363 research outputs found
Fixed-complexity quantum-assisted multi-user detection for CDMA and SDMA
In a system supporting numerous users the complexity of the optimal Maximum Likelihood Multi-User Detector (ML MUD) becomes excessive. Based on the superimposed constellations of K users, the ML MUD outputs the specific multilevel K-user symbol that minimizes the Euclidean distance with respect to the faded and noise-contaminated received multi-level symbol. Explicitly, the Euclidean distance is considered as the Cost Function (CF). In a system supporting K users employing M-ary modulation, the ML MUD uses MK CF evaluations (CFE) per time slot. In this contribution we propose an Early Stopping-aided Durr-HΓΈyer algorithm-based Quantum-assisted MUD (ES-DHA QMUD) based on two techniques for achieving optimal ML detection at a low complexity. Our solution is also capable of flexibly adjusting the QMUD's performance and complexity trade-off, depending on the computing power available at the base station. We conclude by proposing a general design methodology for the ES-DHA QMUD in the context of both CDMA and SDMA systems
OFDMA/SC-FDMA aided space-time shift keying for dispersive multi-user scenarios
Motivated by the recent concept of Space-Time Shift Keying (STSK) developed for achieving a flexible diversity versus multiplexing gain trade-off, we propose a novel Orthogonal Frequency Division Multiple Access (OFDMA)/Single Carrier Frequency Division Multiple Access (SC-FDMA) aided multi-user STSK scheme for frequency-selective channels. The proposed OFDMA/SC-FDMA STSK scheme is capable of providing an improved performance in dispersive channels, while supporting multiple users in a multiple antenna aided wireless system. Furthermore, the scheme has the inherent potential of benefitting from the low-complexity single-stream Maximum-likelihood (ML) detector. Both an uncoded and a sophisticated near-capacity coded OFDMA/SC-FDMA STSK scheme were studied and their performances were compared in multiuser wideband Multiple-Input Multiple-Output (MIMO) scenarios. Explicitly, OFDMA/SC-FDMA aided STSK exhibits an excellent performance even in the presence of channel impairments due to the frequency-selectivity of wideband channels and proves to be a beneficial choice for high capacity multi-user MIMO systems
Rate-Splitting Multiple Access for 6G -- Part I: Principles, Applications and Future Works
This letter is the first part of a three-part tutorial focusing on
rate-splitting multiple access (RSMA) for 6G. As Part I of the tutorial, the
letter presents the basics of RSMA and its applications in light of 6G. To
begin with, we first delineate the design principle and basic transmission
frameworks of downlink and uplink RSMA. We then illustrate the applications of
RSMA for addressing the challenges of various potential enabling technologies
and use cases, consequently making it a promising next generation multiple
access (NGMA) scheme for future networks such as 6G and beyond. We briefly
discuss the challenges of RSMA and conclude the letter. In continuation of Part
I, we will focus on the interplay of RSMA with integrated sensing and
communication, and reconfigurable intelligent surfaces, respectively in Part II
and Part III of this tutorial
Multiuser MIMO-OFDM for Next-Generation Wireless Systems
This overview portrays the 40-year evolution of orthogonal frequency division multiplexing (OFDM) research. The amelioration of powerful multicarrier OFDM arrangements with multiple-input multiple-output (MIMO) systems has numerous benefits, which are detailed in this treatise. We continue by highlighting the limitations of conventional detection and channel estimation techniques designed for multiuser MIMO OFDM systems in the so-called rank-deficient scenarios, where the number of users supported or the number of transmit antennas employed exceeds the number of receiver antennas. This is often encountered in practice, unless we limit the number of users granted access in the base stationβs or radio portβs coverage area. Following a historical perspective on the associated design problems and their state-of-the-art solutions, the second half of this treatise details a range of classic multiuser detectors (MUDs) designed for MIMO-OFDM systems and characterizes their achievable performance. A further section aims for identifying novel cutting-edge genetic algorithm (GA)-aided detector solutions, which have found numerous applications in wireless communications in recent years. In an effort to stimulate the cross pollination of ideas across the machine learning, optimization, signal processing, and wireless communications research communities, we will review the broadly applicable principles of various GA-assisted optimization techniques, which were recently proposed also for employment inmultiuser MIMO OFDM. In order to stimulate new research, we demonstrate that the family of GA-aided MUDs is capable of achieving a near-optimum performance at the cost of a significantly lower computational complexity than that imposed by their optimum maximum-likelihood (ML) MUD aided counterparts. The paper is concluded by outlining a range of future research options that may find their way into next-generation wireless systems
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
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