Guide to GSM Interference Analysis

Ｒａｄｉｏ　Ｎｅｔｗｏｒｋ　Ｐｌａｎｎｉｎｇ　

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Ｄｅｐｔ．　ｏｆ　Ｈｕａｗｅｉ　

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Total pages: 17

Guide to GSM Interference Analysis (V1.0) (For internal use only)

Drafted by:

Special Subject Study Team

Date:

2002-04-16

Checked by:

Date:

yyyy/mm/dd

Checked by:

Date:

yyyy/mm/dd

Approve d by:

Date:

yyyy/mm/dd

Ｈｕａｗｅｉ　Ｔｅｃｈｎｏｌｏｇｉｅｓ　Ｃｏ．，　Ｌｔｄ．　Ａｌｌ　ｒｉｇｈｔｓ　ｒｅｓｅｒｖｅｄ

Guide to GSM Interference Analysis (V1.0) Inner disclosure

Revision RecordDat e

Revision version

Description

Author

2002/04/17

1.00

Initial draft completed

He Qun

2003-09-07

All rights reserved Page 2 of 17


Table of Contents 1

Overview............................................................................................................................... 5 1.1

2

3

Interference Source ................................................................................................................ 6 2.1

Classification ........ ........ ......... ......... ........ ......... ........ ......... ........ ......... ........ ......... ......... 6

2.2

Interference Sources Impacting on Mobile Communication... .... .... .... ...................... .... .... .. 8

Discovery of Interference........... ........ ......... ......... ........ ......... ........ ......... ........ ......... ........ ........ 9 3.1

2

Impact on Network ...................................................................................................... 5

Finding Interference through OMC Traffic Measurement..... ........ .... .... .... ............... .... .... 10 3.1.1

Find Potential Interference through Traffic Conditions..... ............... .... .... .... ............. 10

3.1.2

Find Potential Interference through Handover Data............... .... .... .... ............... .... .... 10

3.1.3

Find Potential Interference through Call Drop Items .... .... ............... .... .... .... ............. 11

3.1.4

Find Potential Interference through Interference Band Items ........... .... .... .... ............. 11

1.1

OMC Alarm and Subscriber Complaint ......... ......... ........ ......... ........ ......... ........ ......... .... 12

1.2

Drive Test................................................................................................................. 12

Interference Location and Elimination............ ......... ........ ......... ........ ......... ........ ......... ........ ...... 13 2.1

Procedures ................................................................................................................ 13 2.1.1 Confirm Interfered Cell through KPI............. ......... ........ ......... ........ ......... ........ ...... 13 2.1.2 Check OMC Alarm. ......... ......... ........ ......... ........ ......... ........ ......... ........ ......... ....... 13 2.1.3 Check Frequency Planning ......... ......... ........ ......... ........ ......... ........ ......... ........ ...... 14 2.1.4 Check Cell Parameter Settings ........ ......... ........ ......... ......... ........ ......... ........ ......... . 14 2.1.5 Drive Test .......................................................................................................... 14 2.1.6 Interference Elimination ......... ........ ......... ........ ......... ......... ........ ......... ........ ......... . 14

3

2

Tools for Interference Test......... ........ ......... ......... ........ ......... ........ ......... ........ ......... ........ ...... 15 3.1

Introduction to Spectrum Analyzer. ........ ......... ........ ......... ......... ........ ......... ........ ......... . 15

1.1

Directional Antenna ......... ........ ......... ......... ........ ......... ........ ......... ........ ......... ........ ...... 16

Test Methods for Interference........ ........ ......... ........ ......... ......... ........ ......... ........ ......... ........ ... 16 2.1

Testing Internal Interference........ ......... ........ ......... ......... ........ ......... ........ ......... ........ ... 16

2.2

Testing External Interference......... ........ ......... ........ ......... ......... ........ ......... ........ ......... . 16

2.3

Searching External Interference Source.. ......... ........ ......... ......... ........ ......... ........ ......... . 17

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Guide to GSM Interference Analysis Keywords: GSM, interference, interference band, BaseStation (BTS), order 3 intermodulation Abstract: This document comprehensively describes interference sources, locations and solutions and

presents detailed analysis , depending on previous experiences. List of abbreviations:

This list describes the abbreviations used in the document and provides the

meaning of each abbreviation. List of references: Please fill the following table with the names, authors, titles, Nos., issue dates and

presses of the references quoted by the document.

