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Björnson, Emil, ProfessorORCID iD iconorcid.org/0000-0002-5954-434X
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Publications (10 of 357) Show all publications
Hou, X., Björnson, E., Lee, N. & Heath Jr, R. W. (2025). Guest Editorial: Near-Field MIMO Technologies Toward 6G. IEEE Communications Magazine, 63(1), 20-21
Open this publication in new window or tab >>Guest Editorial: Near-Field MIMO Technologies Toward 6G
2025 (English)In: IEEE Communications Magazine, ISSN 0163-6804, E-ISSN 1558-1896, Vol. 63, no 1, p. 20-21Article in journal, Editorial material (Refereed) Published
Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2025
National Category
Computer and Information Sciences
Identifiers
urn:nbn:se:kth:diva-358750 (URN)10.1109/MCOM.2025.10819471 (DOI)001391605900019 ()
Note

QC 20250121

Available from: 2025-01-21 Created: 2025-01-21 Last updated: 2025-01-21Bibliographically approved
Topal, O. A., Li, Z., Özger, M., Schupke, D., Björnson, E. & Cavdar, C. (2025). Millimeter-Wave Channel Modeling and Coverage Analysis for Indoor Dense Spaces. IEEE Transactions on Vehicular Technology, 74(1), 5-20
Open this publication in new window or tab >>Millimeter-Wave Channel Modeling and Coverage Analysis for Indoor Dense Spaces
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2025 (English)In: IEEE Transactions on Vehicular Technology, ISSN 0018-9545, E-ISSN 1939-9359, Vol. 74, no 1, p. 5-20Article in journal (Refereed) Published
Abstract [en]

Millimeter-wave channel modeling for airplanes, trains, and other in-vehicle environments can be considered jointly as different variations of a general site, namely an indoor dense space (IDS). In this work, by using ray-tracing (RT) simulations, we compare the effect of frame material, user density, and geometry on the channel characteristics at 28, 39, and 60 GHz bands. We observe that temporal and spatial parameters in IDS have unique distributions some depending on the transmitter (TX)-receiver (RX) separation in comparison to the indoor office (IO) channel model. The frame material is the main determining factor of the channel characteristics, while variations in frequency bands and geometries have only a minor impact. We extend our channel modeling effort to MIMO deployment analysis to compare the validity of the proposed model in terms of coverage and spectral efficiency with the IO model. Several dominant angular intervals in the channel cause five times higher spectral efficiency gained by digital beamforming (BF) in comparison to analog BF. We observe that the path loss in IDS is more severe compared with IO, resulting in at least a 50% reduction in the coverage area.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2025
Keywords
Geometry, Millimeter wave communication, Airplanes, Atmospheric modeling, Production facilities, Frequency measurement, Transmitters, Spectral efficiency, Reflection, OFDM, 5G, 6G, in-cabin, intra-wagon, millimeter-wave, channel modeling, ray-tracing (RT)
National Category
Communication Systems
Identifiers
urn:nbn:se:kth:diva-359517 (URN)10.1109/TVT.2024.3463193 (DOI)001396985700030 ()2-s2.0-85205779134 (Scopus ID)
Note

QC 20250205

Available from: 2025-02-05 Created: 2025-02-05 Last updated: 2025-02-05Bibliographically approved
Mahmoudi, A. & Björnson, E. (2024). Adaptive Quantization Resolution and Power Control for Federated Learning over Cell-free Networks. In: : . Paper presented at IEEE Global Communications Conference, 8–12 December 2024, Cape Town, South Africa.
Open this publication in new window or tab >>Adaptive Quantization Resolution and Power Control for Federated Learning over Cell-free Networks
2024 (English)Conference paper, Oral presentation with published abstract (Refereed)
Abstract [en]

