GSM Interference Analysis

GSM Interference Analysis

GSM Interference AnalysisTable of Contents Table of Contents Course Description ....................................................................................... ................................1 Introduction to Course ....................................................................................... .......................1Course Objective ....................................................................................... ...............................1Relevant Materials ....................................................................................... ............................1 Chapter 1 Overview ....................................................................................... ...............................2 1.1 Affection of Interference on Network .................................................................................2 Interference Sources ....................................................................................... ..............................3

1.2 Classification of Interference Sources ...............................................................................31.2. 1 Natural Noise ....................................................................................... ...................31.2.2 Man-made Noise ....................................................................................... .............31.3 Main Interference Sources Affecting Mobile Communication ...........................................4 Discovery of Interference ....................................................................................... .......................6 1.4 Discovering Interference via OMC Traffic Measurement...................................................61.5 OMC Alarm and Subscriber Complaint....................................................................... ......81.6 Discovering Interference via Drive Test............................................................................8 Chapter 2 Location and Clearance of Interference Source .......................................................9 2.1 Recommended Procedures for Location and Clearance of Interference ...........................92.1.1 Determine Interference Cell according to Key Performance Index (KPI) ................92.1.2 Check Alarm of OMC ....................................................................................... .......92.1.3 Frequency Planning Check

.....................................................................................9 2.1.4 Check Parameter Setting of Cell...........................................................................102. 1.5 Drive Test................................................................................. ............................102.1.6 Interference Clearance ....................................................................................... ..102.2 Location and Clearance of Hardware Fault....................................................................102.2.1 Antenna Performance Degradation ......................................................................102.2.2 Water Seepage of Antenna or Feeder ..................................................................112.2.3 Jumper Connector Fault............................................................................... ........122.2.4 Antenna Connected Inversely ...............................................................................122.2 .5 Incorrect Jumper Connection of BTS ....................................................................142.2.6 TRX Fault............................................................................... ..............................142.2.7 Clock Unlocking ....................................................................................... .............162.2.8 Summary ....................................................................................... .......................172.3 Intra-network Interference ....................................................................................... ........182.3.1 Co-channel Interference .......................................................................................

182.3.2 Adjacent-channel Interference ..............................................................................202.3. 3 Interference Caused by Over-coverage ................................................................222.3.4 Interference Caused by Aggressive Reuse ..........................................................232.4 Repeater Interference ....................................................................................... ..............232.5 Off-network Interference ....................................................................................... ..........242.5.1 Microwave Interference ....................................................................................... .242.5.2 High-power Broadcasting Station Interference .....................................................252.6 Other Phenomena Causing Interference .........................................................................262.7 False Interference ....................................................................................... ....................27 Chapter 3 Anti-interference Measures ...................................................................................... 29Chapter 4 Interference Test Tools ....................................................................................... ......30 4.1 Brief introduction to Spectrum Analyzer ..........................................................................304.2 Directional Antenna

....................................................................................... ..................30 Chapter 5 Way to Test Interference ....................................................................................... ....31 GSM Interference AnalysisTable of Contents 5.2 The way to Test External Interference ............................................................................315.3 The way to Search External Interference Sources ..........................................................32 ii

OMF000403 GSM Interference AnalysisISSUE1.0 Error! Use the Home tab to apply 标题 1 tothe text that you want to appear here. Course Description Introduction to Course To introduce affection brought by interference to the GSM system, fault locationmethod and troubleshooting method, this course hereby lists out the

possibleinterference sources, and offers description of cases to facilitate fault location andtroubleshooting. The main contents of the course are as follows: overview,classification of interference source, judgement method, location and troubleshooting,antiinterference measures, introduction to instrument interference test, etc. Course Objective After completed the course, you can:

get the cause of interference and master the method of locating the interferencesource;

Master the method of overcoming interference. Relevant Materials InterferenceStarwiarskiGSM Principle and Network Optimization Han BinjieMobile Communication Engineering Lu Er’Rui 1

