| 1. |
- Fodor, Sebastian, et al.
(författare)
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Optimizing Pilot Spacing in MU-MIMO Systems Operating Over Aging Channels
- 2023
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Ingår i: IEEE Transactions on Communications. - : Institute of Electrical and Electronics Engineers (IEEE). - 0090-6778 .- 1558-0857. ; 71:6, s. 3708-3720
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Tidskriftsartikel (refereegranskat)abstract
- In the uplink of multiuser multiple input multiple output (MU-MIMO) systems operating over aging channels, pilot spacing is crucial for acquiring channel state information and achieving high signal-to-interference-plus-noise ratio (SINR). Somewhat surprisingly, very few works examine the impact of pilot spacing on the correlation structure of subsequent channel estimates and the resulting quality of channel state information considering channel aging. In this paper, we consider a fast-fading environment characterized by its exponentially decaying autocorrelation function, and model pilot spacing as a sampling problem to capture the inherent trade-off between the quality of channel state information and the number of symbols available for information carrying data symbols. We first establish a quasi-closed form for the achievable deterministic equivalent SINR when the channel estimation algorithm utilizes multiple pilot signals. Next, we establish upper bounds on the achievable SINR and spectral efficiency, as a function of pilot spacing, which helps to find the optimum pilot spacing within a limited search space. Our key insight is that to maximize the achievable SINR and the spectral efficiency of MU-MIMO systems, proper pilot spacing must be applied to control the impact of the aging channel and to tune the trade-off between pilot and data symbols.
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| 2. |
- Gürgünoglu, Doga, 1997-
(författare)
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Channel Estimation Aspects of Reconfigurable Intelligent Surfaces
- 2024
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the sixth generation of wireless communication systems (6G), there exist multiple candidate enabling technologies that help the wireless network satisfy the ever-increasing demand for speed, coverage, reliability, and mobility. Among these technologies, reconfigurable intelligent surfaces (RISs) extend the coverage of a wireless network into dead zones, increase capacity, and facilitate integrated sensing and communications tasks by consuming very low power, thus contributing to energy efficiency as well.RISs are meta-material-based devices whose electromagnetic reflection characteristics can be controlled externally to cater to the needs of the communication links. Most ubiquitously, this comes in the form of adding a desired phase shift to an incident wave before reflecting it, which can be used to phase-align multiple incident waves to increase the strength of the signal at the receiver and provide coverage to an area that otherwise would be a dead zone.While this portrays an image of a dream technology that would boost the existing wireless networks significantly, RISs do not come without engineering problems. First of all, the individual elements do not exhibit ideal reflection characteristics, that is, they attenuate the incident signal in a fashion depending on the configured phase shift. This creates the phenomenon called "phase-dependent amplitude". Another problem caused by RISs is the channel estimation overhead. In a multiple-antenna communication system, the channel between two terminals is as complex as the product of the number of antennas at each end. However, when an RIS comes into the equation, the cascade of the transmitter-RIS and RIS-receiver channels has a complexity further multiplied by the number of RIS elements. Consequently, the channel estimation process to utilize the RIS effectively becomes more demanding, that is, more pilot signals are required to estimate the channel for coherent reception. This adversely affects the effective data rate within a communication system since more resources need to be spent for pilot transmission and fewer resources can be allocated for data transmission. While there exists some work on reducing the channel dimensions by exploiting the channel structure, this problem persists for unstructured channels. In addition, for the wireless networks using multiple RISs, a new kind of pilot contamination arises, which is the main topic of this thesis.In the first part of this thesis, we study this new kind of pilot contamination in a multi-operator context, where two operators provide services to their respective served users and share a single site. Each operator has a single dedicated RIS and they use disjoint 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.In the second part of this thesis, we consider a single-operator-two-RIS integrated sensing and communication (ISAC) system where the single user is both a communication terminal and a positioning target. Based on the uplink positioning pilots, the base station aims to estimate both the communication channel and the user's position within the indoor environment by estimating the angle of arrival (AoA) of the impinging signals on both RISs and then exploiting the system and array geometries to estimate the user position and user channels respectively. Although there is a single operator, due to the presence of multiple RISs, pilot contamination occurs through the same physical means as multi-operator pilot contamination unless the channel estimation process is parameterized. Since the communication links are considered to be pure line-of-sight (LOS), their structure allows the reduction of the number of unknown parameters. Consequently, the reduction of information caused by pilot contamination does not affect the channel estimation procedure, hence the pilot contamination is overcome. On the other hand, the position of the user is determined by intersecting the lines drawn along the AoA estimates. We adopt the Cramér-Rao Lower Bound (CRLB), the lower bound on the mean squared error (MSE) of any unbiased estimator, for both channel estimation and positioning. Our numerical results show that it is possible to utilize positioning pilots for parametric channel estimation when the wireless links are LOS.
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| 3. |
- Gurgunoglu, Doga, 1997-, et al.
