### Browsing by Author "Braun, Robin M"

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- ItemOpen AccessArtificial neural network decoding of multi-h CPM(1997) Kannemeyer, Johan Etienne; Braun, Robin MThe purpose of this report is to set out the results of an investigation into the artificial neural network (ANN) decoding of multi-h continuous phase modulation (CPM) schemes. Multi-h CPM schemes offer forward error correction (FEC) capabilities for continuous transmission, digital communication systems. Multi-h CPM is reported to be a bandwidth efficient alternative to other FEC techniques such as convolutional coding, while neural networks allow for high speed decoding. A neural network decoder was found in [12], where it had been used for the decoding of a convolutional code. This neural network structure by Xiao-an Wang and Stephen 'B. Wicker implements the Viterbi Algorithm (VA). All the necessary decoding information is contained in the interconnections of the ANN, and can be found by inspection of the state trellis diagram of the convolutional code. The decoder therefore requires no training. Since all the computation is done by analogue neurons and shift registers, the neural network reduces to a hybrid digital-analogue implementation of the VA. The use of analogue neurons allows the structure to be used for high data rate communications. Furthermore, the decoder is reported to be suitable for VLSI implementation.
- ItemOpen AccessCharacteristics of multi-h coded modulation(1994) Crawford, Brian Patrick; Braun, Robin MMulti-h Coded Continuous-Phase Frequency Shift Keying (Multi-h CPFSK) has gained interest in recent years because it offers an additional degree of freedom in the coding of CPFSK. Similar to Trellis Coded Modulation (TCM), it does not use redundancy to achieve coding gain. Hence with properly chosen modulation indices, impact to spectral occupancy is kept to a minimum. While there has been less attention given of this method as compared with TCM, this method can also be used with data coding. In cases where data coding is to be implemented, simultaneous use of Multi-h coding can be implemented with very little increase in complexity. In this thesis, a thorough mathematical review of this technique is made. A multi-oscillator multih coded modulator is shown similar to one presented by Massey for MSK. A unique analytical tool called a multi-oscillator trellis is presented. This considers the phase transitions with respect to each of the signalling frequencies instead of the center frequency, fâ‚€. The multi-oscillator trellis is used to determine the state machine that will switch a bank of oscillators. The purpose of the state machine is to maintain continuous phase at the multi-oscillator output while generating the proper signal frequencies according to the data and modulation index. The Maximum Likelihood Detection process at the receiver is shown as a partition of an uncoded CPFSK signal. Finally, an analysis is made to determine if a modulator with a non-linear frequencyvoltage characteristic is suitable in a coherent multi-h coded application. Much of the literature on this topic has been comparative to PSK. It is the intent of this work to use FSK and MSK as the baseline to determine how existing structures may be extended to realize the benefits of multi-h coding. The application of this coding to an 8 Mbps 23 GHz CPFSK point-to-point terrestrial communications system is also a topic of this thesis. It is in this context that the analysis is made.
- ItemOpen AccessA hardware implementation of a Viterbi decoder for a (3,2/3) TCM code(1997) Horwitz, Michael Richard; Braun, Robin MThe report details the design of a dedicated Viterbi decoder chip set for an Ungerboek (3,2/3) Trellis Coded Modulation code. It was the specific intention of the thesis to design a system that could be implemented on standard Field Programmable Gate Arrays (FPGA) yet still be able to cope with high bit rates. The focus of the research was to both evaluate and modify the existing VLSI design techniques and to develop new techniques to make this possible. Trellis Coded Modulation refers to a specific sub-class of convolutional codes that ire an example of coded modulation. In coded modulation there is a direct link between the encoding and modulation processes aimed at improving the performance of the code by introducing redundancy in the signal set used to transmit the code. Ungerboek developed a technique for mapping the encoded words onto points in the signal set, called mapping by set partitioning, that maximises the Euclidian distance between adjacent codewords, and hence maximises the minimum distance between any two output sequences in the code. The Viterbi algorithm is a maximum likelihood decoder for convolutional codes such as TCM. The operation of the Viterbi algorithm is based on using soft decision decoding to produce an estimate of how well the received sequence corresponds with any of the allowed code sequences. The code sequences which most closely matches the received sequence is then decoded to form the output of the decoder. A central problem in implementing systems using TCM with Viterbi decoding is that although the encoder is a relatively simple device, the decoder is not. The complexity of the Viterbi decoder for any given TCM scheme will be the major drawback in implementing the scheme. As such techniques for reducing the complexity of Viterbi decoders are of interest to developers of communication systems. The algorithms describing the implementation and operation of the Viterbi algorithm can be categorised into three main layers. The top layer holds the theoretical algorithm itself, in the second layer are the set of algorithms that describe the broad techniques used to manipulate the theoretical algorithm into a form in which it can be implemented, and the third layer of algorithms describe the implementations themselves. The work contained in this thesis concentrates on the second two layers of algorithms.