OMF000403 GSM Interference AnalysisISSUE1.0

Error! Use the Home tab to apply 标题 1 to the text thatyou want to appear here.Error! Use the Home tab toapply 标题 1 to the text that you want to appear here. Chapter 1 Overview Frequency resource is a kind of rare resource. In GSM system, frequency reuse isvery necessary for GSM system to provide enough capacity. Frequency reuse meansthat same frequency can be used simultaneously in several cells when the distancebetween them is far enough. The distance between the cells allocating the samefrequency is called the reuse distance. And the ratio between the reuse distance andthe cell radius is called co-frequency interference factor. For certain frequencyresource, the network capacity will be larger if the frequency reuse is more tight, butthe interference will be stronger because the reuse distance becomes nearer.The interference caused by frequency reuse is called internal network interference (itis also called internal system interference). Besides, the GSM network may beaffected by interference from other communication systems.Judging from the aspects such as conversation quality, call drop, handover

andcongestion, it is found that interference situation is one of the most important factor that affects the network. How to reduce or eliminate interference is now becomingthe principal task of network planning and optimization. This document hereinprovides a systemic description of interference source, interference location andtroubleshooting on the basis of experience of experts. 1.1 Affection of Interference on Network When there is interference in the network, the subscribers usually encounter thefollowing phenomenon:

During conversation, the subscriber usually cannot hear the voice , and thebackground noise is too loud.

When fixed telephone subscriber calls MS subscriber, or MS subscriber callsfixed telephone subscriber, call drop occurs after “Du, du , du” is heard.

The conversation cannot be carried on smoothly, and call drop often happens.

When interference exists in the network, from the result of traffic statistic, thereare some characteristic as following. 1. There are Level 4~Level 5 interference band in TCH measurementfunction , and the measurement value is more than 1. 2. Congestion rate is comparatively high (As there is interference in SDCCHchannel, immediate assignment or TCH assignment will fail). 3. The call drop rate is higher than other cells. 4. The handover success rate is low. 5. Through Drive Test, it is found that: 6. it is difficult to handover . 7.

The Rx level is high, but the quality is bad. 8. Through tracing the Abis interface signaling with signaling analyzer (MA10/K1205), it is found that The bit error rate is higher than other cells. 2

OMF000403 GSM Interference AnalysisISSUE1.0 Error! Use the Home tab to apply 标题 1 to the text thatyou want to appear here.Error! Use the Home tab toapply 标题 1 to the text that you want to appear here. Interference Sources 1.2 Classification of Interference Sources The interference sources of mobile communication system/noise are mainly classedas follows: 1.2.1 Natural Noise

Atmosphere noise

Galaxy noise

Solar noise (quiet period) 1.2.2 Man-made Noise

Interference of ignition systems of vehicles or other engines

Interference of electronic communication system;

Interference of power line

Interference of scientific research, medical and household appliances;Research data of ITT, America on the above noise/interference is shown as follows. Figure 1.1

Environment noise In the figure, Ta denotes the noise temperature, Fa denotes equivalent noise factor,and the relation between the two is as follows: Fa = 10logloglog TaTo Wherein, To=290 0 K.From the research data of ITT, it can be viewed that the atmosphere noise and solar noise is comparatively low in 30~1000MHz, which can be ignored. And above100MHz, as the radio noise of galaxy is lower than the thermal noise of typicalreceiver, the interference can be ignored as well. Thus, natural noises (atmosphere 3

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标题 1 to the text thatyou want to appear here.Error! Use the Home tab toapply 标题 1 to the text that you want to appear here. noise, galaxy noise and solar noise) are unnecessarily considered for the mobilesystems above 450MHz, 800MHz, 900MHz, 1800MHz and 2000MHz [3] . The affectionof sunspot fastigium on mobile communication is temporarily unknown, but scientistsbelieve that sunspot fastigium has great affection on the electric power andcommunication.According to the research of National Bureau of Standards (NBS), it is found that theman-made noise is one of the main interference sources. Among those man-madeinterference/noise sources, certain interference is uncontrollable, such as ignitioninterference of vehicle engine, electric power interference and industrial electricequipment interference. However, certain interference, such as interference betweencommunication equipment and internal network interference, which can be overcomeby reasonable network planning and system

optimization. The latter is main researchfield of this document. 1.3 Main Interference Sources Affecting MobileCommunication In mobile communication system, when BTS receives the signals from the MScomparatively far away, it will encounter interference from the ambient communicationequipment,and BTS or MS of the same system [3] , as shown in the figure below. Figure 1.2 Schematic diagram of mobile communication interference The main interference of this part is as follows:Hardware fault:

TRX fault: if the performance of TRX is reduced due to manufacture cause or application, which will cause selfexcitation of TRX amplification circuit resultingin interference.

