6,258 research outputs found
Cluster-based Wireless Energy Transfer for Low Complex Energy Receivers
This paper proposes a novel channel estimation method and a cluster-based
opportunistic scheduling policy, for a wireless energy transfer (WET) system
consisting of multiple low-complex energy receivers (ERs) with limited
processing capabilities. Firstly, in the training stage, the energy transmitter
(ET) obtains a set of Received Signal Strength Indicator (RSSI) feedback values
from all ERs, and these values are used to estimate the channels between the ET
and all ERs. Next, based on the channel estimates, the ERs are grouped into
clusters, and the cluster that has its members closest to its centroid in phase
is selected for dedicated WET. The beamformer that maximizes the minimum
harvested energy among all ERs in the selected cluster is found by solving a
convex optimization problem. All ERs have the same chance of being selected
regardless of their distances from the ET, and hence, this scheduling policy
can be considered to be opportunistic as well as fair. It is shown that the
proposed method achieves significant performance gains over benchmark schemes.Comment: 7 pages, 3 figures, To appear in International Symposium on Modeling
and Optimization in Mobile, Ad Hoc and Wireless Networks (WiOpt), 201
Millimeter Wave MIMO Channel Estimation using Overlapped Beam Patterns and Rate Adaptation
This paper is concerned with the channel estimation problem in Millimeter
wave (mmWave) wireless systems with large antenna arrays. By exploiting the
inherent sparse nature of the mmWave channel, we first propose a fast channel
estimation (FCE) algorithm based on a novel overlapped beam pattern design,
which can increase the amount of information carried by each channel
measurement and thus reduce the required channel estimation time compared to
the existing non-overlapped designs. We develop a maximum likelihood (ML)
estimator to optimally extract the path information from the channel
measurements. Then, we propose a novel rate-adaptive channel estimation (RACE)
algorithm, which can dynamically adjust the number of channel measurements
based on the expected probability of estimation error (PEE). The performance of
both proposed algorithms is analyzed. For the FCE algorithm, an approximate
closed-form expression for the PEE is derived. For the RACE algorithm, a lower
bound for the minimum signal energy-to-noise ratio required for a given number
of channel measurements is developed based on the Shannon-Hartley theorem.
Simulation results show that the FCE algorithm significantly reduces the number
of channel estimation measurements compared to the existing algorithms using
non-overlapped beam patterns. By adopting the RACE algorithm, we can achieve up
to a 6dB gain in signal energy-to-noise ratio for the same PEE compared to the
existing algorithms.Comment: This is an extended version of a paper accepted to appear in IEEE
Transactions on Signal Processin
IEEE 802.11ay based mmWave WLANs: Design Challenges and Solutions
Millimeter-wave (mmWave) with large spectrum available is considered as the
most promising frequency band for future wireless communications. The IEEE
802.11ad and IEEE 802.11ay operating on 60 GHz mmWave are the two most expected
wireless local area network (WLAN) technologies for ultra-high-speed
communications. For the IEEE 802.11ay standard still under development, there
are plenty of proposals from companies and researchers who are involved with
the IEEE 802.11ay task group. In this survey, we conduct a comprehensive review
on the medium access control layer (MAC) related issues for the IEEE 802.11ay,
some cross-layer between physical layer (PHY) and MAC technologies are also
included. We start with MAC related technologies in the IEEE 802.11ad and
discuss design challenges on mmWave communications, leading to some MAC related
technologies for the IEEE 802.11ay. We then elaborate on important design
issues for IEEE 802.11ay. Specifically, we review the channel bonding and
aggregation for the IEEE 802.11ay, and point out the major differences between
the two technologies. Then, we describe channel access and channel allocation
in the IEEE 802.11ay, including spatial sharing and interference mitigation
technologies. After that, we present an in-depth survey on beamforming training
(BFT), beam tracking, single-user multiple-input-multiple-output (SU-MIMO)
beamforming and multi-user multiple-input-multiple-output (MU-MIMO)
beamforming. Finally, we discuss some open design issues and future research
directions for mmWave WLANs. We hope that this paper provides a good
introduction to this exciting research area for future wireless systems.Comment: 27 pages, 33 figures. Accepted for publication in IEEE Communications
Surveys and Tutorial
A Survey of Millimeter Wave (mmWave) Communications for 5G: Opportunities and Challenges
With the explosive growth of mobile data demand, the fifth generation (5G)
mobile network would exploit the enormous amount of spectrum in the millimeter
wave (mmWave) bands to greatly increase communication capacity. There are
fundamental differences between mmWave communications and existing other
communication systems, in terms of high propagation loss, directivity, and
