CSA - GSM Timing advance and absolute delay

The notion of GSM Timing advance and absolute delay as considered in relation to cell site analysis (CSA) requires not only understanding the reason and purpose for these linked but seperate elements to be measured, but equally being aware of the components identified to make the measurements.

Within the scope of the work relating to mobile communications forensics and evidence examiners cannot create or make up the measurements and components but should look, as a starting point, to the GSM technical standards for guidance from which the examiner could identify a qualifying statement how GSM Timing advance and absolute delay may be performed and then qualify the guidance given in the testing schedules defined within GSM conformance testing.

Importantly, the defintion Timing advance (TA) is a time offset in bits as sent to the MS by the BS. The MS shall advance its transmissions to the BS by the timing advance relative to 3 timeslots behind transmissions received from the BS. The absolute delay is the delay between a common burst reference point within the received and the transmitted RF burst.

To determine the measurements the components used in normal or dummy bursts is, as mentioned above, the common burst reference point and defined as the transition from bit 13 to bit 14 of the midamble.

For an access burst the common burst reference point is defined to be the transition from bit 48 to bit 49 of the burst.

Basic Terrain Plot, GPS & CSA

Basic Terrain Plot, GPS & CSA

A question emailed to me recently raised a useful point, which I thought would be useful to mention. By using a GPS receiver to trace the route along which test mesaurements are taken in a geographical area, would that reduce the need for terrain and clutter maps? The answer is absolutely not. Given the nature and purpose of a GPS receiver it does not and could not replace terrain and clutter maps. The purpose of the latter have an entirely different function to perform for cell site analysis (CSA) and evidence.

Below is an example of a basic terrain plot. I should point out there are many different versions that can be produced, but the one below will do for the purposes of this blog discussion. 

PRIMER - how to read the plot
The basic plot incorporates the following. Mast location (eg postcode, NGR etc), height above ground level of the antennas (trx) on the Mast, azimuth and a target reference location (eg postcode, NGR etc) where an incident is alleged to have taken place. The raw geo data can be obtained from national sources (eg Ordnance Survey or equivalent in other countries). The TERRAIN path profile has been mapped between those two locations in the plot below. The GREEN LINE represents the linear radio path between both reference locations. The RED LINE represents the TERRAIN between both locations. The BLACK ELLIPSOIDS identify obstructions in the line of the radio path. The BLACK LINE represents GSM "maximum acceptable boundary" (35 km).

The basic terrain plot should be prepared prior to conducting any site surveys, intended radio test measurements, etc. Importantly, this rules out the use of a GPS receiver replacing terrain and clutter maps because:

(a) the plot is produced prior to a site visit, so no e.g. GPS waypoints can be logged at this early stage; and
(b) the plot identifies aspects on the terrain path that need to be known; a GPS receiver can not provide such important information;      
(c) the plot also provides an advanced warning of potential locations where testing maybe needed and where a GPS receiver could be used to mark particular waypoints. Three examples from the plot above for waypoints (i) the small ellipsoid identifies the possibility of radio path interference such as potential attenuation, diffraction, reflection, shadowing, etc (ii) the large ellipsoid suggests blocking of the radio path with coverage being directed into the earth's surface and (iii) the target location is 48km distance from the Mast, thus beyond the maximum acceptable boundary. 

There is a considerable body of material and further discussion I could add about information and inferences about terrain plots (which is covered in the cell site analysis (CSA) course material), but I do not wish to give the impression, nor do I have the intention, of building a case against the use of GPS receivers. A GPS receiver is not a redundant tool. It is, though, only one very useful tool of many useful tools that are needed for cell site analysis (CSA). GPS should not be considered the tool that is capable of replacing other knowledge-based sources or test information gathering tools. The latter are as equally required for cell site analysis, and, in some instances, even more so.