List of references Ｎａｍｅ

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Ｎｏ．

Ｉｓｓｕｅ　ｄａｔｅ

Ｗａｙ　ｔｏ　ｌｏｏｋ　ｕｐ

Ｐｒｅｓｓ　（ｉｆ　ｔｈｅ　ｒｅｆｅｒｅｎｃｅ　ｉｓ　ｎｏｔ　ｉｓｓｕｅｄ　ｂｙ　Ｈｕａｗｅｉ，　ｐｌｅａｓｅ　ｆｉｌｌ　ｔｈｉｓ　ｉｔｅｍ）

Interference

Starwiarski

1

GSM Principle and Network Optimization Mobile Communication Engineering Analysis of BTS Interference Case

Han Binjie

2

China Machine Press

Lu Er'rui etc.

3

Post & Telecom Press

Fang Chao

4

Multiple

5

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2001.1 Www.support.com


1. Overview Frequency resource is quite rare. In the GSM system, frequency multiplexing is required to improve the system capacity. Frequency multiplexing refers to that the same frequency is shared by several cells which are far apart from each other. The distance between the cells sharing the same frequency is called multiplexing distance. The ratio of multiplexing distance to cell radius is called co-frequency interference factor. For a frequency resource, the more closely the frequency multiplexing is adopted, the higher the network capacity shall be. The shorter the multiplexing distance, the greater the interference. The frequency multiplexing above will cause intra-network interference (also called intra-system interference). Besides, the GSM system might suffer the interference from other communication systems. Interference is the key factor impacting the quality of network since it greatly impacts quality of conversion, call drop ratio, handover and congestion ratio etc. How to suppress even eliminate interference is the primary task for network planning and optimization. This document summarizes the previous experiences and comprehensively describes interference sources, locations and solutions.

1.1 Impact on Network If interference exists in the network, the MS subscriber will feel the following phenomena. ²

The MS subscriber in conversation cannot hear the voice and the background noise is quite

high. ²

When a fixed phone calls a MS or a MS calls another MS, call drop often occurs after beep. .

²

Conversation is intermittent, often accompanied by call drop.

If interference exists in a cell, the following phenomena may happen in traffic measurement. ² Level 4~5 interference band appears, with statistic value greater than 1. ² High congestion ratio (As SDCCH is interfered, immediate assignment or TCH assignment failure

shall occur). ² Call drop rate far higher than other cells. ² Low handover success rate.

After drive test, the following problems shall be found out. ² Difficult handover.

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² High level and low quality.

By using a signaling analyzer (MA10/K1205) to trace Abis interface signaling, it shall be found that: ² The BER in this cell is higher than those in other cells.

2. Interference Source 1.1 Classification The interference sources/noises in the mobile communication system are classified into: 1. Natural noise Ÿ

Atmospheric noise

Ÿ

Galactic noise

Ÿ

Solar noise (silent period)

1. manual interference Ÿ

Ignition interference of motor or other engines

Ÿ

Communication electronic interference

Ÿ

Electrical line interference

Ÿ

Interference of electrical equipment and appliances used for industry, scientific research, medical

and household purposes. The following figure illustrates the research data of interference/noises above from ITT.

