WCDMA optimization case study

WCDMA RF Optimization &Case Study

ZTE UniversityTD&W&PCS BSS Course Team

Content

WCDMA RF Optimization Process

WCDMA RF Optimization Policy

WCDMA DT Cases

WCDMA Network optimization Cases

Single station check

Base station group optimization

Whole network optimization

Satisfy the indexes or not?

Find out base station group that do not

satisfy requirements

No

Common RF Optimization Process


RF Optimization Step 1: Single Station Check

Confirm site information Longitude and latitude, configuration, height above sea level, peripheral

environments and so on;

Confirm antenna feeder information Antenna type, azimuth, down-tile angle and height;

Check antenna feeder link Standing wave ratio, primary set and diversity RSSI check, primary set and

diversity lock balance;

Confirm system parameters List of adjacent areas, overhead channel transmitting power, SC

configuration, switching parameters;

Check and test basic functions Basic call process, soft switching, softer switching;

Check station coverage


RF Optimization Step 2: Base Station Group Optimization

Spectrum scanning

Load-free test

Load test


RF Optimization Step 3: Whole Network Optimization

Test on various radio indices of the system

Analysis on test results

Confirm whole network adjustment scheme


Performance Test Indexes

Voice quality--BLER

Call connection rate (call completion rate and paging response

rate)

Resource utilization—CPU utilization-

Handover success rate

Call drop rate

Network coverage rate Forward coverage

Pilot coverage

Service coverage

Backward coverage


Content



WCDMA DT Cases


Common RF Problems

Call Drop

Discontinuity

Access Failure


Call Drop Analysis

UL/DL coverage is not satisfactory (Ec/Io and Ec) Improve the coverage of the points.

List of adjacent cells are not complete Configuration of list of adjacent areas is not complete.

Interference There is internal interference source.

Pilot pollution is serious

Faults with base stations Incorrect connection of antenna feeders, GPS fault causes

asynchrony between the time and the system, interruption of transmission.

Hard handover failure


Discontinuity Analysis

UL/DL coverage is not satisfactory (Ec/Io and Ec)

Improve the coverage of the points.

List of adjacent cells are not complete

Configuration of list of adjacent areas is not complete.

Interference

There is in-band interference source.

Pilot pollution is serious


Access Failure

Interference

Coverage over weak areas, blind zones or pilot pollution

areas makes it impossible for signaling interaction

between the base station and the mobile phone to be

completed during the access.

Mobile phone performance


RF Optimization Policy

Adjust the antenna down-tilt angle

Adjust the antenna directional angle

Adjust the antenna height

Change the antenna type

Appropriately adjust the base station transmitting power

Adjust the base station location

Increase the base stations



Antenna directional angle During optimization, attention

should be paid to antenna directional angle, as shown in the figure on the right:

If the antenna coverage area is a vast space of residence, and the buildings are of the similar structure, the antenna direction shall be alongside the direction of the buildings (as the red arrow on the left); if the antenna direction is the same as the arrow on the right, the quality of signals in the coverage area may not be good.



RF Optimization Policy for Pilot Pollution

Pilot pollution is caused by too strong signals in some cells out of the coverage area in most cases. Measures for RF optimizations are as follows:

Adjust the antenna down-tilt angle, so as to reduce the coverage area, and further reduce the number of pilots in the pilot pollution area;

Appropriately reduce the transmitting power of the cell, so as to reduce the signal strength to narrow the coverage area, and also further reduce the number of pilots in the pilot pollution area;

If the two measures are of no use, we can increase base stations in the pollution areas, so that there will be a master pilot signal, to solve the pollution. But be careful in taking this measure, as it may impose great influence on the entire network .


Content



WCDMA DT Cases


导频强度分析Weak coverage area

DT （ Best Ec）WCDMA DT Cases

Ant reverse

No domi server

Over cover

DT （ Best SC）WCDMA DT Cases

TX is high

Uplink/downlink unbalance

WCDMA DT Cases

Over Shoot Analysis

PSC’s coverage exceeds defined threshold, and may

cause negative influence on remote region.

WCDMA DT Cases

Call drop

Monitor set 267&283 arestrong

Adjacent cell problem

Handover problem analyze

WCDMA DT Cases

Content



WCDMA DT Cases


Case1(Handover problem)

In especially urban environment, the handover region between two cells might be too small.

If the UE passes such an area in a very quick speed, the call might be dropped.

There might be sudden changes of signal strength at crossroads of the city.


Case1

For successful handover, the

increase and decrease speed of

the signals received by the UE

can enable the UE to complete

the necessary active set

updating process.


Case1

The handover region should

be big enough to ensure the

UE to complete the active set

up-dating process before

being interfered or

compressed.


Case1

Cover the crossroad with one

cell.


Case1

The antenna of the cell should be

put higher than the buildings along

the street so that the cell coverage

area can be bigger.


Case1

How to determine that the call-drop is caused by too small

handover region or sudden change of signal strength?

