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3G WCDMA interference analysis guide

Interference Analysis Guide
Key words
WCDMA, interference, main, diversity, RTWP, inter-modulation

Abstract
The document describes how to locate and solve interference issue in WCDMA network optimization.

Acronyms and abbreviations:
Acronyms and abbreviations

Full Spelling

PIM

Passive Interactive modulation

RTWP

Received Total Wideband Power

BCCH


Broadcasting Channel

FNE

Fixed Network Element

AOA

Angle of Arrival

1.1 Judging Types of Interferences
1.1.1 Interference Types
The interference includes internal interference and external interference.
The interference occurring on NodeB to the antenna-feeder system is internal
interference.
The external interference includes in-band signal interference and out-band strong signal
interference. The typical types are Personal Handy phone System (PHS) interference,
repeater interference, and interference from handset interferer.

1.1.2 Criteria
The interference belongs to external interference if it meets the following judgment criteria:
The interference to main or diversity is relevant. Namely, in terms of time, the
interference to main or diversity trends similarly, and the difference between them is
within 5 dB.
The external interference affects multiple cells that are geographically bordering.
In terms of time feature of RTWP, the external interference is mutational, the
interference occurs at a regular point and in a regular period, and lasts for a regular
period (exceptions are microwave interference, improperly configured gain of repeaters,
so the RTWP is not mutational)
The interference which is not external interference is internal interference, so it follows the internal interference
processing procedures. Locating external interference takes more effort and time than locating internal
interference. Therefore, if the interference is not confirmed to be internal interference, it must be rechecked.
The inter-modulation interference which takes a high ratio in internal interference features typically as below:
The RTWP of main and diversity is usually irrelevant. If the RTWP is relevant, there
must be special causes, such as the main and diversity are combined at some point.
The interference is related to traffic. The interference occurs less probably when traffic is
lower.
The RTWP fluctuates sharply, as great as about 10 dB, or even greater than 10 dB.



The interference will last for a period, without mutational change, which is different
from that of external interference.
In terms of time feature of RTWP, the RTWP changes irregularly.
The inter-modulation usually meets one or more of the previous five features. If the five features are all met, it
must be inter-modulation.
For better understanding of the previous judgment criteria, the following examples provide direct phenomena of
various interference from actual networks. Therefore no specific locating process is provided.

1.2 Equipment and Documents Needed In Interference
Test
1.2.1 Sturcture of Interference Test
The interference test uses the following structure of test equipment.
Figure 1-1 Structure of interference test

Directional Antenna

1.2.2 Equipment and Documents
Table 1-1 lists the equipment and documents used in interference test.
Table 1-1 Equipment and Documents
Equipment or document

Type of connector

Directional antenna

N-type female connector,
For more info, see 4.1.3

Omnidirectional small antenna

SMA-type female connector,
For more info, see 4.1.3


Equipment or document

Type of connector

Bandpass filter

N-type female connector,
For more info, see 4.1.1

YBT250 spectrum analyzer

N-type female connector

1/2 jumper x3

N-type male connector

1/2 jumper x2

N-type male connector/SMA male
connector

50 Ohm Dummy Load x2

N-type male connector

DIN-type male connector -> N-type
female connector x2
DIN-type female connector -> N-type
female connector x2
N-type dual-female connector x2
Laptop (installed with NodeB LMT
software)
GPS
Compass
Test car
FNE map of sites
Historic RTWP map of sites
Distribution map of adjacent sites
Camera
PHS handset (if to locate PHS
interference)

Preparation for different connectors is a little complex and patient work. It’s better try each connection
before go to test. Otherwise, you have to waste your time on spot.


2

Locating Interference

2.1 Locating Internal Interference
Locating internal interference includes initial location and on-site location.

2.1.1 Initial Location
The initial location proceeds as below:
Step 1 Check the configuration of diversity reception if you fail to observe the diversity signals.
Figure 2-1 shows the RTWP variation when the diversity reception is not configured.
Figure 2-1 RTWP variation when the diversity reception is not configured

Step 2 If the uplink RF channel has not been adjusted, check whether the configured gain (especially
TMAs are used) of RF channel is correct. It is better to adjust uplink RF channel gain so that
these problems will not bother locating interference. To check UL RF channel gain , the
following steps has to be taken:
Install dummy load to the port to be checked;
Start NodeB LMT, Input MML command: SET TXSW: TXSW = OFF to switch off
related PA in case of unexpected damage;


