High traffic load over the unlicensed portion of the radiospectrum (a.k.a., ISM bands) along with inefficient usage of thelicensed spectrum gave impetus for a new paradigm in spectrumallocation, whose main purpose is to improve spectrum efficiencythrough opportunistic access. Cognitive radios (CRs) havebeen proposed as a key enabling technology for such paradigm.Operating a CR network (CRN) without impacting the performance oflicensed (primary) users requires new protocols for informationexchange as well as mathematical tools to optimize thecontrollable parameters of the CRN. In this dissertation, wetarget the design of such protocols. First, we develop adistributed CRN MAC (COMAC) protocol that enables unlicensed usersto dynamically utilize the spectrum while limiting theinterference they inflict on primary (PR) users. The main noveltyin COMAC lies in not assuming a predefined CR-to-PR power mask andnot requiring coordination with PR users. Second, we propose anovel distance-dependent MAC protocol for CRNs in whicheach CR is equipped with multiple transceivers. Our protocol(called DDMAC) attempts to maximize the CRN throughput byfollowing a novel probabilistic channel assignment mechanism. Thismechanism exploits the dependence between the signal's attenuationmodel and the transmission distance while considering the trafficprofile. We show that through its distance- and traffic-aware,DDMAC significantly improves network throughput. Finally, weaddress the problem of assigning channels to CR transmissions,assuming one transceiver per CR. The main goal of our design is tomaximize the CRN throughput with respect to both spectrumassignment and transmission power. Specifically, we presentcentralized and distributed solutions that leverage the uniquecapabilities of CRs. Compared with previously proposed protocols,our schemes are shown to significantly improve network throughput.

Diversity in the expression of ion channel proteins among neurons allows a wide range of excitability, growth and functional regulation. Ether-a-go-go (EAG), a member of the voltage-gated K+ channels, was characterized by spontaneous firing in nerve terminals and enhanced neurotransmitter release. In situ whole-cell patch-clamp recordings performed from the somata of Drosophila larval thoracic aCC motoneurons revealed spontaneous spike-like events in eag mutants. Spontaneous events were absent in wild type motoneurons. Spikes evoked by somatic current injection in to the cell body were not altered and comparable to wild type. Spontaneous spike-like events could be due to increased synaptic drive or altered intrinsic excitability of the motoneuron. Reduction of EAG function with selective expression of eag double stranded RNAi transgene in motoneurons only did not cause spontaneous spike-like events or alter evoked firing. This suggests increased synaptic drive contributes to spontaneous events.Both transient and sustained voltage-activated K+ currents, each with Ca++-sensitive (IA(Ca) and IK(Ca)) and Ca++ -insensitive components (IA and IK), were isolated in thoracic aCC motoneurons. In wild type motoneurons, IA was larger than IA(Ca). Conversely, IK(Ca) was larger than IK. Both eag mutants and eag RNAi expression resulted in a decrease in IA , IK and a slow sustained K+ current. Further, EAG and Shal demonstrate a potential functional interaction and contribute to IA. The voltage sensitivity for inactivation was reduced in Shal only and EAG-Shal double knock down compared to controls and EAG only knock down. In addition, a Ca++ sensitive EAG dependent K+ current was blocked by cAMP. Thus, both voltage-dependent and modulatory functions of EAG influence excitability in motoneurons.Firing properties and K+ currents distinguish aCC motoneurons in thoracic segments, T1 and T3. T3aCC had a shorter delay to spike, higher input resistance and were more easily recruited than T1aCC. T1aCC had a larger IA than T3aCC, but comparable IA(Ca). IK(Ca) was larger in T3aCC compared to T1aCC. These differences reflect cell-specific ion channel distribution that could contribute to patterned segmental motor output.

The presence of dispersive fading and inter-carrier interference (ICI) constitute the major impediment to reliable communications in orthogonal frequency-division multiplexing (OFDM) systems. Recently iterative (``Turbo'') processing techniques, which have been successfully applied to many detection/decoding problems, have received considerable attention. In this thesis, we first aim on the design of iterative receiver for single antenna OFDM system with frequency offset and dispersive fading. Further work is then extended to space-time block coded (STBC) OFDM system. At last, the technique is applied to STBC-OFDM system through a newly built channel model, which is based on a physical description of the propagation environment. The performance of such systems are verified by computer simulations. The simulation results show that the iterative techniques work well in OFDM systems.

Device authentication in ad hoc networks is becoming more and more important. Nowadays, there are many interesting applications which communicate via the short-rang wireless communication channel (such as Bluetooth or WiFi). In the communication, a great deal of sensitive information is required to be transmitted. Therefore, device authentication is significant. In order to build a secure authentication mechanism, protocols are proposed using human control visual channels. However, this method brings many challenges, the main one being the burden placed on humans. Therefore, in this thesis, these protocols are optimized using visible light communication techniques, which significantly reduce the work faced by humans.

