207 research outputs found
Intelligent Wireless Communications Enabled by Cognitive Radio and Machine Learning
The ability to intelligently utilize resources to meet the need of growing
diversity in services and user behavior marks the future of wireless
communication systems. Intelligent wireless communications aims at enabling the
system to perceive and assess the available resources, to autonomously learn to
adapt to the perceived wireless environment, and to reconfigure its operating
mode to maximize the utility of the available resources. The perception
capability and reconfigurability are the essential features of cognitive radio
while modern machine learning techniques project great potential in system
adaptation. In this paper, we discuss the development of the cognitive radio
technology and machine learning techniques and emphasize their roles in
improving spectrum and energy utility of wireless communication systems. We
describe the state-of-the-art of relevant techniques, covering spectrum sensing
and access approaches and powerful machine learning algorithms that enable
spectrum- and energy-efficient communications in dynamic wireless environments.
We also present practical applications of these techniques and identify further
research challenges in cognitive radio and machine learning as applied to the
existing and future wireless communication systems
Discovery Signal Design and Its Application to Peer-to-Peer Communications in OFDMA Cellular Networks
This paper proposes a unique discovery signal as an enabler of peer-to-peer
(P2P) communication which overlays a cellular network and shares its resources.
Applying P2P communication to cellular network has two key issues: 1.
Conventional ad hoc P2P connections may be unstable since stringent resource
and interference coordination is usually difficult to achieve for ad hoc P2P
communications; 2. The large overhead required by P2P communication may offset
its gain. We solve these two issues by using a special discovery signal to aid
cellular network-supervised resource sharing and interference management
between cellular and P2P connections. The discovery signal, which facilitates
efficient neighbor discovery in a cellular system, consists of un-modulated
tones transmitted on a sequence of OFDM symbols. This discovery signal not only
possesses the properties of high power efficiency, high interference tolerance,
and freedom from near-far effects, but also has minimal overhead. A practical
discovery-signal-based P2P in an OFDMA cellular system is also proposed.
Numerical results are presented which show the potential of improving local
service and edge device performance in a cellular network.Comment: arXiv admin note: text overlap with arXiv:1112.1990, arXiv:1207.0557
add reference in page 5 add text in page 5 for explainatio
Latency-Optimal Uplink Scheduling Policy in Training-based Large-Scale Antenna Systems
In this paper, an uplink scheduling policy problem to minimize the network
latency, defined as the air-time to serve all of users with a
quality-of-service (QoS), under an energy constraint is considered in a
training-based large-scale antenna systems (LSAS) employing a simple linear
receiver. An optimal algorithm providing the exact latency-optimal uplink
scheduling policy is proposed with a polynomial-time complexity. Via numerical
simulations, it is shown that the proposed scheduling policy can provide
several times lower network latency over the conventional ones in realistic
environments. In addition, the proposed scheduling policy and its network
latency are analyzed asymptotically to provide better insights on the system
behavior. Four operating regimes are classified according to the average
received signal quality, , and the number of BS antennas, . It turns
out that orthogonal pilots are optimal only in the regime and . In other regimes ( or ), it turns out
that non-orthogonal pilots become optimal. More rigorously, the use of
non-orthogonal pilots can reduce the network latency by a factor of
when or by a factor of when
and , which would be a critical guideline for designing 5G
future cellular systems.Comment: submitted to IEEE Transactions on Information Theor
Total Energy Efficiency of TD- and FD-MRC Receivers for Massive MIMO Uplink
This paper proposes a detailed investigation on the uplink (UL) performance
of massive multiple-input-multiple-output (maMIMO) systems employing
maximum-ratio combining at the receiver. While most papers in maMIMO literature
assume orthogonal frequency-division multiplexing (OFDM), current standards
like LTE employ single-carrier (SC) waveform in the UL due to several benefits.
We thus perform a systemic comparison between two fundamental schemes: the
time-reversal MRC (TRMRC) operating under SC, and the frequency-domain MRC
(FDMRC) employed with OFDM. It was recently shown that TRMRC outperforms FDMRC
in terms of achievable rates, since SC systems do not require the cyclic prefix
(CP) of OFDM. On the other hand, the computational complexity of TRMRC
algorithm is higher than that of FDMRC, even when efficient solutions are
employed (e.g., fast convolution with the overlap-and-add method). Hence, the
best scheme for the UL maMIMO systems still remains an open question. The main
contribution of this paper is the comparison of the total energy efficiency of
both TRMRC and FDMRC when used in the UL of maMIMO systems. Our results show
that, for current typical system parameters, FDMRC/OFDM achieves a higher total
energy efficiency than TRMRC/SC. However, if the cell radius is below 300m
and/or the computational efficiency increases by 30% regarding the current
processors, the TRMRC under SC waveform becomes more attractive for the UL of
maMIMO systems.Comment: 27 pages, 03 tables, and 08 figure
Simultaneous Wireless Information and Power Transfer Under Different CSI Acquisition Schemes
In this work, we consider a multiple-input single-output system in which an
access point (AP) performs a simultaneous wireless information and power
transfer (SWIPT) to serve a user terminal (UT) that is not equipped with
external power supply. In order to assess the efficacy of the SWIPT, we target
a practically relevant scenario characterized by imperfect channel state
information (CSI) at the transmitter, the presence of penalties associated to
the CSI acquisition procedures, and non-zero power consumption for the
operations performed by the UT, such as CSI estimation, uplink signaling and
data decoding. We analyze three different cases for the CSI knowledge at the
AP: no CSI, and imperfect CSI in case of time-division duplexing and
frequency-division duplexing communications. Closed-form representations of the
ergodic downlink rate and both the energy shortage and data outage probability
are derived for the three cases. Additionally, analytic expressions for the
ergodically optimal duration of power transfer and channel estimation/feedback
phases are provided. Our numerical findings verify the correctness of our
derivations, and also show the importance and benefits of CSI knowledge at the
AP in SWIPT systems, albeit imperfect and acquired at the expense of the time
available for the information transfer
Optimal User-Cell Association for Massive MIMO Wireless Networks
The use of a very large number of antennas at each base station site
(referred to as "Massive MIMO") is one of the most promising approaches to cope
with the predicted wireless data traffic explosion. In combination with Time
Division Duplex and with simple per-cell processing, it achieves large
throughput per cell, low latency, and attractive power efficiency performance.
