55 research outputs found
ERSVC: An Efficient Routing Scheme for Satellite Constellation Adapting Vector Composition
AbstractCompared with GEO and MEO satellites, LEO satellite constellation is able to provide low-latency, broadband communications which is difficult to be provided by the GEO or MEO satellites. However, one of the challenges in LEO constellation is the development of an efficient and specialized routing scheme. This paper takes transmission rate and data transmission time into consideration, and proposes ERSVC, an efficient routing scheme for satellite constellation adapting vector composition. ERSVC reduces routing table computation complexity, and saves restricted satellite resources. By adapting vector composition method, the amount of data flowing into satellite constellation is maximized while the data traffic is well controlled. Correlative and comprehensive simulation indicates that ERSVC is superior to existing schemes for LEO satellite constellation, especially in balancing data flow
Hybrid Digital-Wave Domain Channel Estimator for Stacked Intelligent Metasurface Enabled Multi-User MISO Systems
Stacked intelligent metasurface (SIM) is an emerging programmable metasurface
architecture that can implement signal processing directly in the
electromagnetic wave domain, thereby enabling efficient implementation of
ultra-massive multiple-input multiple-output (MIMO) transceivers with a limited
number of radio frequency (RF) chains. Channel estimation (CE) is challenging
for SIM-enabled communication systems due to the multi-layer architecture of
SIM, and because we need to estimate large dimensional channels between the SIM
and users with a limited number of RF chains. To efficiently solve this
problem, we develop a novel hybrid digital-wave domain channel estimator, in
which the received training symbols are first processed in the wave domain
within the SIM layers, and then processed in the digital domain. The wave
domain channel estimator, parametrized by the phase shifts applied by the
meta-atoms in all layers, is optimized to minimize the mean squared error (MSE)
using a gradient descent algorithm, within which the digital part is optimally
updated. For an SIM-enabled multi-user system equipped with 4 RF chains and a
6-layer SIM with 64 meta-atoms each, the proposed estimator yields an MSE that
is very close to that achieved by fully digital CE in a massive MIMO system
employing 64 RF chains. This high CE accuracy is achieved at the cost of a
training overhead that can be reduced by exploiting the potential low rank of
channel correlation matrices
Stacked Intelligent Metasurfaces for Multiuser Beamforming in the Wave Domain
Reconfigurable intelligent surface has recently emerged as a promising
technology for shaping the wireless environment by leveraging massive low-cost
reconfigurable elements. Prior works mainly focus on a single-layer metasurface
that lacks the capability of suppressing multiuser interference. By contrast,
we propose a stacked intelligent metasurface (SIM)-enabled transceiver design
for multiuser multiple-input single-output downlink communications.
Specifically, the SIM is endowed with a multilayer structure and is deployed at
the base station to perform transmit beamforming directly in the
electromagnetic wave domain. As a result, an SIM-enabled transceiver overcomes
the need for digital beamforming and operates with low-resolution
digital-to-analog converters and a moderate number of radio-frequency chains,
which significantly reduces the hardware cost and energy consumption, while
substantially decreasing the precoding delay benefiting from the processing
performed in the wave domain. To leverage the benefits of SIM-enabled
transceivers, we formulate an optimization problem for maximizing the sum rate
of all the users by jointly designing the transmit power allocated to them and
the analog beamforming in the wave domain. Numerical results based on a
customized alternating optimization algorithm corroborate the effectiveness of
the proposed SIM-enabled analog beamforming design as compared with various
benchmark schemes. Most notably, the proposed analog beamforming scheme is
capable of substantially decreasing the precoding delay compared to its digital
counterpart.Comment: 6 pages, 4 figures, accepted by IEEE ICC 202
K-Means Based Constellation Optimization for Index Modulated Reconfigurable Intelligent Surfaces
Reconfigurable intelligent surface (RIS) has recently emerged as a promising
technology enabling next-generation wireless networks. In this letter, we
develop an improved index modulation (IM) scheme by utilizing RIS to convey
information. Specifically, we study an RIS-aided multiple-input single-output
(MISO) system, in which the information bits are conveyed by reflection
patterns of RIS rather than the conventional amplitude-phase constellation.