CDU or divider fault: as active amplifier has been adopted for the divider anddivider module of CDU, selfexcitation might be caused when fault occurs.

Spurious emission and inter-modulation: if out-band spurious emission index of BTS TRX or amplifier exceeds the limit, or isolation between TX and RX of theduplexer in CDU is not enough, all these will form interference on the receivingchannel. Inter-modulation may also occur in passive equipment such as passiveantenna and feeder.

Internal network interference:

Improper frequency planning:

Co-frequency interference

Adjacent frequency interferenceRepeater interference:It is commonly adopted that coverage of BTS is extended with repeater in the earlystage network

construction. Due to its own characteristics, it will easily cause 4

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1 to the text that you want to appear here. interference to the BTS if it is improperly used. For repeater, there are mainly twointerference modes:

As the installation of repeater is non standard, causing insufficient isolationbetween the donor antenna and the subscriber antenna, so self-excitation iscaused. This affects normal working of BTS that the repeater belongs to.

For the repeater adopting wideband non-linear amplifier, the inter-modulationindex far exceeds the requirement of the protocol. If the power is comparativelyhigh, the inter-modulation component will be large, and this will causeinterference to the BTS around.

Interference of other communication equipment with high power:

Radar station: From 70s~80s of the 20 th

century, the frequency used by thedecimeter wave radar was similar to that of GSM, and its transmitting power wasvery high, which generally reached tens and hundreds of kilowatts, so the out-band spurious emission is comparatively large. Thus, it easily causesinterference to the BTS.

Analog BTS: The frequency band used by the analog mobile BTS overlaps withthe GSM frequency band in certain segment.

Communication equipment at same frequency band: As the types of communication equipment are so many, some manufacturers maybe adopt thefrequency band but doesn’t comply with the current communication standards.As the GSM frequency band is occupied by the equipments, interference will becaused within the GSM system coverage area. 5

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1 to the text thatyou want to appear here.Error! Use the Home tab toapply 标题 1 to the text that you want to appear here. Discovery of Interference To solve the interference problem and improve the conversation quality, find out theinterference first, and the n use proper means for locating the interference, and at lasteliminating or reducing interference.The methods available for discovering the interference in GSM system are as follows:OMC traffic measurement, OMC alarm, Drive Test and subscriber complaint, etc. Asthe puissant tools, special test equipment such as signaling analyzer and spectrumanalyzer are commonly unused in this stage. 1.4 Discovering Interference via OMC Traffic Measurement After a network on srvice, following traffic measurement tasks should be registered todiscover the problems in time: TCH measuremen functiont, SDCCH measurementfunction and handover measurement

function. After task registration, check the trafficstatus of various cells, handover and traffic measurement indices related to cellquality to discover the cells with potential interference.What needs to be mentioned is that only the existence of the potential interferencecan be judged according to the checking results. To find out whether there isinterference actually, or there are other problems, location and analysis is needed. 1. Discovering potential interference via traffic statistics result Check the “Average TCH busy time (second)” in TCH measurement function of eachcell, the reason is that this index can show the TCH mean occupied time (s”), which isusually called “TCH mean holding time” in the BSC of other manufacturer”, within themeasurement period. If it is found that the Average TCH busy time (second) of certain cell is comparatively short (such as less than 10s), then maybe there is stronginterference in the cell, causing that handover/call drop happens due to bad qualityafter TCH channel occupied to MS.Certainly, if hardware fault occurs in certain TRX (non-BCCH or non-SDCCH carrier)of a cell, the case mentioned above will also appear. 2. Discovering interference via handover data The handover measurement data reflects the mobility of the subscribers within thecell under measurement.

Generally, we can divide the handover data into twocategories for analysis, intra-cell handover and inter-cell handover.

