DSpace Community:http://dspace.cityu.edu.hk:80/handle/2031/7102015-08-08T02:23:32Z2015-08-08T02:23:32ZNumerical analysis of energy consumption and lifetime in complex sensor networksYan, Fan (閻帆)http://dspace.cityu.edu.hk:80/handle/2031/75762015-07-30T01:50:56Z2013-01-01T00:00:00ZTitle: 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 networksThong, Wang Kit Wilson (唐宏傑)http://dspace.cityu.edu.hk:80/handle/2031/75632015-07-17T05:07:57Z2013-01-01T00:00:00ZTitle: 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)2013-01-01T00:00:00ZPhotonic microwave frequency mixing using an optically injected semiconductor laserFu, Xuelei (傅雪蕾)http://dspace.cityu.edu.hk:80/handle/2031/73092014-08-04T10:22:18Z2010-01-01T00:00:00ZTitle: Photonic microwave frequency mixing using an optically injected semiconductor laser
Authors: Fu, Xuelei (傅雪蕾)
Abstract: The nonlinear dynamics of an optically injected semiconductor laser are investigated
for radio-over-fiber (RoF) downlink transmission through photonic microwave frequency
mixing. Subject to optical injection from a master laser, the single-mode slave laser ex-
hibits a variety of nonlinear dynamical behaviors such as stable-locking, period-one (P1)
oscillation, period-two (P2) oscillation, and chaos.
By properly adjusting the injection conditions, the slave laser in the P1 oscillation acts
as a broadly tunable photonic microwave oscillator. When an external current modulation
at an intermediate frequency (IF) is applied to the slave laser, the P1 oscillation is
modulated accordingly. Frequency mixing yields up-converted frequencies at both
the upper sideband (USB) and the lower sideband (LSB). As the photonic microwave
generated through P1 oscillation can go beyond the relaxation resonance frequency due
to nonlinear dynamics, photonic microwave frequency up-conversion beyond the laser
bandwidth is achieved. Experimentally, a 2.5-Gbps-grade semiconductor laser is applied
to generate P1 oscillation at 18 GHz. It is shown that the up-conversion bandwidth is
limited by the direct modulation response of the slave laser with a linear operational range
of more than 20 dB. When binary phase-shift keying (BPSK) data is modulated on the IF
signal, it is up-converted to the USB and LSB for downlink transmission simultaneously.
Data up-conversion from an IF of 2 GHz to a subcarrier frequency as high as 20 GHz
is experimentally demonstrated. After transmission over a 4-km single-mode fiber to a
base station, the data is recovered with a bit-error rate (BER) below 10−9. Only low-cost
lasers are used and there is no requirement on high-speed electronic mixers or external
modulators. Numerical simulations support the feasibility of extending the approach
to the technologically important window at 60 GHz. Furthermore, the chaotic state the optically injected semiconductor laser is examined for power-over-fiber (PoF)
applications. The stimulate Brillouin scattering (SBS) threshold is significantly increased
by driving the laser into chaotic oscillation, which has a bandwidth much broader than the
Brillouin linewidth. By comparing with the free-running state, the chaotic state provides
a 22-fold increase of the maximum output power to 838 mW, after transmission over a
5-km single-mode fiber. Upon optoelectronic conversion using a photovoltaic (PV) cell,
the obtained electrical power of 44 mW is adequate for advanced low-power RoF remote
units.
Notes: CityU Call Number: TK7872.M5 F8 2010; xvii, 98 leaves : ill. 30 cm.; Thesis (M.Phil.)--City University of Hong Kong, 2010.; Includes bibliographical references.2010-01-01T00:00:00ZImpact of antenna correlation on multi-user MIMO systemsWang, Hao (汪浩)http://dspace.cityu.edu.hk:80/handle/2031/70132013-06-13T02:39:28Z2012-01-01T00:00:00ZTitle: 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)2012-01-01T00:00:00Z