Pub Date : 2025-10-07DOI: 10.1109/JSAC.2025.3618797
Shupei Zhang;Yuze Zhang;Hiroaki Hashida;Yonina C. Eldar;Marco Di Renzo;Boya Di
Fluid antenna systems (FASs) have emerged as a promising antenna technology for 6G networks by tapping into new degrees of freedom (DoF) in antenna positions to harness multiplexing gains. In this paper, we propose an implementation of FASs enabled by reconfigurable holographic surfaces (RHSs) and construct a 384-element prototype with amplitude-modulation capabilities, RHSs can adjust the antenna positions by activating different subsets of elements. Due to unknown user locations, multiple antenna ports, and varying channel environments, however, the design of beamforming schemes relying on acquiring accurate channel state information (CSI) suffers high complexity. To avoid CSI acquisition, we design a low-overhead fluid beam training scheme for RHS-enabled FAS. Unlike fixed-position antennas, fluid beam training employs different antenna positions via element activation, namely sliding windows, for each codeword to improve the channel quality, thereby enhancing the received signal strength (RSS). Such an element activation method is also applied to reprogram the effective array aperture, hence the fluid beam training adopts a hierarchical structure where the beamwidth of codewords narrows across layers. Experimental and simulation results verify the variation of RSS with sliding windows. Compared to traditional schemes, the proposed fluid beam training utilizing sliding windows achieves higher training accuracy and data rates.
{"title":"Fluid Antenna Systems Enabled by Reconfigurable Holographic Surfaces: Beamforming Design and Experimental Validation","authors":"Shupei Zhang;Yuze Zhang;Hiroaki Hashida;Yonina C. Eldar;Marco Di Renzo;Boya Di","doi":"10.1109/JSAC.2025.3618797","DOIUrl":"10.1109/JSAC.2025.3618797","url":null,"abstract":"Fluid antenna systems (FASs) have emerged as a promising antenna technology for 6G networks by tapping into new degrees of freedom (DoF) in antenna positions to harness multiplexing gains. In this paper, we propose an implementation of FASs enabled by reconfigurable holographic surfaces (RHSs) and construct a 384-element prototype with amplitude-modulation capabilities, RHSs can adjust the antenna positions by activating different subsets of elements. Due to unknown user locations, multiple antenna ports, and varying channel environments, however, the design of beamforming schemes relying on acquiring accurate channel state information (CSI) suffers high complexity. To avoid CSI acquisition, we design a low-overhead fluid beam training scheme for RHS-enabled FAS. Unlike fixed-position antennas, fluid beam training employs different antenna positions via element activation, namely sliding windows, for each codeword to improve the channel quality, thereby enhancing the received signal strength (RSS). Such an element activation method is also applied to reprogram the effective array aperture, hence the fluid beam training adopts a hierarchical structure where the beamwidth of codewords narrows across layers. Experimental and simulation results verify the variation of RSS with sliding windows. Compared to traditional schemes, the proposed fluid beam training utilizing sliding windows achieves higher training accuracy and data rates.","PeriodicalId":73294,"journal":{"name":"IEEE journal on selected areas in communications : a publication of the IEEE Communications Society","volume":"44 ","pages":"1417-1431"},"PeriodicalIF":17.2,"publicationDate":"2025-10-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145241555","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-10-06DOI: 10.1109/JSAC.2025.3617473
Javier Otero Martinez;Borja Genoves Guzman;Ana Garcia Armada
Emerging technologies like Fluid Antenna Systems (FAS) are important ingredients toward new generation communications. In particular, we focus on systems based on the reconfigurability capabilities of liquid antennas, which are a subset of FAS that use a liquid as their main conductor. Despite recent pertinent advances in the field, the reconfiguration speed was not directly addressed in the literature, as it is assumed to be very high in most works. However, it is a crucial parameter that defines the system’s capabilities and performance bounds. In addition, a few approaches to spatial correlation modeling are presented here that are also needed for a holistic performance evaluation. Hence, a numerical methodology is proposed to realistically model both parameters. Performance metrics are also addressed, which aid in identifying and comprehending the system’s strengths and weaknesses. The importance of these models and metrics is showcased by reconfiguring the FAS, proposing several algorithms and illustrating their features. As a result, we show that it is possible to obtain a realistic performance evaluation for FAS and improve currently available models and performance to further progress on this topic.