Federated learning (FL) is a distributed learning framework where users train a global model by exchanging local model updates with a server instead of raw datasets, preserving data privacy and reducing communication overhead. However, the latency grows with the number of users and the model size, impeding the successful FL over traditional wireless networks with orthogonal access. Cell-free massive multiple-input multipleoutput (CFmMIMO) is a promising solution to serve numerous users on the same time/frequency resource with similar rates. This architecture greatly reduces uplink latency through spatial multiplexing but does not take application characteristics into account. In this paper, we co-optimize the physical layer with the FL application to mitigate the straggler effect. We introduce a novel adaptive mixed-resolution quantization scheme of the local gradient vector updates, where only the most essential entries are given high resolution. Thereafter, we propose a dynamic uplink power control scheme to manage the varying user rates and mitigate the straggler effect. The numerical results demonstrate that the proposed method achieves test accuracy comparable to classic FL while reducing communication overhead by at least 93% on the CIFAR-10, CIFAR-100, and Fashion-MNIST datasets. We compare our methods against AQUILA, Top-q, and LAQ, using the max-sum rate and Dinkelbach power control schemes. Our approach reduces the communication overhead by 75% and achieves 10% higher test accuracy than these benchmarks within a constrained total latency budget. 

National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-358331 (URN)
Conference
IEEE Global Communications Conference, 8–12 December 2024, Cape Town, South Africa
Note

QC 20250115

Available from: 2025-01-15 Created: 2025-01-15 Last updated: 2025-01-15Bibliographically approved
Kennerland, E., Lindoff, B. & Björnson, E. (2024). Analysis of Quantization Noise Suppression Gains in Digital Phased Arrays. In: 2024 IEEE Wireless Communications and Networking Conference, WCNC 2024 - Proceedings: . Paper presented at 25th IEEE Wireless Communications and Networking Conference, WCNC 2024, Dubai, United Arab Emirates, Apr 21 2024 - Apr 24 2024. Institute of Electrical and Electronics Engineers (IEEE)
Open this publication in new window or tab >>Analysis of Quantization Noise Suppression Gains in Digital Phased Arrays
2024 (English)In: 2024 IEEE Wireless Communications and Networking Conference, WCNC 2024 - Proceedings, Institute of Electrical and Electronics Engineers (IEEE) , 2024Conference paper, Published paper (Refereed)
Abstract [en]

Digital phased arrays have often been disregarded for millimeter-wave communications since the analog-to-digital converters (ADCs) are power-hungry. In this paper, we provide a different perspective on this matter by demonstrating analytically and numerically how the ADC resolution can be reduced when using digital phased arrays. We perform a theoretical analysis of the quantization noise characteristics for an OFDM signal received and processed by a digital phased array, using Gaussian approximation of the OFDM signal. In particular, we quantify the quantization noise suppression factor analytically and numerically. This factor describes how much the coherent combining reduces the quantization noise as a function of the number of antennas, which allows for reducing the ADC bit resolution. For instance in a 8-16 antenna digital phased array the ADC resolution can be reduced with 1-2 bits compared to the ADC required for an analog phased array.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2024
Keywords
ADC, Digital Phased Array, Quantization Noise
National Category
Signal Processing Telecommunications
Identifiers
urn:nbn:se:kth:diva-350997 (URN)10.1109/WCNC57260.2024.10570796 (DOI)001268569301128 ()2-s2.0-85198837874 (Scopus ID)
Conference
25th IEEE Wireless Communications and Networking Conference, WCNC 2024, Dubai, United Arab Emirates, Apr 21 2024 - Apr 24 2024
Note

Part of ISBN 9798350303582

QC 20240725

Available from: 2024-07-24 Created: 2024-07-24 Last updated: 2024-10-03Bibliographically approved
Khorsandmanesh, Y., Björnson, E., Jaldén, J. & Lindoff, B. (2024). Beam Coherence Time Analysis for Mobile Wideband mmWave Point-to-Point MIMO Channels. IEEE Wireless Communications Letters, 13(6), 1546-1550
Open this publication in new window or tab >>Beam Coherence Time Analysis for Mobile Wideband mmWave Point-to-Point MIMO Channels
2024 (English)In: IEEE Wireless Communications Letters, ISSN 2162-2337, E-ISSN 2162-2345, Vol. 13, no 6, p. 1546-1550Article in journal (Refereed) Published
Abstract [en]

