797 research outputs found
Continuous Analog Channel Estimation Aided Beamforming for Massive MIMO Systems
Analog beamforming greatly reduces the implementation cost of massive antenna
transceivers by using only one up/down-conversion chain. However, it incurs a
large pilot overhead when used with conventional channel estimation (CE)
techniques. This is because these CE techniques involve digital processing,
requiring the up/down-conversion chain to be time-multiplexed across the
antenna dimensions. This paper introduces a novel CE technique, called
continuous analog channel estimation (CACE), that avoids digital processing,
enables analog beamforming at the receiver and additionally provides resilience
against oscillator phase-noise. By avoiding time-multiplexing of
up/down-conversion chains, the CE overhead is reduced significantly and
furthermore becomes independent of the number of antenna elements. In CACE, a
reference tone is transmitted continuously with the data signals, and the
receiver uses the received reference signal as a matched filter for combining
the data signals, albeit via analog processing. We propose a receiver
architecture for CACE, analyze its performance in the presence of oscillator
phase-noise, and derive near-optimal system parameters and power allocation.
Transmit beamforming and initial access procedure with CACE are also discussed.
Simulations confirm that, in comparison to conventional CE, CACE provides
phase-noise resilience and a significant reduction in the CE overhead, while
suffering only a small loss in signal-to-interference-plus-noise-ratio.Comment: Accepted to IEEE Transactions on Wireless Communications, 201
Two-Timescale Hybrid Compression and Forward for Massive MIMO Aided C-RAN
We consider the uplink of a cloud radio access network (C-RAN), where massive
MIMO remote radio heads (RRHs) serve as relays between users and a centralized
baseband unit (BBU). Although employing massive MIMO at RRHs can improve the
spectral efficiency, it also significantly increases the amount of data
transported over the fronthaul links between RRHs and BBU, which becomes a
performance bottleneck. Existing fronthaul compression methods for conventional
C-RAN are not suitable for the massive MIMO regime because they require
fully-digital processing and/or real-time full channel state information (CSI),
incurring high implementation cost for massive MIMO RRHs. To overcome this
challenge, we propose to perform a two-timescale hybrid analog-and-digital
spatial filtering at each RRH to reduce the fronthaul consumption.
Specifically, the analog filter is adaptive to the channel statistics to
achieve massive MIMO array gain, and the digital filter is adaptive to the
instantaneous effective CSI to achieve spatial multiplexing gain. Such a design
can alleviate the performance bottleneck of limited fronthaul with reduced
hardware cost and power consumption, and is more robust to the CSI delay. We
propose an online algorithm for the two-timescale non-convex optimization of
analog and digital filters, and establish its convergence to stationary
solutions. Finally, simulations verify the advantages of the proposed scheme.Comment: 15 pages, 8 figures, accepted by IEEE Transactions on Signal
Processin
Joint Channel-Estimation/Decoding with Frequency-Selective Channels and Few-Bit ADCs
We propose a fast and near-optimal approach to joint channel-estimation,
equalization, and decoding of coded single-carrier (SC) transmissions over
frequency-selective channels with few-bit analog-to-digital converters (ADCs).