The RTWP normal value range is -105~-106dBm, Check whether measured RTWP
value is in this range or not; If not, set attenuation of this channel to adjust RTWP to
normal value. MML command: SET RXATTEN: ATTEN=***;

Uplink RF channel gain adjustment is not routine operation, which should be performed by RAN
engineer when RAN engineer is available.
Step 3 If a DCS1800M network and a WCDMA network are combined, you must check the
frequency configuration with operators. Meanwhile you must check whether the third order
intermodulation (2f1-f2 and 2f2-f1) of the combined DCS1800M frequency is within the RX
band (1920 MHz to 1980 MHz). If yes, negotiate with operators to change the improper
frequency configuration.
If the interference remains after the previous operations, you must locate interference on site.

2.1.2 On-site Location
The on-site location proceeds as below:
Step 1 Start NodeB LMT and measure the real-time RTWP of the cell to be located. This allows you
to observe real-time RTWP variation after using consequent locating methods.
Step 2 If a DCS network is combined to a WCDMA network, you must know the DCS carrier
features (the carriers on a channel, the channel number, and the channel where BCCH is) and
mark the BCCH channel.
Step 3 If a DCS network is combined to a WCDMA network, you need adjust BCCH to the channel
where interference is located under assistance by the operator according to the result of
interference. The reason is that if BCCH does not use the problematic channel (The GSM
network might transmit signals in both channel, but the BCCH uses only one channel), it is
very hard to locate DCS-traffic-related interference.
Step 4 Knock every RF connector gently on the channel (especially the connectors of jumper, load,
and antenna) and check the RTWP variation. If RTWP changes, the connector is problematic.
Tasks to improve project quality, such as fastening connectors and reconnections, must be
perform under cooperation of the operators' engineers. Ensure to power off power amplifiers
of corresponding cells before performing tasks to avoid radiation injury.
Step 5 When the connector are normal and interference is present, use YBT250, filter, and
directional antenna to check at WCDMA antenna whether interference signals are received
(for requirements on filter and directional antenna, see Appendix 4.1 In special situations, you
must customize the filter according to the local WCDMA receiver band and other radio
network transmission frequency band). If YBT cannot detect special interference, you need
change the NodeB antenna and check whether the interference is caused inside the antenna. If
the interference still exists after changing antennas, turn to judgment of interference types.
Step 6 If interference signals are receives at the WCDMA antenna by using YBT250, filter, and
directional antenna, you can solve the problem by Locating External Interference.
Step 7 If the interference cannot be located after repeated checks, solve it by Judging Types of
Interferences. Stop on-site location and restore the original configurations.
Step 8 Record the previous locating steps in the form of "xx Interference Location Detailed Record".


If successful in locating the interference, you can summarize the problem in the form of
interference location cases based on "xx Interference Location Detailed Record". Send
the cases to the Headquarter for filing.
If failing in locating the interference, you can send the "xx Interference Location
Detailed Record" to technical support engineers in the Headquarter for help.

2.2 Locating External Interference
2.2.1 Preparations before On-site Location
It is hard to know when the external interference appears or disappears, so detailed preparations and analysis must
be performed before on-site location. Otherwise, the on-site location will be less efficient.

1. Needed Data
You need the following data:
The RTWP data for 7 (days, at least 3 days) x 24 (hours) of cells to be located
The data is obtainable in "Collecting Data and Confirming Interference" section.
The MapInfo map of site distribution, the relative location of sites, and the distance
between sites
You can use Nastar to obtain these information.
Antenna azimuth and height of cells
Photos for surveying sites
Whether the cell to be located is the host cell of a repeater
The distribution of 2G and 3G repeaters around the cell to be located
The distribution of PHS BTSs around the cell to be located
The antenna-feeder structure diagram of the cell to be located

2. Needed Analysis and Initial Conclusion
Analysis: the long-time feature and short-time feature of RTWP data for the cell to be located in different periods
Conclusion: the locating time (the periods when interference occurs intensively is obtainable according to RTWP
time feature.
Analyze the following aspects:
Analyze the long-time feature and short-time feature of RTWP data for the cell to be
located in different periods
Analyze the environment of the cell to be located with cell distribution diagram and
surveying photo
Analyze the relativity of main and diversity of the cell to be located according to the
antenna-feeder structure diagram
Use angle of arrival (AOA) to summarize the RTWP data of the cell to be located, the
RTWP data of adjacent cells, antenna azimuth, and antenna height so that the location of
the interference source can be estimated.
Locate the direction of the interference source by cell antennas of multiple NodeBs. Draw on a map, the crossing
point of the direction of each antenna is the interference source.
Conclusion: where to locate.