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Ｆｉｇｕｒｅ１　Ａｍｂｉｅｎｔ　ｎｏｉｓｅ　

In the figure above, Ta is noise temperature and Fa is an equivalent noise figure. The relationship between them is

Fa

=

10 log

Ta To

In which, To = 290 癒 . From the research data it can be seen that in 30 MHz ~ 100 MHz, the atmospheric noise and solar noise are very low and can be ignored. At 100 MHz or higher, the cosmic noise of Milky Way galaxy is lower than the typical receiver thermal noise, so it can also be ignored. Therefore, for 450 MHz, 800 MHz, 900 MHz, 1800 MHz and 2000 MHz mobile communication systems, natural noises (atmospheric noise, galactic noise and solar noise) can be disregarded

【

3】

.

The impact of the solar macular noise at activity peak hours on mobile communication is not understood at present, but most of scientists believe such noise can badly impact on electrical and communication systems. According to the research outcome of National Bureau of Standards (NBS), it can be known that human oise is one of the interference sources impacting the mobile communication system. In these interference/noise sources, some are uncontrollable, e.g. ignition interference of motor engine, electric interference, industrial electric equipment interference, etc. While some can be suppressed by means of

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reasonable network planning and system optimization, e.g. interference between communication devices and that in the communication equipment. This document will focus on the latter.

1.2 Interference Sources Impacting on Mobile Communication In the mobile communication system, a BTS may be interfered not only by other communication devices around but also by another BTS or MS in the same system when receiving signals from a MS far away from it

【

3】

, as shown in the following figure.

Ｆｉｒｇｕｒｅ２　Ｉｎｔｅｒｆｅｒｅｎｃｅ　ｔｏ　ｔｈｅ　ｍｏｂｉｌｅ　ｃｏｍｍｕｎｉｃａｔｉｏｎ　ｓｙｓｔｅｍ　　

The interference sources include: Hardware fault: ² TRX fault: If the performance of a TRX degrades due to poor manufacturing technique or long-

time usage, the amplification circuit of the TRX might be self-excited, thus to generate interference. ² CDU or divider fault: Active amplifiers are used in the divider and divider module of CDU. Once

these amplifiers get faulty, self-excitation shall occur. ² Spurious signal and intermodulation: If the out-band spurious signal of TRX or power amplifier

(PA) exceeds the specified range, or the receive-transmit isolation of the duplexer in CDU is too low, interference shall be generated to impact the receiving channel. Intermodulation may occur to some passive devices, e.g. antenna, feeder, etc. Intra-network interference:

Improper frequency planning may cause:

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² Co-channel interference ² Adjacent-channel interference

Repeater interference:

Repeaters are often used in the early network construction to effectively expand coverage of BTS. Improper application of repeaters may easily interfere BTS. The repeater interferes BTS in two ways: ² If a repeater is installed improperly and the isolation between the donor antenna and service

antenna does not meet the requirement, self-excitation shall occur, thus to impact the normal operation of the BTS to which the repeater is attached. ² For the repeater with broadband non-linear amplifier, its intermodulation requirement is far higher

than that specified in the protocol. If the power is quite high, the strength of the harmonic signal will be great, which may cause interference to the BTSs around. Interference caused by other communication devices with high power : ² Radar station: The frequency of some decimeter radars designed in the 70s or 80s of the 20th

century is the same as or approximate to that of the GSM system. The high transmitting power ranging from decades of kilowatts to hundreds of kilowatts and high out-band spurious signals of these radars tend to interfere the BTSs around. ² Analog BTS: The band of the analog mobile BTS partly overlaps that of the GSM system.

According to the requirement, the analog BTS should not use the frequency same as GSM. However, some analog BTSs do not meet the requirement in fact. When GSM use the same frequency as an analog BTS, it will be interfered by this BTS. (Now the bands of all the analog BTSs in China do not overlap that of GSM, while in overseas field, whether the requirement is met is unknown). ² Other communication devices of the same frequency: Some kinds of communication devices

adopt the bands not complying with the present communication standards and occupy the band of GSM, which may cause interference to the coverage of GSM.

3. Discovery of Interference To improve quality of conversation, the engineer should find the interference first, then locate it adopting proper method and finally suppress or eliminate interference. The methods used to find interference sources for the GSM system include: OMC traffic measurement, OMC alarm, drive test, subscriber complaint. The signaling analyzer and spectrum analyzer

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are often used for interference location ,considering these devices are heavy and expensive. usually we don't use them at the beginning.

