2,516 research outputs found
Impact of Channel Asymmetry on Performance of Channel Estimation and Precoding for Downlink Base Station Cooperative Transmission
Base station (BS) cooperative transmission can improve the spectrum
efficiency of cellular systems, whereas using which the channels will become
asymmetry. In this paper, we study the impact of the asymmetry on the
performance of channel estimation and precoding in downlink BS cooperative
multiple-antenna multiple-carrier systems. We first present three linear
estimators which jointly estimate the channel coefficients from users in
different cells with minimum mean square error, robust design and least square
criterion, and then study the impact of uplink channel asymmetry on their
performance. It is shown that when the large scale channel information is
exploited for channel estimation, using non-orthogonal training sequences among
users in different cells leads to minor performance loss. Next, we analyze the
impact of downlink channel asymmetry on the performance of precoding with
channel estimation errors. Our analysis shows that although the estimation
errors of weak cross links are large, the resulting rate loss is minor because
their contributions are weighted by the receive signal to noise ratio. The
simulation results verify our analysis and show that the rate loss per user is
almost constant no matter where the user is located, when the channel
estimators exploiting the large scale fading gains.Comment: Submitted to the Transactions on Communication
Cross-Technology Communications for Heterogeneous IoT Devices Through Artificial Doppler Shifts
Recent years have seen major innovations in developing energy-efficient
wireless technologies such as Bluetooth Low Energy (BLE) for Internet of Things
(IoT). Despite demonstrating significant benefits in providing low power
transmission and massive connectivity, hardly any of these technologies have
made it to directly connect to the Internet. Recent advances demonstrate the
viability of direct communication among heterogeneous IoT devices with
incompatible physical (PHY) layers. These techniques, however, require
modifications in transmission power or time, which may affect the media access
control (MAC) layer behaviors in legacy networks. In this paper, we argue that
the frequency domain can serve as a free side channel with minimal
interruptions to legacy networks. To this end, we propose DopplerFi, a
communication framework that enables a two-way communication channel between
BLE and Wi-Fi by injecting artificial Doppler shifts, which can be decoded by
sensing the patterns in the Gaussian frequency shift keying (GFSK) demodulator
and Channel State Information (CSI). The artificial Doppler shifts can be
compensated by the inherent frequency synchronization module and thus have a
negligible impact on legacy communications. Our evaluation using commercial
off-the-shelf (COTS) BLE chips and 802.11-compliant testbeds have demonstrated
that DopplerFi can achieve throughput up to 6.5~Kbps at the cost of merely less
than 0.8% throughput loss
Large-scale Antenna Operation in Heterogeneous Cloud Radio Access Networks: A Partial Centralization Approach
To satisfy the ever-increasing capacity demand and quality of service (QoS)
requirements of users, 5G cellular systems will take the form of heterogeneous
networks (HetNets) that consist of macro cells and small cells. To build and
operate such systems, mobile operators have given significant attention to
cloud radio access networks (C-RANs) due to their beneficial features of
performance optimization and cost effectiveness. Along with the architectural
enhancement of C-RAN, large-scale antennas (a.k.a. massive MIMO) at cell sites
contribute greatly to increased network capacity either with higher spectral
efficiency or through permitting many users at once. In this article, we
discuss the challenging issues of C-RAN based HetNets (H-CRAN), especially with
respect to large-scale antenna operation. We provide an overview of existing
C-RAN architectures in terms of large-scale antenna operation and promote a
partially centralized approach. This approach reduces, remarkably, fronthaul
overheads in CRANs with large-scale antennas. We also provide some insights
into its potential and applicability in the fronthaul bandwidthlimited H-CRAN
with large-scale antennas.Comment: To appear in IEEE Wireless Communications Magazine June 201
A Journey from Improper Gaussian Signaling to Asymmetric Signaling
The deviation of continuous and discrete complex random variables from the
traditional proper and symmetric assumption to a generalized improper and
asymmetric characterization (accounting correlation between a random entity and
its complex conjugate), respectively, introduces new design freedom and various
potential merits. As such, the theory of impropriety has vast applications in
medicine, geology, acoustics, optics, image and pattern recognition, computer
vision, and other numerous research fields with our main focus on the
communication systems. The journey begins from the design of improper Gaussian
signaling in the interference-limited communications and leads to a more
elaborate and practically feasible asymmetric discrete modulation design. Such
asymmetric shaping bridges the gap between theoretically and practically
achievable limits with sophisticated transceiver and detection schemes in both
coded/uncoded wireless/optical communication systems. Interestingly,
introducing asymmetry and adjusting the transmission parameters according to
some design criterion render optimal performance without affecting the
bandwidth or power requirements of the systems. This dual-flavored article
initially presents the tutorial base content covering the interplay of
reality/complexity, propriety/impropriety and circularity/noncircularity and
then surveys majority of the contributions in this enormous journey.Comment: IEEE COMST (Early Access
Uplink CoMP under a Constrained Backhaul and Imperfect Channel Knowledge
Coordinated Multi-Point (CoMP) is known to be a key technology for next
generation mobile communications systems, as it allows to overcome the burden
of inter-cell interference. Especially in the uplink, it is likely that
interference exploitation schemes will be used in the near future, as they can
be used with legacy terminals and require no or little changes in
standardization. Major drawbacks, however, are the extent of additional
backhaul infrastructure needed, and the sensitivity to imperfect channel
knowledge. This paper jointly addresses both issues in a new framework
incorporating a multitude of proposed theoretical uplink CoMP concepts, which
are then put into perspective with practical CoMP algorithms. This
comprehensive analysis provides new insight into the potential usage of uplink
CoMP in next generation wireless communications systems.Comment: Submitted to IEEE Transactions on Wireless Communications in February
201
Resilient networking in wireless sensor networks
This report deals with security in wireless sensor networks (WSNs),
especially in network layer. Multiple secure routing protocols have been
proposed in the literature. However, they often use the cryptography to secure
routing functionalities. The cryptography alone is not enough to defend against
multiple attacks due to the node compromise. Therefore, we need more
algorithmic solutions. In this report, we focus on the behavior of routing
protocols to determine which properties make them more resilient to attacks.