{"title":"Correlation and Drop Velocity in Fluid Antenna Systems: Modeling and Performance","authors":"Javier Otero Martinez;Borja Genoves Guzman;Ana Garcia Armada","doi":"10.1109/JSAC.2025.3617473","DOIUrl":"10.1109/JSAC.2025.3617473","url":null,"abstract":"Emerging technologies like Fluid Antenna Systems (FAS) are important ingredients toward new generation communications. In particular, we focus on systems based on the reconfigurability capabilities of liquid antennas, which are a subset of FAS that use a liquid as their main conductor. Despite recent pertinent advances in the field, the reconfiguration speed was not directly addressed in the literature, as it is assumed to be very high in most works. However, it is a crucial parameter that defines the system’s capabilities and performance bounds. In addition, a few approaches to spatial correlation modeling are presented here that are also needed for a holistic performance evaluation. Hence, a numerical methodology is proposed to realistically model both parameters. Performance metrics are also addressed, which aid in identifying and comprehending the system’s strengths and weaknesses. The importance of these models and metrics is showcased by reconfiguring the FAS, proposing several algorithms and illustrating their features. As a result, we show that it is possible to obtain a realistic performance evaluation for FAS and improve currently available models and performance to further progress on this topic.","PeriodicalId":73294,"journal":{"name":"IEEE journal on selected areas in communications : a publication of the IEEE Communications Society","volume":"44 ","pages":"1322-1334"},"PeriodicalIF":17.2,"publicationDate":"2025-10-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145235771","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
In this paper, we propose an energy efficient wireless communication system based on fluid antenna relay (FAR) to solve the problem of non-line-of-sight (NLoS) links caused by blockages with considering the physical properties. Driven by the demand for the sixth generation (6G) communication, fluid antenna systems (FASs) have become a key technology due to their flexibility in dynamically adjusting antenna positions. Existing research on FAS primarily focuses on line-of-sight (LoS) communication scenarios, and neglects the situations where only NLoS links exist. To address the issues posted by NLoS communication, we design an FAR-assisted communication system combined with amplify-and-forward (AF) protocol. In order to alleviate the high energy consumption introduced by AF protocol while ensuring communication quality, we formulate an energy efficiency (EE) maximization problem. By optimizing the positions of the fluid antennas (FAs) on both sides of the FAR, we achieve controllable phase shifts of the signals transmitting through the blockage which causes the NLoS link. Besides, we establish a channel model that jointly considers the blockage-through matrix, large-scale fading, and small-scale fading. To maximize the EE of the system, we jointly optimize the FAR position, FA positions, power control, and beamforming design under given constraints, and propose an iterative algorithm to solve this formulated optimization problem. Simulation results show that the proposed algorithm outperforms the traditional schemes in terms of EE, achieving up to 23.39% and 39.94% higher EE than the conventional reconfigurable intelligent surface (RIS) scheme and traditional AF relay scheme, respectively.
{"title":"Energy Efficient Fluid Antenna Relay (FAR)-Assisted Wireless Communications","authors":"Ruopeng Xu;Zhaohui Yang;Zhaoyang Zhang;Mohammad Shikh-Bahaei;Kaibin Huang;Dusit Niyato","doi":"10.1109/JSAC.2025.3617892","DOIUrl":"10.1109/JSAC.2025.3617892","url":null,"abstract":"In this paper, we propose an energy efficient wireless communication system based on fluid antenna relay (FAR) to solve the problem of non-line-of-sight (NLoS) links caused by blockages with considering the physical properties. Driven by the demand for the sixth generation (6G) communication, fluid antenna systems (FASs) have become a key technology due to their flexibility in dynamically adjusting antenna positions. Existing research on FAS primarily focuses on line-of-sight (LoS) communication scenarios, and neglects the situations where only NLoS links exist. To address the issues posted by NLoS communication, we design an FAR-assisted communication system combined with amplify-and-forward (AF) protocol. In order to alleviate the high energy consumption introduced by AF protocol while ensuring communication quality, we formulate an energy efficiency (EE) maximization problem. By optimizing the positions of the fluid antennas (FAs) on both sides of the FAR, we achieve controllable phase shifts of the signals transmitting through the blockage which causes the NLoS link. Besides, we establish a channel model that jointly considers the blockage-through matrix, large-scale fading, and small-scale fading. To maximize the EE of the system, we jointly optimize the FAR position, FA positions, power control, and beamforming design under given constraints, and propose an iterative algorithm to solve this formulated optimization problem. Simulation results show that the proposed algorithm outperforms the traditional schemes in terms of EE, achieving up to 23.39% and 39.94% higher EE than the conventional reconfigurable intelligent surface (RIS) scheme and traditional AF relay scheme, respectively.","PeriodicalId":73294,"journal":{"name":"IEEE journal on selected areas in communications : a publication of the IEEE Communications Society","volume":"44 ","pages":"1160-1176"},"PeriodicalIF":17.2,"publicationDate":"2025-10-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145235768","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Beam alignment for multiple-input and multiple-output fluid antenna systems (MIMO-FAS) is studied, where two-sided beamforming and port activation are optimized without channel estimation to enhance transmission rate. In contrast to conventional position-fixed MIMO setups, MIMO-FAS leverages flexible beamforming to achieve higher gains with a smaller number of antennas. However, realizing these gains typically requires high-complexity channel estimation methods, especially in MIMO scenarios. To overcome this challenge, a channel estimation-free active-sensing framework for beam alignment in MIMO-FAS is proposed, which consists of three components: 1) A new ping-pong transmission protocol is conceived, enabling full-dimensional pilot reception through sequential sub-array activation. 2) Based on this protocol, two learning-based active-sensing algorithms are proposed for full-dimensional beam alignment via online and offline learning, respectively. 3) A greedy-policy-based method is developed to design the port activation matrices and associated beamforming vectors based on the active-sensing results. Numerical results demonstrate that: 1) the proposed active-sensing framework can effectively utilize the advantages of FAS over conventional MIMO systems without channel estimations; and: 2) the online-learning method enhances generalizability by eliminating the need for extensive centralized offline training, while the offline-learning method ensures robustness and low-complexity beam alignment by leveraging prior knowledge from the training phase.
