UMTS Network Optimization Case

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UMTS Network Optimization Case ZTE University

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Handover Case
Voice Quality Case
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Coverage Case
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Problem analysis
Spot A is about 2.7km from Sousse2 site. A is the entrance of a
uptown highway and has a turn of about 90 degrees. Signals of cell
228 of Erriadh TT site become weak suddenly because the cell is
sheltered.
Spot B is about 2km from CTT Skanes site. The seaside road that
B located is at a lower sea level than the CTT Skanes site. Signals
of cell 332 of CTT Skanes site can be received by the mobile
phone after penetrating several 2~3-layer buildings. At around spot
B, the pilot signal strength is reduced to be below -100dBm.
The NodeB in Sahaling is quite restricted by the environment. The
site height is only 25m; there is little space for increasing the height.
Coverage Case
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Coverage Case
Increase the pilot transmit power
Effect assessment
The coverage effect and the call-drop rate is optimized. There is
almost no dropped call along the express way.
Channel Before the Adjustment After the Adjustment
CPICH 10% 15%
BCH -3dB -3dB
FACH 0dB 0dB
PCH -3dB -3dB
PSCH -4dB -4dB
SSCH -4dB -4dB
PICH -7dB -7dB
AICH -7dB -7dB
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Handover Case
Voice Quality Case
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Signal distribution in the Donghu Road area before the optimization
Problem
During the coverage optimization DT along Zhongshan Road No. 1 and
Donghu Road, it is found that the receiving power of the UE one Donghu
Road between the Donghu base station and Shuqian Road base station is
weak and less than -85dBm. In addition, the pilot signal quality Ec/Io is
also poor and less than -13dB in this area.
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Effect after optimization From the analysis of DT data, it can be seen that in this part of the
Donghu Road, the UE receiving power is >-85dBm and the pilot
Ec/Io>-13dB, which meets the coverage requirement.
 Antenna adjusting Case 1
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 Antenna adjusting Case 2

Problem Through the analysis of the DT data of Baishi Road, it is found that
pilot strength received in the middle part of road is less than -
95dBm, as shown in Area A in the figure below:
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 Analysis: It is found that the coverage of this area is provided by Sector 2 of
Shenzhen University base station. The direction angle of Sector 2
is 110° and the downward tilt angle is 4°. Both shall be adjusted to
enhance the coverage of Baishi Road.
Solution  Adjust the antenna direction angle of Sector from 110°to 120° and
the downward tilt angle from 4°to 12°.
 Antenna adjusting Case 2
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Effect after optimization Conduct DT on the Baishi Road after the optimization. From the DT
result below it can be seen that the pilot strength is improved to
more than 90dBm.
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Pilot pollution Case
Flower hall site is located on the Gaoxun Tower beside the Quzhuang cloverleaf junction. Its is at a
height of 70m. After line testing, it is found that the 425 (scramble) cell of the site provides cross-cell coverage. Cell signals are still strong in the First Zhongshan Road, which is far from the Flower hall
site. As the 425 cell is not configured as the Neighbor-Cell of cell 436 in the first sector of the
Shuqianlu site located on the First Zhongshan Road, calls are easily dropped in this area.
The above figure shows the pilot Ec/Io route testing result on the First Zhongshan
Road (affected by signals from the Flower hall site, Ec/Io in area A is very poor; call-
drop rate in the area is high; however, the pilot strength of the area is good.)
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 Analysis of the call-drop reason
 As there is shadow fading, the occurrence of the following events can be detected
from the active set upgrading report.
Cell2 is the best service area;
Cell1 is deleted from the activation cell;
Cell3 is not in the Neighbor-Cell list of Cell2; strong signals from Cell3 result in poor
Ec/Io; Poor Ec/Io results in call-drops.
Pilot pollution Case
 As Cell3 is in a far distance, it is not
expected to be a member of the active
set in the problematic area;
Reduce the transmit power of Cell3 and
increase its tilt angle in order to control
its signal coverage range. At the same
time, take into consideration the
coverage range to be provided by Cell3.