Figure 2-2 Locating interference source by using AOA

3. Methods and Procedures for On-site Location
On-site location proceeds as below:
Step 1 Start NodeB LMT and monitor realtime RTWP of the cell to be located for the features and
time when the external interference occurs.
Step 2 Check the environment of the antenna for metal blockings, antenna of other networks or
systems, the antenna distribution of other operators. Check the potential adjacent blockings to
signals.
Step 3 Measure the interference strength, direction, and frequency spectrum by using YBT250, filter,
and antenna.
Step 4 Find the rough location of the interference source. For more interference-locating methods
and experience, see chapter Method and experience in external interference location.
Step 5 Fix the potential interference source according to the previous analysis.
Step 6 Verify the relationship between the interference and the state variation of the potential
interference source (such as on, off, starting, and stopping)
For the equipment that is controlled by the operator, such as repeaters, you can verify the
relations between the equipment and the interference by powering on or off the equipment in
a proper time. For the uncontrollable equipment, you need to wait to observe the interference.
Step 7 Record the previous locating steps in the form of "xx Interference Location Detailed Record".
If successful in locating the interference, you can summarize the problem in the form of
interference location cases based on "xx Interference Location Detailed Record". Send
the cases to the Headquarter for filing.
If failing in locating the interference, you can send the "xx Interference Location
Detailed Record" to technical support engineers in the Headquarter for help.


3

Case study

3.1 Cases of Internal Interference
3.1.1 Multi-frequency Intermodulation Due to Load
In an indoor distributed system, the 3G signals, 2G signals of the operator S, and 2G signals of the operator P are
combined. The operator P uses the absolute radio frequency channel number (ARFCN) 747. The operator S uses
the ARFCN 850 and hopping frequency ARFCN 815.
Figure 3-1 shows the variation of RTWP.
Figure 3-1 Variation of RTWP due to load

The interference in the cell is caused by a load with loose connection. Once the load is touched, the RTWP
changes sharply.
The RTWP changes as below:
The main and diversity are irrelevant
The RTWP fluctuates sharply
The interference lasts for a period
The RTWP changes irregularly in terms of time


3.1.2 Multi-frequency Intermodulation Due to Improper
Connection of Multiple RF
The multiple RF connection involves duplexer, feeder, and jumper connector.
The site is constructed with indoor distribution system shared by multiple operators. The antenna-feeder structure
is complex. Wherein, multiple hybrid couplers, feeders, and jumpers are improperly connected, so the RTWP is as
shown in Figure 3-2.
Figure 3-2 Variation of RTWP due to improper connection of multiple RF

The RTWP changes as below:
The RTWP fluctuates sharply
The interference lasts for a period
The RTWP changes irregularly in terms of time

3.1.3 Single Frequency Intermodulation Due to Improper
Connection of Feeder and Jumper
The 3G signals and 2G signals are combined. The 2G network uses only one channel number. Intermodulation
occurs due to improper connection of feeder and jumper.
Figure 3-3 shows the antenna-feeder structure.


Figure 3-3 Antenna-feeder structure

Figure 3-4 shows the variation of RTWP due to improper connection of feeder and jumper.
Figure 3-4 Variation of RTWP

The RTWP changes as below:
The main and diversity are irrelevant
The RTWP fluctuates sharply
The interference lasts for a period
The RTWP changes irregularly in terms of time

3.1.4 Multi-frequency Intermodulation Due to Interaction of 2G
and 3G signals
This is an indoor site, with 2G and 3G signals combined. It is an indoor distributed system shared with other
operators.
Figure 3-5 shows the variation of RTWP due to interaction of 2G and 3G signals.


Figure 3-5 Variation of RTWP due to interaction of 2G and 3G signals

In Figure 3-5, the main interference (in red) is caused by intermodulation of DCS signals and 3G signals at a
connector.
The diversity is not connected to antenna. The external signals near cabinet interferes diversity.

