63 research outputs found
Rate enhancement and multi-relay selection schemes for application in wireless cooperative networks
In this thesis new methods are presented to achieve performance enhancement in wireless cooperative networks. In particular, techniques to improve transmission rate, mitigate asynchronous transmission and maximise end-to-end signal-to-noise ratio are described.
An offset transmission scheme with full interference cancellation for a two-hop synchronous network with frequency flat links and four relays is introduced. This approach can asymptotically, as the symbol block size increases, achieve maximum transmission rate together with full cooperative diversity provided the destination node has multiple antennas. A novel full inter-relay interference cancellation method that also achieves asymptotically maximum rate and full cooperative diversity is then designed which only requires a single antenna at the destination node.
Extension to asynchronous networks is then considered through the use of orthogonal frequency division multiplexing (OFDM) type transmission with a cyclic prefix, and interference cancellation techniques are designed for situations when synchronization errors are present in only the second hop or both the first and second hop. End-to-end bit error rate evaluations, with and without outer coding, are used to assess the performance of the various offset transmission schemes.
Multi-relay selection methods for cooperative amplify and forward type networks are then studied in order to overcome the degradation of end-to-end bit error rate performance in single-relay selection networks when there are feedback errors in the destination to relay node links. Outage probability analysis for two and four relay selection is performed to show the advantage of multi-relay selection when no interference occurs and when adjacent cell interference is present both at the relay nodes and the destination node. Simulation studies are included which support the theoretical expressions.
Finally, outage probability analysis of a cognitive amplify and forward type relay network with cooperation between certain secondary users, chosen by single and multi-relay (two and four) selection is presented. The cognitive relays are assumed to exploit an underlay approach, which requires adherence to an interference constraint on the primary user. The relay selection is performed either with a max-min strategy or one based on maximising exact end-to-end signal-to-noise ratio. The analyses are again confirmed by numerical evaluations
Joint Spatial and Spectrum Cooperation in Wireless Network.
PhDThe sky-rocketing growth of multimedia infotainment applications and broadband-hungry
mobile devices exacerbate the stringent demand for ultra high data rate and more spectrum resources. Along with it, the unbalanced temporal and geographical variations
of spectrum usage further inspires those spectral-efficient networks, namely, cognitive
radio and heterogeneous cellular networks (HCNs). This thesis focuses on the system
design and performance enhancement of cognitive radio (CR) and HCNs. Three different
aspects of performance improvement are considered, including link reliability of cognitive
radio networks (CNs), security enhancement of CNs, and energy efficiency improvement
of CNs and HCNs.
First, generalized selection combining (GSC) is proposed as an effective receiver design
for interference reduction and reliability improvement of CNs with outdated CSI. A uni-
ed way for deriving the distribution of received signal-to-noise ratio (SNR) is developed
in underlay spectrum sharing networks subject to interference from the primary trans-
mitter (PU-Tx) to the secondary receiver (SU-Rx), maximum transmit power constraint
at the secondary transmitter (SU-Tx), and peak interference power constraint at the
PU receiver (PU-Rx), is developed. Second, transmit antenna selection with receive
generalized selection combining (TAS/GSC) in multi-antenna relay-aided communica-
tion is introduced in CNs under Rayleigh fading and Nakagami-m fading. Based on
newly derived complex statistical properties of channel power gain of TAS/GSC, exact
ergodic capacity and high SNR ergodic capacity are derived over Nakagami-m fading.
Third, beamforming and arti cial noise generation (BF&AN) is introduced as a robust
scheme to enhance the secure transmission of large-scale spectrum sharing networks
with multiple randomly located eavesdroppers (Eves) modeled as homogeneous Poisson
Point Process (PPP). Stochastic geometry is applied to model and analyze the impact of
i
BF&AN on this complex network. Optimal power allocation factor for BF&AN which
maximizes the average secrecy rate is further studied under the outage probability con-
straint of primary network. Fourth, a new wireless energy harvesting protocol is proposed
for underlay cognitive relay networks with the energy-constrained SU-Txs. Exact and
asymptotic outage probability, delay-sensitive throughput, and delay-tolerant through-
put are derived to explore the tradeoff between the energy harvested from the PU-Txs
and the interference caused by the PU-Txs. Fifth, a harvest-then-transmit protocol is
proposed in K-tier HCNs with randomly located multiple-antenna base stations (BSs)
and single antenna mobile terminals (MTs) modeled as homogeneous PPP. The average
received power at MT, the uplink (UL) outage probability, and the UL average ergodic
rate are derived to demonstrate the intrinsic relationship between the energy harvested
from BSs in the downlink (DL) and the MT performance in the UL. Throughout the
thesis, it is shown that link reliability, secrecy performance, and energy efficiency of
CNs and HCNs can be signi cantly leveraged by taking advantage of multiple antennas,
relays, and wireless energy harvesting
Cognitive Radio Systems
Cognitive radio is a hot research area for future wireless communications in the recent years. In order to increase the spectrum utilization, cognitive radio makes it possible for unlicensed users to access the spectrum unoccupied by licensed users. Cognitive radio let the equipments more intelligent to communicate with each other in a spectrum-aware manner and provide a new approach for the co-existence of multiple wireless systems. The goal of this book is to provide highlights of the current research topics in the field of cognitive radio systems. The book consists of 17 chapters, addressing various problems in cognitive radio systems
Physical Layer Security in Wireless Networks: Design and Enhancement.