Inter-cell handover There are many reasons of MS triggering handover The handover measurementindices mainly used for judging whether there is interference are as follows: times of attempt to initiate handover (downlink quality), times of attempt to initiate handover (uplink quality), times of receiving quality level (0~7) when initiating handover (uplink)and average receiving quality when initiating handover (uplink).When certain cell initiates handover, if the average receiving quality (uplink) is ≥ 4(this is true when there is no frequency hopping, if there is, it should be ≥ 5), and themean receiving level is ≥ 25(-85dBm), then it is possibly caused by uplinkinterference.When certain cell initiates handover, if the times of receiving quality level above 5 ismore

than that below 4, then there may be uplink interference as well.If the times of certain cell attempts to initiate handover (uplink/downlink quality) is 6

OMF000403 GSM Interference AnalysisISSUE1.0 Error! Use the Home tab to apply 标题 1 to the text thatyou want to appear here.Error! Use the Home tab toapply 标题 1 to the text that you want to appear here. more than 10% of total handover attempt times, then there may be interference in thecell. The two indices are all related to quality handover threshold and interferencehandover threshold within the cell parameters.

Intra-cell handover For intra-cell handover, there is also measurement items such as intra-cell handover request times (uplink/downlink quality) and the interference

situation. If the intra-cellhandover is caused by uplink/downlink quality, and the proportion taken up by totalintra-cell handover times among the total intercell handover times is comparativelyhigher than that of others, then there may be interference in the cell.The handover measurement index is closely related to the setting of cell parameters.The reducing of handover judgment threshold and P/N duration can make thehandover more sensitive, and cause more handovers. And on the contrary, handover times will be reduced. Too few handover times maybe no good to the networkperformance at whiles, and affect the handover success rate directly. However, toomany handover is no good either. As for the hard handover attribute of GSM,handover is also the main cause of call drop. According to the online datameasurement, the proportion that one handover per conversation will becomparatively reasonable. 3. Discovering potential interference via call drop index Call drop is one of the network faults un-acceptable for the subscribers. Themeasurement indices related to call drop are as follows: SDCCH/TCH call drop times,wireless link broken times when SDCCH/TCH occupied (connection failed) and meanuplink/downlink quality in case of SDCCH/TCH call drop.If the call drop times of certain cell is rather higher the the other with same traffic load,and the main cause of call drop is owning to

connection fault, then it is possiblycaused by interference.If the average receiving level during call drop is comparatively high ( ≥ 25), while theaverage receiving quality level is ≥ 6, then the cell should be listed into theinterference source. 4. Discovering potential interference via interference band BTS will utilize an idle TS in a frame to scan the uplink frequencies of the frequenciesused by TRX, and then make measurement to the level 5 interference band. Thedefault setting of interference bands in BSC of Huawei is as follows: 110, 105, 98, 90,87 and 85 (unit: dBm), corresponding to the following interference bands in trafficmeasurement: Table 2.1 Interference bandInterference bandLevel scope (dBm)Interference band one-105 ~ -98Interference band 2-97 ~ -90Interference band 3-89 ~ -87Interference band 4-86 ~ -85Interference band 5 ≥

-84 Compared with other measurement indices, the measurement index of interferenceband can reflect the cell interference situation more directly, but it can only reflectwhether there is interference in the uplink.If the values of interference band 4 and interference band 5 are comparatively large( ≥ 1), then, there may be co-frequency interference in the cell. If the measurementvalues mainly distribute in interference band 1 and interference band 2, then thepossibility of interference will be small. However, if there is comparatively high valuein band 3, then attention should be paid to this. 7

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1 to the text that you want to appear here. Note: As Huawei’s measurement method of interference band is based on cell, thevalue of interference band of certain TRX that suffers serious cofrequencyinterference will be comparatively small in large site type (S8/8/8). The reason is thatthis value has been averaged by the values of other seven TRX without interference.Thus, the values cannot reflect the actual interference condition. 1.5 OMC Alarm and Subscriber Complaint OMC alarm subsystem can report the hardware fault of BTS side. Before starting tolocate the interference source, analysis must be performed to the alarm informationfirst. Before implementation of any optimization works, it is wise to eliminate hardwarefault first.What needs to be mentioned is that the interference, whether from MS or other BTS,cannot be judged via the alarm information of alarm subsystem.Subscriber complaint is also important clew for finding the potential interference.Information which should be collected from user complaint includes MS number, MSmodel, called number and fault phenomenon of calling side and fault phenomenon of called side and particular fault location, etc. If the alarm information is more detailed,it will be more easily to find out the network problems.The description of subscriber complaint is comparatively fuzzy. However, it

