Split and normal gate A1GaAs /GaAs MODFETs were fabricated along with the ohmic test structures and the Hall bar geometries. The DC characteristics of normal gate transistors were evaluated at room temperature and at 77K and the threshold voltages were extracted from the measurements and compared to the theoretical results. The performance of normal gate transistors was reasonable. The sheet carrier density and the mobility extracted from Hall measurements using the Hall bar geometry showed increase of carrier density with increasing gate voltage and an increase of mobility with increasing carrier density. The contact resistance obtained from the ohmic test structure was high and not uniform within the sample. / Graduation date: 1991

The objective of the proposed research is to investigate new and more efficient techniques in numerical evaluation and experimental measurement of phase jitter impact on more general communication systems including dispersion management, filtering, and spectral inversion schemes. There has recently been a renewed effort to develop coherent optical communication systems. In particular, differential phase-shift keying (DPSK), which does not require a local oscillator to perform decoding, has focused the attention and is perceived to be the promising candidate for future optical communication systems updates. This motivates us to exploit DPSK in wavelength-division multiplexed systems. First, modulation formats based on phase show an increased robustness to nonlinear impairments such as cross-phase modulation (XPM) and nonlinear polarization rotation, primarily because the time-dependence of optical power is deterministic and periodic. Second, coherent formats allow a higher spectral efficiency since both in-phase and quadrature dimensions of the signal space are available to encode information. Optical phase is also used in intensity-modulated direct detection systems as an extra degree of freedom, for example to provide better resistance to intrachannel four-wave mixing (FWM), or to increase spectral efficiency in duobinary modulation. Finally, phase modulation outperforms its intensity counterpart in terms of sensitivity since a 3 dB improvement can be achieved when balanced detection is used. Nevertheless, DPSK-based formats show a different behavior to noise accumulated along the propagation. Noise-induced power fluctuations are converted into phase fluctuations by the Kerr effect and become a penalty source which limits the transmission system reach. In this context, there have been intense research activities for evaluating phase uncertainties but the previous studies assume an analytically determined pulse shape and a constant-dispersion optical link which is far from reflecting the actual and future structures of transmission lines.

In this thesis, we investigate the performance of adaptive loading orthogonal frequency division multiplexing (OFDM) under Nakagami fading with maximal ratio combining (MRC) diversity at receiver. We not only expound the principles and structures of the system, but also analyze its performance of the lower bound on the average capacity under Nakagami fading. First, we defined the lower bound on the average capacity under Nakagami fading with ideal MRC diversity. Then, we fixed the values of bit error rate. A maximum rate adaptive loading strategy is derived for uncoded quadrature-amplitude-modulation modulated OFDM. Simple lower bound expressions are provided for average spectral efficiency of the maximum rate adaptive loading OFDM under Nakagami fading channel. Finally, the numerical results will be also shown.

I experimentally studied ultrasound-modulated optical tomography, which holds the promise for biomedical diagnosis. I measured the degree of polarization of laser speckles generated by scattered light transmitted through turbid media, investigated three signal-detection schemes for extracting the intensity of the ultrasound-modulated light, carried out experiments to image thick biological-tissue samples, and studied two techniques providing resolution in the cross-sections containing the ultrasonic axis. The study of degree of polarization presented results important for the understanding of polarization phenomena in turbid media. I explored an optical-filtering based signal detection scheme, improved the parallel-lock-in speckle detection scheme and proposed a speckle-contrast detection scheme. With the speckle-contrast detection scheme, I successfully obtained images of biological-tissue samples up to 50 mm thick. Further I studied frequency-swept ultrasound-modulated optical tomography for sub-millimeter resolution imaging, and developed ultrasound-modulated optical computed tomography that was based on a back-projection image reconstruction method and obtained clear images of biological-tissue samples.

This thesis consists of two subjects of research: A Digital Modulation bit error rate study for transmission using the human body as the transmission channel, and signal coupling using a piezo transducer. The first topic involves studying several kinds of Digital Modulation and application; the transceiver is implemented using LabView. The interface for transmission uses Piezo film transducers. A high pass filter is used to remove noise and interference. The bit error rate using Manchester-coding is measured. A brief discussion is presented justifying the choice of On-Off Keying (OOK) modulation to transmit data on the human body. The second topic is a study on the design and implementation of a human body transmission system. A test chip is designed in TSMC 0.35£gm 2P4M CMOS process and first measured results are presented.

Thesis (Ph.D.)--City University of Hong Kong, 2009. / "Submitted to Department of Electronic Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy." Includes bibliographical references (leaves [142]-152)

Thesis (M. Phil.)--University of Hong Kong, 1989.

Thesis (M. Phil.)--University of Hong Kong, 1986.

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