{"title":"Beam Alignment for MIMO Fluid Antenna Systems","authors":"Hao Jiang;Zhaolin Wang;Yuanwei Liu;Arumugam Nallanathan;Hyundong Shin","doi":"10.1109/JSAC.2025.3618212","DOIUrl":"10.1109/JSAC.2025.3618212","url":null,"abstract":"Beam alignment for multiple-input and multiple-output fluid antenna systems (MIMO-FAS) is studied, where two-sided beamforming and port activation are optimized without channel estimation to enhance transmission rate. In contrast to conventional position-fixed MIMO setups, MIMO-FAS leverages flexible beamforming to achieve higher gains with a smaller number of antennas. However, realizing these gains typically requires high-complexity channel estimation methods, especially in MIMO scenarios. To overcome this challenge, a channel estimation-free active-sensing framework for beam alignment in MIMO-FAS is proposed, which consists of three components: 1) A new ping-pong transmission protocol is conceived, enabling full-dimensional pilot reception through sequential sub-array activation. 2) Based on this protocol, two learning-based active-sensing algorithms are proposed for full-dimensional beam alignment via online and offline learning, respectively. 3) A greedy-policy-based method is developed to design the port activation matrices and associated beamforming vectors based on the active-sensing results. Numerical results demonstrate that: 1) the proposed active-sensing framework can effectively utilize the advantages of FAS over conventional MIMO systems without channel estimations; and: 2) the online-learning method enhances generalizability by eliminating the need for extensive centralized offline training, while the offline-learning method ensures robustness and low-complexity beam alignment by leveraging prior knowledge from the training phase.","PeriodicalId":73294,"journal":{"name":"IEEE journal on selected areas in communications : a publication of the IEEE Communications Society","volume":"44 ","pages":"1193-1208"},"PeriodicalIF":17.2,"publicationDate":"2025-10-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145235773","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
In this work, we study a fluid antenna multiple access (FAMA) system, where a base station (BS) with multiple fluid antennas is responsible for the communication service supply to multiple users also equipped with fluid antennas. We concentrate on the optimal joint antenna configuration and resource allocation design, where the transmit power control is jointly optimized with the antenna configuration including BS antenna assignment and port selection at all activated fluid antennas. The large number of discrete variables needed for antenna configuration makes the joint optimization very challenging. To address these challenges without loss of optimality, we develop in this work a novel methodology for globally optimal FAMA designs. We first focus on FAMA throughput maximization while taking user fairness into account and accordingly formulate a mixed-integer nonlinear problem. To facilitate the optimal design, we characterize the optimal power control with given antenna configuration, which enables us to build up a system of equations and inequalities (SEI) tailored for examining the achievability of any throughput level. A fixpoint-based approach is subsequently proposed for effectively inferring the solvability of established SEI, as well as the throughput achievability. Leveraging the proposed fixpoint-based inference approach, we develop an efficient iterative algorithm for the optimal antenna configuration filtering, where all nonoptimal configuration candidates are efficiently filtered and removed via fixpoint inspections. The optimal power control associated with the optimal antenna configuration finalizes the globally optimal FAMA design. Afterwards, we extend the whole design methodology to a scenario requesting energy efficiency maximization, achieving globally optimal energy-efficient FAMA design. Finally, the obtained FAMA solutions are examined via numerical simulations, verifying the global optimality and spotlighting the high benefits of considering joint antenna configuration and power control in FAMA.