isreasonable if the subscriber cannot tell you where the interference is, the reason isthat they don’t know much about the cellular network actually. When there isinterference in the network, the direct feelings of the subscriber may be: heavy noise,both parties or either party cannot hear each other clearly, call drops and call cannotbe put through, etc. Thus, when many subscribers within the same area complain thesame problem, then work should be done to check whether there is interference inthe area. 1.6 Discovering Interference via Drive Test Drive Test is a method that is most commonly used to find out problems. Thesuspicion of interference gained by the methods described above, namely, analysis of traffic measurement and subscriber complaint, should also be verified via drive test.Note that the Drive Test can only check the downlink interference. During actualimplementation, there are two Drive Test methods available: idle mode test andspecial mode test.Under the idle mode, the test equipment can measure the signal level of both servingcell and adjacent cell. In addition, the equipment can also perform frequencyscanning test to the specified frequency or frequency band.During test, round-trip test should be performed to the object BTS.Under special mode test, the test equipment can measure the signal levels, receivingqualities, power control

registrations and time advance, etc. of both the serving celland adjacent cell. When high level ( ≥ 30) and low quality (Rx_Qual ≥ 6) remain incertain section of highway, then, it can be concluded that interference exists in thesection. Further, part test equipment can directly display the frame elimination rate(FER). Generally when the FER ≥ 25%, subscribers can feel the discontinuous voice,that is to say, interference exists in this section of highway (FER measurement of ANTis inaccurate.). 8

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标题 1 to the text that you want to appear here. Chapter 2 Location and Clearance of InterferenceSource The most important process during optimization is how to locate the fault in thenetwork which requires much experience. The above section only reveals that theinterference may exist in the network, but the causes of interference vary such as co-frequency or adjacent frequency interference in the system, spurious interference of high-power transmitting equipment around, the self-excitation of the transmitter, etc. 2.1 Recommended Procedures for Location and Clearanceof Interference 2.1.1 Determine Interference Cell according to Key Performance Index (KPI) The sudden deterioration of such indices as call drop rate, handover success rate,traffic, congestion rate, interference band indicates that interference exists in the cell.At this time, check the log of the operation on the cells is required. Check whether BTS hardware is added or modified, and data is modified recently, or whether theoccurrence of the interference is associated with these operations in terms of time.If no data modification is performed in this duration, it can be sure that theinterference comes from the hardware or

external of the network. It is recommendedto check the hardware first, if interference still exists after the hardware fault isexcluded, then check whether external interference exists (the method for checkingexternal interference, please refer to the chapter infra). 2.1.2 Check Alarm of OMC Sometimes, the high call drop rate, low handover success rate and high congestionrate may be relevant with the equipment fault, check of OMC alarm record will sparemuch time spent in judgment and analysis. Similarly, the association of alarm recordwith the deterioration of these indices in terms of time is analyzed here.It should be noted that most alarms of OMC aim at the hardware fault of the such asTRX no power output, etc. For most potential fault in optimization such as TRX or CDU receiving performance degradation, self-excitation, etc., the alarm informationcannot be reported. (Compared with digital components, the analog components suchas microwave and so on are more difficult to detect) 2.1.3 Frequency Planning Check If one cell is doubted with interference, check the frequency planning of this cell andcells around. First you should be very clear about the BTS location distribution andazimuth of each cell, then make a drawing of the topology map, and mark theBCCH/TCH frequency and

BSIC. At the same time, make a comparison between theplanned frequency and actually configured frequency in BSC to check whether thereis discrepancy.Generally you can judge whether there is co-frequency and adjacent frequencyinterference according to accurate topology map of frequency planning. 9

