2G-3G_Interference Analysis Caused by RTWP

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 1.2.2 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 2

Check the configuration of diversity reception if you fail to observe the diversity signals. 2.1.1 shows the RTWP variation when the diversity reception is not configured. Figure 1.1 RTWP variation when the diversity reception is not configured

Step 3

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～-107dBm ( Tolerance ), 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 4

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 5

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 6

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 7

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 8

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 9

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 10

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 11

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 12

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.

2. 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

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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

3. 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. 

4. 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. 3.1.1 shows the variation of RTWP. Figure 1.1 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 3.1.2. Figure 1.2 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. 3.1.3 shows the antenna-feeder structure.

Figure 1.3 Antenna-feeder structure

3.1.3 shows the variation of RTWP due to improper connection of feeder and jumper. Figure 1.4 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. 3.1.4 shows the variation of RTWP due to interaction of 2G and 3G signals.

Figure 1.5 Variation of RTWP due to interaction of 2G and 3G signals

In 3.1.4, 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 3.2.1 shows the site distribution around the site 501800.

Figure 1.6 Site distribution around the site 12PKP012

In the network as shown in 3.2.1, a 3G NodeB 12PKP012 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 1.7 Variation of RTWP in adjacent cells (1)

Figure 1.8 Variation of RTWP in adjacent cells (2)

Figure 1.9 Variation of RTWP in adjacent cells (3)

Figure 1.10 Variation of RTWP in adjacent cells (4)

Figure 1.11 Variation of RTWP in adjacent cells (5)

Figure 1.12 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. 3.2.2 shows the RTWP variation of cell 54291.

Figure 1.13 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 Selfexcitement The gain of the repeater is 90 dB. 3.2.3 shows the RTWP variation of cell 45680. Figure 1.14 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 3.2.4, the 3G antenna is a diversity antenna and the main antenna is far from 2G antenna. Figure 1.15 Antenna location

Figure 1.16 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. 3.2.5 shows the site location near the railway.

Figure 1.17 Site location

Figure 1.18 RTWP variation of a NodeB near railway

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

Figure 1.19 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 3.2.7 shows the RTWP variation due to power on or off of outdoor air-conditioner of other operator. Figure 1.20 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 3.2.8 shows the RTWP variation due to power on or off of indoor emergency lights, marked in red.

Figure 1.21 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. 3.2.9 shows the long-time RTWP variation. Figure 1.22 Long-time RTWP variation

3.2.9 shows the short-time RTWP variation.

Figure 1.23 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 3.2.10 shows the interference caused by the spectrum analyzer YBT250 at 1924.3 MHz when the directional antenna approaches the YBT250. Figure 1.24 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 3.2.11 and 3.2.11 show the uplink interference due to transmission line.

Figure 1.25 Uplink interference due to transmission line (1)

Figure 1.26 Uplink interference due to transmission line (2)

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

Figure 1.27 Long-time RTWP variation of the interference like self-excitation

3.2.12 shows the short-time RTWP variation of the interference like self-excitation. Figure 1.28 Short-time RTWP variation of the interference like self-excitation

3.2.12 shows the frequency spectrum feature. Figure 1.29 Frequency spectrum feature

3.2.12 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 4.1.1. The bandpass filter allows the signals within the useful frequency band to pass and suppresses interference signals beyond the useful frequency band. 4.1.1 shows a bandpass filter. Generally, the two ends of the bandpass filter can serve as an input or output end. Figure 1.30 Bandpass filter

Table 30.1 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 30.2 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 4.1.3) made by Shenglu Antenna Co., Ltd. is used. For the major technical specifications, see 4.1.3.

Figure 1.31 Log periodic antenna SL14088A

Table 31.1 Major technical specifications of a log periodic antenna Major Technical Specifications Model

SL14088A

Frequency Range

806 MHz to 960 MHz, 1710 MHz to 2500 MHz

VSWR

< 1.5

Input Impedance

50 Ω

Gain

11 dBi

Front-to-Back Ratio

12 dB

Horizontal Lobe Width

50°±10°

Vertical Lobe Width

40°

Polarization Mode

Vertical or horizontal

Maximum Power

50 W

Grounding Mode

DC grounding

Input Interface

N female connector

If no log periodic antenna is available, a Yagi antenna can be used. 4.1.3 lists the major technical specifications of a Yagi antenna manufactured by Shenglu Antenna Co., Ltd. Table 31.2 Major technical specifications of a Yagi antenna Major Technical Specifications Model

TDJ-1800/2000B14G13

Frequency Range

1710 MHz to 2150 MHz

VSWR