{"title":"Optimal Antenna Configuration Filtering and Joint Power Control in Fluid Antenna Multiple Access Networks","authors":"Xiaopeng Yuan;Ning Guo;Yulin Hu;Robert Schober;Anke Schmeink","doi":"10.1109/JSAC.2025.3617023","DOIUrl":"10.1109/JSAC.2025.3617023","url":null,"abstract":"In this work, we study a fluid antenna multiple access (FAMA) system, where a base station (BS) with multiple fluid antennas is responsible for the communication service supply to multiple users also equipped with fluid antennas. We concentrate on the optimal joint antenna configuration and resource allocation design, where the transmit power control is jointly optimized with the antenna configuration including BS antenna assignment and port selection at all activated fluid antennas. The large number of discrete variables needed for antenna configuration makes the joint optimization very challenging. To address these challenges without loss of optimality, we develop in this work a novel methodology for globally optimal FAMA designs. We first focus on FAMA throughput maximization while taking user fairness into account and accordingly formulate a mixed-integer nonlinear problem. To facilitate the optimal design, we characterize the optimal power control with given antenna configuration, which enables us to build up a system of equations and inequalities (SEI) tailored for examining the achievability of any throughput level. A fixpoint-based approach is subsequently proposed for effectively inferring the solvability of established SEI, as well as the throughput achievability. Leveraging the proposed fixpoint-based inference approach, we develop an efficient iterative algorithm for the optimal antenna configuration filtering, where all nonoptimal configuration candidates are efficiently filtered and removed via fixpoint inspections. The optimal power control associated with the optimal antenna configuration finalizes the globally optimal FAMA design. Afterwards, we extend the whole design methodology to a scenario requesting energy efficiency maximization, achieving globally optimal energy-efficient FAMA design. Finally, the obtained FAMA solutions are examined via numerical simulations, verifying the global optimality and spotlighting the high benefits of considering joint antenna configuration and power control in FAMA.","PeriodicalId":73294,"journal":{"name":"IEEE journal on selected areas in communications : a publication of the IEEE Communications Society","volume":"44 ","pages":"1227-1242"},"PeriodicalIF":17.2,"publicationDate":"2025-10-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145215669","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-10-03DOI: 10.1109/JSAC.2025.3617021
Xiazhi Lai;Tuo Wu;Lifeng Mai;Maged Elkashlan;Naofal Al-Dhahir;Mérouane Debbah;George K. Karagiannidis;Chau Yuen
Fluid antenna systems (FAS) have emerged as a promising technology to achieve high spatial diversity by dynamically reconfiguring multiple closely spaced $N$ antenna ports. However, the inherent spatial correlation among these ports poses significant challenges for accurate performance analysis. Traditional block-correlation modeling algorithms, which partition the $Ntimes N$ Toeplitz-structured correlation matrix into independent $D$ blocks with constant correlation coefficients, often yield substantial approximation errors to block-correlation models, especially in scenarios with limited ports. In this paper, we revisit the spatial block-correlation model for FAS and introduce a novel block-correlation modeling algorithm in tuning the model parameters, which realizes the variable block-correlation model in practice. Our proposed approach derives closed-form expressions for the optimal block-specific correlation coefficients and develops a low-complexity heuristic algorithm that reduces the computational complexity from exponential $D^{N-D}$ to linear $(N-D)times D$ searches, thereby achieving significantly lower approximation error compared to constant correlation models. To validate the effectiveness of our variable block-correlation modeling algorithm, we first apply it to point-to-point FAS communications with closely spaced ports, deriving analytical expressions for the joint probability density function (PDF) of channel amplitudes and outage probability. Our analysis shows that the proposed algorithm offers tractable performance evaluation and superior accuracy, particularly when the number of ports is small ($Nlt 20$ ). Furthermore, we extend our framework to classical FAS-assisted reconfigurable intelligent surface (FAS-RIS) communications, where the interplay between direct and RIS-assisted links complicates the channel statistics. By integrating the central limit theorem (CLT) with variable block-correlation model, we derive tractable outage probability expressions that capture the coupling effects among different channel coefficients. Extensive numerical simulations under various system configurations demonstrate that variable block-correlation model not only outperforms conventional constant-correlation approaches in terms of approximation accuracy and robustness, but also provides reliable performance prediction in threshold-sensitive and challenging propagation environments. These results underscore the practical value of our approach for the design and optimization of next-generation FAS-based wireless networks.