sensitivity to blockage. These characteristics of mmWave communications pose
several challenges to fully exploit the potential of mmWave communications,
including integrated circuits and system design, interference management,
spatial reuse, anti-blockage, and dynamics control. To address these
challenges, we carry out a survey of existing solutions and standards, and
propose design guidelines in architectures and protocols for mmWave
communications. We also discuss the potential applications of mmWave
communications in the 5G network, including the small cell access, the cellular
access, and the wireless backhaul. Finally, we discuss relevant open research
issues including the new physical layer technology, software-defined network
architecture, measurements of network state information, efficient control
mechanisms, and heterogeneous networking, which should be further investigated
to facilitate the deployment of mmWave communication systems in the future 5G
networks.Comment: 17 pages, 8 figures, 7 tables, Journal pape
Symbol-level and Multicast Precoding for Multiuser Multiantenna Downlink: A Survey, Classification and Challenges
Precoding has been conventionally considered as an effective means of
mitigating the interference and efficiently exploiting the available in the
multiantenna downlink channel, where multiple users are simultaneously served
with independent information over the same channel resources. The early works
in this area were focused on transmitting an individual information stream to
each user by constructing weighted linear combinations of symbol blocks
(codewords). However, more recent works have moved beyond this traditional view
by: i) transmitting distinct data streams to groups of users and ii) applying
precoding on a symbol-per-symbol basis. In this context, the current survey
presents a unified view and classification of precoding techniques with respect
to two main axes: i) the switching rate of the precoding weights, leading to
the classes of block- and symbol-level precoding, ii) the number of users that
each stream is addressed to, hence unicast-/multicast-/broadcast- precoding.
Furthermore, the classified techniques are compared through representative
numerical results to demonstrate their relative performance and uncover
fundamental insights. Finally, a list of open theoretical problems and
practical challenges are presented to inspire further research in this area.Comment: Submitted to IEEE Communications Surveys & Tutorial
Electromagnetic Lens-focusing Antenna Enabled Massive MIMO: Performance Improvement and Cost Reduction
Massive multiple-input multiple-output (MIMO) techniques have been recently
advanced to tremendously improve the performance of wireless communication
networks. However, the use of very large antenna arrays at the base stations
(BSs) brings new issues, such as the significantly increased hardware and
signal processing costs. In order to reap the enormous gain of massive MIMO and
yet reduce its cost to an affordable level, this paper proposes a novel system
design by integrating an electromagnetic (EM) lens with the large antenna
array, termed the EM-lens enabled MIMO. The EM lens has the capability of
focusing the power of an incident wave to a small area of the antenna array,
while the location of the focal area varies with the angle of arrival (AoA) of
the wave. Therefore, in practical scenarios where the arriving signals from
geographically separated users have different AoAs, the EM-lens enabled system
provides two new benefits, namely energy focusing and spatial interference
rejection. By taking into account the effects of imperfect channel estimation
via pilot-assisted training, in this paper we analytically show that the
average received signal-to-noise ratio (SNR) in both the single-user and
multiuser uplink transmissions can be strictly improved by the EM-lens enabled
system. Furthermore, we demonstrate that the proposed design makes it possible
to considerably reduce the hardware and signal processing costs with only
slight degradations in performance. To this end, two complexity/cost reduction
schemes are proposed, which are small-MIMO processing with parallel receiver
filtering applied over subgroups of antennas to reduce the computational
complexity, and channel covariance based antenna selection to reduce the
required number of radio frequency (RF) chains. Numerical results are provided
to corroborate our analysis.Comment: 30 pages, 9 figure
Wireless Energy Beamforming Using Signal Strength Feedback
Multiple antenna techniques, that allow energy beamforming, have been looked
upon as a possible candidate for increasing the efficiency of the transfer
process between the energy transmitter (ET) and the energy receiver (ER) in
wireless energy transfer. This paper introduces a novel scheme that facilitates
energy beamforming by utilizing Received Signal Strength Indicator (RSSI)
values to estimate the channel. Firstly, in the training stage, the ET will
transmit sequentially using each beamforming vector in a codebook, which is
pre-defined using a Cramer-Rao lower bound analysis. The RSSI value
corresponding to each beamforming vector is fed back to the ET, and these
values are used to estimate the channel through a maximum likelihood analysis.