The RTWP changes as below:
The main and diversity are irrelevant
The RTWP fluctuates sharply
The interference lasts for a period
The RTWP changes irregularly in terms of time

3.2 Cases of External Interference
3.2.1 Sites Around Repeaters of Self-excitation Interference
Figure 3-6 shows the site distribution around the site 501800.


Figure 3-6 Site distribution around the site 501800

In the network as shown in Figure 3-6, a 3G repeater close to the NodeB 501800 transmits a self-excitation signal
every hour approximately. Therefore the uplink in multiple cells is interfered. The uplink interference varies
according to the direction and the distance between the cell and the repeater. However, it is clear that the uplink
interference occurs every hour approximately.
Figure 3-7 Variation of RTWP in adjacent cells (1)


Figure 3-8 Variation of RTWP in adjacent cells (2)

Figure 3-9 Variation of RTWP in adjacent cells (3)

Figure 3-10 Variation of RTWP in adjacent cells (4)


Figure 3-11 Variation of RTWP in adjacent cells (5)

Figure 3-12 Variation of RTWP in adjacent cells (6)

The site 501800 is an indoor site with a single antenna.

The RTWP changes as below:
The main and diversity are relevant
The interference influences multiple cells that are close to each other
The interference is mutational
The interference changes with a regular internal

3.2.2 Uplink Interference to Host Cell Due to Repeater
Self-excitation
The NodeB 45680 uses a 3G repeater. The host cell of the repeater is the first cell 54291 of the NodeB 45680. The
occurrence time of self-excitation of the repeater is irregular.
Figure 3-13 shows the RTWP variation of cell 54291.


Figure 3-13 Variation of RTWP

The RTWP changes as below:
The main and diversity are relevant
The interference is mutational

3.2.3 Uplink Interference to Host Cell Due to Improperly
Configured Gain of Repeater and Self-excitement
The gain of the repeater is 90 dB. Figure 3-14 shows the RTWP variation of cell 45680.
Figure 3-14 RTWP variation of cell 45680

After adjustment of the repeater gain to 70 dB, the RTWP becomes normal.
The RTWP variations feature the same as that of improperly configured gain of repeater. Namely, the interference
is strong and stable.

3.2.4 Uplink Interference to 3G Antenna Due to Close Radiation
from 2G Repeater Antenna
The 3G antenna is interfered by a 2G repeater antenna another operator. The 3G antenna uses space diversity. As


shown in Figure 3-15, the 3G antenna is a diversity antenna and the main antenna is far from 2G antenna.
Figure 3-15 Antenna location

Figure 3-16 RTWP variation

The RTWP changes as below:
The main and diversity are relevant
The interference is mutational

3.2.5 RTWP Variation Due to Passing Trains
The NodeB is close to the railway with intensive trains passing by. Figure 3-17 shows the site location near the
railway.


Figure 3-17 Site location

Figure 3-18 RTWP variation of a NodeB near railway

3.2.6 Uplink Interference Due to State Switch of Indoor
Air-conditioner Controller
Figure 3-19 shows the uplink interference fluctuation upon state switch of indoor air-conditioner controller.


Figure 3-19 RTWP variation due to indoor air-conditioner

3.2.7 Uplink Interference Due to Power On or Off of Outdoor
Air-conditioner of Other Operator
Figure 3-20 shows the RTWP variation due to power on or off of outdoor air-conditioner of other operator.
Figure 3-20 RTWP variation due to power on or off of outdoor air-conditioner of other operator

3.2.8 Uplink Interference Due to Power On or Off of Indoor
Emergency Lights
Figure 3-21 shows the RTWP variation due to power on or off of indoor emergency lights, marked in red.


Figure 3-21 RTWP variation due to power on or off of indoor emergency lights

3.2.9 Uplink Interference with Period of 200 Seconds
This uplink interference is probably due to air-conditioner compressor, but this cannot be confirmed due to
property restriction.
Figure 3-22 shows the long-time RTWP variation.
Figure 3-22 Long-time RTWP variation

Figure 3-23 shows the short-time RTWP variation.