Following the current wireless technology trend of moving to higher frequency
bands and denser small cell deployments, a large number of antennas can be
implemented within a small form factor even in small cell base stations. In a
heterogeneous network formed by large (macro) and small cell BSs, a key system
optimization problem consists of "load balancing", that is, associating users
to BSs in order to avoid congested hot-spots and/or under-utilized
infrastructure. In this paper, we consider the user-BS association problem for
a massive MIMO heterogeneous network. We formulate the problem as a network
utility maximization, and provide a centralized solution in terms of the
fraction of transmission resources (time-frequency slots) over which each user
is served by a given BS. Furthermore, we show that such a solution is
physically realizable, i.e., there exists a sequence of integer scheduling
configurations realizing (by time-sharing) the optimal fractions. While this
solution is optimal, it requires centralized computation. Then, we also
consider decentralized user-centric schemes, formulated as non-cooperative
games where each user makes individual selfish association decisions based only
on its local information. We identify a class of schemes such that their Nash
equilibrium is very close to the global centralized optimum. Hence, these
user-centric algorithms are attractive not only for their simplicity and fully
decentralized implementation, but also because they operate near the system
"social" optimum
Massive MIMO for Internet of Things (IoT) Connectivity
Massive MIMO is considered to be one of the key technologies in the emerging
5G systems, but also a concept applicable to other wireless systems. Exploiting
the large number of degrees of freedom (DoFs) of massive MIMO essential for
achieving high spectral efficiency, high data rates and extreme spatial
multiplexing of densely distributed users. On the one hand, the benefits of
applying massive MIMO for broadband communication are well known and there has
been a large body of research on designing communication schemes to support
high rates. On the other hand, using massive MIMO for Internet-of-Things (IoT)
is still a developing topic, as IoT connectivity has requirements and
constraints that are significantly different from the broadband connections. In
this paper we investigate the applicability of massive MIMO to IoT
connectivity. Specifically, we treat the two generic types of IoT connections
envisioned in 5G: massive machine-type communication (mMTC) and ultra-reliable
low-latency communication (URLLC). This paper fills this important gap by
identifying the opportunities and challenges in exploiting massive MIMO for IoT
connectivity. We provide insights into the trade-offs that emerge when massive
MIMO is applied to mMTC or URLLC and present a number of suitable communication
schemes. The discussion continues to the questions of network slicing of the
wireless resources and the use of massive MIMO to simultaneously support IoT
connections with very heterogeneous requirements. The main conclusion is that
massive MIMO can bring benefits to the scenarios with IoT connectivity, but it
requires tight integration of the physical-layer techniques with the protocol
design.Comment: Submitted for publicatio
Throughput Optimization for Massive MIMO Systems Powered by Wireless Energy Transfer
This paper studies a wireless-energy-transfer (WET) enabled massive
multiple-input-multiple-output (MIMO) system (MM) consisting of a hybrid
data-and-energy access point (H-AP) and multiple single-antenna users. In the
WET-MM system, the H-AP is equipped with a large number of antennas and
functions like a conventional AP in receiving data from users, but additionally
supplies wireless power to the users. We consider frame-based transmissions.
Each frame is divided into three phases: the uplink channel estimation (CE)
phase, the downlink WET phase, as well as the uplink wireless information
transmission (WIT) phase. Firstly, users use a fraction of the previously
harvested energy to send pilots, while the H-AP estimates the uplink channels
and obtains the downlink channels by exploiting channel reciprocity. Next, the
H-AP utilizes the channel estimates just obtained to transfer wireless energy
to all users in the downlink via energy beamforming. Finally, the users use a
portion of the harvested energy to send data to the H-AP simultaneously in the
uplink (reserving some harvested energy for sending pilots in the next frame).
To optimize the throughput and ensure rate fairness, we consider the problem of
maximizing the minimum rate among all users. In the large- regime, we obtain
the asymptotically optimal solutions and some interesting insights for the
optimal design of WET-MM system. We define a metric, namely, the massive MIMO
degree-of-rate-gain (MM-DoRG), as the asymptotic UL rate normalized by
. We show that the proposed WET-MM system is optimal in terms of
MM-DoRG, i.e., it achieves the same MM-DoRG as the case with ideal CE.Comment: 15 double-column pages, 6 figures, 1 table, to appear in IEEE JSAC in
February 2015, special issue on wireless communications powered by energy
harvesting and wireless energy transfe
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