Digital Mobile Communications
Module P00330
Jorge Pinto
09097562
Dec 2010
The goal of radiolocation via a mobile terminal is positioned at the same level with any
degree of accuracy to know exactly where he stands.
This is to use a radio network that can provide certain equipment through its infrastructure
enough information to perform calculations that lead to obtain the coordinates of the terminal
involved.
When applying different methods on the GSM network we must take into account that the
measurement accuracy is determined by the radius of coverage of cells attempting to locate
the mobile terminal. In cities, a BS can have up to a coverage radius of approximately 100
meters, which obviously reduces significantly the margin of error of measurement. For rural
areas, with BS that cover a radius of ± 100 km, the precision of measurement error may also
increase.
Timing Advance is equivalent to round trip time, when the mobile is switched ON, it start to
tune in with a particular frequency and also synchronise with a particular timing slot. This
process takes some time to the MS. In this stage the MS still does not know where the BS is.
The mobile station (MS) sends random access message, which tell to the base station about
the timing advance because receiving those messages the base station (BS) could calculate
the distance from the MS to the BS base on the delay. This random messages does not fill up
the time slot completely due to the time advancing so the distant MS need to start to
synchronize a bit early and it uses the free time remained in the slot.
The maximum allowed bit is 63-bit period, which is equal to 232.47!" hence:
!"##$ =!"#$%&'(!"#$
Distance (round trip) = 3!10! x 232.47!" = 69.74 km
The maximum timing advance of 232.47µμs is 63 bit times 3.69!" therefore it corresponds to
a round trip approximately around 70 !" hence a single trip distance is 35 !". By bit units
the base station tells the mobile station to advance their timing. For a unit bit, round trip
distance and single trip distance are calculated.
For a single bit delay the distance is:
!"##$ = !"#$%&'(!"#$
!"#$% !"#$ !"#$%&'( = 3!10! ! 3.69!" = 1.107!"
The single trip distance will be the half of the round trip distance and it will be around
0.554 !". Hence, for a 0 bit delay the distance will definitely zero (0) it means technically
the mobile station is closer to the base station, base on that a range of distances has been
calculated according to the range of bit delays. Considering those ranges the timing advanced
has been set into a table in order to shows the theoretical calculation. (See table 1)
Bit delay Timing advance Distance range
0-1 0 0 < !"#$%&'( < 0.54 !"
1-2 1 0.54 !" < !"#$%&'( < 1.11 !"
2-3 2 1.11 !" < !"#$%&'( < 1.66 !"
3-4 3 1.66 !" < !"#$%&'( < 2.21 !"
4-5 4 2.21 !" < !"#$%&'( < 2.77 !"
5-6 5 2.77 !" < !"#$%&'( < 3.32 !"
6-7 6 3.32 !" < !"#$%&'( < 3.87 !"
Table 1. - Theoretical calculation of timing advanced related with the distance.
In Figure 1, see all drive from Headington hill until Wheatley Campus. It is showing the
Timing Advance signal recorded by RANOPT.
Figure 1. – Screenshot taken from Ranopt showing the timing advance values
Looking the Figure 1, note how the timing advance signal behave through the drive, where
green means values from 0 to 3 (close the base station) and yellow between 4 to 7 (further
away from base station or signal been cut by any obstacle).
Figure 2. - Base Station Location
Figure 3. - Identifying Orange Base station along the drive. BS on Gipsy Lane.
Figure 4. - Base Station Location
Figure 5. - Base Station Identified for Headington
Figure 6. - Orange base station around Risinghurst.
Figure 7. - Orange Base station in Wheatley.
After the identification of base station is finished, let’s show the positioning of Orange’s base
station along the drive. See Figure 8
Figure 8. All the Orange Base station identified along the drive.
Figure 9. - Different point chosen to calculate the timing advance (see result on table 2)
BSIC/BCCH Distance (km) Timing Advance Rx Power (dBm)
56/771 0.092 km 0 -62
56/810 0.6418 km 2 -79
57/856 2.25 km 5 -81
56/849 3.25 km 6 -92
Table 2. Timing Advance in different point along the drive
Distance measurement using Timing Advance Formula.
Since 1 bit of the TA represents a difference of of the signal BTS - MS - BTS, and the
refraction index of air is approximately 1 [6], the distance per bit of TA is:
Since the TA is rounded to the nearest bit-period during calculation, the actual BTS-MS
distance, d, is:
According with Figure 9 and Table 2, the timing advance varies depending the distance and
obstacles that the signal pass through. There is a correlation between timing advance and RX
power; these processes are very similar and correlated, because of their behaviour and how to
act in terms of distance, FSPL and the real world. One can say that if the two signal interact
in an unobstructed space between the base station and mobile station will both be strong, but
if we think in the real world where the signal must travel through different media and go
through a number of obstacles, is normal to see that as the distance between the base station
and mobile increase both signals will be dissipate. The distance is a factor behind this
correlation. As we know the further the mobile station is from any particular base station the
lower the power it will receive and that gives an idea about the serving base station. The
longer distance the mobile station is from a base station the higher the timing advance.
References
[1] Redl, M. Weber, M. & Oliphant, M., 1995. An Introduction to GSM. Mobile Communications Series. Massachusetts: Artech House.
[2] GSM For Dummies. 2009. Frequency Division Multiple Access (Introduction to GSM). [Online] Available at: http://www.gsmfordummies.com/intro/intro.shtml [Accessed 15/12/2009].
[3] Schwartz, M., 2005. Mobile Wireless Communications. 1st ed. Cambridge: Cambridge University Press.
[4] Wigard, J., & Nielsen, T., 2000. Performance enhancements in a frequency hopping GSM network. Dordrecht: Kluwer Academic Publishers.
[5] GSM: Radio Interface. [Online] Available at: http://www.cs.ucl.ac.uk/staff/t.pagtzis/wireless/gsm/radio.html [Accessed 15/12/2009].
[6] Douglas C. Giancoli. Physics for scientists and engi neers. Prentice Hall, 1989.
