874 research outputs found
Improving Performance Through Channel Diversity in the Presence of Bursty Losses
As more applications migrate to IP networks, ensuring a consistent level of service is increasingly important. One option is for the network to offer service guarantees. Another is to leverage the path diversity that the Internet intrinsically offers. Our focus is on understanding if and when one can indeed take advantage of multiple disjoint paths to improve performance. We consider an environment where loss patterns are bursty and where coding is used to provide robustness against packet losses. We assume that only long-term loss statistics are known about each path, and we seek to identify the best strategy for sending packets over the available paths. Our contributions are two-fold. First we demonstrate that even with minimal knowledge of channel characteristics and using simple transmission policies, path diversity can help significantly improve performance. Second, we derive an efficient method for identifying optimal policies, and more importantly characterize when having access to multiple paths can be of benefit
Channel-Aware Random Access in the Presence of Channel Estimation Errors
In this work, we consider the random access of nodes adapting their
transmission probability based on the local channel state information (CSI) in
a decentralized manner, which is called CARA. The CSI is not directly available
to each node but estimated with some errors in our scenario. Thus, the impact
of imperfect CSI on the performance of CARA is our main concern. Specifically,
an exact stability analysis is carried out when a pair of bursty sources are
competing for a common receiver and, thereby, have interdependent services. The
analysis also takes into account the compound effects of the multipacket
reception (MPR) capability at the receiver. The contributions in this paper are
twofold: first, we obtain the exact stability region of CARA in the presence of
channel estimation errors; such an assessment is necessary as the errors in
channel estimation are inevitable in the practical situation. Secondly, we
compare the performance of CARA to that achieved by the class of stationary
scheduling policies that make decisions in a centralized manner based on the
CSI feedback. It is shown that the stability region of CARA is not necessarily
a subset of that of centralized schedulers as the MPR capability improves.Comment: The material in this paper was presented in part at the IEEE
International Symposium on Information Theory, Cambridge, MA, USA, July 201
Secure Communications for the Two-user Broadcast Channel with Random Traffic
In this work, we study the stability region of the two-user broadcast channel
(BC) with bursty data arrivals and security constraints. We consider the
scenario, where one of the receivers has a secrecy constraint and its packets
need to be kept secret from the other receiver. This is achieved by employing
full-duplexing at the receiver with the secrecy constraint, so that it
transmits a jamming signal to impede the reception of the other receiver. In
this context, the stability region of the two-user BC is characterized for the
general decoding case. Then, assuming two different decoding schemes the
respective stability regions are derived. The effect of self-interference due
to the full-duplex operation on the stability region is also investigated. The
stability region of the BC with a secrecy constraint, where the receivers do
not have full duplex capability can be obtained as a special case of the
results derived in this paper. In addition, the paper considers the problem of
maximizing the saturated throughput of the queue, whose packets does not
require to be kept secret under minimum service guarantees for the other queue.
The results provide new insights on the effect of the secrecy constraint on the
stability region of the BC. In particular, it is shown that the stability
region with secrecy constraint is sensitive to the coefficient of
self-interference cancelation under certain cases.Comment: Submitted for journal publicatio
Error models for digital channels and applications to wireless communication systems
Digital wireless channels are extremely prone to errors that appear in bursts or clusters.
Error models characterise the statistical behaviour of bursty profiles derived from
digital wireless channels. Generative error models also utilise those bursty profiles in
order to create alternatives, which are more efficient for experimental purposes. Error
models have a tremendous value for wireless systems. They are useful for the design
and performance evaluation of error control schemes, in addition to higher layer protocols
in which the statistical properties of the bursty profiles are greatly functional.
Furthermore, underlying wireless digital channels can be substituted by generated
error profiles. Consequently, computational load and simulation time can be significantly
reduced when executing experiments and performing evaluation simulations
for higher layer communications protocols and error control strategies.
The burst error statistics are the characterisation metrics of error models. These
statistics include: error-free run distribution; error-free burst distribution; error burst
distribution; error cluster distribution; gap distribution; block error probability distribution;
block burst probability distribution; bit error correlation function; normalised
covariance function; gap correlation function; and multigap distribution. These burst
error statistics scrutinise the error models and differentiate between them, with regards
to accuracy. Moreover, some of them are advantageous for the design of digital
components in wireless communication systems.
This PhD thesis aims to develop accurate and efficient error models and to find applications
for them. A thorough investigation has been conducted on the burst error
statistics. A breakdown of this thesis is presented as follows.
Firstly, an understanding of the different types of generative error models, namely,
Markovian based generative models, context-free grammars based generative models,
chaotic models, and deterministic process based generative models, has been presented.
The most widely used models amongst the generative models have been
compared with each other consulting the majority of burst error statistics. In order
to study generative error models, error burst profiles were obtained mainly from the
Enhanced General Packet Radio Service (EGPRS) system and also the Long Term
Evolution (LTE) system.
Secondly, more accurate and efficient generative error models have been proposed.
Double embedded processes based hidden Markov model and three-layered processes
based hidden Markov model have been developed. The two types of error profiles,
particularly the bit-level and packet-level error profiles were considered.
Thirdly, the deterministic process based generative modelsā parameters have been
tuned or modified in order to generate packet error sequences rather than only bit
error sequences. Moreover, a modification procedure has been introduced to the same
models to enhance their generation process and to make them more desirable.
Fourthly, adaptive generative error models have been built in order to accommodate
widely used generative error models to different digital wireless channels with different
channel conditions. Only a few reference error profiles have been required in order to
produce additional error profiles in various conditions that are beneficial for the design
and performance evaluation of error control schemes and higher layer protocols.
Finally, the impact of the Hybrid Automatic Repeat reQuest (HARQ) on the burst
error statistics of physical layer error profiles has been studied. Moreover, a model that
can generate predicted error sequences with burst error statistics similar to those of
error profiles when HARQ is included has been proposed. This model is constructive
in predicting the behaviour of the HARQ in terms of a set of higher order statistics
rather than only predicting a first order statistic. Moreover, the whole physical layer
is replaced by adaptively generated error profiles in order to check the performance
of the HARQ protocol.
The developed generative error models as well as the developed adaptive generative
error models are expected to benefit future research towards the testing of many
digital components in the physical layer as well as the wireless protocols of the link
and transport layers for many existing and emerging systems in the field of wireless
communications
Performance study of voice over frame relay : a thesis presented in partial fulfilment of the requirements for the degree of Master of Engineering in Information Engineering, Massey University, Albany, New Zealand
Frame Relay (FR) represents an important paradigm shift in modern telecommunication. This technology is beginning to evolve from data only application to broad spectrum of multimedia users and potential to provide end users with cost effective transport of voice traffic for intra office communication. In this project the recent development in voice communication over Frame relay is investigated. Simulations were carried out using OPNET, a powerful simulation software. Following the simulation model, a practical design of the LAN-to-LAN connectivity experiment was also done in the Net Lab. From the results of the simulation, Performance measures such as delay, jitter, and throughput are reported. It is evident from the results that real-time voice or video across a frame relay network can provide acceptable performance
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