Detection for a Statistically-Known, Time-Varying Dispersive Channel

Detection for the statistically known channel (SKC) is aimed at obtaining good performance in situations where our statistical knowledge of a time-varying channel is good, and where other equalization/detection schemes are either too complex to implement, or their performance is limited due to the rapidity of channel fading, or where we are simply unable to perform channel estimation. By using a statistical characterization of the channel, we develop a new detector that performs maximum-likelihood sequence estimation (MLSE) (given the channel model) on blocks of N symbols. Both symbol-spaced and fractionally spaced samples are used, to obtain two different detectors, that are generalizations of those devised for optimal block schemes on nondispersive channels. The detector that uses fractionally spaced samples is shown to outperform the detector that uses symbol-spaced samples. The performance of both appears to approach that of the corresponding known channel (KC) detector as the block length increases. We also numerically evaluate the SKC detector performance under conditions where the channel parameters (statistics) are incorrectly estimated, and show that the fractionally spaced detector is fairly robust to modeling errors. Finally, we devise a sliding block algorithm, for use when transmitting more than N symbols

Adaptive equalisation for fading digital communication channels

This thesis considers the design of new adaptive equalisers for fading digital communication channels. The role of equalisation is discussed in the context of the functions of a digital radio communication system and both conventional and more recent novel equaliser designs are described. The application of recurrent neural networks to the problem of equalisation is developed from a theoretical study of a single node structure to the design of multinode structures. These neural networks are shown to cancel intersymbol interference in a manner mimicking conventional techniques and simulations demonstrate their sensitivity to symbol estimation errors. In addition the error mechanisms of conventional maximum likelihood equalisers operating on rapidly time-varying channels are investigated and highlight the problems of channel estimation using delayed and often incorrect symbol estimates. The relative sensitivity of Bayesian equalisation techniques to errors in the channel estimate is studied and demonstrates that the structure's equalisation capability is also susceptible to such errors. Applications of multiple channel estimator methods are developed, leading to reduced complexity structures which trade performance for a smaller computational load. These novel structures are shown to provide an improvement over the conventional techniques, especially for rapidly time-varying channels, by reducing the time delay in the channel estimation process. Finally, the use of confidence measures of the equaliser's symbol estimates in order to improve channel estimation is studied and isolates the critical areas in the development of the technique — the production of reliable confidence measures by the equalisers and the statistics of symbol estimation error bursts

Estimation and detection techniques for doubly-selective channels in wireless communications

A fundamental problem in communications is the estimation of the channel. The signal transmitted through a communications channel undergoes distortions so that it is often received in an unrecognizable form at the receiver. The receiver must expend significant signal processing effort in order to be able to decode the transmit signal from this received signal. This signal processing requires knowledge of how the channel distorts the transmit signal, i.e. channel knowledge. To maintain a reliable link, the channel must be estimated and tracked by the receiver. The estimation of the channel at the receiver often proceeds by transmission of a signal called the 'pilot' which is known a priori to the receiver. The receiver forms its estimate of the transmitted signal based on how this known signal is distorted by the channel, i.e. it estimates the channel from the received signal and the pilot. This design of the pilot is a function of the modulation, the type of training and the channel. [Continues.

Iterative receivers and multichannel equalisation for time division multiple access systems

The thesis introduces receiver algorithms improving the performance of TDMA mobile radio systems. Particularly, we consider receivers utilising side information, which can be obtained from the error control coding or by having a priori knowledge of interference sources. Iterative methods can be applied in the former case and interference suppression techniques in the latter. Convolutional coding adds redundant information into the signal and thereby protects messages transmitted over a radio channel. In the coded systems the receiver is usually comprised of separate channel estimation, detection and channel decoding tasks due to complexity restrictions. This suboptimal solution suffers from performance degradation compared to the optimal solution achieved by optimising the joint probability of information bits, transmitted symbols and channel impulse response. Conventional receiver utilises estimated channel state information in the detection and detected symbols in the channel decoding to finally obtain information bits. However, the channel decoder provides also extrinsic information on the bit probabilities, which is independent of the received information at the equaliser input. Therefore it is beneficial to re-perform channel estimation and detection using this new extrinsic information together with the original input signal. We apply iterative receiver techniques mainly to Enhanced General Packet Radio System (EGPRS) using GMSK modulation for iterative channel estimation and 8-PSK modulation for iterative detection scheme. Typical gain for iterative detection is around 2 dB and for iterative channel estimation around 1 dB. Furthermore, we suggest two iteration rounds as a reasonable complexity/performance trade-off. To obtain further complexity reduction we introduce the soft trellis decoding technique that reduces the decoder complexity significantly in the iterative schemes. Cochannel interference (CCI) originates from the nearby cells that are reusing the same transmission frequency. In this thesis we consider CCI suppression by joint detection (JD) technique, which detects simultaneously desired and interfering signals. Because of the complexity limitations we only consider JD for two binary modulated signals. Therefore it is important to find the dominant interfering signal (DI) to achieve the best performance. In the presence of one strong DI, the JD provides major improvement in the receiver performance. The JD requires joint channel estimation (JCE) for the two signals. However, the JCE makes the implementation of the JD more difficult, since it requires synchronised network and unique training sequences with low cross-correlation for the two signals.reviewe