Our approach leverages parametric bilinear generalized approximate message
passing (PBiGAMP) to reduce the implementation complexity of joint channel
estimation and (soft) symbol decoding to that of a few fast Fourier transforms
(FFTs). Furthermore, it learns and exploits sparsity in the channel impulse
response. Our work is motivated by millimeter-wave systems with bandwidths on
the order of Gsamples/sec, where few-bit ADCs, SC transmissions, and fast
processing all lead to significant reductions in power consumption and
implementation cost. We numerically demonstrate our approach using signals and
channels generated according to the IEEE 802.11ad wireless local area network
(LAN) standard, in the case that the receiver uses analog beamforming and a
single ADC
Terahertz Multi-User Massive MIMO with Intelligent Reflecting Surface: Beam Training and Hybrid Beamforming
Terahertz (THz) communications open a new frontier for the wireless network
thanks to their dramatically wider available bandwidth compared to the current
micro-wave and forthcoming millimeter-wave communications. However, due to the
short length of THz waves, they also suffer from severe path attenuation and
poor diffraction. To compensate the THz-induced propagation loss, this paper
proposes to combine two promising techniques, viz., massive multiple input
multiple output (MIMO) and intelligent reflecting surface (IRS), in THz
multi-user communications, considering their significant beamforming and
aperture gains. Nonetheless, channel estimation and low-cost beamforming turn
out to be two main obstacles to realizing this combination, due to the
passivity of IRS for sending/receiving pilot signals and the large-scale use of
expensive RF chains in massive MIMO. In view of these limitations, this paper
first develops a cooperative beam training scheme to facilitate the channel
estimation with IRS. In particular, we design two different hierarchical
codebooks for the proposed training procedure, which are able to balance
between the robustness against noise and searching complexity. Based on the
training results, we further propose two cost-efficient hybrid beamforming (HB)
designs for both single-user and multi-user scenarios, respectively. Simulation
results demonstrate that the proposed joint beam training and HB scheme is able
to achieve close performance to the optimal fully digital beamforming (FDB)
which is implemented even under perfect channel state information (CSI)
Towards Smart and Reconfigurable Environment: Intelligent Reflecting Surface Aided Wireless Network
Although the fifth-generation (5G) technologies will significantly improve
the spectrum and energy efficiency of today's wireless communication networks,
their high complexity and hardware cost as well as increasingly more energy
consumption are still crucial issues to be solved. Furthermore, despite that
such technologies are generally capable of adapting to the space and time
varying wireless environment, the signal propagation over it is essentially
random and largely uncontrollable. Recently, intelligent reflecting surface
(IRS) has been proposed as a revolutionizing solution to address this open
issue, by smartly reconfiguring the wireless propagation environment with the
use of massive low-cost, passive, reflective elements integrated on a planar
surface. Specifically, different elements of an IRS can independently reflect
the incident signal by controlling its amplitude and/or phase and thereby
collaboratively achieve fine-grained three-dimensional (3D) passive beamforming
for signal enhancement or cancellation. In this article, we provide an overview
of the IRS technology, including its main applications in wireless
communication, competitive advantages over existing technologies, hardware
architecture as well as the corresponding new signal model. We focus on the key
challenges in designing and implementing the new IRS-aided hybrid (with both
active and passive components) wireless network, as compared to the traditional
network comprising active components only. Furthermore, numerical results are
provided to show the potential for significant performance enhancement with the
use of IRS in typical wireless network scenarios.Comment: A short version of this work was accepted by IEEE Communications
Magazine. Several other technical works on Beamforming, discrete phase
shifts, wireless power transfer are available at
https://elewuqq.wixsite.com/mysit
Periodic Analog Channel Estimation Aided Beamforming for Massive MIMO Systems
Analog beamforming is an attractive and cost-effective solution to exploit
the benefits of massive multiple-input-multiple-output systems, by requiring
only one up/down-conversion chain. However, the presence of only one chain
imposes a significant overhead in estimating the channel state information
required for beamforming, when conventional digital channel estimation (CE)
approaches are used. As an alternative, this paper proposes a novel CE
technique, called periodic analog CE (PACE), that can be performed by analog
hardware. By avoiding digital processing, the estimation overhead is
significantly lowered and does not scale with number of antennas. PACE involves
periodic transmission of a sinusoidal reference signal by the transmitter,
estimation of its amplitude and phase at each receive antenna via analog
hardware, and using these estimates for beamforming. To enable such non-trivial
operation, two reference tone recovery techniques and a novel receiver
architecture for PACE are proposed and analyzed, both theoretically and via