Multi-Gbps data rates are achievable in millimeter-wave (mmWave) bands, but a prominent issue is the tiny wavelength that results in rapid fading variations and significant pilot signaling for channel estimation. In this letter, we recognize that the angles of scattering clusters seen from the UE vary slowly compared to the small-scale fading. We characterize the beam coherence time, which quantifies how frequently the UE must update its downlink receive combining matrix. The exact beam coherence time is derived in the single-cluster case, and an achievable lower bound is proposed for the multi-cluster case. These values are determined so that at least half of the received signal gain is maintained in between the combining updates. We demonstrate how the beam coherence time can be hundreds of times larger than the channel coherence time of the small-scale fading.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2024
Keywords
Coherence time, Millimeter wave communication, Fading channels, Scattering, OFDM, MIMO communication, Array signal processing, Millimeter wave (mmWave), beam coherence time, half-power beamwidth, user mobility
National Category
Telecommunications
Identifiers
urn:nbn:se:kth:diva-349619 (URN)10.1109/LWC.2024.3381434 (DOI)001246583100047 ()2-s2.0-85189303602 (Scopus ID)
Note

QC 20240702

Available from: 2024-07-02 Created: 2024-07-02 Last updated: 2024-07-02Bibliographically approved
Chen, Y., Han, C. & Björnson, E. (2024). Can Far-Field Beam Training Be Deployed for Cross-Field Beam Alignment in Terahertz UM-MIMO Communications?. IEEE Transactions on Wireless Communications, 23(10), 14972-14987
Open this publication in new window or tab >>Can Far-Field Beam Training Be Deployed for Cross-Field Beam Alignment in Terahertz UM-MIMO Communications?
2024 (English)In: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, Vol. 23, no 10, p. 14972-14987Article in journal (Refereed) Published
Abstract [en]

Ultra-massive multiple-input multiple-output (UM-MIMO) is the enabler of Terahertz (THz) communications in next-generation wireless networks. In THz UM-MIMO systems, a new paradigm of cross-field communications spanning from near-field to far-field is emerging, since the near-field range expands with higher frequencies and larger array apertures. Precise beam alignment in cross-field is critical but challenging. Specifically, unlike far-field beams that rely only on the angle domain, the incorporation of dual-domain (angle and distance) training significantly increases overhead. A natural question arises of whether far-field beam training can be deployed for cross-field beam alignment. In this paper, this question is answered, by demonstrating that the far-field training enables sufficient signal-to-noise ratio (SNR) in both far- and near-field scenarios, while exciting all channel dimensions. Based on that, we propose a subarray-coordinated hierarchical (SCH) training with greatly reduced overhead. To further obtain high-precision beam designs, we propose a two-phase angle and distance beam estimator (TPBE). Extensive simulations demonstrate the effectiveness of the proposed methods. Compared to near-field exhaustive search, the SCH possesses 0.2% training overhead. The TPBE achieves 0.16 degrees and 0.01 meters estimation root-mean-squared errors for angle and distance. Furthermore, with the estimated beam directions, a near-optimal SNR with 0.3 dB deviation is attained after beam alignment.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2024
Keywords
Training, Estimation, Terahertz communications, Signal to noise ratio, Channel models, Wireless communication, Signal processing algorithms, Terahertz (THz) communications, near-field, ultra-massive multiple-input multiple-output (UM-MIMO) systems, cross far- and near-field
National Category
Communication Systems
Identifiers
urn:nbn:se:kth:diva-356078 (URN)10.1109/TWC.2024.3422257 (DOI)001338574900160 ()2-s2.0-85198388562 (Scopus ID)
Note