OMF000403 GSM Interference AnalysisISSUE1.0 Error! Use the Home tab to apply 标题 1 to the text thatyou want to appear here.Error! Use the Home tab toapply 标题 1 to the text that you want to appear here. 2.1.4 Check Parameter Setting of Cell Some cell parameters have impact on the interference such as CRO, handover threshold, handover judgment/statistics duration (P/N criteria), and adjacent cellrelation.If CRO is set too larger than the neighbor cells, MS is drawn in this cell with in idlemode

even the actual Rx level lower than the cells around, so once C/I cannot bemore than the threshold 12dB in case of conversation, interference will emerge.If configuration of adjacent cell is omitted in adjacent cell relation, MS will fail tohandover to the cell with better signal level and quality, this will also lead tointerference. Slight interference may also occur in case of large handover threshold,and P/N criteria as well as handover difficulty between cells. But it is far moredangerous if the value is too small, the frequent handover not only increases the calldrop rate, but also the system load, even leads to disastrous result, i.e., BSCbreakdown. 2.1.5 Drive Test Drive Test is an effective way to locate the interference. The method is the same asproblem location in 3.3. The difference is that the cell with interference is highlightedduring the test when locate the interference. 2.1.6 Interference Clearance Adjustments should be made respectively according to the above location results. For details, please refer to the technical cases. Finally the effect of interference clearancewill be evaluated according to KPI, and Drive Test result.The specific methods for interference location and clearance are described in form of cases as

follows, at the same time the case ID is provided for the convenience of consultation. 2.2 Location and Clearance of Hardware Fault When a certain cell is doubted with interference through the above analysis, it isrequired to check whether the BTS first where the cell is located works normally or not. Check whether there is antenna feeder alarm, TRX alarm, BTS clock alarm andso on remotely; check whether there is antenna damage, water seepage, feeder (including jumper) damage, water seepage, CDU fault, TRX fault, incorrect BTS jumper connection, system clock unlocked. 2.2.1 Antenna Performance Degradation As one of the passive devices, the possibility of antenna damage is very little, but thereal antenna damage or performance degradation will lead to poor communicationquality.Case 0010761. Fault description: There are 5 BTSs for a certain network in a county configured asS4/4/4 and 6/6/6, the BTS type includes BTS20 and BTS30. The interference band 5in TCH measurement function of some cells is over 15, and there is no alarminformation in OMC Fault location process

1) Register the statistics task of interference band of 24 hours for the cell with 10

OMF000403 GSM Interference AnalysisISSUE1.0 Error! Use the Home tab to apply 标题 1 to the text thatyou want to appear here.Error! Use the Home tab toapply 标题 1 to the text that you want to appear here. problems, it is found that interference band 5 mainly occurs in daylight, and inthe small hours near middle night, the interference band value is almost 0. 2) After opening the idle BURST of all BTSs and transmitting it in the early morning,it is found that the interference band occurs. It disappears after transmitting isstopped. It can be judged from this

phenomenon that the interference comesfrom internal network and has nothing to do with other equipment. 3) No frequency in the network and data are modified before the interferenceoccurs, accordingly, the interference is irrelevant with the frequency planning. 4) It can be seen from the above second and third points that the problem isrelevant with the BTS equipment. 5) Observe the RXM test interface of CDU with the spectrum analyzer in peak hour in the daylight, it can be seen that unstable strong broadband interference andrise of back noise occur. 6) First replace all boards (TRX, CDU, FPU, HPA, and power board) of this BTS(BTS20, with Tower Top Amplifier) one by one, at the same time observe thespectrum signal of RXM test interface, it can be seen that interference exists allthe time. This indicates that the interference is relevant with the antenna feeder (including divider, combiner, feeder, antenna, lightning

arrester, Tower TopAmplifier, jumper and connector) instead of the board. 7) Since the above BTS under test has the Tower Mount Amplifier, the antenna andfeeder check is inconvenient, replace another BTS30(S4/4/4) (dual-CDU, anddualpolarization antenna) with interference and check the antenna feeder. 8) Since no interference exists in one of the cells while strong interference exists inanother two cells in the BTS, interchange the antenna and feeder (changing the jumper at the top of the cabinet) of the cells which are with and withoutinterference in the BTS in the evening. Then send idle BURST, it is found thatthe interference follows the antenna and feeder. This step helps further locatethe fault which should exist in antenna and feeder system. 9) The situation remains the same even after replacing lightning arrester of antennafeeder and checking all jumper connectors. Then it can be sure that fault existsin the feeder or antenna. 10)

