<p> This thesis deals with the problem of digital communication over noisy dispersive channels. The dispersion causes the overlapping of successive received pulses thus creating intersymbol interference which severely limits the performance of conventional receivers designed to combat only additive interference or noise.</p> <p> In this thesis Bayes estimation theory has been applied to obtain a new, optimum, unrealizable receiver structure for the improved reception of noisy, dispersed, pulse amplitude-modulated (PAM) signals. By making certain approximations, a realization of this structure, known as the estimate feedback receiver or equalizer, is obtained. It consists of the combination of a matched filter and a nonlinear, recursive equalizer having, in the case of binary signals, a hyperbolic tangent nonlinearity in the feedback path. The well known decision feedback equalizer is shown to be a small noise limiting case of the estimate feedback equalizer. A saturating limiter is also considered as an approximation to the hyperbolic tangent nonlinearity.</p> <p> A new adaptive algorithm for the iterative adjustment of the estimate feedback equalizer is derived. It incorporates an extrapolation process which has the purposes of accelerating convergence of the equalizer's parameters to their optimum values and of maintaining the equalizer's frame of reference. It is constrained so that the equalizers parameters always move toward their optimum values.</p> <p> Computer simulations are used to demonstrate the properties of the adaptive estimate feedback equalizer and to compare them to those of presently known equalizers. When the estimate feedback equalizer is used, without a matched filter preceding it, to equalize phase distorted channels, its performance is seen to be superior to that of existing equalizers. The performance of an equalizer using a saturating limiter in place of the optimum hyperbolic tangent nonlinearity is seen to be almost as good as that of the estimate feedback equalizer.</p> / Thesis / Doctor of Philosophy (PhD)

Most existing transceivers are Fourier-centric where complex sinusoids play a central role in the internals of the core building blocks. From the channel perspective, complex sinusoids constitute the fundamental effects in the wireless baseband equivalent channel model; exemplified by the time-invariant and time-varying transfer functions in static and time-varying channel conditions respectively. In addition, complex sinusoids are used as signaling waveforms for data transmission through the channel. The dominant mode of transmission in modern communications is in the form of finite time duration blocks having approximately finite bandwidth. As a result, the time-frequency space becomes projected to a time-frequency subspace having essentially limited support where complex sinusoids suffer from leakage effects due to the finite time extent of a block. In addition, Kronecker delta signals (duals of complex sinusoids) suffer from the same vulnerability due to the finite extent bandwidth. Gabor signaling bases using non-rectangular pulse shapes can attain good confinement in the time-frequency space, however, at the expense of completeness which reduces the utilization efficiency of the time-frequency signaling resources. Over a signaling block period, a doubly dispersive (DD) channel is projected onto an essentially limited time-frequency subspace. In this subspace, the Discrete Prolate Spheroidal (DPS) basis matched to the channel parameters is known to be optimally compact in representing the channel using a basis expansion decomposition. Unlike the Discrete Fourier Transform (DFT) basis which lacks compactness due to the leakage effect. Leakage in the expansion coefficients of a particular channel using the DFT basis has a direct correspondence with the Inter-Symbol Interference (ISI) between the DFT signaling components when transmitted through the same channel. For the DPS basis, however, the correspondence is not as obvious. Nevertheless, DPS when used for signaling results in ISI compactness in the form of an exponential decay of distant ISI components. The efficacy of DPS signaling in DD channels in addition to its efficiency in modeling DD channels motivates the investigation of a new transceiver baseband architecture where DFT is supplanted by DPS. / Ph. D.

This paper offers a comprehensive and integrative overview of all empirical data available from the open literature on the in-tunnel radiowave-communication channel's power loss characteristics, as a function of the tunnel's cross-sectional shape, cross-sectional size, longitudinal shape, wall materials, presence or absence of vehicular/human traffic, and presence/absence of branches. These data were originally presented in about 50 papers in various journals, conferences, and books.

This paper offers a comprehensive and integrative overview of all empirical data available from the open literature on the in-tunnel radiowave-communication channel's power loss characteristics, as a function of the tunnel's cross-sectional shape, cross-sectional size, longitudinal shape, wall materials, presence or absence of vehicular/human traffic, and presence/absence of branches. These data were originally presented in about 50 papers in various journals, conferences, and books.

With the recent advancements of wireless networks to satisfy new requirements, the investigation of novel transmission schemes to improve the link level performance is of major importance. A very common technique utilized in nowadays systems is the Orthogonal frequency division multiplexing (OFDM) waveform, which has been adopted by several standards, including WiFi, LTE, and more recently 5G, due to its simple equalization process. Despite its success, this dissertation shows that OFDM is a sub-optimal scheme under frequency-selective channel (FSC), when channel state information (CSI) is available at the receiver only. Based on the coded modulation capacity approach, this work demonstrates that the data symbols should experience the same channel gain in order to achieve the best performance, leading to the equal gain criterion (EGC). However, this comes at a cost in terms of losing orthogonality among data symbols. The result is valid for linear modulation matrices under the assumptions of CSI at only at the receiver with perfect feedback equalization. In order to attain the EGC for doubly-dispersive channels, the block multiplexing (BM) waveform is proposed in this thesis, where the data symbols are spread in frequency and time. For instance, the recently conceived orthogonal time frequency space (OTFS) is shown to be a particular case of BM with the classical single-carrier (SC). Regarding the equalization for the robust waveforms, it is shown that the minimum mean squared error with parallel interference cancellation (MMSE-PIC) employed together with convolutional encoder and soft decoder can completely remove the inter-symbol interference (ISI), where a low-complexity implementation is designed. In addition, a waveform with decreased complexity based on the sparse Walsh-Hadamard (SWH) is proposed for two reasons, i) sparse spreading requires a transform with lower size, ii) the Walsh-Hadamard transform is implemented with 1s and −1s, which requires less complexity than fast Fourier transform (FFT) based waveforms. Furthermore, the problem of estimating the time varying channel is considered, where a unique word (UW) or (pilot block) based approach is studied. In this regard, another main contribution of this dissertation is to develop an optimization framework, where the combination of channel estimation plus Doppler spread error is minimized. In particular, the composite error minimization is achieved by properly setting the FFT size of the system, for a fixed data length. Lastly, cyclic prefix (CP)-free system is considered such that the transmission time is decreased, and therefore provides a better channel estimation. Naturally, the CP-free system has undesirable interference, which is resolved by an iterative CP-Restoration algorithm. In this case, we extend the EGC to equal reliability criterion (ERC), i.e., the data symbols should be equally reliable and not only have equal gain. As a consequence, the BM with orthogonal chirp division multiplexing (OCDM) waveform has the best performance due to equal time and frequency spreading. In conclusion, the coded modulation capacity approach of this dissertation provides new insights and solutions to improve the performance of wireless systems.

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