June 2000 | NTIA Technical Report TR-00-379
Michael G. Cotton; Edward F. Kuester; Christopher L. Holloway
Abstract: In this study we investigated the geometric optics (GO) approximation to the fields of an incremental electric dipole above a half plane for geometries typical of wireless indoor communications. This inspection was motivated by efforts to establish a ray-trace model to characterize indoor radio propagation channels. Eight canonical geometries were examined to isolate near-surface and near-field effects that are not accounted for in the GO approximation. Common building materials and physical dimensions (i.e., antenna separation and height) as small as 1 cm were investigated for frequencies up to 8 GHz. Theoretical fields were calculated via numerical evaluation of Sommerfeld integrals and compared to corresponding GO approximations. As expected, GO approximations agreed with theoretical results when the source and observation points were multiple wavelengths above the surface and relatively far apart. Close to the surface, an interesting interference pattern in the frequency domain was caused by adjacent fields in the two media propagating at different speeds. This so-called "pseudo-lateral wave" phenomenon is discussed and demonstrated in various examples. Next, we emulated system specifications (i.e., center frequency and bandwidth), computed time-domain impulse responses, and used delay spread as a metric to quantify GO error. Results show that mechanisms exist under certain circumstances which invalidate GO assumptions; conventional expressions to complement GO approximations are summarized.
Keywords: impulse response; delay spread; geometric optics; indoor propagation channel; propagation over ground; Sommerfeld integral; numerical integration; ray-trace model
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Michael G. Cotton
Institute for Telecommunication Sciences
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