Scorpion toxins specific for Na+ and K+ channels, have been studied extensively but relatively little has been done on Ca2+ channel toxins. Toxins in the venom of only two South African scorpions P. transvaalicus and P. granulatus have been found to interact with Ca2+ channels. Kurtoxin isolated from the venom of P. transvaalicus inhibits the T and L-type neuronal Ca2+ channels, whereas KLI and KLII (Kurtoxin-like peptide I and II), isolated from P. granulatus, inhibits T-type Ca2+ channel activity in mouse male germ cells. In this study the effects of fractions isolated from the venom of P. granularus on Cca2+ channels in rat ventricular myocytes were investigated by means of the whole-cell patch clamp technique. Fractions of P. granulatus crude venom were isolated with Sephadex G50 columns (fraction I-IV). Fraction III (PgIII) showed a voltage dependent increase of the inward Ca2+ current and influenced the channel kinetics by shifting the voltage dependence of activation towards more hyperpolarizing membrane potentials and decreased the rate of inactivation and deactivation. The time of the current to reach peak was also delayed. PgIII was further separated by HPLC in an attempt to identify the subfraction/s responsible for the agonistic effect. Subfraction I had an agonistic effect similar to PgIII, whereas subfraction II and III, decreased the Ca2+ current. The observed agonistic effect has not been described in the literature. The identification of new peptide structures with unique functions are important in the field of toxin research. Peptides that target Ca2+ channels can be valuable tools to characterize Ca2+ channels. Ca2+ channels in the heart are implicated in a number of pathological disorders like angina, ischemia, some arrhythmias and hypertension. / Thesis (M.Sc. (Physiology))--North-West University, Potchefstroom Campus, 2005.

With the rapid adoption and development of new standards, Multiple-Input Multiple-Output (MIMO) technology is becoming a necessity in current wireless systems. One problem posed by using multiple antennas at a transmitter or receiver is the undesirable effect of signal correlation between closely-spaced radiating elements. This thesis presents the concept, design, and evaluation of a hemispherical lens antenna for use in MIMO systems. A high-permittivity dielectric material allows radiating elements to be placed in close proximity with reduced spatial correlation effects. An intermediate matching layer and a hemispherical lens design facilitate the preservation of the pattern characteristics in the transition between the dielectric and free-space. The antenna was simulated against benchmark antenna arrays in free-space and showed a 35%-70% improvement in channel capacity in multipath-rich environments, showing strength as a candidate for further development in MIMO applications.

Neuronal excitability is controlled by multiple ion channels at the plasma membrane of neurons. Recently, the Na+ leak channel, NCA in C. elegans and NALCN in M. Musculus, has been identified as the molecular entity responsible for the background Na+ leak at rest in neurons. In this thesis, I show that NLF-1 (NCA Localization Factor) and mouse NLF-1, a group of newly defined, uncharacterized proteins, are endoplasmic reticular proteins required for the trafficking of NCA-1 and NCA-2 to their target axonal membrane. In primary mouse cortical neurons, knockdown of mNLF-1 partially abolishes the background Na+ leak current. Furthermore, NLF-1 and mNLF-1 directly interact with domain II S5/P-loop/S6 of NALCN through a membrane yeast-two-hybrid assay. In C. elegans, this region is required in vivo in NCA-1 for it’s trafficking. Finally, I identify novel NLF-1 interacting partners through a MYTH assay.

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The well known geometrical scattering channel modeling technique has been suggested to describe the spatial statistical distribution of the received multipath signals at various types of wireless communication environments and for different wireless system applications. This technique is based on the assumption that the scatterers, i.e. objects that give rise to the multipath signals, are randomly distributed within a specified geometry that may include the base station and/or the mobile station. The geometrical scattering channel models can provide convenient and simple statistical functions for some of the important physical quantities of the received multipath fading signals, such as: angle of arrival, time of arrival, angular spread, delay spread and the spatial correlation function. In this thesis, a new three dimensional geometrical scattering channel model has been developed for outdoor and indoor wireless communication environments. The probability density functions of the angle of arrival of the received multipath signals are provided in compact forms. These functions facilitate independent control of the angular spread in both the azimuth and the elevation angles via the model's parameters. To establish the model verification, the developed model has been compared against the results from a site-specific propagation prediction technique in indoor and outdoor wireless communication environments. The developed three dimensional model has been extended to include the temporal statistical distribution of the received multipath signals for uniform and non-uniform distributions of the scatterer. Several of the probability density functions of the angle of arrival and time of arrival of the received multipath signals are provided. The probability density functions of the angle of arrival have been validated by comparing them against the results from real channel measurements data. In addition, the developed three dimensional geometrical scattering channel model has been extended for multiple input multiple output wireless channel modeling applications. A three dimensional spatial correlation function has been developed in terms of some of the physical channel's parameters, such as: displacements and orientation of the employed antenna elements. The developed correlation function has been used to simulate and investigate the performance of wireless multiple input multiple output systems in different scenarios.

Neuronal excitability is controlled by multiple ion channels at the plasma membrane of neurons. Recently, the Na+ leak channel, NCA in C. elegans and NALCN in M. Musculus, has been identified as the molecular entity responsible for the background Na+ leak at rest in neurons. In this thesis, I show that NLF-1 (NCA Localization Factor) and mouse NLF-1, a group of newly defined, uncharacterized proteins, are endoplasmic reticular proteins required for the trafficking of NCA-1 and NCA-2 to their target axonal membrane. In primary mouse cortical neurons, knockdown of mNLF-1 partially abolishes the background Na+ leak current. Furthermore, NLF-1 and mNLF-1 directly interact with domain II S5/P-loop/S6 of NALCN through a membrane yeast-two-hybrid assay. In C. elegans, this region is required in vivo in NCA-1 for it’s trafficking. Finally, I identify novel NLF-1 interacting partners through a MYTH assay.