The results that are obtained are remarkably simple, requires minimal
processing, and can be easily implemented. Also, the results are general and
hold for all well known fading models. The paper also validates the analytical
results numerically, as well as experimentally, and it is shown that the
proposed method achieves impressive results in wireless energy transfer.Comment: 6 pages, 6 figures, in Proc. IEEE Global Communications Conference
(GLOBECOM), 201
MAC Protocols for Terahertz Communication: A Comprehensive Survey
Terahertz communication is emerging as a future technology to support
Terabits per second link with highlighting features as high throughput and
negligible latency. However, the unique features of the Terahertz band such as
high path loss, scattering and reflection pose new challenges and results in
short communication distance. The antenna directionality, in turn, is required
to enhance the communication distance and to overcome the high path loss.
However, these features in combine negate the use of traditional Medium access
protocols. Therefore novel MAC protocol designs are required to fully exploit
their potential benefits including efficient channel access, control message
exchange, link establishment, mobility management, and line-of-sight blockage
mitigation. An in-depth survey of Terahertz MAC protocols is presented in this
paper. The paper highlights the key features of the Terahertz band which should
be considered while designing an efficient Terahertz MAC protocol, and the
decisions which if taken at Terahertz MAC layer can enhance the network
performance. Different Terahertz applications at macro and nano scales are
highlighted with design requirements for their MAC protocols. The MAC protocol
design issues and considerations are highlighted. Further, the existing MAC
protocols are also classified based on network topology, channel access
mechanisms, and link establishment strategies as Transmitter and Receiver
initiated communication. The open challenges and future research directions on
Terahertz MAC protocols are also highlighted.Comment: Submitted to IEEE Communication Surveys and Tutorials Journa
Joint Spatial Division and Multiplexing for mm-Wave Channels
Massive MIMO systems are well-suited for mm-Wave communications, as large
arrays can be built with reasonable form factors, and the high array gains
enable reasonable coverage even for outdoor communications. One of the main
obstacles for using such systems in frequency-division duplex mode, namely the
high overhead for the feedback of channel state information (CSI) to the
transmitter, can be mitigated by the recently proposed JSDM (Joint Spatial
Division and Multiplexing) algorithm. In this paper we analyze the performance
of this algorithm in some realistic propagation channels that take into account
the partial overlap of the angular spectra from different users, as well as the
sparsity of mm-Wave channels. We formulate the problem of user grouping for two
different objectives, namely maximizing spatial multiplexing, and maximizing
total received power, in a graph-theoretic framework. As the resulting problems
are numerically difficult, we proposed (sub optimum) greedy algorithms as
efficient solution methods. Numerical examples show that the different
algorithms may be superior in different settings.We furthermore develop a new,
"degenerate" version of JSDM that only requires average CSI at the transmitter,
and thus greatly reduces the computational burden. Evaluations in propagation
channels obtained from ray tracing results, as well as in measured outdoor
channels show that this low-complexity version performs surprisingly well in
mm-Wave channels.Comment: Accepted for publication in "JSAC Special Issue in 5G Communication
Systems
Efficient Downlink Channel Probing and Uplink Feedback in FDD Massive MIMO Systems
Massive Multiple-Input Multiple-Output (massive MIMO) is a variant of
multi-user MIMO in which the number of antennas at each Base Station (BS) is
very large and typically much larger than the number of users simultaneously
served. Massive MIMO can be implemented with Time Division Duplexing (TDD) or
Frequency Division Duplexing (FDD) operation. FDD massive MIMO systems are
particularly desirable due to their implementation in current wireless networks
and their efficiency in situations with symmetric traffic and delay-sensitive
applications. However, implementing FDD massive MIMO systems is known to be
challenging since it imposes a large feedback overhead in the Uplink (UL) to
obtain channel state information for the Downlink (DL). In recent years, a
considerable amount of research is dedicated to developing methods to reduce
the feedback overhead in such systems. In this paper, we use the sparse spatial
scattering properties of the environment to achieve this goal. The idea is to
estimate the support of the continuous, frequency-invariant scattering function
from UL channel observations and use this estimate to obtain the support of the
DL channel vector via appropriate interpolation. We use the resulting support
estimate to design an efficient DL probing and UL feedback scheme in which the
feedback dimension scales proportionally with the sparsity order of DL channel
vectors. Since the sparsity order is much less than the number of BS antennas
in almost all practically relevant scenarios, our method incurs much less
feedback overhead compared with the currently proposed methods in the
literature, such as those based on compressed-sensing. We use numerical
simulations to assess the performance of our probing-feedback algorithm and
compare it with these methods.Comment: 24 pages, 10 figure
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