Publications
In light of recent interests and activities by cellular networks operators to exploit the unlicensed bands to boost network capacity, never has the issue of fair coexistence and spectrum sharing been at the heart of most feasibility and performance studies of radio access technologies coexisting in unlicensed bands. This has been the case because overall system performance by incumbent radio access technologies is not expected to be compromised in the spirit of fairness. Therefore, fair coexistence or spectrum sharing has become a key performance metric in evaluating the performance of most solutions proposed to permit radio access technology coexistence in the unlicensed bands. Time- based fairness, the focus of this thesis, refers to a mechanism which can by adopted to evaluate fairness performance among coexisting radio access technologies in the time domain, however, limited studies have been conducted and practical implementation solutions are still out of reach. In this thesis, the objectives are to address existing gaps in accessing practical time-based fairness solutions. Firstly, a review of the state of the art on fairness issues, metrics and approaches are discussed, providing an overview of current approaches and solutions and identifies their shortcomings to practical implementation. Secondly, the estimation of number of nodes contending over the unlicensed spectrum, which is a requirement for many fairness oriented schemes proposed for radio access technologies coexisting in unlicensed bands, is addressed. A novel technology-neutral estimation method for node numbers is proposed. The transmission interval observed over the unlicensed channel represents a probabilistic distribution, which can be obtained via the uniform difference distribution. The characteristic features of the uniform difference distribution are exploited to aid estimation of node numbers in scenarios where nodes contend for the channel within the same contention window and under multiple contention windows. The benefit of the proposed method over existing methods is the level of accuracy and its ability to provide a tighter estimate to small increase in numbers of contenders on the channel. Thirdly, two approaches to achieving time-based fairness are proposed. The first being a deterministic approach and the second a probabilistic one. The deterministic approach aims to study the upper bound performance of time-based fairness utilizing the estimation method for node numbers proposed in this thesis. An optimal value is computed as a backoff value for each transmission cycle while keeping the transmission opportunity of all coexisting radio access technologies the same. The results show time-based fairness improves spectrum utilization and overall throughput performance. The probabilistic approach seeks for a practical and implementable solution to achieving time-based fairness based on the proven performance benefits shown from the deterministic approach. The transmission interval distribution obtained from observation of the channel activity during the node number estimation serves as the building block towards a practical solution. The distribution is continually monitored and mapping of the occurrence of transmission intervals to its probability density functions are performed. The mean of the distribution is then optimized to provide a balance between the channel access probabilities in order to achieve approximately equal transmission time by all nodes contending over the channel. Two parameters are adjusted to attain time-based fairness, which are the contention window sizes and transmission opportunity. Simulation results show that time-based fairness under the proposed scheme can improve spectrum utilization, reduce the disparity in throughput performance and guarantee fairness among coexisting radio access technologies.
Perovskite solar cells (PSCs) have emerged as a ‘rising star’ in recent years due to their high-power conversion efficiency (PCE), extremely low cost and facile fabrication techniques. To date, PSCs have achieved a certified PCE of 25.2% on rigid conductive substrates, and 19.5% on flexible substrates. The significant advancement of PSCs has been realized through various routes, including perovskite composition engineering, interface modification, surface passivation, fabrication process optimization, and exploitation of new charge transport materials. However, compared with rigid counterparts, the efficiency record of flexible perovskite solar cells (FPSCs) is advancing slowly, and therefore it is of great significance to scrutinize recent work and expedite the innovation in this field. In this article, we comprehensively review the recent progress of FPSCs. After a brief introduction, the major features of FPSCs are compared with other types of flexible solar cells in a broad context including silicon, CdTe, dye-sensitized, organic, quantum dot and hybrid solar cells. In particular, we highlight the major breakthroughs of FPSCs made in 2019/2020 for both laboratory and large-scale devices. The constituents of making a FPSC including flexible substrates, perovskite absorbers, charge transport materials, as well as device fabrication and encapsulation methods have been critically assessed. The existing challenges of making high performance and long-term stable FPSCs are discussed. Finally, we offer our perspectives on the future opportunities of FPSCs in the field of photovoltaics.
Silicon-germanium (SiGe) heterojunction metal-oxide-semiconductor field-effect transistors (SiGe HMOSFETs) have been successfully fabricated on Si substrate. The semiconductor heterostructure, which was grown by gas-source molecular beam epitaxy (GS-MBE), was initiated by the deposition of a Si Ge "virtual substrate". The n-type transistors were fabricated using a standard MOS process. The channel is a thin, undoped layer of strained Si and is buried below an arsenic-doped Si Ge layer, which provides the carriers. The devices exhibited excellent current-voltage (I-V) characteristics in terms of transconductance and drain current, with no breakdown or leakage. A level-1 model was extracted, for use in circuit design. The results suggest that the realisation of buried-channel SiGe n-HMOSFETs is feasible in MOS processes. These devices are of particular importance in analogue applications. © 2003 Elsevier B.V. All rights reserved.
N-channel enhancement mode Si/SiGe MOSFETs are characterised and studied over a wide temperature range of 10K
An efficient and robust video watermarking algorithm for the state-of-the-art video coding standard H.264/AVC is proposed for copyright protection. Grayscale 2-D watermark patterns such as detailed trademarks or logos can be highly compressed by a proposed grayscale watermark pre-processing, and inserted into the low bit-rate H.264/AVC videos in the compressed domain. The marked video sequences maintain good visual quality and the same overall consuming bit-rate. The proposed algorithm can robustly survive transcoding process and common signal processing, such as bit-rate reduction, Gaussian filtering and contrast enhancement.
