120 research outputs found
High Performance WLAN Using Smart Antenna
The need for higher data rates in WLANs boosts drastically because tremendous consumer interest in emerging multimedia applications, such as HDTV, has been increased. Currently, the IEEE 802.11a/b/g WLANs provide a limited data rate for the current user application requirements. In order to overcome substantial limitations of the existing WLANs, the next generation of WLANs, IEEE 802.11n, is in the course of development and expected to support higher throughput, larger coverage area and better QoS. The high performance IEEE 802.11n WLAN can improve data rate significantly by using smart antenna systems in the physical layer to take advantage of multi-path fading of wireless channels.
In this thesis, an analytical model is developed to study the MAC performance and
the underlying smart antenna technologies used in multi-path fading channels. Multiple
antennas employed in the AP arise two popular approaches to provide a significant performance improvement, diversity and multiplexing. Considering the diversity gain of multiple antennas at the AP in which the AP with multiple antennas serves one user at a time, the capacity and throughput can be obtained. In addition, the AP is possible to serve multiple users in the downlink, by exploiting the multiplexing gain of the wireless channel. We investigate the maximum network throughput when the traffic intensity of the AP approaches to one. Unlike most of previous research which focus on either the physical or the MAC layer performance, our analytical model jointly considers the MAC protocol and the smart antenna technology
A General Framework for Analyzing, Characterizing, and Implementing Spectrally Modulated, Spectrally Encoded Signals
Fourth generation (4G) communications will support many capabilities while providing universal, high speed access. One potential enabler for these capabilities is software defined radio (SDR). When controlled by cognitive radio (CR) principles, the required waveform diversity is achieved via a synergistic union called CR-based SDR. Research is rapidly progressing in SDR hardware and software venues, but current CR-based SDR research lacks the theoretical foundation and analytic framework to permit efficient implementation. This limitation is addressed here by introducing a general framework for analyzing, characterizing, and implementing spectrally modulated, spectrally encoded (SMSE) signals within CR-based SDR architectures. Given orthogonal frequency division multiplexing (OFDM) is a 4G candidate signal, OFDM-based signals are collectively classified as SMSE since modulation and encoding are spectrally applied. The proposed framework provides analytic commonality and unification of SMSE signals. Applicability is first shown for candidate 4G signals, and resultant analytic expressions agree with published results. Implementability is then demonstrated in multiple coexistence scenarios via modeling and simulation to reinforce practical utility
Physical layer model design for wireless networks
Wireless network analysis and simulations rely on accurate physical layer models.
The increased interest in wireless network design and cross-layer design require an
accurate and efficient physical layer model especially when a large number of nodes
are to be studied and building the real network is not possible. For analysis of upper
layer characteristics, a simplified physical layer model has to be chosen to model the
physical layer.
In this dissertation, the widely used two-state Markov model is examined and
shown to be deficient for low to moderate signal-to-noise ratios. The physical layer
statistics are investigated, and the run length distributions of the good and bad
frames are demonstrated to be the key statistics for accurate physical layer modeling. A four-state Markov model is proposed for the flat Rayleigh fading channel by
approximating the run length distributions with a mixture of exponential distributions. The transition probabilities in the four-state Markov model can be established
analytically without having to run extensive physical layer simulations, which are
required for the two-state Markov model. Physical layer good and bad run length
distributions are compared and it is shown that the four-state Markov model reasonably approximates the run length distributions. Ns2 simulations are performed and
the four-state Markov model provides a much more realistic approximation compared
to the popular two-state Markov model. Achieving good results with the flat Rayleigh fading channel, the proposed four-state Markov model is applied to a few diversity channels. A coded orthogonal fre-
quency division multiplexing (OFDM) system with a frequency selective channel and
the Alamouti multiple-input multiple-output system are chosen to verify the accuracy of the four-state Markov model. The network simulation results show that the
four-state Markov model approximates the physical layer with diversity channel well
whereas the traditional two-state Markov model estimates the network throughput
poorly. The success of adapting the four-state Markov model to the diversity channel
also shows the flexibility of adapting the four-state Markov model to various channel
conditions
Recommended from our members
IEEE 802.11 wireless LAN traffic analysis: a cross-layer approach
textThe deployment of broadband wireless data networks, e.g., wireless local area
networks (WLANs) [29], experienced tremendous growth in the last several
years, and this trend is continuously gaining momentum. In fact, WLAN is
becoming an indispensable component of the modern telecommunication infrastructure.
Despite this optimistic outlook, however, little is known about
the impact of the wireless channel on the characteristics of WLAN traffic.
This dissertation characterizes the correlation structures of WLAN channel
with traffic statistics from a cross-layer point of view, and provides new measurement
methodologies and statistical models for WLAN networks.
Currently WLAN standards are designed within the paradigm of the
layered network architecture. For example, the architecture of IEEE 802.11
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is almost identical to the Ethernet. However, wireless networks are fundamentally
different from their wired peers due to the shift of transmission media
from cables to over-the-air radio waves. This transition exposes wireless
systems to the influence of radio propagation, and more importantly, to the
temporal and spacial fluctuations of the radio channel that can actually be
propagated up to upper layers. However, the current WLAN architecture isolates
network layers, and largely ignores this impact. Therefore, we believe
that a cross-layer based approach is necessary to understand and reflect this
underlying impact of the channel to the upper layers of the network, especially
in relation to WLAN traffic behavior.
Measurement is one of the fundamental tools used to quantify radio
propagation. As part of this dissertation, a complete framework for a measurement
methodology, including hardware, software, and measurement procedures,
is established. Characteristics of the propagation channel are estimated
from measurement data, and the channel knowledge is applied to the upper
layers for more realistic and accurate modeling.