Furthermore, the K-means algorithm is employed to optimize the reflection
constellation to improve the error performance. Also, we propose a generalized
Gray coding method for mapping information bits to an appropriate reflection
constellation and analytically evaluate the error performance of the proposed
scheme by deriving a tight upper bound of the average bit error rate (BER).
Finally, numerical results verify the accuracy of our theoretical analysis as
well as the substantially improved BER performance of the proposed RIS-based IM
transmission scheme.Comment: 5 pages, 3 figures, accepted by IEEE C
Stacked Intelligent Metasurfaces for Multiuser Downlink Beamforming in the Wave Domain
Intelligent metasurface has recently emerged as a promising technology that
enables the customization of wireless environments by harnessing large numbers
of inexpensive configurable scattering elements. However, prior studies have
predominantly focused on single-layer metasurfaces, which have limitations in
terms of the number of beam patterns they can steer accurately due to practical
hardware restrictions. In contrast, this paper introduces a novel stacked
intelligent metasurface (SIM) design. Specifically, we investigate the
integration of SIM into the downlink of a multiuser multiple-input
single-output (MISO) communication system, where a SIM, consisting of a
multilayer metasurface structure, is deployed at the base station (BS) to
facilitate transmit beamforming in the electromagnetic wave domain. This
eliminates the need for conventional digital beamforming and high-resolution
digital-to-analog converters at the BS. To this end, we formulate an
optimization problem that aims to maximize the sum rate of all user equipments
by jointly optimizing the transmit power allocation at the BS and the
wave-based beamforming at the SIM, subject to both the transmit power budget
and discrete phase shift constraints. Furthermore, we propose a computationally
efficient algorithm for solving this joint optimization problem and elaborate
on the potential benefits of employing SIM in wireless networks. Finally, the
numerical results corroborate the effectiveness of the proposed SIM-enabled
wave-based beamforming design and evaluate the performance improvement achieved
by the proposed algorithm compared to various benchmark schemes. It is
demonstrated that considering the same number of transmit antennas, the
proposed SIM-based system achieves about 200\% improvement in terms of sum rate
compared to conventional MISO systems.Comment: 32 pages, 6 figures, submitted to IEEE TW
Revealing two radio active galactic nuclei extremely near PSR J04374715
Newton's gravitational constant may vary with time at an extremely low
level. The time variability of will affect the orbital motion of a
millisecond pulsar in a binary system and cause a tiny difference between the
orbital period-dependent measurement of the kinematic distance and the direct
measurement of the annual parallax distance. PSR J04374715 is the nearest
millisecond pulsar and the brightest at radio. To explore the feasibility of
achieving a parallax distance accuracy of one light-year, comparable to the
recent timing result, with the technique of differential astrometry, we
searched for compact radio sources quite close to PSR J04374715. Using
existing data from the Very Large Array and the Australia Telescope Compact
Array, we detected two sources with flat spectra, relatively stable flux
densities of 0.9 and 1.0 mJy at 8.4 GHz and separations of 13 and 45 arcsec.
