Digital communications

Time Average Fourier Telescopy (TAFT) has been proposed as a means for obtaining high-resolution, diffraction-limited images over large distances through ground-level horizontal-path atmospheric turbulence. Image data is collected in the spatial-frequency, or Fourier, domain by means of Fourier Telescopy; an inverse two dimensional Fourier transform yields the actual image. TAFT requires active illumination of the distant object by moving interference fringe patterns. Light reflected from the object is collected by a “light-bucket” detector, and the resulting electrical signal is digitized and subjected to a series of signal processing operations, including an all-critical averaging of the amplitude and phase of a number of narrow-band signals.

The design of any communication receiver needs to addresses the issues of operating under the lowest possible signal-to-noise ratio. Among various algorithms that facilitate this objective are those used for iterative decoding of two-dimensional systematic convolutional codes in applications such as spread spectrum communications and Code Division Multiple Access (CDMA) detection. A main theme of any decoding schemes is to approach the Shannon limit in signal-to-noise ratio. All these decoding algorithms have various complexity levels and processing delay issues. Hence, the optimality depends on how they are used in the system. The technique used in various decoding algorithms is termed as iterative decoding. Iterative decoding was first developed as a practical means for decoding turbo codes. With the Log-Likelihood algebra, it is shown that a decoder can be developed that accepts soft inputs as a priori information and delivers soft outputs consisting of channel information, a posteriori information and extrinsic information to subsequent stages of iteration. Different algorithms such as Soft Output Viterbi Algorithm (SOVA), Maximum A Posteriori (MAP), and Log-MAP are compared and their complexities are analyzed in this thesis. A turbo decoder is implemented on the Digital Signal Processing (DSP) chip, TMS320C30 by Texas Instruments using a Modified-Log-MAP algorithm. For the Modified-Log-MAP-Algorithm, the optimal choice of the lookup table (LUT) is analyzed by experimenting with different LUT approximations. A low complexity decoder is proposed for a (7,5) code and implemented in the DSP chip. Performance of the decoder is verified under the Additive Wide Gaussian Noise (AWGN) environment. Hardware issues such as memory requirements and processing time are addressed for the chosen decoding scheme. Test results of the bit error rate (BER) performance are presented for a fixed number of frames and iterations.

Based on the theoretical method developed by Clark and Greenstein for frequency-selective Rayleigh fading channel, we develop a general model for frequency-selective Nakagami fading channel. We derive analytical expressions of the average bit-error-rate in an ideal space diversity mobile radio receiver using the matched filter bound. Our simulation results show that the influences of the diversity order, the shape of the multipath delay profile, and delay spread of the multipath delay profile. Five shapes are considered in our simulation. Our simulation results highlight the importance of the ratio the normalized delay spread d. The results show that the delay profile is of no importance for $d<0.3,$ but can have a profound influence for $d\geq0.3.$

A test system was designed to determine the performance a QPSK satellite burst modem when using the Ka band link of the Advanced Communications Technology Satellite. Interface circuitry, largely based on the 22V10 programmable logic device, was designed to allow the modem to be controlled by a personal computer. Communication between the interface and the computer was accomplished through the computer's parallel port. TDMA frame timing was automatically controlled by the interface. A C language program provided operator control of the interface itself. Tests using this system showed that a severe night-time fading problem is experienced at the FAU receiver site. Very low error rates were recorded by this system in a loop-back transmission at the NASA satellite control terminal.

In the current communications age, the capabilities of mobile devices are increasing. The mobiles are capable of communicating at data rates of hundreds of mbps on 4G networks. This enables playback of rich multimedia content comparable to internet and television networks. However, mobile networks need to be spectrum-efficient to be affordable to users. Multimedia Broadcast Multicast Systems (MBMS) is a wireless broadcasting standard that is being drafted to enable multimedia broadcast while focusing on being spectrum-efficient. The hybrid video coding techniques facilitate low bitrate transmission, but result in dependencies across frames. With a mobile environment being error prone, no error correction technique can guarantee error free transmission. Such errors propagate, resulting in quality degradation. With numerous mobiles sharing the broadcast session, any error resilient scheme should account for heterogeneous device capabilities and channel conditions. The current research on wireless video broadcasting focuses on network based techniques such as FEC and retransmissions, which add bandwidth overhead. There is a need to design innovative error resilient techniques that make video codec robust with minimal bandwidth overhead. This Dissertation introduces novel techniques in the area of MBMS systems. First, robust video structures are proposed in Periodic Intra Frame based Prediction (PIFBP) and Periodic Anchor Frame based Prediction (PAFBP) schemes. In these schemes, the Intra frames or anchor frames serve as reference frames for prediction during GOP period. The intermediate frames are independent of others; any errors in such frames are not propagated, thereby resulting in error resilience. In prior art, intra block rate is adapted based on the channel characteristics for error resilience. This scheme has been generalized in multicasting to address a group of users sharing the same session. Average packet loss is used to determine the intra block rate. This improves performance of the overall group and strives for consistent performance. Also, the inherent diversity in the broadcasting session can be used for its advantage. With mobile devices capable of accessing a WLAN during broadcast, they form an adhoc network on a WLAN to recover lost packets. New error recovery schemes are proposed for error recovery and their performance comparison is presented.

