7 research outputs found
Adaptive Equalization for CO-Channel Interference in a Multipath Fading Environment
This paper concerns the feasibility and achievable performance of adaptive filtering in an interference-limited multipath fading environment as encountered in indoor wireless communications. In a typical cellular radio application, the performance-limiting impairment is interference due to synchronous data streams from other co-channel and adjacent channel users (CCI and ACI). The receiver under consideration employs an adaptive fractionally spaced decision feedback equalizer (DFE) which exploits the correlation of the cyclostationary interference to achieve superior performance relative to the worst case when the interference is stationary noise. This paper presents ideal calculations which confirm that significant DFE performance gains are potentially achievable by explicitly accounting for the cyclostationary CCI. Two adaptive DFE strategies are considered. One approach is to adapt the DFE directly using iterative algorithms such as least mean square (LMS) or recursive least squares (RLS). Another approach is to compute the MMSE DFE using an RLS CIR estimate and a sample estimate of the CCI autocorrelation obtained from the channel impulse response (CIR) estimation error during training. The best approach for adaptive equalization, in terms of adaptation speed and system performance, is to employ an RLS DFE which does not explicitly estimate the CIR or the CCI autocorrelation
Development of Fuzzy System Based Channel Equalisers
Channel equalisers are used in digital communication receivers to mitigate the effects of inter symbol interference (ISI) and inter user interference in the form of co-channel interference (CCI) and adjacent channel interference (ACI) in the presence of additive white Gaussian noise (AWGN). An equaliser uses a large part of the computations involved in the receiver. Linear equalisers based on adaptive filtering techniques have long been used for this application. Recently, use of nonlinear signal processing techniques like artificial neural networks (ANN) and radial basis functions (RBF) have shown encouraging results in this application. This thesis
presents the development of a nonlinear fuzzy system based equaliser for digital communication receivers.
The fuzzy equaliser proposed in this thesis provides a parametric implementation of symbolby-symbol maximum a-posteriori probability (MAP) equaliser based on Bayes’s theory. This MAP equaliser is also called Bayesian equaliser. Its decision function uses an estimate of the noise free received vectors, also called channel states or channel centres. The fuzzy equaliser developed here can be implemented with lower computational complexity than the RBF implementation of the MAP equaliser by using scalar channel states instead of channel states. It also provides schemes for performance tradeoff with complexity and schemes for subset centre
selection. Simulation studies presented in this thesis suggests that the fuzzy equaliser by using only 10%-20% of the Bayesian equaliser channel states can provide near optimal performance.
Subsequently, this fuzzy equaliser is modified for CCI suppression and is termed fuzzy–CCI equaliser. The fuzzy–CCI equaliser provides a performance comparable to the MAP equaliser designed for channels corrupted with CCI. However the structure of this equaliser is similar to the MAP equaliser that treats CCI as AWGN. A decision feedback form of this equaliser which uses a subset of channel states based on the feedback state is derived. Simulation studies presented in this thesis demonstrate that the fuzzy–CCI equaliser can effectively remove CCI without much increase in computational complexity. This equaliser is also successful in removing
interference from more than one CCI sources, where as the MAP equalisers treating CCI as AWGN fail. This fuzzy–CCI equaliser can be treated as a fuzzy equaliser with a preprocessor for CCI suppression, and the preprocessor can be removed under high signal to interference ratio condition
A study of RF-over-fibre based active RFID indoor location system
Location systems developed for indoor environments have attracted increasing
interest, as a result of the rapidly growing location and navigation services provided
by the Global Positioning System (GPS). Location information of people and objects
can be used to cooperate with existing communication or database systems to provide
abundant services to system operators and end users. For example, equipment tracking
in hospitals ensure that location of the appropriate equipment can be provided
simultaneously with necessary medical services; attendee tracking at conferences may
encourage more efficient communications and networking; location of valuable assets
in factories or warehouses aids logistics and protects these assets from theft. Since
established global and terrestrial navigation systems cannot provide reliable location
services in indoor environments, these demands are increasingly being met by
wireless indoor location systems.
A review of the existing systems reveals that the current systems are able to provide
either an accurate location service with sophisticated system design at higher cost or a
less accurate location service by means of integrated systems supplemented by
existing facilities.
This thesis presents a novel design of an indoor location system that is based on an
RF-over-fibre backbone network, which is able to provide high location accuracy
while the network infrastructure can be shared with multiple wireless systems. It is the
first such demonstrator in this area. This research has been conducted by the author
through a research project called The Intelligent Airport (TINA), which is the
motivation for this research.
The TINA project seeks to develop a new seamless wireless/wired ubiquitous
infrastructure with high levels of computational capability to meet the application
requirements of future airport environments. In the TINA system, multiple wireless
services are provided through an integrated system supported by an RF-over-Fibre
network, which transports RF signals through optical fibres. The active RFID indoor location unit is an essential part of the TINA system, which will facilitate the
infrastructure to provide location-based services.
The thesis describes the detailed design of the active RFID indoor location system
proposed for the TINA project, and a few key issues discovered during trials of the
demonstration system developed. The overall system design, including ranging
technique, TDOA location finding algorithm, and hardware implementation, is
presented in this thesis. Particular contributions also include a numerical algorithm for
solving target location from TDOA measurement and a technique to determine the
chirp linearity requirement. The field trial results of the system design demonstrate the
principals and their location performance. The system has the potential to be extended
to other scenarios where RF-over-fibre networks are employed and accurate location
ability is desired
Frequency shift filtering for cyclostationary signals.
The frequency-shift (FRESH) filter is a structure which exploits the spectral correlation of cyclostationary
signals for removing interference and noise from a wanted signal. As most digital
communication signals are cyclostationary, FRESH filtering offers certain advantages for interference
rejection in a communications receiver.
This thesis explores the operation and application of FRESH filters in practical interference scenarios.
The theoretical background to cyclostationarity is clarified with graphical interpretations
of what cyclostationarity is, and how a FRESH filter exploits it to remove interference. The effects
of implementation in a sampled system are investigated, in filters which use baud rate related cyclostationarity,
leading to efficiency improvements. The effects of varying the wanted signal pulse
shape to enhance the cyclostationarity available to the FRESH filter are also investigated.
A consistent approach to the interpretation of the FRESH filter's operation is used throughout,
while evaluating the performance in a wide range of realistic channel conditions.
VLF radio communication is proposed as one area where interference conditions are particularly
suitable for the use of FRESH filtering. In cases of severe adjacent channel interference it is found
that a FRESH filter can almost completely remove the interferer. The effects of its use with an
impulse rejection technique are also investigated.
Finally, blind adaptation of FRESH filters through exploitation of carrier related cyclostationarity
is investigated. It is found that one existing method loses the advantage of FRESH filtering over
time invariant linear filtering. An improvement is proposed to the latter which restores its performance
to that of a trained FRESH filter, and also reveals that carrier related cyclostationarity can
be exploited, in some cases, by a simpler method.
J