QC 20241108

Available from: 2024-11-08 Created: 2024-11-08 Last updated: 2024-11-08Bibliographically approved
Gurgunoglu, D., Björnson, E. & Fodor, G. (2024). Combating Inter-Operator Pilot Contamination in Reconfigurable Intelligent Surfaces Assisted Multi-Operator Networks. IEEE Transactions on Communications, 72(9), 5884-5895
Open this publication in new window or tab >>Combating Inter-Operator Pilot Contamination in Reconfigurable Intelligent Surfaces Assisted Multi-Operator Networks
2024 (English)In: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, Vol. 72, no 9, p. 5884-5895Article in journal (Refereed) Published
Abstract [en]

In this paper, we study a new kind of pilot contamination appearing in multi-operator reconfigurable intelligent surfaces (RIS) assisted networks, where multiple operators provide services to their respective served users. The operators use dedicated frequency bands, but each RIS inadvertently reflects the transmitted uplink signals of the user equipment devices in multiple bands. Consequently, the concurrent reflection of pilot signals during the channel estimation phase introduces a new inter-operator pilot contamination effect. We investigate the implications of this effect in systems with either deterministic or correlated Rayleigh fading channels, specifically focusing on its impact on channel estimation quality, signal equalization, and channel capacity. The numerical results demonstrate the substantial degradation in system performance caused by this phenomenon and highlight the pressing need to address inter-operator pilot contamination in multi-operator RIS deployments. To combat the negative effect of this new type of pilot contamination, we propose to use orthogonal RIS configurations during uplink pilot transmission, which can mitigate or eliminate the negative effect of inter-operator pilot contamination at the expense of some inter-operator information exchange and orchestration.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2024
Keywords
Reconfigurable intelligent surface, channel estimation, pilot contamination.
National Category
Communication Systems
Identifiers
urn:nbn:se:kth:diva-345652 (URN)10.1109/TCOMM.2024.3390095 (DOI)001319557300012 ()2-s2.0-85190752815 (Scopus ID)
Note

QC 20241008

Available from: 2024-04-16 Created: 2024-04-16 Last updated: 2025-01-27Bibliographically approved
Kama, E. B., Kim, J. & Björnson, E. (2024). Downlink Pilots are Essential for Cell-Free Massive MIMO with Multi-Antenna Users. In: 2024 IEEE Wireless Communications and Networking Conference, WCNC 2024 - Proceedings: . Paper presented at 25th IEEE Wireless Communications and Networking Conference, WCNC 2024, Dubai, United Arab Emirates, Apr 21 2024 - Apr 24 2024. Institute of Electrical and Electronics Engineers (IEEE)
Open this publication in new window or tab >>Downlink Pilots are Essential for Cell-Free Massive MIMO with Multi-Antenna Users
2024 (English)In: 2024 IEEE Wireless Communications and Networking Conference, WCNC 2024 - Proceedings, Institute of Electrical and Electronics Engineers (IEEE) , 2024Conference paper, Published paper (Refereed)
Abstract [en]

We consider a cell-free massive MIMO system with multiple antennas on the users and access points. In previous works, the downlink spectral efficiency (SE) has been evaluated using the hardening bound that requires no downlink pilots. This approach works well when having single-antenna users. In this paper, we show that much higher SEs can be achieved if downlink pilots are sent since the effective channel matrix does not harden when having multi-antenna users. We propose a pilot-based downlink estimation scheme and derive a new SE expression that utilizes zero-forcing combining. We show numerically how the number of users and user antennas affects the SE.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2024
Keywords
Cell-free massive MIMO, downlink pilots, multi-antenna users, spectral efficiency
National Category
Telecommunications Signal Processing
Identifiers
urn:nbn:se:kth:diva-350998 (URN)10.1109/WCNC57260.2024.10570902 (DOI)001268569302063 ()2-s2.0-85198827886 (Scopus ID)
Conference
25th IEEE Wireless Communications and Networking Conference, WCNC 2024, Dubai, United Arab Emirates, Apr 21 2024 - Apr 24 2024
Note

