123 research outputs found
Array communications in wireless sensor networks
Imperial Users onl
Multi-hop relaying networks in TDD-CDMA systems
The communications phenomena at the end of the 20th century were the Internet and mobile
telephony. Now, entering the new millennium, an effective combination of the two should
become a similarly everyday experience. Current limitations include scarce, exorbitantly priced
bandwidth and considerable power consumption at higher data rates.
Relaying systems use several shorter communications links instead of the conventional
point-to-point transmission. This can allow for a lower power requirement and, due to the
shorter broadcast range, bandwidth re-use may be more efficiently exploited. Code division
multiple access (CDMA) is emerging as one of the most common methods for multi user
access. Combining CDMA with time division duplexing (TDD) provides a system that
supports asymmetric communications and relaying cost-effectively. The capacity of CDMA
may be reduced by interference from other users, hence it is important that the routing of
relays is performed to minimise interference at receivers.
This thesis analyses relaying within the context of TDD-CDMA systems. Such a system was
included in the initial draft of the European 3G specifications as opportunity driven multiple
access (ODMA). Results are presented which demonstrate that ODMA allows for a more
flexible capacity coverage trade-off than non-relaying systems. An investigation into the
interference characteristics of ODMA shows that most interference occurs close to the base
station (BS). Hence it is possible that in-cell routing to avoid the BS may increase capacity.
As a result, a novel hybrid network topology is presented. ODMA uses path loss as a metric
for routing. This technique does not avoid interference, and hence ODMA shows no capacity
increase with the hybrid network. Consequently, a novel interference based routing algorithm
and admission control are developed. When at least half the network is engaged in in-cell
transmission, the interference based system allows for a higher capacity than a conventional
cellular system. In an attempt to reduce transmitted power, a novel congestion based routing algorithm is introduced. This system is shown to have lower power requirement than any other analysed system and, when more than 2 hops are allowed, the highest capacity.
The allocation of time slots affects system performance through co-channel interference. To
attempt to minimise this, a novel dynamic channel allocation (DCA) algorithm is developed
based on the congestion routing algorithm. By combining the global minimisation of system
congestion in both time slots and routing, the DCA further increases throughput. Implementing
congestion routed relaying, especially with DCA, in any TDD-CDMA system with in-cell calls
can show significant performance improvements over conventional cellular systems
Cross-layer design of multi-hop wireless networks
MULTI -hop wireless networks are usually defined as a collection of nodes
equipped with radio transmitters, which not only have the capability to
communicate each other in a multi-hop fashion, but also to route each others’ data
packets. The distributed nature of such networks makes them suitable for a variety of
applications where there are no assumed reliable central entities, or controllers, and
may significantly improve the scalability issues of conventional single-hop wireless
networks.
This Ph.D. dissertation mainly investigates two aspects of the research issues
related to the efficient multi-hop wireless networks design, namely: (a) network
protocols and (b) network management, both in cross-layer design paradigms to
ensure the notion of service quality, such as quality of service (QoS) in wireless mesh
networks (WMNs) for backhaul applications and quality of information (QoI) in
wireless sensor networks (WSNs) for sensing tasks. Throughout the presentation of
this Ph.D. dissertation, different network settings are used as illustrative examples,
however the proposed algorithms, methodologies, protocols, and models are not
restricted in the considered networks, but rather have wide applicability.
First, this dissertation proposes a cross-layer design framework integrating
a distributed proportional-fair scheduler and a QoS routing algorithm, while using
WMNs as an illustrative example. The proposed approach has significant performance
gain compared with other network protocols. Second, this dissertation proposes
a generic admission control methodology for any packet network, wired and
wireless, by modeling the network as a black box, and using a generic mathematical
0. Abstract 3
function and Taylor expansion to capture the admission impact. Third, this dissertation
further enhances the previous designs by proposing a negotiation process,
to bridge the applications’ service quality demands and the resource management,
while using WSNs as an illustrative example. This approach allows the negotiation
among different service classes and WSN resource allocations to reach the optimal
operational status. Finally, the guarantees of the service quality are extended to
the environment of multiple, disconnected, mobile subnetworks, where the question
of how to maintain communications using dynamically controlled, unmanned data
ferries is investigated
Low-latency Networking: Where Latency Lurks and How to Tame It
While the current generation of mobile and fixed communication networks has
been standardized for mobile broadband services, the next generation is driven
by the vision of the Internet of Things and mission critical communication
services requiring latency in the order of milliseconds or sub-milliseconds.
However, these new stringent requirements have a large technical impact on the
design of all layers of the communication protocol stack. The cross layer
interactions are complex due to the multiple design principles and technologies
that contribute to the layers' design and fundamental performance limitations.
