Pang Shyan Kooi

Analysis of radiowave propagation in a four-layered anisotropic forest environment

Propagation of electromagnetic waves in forest environments is examined in which both the transmitting and receiving points are located in the trunk layer of a four-layered anisotropic forest model. This propagation model considers the forest as a horizontally stratified, anisotropic media of canopy and trunk, bounded by ground below and air above. The electromagnetic fields are obtained using dyadic Greens functions in their eigenfunction expansion forms for an anisotropic four-layered geometry. Analytical results are found for the fields, which consist primarily of three wave modes: a direct wave, multiply reflected waves, and lateral waves. These field constituents are compared, and their domains of preponderance are calculated; it is found that the lateral wave plays a major role in communication at large distances. Radio losses for typical forest are calculated to illustrate numerical application of the forest model.

Time-varying step-transform algorithm for high squint SAR imaging

High-squint angle synthetic aperture radar (SAR) imaging has applications in specific situations such as providing looking angle-dependent scattering information and revisiting an area of interest. It is very difficult for current SAR-imaging algorithms to be used directly in high-squint configurations. This is due to the large range-cell migration and the nonnegligible cubic-phase term in the signal. In this paper, an improved algorithm called the time-varying step-transform algorithm is proposed. It uses different chirp rates to deramp the signal in different subapertures to alleviate the range-focus problem. The interval between subapertures is also varied according to the deramping chirp rate. The nonnegligible cubic-phase term is included in the focusing procedure. The performance of the proposed algorithm in high-squint-angle mode is excellent. By reducing the subaperture length as the squint angle is increased, the performance of the algorithm is almost independent of squint angle. The proposed algorithm is also flexible for multilook processing and motion-error compensation.

Generalized perturbation analysis of distortion in semiconductor lasers

We have carried out a generalized perturbation analysis of the second‐order nonlinear equation for photon density for semiconductor lasers. The second‐order equation is obtained with minimal approximations, from the single‐mode semiconductor laser rate equations, which unlike previous work, include both the effect of gain compression and the coupling of the spontaneous emission to the lasing mode. After deriving the general perturbation formula, expressions for second harmonic distortion as well as third‐order intermodulation distortion are obtained. Unlike previous work which is restricted to two subcarriers, the present work gives results for an arbitrary number of subcarriers.

Characterization and modeling of avalanche multiplication in HBTs

An accurate modeling of avalanche breakdown of HBTs in compact bipolar transistor models for circuit simulation is presented. Based on various device electrical characteristics that are grouped into three classes, a modified VBIC avalanche multiplication model is proposed. By simply replacing one constant avalanche model parameter with current linear dependence, the new model predicts well broad behaviors of breakdown from weak avalanche up into high injections.

SiGe HBTs model converting technique from SGP to VBIC model

Abstract A new parameter extraction methodology — local ratio evaluation is presented which is well suitable for converting one model to another. An example is given for VBIC model extraction by going through SPICE Gummel–Poon (SGP) model. It is based on the fact that VBIC model is a direct enhancement and extension of SGP model. First, the standard SGP model is extracted in the standard way. Next, SGP model parameters are directly converted to those in VBIC model. Local modifications are subsequently carried out for those parameters that are affected by different equations used in the two models. Finally, new model parameters for enhanced modeling features are introduced by evaluating the difference between measurement and simulation.

Extraction of VBIC model for SiGe HBTs made easy by going through Gummel-Poon model

A new parameter extraction methodology - local ratio evaluation is presented which is well suited for converting tone model to another. An example is given for VBIC model extraction by going through Spice Gummel-Poon (SGP) model. It is based on the fact that the VBIC model is a direct enhancement and extension of SGP model. Firstly, the standard SGP model is extracted in the standard way. Then, SGP model parameters are directly converted to VBIC model. Next, local modifications are carried out for those parameters that are affected by different equations used in the two models. Finally, new model parameters for enhanced modeling features are introduced by evaluating the difference between measurement and simulation.

SAR real time motion compensation: average cancellation method for aircraft

Effective motion compensation is the key for achieving high quality SAR images. A frequency domain method which is based on an analysis of the azimuth spectrum of SAR raw data to extract aircrafts motion-error, has the advantage of having the ability to operate without accelerometer and INS. However, an antenna with a wide azimuth beam is required. Furthermore,its accuracy depends heavily on the contrast of the ground reflectivity function. A new method based on the time domain approach is presented. Compared with the existing method, the new approach not only has all the mentioned features, but also the advantages of low computation requirement as well as the wide-band motion-error extraction capability. Furthermore, it relaxes some requirements on azimuth antenna beamwidth as well as the ground reflectivity function contrast.<<ETX>>

A new general formulation for the entire-domain analysis of an array of arbitrarily oriented circular loop antennas

A general formulation for the entire-domain analysis of arbitrarily oriented coupled circular loops has been presented for the first time. The Eulerian angles are used to unambiguously define the orientation of each loop with respect to another. The formulation gives a solution of the Fourier coefficients of the current on each loop after taking mutual coupling into account. The current coefficients can be used to directly obtain the input impedance and radiation characteristics of a coupled loop array. The formulation has been illustrated by applying it to two scenarios involving a pair of coplanar and coaxial loops. It is seen that the rotation of one loop about each axis could have rather different impacts on the mutual coupling between the loops. Thus the results highlight the necessity for the general formulation presented in this paper for correctly and conveniently analyzing all the various possible orientations in coupled circular loop arrays.

Narrowband Edge-Coupled Microstrip Filter

The design of a bandpass type microstrip filter using phase velocity compensated wiggly edge-coupled sections is presented. Narrow bandwidth is achieved by employing identical coupled sections for the centremost and outermost sections of the filter thereby eliminating the need for extremely loose coupler.

Intermodulation distortion in semiconductor lasers in application to subcarrier multiplexed fiber optic video systems

Analytical techniques for determining intermodulation distortion in directly modulated semiconductor lasers are examined. Direct expressions for the two- and three-frequency third-order intermodulation distortion-to-carrier ratios are used to check the validity of a well-known 6-dB difference formula. It is found that this formula is not always valid, the discrepancy being a function of laser diode parameters and subcarrier spacing. The carrier-to-intermodulation-noise ratios are computed for a 25-channel subcarrier multiplexed system, with subcarrier spacings of 6 MHz for amplitude modulation and 40 MHz for frequency modulation, and it is found that the central subcarrier is not always the worst affected, although it has the maximum number of interfering intermodulation products. This is in contrast to memoryless nonlinearities where the central subcarrier may be expected to have the worst carrier-to-intermodulation-noise ratio.