836 research outputs found
Massive MIMO is a Reality -- What is Next? Five Promising Research Directions for Antenna Arrays
Massive MIMO (multiple-input multiple-output) is no longer a "wild" or
"promising" concept for future cellular networks - in 2018 it became a reality.
Base stations (BSs) with 64 fully digital transceiver chains were commercially
deployed in several countries, the key ingredients of Massive MIMO have made it
into the 5G standard, the signal processing methods required to achieve
unprecedented spectral efficiency have been developed, and the limitation due
to pilot contamination has been resolved. Even the development of fully digital
Massive MIMO arrays for mmWave frequencies - once viewed prohibitively
complicated and costly - is well underway. In a few years, Massive MIMO with
fully digital transceivers will be a mainstream feature at both sub-6 GHz and
mmWave frequencies. In this paper, we explain how the first chapter of the
Massive MIMO research saga has come to an end, while the story has just begun.
The coming wide-scale deployment of BSs with massive antenna arrays opens the
door to a brand new world where spatial processing capabilities are
omnipresent. In addition to mobile broadband services, the antennas can be used
for other communication applications, such as low-power machine-type or
ultra-reliable communications, as well as non-communication applications such
as radar, sensing and positioning. We outline five new Massive MIMO related
research directions: Extremely large aperture arrays, Holographic Massive MIMO,
Six-dimensional positioning, Large-scale MIMO radar, and Intelligent Massive
MIMO.Comment: 20 pages, 9 figures, submitted to Digital Signal Processin
Full-Duplex Wireless for 6G: Progress Brings New Opportunities and Challenges
The use of in-band full-duplex (FD) enables nodes to simultaneously transmit
and receive on the same frequency band, which challenges the traditional
assumption in wireless network design. The full-duplex capability enhances
spectral efficiency and decreases latency, which are two key drivers pushing
the performance expectations of next-generation mobile networks. In less than
ten years, in-band FD has advanced from being demonstrated in research labs to
being implemented in standards and products, presenting new opportunities to
utilize its foundational concepts. Some of the most significant opportunities
include using FD to enable wireless networks to sense the physical environment,
integrate sensing and communication applications, develop integrated access and
backhaul solutions, and work with smart signal propagation environments powered
by reconfigurable intelligent surfaces. However, these new opportunities also
come with new challenges for large-scale commercial deployment of FD
technology, such as managing self-interference, combating cross-link
interference in multi-cell networks, and coexistence of dynamic time division
duplex, subband FD and FD networks.Comment: 21 pages, 15 figures, accepted to an IEEE Journa
Integrated Sensing and Communications with Reconfigurable Intelligent Surfaces
Integrated sensing and communications (ISAC) are envisioned to be an integral
part of future wireless networks, especially when operating at the
millimeter-wave (mmWave) and terahertz (THz) frequency bands. However,
establishing wireless connections at these high frequencies is quite
challenging, mainly due to the penetrating pathloss that prevents reliable
communication and sensing. Another emerging technology for next-generation
wireless systems is reconfigurable intelligent surfaces (RISs), which are
capable of modifying harsh propagation environments. RISs are the focus of
growing research and industrial attention, bringing forth the vision of smart
and programmable signal propagation environments. In this article, we provide a
tutorial-style overview of the applications and benefits of RISs for sensing
functionalities in general, and for ISAC systems in particular. We highlight
the potential advantages when fusing these two emerging technologies, and
identify for the first time that: i) joint sensing and communications designs
are most beneficial when the channels referring to these operations are
coupled, and that ii) RISs offer means for controlling this beneficial
coupling. The usefulness of RIS-aided ISAC goes beyond the individual obvious
gains of each of these technologies in both performance and power efficiency.
We also discuss the main signal processing challenges and future research
directions which arise from the fusion of these two emerging technologies.Comment: 37 pages, 9 figure
Joint Radar and Communication Design: Applications, State-of-the-Art, and the Road Ahead
Sharing of the frequency bands between radar and communication systems has attracted substantial attention, as it can avoid under-utilization of otherwise permanently allocated spectral resources, thus improving efficiency. Further, there is increasing demand for radar and communication systems that share the hardware platform as well as the frequency band, as this not only decongests the spectrum, but also benefits both sensing and signaling operations via the full cooperation between both functionalities. Nevertheless, the success of spectrum and hardware sharing between radar and communication systems critically depends on high-quality joint radar and communication designs. In the first part of this paper, we overview the research progress in the areas of radar-communication coexistence and dual-functional radar-communication (DFRC) systems, with particular emphasis on application scenarios and technical approaches. In the second part, we propose a novel transceiver architecture and frame structure for a DFRC base station (BS) operating in the millimeter wave (mmWave) band, using the hybrid analog-digital (HAD) beamforming technique. We assume that the BS is serving a multi-antenna user equipment (UE) over a mmWave channel, and at the same time it actively detects targets. The targets also play the role of scatterers for the communication signal. In that framework, we propose a novel scheme for joint target search and communication channel estimation, which relies on omni-directional pilot signals generated by the HAD structure. Given a fully-digital communication precoder and a desired radar transmit beampattern, we propose to design the analog and digital precoders under non-convex constant-modulus (CM) and power constraints, such that the BS can formulate narrow beams towards all the targets, while pre-equalizing the impact of the communication channel. Furthermore, we design a HAD receiver that can simultaneously process signals from the UE and echo waves from the targets. By tracking the angular variation of the targets, we show that it is possible to recover the target echoes and mitigate the resulting interference to the UE signals, even when the radar and communication signals share the same signal-to-noise ratio (SNR). The feasibility and efficiency of the proposed approaches in realizing DFRC are verified via numerical simulations. Finally, the paper concludes with an overview of the open problems in the research field of communication and radar spectrum sharing (CRSS)
Joint Communication and Sensing in RIS-enabled mmWave Networks
Empowering cellular networks with augmented sensing capabilities is one of
the key research areas in 6G communication systems. Recently, we have witnessed
a plethora of efforts to devise solutions that integrate sensing capabilities
into communication systems, i.e., joint communication and sensing (JCAS).
However, most prior works do not consider the impact of reconfigurable
intelligent surfaces (RISs) on JCAS systems, especially at millimeter-wave
(mmWave) bands. Given that RISs are expected to become an integral part of
cellular systems, it is important to investigate their potential in cellular
networks beyond communication goals. In this paper, we study mmWave orthogonal
frequency-division multiplexing (OFDM) JCAS systems in the presence of RISs.
Specifically, we jointly design the hybrid beamforming and RIS phase shifts to
guarantee the sensing functionalities via minimizing a chordal-distance metric,
subject to signal-to-interference-plus-noise (SINR) and power constraints. The
non-convexity of the investigated problem poses a challenge which we address by
proposing a solution based on the penalty method and manifold-based alternating
direction method of multipliers (ADMM). Simulation results demonstrate that
under various settings both sensing and communication experience improved
performance when the RIS is adequately designed. In addition, we discuss the
tradeoff between sensing and communication
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