2,304 research outputs found
Cell-Free XL-MIMO Meets Multi-Agent Reinforcement Learning: Architectures, Challenges, and Future Directions
Cell-free massive multiple-input multiple-output (mMIMO) and extremely
large-scale MIMO (XL-MIMO) are regarded as promising innovations for the
forthcoming generation of wireless communication systems. Their significant
advantages in augmenting the number of degrees of freedom have garnered
considerable interest. In this article, we first review the essential
opportunities and challenges induced by XL-MIMO systems. We then propose the
enhanced paradigm of cell-free XL-MIMO, which incorporates multi-agent
reinforcement learning (MARL) to provide a distributed strategy for tackling
the problem of high-dimension signal processing and costly energy consumption.
Based on the unique near-field characteristics, we propose two categories of
the low-complexity design, i.e., antenna selection and power control, to adapt
to different cell-free XL-MIMO scenarios and achieve the maximum data rate. For
inspiration, several critical future research directions pertaining to green
cell-free XL-MIMO systems are presented
Design and Analysis of a Cylindrical Dielectric Resonator Antenna Array and Its Feed Network
There is an ever increasing need for smaller, lighter, more efficient antennas for commercial and military applications. One such antenna that meets these requirements is the dielectric resonator antenna (DRA). In recent years there has been an abundance of research on the utilization of the DRA as a radiating element. However, its practical application - especially pertaining to DRA arrays - is still considered to be at its infancy. The purpose of this work is to present a systematic process to be used in the design, simulation, optimization, fabrication, and testing of a cylindrical DRA array including its associated feed network. The DRA array development cycle begins with a single cylindrical radiating element. ComDRA parameters such as DRA radius, feed type, feed location, and element spacing are investigated. A DRA element in this research is optimized for bandwidth and gain for use at x-band (8-12 GHz). The antenna feed network, being an integral part of all antenna arrays, is also considered. The primary causes of impedance mismatch in the feed network are identified and techniques to improve performance are explored. An improvement in impedance bandwidth is gained through traditional transmission line matching methods. Ultimately, a 16 (4x4) element and 256 (16x16) element array is fabricated, tested, and compared to an existing commercial technology
Holographic MIMO Communications: Theoretical Foundations, Enabling Technologies, and Future Directions
Future wireless systems are envisioned to create an endogenously
holography-capable, intelligent, and programmable radio propagation
environment, that will offer unprecedented capabilities for high spectral and
energy efficiency, low latency, and massive connectivity. A potential and
promising technology for supporting the expected extreme requirements of the
sixth-generation (6G) communication systems is the concept of the holographic
multiple-input multiple-output (HMIMO), which will actualize holographic radios
with reasonable power consumption and fabrication cost. The HMIMO is
facilitated by ultra-thin, extremely large, and nearly continuous surfaces that
incorporate reconfigurable and sub-wavelength-spaced antennas and/or
metamaterials. Such surfaces comprising dense electromagnetic (EM) excited
elements are capable of recording and manipulating impinging fields with utmost
flexibility and precision, as well as with reduced cost and power consumption,
thereby shaping arbitrary-intended EM waves with high energy efficiency. The
powerful EM processing capability of HMIMO opens up the possibility of wireless
communications of holographic imaging level, paving the way for signal
processing techniques realized in the EM-domain, possibly in conjunction with
their digital-domain counterparts. However, in spite of the significant
potential, the studies on HMIMO communications are still at an initial stage,
its fundamental limits remain to be unveiled, and a certain number of critical
technical challenges need to be addressed. In this survey, we present a
comprehensive overview of the latest advances in the HMIMO communications
paradigm, with a special focus on their physical aspects, their theoretical
foundations, as well as the enabling technologies for HMIMO systems. We also
compare the HMIMO with existing multi-antenna technologies, especially the
massive MIMO, present various...Comment: double column, 58 page
Optimizing Antenna Arrays for Spatial Multiplexing: Towards 6G Systems
In this paper we discuss the design of antenna arrays to be used for multiplexing applications. In particular, we introduce a suitable performance index to analyze the effect of the antenna geometry and the distribution of users for the overall performance of Multi-User Multiple Input Multiple Output systems. By means of such performance index, antenna arrays can be designed so as to increase the number of multiplexed parallel sub-channels. Numerical results show that a proper design could allow to double the contemporary served users and the overall system throughput
Wide Band Embedded Slot Antennas for Biomedical, Harsh Environment, and Rescue Applications
For many designers, embedded antenna design is a very challenging task when designing embedded systems. Designing Antennas to given set of specifications is typically tailored to efficiently radiate the energy to free space with a certain radiation pattern and operating frequency range, but its design becomes even harder when embedded in multi-layer environment, being conformal to a surface, or matched to a wide range of loads (environments).
