Educational Considerations, vol. 20 (2) Full Issue

Educational Considerations, vol. 20 (2) Spring 1993 - Full issu

Joint signal detection and channel estimation in rank-deficient MIMO systems

L'évolution de la prospère famille des standards 802.11 a encouragé le développement des technologies appliquées aux réseaux locaux sans fil (WLANs). Pour faire face à la toujours croissante nécessité de rendre possible les communications à très haut débit, les systèmes à antennes multiples (MIMO) sont une solution viable. Ils ont l'avantage d'accroître le débit de transmission sans avoir recours à plus de puissance ou de largeur de bande. Cependant, l'industrie hésite encore à augmenter le nombre d'antennes des portables et des accésoires sans fil. De plus, à l'intérieur des bâtiments, la déficience de rang de la matrice de canal peut se produire dû à la nature de la dispersion des parcours de propagation, ce phénomène est aussi occasionné à l'extérieur par de longues distances de transmission. Ce projet est motivé par les raisons décrites antérieurement, il se veut un étude sur la viabilité des transcepteurs sans fil à large bande capables de régulariser la déficience de rang du canal sans fil. On vise le développement des techniques capables de séparer M signaux co-canal, même avec une seule antenne et à faire une estimation précise du canal. Les solutions décrites dans ce document cherchent à surmonter les difficultés posées par le medium aux transcepteurs sans fil à large bande. Le résultat de cette étude est un algorithme transcepteur approprié aux systèmes MIMO à rang déficient

Dynamics of DNA Breathing and Folding for Molecular Recognition and Computation

This thesis is centered on the development of the molecular beacon, as a new DNA probe for DNA genotyping, D N A computation and biophysical studies of DNA conformations. Molecular beacons are single-stranded DNA molecules that form a stem-and-loop structure. A fluorophore and a quencher are grafted at their two ends to report their conformations: when the molecular beacon is closed, fluorophore and quencher are held in close proximity and the fluorescence is quenched; when the molecular beacon is open, fluorophore and quencher are far apart, and the fluorescence is restored. Molecular beacons are ideal DNA probes coupling conformational switch with fluorescence signal turning-ON. We use molecular beacons to study the molecular recognition of single-stranded DNA (ssDNA) oligonucleotide. We present a thermodynamic diagram to show that structural constraints make the molecular beacon highly sensitive to the presence of mismatches in its target. We introduce a sequence sensitivity parameter to quantitatively compare different DNA probes, and propose an algorithm to optimally tune the probe\u27s structure for enhanced sequence discrimination. Logic gates (OR and AND gates) using molecular beacons are designed to carry most elementary molecular computations. The conformational changes associated with such computations can be used to concatenate many chemical reactions, and carry out complex molecular computations. Molecular beacons are also ideal probes to study DNA secondary structures and their fluctuations. We develop the fluorescence correlation spectroscopy (FCS) technique to monitor the dynamics of relaxation of DNA conformational fluctuations. We first measure the opening and closing timescales of DNA hairpin-loops. Activation barriers for opening and closing for different loop lengths and sequences are analyzed to better account for the stability of DNA secondary structures. A sequence dependent rigidity of ssDNA has been discovered, and analyzed in terms of base stacking. We then use F C S to study the dynamics of double-stranded DNA (dsDNA) breathing modes with synthetic DNA constructs. The analysis of the base pairing fluctuation dynamics, monitored by fluorescence, unravels lifetimes of breathing modes ranging from 1/us to 1ms. Long-range distortions of the d s DNA have been unraveled for purine-rich sequences, of relevance to the specificity of transcription initiation in prokaryotes

Reports to the President

A compilation of annual reports for the 1986-1987 academic year, including a report from the President of the Massachusetts Institute of Technology, as well as reports from the academic and administrative units of the Institute. The reports outline the year's goals, accomplishments, honors and awards, and future plans