We will be able to develop low-latency networks only if we address the problem
of these complex interactions from the new point of view of sub-milliseconds
latency. In this article, we propose a holistic analysis and classification of
the main design principles and enabling technologies that will make it possible
to deploy low-latency wireless communication networks. We argue that these
design principles and enabling technologies must be carefully orchestrated to
meet the stringent requirements and to manage the inherent trade-offs between
low latency and traditional performance metrics. We also review currently
ongoing standardization activities in prominent standards associations, and
discuss open problems for future research
Congestion Control and Routing over Challenged Networks
This dissertation is a study on the design and analysis of novel, optimal
routing and rate control algorithms in wireless, mobile communication networks.
Congestion control and routing algorithms upto now have been designed and
optimized for wired or wireless mesh networks. In those networks, optimal
algorithms (optimal in the sense that either the throughput is maximized or
delay is minimized, or the network operation cost is minimized) can be
engineered based on the classic time scale decomposition assumption that the
dynamics of the network are either fast enough so that these algorithms
essentially see the average or slow enough that any changes can be tracked to
allow the algorithms to adapt over time. However, as technological advancements
enable integration of ever more mobile nodes into communication networks, any
rate control or routing algorithms based, for example, on averaging out the
capacity of the wireless mobile link or tracking the instantaneous capacity
will perform poorly. The common element in our solution to engineering
efficient routing and rate control algorithms for mobile wireless networks is
to make the wireless mobile links seem as if they are wired or wireless links
to all but few nodes that directly see the mobile links (either the mobiles or
nodes that can transmit to or receive from the mobiles) through an appropriate
use of queuing structures at these selected nodes. This approach allows us to
design end-to-end rate control or routing algorithms for wireless mobile
networks so that neither averaging nor instantaneous tracking is necessary
Wireless sensor networks
Wireless sensor networks promise an unprecedented fine-grained interface between the virtual and the physical world. They are one of the most rapidly developing new information technologies, with applications in a wide range of fields including industrial process control, security and surveillance, environmental sensing, and structural health monitoring.
The subject of this project is motivated by the urgent need to provide a comprehensive and organized survey of the field. It shows how the core challenges of energy efficiency, robustness, and autonomy are addressed in these systems by networking techniques across multiple layers.
The topics covered include network deployment, wireless characteristics, time synchronization, congestion and error control, medium access, standards, topology control, routing, security, data transfer, transport protocols and new technologies and materials in fabricating sensors
QoS-driven adaptive resource allocation for mobile wireless communications and networks
Quality-of-service (QoS) guarantees will play a critically important role in future
mobile wireless networks. In this dissertation, we study a set of QoS-driven resource
allocation problems for mobile wireless communications and networks.
In the first part of this dissertation, we investigate resource allocation schemes
for statistical QoS provisioning. The schemes aim at maximizing the system/network
throughput subject to a given queuing delay constraint. To achieve this goal, we
integrate the information theory with the concept of effective capacity and develop
a unified framework for resource allocation. Applying the above framework, we con-sider a number of system infrastructures, including single channel, parallel channel,
cellular, and cooperative relay systems and networks, respectively. In addition, we
also investigate the impact of imperfect channel-state information (CSI) on QoS pro-visioning. The resource allocation problems can be solved e±ciently by the convex
optimization approach, where closed-form allocation policies are obtained for different
application scenarios.
Our analyses reveal an important fact that there exists a fundamental tradeoff
between throughput and QoS provisioning. In particular, when the delay constraint
becomes loose, the optimal resource allocation policy converges to the water-filling
scheme, where ergodic capacity can be achieved. On the other hand, when the
QoS constraint gets stringent, the optimal policy converges to the channel inversion scheme under which the system operates at a constant rate and the zero-outage
capacity can be achieved.
In the second part of this dissertation, we study adaptive antenna selection for
multiple-input-multiple-output (MIMO) communication systems. System resources
such as subcarriers, antennas and power are allocated dynamically to minimize the
symbol-error rate (SER), which is the key QoS metric at the physical layer. We
propose a selection diversity scheme for MIMO multicarrier direct-sequence code-
division-multiple-access (MC DS-CDMA) systems and analyze the error performance
of the system when considering CSI feedback delay and feedback errors. Moreover,
we propose a joint antenna selection and power allocation scheme for space-time
block code (STBC) systems. The error performance is derived when taking the CSI
feedback delay into account. Our numerical results show that when feedback delay
comes into play, a tradeoff between performance and robustness can be achieved by
dynamically allocating power across transmit antennas
- …