Replace the jumper (i.e., antenna) at the top of the tower, it is found that theinterference follows the antenna, so the feeder fault can be excluded while theantenna fault is quite possible. (It should be noted that the external interferenceat this step cannot be excluded because the actual installation place of theantenna does not change, but the external interference has already beenexcluded in the above step. ) 11) Finally check the antenna. The strong interference disappears immediately after the antenna is replaced on the tower by using the dual-polarization antenna. For further verification, replace the antenna of one cell with strong interference inanother BTS20 with a new one, and then the interference disappears, thus theproblem is solved here.Sometimes the alarm cannot be reported to the OMC alarm console after the antennais damaged. While the antenna damage will lead to the degradation of the radiationperformance and inter-modulation. And the inter-modulation product is fed back to thereceiving channel of the BTS, generating interference and effect the conversationquality. Similar case also includes case 0017185. 2.2.2 Water Seepage of Antenna or Feeder

The water seepage of antenna and feeder will change the media structure and bringhigh loss of radio signals. Fault cases in the on service network indicate that thepossibility for water seepage of antenna and feeder is less than that of antennadamage, while the former often leads to the reduction of service range. And no casethat interference is caused by water seepage of antenna and feeder till now. 11

OMF000403 GSM Interference AnalysisISSUE1.0 Error! Use the Home tab to apply 标题 1 to the text thatyou want to appear here.Error! Use the Home tab toapply 标题 1 to the text that you want to appear here. For case of water seepage of feeder, please refer to case 0009057. 2.2.3 Jumper Connector Fault

The RF signal of GSM falls into the UHF signal, if loose contact exists in any sectionfrom TRX, CDU, feeder to antenna will lead to high VSWR, increase of intermodulation and interference.Case 0015118 Fault description: the type of certain BTS is BTS2.0,site configuration is S242, MS isdifficult to access cell 2, and also call drop often occurs during conversation. It isfound that the interference band 4 and 5 appear in cell 2 interference band of the BTSby viewing the traffic measurement. Interference of different value occur on severalcarriers of this cell through signaling tracing, but interference band only occurs tointerference band 2 and 3 several days later. Though the conversation is notaffected, the interference does not disappear. there is no alarm message in OMCsystem Fault location process 1) Check carefully the frequency planning of the BTS. The possibility of internal co-frequency and adjacent frequency interference is excluded because the BTSs inthis area are not so dense and with loose frequency reuse. 2)

It is found that though interference spectrum occurs, but the interference signallevel is not too high when checking with spectrum analyzer. 3) The interference still exists after replacing carrier board, power amplificationboard, power board, and CDU. Subsequently the interference occurs anddisappears along with the operations of screwing up and screwing off theconnector at the exit of CDU. 4) It is found that some scraps exist on the CDU output connector. Screw up theconnector after blowing off the scraps, and since then, interference never occurs.This problem belongs to the soft fault. The scraps in the connector will bringunconspicuous interference, so experience is quite important here. On the other hand, this problem reminds us of the importance of checking the engineering qualitycarefully. 2.2.4 Antenna Connected Inversely It is quite usually that the antenna is connected inversely, which will lead to completedifference between the using frequency and the planned frequency. It will also lead toco-frequency and adjacent frequency interference, call drop, handover failure, etc. If the frequency resources for operator is a few,

antenna connected inversely will affectthe network quality much more.Case 0011108 Fault description: the interference bands 4 and 5 often occur in the trafficmeasurement after certain BTS is on service, the inter-cell handover success rate isvery low and the congestion rate is up to 5%. There is no alarm in OMC. Fault location process 1) Since the interference bands 4 and 5 occur along with low handover successrate and congestion, it is doubted that the interference causes the abovephenomena. 2) Check the frequency planning first, no problem is found. The externalinterference becomes the chief consideration after the frequency planningproblem is excluded. Change the original used frequency 9 into the far-awayfrequency 94 to avoid external interference, but situation si the same.Confirmation made with the operator’s branch office indicates that the BTS is 12