In an effort to clarify the design process, we took a closer look at the key considerations for designing an embedded antenna. The design could be geared towards wireless/mobile platforms, wearable antennas, or body area network.
Our group at UT has been involved in developing portable and embedded systems for multi-band operation for cell phones or laptops. The design of these antennas addressed single band/narrowband to multiband/wideband operation and provided over 7 bands within the cellular bands (850 MHz to 2 GHz). Typically the challenge is: many applications require ultra wide band operation, or operate at low frequency. Low frequency operation is very challenging if size is a constraint, and there is a need for demonstrating positive antenna gain
MICROPHONE ARRAY OPTIMIZATION IN IMMERSIVE ENVIRONMENTS
The complex relationship between array gain patterns and microphone distributions limits the application of traditional optimization algorithms on irregular arrays, which show enhanced beamforming performance for human speech capture in immersive environments. This work analyzes the relationship between irregular microphone geometries and spatial filtering performance with statistical methods. Novel geometry descriptors are developed to capture the properties of irregular microphone distributions showing their impact on array performance. General guidelines and optimization methods for regular and irregular array design are proposed in immersive (near-field) environments to obtain superior beamforming ability for speech applications. Optimization times are greatly reduced through the objective function rules using performance-based geometric descriptions of microphone distributions that circumvent direct array gain computations over the space of interest. In addition, probabilistic descriptions of acoustic scenes are introduced to incorporate various levels of prior knowledge for the source distribution. To verify the effectiveness of the proposed optimization methods, simulated gain patterns and real SNR results of the optimized arrays are compared to corresponding traditional regular arrays and arrays obtained from direct exhaustive searching methods. Results show large SNR enhancements for the optimized arrays over arbitrary randomly generated arrays and regular arrays, especially at low microphone densities. The rapid convergence and acceptable processing times observed during the experiments establish the feasibility of proposed optimization methods for array geometry design in immersive environments where rapid deployment is required with limited knowledge of the acoustic scene, such as in mobile platforms and audio surveillance applications
1-D broadside-radiating leaky-wave antenna based on a numerically synthesized impedance surface
A newly-developed deterministic numerical technique for the automated design of metasurface antennas is applied here for the first time to the design of a 1-D printed Leaky-Wave Antenna (LWA) for broadside radiation. The surface impedance synthesis process does not require any a priori knowledge on the impedance pattern, and starts from a mask constraint on the desired far-field and practical bounds on the unit cell impedance values. The designed reactance surface for broadside radiation exhibits a non conventional patterning; this highlights the merit of using an automated design process for a design well known to be challenging for analytical methods. The antenna is physically implemented with an array of metal strips with varying gap widths and simulation results show very good agreement with the predicted performance
Beam scanning by liquid-crystal biasing in a modified SIW structure
A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium
Asynchronous RIS-assisted Localization: A Comprehensive Analysis of Fundamental Limits
The reconfigurable intelligent surface (RIS) has drawn considerable attention
for its ability to enhance the performance of not only the wireless
communication but also the indoor localization with low-cost. This paper
investigates the performance limits of the RIS-based near-field localization in
the asynchronous scenario, and analyzes the impact of each part of the cascaded
channel on the localization performance. The Fisher information matrix (FIM)
and the position error bound (PEB) are derived. Besides, we also derive the
equivalent Fisher information (EFI) for the position-related intermediate
parameters. Enabled by the derived EFI, we verify that both the ranging and
bearing information of the user can be obtained when the near-field model is
considered for the RIS-User equipment (UE) part of the channel, while only the
direction of the UE can be inferred in the far-field scenario. This result is
well known in the scenario that the curvature of arrival (COA) is directly
sensed by the traditional active large-scale array, and we prove that it still
holds when the COA is sensed passively by the large RIS. For the base station
(BS)-RIS part of the channel, we reveal that this part of the channel
determines the type of the gain provided by the BS antenna array. Besides, in
the single-carrier, single snapshot case, it requires both the BS-RIS and the
RIS-UE part of the channel works in the near-field scenario to localize the UE.
We also show that the well-known focusing control scheme for RIS, which
maximizes the received SNR, is not always a good choice and may degrade the
localization performance in the asynchronous scenario. The simulation results
validate the analytic work. The impact of the focusing control scheme on the
PEB performances under synchronous and asynchronous conditions is also
investigated
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