With a network consisting of the Long Baseline Array and the Kunming 40-m radio
telescope, we found that both sources have a point-like structure and a
brightness temperature of 10 K. According to these radio inputs and
the absence of counterparts in the other bands, we argue that they are most
likely the compact radio cores of extragalactic active galactic nuclei rather
than Galactic radio stars. The finding of these two radio active galactic
nuclei will enable us to achieve a sub-pc distance accuracy with the in-beam
phase-referencing very-long-baseline interferometric observations and provide
one of the most stringent constraints on the time variability of in the
near future.Comment: 9 pages, 3 tables, 3 figures. Accepted for publication in MNRA
Toward Beamfocusing-Aided Near-Field Communications: Research Advances, Potential, and Challenges
Next-generation mobile networks promise to support high throughput, massive
connectivity, and improved energy efficiency. To achieve these ambitious goals,
extremely large-scale antenna arrays (ELAAs) and terahertz communications
constitute a pair of promising technologies. This will result in future
wireless communications occurring in the near-field regions. To accurately
portray the channel characteristics of near-field wireless propagation,
spherical wavefront-based models are required and present both opportunities as
well as challenges. Following the basics of near-field communications (NFC), we
contrast it to conventional far-field communications. Moreover, we cover the
key challenges of NFC, including its channel modeling and estimation,
near-field beamfocusing, as well as hardware design. Our numerical results
demonstrate the potential of NFC in improving the spatial multiplexing gain and
positioning accuracy. Finally, a suite of open issues are identified for
motivating future research.Comment: 8 pages, 5 figures, 1 tabl
Low-complexity channel estimation and passive beamforming for RIS-assisted MIMO systems relying on discrete phase shifts
Reconfigurable intelligent surfaces (RISs) are capable of enhancing the capacity of wireless networks at a low cost. In practical RIS-assisted communication systems, the acquisition of channel state information (CSI) and RIS reflection optimization constitute a pair of challenges. In this paper, a low complexity channel estimation and passive beamforming design is proposed. First of all, we conceive a low-complexity framework for maximizing the achievable rate of RIS-assisted multiple input multiple-output (MIMO) systems having discrete phase shifts at each RIS element. In contrast to existing solutions, the proposed arrangement partitions the channel training stage into several phases, where the RIS reflection coefficients are pre-designed and the effective superposed channel is estimated instead of separately training the source-destination and source-RIS-destination links. Based on this, the active beamformer can be designed at low complexity and the RIS reflection optimization is performed by selecting that one from the pre-designed training set which maximizes the achievable rate. Secondly, we propose novel techniques for generating the training set of RIS reflection coefficients. The theoretical performance of the proposed scheme is analyzed and compared to the optimal RIS configuration. Finally, our simulation results demonstrate that the proposed framework is more competitive than its existing counterparts when relying on imperfect CSI, especially for rapidly time varying channels having short channel coherence time
The achievable rate analysis of generalized quadrature spatial modulation and a pair of low-complexity detectors
Generalized quadrature spatial modulation (GQSM) seamlessly amalgamates the generalized spatial modulation (GSM), quadrature spatial modulation (QSM) and vertical Bell Laboratories layered space-time (V-BLAST) techniques. In contrast to traditional multiple-input multiple-output (MIMO) schemes transmitting information bits only through the constellation defined on the complex plane, GQSM transmits additional information bits implicitly by selecting the transmit antenna (TA) activation pattern. The philosophy of QSM is that of separating the indices of the real and the imaginary parts of the transmit symbols, which increases the attainable throughput.In this paper, we analyze the inherent benefits of GQSM in terms of throughput. More specifically, we first derive the achievable rate expression of the recent GQSM scheme and unveil the condition for GQSM to approach the maximum rate. Furthermore, we compare the rate of the GQSM scheme to that of other transmission schemes and reveal the conditions for the GQSM’s maximum throughput to exceed that of the benchmark schemes. Simulation results show that when the number of TAsis 40 and quadrature phase shift keying (QPSK) is adopted, the rate of the GQSM scheme may reach 150% of that of V-BLAST. Additionally, a pair of low-complexity detectors are conceived one based on ordered successive interference cancellation (OSIC) and another one on the orthogonal matching pursuit (OMP) algorithm
Low-complexity improved-rate generalised spatial modulation: Bit-to-symbol mapping detection and performance analysis
Low-complexity improved-rate generalised spatial modulation (LCIR-GSM) is proposed to mitigate the high complexity of the mapping book design and demodulation of variable GSM. Specifically, first of all, we propose two efficient schemes for mapping the information bits to the transmit antenna activation patterns, which can be readily scaled to massive MIMO setups. Secondly, we derive a pair of low-complexity near-optimal detectors, one of which has a reduced search scope, while the other benefits from a decoupled single-stream based signal detection algorithm. Finally, the performance of the proposed LCIR-GSM system is characterised by the improved error probability upper bound. Our simulation results confirm the improved error performance of our proposed scheme, despite its reduced signal detection complexity
- …