{"title":"Revisiting Spatial Block-Correlation Model for Fluid Antenna Systems: From Constant to Variable Correlations","authors":"Xiazhi Lai;Tuo Wu;Lifeng Mai;Maged Elkashlan;Naofal Al-Dhahir;Mérouane Debbah;George K. Karagiannidis;Chau Yuen","doi":"10.1109/JSAC.2025.3617021","DOIUrl":"10.1109/JSAC.2025.3617021","url":null,"abstract":"Fluid antenna systems (FAS) have emerged as a promising technology to achieve high spatial diversity by dynamically reconfiguring multiple closely spaced <inline-formula> <tex-math>$N$ </tex-math></inline-formula> antenna ports. However, the inherent spatial correlation among these ports poses significant challenges for accurate performance analysis. Traditional block-correlation modeling algorithms, which partition the <inline-formula> <tex-math>$Ntimes N$ </tex-math></inline-formula> Toeplitz-structured correlation matrix into independent <inline-formula> <tex-math>$D$ </tex-math></inline-formula> blocks with constant correlation coefficients, often yield substantial approximation errors to block-correlation models, especially in scenarios with limited ports. In this paper, we revisit the spatial block-correlation model for FAS and introduce a novel block-correlation modeling algorithm in tuning the model parameters, which realizes the variable block-correlation model in practice. Our proposed approach derives closed-form expressions for the optimal block-specific correlation coefficients and develops a low-complexity heuristic algorithm that reduces the computational complexity from exponential <inline-formula> <tex-math>$D^{N-D}$ </tex-math></inline-formula> to linear <inline-formula> <tex-math>$(N-D)times D$ </tex-math></inline-formula> searches, thereby achieving significantly lower approximation error compared to constant correlation models. To validate the effectiveness of our variable block-correlation modeling algorithm, we first apply it to point-to-point FAS communications with closely spaced ports, deriving analytical expressions for the joint probability density function (PDF) of channel amplitudes and outage probability. Our analysis shows that the proposed algorithm offers tractable performance evaluation and superior accuracy, particularly when the number of ports is small (<inline-formula> <tex-math>$Nlt 20$ </tex-math></inline-formula>). Furthermore, we extend our framework to classical FAS-assisted reconfigurable intelligent surface (FAS-RIS) communications, where the interplay between direct and RIS-assisted links complicates the channel statistics. By integrating the central limit theorem (CLT) with variable block-correlation model, we derive tractable outage probability expressions that capture the coupling effects among different channel coefficients. Extensive numerical simulations under various system configurations demonstrate that variable block-correlation model not only outperforms conventional constant-correlation approaches in terms of approximation accuracy and robustness, but also provides reliable performance prediction in threshold-sensitive and challenging propagation environments. These results underscore the practical value of our approach for the design and optimization of next-generation FAS-based wireless networks.","PeriodicalId":73294,"journal":{"name":"IEEE journal on selected areas in communications : a publication of the IEEE Communications Society","volume":"44 ","pages":"1335-1351"},"PeriodicalIF":17.2,"publicationDate":"2025-10-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145215670","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Respiration monitoring via radio signals enables contactless health sensing but suffers from interference caused by nearby motion. We propose a robust respiration sensing framework using Cell-free Massive MIMO (CF-mMIMO), which leverages spatial macro-diversity for interference resilience. Specifically, we analyze respiration sensing in single-antenna channels using Power Spectral Density (PSD) to reveal the impact of interference on the breathing channel’s movement spectrum. Based on this, we introduce a new metric, Sensing-Signal-to-Interference Ratio (SSIR), to evaluate local channel quality without requiring ground truth. Then, we design a Weighted Antenna Combining (WAC) method to prioritize reliable sensing links and suppress distortion. Experimental validation using a 64-antenna CF-mMIMO testbed with 100 Orthogonal Frequency-Division Multiplexing (OFDM) subcarriers over an 18 MHz bandwidth confirms the framework’s robustness. In the presence of interference, the WAC method achieves a mean waveform correlation of 0.81 with ground truth, significantly outperforming single-antenna (0.52), averaging-based methods (0.53), and existing Wi-Fi approaches. Finally, we analyze the impact of time, frequency, and spatial resource allocation on both communication and sensing performance. Results show that increasing bandwidth and antenna count benefits both communication and sensing. With a sufficient number of antennas, respiration sensing remains accurate even with long coherence times (1 second) and narrow bandwidths (3 subcarriers), enabling its integration into communication systems with negligible overhead, making it practically “for free”. This makes CF-mMIMO a promising architecture for robust and scalable Integrated Sensing and Communication (ISAC) health monitoring.