- ItemOpen AccessHigh performance non-symmetric multi-h CPFSK modulator and demodulator design(1997) Cuthbert, James; Braun, Robin MThe continuity properties of the CPFSK signal at it's symbol-period boundaries reveals an inherent memory contained in the transmitted signal. This is utilized as an error correction property. Furthermore, it was shown that the Multi-h CPFSK construction can be accomplished through the combination of a block constructing the memoryless component of the signal and either a block of digital logic circuitry or a continuous phase encoder constructing the memory component. The implementation of the first method was seen to function through simulations performed by using the TESLA simulation package. An extensive search for good Multi-h CPFSK h-sets was performed. The criteria for determining the performance of these h-sets was the Probability of Error gains over Minimum Shift Keying. The method of search was novel to this work. Specifically, a genetic search algorithm known as the Population Based Incremental Learning algorithm was utilized. The algorithm was implemented through the C++ programming language Faster error correction convolutional decoding algorithms were reviewed. Certain decoders exhibit lighter hardware demands, and in specific applications, are less susceptible to erasure problems. The Fano algorithm was selected as the best alternative to the Viterbi algorithm and was modified for the CPFSK implementation. The functionality of the implementation was tested using a C++ simulation. Various structures used to implement the synchronization and demodulation of Multi-hCPFSK were investigated. The most comprehensive structure that could be found was a scheme developed by Premji and Taylor using maximum likelihood techniques. This scheme was selected as it can be easily modified for the use with the large state, high speed implementation of non-symmetric Multi-h CPFSK investigated in this thesis. The PBIL algorithm was found to be an efficient method for finding good h-sets with large numbers of phase states. Theoretical gains over MSK using this method were found to be significant. It was concluded that the Fano decoder is highly applicable to the demodulator structure proposed in this thesis and is a preferred alternative to the Viterbi decoder under specific circumstances.
- ItemOpen AccessAn investigation into non-linear propagation of MSK with a view to specifying an adaptive equalizer(1989) Howie, David Baird; Braun, Robin MThe purpose of this dissertation is to investigate the effects of non-linear propagation on a proposed digital microwave radio link which employs MSK modulation, in order to specify a suitable form of adaptive equalization. MSK is a coherent modulation technique, having improved spectral roll-off over FSK because it avoids the abrupt phase changes at the bit transitions. However computer simulations and field results indicate that MSK digital radio links do suffer from intersymbol interference and crosstalk. Software and hardware simulations of multipath propagation are based on Rummlers simplified three path model and statistics. The results obtained from the computer simulations of the MSK link and multipath propagation confirm that there is no simple relationship between the multipath parameters and the BER degradation which could be used in the design of an equalizer. The choice of adaptive equalizer is made based on criteria such as construction cost, circuit complexity, and performance improvement. It is known from ray model analysis that at a transmitting frequency of 23 GHz deep fading will only occur on links longer than 5.24 kms. However even on hops of length 5 km' s the fade time is-in the order of 1612 seconds/month (calculated using Rummlers model). A ldB increase in theoretical Eb/No will also be required to overcome potential modem imperfections. It is necessary to have a time domain equalizer which can compensate for both amplitude and phase distortions simultaneously by acting directly on the ISI. The equalizer structure chosen is a 2-by-2, fractionally spaced, decision feedback, complex adaptive equalizer with zero forcing control algorithm.
- ItemOpen AccessAn investigation into the performance capabilities of multi-h CPFSK digital modulation(1997) Airey, Garth; Braun, Robin MThis thesis sets out the procedures and results of an investigation into the performance capabilities of multi-h CPFSK digital modulation. Multi-h CPFSK is a coded modulation scheme. It exhibits high bandwidth efficiency and good envelope properties and is therefore well suited to use in satellite communications channels. However, the power efficiency of multi-h CPFSK signals is largely dependent on the particular multi-h code used, as well as the optimality of the decoder. Optimal decoding of multi-h CPFSK signals is achieved by means of the Viterbi Algorithm, applied to the multi-h phase trellis diagram. The Viterbi path metric is given by the squared euclidean distance between the particular path and the received signal.
- ItemOpen AccessNovel low cost synchronisation network for spread spectrum systems(1995) Van de Groenendaal, Joannes Gerard; Braun, Robin MSpread Spectrum systems are found in many flavours, used in many applications and have existed since the early days of radio communications. The properties of spread spectrum do however place restrictions on the design, and often make the implementation expensive and complex. When using spread spectrum to provide a basic communications infrastructure, many factors need to be considered. These include supplying the appropriate technology at the right cost. To achieve this a trade-off against performance is often required. One of the more difficult aspects of Spread Spectrum design is the synchronisation of the spreading waveform. The primary characteristic of pseudonoise sequence synchronisation is the need for two levels of synchronisation namely acquisition (course synchronisation) and tracking (fine synchronisation). In these networks (the term network is used to describe a circuit or system throughout the thesis.) a decision is required to switch between the two synchronisation modes. The two layer structure of the typical pseudonoise sequence synchronisation network can increase the overall cost of spread spectrum systems. The objective of the research was therefore to find solutions to reduce the overall cost and complexity of the synchronisation network. The synchronisation structure should perform acquisition and tracking in a single structure, and thereby be low cost. To achieve the primary objective of this dissertation a. mixture of theory, simulations and practical implementation was used. The basis of the investigation was a time-variant spectral evaluation of pseudonoise sequences. It is shown that by multiplying a differentiated pseudonoise sequence with another pseudonoise sequence, useful information is obtained that can form the basis of a synchronisation network.
- ItemOpen AccessA study of the minimum shift keying modulation scheme(1988) Aldera, Mario Antonio; Braun, Robin MThis thesis concerns itself with the study of the Minimum Shift Keying (MSK) modulation scheme. The aspects considered are its operation under non-linear conditions as well as an investigation into the hardware implementation of both coherent and non-coherent MSK modems. The literature on digital data transmission and MSK in particular is surveyed, and a comprehensive theoretical description of MSK is given. In addition, papers on the operation of MSK under non-linear conditions were studied, and their major findings are presented. Due to the lack of theory on the effects of incorrect modulation index on the error performance of MSK, an investigation into this avenue was performed. The design of a correction mechanism for maintaining the modulation index at its correct value is described, and aspects of its implementation are considered. Using the available literature, various modules of which a coherent MSK modem is comprised were developed, and their design is discussed. The design of a non-coherent MSK demodulator is also described.