(författare)
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Combating Inter-Operator Pilot Contamination in Reconfigurable Intelligent Surfaces Assisted Multi-Operator Networks
- 2024
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Ingår i: IEEE Transactions on Communications. - 0090-6778 .- 1558-0857.
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 4. |
- Gurgunoglu, Doga, 1997-, et al.
(författare)
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Impact of Pilot Contamination Between Operators With Interfering Reconfigurable Intelligent Surfaces
- 2023
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Ingår i: 2023 IEEE International Black Sea Conference on Communications and Networking, BlackSeaCom 2023. - : Institute of Electrical and Electronics Engineers (IEEE). - 9798350337822 ; , s. 27-32
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Konferensbidrag (refereegranskat)abstract
- In this paper, we study the impact of pilot contamination in a system where two operators serve their respective users with the assistance of two wide-band reconfigurable intelligent surfaces (RIS), each belonging to a single operator. We consider one active user per operator and they use disjoint narrow frequency bands. Although each RIS is dedicated to a single operator, both users' transmissions are reflected by both RISs. We show that this creates a new kind of pilot contamination effect when pilots are transmitted simultaneously. Since combating inter-operator pilot contamination in RIS-assisted networks would require long pilot signal sequences to maintain orthogonality among the users of different operators, we propose the orthogonal configurations of the RISs. Numerical results show that this approach completely eliminates pilot contamination, and significantly improves the performance in terms of channel estimation and equalization by removing the channel estimation bias.
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| 5. |
- Gurgunoglu, Doga, 1997-, et al.
(författare)
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Joint Pilot-Based Localization and Channel Estimation in RIS-Aided Communication Systems
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In this letter, we investigate the use of reconfigurable intelligent surfaces (RISs) to jointly estimate the position and channel of a user in an indoor environment using uplink pilot signals. We consider a setup with a user and a base station (BS), where the BS is located outside an indoor environment, and virtual line-of-sight (LOS) links are created over two reconfigurable intelligent surfaces (RISs). We propose exploiting the channel and indoor geometries to estimate the user's position and the user-RIS channels jointly by estimating the azimuth angle of arrivals of the impinging signals at the two RISs. Next, the BS uses triangulation to estimate the user location, and the RIS geometries to estimate the channels. As our performance metric for channel estimation and localization, we consider the Cram\'{e}r-Rao Lower Bound. Our numerical results show that by exploiting the LOS structure, it is possible to perform joint channel estimation and localization while avoiding multi-RIS pilot contamination using much fewer pilots compared to non-parametric channel estimation.
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| 6. |
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| 7. |
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| 8. |
- Saglam, Baturay, et al.
(författare)
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Deep Reinforcement Learning Based Joint Downlink Beamforming and RIS Configuration in RIS-aided MU-MISO Systems Under Hardware Impairments and Imperfect CSI
- 2023
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Ingår i: 2023 IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS WORKSHOPS, ICC WORKSHOPS. - : Institute of Electrical and Electronics Engineers (IEEE). ; , s. 66-72
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Konferensbidrag (refereegranskat)abstract
- We introduce a novel deep reinforcement learning (DRL) approach to jointly optimize transmit beamforming and reconfigurable intelligent surface (RIS) phase shifts in a multiuser multiple input single output (MU-MISO) system to maximize the sum downlink rate under the phase-dependent reflection amplitude model. Our approach addresses the challenge of imperfect channel state information (CSI) and hardware impairments by considering a practical RIS amplitude model. We compare the performance of our approach against a vanilla DRL agent in two scenarios: perfect CSI and phase-dependent RIS amplitudes, and mismatched CSI and ideal RIS reflections. The results demonstrate that the proposed framework significantly outperforms the vanilla DRL agent under mismatch and approaches the golden standard. Our contributions include modifications to the DRL approach to address the joint design of transmit beamforming and phase shifts and the phase-dependent amplitude model. To the best of our knowledge, our method is the first DRL-based approach for the phase-dependent reflection amplitude model in RIS-aided MU-MISO systems. Our findings in this study highlight the potential of our approach as a promising solution to overcome hardware impairments in RIS-aided wireless communication systems.
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| 9. |
- Saglam, Baturay, et al.
(författare)
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Deep Reinforcement Learning Based JointDownlink Beamforming and RIS Configuration in RIS-aided MU-MISO Systems Under HardwareImpairments and Imperfect CSI
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We investigate the joint transmit beamforming and reconfigurable intelligent surface (RIS) configuration problem to maximize the sum downlink rate of a RIS-aided cellular multiuser multiple input single output (MU-MISO) system under imperfect channel state information (CSI) and hardware impairments by considering a practical phase-dependent RIS amplitude model. To this end, we present a novel deep reinforcement learning (DRL) framework and compare its performance against a vanilla DRL agent under two scenarios: the golden standard where the base station (BS) knows the channel and the phasedependentRIS amplitude model perfectly, and the mismatch scenario where the BS has imperfect CSI and assumes idealRIS reflections. Our numerical results show that the introduced framework substantially outperforms the vanilla DRL agent under mismatch and approaches the golden standard.
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