Before the call is dropped, the Ec/Io reported by the UE is very

poor.

Once in the idle mode, the UE sets up connection with the new

cell.

The Ec/Io reported by the UE becomes very good.

The big difference of Ec/Io indicates that the call-drop is caused by

these reasons.

The pilot strength data of the two cells recorded by the Scanner

also proves the above conclusion.


Case2

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.


Case2

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.


Case2

Problem analysis:

As the handover region is short and the call-drop venue on

the seaside road is close to the TRI002 site (only 400m),

signals might be strong at first but disappear quickly. This

can cause slow speed of strong signals of the adjacent 404

cell in adding the active set. It can also cause a lot of ping-

pang handover and result in call-drop.


Case2

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 such adjustment is to

enable high percentage of the user’s using

strongest and stable scramble.

Solution:

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


Effect after optimization:

According to the driving testing after handover parameter adjustment, the handover success rate on this section is greatly improved; the call-drop rate is reduced.


Case3


Case3

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.


Case3

Adjust the transmit power of

common channels;

Increase the pilot transmit

power

Solution:


The coverage effect and the call-drop rate is optimized. There is almost no dropped call along the express way.

Channel Before adjustment Afteradjustment

CPICH 10% 15%

BCH -3dB 0dB

FACH 0dB 3dB

PCH -3dB 0dB

PSCH -4dB -3dB

SSCH -4dB -3dB

PICH -7dB -4dB

AICH -7dB -4dB


Case 4 (High Sites problem) Flower hall site is located on the Gaoxun Tower beside the Quzhuang cloverleaf junction. Its is at a height of

70m. After driving testing, it is found that the 425 (scramble) cell of the site provides overshoot 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 driving 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.)


Case 4 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.


Case 4

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, common channel power

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


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

After optimization, there is no call-drop.


Case 4

There is no strict definition for the high site. It is a relative

concept.

It is not necessarily wrong to put the UMTS base station on the

top of the hill.

The high site can easily receive uplink interference generated

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 the high

site is not obvious. But it still cause overshoot coverage, pilot

pollution and call-drop.


Case 4

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. Of course, 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.


Case 5

the Neighbour-Cell list problems

The Neighbour-Cell list is a cell list that might be added into

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.


Case 5

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 MS;

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 driving testing data.


Case 5 According to repeated driving tests, it is found that calls are usually dropped

during the handover in the direction from the Flower hall site to the Yunshan Hotel site; in the opposite direction from the Yunshan Hotel site to the Flower hall site, no call-drop occurs.


Case 5 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, driving tests are made on the road segment between the

Flower hall site and the Yunshan Hotel site. No call-drop occurs.


Case 5

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 driving

tests and statistics analysis of the driving testing data.

The Neighbour-Cell list optimized via driving test data statistics

analysis is a short Neighbour-Cell list. And if necessary, the

preference sequence in the Neighbour-Cell list can be very clear.

By analyzing the driving test data, Neighbour-Cells not configured

in the Neighbour-Cell list via planning tool can be found.


Case 6

If the path loss values from the MS to multiple cells are similar, problems will be

generated as there is no dominant server.

Such problems include poor Ec/Io, low downlink capacity, and frequent updating

of the active set.


Case 6

Add loads on a small network with 7 omni-directional sites; there are

200 voice terminals in average; the connection rate is 100%.


Case 6

Remove the central site; pilot pollution will be generated in the central area, which leads

to the emulated Ec/Io failure (the coverage probability now is 78%).


Case 6

Increase the pilot power from 33 dBm to 38 dBm; simulated Ec/Io failures

disappear; but downlink Eb/No failures occur in the same area.


Zoom in

Pilot pollution

Case 6


Case 6

Solution:

In the precondition of satisfying coverage, adjust the tilt

angle, azimuth and power parameters of Neighbour-Cells so

that signals of one cell become a dominant server.

Add a site in the problematic area; adopt one cell of the new

site into the dominant server.


Signal distribution in Donghu Road after the optimization

Case7(Antenna problem)


Case7

Problem analysis: Through the review of the DT data with optimization analysis

software ZXPOS CNA1 and the survey on the site, it is found that in front of Sector 2 (with the scramble 437) of the Shuqian Road base station, there are dense buildings which form a serious barrier and influences in the coverage of the sector. Besides, the areas within scores of meters in front of Sector 1 (with the scramble 439) of Donghu base station is also completely blocked by a row of high residential buildings, which makes Sector 1 unable to cover that area.

Solution Change the direction angle of Sector 2 in the Shuqian Road base

station from 240o to 230o to enhance the coverage of that area of Donghu Road.


Case7

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

Case7


Case7

Problem analysis: 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

DT test picture. 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

Case7

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.


Excellent and comprehensive Test Result with CMII, CMCC, CNC, CTC, CRC; Rich Planning & Optimization experience is acquired and prepared for the coming large-scale commercial application of 3G.

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WCDMA optimization case study

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