DSpace Collection:
http://dspace.cityu.edu.hk:80/handle/2031/746
2015-08-05T04:26:37ZNumerical analysis of energy consumption and lifetime in complex sensor networks
http://dspace.cityu.edu.hk:80/handle/2031/7576
Title: Numerical analysis of energy consumption and lifetime in complex sensor networks
Authors: Yan, Fan (閻帆)
Abstract: Many complex networks share a common feature that the nodes are resource-limited.
Examples include the Internet which consists of finite-buffer network devices, cooperation
networks which are composed of time-limited people, and wireless sensor
networks which are constituted of self-powered devices. Therefore, studying resourcelimited
networks is important from an engineering perspective.
In the introduction part of this thesis, some theoretical models for generating
complex networks and some important measures on networks and some overview of
energy-saving techniques for sensor networks are reviewed. In the main part of the
thesis, the relationship between energy consumption and time e ciency of sensor
networks are studied. A comprehensive numerical study of sensor networks with five
different structural topologies and four different global and local routing methods are
presented. Their performances and costs are studied. The numerical results showed
that networks with both scale-free and small-world topological features will have the
longest lifetime by using the same routing methods. After that, the local routing
method is studied deeply. The relationship between the weighting of the energy and
the degree of sensor nodes by using the random-walk routing method is studied. Numerical
results show that the size of a living network follows a pattern of exponential
decrease in time. Based on their different behaviors, the networks are categorized
into two types and a functional lifetime criterion to measure the performances of the
networks is proposed. If the energy information is taken into account in the routing
methods, the time of keeping the large portion of networks functional becomes longer.
The findings provide references and guidelines for designing better sensor networks
under various conditions for possible applications.
Notes: viii, 104 p. : ill. (some col.) ; 30 cm.; Thesis (Ph.D.)--City University of Hong Kong, 2013.; Includes bibliographical references (p. 94-102)2013-01-01T00:00:00ZDeflection routing in complex networks
http://dspace.cityu.edu.hk:80/handle/2031/7563
Title: Deflection routing in complex networks
Authors: Thong, Wang Kit Wilson (唐宏傑)
Abstract: Deflection routing refers to a protocol consisting of a table-based routing protocol,
plus a software module responsible for deflection. When a link is congested, or a
queueing delay is too long, a packet can be forwarded by the deflection module.
Routing table is overridden and the deflection module "deflects" the packet away
from the normal path, by-passing the congestion point. The packet then arrives
to its next hop and hooks up to another path toward its destination. The packet
may be deflected more than once if needed.
In contrast to adapting to topology changes, adapting to traffic changes is
a more challenging objective. In this thesis, I study using deflection routing to
achieve the aforementioned objective, and report several studies to address the
technical difficulties. Afterward, I propose an enhanced routing protocol based
upon deflection routing - Random Early Deflection (RED-f) routing protocol,
which allows packets to take different paths preemptively before congestion is
developed.
Notes: CityU Call Number: TK5105.5487 .T45 2013; v, 90 leaves : ill. 30 cm.; Thesis (Ph.D.)--City University of Hong Kong, 2013.; Includes bibliographical references (leaves 80-89)2013-01-01T00:00:00ZImpact of antenna correlation on multi-user MIMO systems
http://dspace.cityu.edu.hk:80/handle/2031/7013
Title: Impact of antenna correlation on multi-user MIMO systems
Authors: Wang, Hao (汪浩)
Abstract: Multiple-input multiple-output (MIMO) techniques provide a promising solution
to enhance the performance of wireless communication systems. Antenna correlation
exists in practical MIMO systems for two reasons. First, there may be no
sufficient space to separate antenna elements due to the limited physical sizes of
the transmitters and/or the receivers. Second, the practical propagation environments
may not provide sufficient scatters. In a conventional single-user MIMO
system, since antenna correlation results in reduced degrees of freedom (DOF)
and may severely degrade the system capacity, it has been generally regarded as
a negative factor. However, the impact of antenna correlation in multi-user environments
is still limited explored. In this thesis, we make a comprehensive study
on theoretical and practical aspects of multi-user MIMO systems with correlated
fading.