In WLAN environments, knowledge of the traffic characteristics is essential
for proper network provisioning, and for improving the performance
of the IEEE 802.11 standard and network devices, e.g., to design improved
MAC schemes, or to build better buffer scheduling algorithms with channel
knowledge, etc. Built upon extensive WLAN traffic traces, this dissertation
work presents cross-layer models for WLAN throughput predictions, traffic
statistics, and link layer characteristics.
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The main goal of this dissertation work is to experiment with and develop
new methods for identifying channel characteristics. Thereby utilizing
this knowledge, we show how to predict and improve WLAN performance.
Within the framework of the developed cross-layer measurement methodology,
we conducted extensive measurements in different physical environments
and different settings such as office buildings and stores, and (1) show that
the impact of the propagation channel can be quantified by using simple large
scale channel metric (throughput over longer period of time), and (2) also
present the existence of a Doppler effect within today’s WLAN packet traffic
at sub-second time scales. We also show the real-world WLAN usage pattern
from our measurement results. From this data, we conclude that the key issues
to study WLAN networks include accurate site-specific propagation channel
modeling and real-time autonomous traffic control.Electrical and Computer Engineerin
Performance Study of Hybrid Spread Spectrum Techniques
This thesis focuses on the performance analysis of hybrid direct sequence/slow frequency hopping (DS/SFH) and hybrid direct sequence/fast frequency hopping (DS/FFH) systems under multi-user interference and Rayleigh fading. First, we analyze the performance of direct sequence spread spectrum (DSSS), slow frequency hopping (SFH) and fast frequency hopping (FFH) systems for varying processing gains under interference environment assuming equal bandwidth constraint with Binary Phase Shift Keying (BPSK) modulation and synchronous system. After thorough literature survey, we show that hybrid DS/FFH systems outperform both SFH and hybrid DS/SFH systems under Rayleigh fading and multi-user interference. Also, both hybrid DS/SFH and hybrid DS/FFH show performance improvement with increasing spreading factor and decreasing number of hopping frequencies
Timing synchronization in MIMO-OFDM systems
OFDM (Orthogonal Frequency Division Multiplexing) provides a promising physical layer for
4G and 3GPP LTE Systems in terms of efficient use of bandwidth and high data rates. It is used
in several applications likeWiFi (IEEE 802.11n),WiMax (IEEE 802.16), Digital Audio Broadcasting
(DAB), Digital Video Broadcasting (DVB) and so on. OFDM suffers from inter-symbol
interference and inter-carrier interference in wireless and fading environments and it is important
to estimate and correct the start of OFDM symbol efficiently to reduce timing and frequency
offset errors. Synchronization issues in OFDM are crucial and can lead to certain amount of
information loss if they are not properly addressed. There are two modes of implementation
forDigital Video Broadcasting-Terrestrial (DVB-T) and this thesis implements the 2K mode. It
highlights the implementation of OFDM in DVB-T according to the European Telecommunications
Standards Institute (ETSI) . It mainly focuses on the timing offset problem present in
OFDM systems and its proposed solution using Cyclic Prefix (CP) as a modified Schmidl and
Cox’s (SC) algorithm. Simulations were performed to compare the different synchronization
methods with different amount of timing offsets and under different channel environments
Coupled Simulation-Measurements Platform for the Evaluation of Frequency-Reuse in the 2.45 GHz ISM band for Multi-mode Nodes with Multiple Antennas
International audienceIn this paper we address the problem of efficiently evaluate performance of concurrent radio links on overlapped channels. In complex network topologies with various standards and frequency channels, simulating a realistic PHY layer communication is a key point. The presented coupled simulation-measurement platform offers a very promising way of rapidly modelling and validating effective performance of multi-mode, multi-channel and multi-antenna radio nodes. An accurate of radio channel is performed and then realistic performance with or without antenna processing is shown, verifying theoretical performance. Finally, available performance of concurrent communications on overlapped channels is exposed, showing that this approach is viable to enhance network capacity
A new hybrid model of dengue incidence rate using negative binomial generalised additive model and fuzzy c-means model: a case study in Selangor
Dengue is one of the top reason for illness and mortality in the world with beyond oneÂthird of the world's population living in the risk areas of dengue infection. In this study, there are five stages to achieve the research objectives. Firstly, the verification of predetem1ined variables. Secondly, the identification of new datasets after clustered by district and Fuzzy C-Means Model (FCM). Thirdly, the development of models using the existing dataset and the new datasets which clustered by the two different clustering categories. Then, to assess the models developed by using three measurement methods which are deviance (D), Akaike Jnfonnation Criteria (AIC) and Bayesian Infonnation Criteria (BIC} Lastly, the validation of model developed by comparing the value of D, AIC and BIC between the existing model and the new models developed which used the new datasets. There are two different clustering techniques applied which are clustering the data by district and by FCM. This study proposed a new modelling hybrid framework by using two statistical models which are FCM and negative binomial Generalised Additive Model (GAM). This study successfully presents the significant difference in the climatic and non-climatic factors that influenced dengue incidence rate (DIR) in Selangor, Malaysia. Results show that the climatic factors such as rainfall with current month up to 3 months and number of rainy days with current month up to lag 3 months are significant to DIR. Besides, the interaction between rainfall and number of rainy days also shows strong positive relationship to DIR. Meanwhile, non-climatic vaiiables such as population density, number of locality and lag DIR from I month until 3 months also show significant relationship towards DIR For both clustering techniques, there are two clusters fonned and there are four new models developed in this study. After comparing the values of D, AIC ai1d BIC between the existing model and the new models, this study concluded that four new models recorded lower values compared to the existing model. Therefore, the four new models are selected to present the dengue incidence in Selangor
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