Part of ISBN 9798350303582

QC 20240725

Available from: 2024-07-24 Created: 2024-07-24 Last updated: 2024-10-07Bibliographically approved
Tavana, M., Chen, Z. & Björnson, E. (2024). Dynamic Queue-Aware RF Charging of Zero-Energy Devices via Reconfigurable Surfaces. IEEE Wireless Communications Letters, 13(9), 2551-2555
Open this publication in new window or tab >>Dynamic Queue-Aware RF Charging of Zero-Energy Devices via Reconfigurable Surfaces
2024 (English)In: IEEE Wireless Communications Letters, ISSN 2162-2337, E-ISSN 2162-2345, Vol. 13, no 9, p. 2551-2555Article in journal (Refereed) Published
Abstract [en]

Reconfigurable intelligent surfaces (RIS) can enhance wireless power transfer efficiency in communication networks by steering electromagnetic waves from a transmitter (TX) toward zero-energy devices (ZEDs) for charging their batteries. In this letter, we use the Lyapunov optimization framework to develop an algorithm that dynamically adjusts the TX power and RIS phase configuration based on the data queue lengths at the ZEDs. This approach aims to maintain queue stability while minimizing the average TX power. Our simulation results demonstrate that the proposed method provides stability and reduces the average TX power, whereas the queue-agnostic benchmark fails to achieve that even with much higher TX power.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2024
Keywords
Reconfigurable intelligent surfaces, Radio frequency, Optimization, Batteries, Heuristic algorithms, Antenna arrays, Vectors, Energy harvesting, Lyapunov optimization, zero-energy devices
National Category
Telecommunications
Identifiers
urn:nbn:se:kth:diva-354331 (URN)10.1109/LWC.2024.3426095 (DOI)001312951400044 ()2-s2.0-85198301402 (Scopus ID)
Note

QC 20241004

Available from: 2024-10-04 Created: 2024-10-04 Last updated: 2024-10-04Bibliographically approved
Tavana, M., Masoudi, M. & Björnson, E. (2024). Dynamic RF Charging of Zero-Energy Devices via Reconfigurable Intelligent Surfaces. IEEE Wireless Communications Letters, 13(8), 2295-2299
Open this publication in new window or tab >>Dynamic RF Charging of Zero-Energy Devices via Reconfigurable Intelligent Surfaces
2024 (English)In: IEEE Wireless Communications Letters, ISSN 2162-2337, E-ISSN 2162-2345, Vol. 13, no 8, p. 2295-2299Article in journal (Refereed) Published
Abstract [en]

Communication networks can achieve self-sustainability with the help of reconfigurable intelligent surfaces (RIS) that direct electromagnetic waves toward zero-energy devices (ZEDs) for energy harvesting. The harvested energy charges the ZEDs' batteries, reduces their dependency on external energy sources and enables energy neutrality. In this letter, we propose a dynamic algorithm that adjusts the joint RIS and TX phases based on the battery levels to maximize a weighted-sum power metric favoring ZEDs with lower battery levels. The simulation results show that threshold-based weighting slightly underperforms compared to the weighting with optimal parameters while requiring less data exchange with the network entity manager.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2024
Keywords
Batteries, Reconfigurable intelligent surfaces, Vectors, Array signal processing, Optimization, Radio frequency, Heuristic algorithms, Analog beamforming, energy harvesting, phased array, reconfigurable intelligent surface, zero-energy devices
National Category
Communication Systems
Identifiers
urn:nbn:se:kth:diva-352530 (URN)10.1109/LWC.2024.3413629 (DOI)001288996600020 ()2-s2.0-85196071752 (Scopus ID)
Note

QC 20240903

Available from: 2024-09-03 Created: 2024-09-03 Last updated: 2024-09-03Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0002-5954-434X

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