Download - UMTS Network Optimization Case

8/9/2019 UMTS Network Optimization Case

http://slidepdf.com/reader/full/umts-network-optimization-case 1/70

UMTS Network Optimization Case

ZTE University



Content

Coverage Case

Antenna Adjusting Case

Pilot Pollution Case

Neighbour-Cell Case Cell Reselection Case

Handover Case

GSM/UMTS Inter-operation Case Call Drop Case

Voice Quality Case



Coverage Case



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



Coverage Case

Solution Adjust the transmit power of common channels

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



Content

Coverage Case




Handover Case


Voice Quality Case



Antenna adjusting Case 1

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.



Signal distribution in Donghu Road after the optimization

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.




Signal distribution of Baishi Road before the optimization


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:



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°.




Pilot coverage of Baishi Road after the optimization

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.




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-cellcoverage. 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.)



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.


Solution

Add Cell3 into Cell2’s Neighbor -Cell list

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.



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 andpilot channel power of Flower hall site 425 cell by 3dB.

Effect after optimization:

After optimization, the pilot Ec/Io of area A is obviously improved.

After optimization, there is no call-drop.





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.




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 betransmitted 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.



Content

Coverage Case




Handover Case


Voice Quality Case



Neighboring cell Case

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.



The network planning tool can use proper algorithm to automatically

plan the Neighbour-Cell list; such planning is always based on theinterference 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 theNeighbour-Cell list. Manual adjustment is needed. The Neighbour-Cell

list should finally be optimized by using the route testing data.




Problem analysis

According to testing data analysis, the section 20m from the call-drop venue ismainly 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.




Summary

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.




Content

Coverage Case Antenna Adjusting Case



Handover Case


Voice Quality Case



Cell reselection Case

Description In drive test, pilot Ec/Io value was normal in continuous call test.




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 cellreselection did not happen on time, as shown in the figure below.




Troubleshooting Procedure Firstly, checked parameter SIntraSearch and found it was set as "NO",

which meant it was invalid in intra-frequency reselection. SIntraSearch

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.




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.



Handover failure Case 1

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.




Cause Analysis In this situation, we traced signaling message of the subscriber

according to IMSI to find out the handover failure reason.




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.




Description

When the UE moves from the

coverage area on Shuqian

Road site (PSC: 436) to that

of Meihuacun Hotel site

(PSC:434), signals on

Shuqian Road site (PSC: 436)deteriorate due to the

blocking of the dual-deck

viaduct. However, the

Meihuacun Hotel site

(PSC:434) enters the activeset slowly for the high

threshold. Therefore, the

handover success rate is low.




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

Time to Trigger 640ms 200ms

Handover threshold 5dB 7dB

Time to Trigger 640ms 1280ms

Event 1A

Event 1B



Content




Handover Case


Voice Quality Case



UMTS to GSM handover failure Case

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.




Cause Analysis Firstly, check neighbor cell configuration. Because miss configuring

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.



inside room

window


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.




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 dropstill existed.

Modified GSM RSSI threshold from -90 dBm to -95 dBm. Handover

to the GSM cell was easier, but call drop still existed.




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 calldrop 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 to1000 ms. Handover happened more quickly, and the call drop

problem was solved.



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.




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.



Content




Handover Case


Voice Quality Case



Call drop Case 1

Problem

It is found that the call-drop rate is very high on the seasideexpress way from TRI002 to TRI004. According to the testing data

analysis, the coverage distance of 404 is very short at the call-drop

venue.



Handling Idea

To take a bird’s-eye view from the sky, it is found that there areseveral tall buildings in front of the 404 cell.

Call drop Case 1



Solution

Optimize the handover parameter:

Adjust 1A and 1B event handover

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

strongest pilot is reduced;

replacement observation duration is

increased. The advantage of suchadjustment is to enable high

percentage of the user’s using

strongest and stable scramble.

Call drop Case 1

Event Setting before

optimization

Setting after

optimization

1A event

Reporting Range

Constant

3 5

Hysteresis 3.5dB 2dB

Time to trigger 200ms 200ms

1B event Reporting Range

Constant

7 6

Hysteresis 3.5dB 4dB


1C event

Hysteresis 6dB 4dB


1D event

Hysteresis 6dB 4dB




Call drop Case 1

Effect after optimization

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.



Call drop Case 2

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.



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.



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 siteBKC0074U is removed from the active set and enters the monitoring

set.



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 SC48reports 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.



Call drop Case 2

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.



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

Verification of Optimization

Effect

The UE moves in the arrow

direction in a call-hold mode,

and its main serving cell is

cell SC53 of site BKC0074U.



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.



Content




Handover Case


Voice Quality Case



Voice quality Case

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.



Voice quality Case

Discovery and solution of voice quality problem

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 returnednormal. 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. Afteranalysis, we found that packet loss occurred at the IUB interface,

and many time adjustment frames were received at the IUB

interface.

V i li C



Voice quality Case

We all know that UMTS uses the receiving window to synchronize

transport channels. If the transport channel synchronization framessent 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



Voice quality Case

The time frame adjustment means that the NodeB finds that the

delay of some packets are outside the receiving window whilesynchronizing 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



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 trafficheaviness, 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



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 togetherwould 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 whenreaching 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



Voice quality Case

Improving call dropping or voice quality degradation

through parameter modification

Parameters involved: MAXDLDPCHPWR, BLERTARGET

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.