Canonical time-frequency, time-scale, and frequency-scale representations of time-varying channels

Mobile communication channels are often modeled as linear time-varying filters or, equivalently, as time-frequency integral operators with finite support in time and frequency. Such a characterization inherently assumes the signals are narrowband and may not be appropriate for wideband signals. In this paper time-scale characterizations are examined that are useful in wideband time-varying channels, for which a time-scale integral operator is physically justifiable. A review of these time-frequency and time-scale characterizations is presented. Both the time-frequency and time-scale integral operators have a two-dimensional discrete characterization which motivates the design of time-frequency or time-scale rake receivers. These receivers have taps for both time and frequency (or time and scale) shifts of the transmitted signal. A general theory of these characterizations which generates, as specific cases, the discrete time-frequency and time-scale models is presented here. The interpretation of these models, namely, that they can be seen to arise from processing assumptions on the transmit and receive waveforms is discussed. Out of this discussion a third model arises: a frequency-scale continuous channel model with an associated discrete frequency-scale characterization.Comment: To appear in Communications in Information and Systems - special issue in honor of Thomas Kailath's seventieth birthda

A Micromechanical Parylene Spiral-Tube Sensor and Its Applications of Unpowered Environmental Pressure/Temperature Sensing

A multi-function micromechanical pressure/temperature sensor incorporating a microfabricated parylene spiral tube is presented. Its visible responses in expression of in situ rotational tube deformation enable unpowered sensing directly from optical device observation without electrical or any powered signal transduction. Sensor characterizations show promising pressure (14.46°/kPa sensitivity, 0.11 kPa resolution) and temperature (6.28°/°C sensitivity, 0.24 °C resolution) responses. Depending on different application requests, this sensor can be individually utilized to measure pressure/temperature of systems having one property varying while the other stabilized, such as intraocular or other in vivo pressure sensing of certain apparatus inside human bodies or other biological targets. A straightforward sensor-pair configuration has also been implemented to retrieve the decoupled pressure and temperature readouts, hence ultimately realizes a convenient environmental pressure and temperature sensing in various systems