Using Propagation Delay in WCDMA RAN Optimization.ppt

Using propagation delay in wcdma ran optimizationUsing propagation delay in wcdma ran optimization
Wcdma ran p6 optimization
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.
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Outline
Definition
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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.
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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.
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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.
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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.
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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.
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Propagation delay: the report
The Business Objects report shows visually the number of samples in each of the BINs, and effectively, we can directly detect when a cell is overshooting.
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After opening the Report, Press the Refresh Data and specify the period and the RBS under investigation.
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You Can right click on the Chart, select Format Filter, then select the sector under investigation.
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Site: 2169
Cell: 3
Status: sudden blocks as attempts on R99 and HS start increasing
Physical Configuration:
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)
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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)
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Sector was downtilted at 9 AM. Immediate effect can be seen in the decreasing propagation delay values in that range.
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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).
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Solution: downtilt the sector from 3 to 5 degrees.
When: the change was done on 7 December 2009 at 9 AM.
Result: the result can be seen directly in slides 16 and 20:
The number of times pmPropagationDelay is pegged at 30-42% decreased considerably, which means that the cell was no longer overshooting at 10 KM.
The R99 accessibility rate improved without affecting the number of users in reach. In the graph of next slide you can see that the number of R99 and HS attempts did not decrease after the change.
It is important to note that the target is not to have zero overshooting. In rural and suburban areas where coverage is poor, a cell can overshoot at close distances without affecting its performance; it provides coverage. We do not want to completely remove that because we do not wish to lose coverage in such areas. We only want to reduce the overshooting that is hurting our performance, i.e overshooting at far distances. So you can see in slide 16 that the counter is still being pegged at 4% and it is harmless.
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Site: 0271
Cell: 3
Status: Dropped calls, and causing Pilot Pollution and drops due to missing Ncells (neighbor cells) for far cells which are not defined as neighbors.
Area: Suburban
As seen from the map, there is a lake just below the site causing lots of reflections, which is the main contribution to the overshooting:
Cell Range = 354 Chips
Cell Range (in Meters) = 354 x 78.125 = 27665 meters
From the BO report we find that the cell is overshooting at 33%-36% of the cell range which corresponds to 9-10 Km.
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Case2: Improving Retainability
On the 13th, the Cpichpower was reduced. After this date, the cell is no longer overshooting.
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Case2: Improving Retainability
On 13/11, the cpichPower was reduced. We can see in the prop. Delay graph (slide 23) that the cell is no longer overshooting.
The overshooting is reduced to 16% of the cell range, which corresponds to 4.5 Km.
Looking at retainability (slide 25), the drop call rate decreased.
In addition, we can find that the drops due to missing neighbor-cells was also reduced.
After reducing the CPICH power, the cell is covering up to 4.5 Km which is the planned coverage for this cell.
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Area: Suburban
The site is covering a city beyond which there is a wide plane of agricultural land. In this area the coverage is poor.
The prop.delay report shows that the cell is overshooting. After downtilting the sector to 5 degrees, we can see in the prop. Delay graph that the cell is no longer overshooting at 30% distance. The drop rate decreases to below 2%.
The downtilt was executed on the 26th of November 2009. in the graphs the improvement is clearly seen after this date.
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After 26/11 the drop rate decreased to less than 2%
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CASE3: improving Retainability
Overshooting at 20 to 30% of the cell range before downtilting
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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.
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references
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Using Propagation Delay in WCDMA RAN Optimization.ppt