{"title":"Fundamentals and Experiments of Robust Respiration Sensing via Cell-Free Massive MIMO","authors":"Haoqiu Xiong;Robbert Beerten;Qing Zhang;Yang Miao;Zhuangzhuang Cui;Sofie Pollin","doi":"10.1109/JSAC.2025.3617012","DOIUrl":"10.1109/JSAC.2025.3617012","url":null,"abstract":"Respiration monitoring via radio signals enables contactless health sensing but suffers from interference caused by nearby motion. We propose a robust respiration sensing framework using Cell-free Massive MIMO (CF-mMIMO), which leverages spatial macro-diversity for interference resilience. Specifically, we analyze respiration sensing in single-antenna channels using Power Spectral Density (PSD) to reveal the impact of interference on the breathing channel’s movement spectrum. Based on this, we introduce a new metric, Sensing-Signal-to-Interference Ratio (SSIR), to evaluate local channel quality without requiring ground truth. Then, we design a Weighted Antenna Combining (WAC) method to prioritize reliable sensing links and suppress distortion. Experimental validation using a 64-antenna CF-mMIMO testbed with 100 Orthogonal Frequency-Division Multiplexing (OFDM) subcarriers over an 18 MHz bandwidth confirms the framework’s robustness. In the presence of interference, the WAC method achieves a mean waveform correlation of 0.81 with ground truth, significantly outperforming single-antenna (0.52), averaging-based methods (0.53), and existing Wi-Fi approaches. Finally, we analyze the impact of time, frequency, and spatial resource allocation on both communication and sensing performance. Results show that increasing bandwidth and antenna count benefits both communication and sensing. With a sufficient number of antennas, respiration sensing remains accurate even with long coherence times (1 second) and narrow bandwidths (3 subcarriers), enabling its integration into communication systems with negligible overhead, making it practically “for free”. This makes CF-mMIMO a promising architecture for robust and scalable Integrated Sensing and Communication (ISAC) health monitoring.","PeriodicalId":73294,"journal":{"name":"IEEE journal on selected areas in communications : a publication of the IEEE Communications Society","volume":"44 ","pages":"959-974"},"PeriodicalIF":17.2,"publicationDate":"2025-10-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145215671","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-09-30DOI: 10.1109/JSAC.2025.3616068
Peng Zhang;Jian Dang;Miaowen Wen;Zaichen Zhang;Liang Wu;Yudong Yao
Fluid antenna-enabled multiple-input multiple-output (FA-MIMO) systems hold significant application potential; however, they also introduce substantial costs in channel state information (CSI) acquisition. In this paper, we propose a novel FA-assisted rectangular differential index modulation (FA-RDIM) scheme for MIMO systems, aiming to achieve high spectral efficiency while addressing the problem of the increased CSI acquisition cost. The transmitted information is mapped to modulation symbols and cyclic shifts of FA pattern indices. A multi-stage detection method is introduced, which reduces computational complexity by identifying the most likely candidates for FA pattern indices. We derive a closed-form expression for the bit error rate (BER) theoretical performance considering error propagation, and present an optimization algorithm based on rank and determinant criterion (RDC), Hamming distance (HD), and gradient descent (GD) to optimize the FA pattern vector set. Simulation results demonstrate that the proposed scheme exhibits a minimal performance loss compared to conventional coherent modulation schemes under static channel conditions, while offering a performance advantage in time-varying channels with outdated CSI.
{"title":"Fluid Antenna-Assisted Rectangular Differential Index Modulation: A Non-Coherent System Design, Optimization, and Performance Analysis","authors":"Peng Zhang;Jian Dang;Miaowen Wen;Zaichen Zhang;Liang Wu;Yudong Yao","doi":"10.1109/JSAC.2025.3616068","DOIUrl":"10.1109/JSAC.2025.3616068","url":null,"abstract":"Fluid antenna-enabled multiple-input multiple-output (FA-MIMO) systems hold significant application potential; however, they also introduce substantial costs in channel state information (CSI) acquisition. In this paper, we propose a novel FA-assisted rectangular differential index modulation (FA-RDIM) scheme for MIMO systems, aiming to achieve high spectral efficiency while addressing the problem of the increased CSI acquisition cost. The transmitted information is mapped to modulation symbols and cyclic shifts of FA pattern indices. A multi-stage detection method is introduced, which reduces computational complexity by identifying the most likely candidates for FA pattern indices. We derive a closed-form expression for the bit error rate (BER) theoretical performance considering error propagation, and present an optimization algorithm based on rank and determinant criterion (RDC), Hamming distance (HD), and gradient descent (GD) to optimize the FA pattern vector set. Simulation results demonstrate that the proposed scheme exhibits a minimal performance loss compared to conventional coherent modulation schemes under static channel conditions, while offering a performance advantage in time-varying channels with outdated CSI.","PeriodicalId":73294,"journal":{"name":"IEEE journal on selected areas in communications : a publication of the IEEE Communications Society","volume":"44 ","pages":"1307-1321"},"PeriodicalIF":17.2,"publicationDate":"2025-09-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145195234","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Fluid Antenna System (FAS) overcomes the spatial degree-of-freedom limitations of conventional static antenna arrays in wireless communications. This capability critically depends on acquiring full Channel State Information across all accessible ports. Existing studies focus exclusively on narrowband FAS, performing channel estimation solely in the spatial domain. This work proposes a channel estimation and spatial equalization framework for wideband FAS, revealing for the first time an inherent group-sparse structure in aperture-limited FAS channels. First, we establish a group-sparse recovery framework for space-frequency characteristics in FAS, formally characterizing leakage-induced sparsity degradation from limited aperture and bandwidth as a structured group-sparsity problem. By deriving dictionary-adapted group restricted isometry property, we prove tight recovery bounds for a convex $ell _{1}/ell _{2}$ -mixed norm optimization formulation that preserves leakage-aware sparsity patterns. Second, we develop a descending correlation group orthogonal matching pursuit algorithm that systematically relaxes leakage constraints to reduce subcoherence. This approach enables FSC recovery with accelerated convergence and superior performance compared to conventional compressive sensing methods like OMP or GOMP. Third, we formulate spatial equalization as a mixed-integer linear programming problem, complement this with a greedy algorithm maintaining near-optimal performance. Simulation results demonstrate the proposed channel estimation algorithm effectively resolves energy misallocation and enables recovery of weak details, achieving superior recovery accuracy and convergence rate. The SE framework suppresses deep fading phenomena and largely reduces time consumption overhead while maintaining equivalent link reliability.
