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."

Saturday, June 21, 2014

CSA - Site Survey Method

If I were to say 'up' you can guarantee someone will say the complete opposite. Cell Site Analysis has this fascinating way of bringing opinion out of the woodwork an opposite response that hitherto such a response had remained hidden. The thought processes that make humans do this are far too complex to discuss in this thread and, frankly, would take a far greater mind than mine to offer to you an acceptable opinion. The best I can do is to offer to you some thoughts, evolved from knowledge skill and experience, and in doing so to invite you to enter the observation/opinion arena to see if you can offer improved solutions how to create a Site Survey Method.

The diagram below represents a sketch of the surrounding area where a murder has taken place. The first observation is that from knowledge the scene of crime is historic. Rarely does CSA come into play in live matters and if it does lawful interception elements are usually active, too. Thus, I am not considering overt, clandestine operations or montioring via bug phones etc.

Dependent on the information gathered during the Cell Site Identification procedure relating to the target area to be investigated, examiners should equip themselves with as much local information as is reasonable in order to conduct a site survey. It is most definitely worth the time and effort obtaining any aerial views that may be available, and that are up-to-date, about the target area.

Before visiting site, preparation is important. Attempt to gain some appreciation about the terrain, man-made clutter and natural form. Obtain local road maps to comprehend the road and street plan. Produce a rough
plan of the area that is to be inspected and approximately highlight those manmade structures that create landmarks.

Green Dotted Area
The diagram above depicts the area identified in the murder scenario discussed in the section on Neighbouring Cell Sites Identification. The localised scene is brought into view, the area identified, and dotted boundaries highlighted. The Red dotted boundary is the first areas in which an examiner may wish to conduct radio tests relevant to the immediate murder scene. Tests may need to be extended, as shown by the Green dotted boundary area.

During compilation of site information an examiner may wish to conduct a wider search so as to exclude other areas where the handset could detect Cells IDs (obtained during enquiries and mast tests) and other Cells d IDs detected by the test device handset. This is a good practice procedure and demonstrates the examiner is not assuming or excluding factors without due consideration. Quite often in criminal cases an
examiner will be instructed to survey a wider area due to information received, usually from a defendant suggesting his movements (Purple areas) at material times on the day of the crime, or an eyewitness thinks s/he sees the handset user at other locations. Always check them out and conduct radio tests.

Red Dotted Area
*Outdoor - Having selected the target zones for investigation, there is still the need to know how to conduct, in practical terms, those tests at the specific site. There are no defined guidelines that state measurements should be taken at certain distances. Experience suggests, considering the front, sides and rear of the block of flats (Fresnel), to select appropriate points of 1M near corners of buildings (denoted by the Green circled crosses) due to reflection or diffraction (e.g. Fresnel–Kirchhoff diffraction, Huygens–Fresnel diffraction etc) of radio signals.

Thereafter, select appropriate measurement points on a case by case basis at 1metres, 5metres, 10 metres or 100 metres (don't forget the increments inbetweeen) depending upon the length of the building and free land space surrounding it. The distance selected is optional, as already mentioned, but for the avoidance of doubt distances are suggested taking into account not only manmade objects, but, natural objects; such as trees or high hedges etc. Endeavour to take a camera to site so that photographs can be taken of all the places where measurements were actually conducted. As a place will represent a location, another useful tool for site surveys is a GPS monitor to identify precise coordinates, however it is best to avoid inferring GPS is part of the GSM or another cellular radio system as it is not the case.

Near-Wide Area Search
Whilst at the block of flats attempt to gain access to the roof and conduct tests, but equally as important is to get a visual bearing of the surrounding area. Take photographs (North-South-East-West) and tilt the camera to get overhead views of the clutter on the landscape surrounding the block of flats.

Finally, remember that where information is received about other locations where the handset user may have been (Purple areas). Identify each
location by using techniques adopted from Cell Site Identification and log of GPS monitor. Indoor and outdoor tests that are required should correspond to the information received.

In preparing this discussion a limited geographical/terrain location has been adopted merely to assist the flow of discussion. The information in this discussion should not be used as definitve for all geo/terrain profiles.

*Indoor will be discussed in another discussion thread .