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Execute solution:  Add the mechanical tilt angle of the antenna of Huachang site 425
cell;
 Add Huachang site 425 cell into the Neighbour-Cell list of
Shuqianlu site;
Reduce the maximum transmit power, public channel power and pilot channel power of Flower hall site 425 cell by 3dB.
Effect after optimization:

Pilot pollution Case
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There is no strict definition for the high site. It is a relativeconcept.
It is not necessarily wrong to put the UMTS base station on
the top of the hill.

The high site can easily receive uplink interferencegenerated by other users.
The bigger the loads in the high site coverage area, the
more possible the problem might occur.

If the network is vacant or lightly loaded, the effect of thehigh site is not obvious. But it still cause cross-cell
coverage, pilot pollution and call-drop.
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Suggestion
In urban areas, buildings are densely located and the penetration loss is big; the
radio transmission environment is complicated and the NodeB coverage distance is
small. Hence the antenna should not be put too high. According to the present
building density and average height, the antenna height can be about 35m; it should
be 10~15m higher than the average height of surrounding buildings. Ofcourse, the
specific height of the antenna should be determined according to the local radio
transmission environment. In rural areas, population is relatively small and buildings are not densely located;
distances between base stations are big. Hence the antenna should be high; in
general, the antenna height in rural areas is around 50m and should be 15m higher
than the average height of its surrounding.
In the sea, the radio transmission model is similar as the transmission model for free
spaces. The radio transmission environment is good; radio electric waves can be transmitted to a far distance. The site can be located on a high hill (higher than
100m) in order to expand its coverage.
In deserts and Gobi areas, signals are transmitted to a farer distance than in
ordinary plains. The antenna height is usually 60m or higher in order to expand the
signal coverage area.
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Handover Case
Voice Quality Case
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The Neighbour-Cell list is a cell list that might be addedinto the active set;
Cells in the Neighbour-Cell list will be measured as
whether they meet the requirement for soft handover or
softer handover with the main service cell;
The number of cells in the Neighbour-Cell list is up to 32;
 Avoid missing Neighbour-Cells with best signals in the
Neighbour-Cell list.
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The network planning tool can use proper algorithm to automatically
plan the Neighbour-Cell list; such planning is always based on the interference among cells;
If the pilot signals of one cell is very strong but the cell is not added in
the active set, signals of the cell will become strong interference;
Either single-directional configuration or bi-directional configuration
might be adopted between Neighbour-Cells;
In setting the Neighbour-Cell list, take into first considerations about
the cell interference and the cell’s possibility of becoming a main
service cell of the UE;
The method of automatically creating the Neighbour-Cell list via the
network planning tool can be regarded as an initial reference of the Neighbour-Cell list. Manual adjustment is needed. The Neighbour-Cell
list should finally be optimized by using the route testing data.
Neighboring cell Case
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Problem analysis
 According to testing data analysis, the section 20m from the call-drop venue is mainly covered by signals from the third sector (scramble 426) of the Flower hall
site instead of signals from the first sector (scramble 424) of the Flower hall site.
The reason might be the third sector (scramble 426) of the Flower hall site is
sheltered by a tall building in front of it; signals of this sector are reflected to the
road segment of 20m between the Flower hall site and the Yunshan Hotel site.
Check the Neighbour-Cell list; it is found that the third sector (scramble 414) of the
Yunshan Hotel site has configured the third sector of the Flower hall site as an
Neighbour-Cell, while the third sector (scramble 426) of the Flower hall site does not
configure the third sector (scramble 414) of the Yunshan Hotel site as an
Neighbour-Cell. This has caused a failure in single-directional handover and
resulted in call-drop.
Solution
Configure the third sector (scramble 414) of the Yunshan Hotel site as an
Neighbour-Cell of the third sector (scramble 426) of the Flower hall site.