在无线通信中，流体天线系统克服了传统静态天线阵列的空间自由度限制。此功能主要依赖于获取所有可访问端口的完整通道状态信息。现有的研究主要集中在窄带FAS上，仅在空间域中进行信道估计。本文提出了一种用于宽带FAS的信道估计和空间均衡框架，首次揭示了有限孔径FAS信道中固有的群稀疏结构。首先，我们建立了FAS空间频率特性的群稀疏恢复框架，将泄漏引起的有限孔径和带宽的稀疏性退化正式表征为结构化群稀疏性问题。通过推导字典适应的群限制等距性质，我们证明了凸$ well _{2}$ / well _{2}$混合范数优化公式保留泄漏感知稀疏模式的紧恢复界。其次，我们开发了一种下降相关群正交匹配追踪算法，系统地放松泄漏约束以降低亚相干性。与传统的压缩感知方法（如OMP或GOMP）相比，该方法可实现FSC恢复，具有加速收敛和卓越性能。第三，我们将空间均衡表述为一个混合整数线性规划问题，并用贪心算法来补充这个问题，从而保持接近最优的性能。仿真结果表明，所提出的信道估计算法有效地解决了能量分配不当的问题，能够恢复弱细节，具有较高的恢复精度和收敛速度。SE框架抑制了深度衰落现象，在保持等效链路可靠性的同时，大大降低了时间消耗开销。
{"title":"Frequency-Space Channel Estimation and Spatial Equalization in Wideband Fluid Antenna System","authors":"Xuehui Dong;Kai Wan;Shuangyang Li;Robert Caiming Qiu;Giuseppe Caire","doi":"10.1109/JSAC.2025.3615912","DOIUrl":"10.1109/JSAC.2025.3615912","url":null,"abstract":"The Fluid Antenna System (FAS) overcomes the spatial degree-of-freedom limitations of conventional static antenna arrays in wireless communications. This capability critically depends on acquiring full Channel State Information across all accessible ports. Existing studies focus exclusively on narrowband FAS, performing channel estimation solely in the spatial domain. This work proposes a channel estimation and spatial equalization framework for wideband FAS, revealing for the first time an inherent group-sparse structure in aperture-limited FAS channels. First, we establish a group-sparse recovery framework for space-frequency characteristics in FAS, formally characterizing leakage-induced sparsity degradation from limited aperture and bandwidth as a structured group-sparsity problem. By deriving dictionary-adapted group restricted isometry property, we prove tight recovery bounds for a convex <inline-formula> <tex-math>$ell _{1}/ell _{2}$ </tex-math></inline-formula>-mixed norm optimization formulation that preserves leakage-aware sparsity patterns. Second, we develop a descending correlation group orthogonal matching pursuit algorithm that systematically relaxes leakage constraints to reduce subcoherence. This approach enables FSC recovery with accelerated convergence and superior performance compared to conventional compressive sensing methods like OMP or GOMP. Third, we formulate spatial equalization as a mixed-integer linear programming problem, complement this with a greedy algorithm maintaining near-optimal performance. Simulation results demonstrate the proposed channel estimation algorithm effectively resolves energy misallocation and enables recovery of weak details, achieving superior recovery accuracy and convergence rate. The SE framework suppresses deep fading phenomena and largely reduces time consumption overhead while maintaining equivalent link reliability.","PeriodicalId":73294,"journal":{"name":"IEEE journal on selected areas in communications : a publication of the IEEE Communications Society","volume":"44 ","pages":"1243-1258"},"PeriodicalIF":17.2,"publicationDate":"2025-09-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145195229","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-09-30DOI: 10.1109/JSAC.2025.3616064
Yanqing Ren;Xiaokun Teng;Mingyong Zhou;Weicong Chen;Wankai Tang;Hao Xu;Xiao Li;Shi Jin
Reconfigurable intelligent surface (RIS) has emerged as a promising technology for enhancing communication systems. This paper investigates a novel flexible-position RIS-assisted communication system, where the RIS is mounted on a slide rail, enabling spatial adaptability. Unlike traditional fixed-position RISs, the proposed system leverages both spatial flexibility and phase reconfigurability to optimize system performance while reducing overhead through strategic position adjustment. To characterize the spatial variations introduced by RIS movement, we propose a generalized RIS channel model that integrates