1.1 Finding Interference through OMC Traffic Measurement When a network begins to operate, such traffic measurement tasks as TCH performance measurement, SDCCH performance measurement and handover performance measurement should be registered to find faults in time. By analyzing cell traffic conditions, handover and traffic measurement items related to quality of cells, the cells where potential interference exists can be found out. It should be noted that these methods can help to judge whether there is suspect interference. To further confirm whether it is real interference, and interference location is required. 1.1.1 Find Potential Interference through Traffic Conditions

Check the item Mean TCH Busy Time in TCH Performance Measurement of each cell. This item is used to measure the mean TCH holding time (unit: second) in a specified period. It is usually called TCH Mean Holding Time in the BSCs from other suppliers. If the mean TCH holding time of a cell is quite short, such as less than 10s it indicates there might be strong interference in this cell. The interference may result in the condition that handover/call drop occurs to a MS due to poor communication quality as soon as the MS seizes TCH. Or, it indicates a TRX not containing BCCH and SDCCH in this cell gets faulty. 1.1.2 Find Potential Interference through Handover Data

Handover data reflects the mobility of subscribers in the cell measured and can be classified into intra-cell handover and inter-cell handover. ² Inter-cell handover

There are multiple reasons resulting in handover occurring to a MS. The handover measurement items that can be used to judge whether there is interference include Number of Handover Attempts (downlink quality), Number of Handover Attempts (uplink quality), Number of Level 0~7 Signals Received in Handover Origination (uplink) and Mean Receive Quality Level in Handover Origination (uplink). If in a cell the mean receive quality (uplink) is greater than or equal to 4 (in case of no frequency hopping) or 5 (in case of frequency hopping), and the mean receive level during handover origination is greater than or equal to 25, uplink interference might occur to this cell. If level 5 signals received is more than level 4 ones when a cell originating a handover, uplink interference might exist in this cell.

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If the number of handover attempts (uplink quality and downlink quality) originated by a cell is greater than 10 percent of the total number of handover attempts, interference might exist in this cell. These two measurement items are related to the parameters "poor quality handover threshold" and "interference handover threshold". ² Intra-cell handover

The measurement items Number of Intra-cell Handover Requests (uplink quality) and Number of Intra-cell Handover Requests (downlink quality) are related to the degree of interference in the cell. If the handovers in a cell are mainly caused by uplink and downlink quality problem and the ratio of number of intra-cell handovers to the total number of inter-cell handovers is higher than other cells, interference might exist in this cell. Handover measurement items for a cell are closely relevant to the parameter settings of this cell. Decrease of the handover decision threshold or P/N duration can make handover more sensitive and cause more handovers. Whereas, increase of the handover decision threshold or P/N duration will reduce the number of handovers. Too few handovers might not benefit network items and can reduce the handover success rate. Too frequent handovers may cause call drop due to the hard handover characteristic of GSM. According to the data statistics, once handover per conversation is reasonable. 1.1.3 Find Potential Interference through Call Drop Items

Call drop is one of the intolerable network faults. The call drop related measurement items include Number of SDCCH/TCH Call Drops, Number of Radio Link Breaks in SDCCH/TCH Occupation (connection failure), Mean Uplink/Downlink Quality in SDCCH/TCH Call Drop. If there are many call drops in a cell and the cause is connection failure, interference might exist in this cell. If in a cell the mean receive level in case of call drop is high ( ＿25) and the mean receive quality level in case of call drop is ＿6, interference might occur to this cell. 1.1.4 Find Potential Interference through Interference Band Items

When in its idle state, a BTS can use the idle timeslots in a frame to scan the uplink frequency of its TRX and classify it into one of 5-level interference bands. The default settings for the interference bands in Huawei BSC are 110, 105, 98, 90, 87 and 85, with the unit of dBm. The corresponding interference bands in traffic measurement are as follows: 表１　Ｉｎｔｅｒｆｅｒｅｎｃｅ　ｂａｎｄｓ　 Interference band Interference band 1

