WCDMA RAN Optimisation

1. Using propagation
delay in wcdma ran
optimization
Wcdma ran p6 optimization

2. 2009-12-22
abstract
This is a Business Objects report that makes use of “propagation delay” to
improve Accessibility and Retainability KPIs in WCDMA RAN P6 by detecting
overshooting cells.
It falls under the domain of WCDMA RAN Optimization and would benefit people
looking into network performance and optimization.

3. 2009-12-22
Outline
› Definition
› The Concept
› The Counter
› The Report
› Case1: Improving Accessibility
› Case2: Improving Retainability
› Case3: Improving Retainability
› Summary

4. 2009-12-22
Propagation delay: Definition
› The propagation delay in radio access is the propagation
time needed by the signal to travel from UE to NodeB.
› If we have the propagation delay values, we can calculate
the distance between UE and NodeB based on the
equation:
Distance [meter] = (speed of light * propagation delay)/chip rate
› This allows us to approximate the user location with
respect to the NodeB.

5. 2009-12-22
Propagation delay: THE concept
› In link budget analysis and radio network design, the preliminary
expected range of a cell is defined, ex. 1Km.
› Using propagation delay, one could detect the actual covering range of
a cell (by calculating distance of UEs).
› If the values of propagation delay reveal UE locations very far from the
NodeB in regions where other NodeBs exist, we can conclude that the
NodeB in question is overshooting.
› In WCDMA RAN Optimization, overshooting often causes pilot pollution
and interference and high transmit power, which lead to Accessibility
(call establishment failures) and Retainability (dropped calls) issues on
the overshooting cell itself and in the areas where it is overshooting.

6. 2009-12-22
Propagation delay: the counter
› The counter that shows the propagation delay of the signal
in Ericsson is “pmPropagationDelay”.
› Propagation delay is measured on RACH messages with
correct CRC. A sample is taken from each detected
preamble with successful detected message.
› The counter is a PDF counter and has 41 Bins.
– Bin 0: includes the Maximum delay in chips.
– Bin 1  Bin 40: each bin shows the number of counted samples
during a specific period with a specific percentage out of the
maximum cell range.

7. 2009-12-22
Propagation delay: the counter
› Bin 0 shows the maximum delay in chips.
› The maximum delay is the time for Initial PRACH to propagate from the
maximum configurable Cell Range to the RBS.
› Accordingly, we can calculate the Cell Range of the cell.
– Cell Range (in meters) = Maximum Delay (in chips) * 3x10^8(in
meters/sec)/3.48x10^6(in chips/sec) = maximum delay x 78.125
– As an Example, if we have in Bin0 the value 350 (the maximum value is 450
chips), the cell range is 27000 meters, which is equal to the configured
parameter CellRange (the maximum Configurable CellRange 35000 m).
– Any user trying to connect to the RBS with a propagation delay > 450 will be
disregarded.

8. 2009-12-22
Propagation delay: the counter
› Bin1 shows the number of samples that have propagation delay
between 0 and 1% of the cell range (if the cell range is 27Km, this
corresponds to distance between zero to 270 m away from the site).
› Bin 2 shows the number of samples that have propagation delay
between 1% and 2% of the cell range (if the cell range is 27Km, this
corresponds to distance between 270 to 540 m away from the site).
› Bin 3 …
› .
› .
› .
› Bin 40 shows the number of samples that have propagation delay
between 96% and 100% of the cell range.

9. 2009-12-22
Case1: improving Accessibility
› Network: 3G Vodafone Netherlands
› Site: 2169
› Cell: 3
› Status: sudden blocks as attempts on R99 and HS start increasing
› Physical Configuration:
– Height: 30 m
– Electrical Tilt: 3
– Mechanical Tilt 0
› Area: Rural
› Neighboring cells are not well covering the area in red (no dominant):
12771(height:40m,Etilt:8); 4523(35m,6); 4792(20m,4); 21602(39m,5);
34551(36m,4)

10. 2009-12-22
Case1: improving Accessibility
› Accessibility is degraded because a lot of R99 attempts are
getting blocked after admission: pmFailureAfterAdmission
is being stepped. (see slide 15)
› After_Admission blocks are caused by one of the following:
– Transport congestion: not existing for this site
– Lack of CE in UL (since UlHwAdm=100); on this site CE UL usage is
low
– Bad coverage: site either overshooting or heavy pilot pollution
› The third option seems to prevail but to be sure we take a
look at “pmpropagationdelay”. (see slide 16)

11. 2009-12-22
Case1: improving Accessibility
› From the chart, we see that the cell coverage is reaching
up to 30-42% of the cell range.
› The Maximum Delay = 447 chips  CellRange =
447x78.125 = 35 Km.
› The results show that the cell is overshooting for a range
between 10Km and 14Km.
› In the Google snapshot (see slide 18), this area is delimited
with a red cone. In that area we see many small residential
spots which are not covered by the surrounding sites.
› Any UE attempting to establish an R99 connection 10 to 14
Km away from the site might get blocked due to weak
coverage (bad RSCP,EcNo).

12. 2009-12-22
summary
› This report helps you detect WHEN and WHERE a cell is overshooting.
› Applying this knowledge to performance monitoring and optimization,
you can:
– Improve accessibility/retainability as shown in the 3 cases.
– Make sure that 2 non-neighbor cells will not be added as Ncells when using
GPEH traces if the cells are far from each others and one of them is
overshooting.
– When Optimizing Ncells and if using GPEH traces, it is possible to find
different cells having the same SC causing drops due to missing neighbors.
Using the Prop. Delay report, you can directly identify which cell should be
added if any.
– Reducing pilot pollution by reducing the coverage of overshooting cells.
– Improve coverage in case a cell shows that it is not covering the planned
area, where it is supposed to be the dominant cell.