simulations. Results suggest that in sparse, wide-band channels and above a
certain signal-to-noise ratio, PACE aided beamforming suffers only a small loss
in beamforming gain and enjoys a much lower CE overhead, in comparison to
conventional approaches. Benefits of using PACE aided beamforming during the
initial access phase are also discussed.Comment: Accepted to IEEE Transactions on Wireless Communications, 201
Fully-/Partially-Connected Hybrid Beamforming Architectures for mmWave MU-MIMO
Hybrid digital analog (HDA) beamforming has attracted considerable attention
in practical implementation of millimeter wave (mmWave) multiuser
multiple-input multiple-output (MU-MIMO) systems due to the low power
consumption with respect to its fully digital baseband counterpart. The
implementation cost, performance, and power efficiency of HDA beamforming
depends on the level of connectivity and reconfigurability of the analog
beamforming network. In this paper, we investigate the performance of two
typical architectures that can be regarded as extreme cases, namely, the
fully-connected (FC) and the one-stream-per-subarray (OSPS) architectures. In
the FC architecture each RF antenna port is connected to all antenna elements
of the array, while in the OSPS architecture the RF antenna ports are connected
to disjoint subarrays. We jointly consider the initial beam acquisition and
data communication phases, such that the latter takes place by using the beam
direction information obtained by the former. We use the state-of-the-art beam
alignment (BA) scheme previously proposed by the authors and consider a family
of MU-MIMO precoding schemes well adapted to the beam information extracted
from the BA phase. We also evaluate the power efficiency of the two HDA
architectures taking into account the power dissipation at different hardware
components as well as the power backoff under typical power amplifier
constraints. Numerical results show that the two architectures achieve similar
sum spectral efficiency, while the OSPS architecture is advantageous with
respect to the FC case in terms of hardware complexity and power efficiency, at
the sole cost of a slightly longer BA time-to-acquisition due to its reduced
beam angle resolution
Millimeter Wave Beam-Selection Using Out-of-Band Spatial Information
Millimeter wave (mmWave) communication is one feasible solution for high
data-rate applications like vehicular-to-everything communication and next
generation cellular communication. Configuring mmWave links, which can be done
through channel estimation or beam-selection, however, is a source of
significant overhead. In this paper, we propose to use spatial information
extracted at sub-6 GHz to help establish the mmWave link. First, we review the
prior work on frequency dependent channel behavior and outline a simulation
strategy to generate multi-band frequency dependent channels. Second, assuming:
(i) narrowband channels and a fully digital architecture at sub-6 GHz; and (ii)
wideband frequency selective channels, OFDM signaling, and an analog
architecture at mmWave, we outline strategies to incorporate sub-6 GHz spatial
information in mmWave compressed beam selection. We formulate compressed
beam-selection as a weighted sparse signal recovery problem, and obtain the
weighting information from sub-6 GHz channels. In addition, we outline a
structured precoder/combiner design to tailor the training to out-of-band
information. We also extend the proposed out-of-band aided compressed
beam-selection approach to leverage information from all active OFDM
subcarriers. The simulation results for achievable rate show that out-of-band
aided beam-selection can reduce the training overhead of in-band only
beam-selection by 4x.Comment: 30 pages, 11 figure
Performance Analysis of Multi-Cell Millimeter Wave Massive MIMO Networks with Low-Precision ADCs
In this paper, we investigate a multi-cell millimeter wave (mmWave) massive
multiple-input multiple-output (MIMO) network with low-precision
analog-to-digital converters (ADCs) at the base station (BS). Each cell serves
multiple users and each user is equipped with multiple antennas but driven by a
single RF chain. We first introduce a channel estimation strategy for the
mmWave massive MIMO network and analyze the achievable rate with imperfect
channel state information. Then, we derive an insightful lower bound for the
achievable rate, which becomes tight with a growing number of users. The bound
clearly demonstrates the impacts of the number of antennas and the ADC
precision, especially for a single-cell mmWave network at low signal-to-noise
ratio (SNR). It characterizes the tradeoff among various system parameters. Our
analytical results are finally confirmed by extensive computer simulations.Comment: 16 pages, 9 figure
Artificial Intelligence-Defined 5G Radio Access Networks
Massive multiple-input multiple-output antenna systems, millimeter wave
communications, and ultra-dense networks have been widely perceived as the
three key enablers that facilitate the development and deployment of 5G
systems. This article discusses the intelligent agent in 5G base station which
combines sensing, learning, understanding and optimizing to facilitate these
enablers. We present a flexible, rapidly deployable, and cross-layer artificial
intelligence (AI)-based framework to enable the imminent and future demands on
5G and beyond infrastructure. We present example AI-enabled 5G use cases that
accommodate important 5G-specific capabilities and discuss the value of AI for
enabling beyond 5G network evolution
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