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Level range (dBm) -105 ~ -98


Interference Interference Interference Interference

band band band band

2 3 4 5

-97 ~ -90 -89 ~ -87 -86 ~ -85 ＿ -84

The interference band measurement items can reflect the degree of interference to a cell more directly than other items, but it can show whether interference occurs to uplink frequency only. If the values of interference bands 4 and 5 are ＿1 in a cell, intra-frequency interference might exist in this cell. If the measured values are mainly in interference bands 1 and 2, it is possible no interference exists in this cell. If the value of interference band 3 is great, interference might occur to this cell. Note: The way used by Huawei to meausre interference bands is based on cell. Therefore, for a large-scale BTS (e.g. S8/8/8), if only one TRX suffers intra-frequency interference, the value of the interference band will be less than the actual value. Thus it cannot truly reflect the degree of interference.

1.1 OMC Alarm and Subscriber Complaint The OMC alarm system can report hardware faults of BTS in time. The operator should analyze alarm information before locating interference. Hardware fault removal is recommended before any optimization. It should be noted that alarm information is not helpful for confirming whether there is potential interference from MS or other BTSs. Subscriber complaint is the useful way to find out potential interference. The information that should be collected for subscriber complaint includes MS number, MS type, the called number, fault information of the calling and the called, specific address where faults happened, etc. The more detailed complaint information is collected, the more easily the network fault can be found. Whether the complaint information given by a subscriber is specific depends on the subscriber's understanding about the cellular network. Once interference exists in the network, a subscriber may complain high noise or call drop, or that he cannot hear the voice of the opposite one, or make a call instead of reporting where the interference is. Therefore, once many subscribers complain such problem in an area, the operator should check whether there is interference exists in this area.

1.2 Drive Test

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Drive test is the commonest way to find interference. If the operator suspects there is interference in a certain area by analyzing traffic measurement items and subscriber complaint information, he should confirm it through drive test (Note: Drive test can be used to find downlink interference only). In actual applications, there are two kinds of drive test models, i.e. idle model and dedicated model. In case of idle model, the test device is used to measure the signal levels of the serving cell and its neighboring cells and perform sweep check on the specified frequency or band. The target BTS should be available for multiple maximum distance tests during drive test. In case of dedicated model, the test device is used to measure signal levels, receive qualities, power control registration and timing advance of the serving cell and its neighboring cells. When high level ( ＿30) and low quality (Rx_Qual＿6) frequently occur in an area, it can be confirmed that there is interference in this area. Some test devices are able to directly display the frame error rate (FER). If FER is ＿25%, the subscriber in conversation may feel that the conversation is intermittent, that is, there is interference. (Note: The FER measured by ANT is not accurate).

4.

Interference Location and Elimination Interference location is the most important step in the optimization procedure. The section above describes how to find potential interference while there are many kinds of interference, e.g. Co-channel or adjacent-channel interference in a system, spurious interference caused by the transmitting device of high

power around, self -excitation of transmitter, etc.

1.1 Procedures 1.1.1 Confirm Interfered Cell through KPI

Unexpected deterioration of call drop rate, handover success rate, traffic, congestion rate or interference band for a cell means there is interference in this cell. Check operation logs of this cell to see whether BTS hardware was added or modified recently, whether the data was modified and whether the interference accompanies with these operations. If the operations above were not performed, the interference should be caused by the hardware or outside. It is recommended to check whether the hardware gets faulty first. If the interference still remains after the removal of the hardware fault, check whether there is off-net interference (See the later section for details). 1.1.2 Check OMC Alarm

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Sometimes high call drop rate, low handover success rate and high congestion rate are related to equipment fault. Checking and analyzing OMC alarm records can save time and help to find out the time correlation between alarm records and item deterioration. Most of OMC alarms are caused by hardware fault, e.g. no power output due to complete wastage of TRX. For most of implicit faults in the optimization, e.g. degrade of TRX/CDU receive performance, TRX/CDU self-excitation, etc., no relevant alarm information will be reported. It is more difficult to locate the faults in analog devices than in digital devices. 1.1.3 Check Frequency Planning