A Summative Comparison of Blind Channel Estimation Techniques for Orthogonal Frequency Division Multiplexing Systems

The OFDM techniquei.e. Orthogonal frequency division multiplexing has become prominent in wireless communication since its instruction in 1950’s due to its feature of combating the multipath fading and other losses. In an OFDM system, a large number of orthogonal, overlapping, narrow band subchannels or subcarriers, transmitted in parallel, divide the available transmission bandwidth. The separation of the subcarriers is theoretically optimal such that there is a very compact spectral utilization. This paper reviewed the possible approaches for blind channel estimation in the light of the improved performance in terms of speed of convergence and complexity. There were various researches which adopted the ways for channel estimation for Blind, Semi Blind and trained channel estimators and detectors. Various ways of channel estimation such as Subspace, iteration based, LMSE or MSE based (using statistical methods), SDR, Maximum likelihood approach, cyclostationarity, Redundancy and Cyclic prefix based. The paper reviewed all the above approaches in order to summarize the outcomes of approaches aimed at optimum performance for channel estimation in OFDM system

Underwater acoustic communications

The underwater acoustic medium poses unique challenges to the design of robust, high throughput digital communications. The aim of this work is to identify modulation and receiver processing techniques to enable the reliable transfer of data at high rate, at range between two, potentially mobile parties using acoustics. More generally, this work seeks to investigate techniques to effectively communicate between two or more parties over a wide range of channel conditions where data rate is a key but not always the absolute performance requirement. Understanding the intrinsic ocean mechanisms that influence signal coherence, the relationship between signal coherence and optimum signal design, and the development of robust modulation and receiver processing techniques are the main areas of study within this work. New and established signal design, modulation, synchronisation, equalisation and spatial processing techniques are investigated. Several new, innovative techniques are presented which seek to improve the robustness of ‘classical’ solutions to the underwater acoustic communications problem. The performance of these techniques to mitigate the severe temporal dispersion of the underwater channel and its unique temporal variability are assessed. A candidate modulation, synchronisation and equalisation architecture is proposed based on a spatial-temporal adaptive signal processing (STAP) receiver. Comprehensive simulation results are presented to demonstrate the performance of the candidate receiver to time selective, frequency selective and spatially selective channel behaviour. Several innovative techniques are presented which maximise system performance over a wider range of operational and environmental conditions. Field trials results are presented based on system evaluation over a wide range of geographically distinct environments demonstrating system performance over a diverse range of ocean bathymetry, topography and background noise conditions. A real time implementation of the system is reported and field trials results presented demonstrating the capability of the system to support a wide range of data formats including video at useful frame rates. Within this work, several novel techniques have been developed which have extended the state of the art in high data rate underwater communications:- • Robust, high fidelity open loop synchronisation techniques capable of operating at marginal signal-to-noise ratios over a wide range of severely time spread environments. These high probability of synchronisation, low probability of false alarm techniques, provide the means for ‘burst’ open loop synchronisation in time, Doppler and space (bearing). The techniques have been demonstrated in communication and position fixing/navigation systems to provide repeatable range accuracy’s to centimetric order. • Novel closed loop synchronisation compensation for STAP receiver architectures. Specifically, this work has demonstrated the performance benefits of including both delay lock loop (DLL) and phase lock loop (PLL) support for acoustic adaptive receivers to offload tracking effort from the fractional feedforward equaliser section. It has been shown that the addition of a DLL/PLL outperforms the PLL only case for Doppler errors exceeding a few fractions of a knot. • Recycling of training data has been demonstrated as a potentially useful means to improve equaliser convergence in difficult acoustic channels. With suitable processing power, training data recycling introduces no additional transmission time overhead, which may be a limiting factor in battery powered applications. • Forward and time reverse decoding of packet data has been demonstrated as an effective means to overcome some non-minimum phase channel conditions. It has also been shown that there may be further benefits in terms of improved bit error performance, by exploiting concurrent forward and backward symbol data under modest channel conditions. • Several wideband techniques have been developed and demonstrated to be effective at resolving and coherently tracking difficult doubly spread acoustic channels. In particular, wideband spread spectrum techniques have been shown to be effective at resolving acoustic multipath, and with the aid of independent delay lock loops, track individual path arrivals. Techniques have been developed which can effect coherent or non-coherent recombination of these paths with a view to improving the robustness of an acoustic link operating at very low signal-to-noise levels. • Demonstrated throughputs of up to 41kbps in a difficult, tropical environment, featuring significant biological noise levels for mobile platforms at range up to 1.5km. • Demonstrated throughputs of between 300bps and 1600bps in a shallow, reverberant environment, at a range up to 21km at LF. • Implemented and demonstrated all algorithms in real time systems

Adaptive estimation and equalisation of the high frequency communications channel

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