Our aim is to find some answers to the following questions. Are there any
existing protocols, not designed initially for security, but which already
contain some inherently resilient properties against attacks under which some
portion of the network nodes is compromised? If yes, which specific behaviors
are making these protocols more resilient? We propose in this report an
overview of security strategies for WSNs in general, including existing attacks
and defensive measures. In this report we focus at the network layer in
particular, and an analysis of the behavior of four particular routing
protocols is provided to determine their inherent resiliency to insider
attacks. The protocols considered are: Dynamic Source Routing (DSR),
Gradient-Based Routing (GBR), Greedy Forwarding (GF) and Random Walk Routing
(RWR)
Backhaul Limited Asymmetric Cooperation for MIMO Cellular Networks via Semidefinite Relaxation
Multicell cooperation has recently attracted tremendous attention because of
its ability to eliminate intercell interference and increase spectral
efficiency. However, the enormous amount of information being exchanged,
including channel state information and user data, over backhaul links may
deteriorate the network performance in a realistic system. This paper adopts a
backhaul cost metric that considers the number of active directional
cooperation links, which gives a first order measurement of the backhaul
loading required in asymmetric Multiple-Input Multiple-Output (MIMO)
cooperation. We focus on a downlink scenario for multi-antenna base stations
and single-antenna mobile stations. The design problem is minimizing the number
of active directional cooperation links and jointly optimizing the beamforming
vectors among the cooperative BSs subject to
signal-to-interference-and-noise-ratio (SINR) constraints at the mobile
station. This problem is non-convex and solving it requires combinatorial
search. A practical algorithm based on smooth approximation and semidefinite
relaxation is proposed to solve the combinatorial problem efficiently. We show
that semidefinite relaxation is tight with probability 1 in our algorithm and
stationary convergence is guaranteed. Simulation results show the saving of
backhaul cost and power consumption is notable compared with several baseline
schemes and its effectiveness is demonstrated.Comment: 14 pages, 7 figures. This paper is accepted by IEEE Transactions on
Signal Processin
Security and Privacy Issues in Wireless Mesh Networks: A Survey
This book chapter identifies various security threats in wireless mesh
network (WMN). Keeping in mind the critical requirement of security and user
privacy in WMNs, this chapter provides a comprehensive overview of various
possible attacks on different layers of the communication protocol stack for
WMNs and their corresponding defense mechanisms. First, it identifies the
security vulnerabilities in the physical, link, network, transport, application
layers. Furthermore, various possible attacks on the key management protocols,
user authentication and access control protocols, and user privacy preservation
protocols are presented. After enumerating various possible attacks, the
chapter provides a detailed discussion on various existing security mechanisms
and protocols to defend against and wherever possible prevent the possible
attacks. Comparative analyses are also presented on the security schemes with
regards to the cryptographic schemes used, key management strategies deployed,
use of any trusted third party, computation and communication overhead involved
etc. The chapter then presents a brief discussion on various trust management
approaches for WMNs since trust and reputation-based schemes are increasingly
becoming popular for enforcing security in wireless networks. A number of open
problems in security and privacy issues for WMNs are subsequently discussed
before the chapter is finally concluded.Comment: 62 pages, 12 figures, 6 tables. This chapter is an extension of the
author's previous submission in arXiv submission: arXiv:1102.1226. There are
some text overlaps with the previous submissio
To Motivate Social Grouping in Wireless Networks
We consider a group of neighboring smartphone users who are roughly at the
same time interested in the same network content, called common interests.
However, ever-increasing data traffic challenges the limited capacity of
base-stations (BSs) in wireless networks. To better utilize the limited BSs'
resources under unreliable wireless networks, we propose local common-interests
sharing (enabled by D2D communications) by motivating the physically
neighboring users to form a social group. As users are selfish in practice, an
incentive mechanism is needed to motivate social grouping. We propose a novel
concept of equal-reciprocal incentive over broadcast communications, which
fairly ensures that each pair of the users in the social group share the same
amount of content with each other. As the equal-reciprocal incentive may
restrict the amount of content shared among the users, we analyze the optimal
equal-reciprocal scheme that maximizes local sharing content. While ensuring
fairness among users, we show that this optimized scheme also maximizes each
user's utility in the social group. Finally, we look at dynamic content
arrivals and extend our scheme successfully by proposing novel on-line
scheduling algorithms.Comment: 32 pages (single column), submitted for possible journal publicatio
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