Check the frequency planning of the cell where interference might exist and its neighboring cells. Make the distribution of the corresponding BTS and azimuths of these cells clear first, then draw a topological diagram and give BCCH/TCH frequency and BSIC. Then compare the planned frequency with the actually configured frequency in BSC to check whether they are consistent. Usually, whether there is Co-channel or adjacent-channel interference can be confirmed according to the accurate frequency planning topologic diagram. 1.1.4 Check Cell Parameter Settings

Improper setting of some cell parameters, e.g. CRO, handover threshold, handover duration (PN criteria), neighborhood, etc., may cause interference. If the value of CRO is too great, MS will be induced to a idle cell whose level is lower than neighboring cells'. When MS is in conversation and C/I is not greater than 12dB (threshold), interference will be brought about. If a neighboring cell is not configured in the parameter neighborhood, MS will not be able to hand over to the cell with better signal level and quality, which will cause interference. If the values of handover threshold and P/N criteria are too great, handover between cells will become more difficult, which may cause slight interference (e.g. more handovers occur due to poor quality). However, if the values are too little, the consequence will be more serious. Too frequent handovers increase the call drop rate and aggravate the system payload and even bring about catastrophic consequence, i.e. BSC breakdown. 1.1.5 Drive Test

Drive test is often used for interference location. The method is similar to that described in Section 3.3. Whereas, for interference location, only the cell where interference exists needs to be tested. 1.1.6 Interference Elimination

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According to the location result to eliminate interference. See the section below for details. After the elimination, evaluate the effects as per KPI and drive test results. The following presents the methods of locating and eliminating interference in the form of case and gives case Nos. for convenient lookup.

5. Tools for Interference Test 1.1 Introduction to Spectrum Analyzer At present the spectrum analyzer is often used by Huawei to test interference signals. It is a high performance broadband signal receiver and can display the spectrums of received signals. Spectrum analyzers of different types are characterized by different receive bands and sensitivities. Proper usage of these spectrum analyzers is required. The following gives several key indices of the spectrum analyzer. 1.

Input frequency: Frequency that can be received by the spectrum analyzer. This index decides the

range of interference signal frequencies that can be tested. 2.

Sensitivity: The minimum receive level of the signal with bandwidth of 1 Hz is defined as the receive

sensitivity. The receive sensitivity of HP85 spectrum analyzer can be ＿-142 dBm. The receive sensitivity of the signal with bandwidth of xHz = the receive sensitivity of 1 Hz signal + 10logx. For example, the receive sensitivity of 200 kHz GSM signal = -142 dBm + 10log(200*1000) = -89 dBm. 1.

Received signal resolution bandwidth (RBW): Minimum signal bandwidth that can be resolved by the

spectrum analyzer. The less the value of this parameter, the higher the receive sensitivity of the spectrum analyzer, that is, the lower the noise of this device. 2.

Video filtering bandwidth (VBW): Bandwidth of the intermediate-frequency filter after frequency

mixing of the spectrum analyzer. The narrower the bandwidth, the smoother the curve. 3.

Central frequency (FO): Central frequency of the spectrum that can be tested by the spectrum

analyzer. 4.

Bandwidth (SPAN): Bandwidth of the spectrum that can be tested by the spectrum analyzer.

5.

Input signal attenuation (ATT): When there are large signals input, these signals should be properly

attenuated. Otherwise, the spectrum analyzer might generate large quantities of intermodulation components, which will influence the accuracy of test results.