PhDSecurity and privacy have become increasingly significant concerns in wireless communication
networks, due to the open nature of the wireless medium which makes the wireless
transmission vulnerable to eavesdropping and inimical attacking. The emergence and
development of decentralized and ad-hoc wireless networks pose great challenges to the
implementation of higher-layer key distribution and management in practice. Against
this background, physical layer security has emerged as an attractive approach for performing
secure transmission in a low complexity manner. This thesis concentrates on
physical layer security design and enhancement in wireless networks.
First, this thesis presents a new unifying framework to analyze the average secrecy
capacity and secrecy outage probability. Besides the exact average secrecy capacity
and secrecy outage probability, a new approach for analyzing the asymptotic behavior is
proposed to compute key performance parameters such as high signal-to-noise ratio slope,
power offset, secrecy diversity order, and secrecy array gain. Typical fading environments
such as two-wave with diffuse power and Nakagami-m are taken into account.
Second, an analytical framework of using antenna selection schemes to achieve secrecy
is provided. In particular, transmit antenna selection and generalized selection combining
are considered including its special cases of selection combining and maximal-ratio
combining.
Third, the fundamental questions surrounding the joint impact of power constraints on
the cognitive wiretap channel are addressed. Important design insights are revealed
regarding the interplay between two power constraints, namely the maximum transmit
at the secondary network and the peak interference power at the primary network.
Fourth, secure single carrier transmission is considered in the two-hop decode-andi
forward relay networks. A two-stage relay and destination selection is proposed to minimize
the eavesdropping and maximize the signal power of the link between the relay and
the destination. In two-hop amplify-and-forward untrusted relay networks, secrecy may
not be guaranteed even in the absence of external eavesdroppers. As such, cooperative
jamming with optimal power allocation is proposed to achieve non-zero secrecy rate.
Fifth and last, physical layer security in large-scale wireless sensor networks is introduced.
A stochastic geometry approach is adopted to model the positions of sensors, access
points, sinks, and eavesdroppers. Two scenarios are considered: i) the active sensors
transmit their sensing data to the access points, and ii) the active access points forward
the data to the sinks. Important insights are concluded
From MANET to people-centric networking: Milestones and open research challenges
In this paper, we discuss the state of the art of (mobile) multi-hop ad hoc networking with the aim to present the current status of the research activities and identify the consolidated research areas, with limited research opportunities, and the hot and emerging research areas for which further research is required. We start by briefly discussing the MANET paradigm, and why the research on MANET protocols is now a cold research topic. Then we analyze the active research areas. Specifically, after discussing the wireless-network technologies, we analyze four successful ad hoc networking paradigms, mesh networks, opportunistic networks, vehicular networks, and sensor networks that emerged from the MANET world. We also present an emerging research direction in the multi-hop ad hoc networking field: people centric networking, triggered by the increasing penetration of the smartphones in everyday life, which is generating a people-centric revolution in computing and communications
Channel assembling and resource allocation in multichannel spectrum sharing wireless networks
Submitted in fulfilment of the academic requirements for the degree of
Doctor of Philosophy (Ph.D.) in Engineering, in the School of Electrical and
Information Engineering, Faculty of Engineering and the Built Environment,
at the University of the Witwatersrand, Johannesburg, South Africa, 2017The continuous evolution of wireless communications technologies has increasingly imposed a
burden on the use of radio spectrum. Due to the proliferation of new wireless networks applications
and services, the radio spectrum is getting saturated and becoming a limited resource. To a large
extent, spectrum scarcity may be a result of deficient spectrum allocation and management policies,
rather than of the physical shortage of radio frequencies. The conventional static spectrum
allocation has been found to be ineffective, leading to overcrowding and inefficient use. Cognitive
radio (CR) has therefore emerged as an enabling technology that facilitates dynamic spectrum
access (DSA), with a great potential to address the issue of spectrum scarcity and inefficient use.
However, provisioning of reliable and robust communication with seamless operation in cognitive
radio networks (CRNs) is a challenging task. The underlying challenges include development of
non-intrusive dynamic resource allocation (DRA) and optimization techniques.
The main focus of this thesis is development of adaptive channel assembling (ChA) and DRA
schemes, with the aim to maximize performance of secondary user (SU) nodes in CRNs, without
degrading performance of primary user (PU) nodes in a primary network (PN). The key objectives
are therefore four-fold. Firstly, to optimize ChA and DRA schemes in overlay CRNs. Secondly, to
develop analytical models for quantifying performance of ChA schemes over fading channels in
overlay CRNs. Thirdly, to extend the overlay ChA schemes into hybrid overlay and underlay
architectures, subject to power control and interference mitigation; and finally, to extend the
adaptive ChA and DRA schemes for multiuser multichannel access CRNs.
Performance analysis and evaluation of the developed ChA and DRA is presented, mainly through
extensive simulations and analytical models. Further, the cross validation has been performed
between simulations and analytical results to confirm the accuracy and preciseness of the novel
analytical models developed in this thesis. In general, the presented results demonstrate improved
performance of SU nodes in terms of capacity, collision probability, outage probability and forced
termination probability when employing the adaptive ChA and DRA in CRNs.CK201
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