Monday, June 30, 2014

CSA - Site Survey Method 1

Mention in CSA - Site Survey Method ( ) the terms Fresnel, Fresnel–Kirchhoff diffraction and Huygens–Fresnel diffractions were used to illustrate to readers that there are various mixed-models of environments which propagation is influnced or reacts.

The image above was produced to present a very simplified view how elements in the environment can influence radio signals. These are elements referred to in the terrain classification discussion ( ) but as radio signal cannot be visually seen requires understanding potential influences on radio signals to be detected or not at a particular location during CSA.

Prior to establishing a radio network a huge range of assessments are made to determine and understand the hostile environment into which cellular radio will be propagated. Integral to these assessments are various path loss models that have already established principles for radio network design.

The CSA technician or expert doesn't need to re-invent the wheel and define new class/es of influences in path loss models, merely work with the existing established models. Working with them actually improves the technique of conducting site surveys.  By way of illustrations, using the image above, a building object is present that may cause diffraction. The technican or expert may wish to exemplify this object in the analysis of the radio test measurement. It maybe useful therefore to record that the results from radio test measurement detected at a particular location appears to be as a consequence of diffraction method (*Deygout 1966)

[*Deygout 1966 proposed a diffraction method that takes two obstacles into account: A primary obstacle between the transmitter (Tx) and the maximum clearance ratio according to the entire line of sight (LOS) between the Tx and Rx (Receiver) ; a secondary obstacle obtained from the maximum clearance ratio but according to the (a) line of sight between the Tx to the primary obstacle and (b) the line of sight between the primary obstacle and the Rx. Whilst diffraction may be defined as a cause for detecting or not e.g. a Cell ID and services coverage at a particular location there is still the requirement to understand the influences in path loss.

As Whitteker, JH noted in Radio Science 1990 "Fresnel-Kirchhoff theory is adapted to the problem of finding the diffraction attenuation at VHF and UHF over terrain profiles of arbitrary shape. Approximations are based on the assumptions of small wavelength and small diffraction angles. As a preliminary step, the theory is applied to the multiple-knife-edge problem. The field is found as a function of height above each knife edge in turn. In an application of Huygens' principle, an integration over the field above one knife edge provides the field at any point above the next. This formulation is equivalent to knife-edge formulations used in the past. Then each pair of neighboring knife edges is bridged with an imperfectly reflecting plane surface, representing the terrain. Huygens' principle is used again for the reflected wave, neglecting backscatter. The field found in this way is accurate for a good reflector but not a poor one."]

The above use of Deygout 1966 proposed diffraction method can be useful when accompanied by a site photograph as the example enables elimination of a vast track of text. Consequently if the findings were to espouse the notion that the "detected coverage is believed to have been adapted due to noted Deygout method with intervening knife-edge (Fresnel–Kirchhoff diffraction) which bare results in the signal strength at the ground floor level of a block flat found to be in the range of -98dBm to -95dBm consistently over a ground level range of 40 metres (horizontal plane), the results indicate the quality indicators might allow the Cell ID's coverage to be used. However, those findings only suggest usage could be on the basis of not being able to 'exclude usage'. Other coverage from different Cell IDs in the same area held significantly improved signal strength and quality indicators making those Cell IDs better placed to provide services at that location as the test results show."

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