Radio Propagation Process Research Articles

Taking a Sharp Turn: Modeling of Diffraction Coupling in Radio Propagation Inside Tunnels

The overall performance of a communication-based train control (CBTC) system largely depends on the performance of its data communications subsystem (DCS). The DCS network in almost all CBTC commercial system products marketed in the last decade uses radio communications in the open industrial, scientific, and medical (ISM) bands (2.4 or 5.8 GHz) to establish the bidirectional data link between central/wayside and onboard segments. Key factors in ensuring stable and sound radio communication are the number of wayside access points and locations of their antennas. Radio propagation modeling aims to provide optimal and reasonably reliable assurance of stable communication. However, the diffraction impact of sharp corners and edges in tunnels on the radio propagation process has been unaccounted for in the majority of models. The purpose of the present research is to incorporate the effect of diffraction coupling due to sharp edges in tunnel sections, including geometrical discontinuities such as cross junctions and L bends, through ray-mode conversion. The proposed modeling approach offers sufficient versatility to assimilate a variety of discontinuous geometries involving sharp edges in a tunnel environment. Numerical and empirical results suggest that the model provides an accurate tool for analyzing diffraction effects of tunnel discontinuities with sharp edges on the process of radio propagation.

Evidence for the Conversion of Noise Envelope Statistics by Radio Propagation Processes

Results of experiments that bear upon the hypothesized transformation of impulsive noise envelope statistics are presented and interpreted in light of the existing theory. In particular, recently published data by Sheikh and Parsons are found to be consistent with the envelope conversion concept.

The Conversion of Area Distributed, Incidental Radio Noise Envelope Distribution Functions by Radio Propagation Processes

The central limit theorem may be invoked for man-made incidental radio noise to establish the limiting statistical distribution which is approached as either the noise source-to-observer distance or the observation frequency is increased. As these parameters separately increase, the distribution of the received noise-voltage envelope for either surface or airborne noise is shown to approach a Rayleigh distribution. Conversion of the impulsive noise voltage envelope distribution to a Rayleigh distribution occurs through a combination of effects associated with both the radio propagation process and the spectral density of the noise sources. Free-space spreading and the dispersion of irregular terrain for low-height antennas are the propagation processes which contribute to the conversion of the noise statistics measured by constant-gain antennas. The noise emission spectra, derivable from a Poisson process below the VHF band and from a white-noise process in the upper VHF and UHF bands contribute the second factor to the noise-dispersion function. Existing in-flight observations, although qualitative, of impulsive noise distribution transformations with increasing range are predicted. Additional predictions of impulsive noise-voltage envelopedistribution conversion to a Rayleigh distribution are made for both airborne and surface observations as either the observation frequency or the range increases.