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Technical indices of the common spectrum analyzers for interference test:

1.1

Type

Working band

Sensitivity (1Hz)

HP8591E HP8594E HP8595E HP8561E

30Hz-1.8GHz 30Hz-2.9GHz 30Hz-6.5GHz 30Hz-6.5GHz

-145dBm -142dBm -142dBm -145dBm

Minimum resolution bandwidth 30Hz 30Hz 30Hz 1Hz

Directional Antenna Directional antenna is used to search interference source. More explicit directionality and higher gain of the antenna will bring about greater search capability. The log-periodical antenna with wide band is recommended due to its wide band, high gain and explicit directionality.

6. Test Methods for Interference 1.1 Testing Internal Interference 1.

Set the parameters of a spectrum analyzer

900 M BTS: f0=902MHz, SPAN=30MHz, ATT=0, RBW=30kHz, VBW=30kHz; 1800 M BTS: f0=1715MHz, SPAN=10MHz, ATT=0, RBW=30kHz, VBW=30kHz. 1.

Unscrew the connector in the output port of the divider of CDU and connect the output signal of the

divider to the spectrum analyzer for test. If the level of the spurious spectrum is less than -80 dBm, it means there is no internal interference. If it is greater than -80 dBm, it indicates the CDU or TRX in the BTS is interfered or suffers self-excitation. 2.

If there is internal interference, confirm whether it occurs to CDU or TRX. Disconnect the TRX

from the divider and use the spectrum analyzer to check the TRX main/diversity connector. If the level of the spurious spectrum is less than -80 dBm, it means the TRX is normal. Otherwise, replace the TRX. The three steps above is for measurement of interference in uplink band. To measure the interference in downlink band, please follow the steps below. 1.

Check whether there is interference in the transmit band. Set the frequency of the spectrum analyzer

to the one in the transmit band of BTS first. Since the output power of BTS is high, input signals should be attenuated. Set ATT to 40 dB and connect the tx_test signal of CDU to the spectrum analyzer. Then observe whether there are interference signals generated.

1.2 Testing External Interference If interference is from outside, first make the position and spectrum distribution of the interference source clear. To do it, use the low-noise amplifier in front of the antenna & feeder system and RF system of BTS to test.

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1.

Properly set the spectrum analyzer. See the previous section.

2.

Select a output port of the divider of the cell interfered. In order not to impact the normal operation

of BTS, it is recommended to select an idle main/diversity output port (loaded). 3.

Unscrew the selected connector and use a coaxial cable to lead the output signals of the divider into

the spectrum analyzer. 4.

Observe the spectrum distribution of the spectrum analyzer and find interference signals. The level of

interference signal is calculated as follows: Interference level of antenna port = interference level actually tested by the spectrum analyzer - 15 dB (TTA gain) + 3 dB (cable loss) - 7 dB (divider gain) For example, interference level of antenna port = - 65 dBm-15+3-7 = -84 dBm Note: Cable loss varies with the length of the cable. Whether interference level impacts system is judged as follows: 1) In order not to impact the system, the maximum interference level of the antenna port is -108 dBm, the sensitivity is -9 dBm and the intra-frequency interference is -117 dBm. 2) In order not to impact the system, the maximum interference level of the output port of the divider is 117 dBm +15-3+7= -98 dBm.

1.3 Searching External Interference Source The approximate position of the interference source can be confirmed according to the output port of the divider of BTS. To further search the specific position, use the directional antenna mentioned previously. The search procedure is as follows: 1.

Select a test point not obstructed by buildings around in the interfered cell.

2.

Correctly set the spectrum analyzer and properly assemble the directional antenna.

3.

If there is a rotating table, put the antenna on the table and make the beam of the antenna face ahead.

If no rotating table is available, raise the antenna above your head. Then rotate it slowly and observe the spectrum analyzer. Once abnormal signals appear, stop rotating the antenna and slowly change its pitch angle until the maximum received signal strength is obtained. 4.

Analyze the spectrum distribution carefully, find out interference signals and record the signal

strength and the azimuth and pitch angle of the directional antenna beam. 5.

Select a new test point in the direction of the antenna beam. Repeat Steps 2~4 until the interference

source is found.

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Guide to GSM Interference Analysis

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