Effect after optimization
 After the Neighbour-Cell is configured, route tests are made on the road segment
between the Flower hall site and the Yunshan Hotel site. No call-drop occurs.
Neighboring cell Case
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In the network planning phase, the Neighbour-Cell list can be
automatically generated via the network planning tool.
Optimization of the Neighbour-Cell list can be executed via route
tests and statistics analysis of the route testing data.

The Neighbour-Cell list optimized via route test data statisticsanalysis is a short Neighbour-Cell list. And if necessary, the
preference sequence in the Neighbour-Cell list can be very clear.
By analyzing the route test data, Neighbour-Cells not configured in
the Neighbour-Cell list via planning tool can be found.
Neighboring cell Case
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Pilot Pollution Case
Handover Case
Voice Quality Case
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Cell reselection Case
Description In drive test, pilot Ec/Io value was normal in continuous call test.
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Cell reselection Case
But the pilot Ec/Io in cycling call test was poor. Between cycling voice
calls, the UE was in idle mode. The reason of Poor Ec/Io was that cell reselection did not happen on time, as shown in the figure below.
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indicates the intra-frequency measurement threshold of cell
reselection.
If Sx > SIntraSearch, UE will not perform intra-frequency measurements. If Sx <= SIntraSearch, UE performs intra-frequency measurements.
Normally, Sx = pilot Ec/Io - Qqualmin
The smaller SIntraSearch, the easier intra-frequency measurement is
triggered. On the contrary, larger one will make it more difficult to
trigger the measurement of intra-frequency cells.
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Troubleshooting Procedure Secondly, checked Treselections and found the value was 1s. To make
the reselection happen earlier, changed its value to 0.
Do the drive test again. It showed that the cell reselection
happened more quickly at the fault location.
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Description UMTS external cells and neighbor cell relation were correctly
configured. The setting of reselection and handover parameters
were suitable, and reselection of GSM or UMTS network was also
normal. However, for CS service, handover from UMTS to GSM
failed.
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Cause Analysis In this situation, we traced signaling message of the subscriber
according to IMSI to find out the handover failure reason.
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Troubleshooting procedure Contacting with Core Network engineers and confirmed that the
MGW sent the RNC integrity algorithm to the MGW of the GSM
network with transparent transmission. But GSM did not use
encrypted algorithm, which caused relocation failure and handover
failure.  After changing the CIPHER option to FLAG in the LAICGI table, the
sent encrypted algorithm was removed. Further tests showed that
handover was normal.
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coverage area on Shuqian
of Meihuacun Hotel site
blocking of the dual-deck
threshold. Therefore, the
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 Adjust the handover threshold and Time to Trigger parameters of
Event 1A and Event 1B: reduce the handover threshold and Time to
Trigger parameters of Event 1A, so that cells with better signal quality
can enter the active set as soon as possible; raise the handover
threshold and Time to Trigger parameters of Event 1B, so that cells
within the active set would be removed for sudden fading of signals.
Effect after the optimization:  After the optimization, cell 434 on Meihuacun Hotel site can speedily enter
the active set and cell 436 on Shuqian Road site would be removed from
the active set due to the sudden fading of signals. Drive test after the
parameter adjustment shows that the success rate of handovers between
Shuqian Road site and Meihuacun Hotel site is greatly improved.
Event Parameter Setting Before Optimization Setting After Optimization
Handover threshold 2dB 4dB
Handover threshold 5dB 7dB
Event 1A
Event 1B
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Pilot Pollution Case
Handover Case
Voice Quality Case
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Description When voice calls were made in UMTS covered area in one building,
call drop happened very frequently when UE moving towards GSM
covered area. In 10 call, 9 calls dropped.
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neighbor cell relation is one of common reasons to cause
GSM/UMTS handover failure.
But it was confirmed that the GSM cell is in the UMTS cell’s
neighbor cell list.
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inside room

Cause Analysis Secondly, check coverage. It was found that the building had no
UMTS indoor distribution system, and was covered by outdoor
UMTS NodeB. When UE moved inside, UMTS signal penetrated
two iron doors to the UE, causing fast fading.