Stephen A. Lynch, Douglas J. Paul, Paul Townsend, Guy Matmon, Zhang Suet, Robert W. Kelsall, Zoran Ikonic, Paul Harrison, Jing Zhang, David J. Norris, Anthony G. Cullis, Carl R. Pidgeon, Pawel Murzyn, Ben Murdin, Mike Bain, Harry S. Gamble, Ming Zhao, Wei-Xin Ni (2006)Toward Silicon-Based Lasers for Terahertz Sources, In: IEEE Journal of Selected Topics in Quantum Electronics12(6)pp. 1570-1578 Producing an electrically pumped silicon-based laser at terahertz frequencies is gaining increased attention these days. This paper reviews the recent advances in the search for a silicon-based terahertz laser. Topics covered include resonant tunneling in p-type Si/SiGe, terahertz intersubband electroluminescence from quantum cascade structures, intersubband lifetime measurements in Si/SiGe quantum wells, enhanced optical guiding using buried silicide layers, and the potential for exploiting common impurity dopants in silicon such as boron and phosphorus to realize a terahertz laser
The role of oxygen concentration on the formation/evolution of residual defects in implanted and rapid thermal annealed silicon was studied in samples with various oxygen concentrations. Photoluminescence (PL) study showed a strong correlation between the D-line intensity and the oxygen concentration. Transmission electron microscopy (TEM) measurements also suggested the extended defects were more favored in the high oxygen sample. High frequency capacitance-voltage (C-V) measurements revealed excess acceptors that were further investigated by deep level transient spectroscopy (DLTS). A hole trap with activation energy of 450 meV was detected and was suggested to relate to agglomerations of point defects associated with more than one type of 3D-metal related deep levels.
Marco Califano, N. Q. Vinh, P. J. Phillips, Z. Ikonić, R. W. Kelsall, P. Harrison, C. R. Pidgeon, B. N. Murdin, D. J. Paul, P. Townsend, J. Zhang, I. M. Ross, A.G. Cullis (2007)Interwell relaxation times in p-Si/SiGe asymmetric quantum well structures: Role of interface roughness, In: Physical Review B75(045338) We report the direct determination of nonradiative lifetimes in Si/SiGe asymmetric quantum well structures designed to access spatially indirect (diagonal) interwell transitions between heavy-hole ground states, at photon energies below the optical phonon energy. We show both experimentally and theoretically, using a six-band k·p model and a time-domain rate equation scheme, that, for the interface quality currently achievable experimentally (with an average step height 1 greater than or equal to Å), interface roughness will dominate all other scattering processes up to about 200 K. By comparing our results obtained for two different structures we deduce that in this regime both barrier and well widths play an important role in the determination of the carrier lifetime. Comparison with recently published experimental and theoretical data obtained for mid-infrared GaAs/AlxGa1−xAs multiple quantum well systems leads us to the conclusion that the dominant role of interface roughness scattering at low temperature is a general feature of a wide range of semiconductor heterostructures not limited to IV-IV materials.
In this paper, we present a novel sequence design for efficient channel estimation in multiple input multiple output filterbank multicarrier (MIMO-FBMC) system with offset QAM modulation. Our proposed sequences, transmitted over one FBMC/OQAM symbol, are real-valued in the frequency domain and display zero-correlation zone properties in the time-domain. The latter property enables optimal channel estimation for a least-square estimator in frequency-selective fading channels. To further improve the system performance, we mitigate the data interference by an iterative feedback loop between channel estimation and FBMC demodulation. Simulation results validate that our proposed real-valued orthogonal sequences and the iterative channel estimation and demodulation scheme provide a practical solution for enhanced performance in preamble-based MIMO-FBMC systems.
H A El Mubarek, J M Bonar, G D Dilliway, P Ashburn, M Karunaratne, A F Willoughby, Y Wang, P L Hemment, R Price, J Zhang, P Ward (2004)Effect of Fluorine Implantation Dose on Boron Thermal Diffusion in Silicon, In: Journal of Applied Physics96(8) This paper investigates how the thermal diffusion of boron in silicon is influenced by a high energy fluorine implant with a dose in the range 5x10(14)-2.3x10(15) cm(-2). Secondary Ion Mass Spectroscopy (SIMS) profiles of boron marker layers are presented for different fluorine doses and compared with fluorine profiles to establish the conditions under which thermal boron diffusion is suppressed. The (SIMS) profiles show significantly reduced boron thermal diffusion above a critical F+ dose of 0.9-1.4x10(15) cm(-2). Fitting of the measured boron profiles gives suppressions of the boron thermal diffusion coefficient by factors of 1.9 and 3.7 for F+ implantation doses of 1.4x10(15) and 2.3x10(15) cm(-2), respectively. The suppression of boron thermal diffusion above the critical fluorine dose correlates with the appearance of a shallow fluorine peak on the (SIMS) profile in the vicinity of the boron marker layer. This shallow fluorine peak is present in samples with and without boron marker layers, and hence it is not due to a chemical interaction between the boron and the fluorine. Analysis of the (SIMS) profiles and cross-section Transmission Electron Microscope micrographs suggests that it is due to the trapping of fluorine at vacancy-fluorine clusters, and that the suppression of the boron thermal diffusion is due to the effect of the clusters in suppressing the interstitial concentration in the vicinity of the boron profile
