Decentralized Multi-Antenna Architectures: The WAX Decomposition
- 2021
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Ingår i: IEEE Transactions on Signal Processing. - 1053-587X. ; 69, s. 3627-3641
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Tidskriftsartikel (refereegranskat)abstract
- Current research on multi-antenna architectures is trending towards increasing the amount of antennas in the base stations (BSs) so as to increase the spectral efficiency. As a result, the interconnection bandwidth and computational complexity required to process the data using centralized architectures is becoming prohibitively high. Decentralized architectures can reduce these requirements by pre-processing the data before it arrives at a central processing unit (CPU). However, performing decentralized processing introduces also cost in complexity/interconnection bandwidth at the antenna end which is in general being ignored. This paper aims at studying the interplay between level of decentralization and the associated complexity/interconnection bandwidth requirement at the antenna end. To do so, we propose a general framework for centralized/decentralized architectures that can explore said interplay by adjusting some system parameters, namely the number of connections to the CPU (level of decentralization), and the number of multiplications/outputs per antenna (complexity/interconnection bandwidth). We define a novel matrix decomposition, the WAX decomposition, that allows information-lossless processing within our proposed framework, and we use it to obtain the operational limits of the interplay under study. We also look into some of the limitations of the WAX decomposition.
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