With the continuing advances in computing and wireless technologies, mobile ad hoc networks (MANETs) are expected to become an indispensable part of the computing environment in the near future. Wireless devices are constantly growing in computing speed, memory, communication capabilities and features, while shrinking in weight and size. With this growth and the proliferation of these devices in every aspect of society, the need for such devices to communicate in a seamless manner is becoming increasingly essential. Multiple routing protocols have been developed for MANETs [51]. As MANETs gain popularity, their need to support real time and multimedia applications is growing as well. Such applications have stringent quality of service (QoS) requirements such as bandwidth, delay, and delay fitter. Design and development of routing algorithms with QoS support is experiencing increased research interest. Several approaches which propose various routing algorithms with QoS support for MANETs have been presented in research. This dissertation addresses the issues and challenges of QoS routing in MANETS and presents three new protocols which provide QoS support for this environment. First, a brief classification of existing QoS routing algorithms is provided. Then, the three new protocols for QoS routing support in MANETs are presented. These protocols focus on resource reservation for QoS provisioning in TDMA-based MANETs. The first protocol improves QoS support by eliminating racing conditions during multiple reservations of QoS paths. The second protocol is a new protocol for resource reservation for QoS support in TDMA-based MANETs using directional antennas. The last protocol provides dynamic range resource reservation for QoS support in MANETs and represents an extension of the previous protocols.

This dissertation is concerned with studies on the performance aspects of digital cellular radio systems operating in non-Gaussian multipath fading environments. The multipath fading channel, modeled as a superposition of sinusoidal random vectors, is the main focus of this work. Elementary phase distributions, which cause quadrature components of the composite received vector to be correlated, are studied and relevant envelope distribution for the resulting non-Gaussian quadrature components is investigated. The Student-t distributed random process is chosen to model the quadrature components in an indoor multipath fading channel when the number of sinusoidal random vectors is small. For the correlated bivariate non-Gaussian quadrature components, the exact probability distribution function, corresponding to those elementary phase distributions as well as the Gaussian approximation for resultant envelope are evaluated. The scenario where the elementary envelope is beta distributed is also considered. Spherically invariant random process (SIRP) is also used to model the multipath fading channel. The performance analysis based on the spherically invariant multipath fading channel, is then evaluated. The system performance, specified in terms of outage probability and average error probability, significantly depends on the choice of characteristic probability distribution function of the random process that describes the RF ambient. It is shown that the optimality of the optimum combining and maximal ratio combining schemes in interference-limited environments is still retained under the spherically invariant multipath fading channel model.

This document serves as a dissertation for the Doctor of Philosophy in Ocean Engineering degree at Florida Atlantic University's Department of Ocean Engineering. This dissertation documents the design, development, and performance analysis of an acoustic receiver system that incorporates a decision directed learning filter for acoustic multi-phase shift keying signals. The system makes use of advanced signal processing, inclusive of spatial diversity and decision directed learning techniques in order to decode digital acoustic data. Error correcting techniques are also employed as well as methods of analyzing the efficiency of the system.

Time Division Multiple Access (TDMA) architecture is an established technology for digital cellular, personal and satellite communications, as it supports variable data rate transmission and simplified receiver design. Due to transmission bandwidth restrictions, increasing user demands and the necessity to operate at lower signal-to-noise ratio (SNR), the TDMA systems employ high order modulation schemes such as M-ary Quadrature Amplitude Modulation (M-QAM) and burst transmission. Use of such techniques in low SNR fading channels causes degradations of carrier frequency error, phase rotation error, and symbol timing jitter. To compensate for the severe degradation due to additive white Gaussian noise (AWGN) and channel impairments, precise and robust synchronization algorithms are required. This dissertation deals with the synchronization techniques for TDMA receivers using short burst mode transmission with emphasis on preamble-less feedforward synchronization schemes. The objective is to develop new algorithms for symbol timing, carrier frequency offset acquisition, and carrier phase tracking using preamble-less synchronization techniques. To this end, the currently existing synchronization algorithms are surveyed and analyzed. The performance evaluation of the developed algorithms is conducted through Monte-Carlo simulations and theoretical analyses. The statistical properties of the proposed algorithms in AWGN and fading channels are evaluated in terms of the mean and variance of the estimated synchronization errors and their Cramer-Rao lower bounds. Based on the investigation of currently employed feedforward symbol timing algorithms, two new symbol timing recovery schemes are proposed for 16-QAM land mobile signals operating in fading channels. Both schemes achieve better performance in fading channels compared to their existing counterparts without increasing the complexity of the receiver implementation. Further, based on the analysis of currently employed carrier offset and carrier phase recovery algorithms, two new algorithms are proposed for carrier acquisition and carrier tracking of mobile satellite systems utilizing short TDMA bursts with large frequency offsets. The proposed algorithms overcome some of the conventional problems associated with currently employed carrier recovery schemes in terms of capture range, speed of convergence, and stability.