In the first contribution, the capacity of correlated MIMO systems with full
channel state information (CSI) at both the transmitters and the receiver over
multiple access channels (MACs) (i.e. uplink multi-user MIMO systems) is analyzed.
In contrast to the common views, we show that antenna correlation is
potentially beneficial in a multi-user environment. The key is that the spatial
diversity related to user locations (i.e., multi-user diversity) can compensate the
loss of DOF due to antenna correlation. More specifically, it is shown numerically
that there is a cross point between the capacity curves for systems with and
without correlation. Below this point, correlation is advantageous and vice versa.
Moreover, such a point occurs at a rate increasing with the number of mobile units
(MUs) (denoted by K in this thesis), which implies that the range where antenna
correlation is beneficial increases with K. We also quantify this advantage analytically
in the limiting case of K → ∞. In the meanwhile, it is shown numerically
and analytically that there is a similar advantage from antenna correlation for
MIMO MACs with rate constraints. (We call this advantage correlation gain in
this thesis.)
In the second contribution, we study the impact of antenna correlation on the
capacity of MIMO MACs with imperfect CSI at the transmitter (CSIT). We first
consider the case of no CSIT.We prove that isotropic inputs (i.e., the covariance of
transmitted signal for each MU is identity matrix) are the most robust and optimal
ones, and therefore achieve the capacity of such systems. Both numerical results
and theoretical analysis show that although antenna correlation is detrimental in all rate or power range, the capacity degradation decreases with K increasing and
vanishes when K → ∞. These results imply that besides multi-user diversity,
CSIT also plays an important role in exploiting correlation gain. We then study
the systems with partial CSIT in form of channel covariance information (CCI).
Our major finding is that, similar to the scenario of perfect CSIT, antenna correlation
is potentially beneficial in a multi-user environment. We also prove that,
when antennas at MUs are fully correlated, systems with CCI at transmitters
can obtain exact the same correlation gain as that in systems with perfect CSIT.
This indicates that CCI may be enough to exploit the potential benefit of antenna
correlation.
In the third contribution, we extend the results from the MAC scenario to
the broadcast channel (BC) scenario (i.e., downlink multi-user MIMO systems).
Numerical results show that, similar to MIMO MACs, antenna correlation can
potentially improve the capacity of MIMO BCs. We point out that, besides multiuser
diversity and power focusing effect, such a gain mainly comes from the fact
that antenna correlation can increase the variance of the channel gain, which has
already been regarded as a advantageous factor in systems with user scheduling.
We also quantify the correlation gain in BCs for the limiting case of K → ∞ when
antennas at the base station (BS) and/or MUs are fully correlated. Theoretical
analysis shows that the asymptotic correlation gain for the case of full correlation
at both the BS and MUs grows linearly and logarithmically with the antenna
number at the BS, and logarithmically with the antenna number at each MU.
In the final contribution, we consider the practical implementation aspects of
correlation gain in coded MIMO multiple-access systems. To decrease the complexity
of MIMO transmission under imperfect CSIT, we propose two types of
low-cost but asymptotically optimal strategies, i.e. the instantaneous maximum
eigenmode beamforming (MEB) and statistical MEB strategy. Interleave-division
multiple-access (IDMA) technique, as a low-cost iterative multi-user detection
(MUD) approach, is adopted to alleviate inter-user interference resulting from
multi-user concurrent transmission. Simulation results demonstrate that our proposed
transceiver (i.e., MEB-based IDMA system) is an effective platform in practice
to obtain the aforementioned correlation gain.
In summary, this thesis presents a comprehensive study on antenna correlation
in multi-user MIMO systems. Both numerical and analytical results show that
antenna correlation is potential advantageous in a multi-user environment. Such
an advantage mainly comes from three aspects. First, the loss of the spatial
DOF due to antenna correlation is compensated by multi-user diversity. Second, antenna correlation enables focusing power. Third, the variance of the channel
gain is enlarged by antenna correlation, which is beneficial in systems with user
scheduling. The finding in this thesis is useful in practice as minimizing the
physical size of MUs and/or the BS is highly desirable, but it may result in antenna
correlation.