a practical visibility region function with near-field spherical wave propagation. This model captures the spatial correlation characteristics influenced by multipath angular spread, scatterer distribution, and RIS positioning. Furthermore, we introduce a measurement scheme using a multi-state RIS hardware to analyze segmented channels across fixed-position and flexible-position scenarios. Our measurement reveals that the intra-cluster power angular spectrum follows a Gaussian distribution, and in strong scattering environments, spatial correlation exhibits an enhanced degree of freedom due to spatial non-stationarity effects. In particular, the experimental results demonstrate that the gain of the received power varies from 0.4 dB to 5.3 dB across different RIS positions, providing empirical evidence that spatial adaptability of RIS effectively resists channel non-stationarity. These findings highlight the potential of flexible-position RIS to enhance future wireless communication systems.
可重构智能表面（RIS）已成为一种很有前途的通信系统增强技术。本文研究了一种新型的柔性位置RIS辅助通信系统，其中RIS安装在滑轨上，具有空间适应性。与传统的固定位置RISs不同，该系统利用空间灵活性和相位可重构性来优化系统性能，同时通过战略性位置调整减少开销。为了描述RIS运动带来的空间变化，我们提出了一种将实际可见区域函数与近场球面波传播相结合的广义RIS通道模型。该模型捕获了受多径角扩展、散射体分布和RIS定位影响的空间相关特征。此外，我们还介绍了一种使用多状态RIS硬件来分析固定位置和灵活位置场景下的分段信道的测量方案。我们的测量结果表明，簇内功率角谱服从高斯分布，并且在强散射环境中，由于空间非平稳性效应，空间相关性表现出增强的自由度。特别是，实验结果表明，接收功率增益在不同RIS位置的变化范围为0.4 dB ~ 5.3 dB，这为RIS的空间适应性有效抵抗信道非平稳性提供了经验证据。这些发现突出了灵活位置RIS增强未来无线通信系统的潜力。
{"title":"Flexible-Position Multi-State RIS-Assisted Wireless Communication: Channel Modeling and Spatial Characteristic Measurements","authors":"Yanqing Ren;Xiaokun Teng;Mingyong Zhou;Weicong Chen;Wankai Tang;Hao Xu;Xiao Li;Shi Jin","doi":"10.1109/JSAC.2025.3616064","DOIUrl":"10.1109/JSAC.2025.3616064","url":null,"abstract":"Reconfigurable intelligent surface (RIS) has emerged as a promising technology for enhancing communication systems. This paper investigates a novel flexible-position RIS-assisted communication system, where the RIS is mounted on a slide rail, enabling spatial adaptability. Unlike traditional fixed-position RISs, the proposed system leverages both spatial flexibility and phase reconfigurability to optimize system performance while reducing overhead through strategic position adjustment. To characterize the spatial variations introduced by RIS movement, we propose a generalized RIS channel model that integrates a practical visibility region function with near-field spherical wave propagation. This model captures the spatial correlation characteristics influenced by multipath angular spread, scatterer distribution, and RIS positioning. Furthermore, we introduce a measurement scheme using a multi-state RIS hardware to analyze segmented channels across fixed-position and flexible-position scenarios. Our measurement reveals that the intra-cluster power angular spectrum follows a Gaussian distribution, and in strong scattering environments, spatial correlation exhibits an enhanced degree of freedom due to spatial non-stationarity effects. In particular, the experimental results demonstrate that the gain of the received power varies from 0.4 dB to 5.3 dB across different RIS positions, providing empirical evidence that spatial adaptability of RIS effectively resists channel non-stationarity. These findings highlight the potential of flexible-position RIS to enhance future wireless communication systems.","PeriodicalId":73294,"journal":{"name":"IEEE journal on selected areas in communications : a publication of the IEEE Communications Society","volume":"44 ","pages":"1366-1381"},"PeriodicalIF":17.2,"publicationDate":"2025-09-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145195454","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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