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Optimization measures Modified Cell Independent Offset (CIO) of the neighbor GSM cell
from 0 to 5 dB. The handover happened easier, but call drop still
existed.
Modified 2d RSCP threshold from -95 dBm to -85 dBm, and then -
75 dBm. The measurement of the GSM started earlier, but call drop still existed.
Modified GSM RSSI threshold from -90 dBm to -95 dBm. Handover
to the GSM cell was easier, but call drop still existed.
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Optimization measures Modified event 2d Time-to-Trigger from 640 to 320 ms, and then to
0 ms. The measurement of GSM started easier, but call drop still
existed. We changed it back to 640 ms.
Changed the measurement quantity from RSCP to Ec/Io, and
changed event 2d Ec/Io Threshold from -24 dB to -10 dB. But call drop still existed.
Changed event 3a Time-to-Trigger from 5000 to 2000 ms.
Handover happened more quickly, and the call drop problem
relieved.
Furthermore, changed event 3a Time-to-Trigger from 2000 ms to 1000 ms. Handover happened more quickly, and the call drop
problem was solved.
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UMTS to GSM call drop Case
Description Voice call drop happened during Handover from UMTS to GSM.
Checking the signaling in drive test, it was found that inter-RAT
measurement was started.
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Troubleshooting Troubleshooting level by level from BSC to the Core Network.
checked the BSC of another vendor, and found that the BSC
rejected the handover command from the Core network. And then
check BSC data. The result was that the inter-RAT service
handover function on the BSC of another vendor was not activated.
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Pilot Pollution Case
Handover Case
Voice Quality Case
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Problem
It is found that the call-drop rate is very high on the seaside express way from TRI002 to TRI004. According to the testing data
analysis, the coverage distance of 404 is very short at the call-drop
venue.
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Handling Idea
To take a bird’s-eye view from the sky, it is found that there are several tall buildings in front of the 404 cell.
Call drop Case 1
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parameters so that adding events
can easily occur and deleting events
occur slowly and difficultly. The
values of handover parameters 1C
and 1D events are adjusted.
Replacement threshold with
increased. The advantage of such adjustment is to enable high
percentage of the user’s using
strongest and stable scramble.
Call drop Case 1
1B event  Reporting Range
1C event
1D event
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 According to the route testing after handover parameter adjustment,
the handover success rate on this section is greatly improved; the
call-drop rate is reduced.
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Description
In this case, the UE would move in the directions marked by the
red arrow in the following figure. If call drop happens, the two sites
nearby would be marked as BKC0044U and BKCOO74U.
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Call drop Case 2
The main serving cell of the UE is the third cell (SC53) of
site BKC0074U, its Ec/Io is -9.83dB.
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Call drop Case 2
 As the UE moves on, the main serving cell changes to the third cell
(SC48) of site BKC0044U, its Ec/Io is -10.31dB. Cell SC53 of site BKC0074U is removed from the active set and enters the monitoring
set.
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Call drop Case 2
 After 1s, the signal quality of cell SC53 of site BKC0074U is stronger than cell
SC48 of site BKC0044U, and the Ec/Io of SC48 reaches -2.39dB. Cell SC48 reports to Event 1A and tries to enter the active set again. At this moment, the
pilot quality of the cell SC48 of site BKC0044U is very bad, with its Ec/Io down
to -21.05dB. The UE reports to Event 1A, but cannot receive the handover
command, then the call drops.
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Optimization Solution
To avoid the condition that cell SC53 of site BKC0074U cannot
enter the active set after being removed, the value of
CellIndivOffset(utranCell) of cell SC53 of site BKC0074U is
changed from 0dB to 3dB to prevent the cell from being removed
from the active set.
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Then the signal quality of cell SC53 of
site BKC0074U declines, with Ec/Iodown to -13.23dB, which is worse than
that of cell SC48 of site BKC0044U.