Figure 3-23 Short-time RTWP variation

The RTWP changes as below:
The main and diversity are relevant
The interference is mutational
The interference changes with a regular internal

3.2.10 Interference Caused by the Spectrum Analyzer YBT250 at
1924.3 MHz
Figure 3-24 shows the interference caused by the spectrum analyzer YBT250 at 1924.3 MHz when the directional
antenna approaches the YBT250.
Figure 3-24 Frequency spectrum when the directional antenna approaches the YBT250

When locating interference, pay attention to the feature of YBT250.

3.2.11 Uplink Interference Due to Transmission Line
Figure 3-25 and Figure 3-26 show the uplink interference due to transmission line.


Figure 3-25 Uplink interference due to transmission line (1)

Figure 3-26 Uplink interference due to transmission line (2)

3.2.12 Interference Like Self-excitation
Figure 3-27 shows the long-time RTWP variation of the interference like self-excitation.


Figure 3-27 Long-time RTWP variation of the interference like self-excitation

Figure 3-28 shows the short-time RTWP variation of the interference like self-excitation.
Figure 3-28 Short-time RTWP variation of the interference like self-excitation

Figure 3-29 shows the frequency spectrum feature.
Figure 3-29 Frequency spectrum feature

Figure 3-29 is a static figure. Its frequency spectrum features as below:


The scanned frequency ranges from 1914 MHz to 1951 MHz
The amplitude of different frequencies is different
The frequency jumps after a scanning period
The RTWP changes as below:
The main and diversity are relevant
The interference is mutational.


4

Appendix

4.1 Key Device Parameters
4.1.1 Filter
The bandwidth of the bandpass filters used for uplink/downlink electromagnetic interference tests is 60 MHz. For
the specifications, see Table 4-1. The bandpass filter allows the signals within the useful frequency band to pass
and suppresses interference signals beyond the useful frequency band. Figure 4-1 shows a bandpass filter.
Generally, the two ends of the bandpass filter can serve as an input or output end.
Figure 4-1 Bandpass filter

Table 4-1 Specifications for uplink and downlink bandpass filters
Operating Frequency
(MHz)

1920 to 1980

2110 to 2170

Insertion Loss

0.7 dB at 1920 MHz

0.9 dB at 2110 MHz

0.4 dB at 1950 MHz

0.3 dB at 2140 MHz

0.7 dB at 1980 MHz

0.7 dB at 2170 MHz

–3-dB Bandwidth (MHz)

1912–1987

2106–2177

Out-of-band suppression

≥ 70 dB (1900–1999 MHz)

≥ 70 dB (2090–2192 MHz)


Operating Frequency
(MHz)

1920 to 1980

2110 to 2170

VSWR

< 1.15

< 1.15

Group Delay (ns)

30–50

30–50

4.1.2 Low-Noise Amplifier
Although a low-noise amplifier (LNA) is built in the YBT250, the noise figure is so high that it is difficult for the
receiver sensitivity to meet the requirement. Therefore, an external LNA is required to improve the receiver
sensitivity of the tester. For a cascade network, with a high-gain amplifier installed at the front end of the system,
the noise figure of the system depends on the noise figure of the first LNA. Therefore, a ZRL-2400LN low-noise
amplifier of Mini-Circuits is selected. The gain is 25 dB and the noise figure is 1.2 dB. However, an extra power
supply is required for the LNA.
Table 4-2 Specifications for the LNA

4.1.3 Antenna
An omni-directional antenna can be used for electromagnetic interference measurement, but it is not favorable for
locating interference sources. It is recommended that a directional antenna be used to locate interference sources.
The higher the antenna directivity is, the higher the gain and the search capability are. Common directional
antennas include plate antennas, Yagi antennas, and log periodic antennas. For convenience, only a directional
antenna is used for tests in different directions. A Yagi antenna is used for narrowband and high-gain short-wave
communication. A Yagi antenna is simple, light but solid, and convenient to feed, however, the frequency band of
such an antenna is narrow and the resistance to interference is low. A log periodic antenna is a broadband antenna
or a frequency-independent antenna. Compared with other high-gain antennas, a log periodic antenna has a higher
directivity and a larger attenuation to signals in unexpected directions. Therefore, a log periodic antenna is
preferred in interference tests. A log periodic antenna (see Figure 4-2) made by Shenglu Antenna Co., Ltd. is used.
For the major technical specifications, see Table 4-3.


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