Notes: CityU Call Number: TK5103.4836 .W36 2012; xi, 91, iii leaves : ill. 30 cm.; Thesis (Ph.D.)--City University of Hong Kong, 2012.; Includes bibliographical references (leaves [83]-91)2012-01-01T00:00:00ZAn investigation into the use of coil-resonators in domino forms for wireless power transfer
http://dspace.cityu.edu.hk:80/handle/2031/6960
Title: An investigation into the use of coil-resonators in domino forms for wireless power transfer
Authors: Zhong, Wenxing (鍾文興)
Abstract: This thesis presents a study on the wireless power transfer systems using coil-resonators in domino forms (called domino-resonator systems or domino-systems in this thesis). The study is based on the lumped circuit model of domino-resonator systems and aims to analyze the operating principles and find out the optimum operations of the systems under different operating conditions. Straight and circular domino-resonator systems with one source resonator (transmitter) and one loaded resonator (receiver) are firstly studied. Then domino-resonator systems consisting of one transmitter and two receivers are investigated. Analysis methods based on superposition theorem, vector diagrams and power path are proposed. From theoretical analysis, simulation results and practical measurements, a number of findings are reported on the operation and optimization of the domino-resonator systems.
Straight domino-resonator systems are studied firstly assuming that the resonators in each system are identical and the operating frequency of the system equals to the resonant frequency of the resonators. Analytical expressions for the optimum load resistance and maximum efficiency are derived for a system with more than 2 resonators when the cross-couplings between non-adjacent resonators are negligible. The optimum arrangement of the resonators in a straight domino-resonator system is then studied. It is shown from theoretical analysis and numerical simulations that the optimum arrangement of the resonators is dependent on the load resistance and that if the load resistance is also optimized then equal-spacing is the best arrangement for a system with 3 resonators, but for a system with more than 3 resonators, the optimum arrangement is not equal-spacing which is different from the equal-spacing arrangement used in domino-resonator systems for waveguide applications.
Next the operating frequency is taken into consideration in the study on the straight domino-resonator system. Power flow analysis shows that there exist more than one power paths in a domino-system with more than 2 resonators due to the cross-couplings of non-adjacent resonators. As verified by analysis and practical measurements, the cross-coupling effects of the non-adjacent resonators cause the optimum operating frequencies for both the equally-spaced and the optimally-spaced resonator systems with identical resonators to shift away from the resonant frequency of the resonators so that all the power paths in the system can be utilized optimally to achieve the maximum efficiency. The cross-coupling effects of the non-adjacent resonators increase with increasing number of resonators and decreasing distances of the resonators.
Apart from the straight domino-resonator system which normally contains only one main power flow path, the domino-resonator system with two main power flow paths is also studied. One typical example of such systems is the circular domino-resonator system. Based on superposition theorem, individual power flow path of a circular domino-system is analyzed and then their interactions are merged and explained with vector diagrams. It is demonstrated that the optimum operating frequency could be far away from the resonant frequency of the resonators. However, if the operating frequency is fixed, the individual resonant frequency of each resonator in the system should be adjusted, for example by varying the compensating capacitances of the resonators, in order to regulate the power flows of the circular domino-resonator system to an optimum distribution.
As the simplest and typical multi-receiver domino-system, a system consisting of one transmitter and two receivers is studied. Theoretical analysis and numerical simulations show that the coupling between two receivers could be either helpful or harmful or even irrelevant to the power transfer efficiency of a system. All possibilities have been tabulated and explained. When the effect of the output power ratio is considered, three methods to obtain a desired output power ratio are introduced. It is proven from the simulation and experimental results that the efficiency can be improved significantly by optimizing the compensating capacitances in some cases, while in other cases operating at the resonant frequency of the resonators is already good enough. This phenomenon is favorably explained with the help of the power path concept. In addition, it has been found that a strong coupling between the receivers can help achieve a higher efficiency within a large variation of the output power ratio.
Notes: CityU Call Number: TK7895.P68 Z45 2012; vii, 159 leaves : ill. 30 cm.; Thesis (Ph.D.)--City University of Hong Kong, 2012.; Includes bibliographical references (leaves 153-159)2012-01-01T00:00:00Z