Then, the main serving cell of the UE
changes to cell SC48, but cell SC53
remains in the active set.
Call drop Case 2
direction in a call-hold mode,
and its main serving cell is
cell SC53 of site BKC0074U.
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Call drop Case 2
In the end, the main serving cell of the UE changes back to cell SC53
as shown in the following figure, and no call-drop happens.
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Pilot Pollution Case
Handover Case
Voice Quality Case
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Description
When using iPhone to dial 181 for time inquiry service at every
location in the network, the subscribers sometimes can only hear
noise. The problem did not happen frequently, usually one out of
200 calls.
 After test, we found that both the pilot strength and quality of the
serving cell were good. Besides, UE transmitted power was also
normal, and SIR was stable.
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 Abnormality was found when checking BLER. In normal cases, the
number of received blocks should remain steadily around 100 (this
value has been normalized). However, the figure showed that when
the voice quality problem occurred, the number of received blocks
was smaller than normal and kept fluctuating. Once this number
stopped fluctuating and returns to 100, voice quality returned normal. Meanwhile, the number of wrong blocks remained 0, which
meant that block error rate was 0. Thus, the possibility of downlink
interference was ruled out. We checked RTWP and found it was at
the normal level.
Then we opened the subscriber signaling tracing interface. After analysis, we found that packet loss occurred at the IUB interface,
and many time adjustment frames were received at the IUB
interface.
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Voice quality Case
We all know that UMTS uses the receiving window to synchronize
transport channels. If the transport channel synchronization frames sent by the RNC is within the receiving window, then other data
should also be within the receiving window. In this case, the
transmission in the transport channels of RNC and NodeB is
synchronous. If the said frames are outside the receiving window,
then the transport channels are not synchronous. In this case, NodeB needs to calculate the offset value and notify the RNC to
adjust the sending times of data frames through the uplink
synchronization frames on the transport channel in order to re-
synchronize data. When establishing the transport channel, the
RNC gives the starting point TOAWS and the ending point TOAWEof the receiving window.
V i lit C
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Voice quality Case
The time frame adjustment means that the NodeB finds that the
delay of some packets are outside the receiving window while synchronizing with the RNC, so the NodeB needs to repeatedly
send time adjustment frames to the RNC for the RNC to change
the frame sending time, so that NodeB can catch the desired data
that is sent at more accurate times. Data that is still outside the
receiving window outside adjustment is probably dropped.
V i lit C
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Voice quality Case
We first attempted to modify the receiving window of NodeB, which
did not solve the problem. Voice quality is related to call traffic heaviness, which illuminated us that the busiest service in the
carrier's building is HSPA service. We changed the receiving
window back to 16/8 and then reduced the number of HSDPA
channels from ten to five. Thus, we forcibly lowered the traffic on
the IUB interface. As a result, the voice quality returned normal.
V i lit C
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Voice quality Case
We observed that the configured traffic for OMCR was 15 Mbps but
the actual traffic was 5 Mbps. The traffic of two NodeBs together would no doubt exceed 10 Mbps. Therefore, the sent data
exceeded the bearer capacity and large amounts of voice data
were queuing in the transmission equipment. Some data was
dropped when the waiting timed out, and some data was dropped
by NodeB because it was outside the receiving window when reaching NodeB due to the long queue time. This explains why the
number of received blocks decreased, i.e. voice packet loss
occurred. Delay jittering explains the severity of jamming within the
transport equipment. The more severe the jamming, the longer the
queue and the longer the delay. Delay jitters as data traffic varies.
V i lit C
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through parameter modification
When signal strength or quality reaches a threshold, call dropping
or severe voice quality problems may occur. If this phenomenon
occurs in coverage holes and cannot be improved by RFadjustment, we can modify some parameters to make some
improvements.
Call drop and voice quality are related with AMR channel power
and downlink BLER. AMR channel power and downlink BLER are
directly correlated with signal strength and quality. So when signalstrength and quality cannot be improved, we can increase
DLDPCH power and set a higher BLERTarget.
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