Scott Beaver

Optimal OFDM design for time-frequency dispersive channels

Transmission over wireless channels is subject to time dispersion due to multipath propagation and to frequency dispersion due to the Doppler effect. Standard orthogonal frequency-division multiplexing (OFDM) systems, using a guard-time interval or cyclic prefix, combat intersymbol interference (ISI), but provide no protection against interchannel interference (ICI). This drawback has led to the introduction of pulse-shaping OFDM systems. We first present a general framework for pulse shape design. Our analysis shows that certain pulse shapes proposed in the literature are, in fact, optimal in a well-defined sense. Furthermore, our approach provides a simple way to adapt the pulse shape to varying channel conditions. We then show that (pulse-shaping) OFDM systems based on rectangular time-frequency lattices are not optimal for time- and frequency-dispersive wireless channels. This motivates the introduction of lattice-OFDM (LOFDM) systems which are based on general time-frequency lattices. Using results from sphere packing theory, we show how to design LOFDM systems (lattice and pulse shape) optimally for timeand frequency-dispersive channels in order to minimize the joint ISI/ICI. Our theoretical analysis is confirmed by numerical simulations, showing that LOFDM systems outperform traditional pulse-shaping OFDM systems with respect to robustness against ISI/ICI.

Cluster Analysis of Hourly Wind Measurements to Reveal Synoptic Regimes Affecting Air Quality

Abstract A clustering algorithm is developed to study hourly, ground-level wind measurements obtained from a network of monitoring stations positioned throughout the San Francisco Bay Area of California. A statistical model based on principal components analysis (or empirical orthogonal functions) is used to cluster these autocorrelated and cross-correlated observations. Patterns at the synoptic time scale are isolated by using windowing and scaling operations to treat the data. Four dominant wind patterns that affect air quality are identified for the study region, and summer days from 8 yr of historical data are assigned to these modes. One cluster captures a high pressure system over the western United States, the anticyclonic winds of which block the typical marine flow through the study region. Differential heating convects a polluted air mass to a nearby valley in which severe episodes of higher-than-average ozone composition occur. A second pattern represents a seasonal, offshore ridge of high pres...

Cluster Sequencing to Analyze Synoptic Transitions Affecting Regional Ozone

Abstract Synoptic conditions are known to strongly influence mesoscale flows and hence local ozone levels in the San Francisco Bay Area of California. Sets of individual days sharing certain combinations of synoptic features constitute “static” meteorological patterns (or regimes) that account for much of the variability in regional ozone levels. Upon labeling each day of the core Bay Area ozone season using a small number of synoptic regimes, the resulting daily sequence of static patterns indicates the time evolution of the synoptic system through a series of regimes realized for various durations. Further insight into ozone buildup processes can be gained by examining this sequence of static labels to identify “dynamic” meteorological patterns affecting local ozone levels. Transition probabilities between each pair of synoptic regimes are modeled using binomial statistics to determine transitions that are either energetically favored or disfavored to occur. The persistency of the meteorological regimes...

Optimal OFDM pulse and lattice design for doubly dispersive channels

We introduce a general framework for OFDM pulse shape design and demonstrate how to obtain a pulse optimally matched to given channel properties. Furthermore, standard OFDM systems are associated with a rectangular grid in the time-frequency plane, which is not optimal for pulse-shaping OFDM systems for doubly dispersive channels. We introduce lattice OFDM systems which are OFDM systems using general lattices in the time-frequency plane.

Pattern-Based Evaluation of Coupled Meteorological and Air Quality Models

Abstract A novel pattern-based model evaluation technique is proposed and demonstrated for air quality models (AQMs) driven by meteorological model (MM) output. The evaluation technique is applied directly to the MM output; however, it is ultimately used to gauge the performance of the driven AQM. This evaluation of AQM performance based on MM performance is a major advance over traditional evaluation methods. First, meteorological cluster analysis is used to assign the days of a historical measurement period among a small number of weather patterns having distinct air quality characteristics. The clustering algorithm groups days sharing similar empirical orthogonal function (EOF) representations of their measurements. In this study, EOF analysis is used to extract space–time patterns in the surface wind field reflecting both synoptic and mesoscale influences. Second, simulated wind fields are classified among the determined weather patterns using the measurement-derived EOFs. For a given period, the leve...

Optimal OFDM system design through optimal sphere coverings

Standard OFDM systems are associated with a rectangular grid in the time-frequency plane. However such a setup is in general not optimal for pulse shaping OFDM systems for doubly dispersive channels. We introduce lattice-OFDM systems (LOFDM), which are OFDM systems constructed with respect to general lattices in the time-frequency plane. We show how to design optimal pulse shapes for LOFDM systems. Furthermore we demonstrate by theoretical considerations and numerical simulations that LOFDM systems using hexagonal-type lattices outperform ordinary OFDM systems with regard to robustness against ISI/ICI.