GSM Interference Analysis

GSM Interference Analysis

GSM Interference Analysis Table of Contents

Table of Contents Course Description.......................................................................................................................1 Introduction to Course..............................................................................................................1 Course Objective......................................................................................................................1 Relevant Materials...................................................................................................................1 Chapter 1 Overview......................................................................................................................2 1.1 Affection of Interference on Network.................................................................................2 Interference Sources.....................................................................................................................3 1.2 Classification of Interference Sources...............................................................................3 1.2.1 Natural Noise..........................................................................................................3 1.2.2 Man-made Noise....................................................................................................3 1.3 Main Interference Sources Affecting Mobile Communication...........................................4 Discovery of Interference..............................................................................................................6 1.4 Discovering Interference via OMC Traffic Measurement...................................................6 1.5 OMC Alarm and Subscriber Complaint.............................................................................8 1.6 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...........................9 2.1.1 Determine Interference Cell according to Key Performance Index (KPI)................9 2.1.2 Check Alarm of OMC..............................................................................................9 2.1.3 Frequency Planning Check.....................................................................................9 2.1.4 Check Parameter Setting of Cell...........................................................................10 2.1.5 Drive Test.............................................................................................................10 2.1.6 Interference Clearance.........................................................................................10 2.2 Location and Clearance of Hardware Fault....................................................................10 2.2.1 Antenna Performance Degradation......................................................................10 2.2.2 Water Seepage of Antenna or Feeder..................................................................11 2.2.3 Jumper Connector Fault.......................................................................................12 2.2.4 Antenna Connected Inversely...............................................................................12 2.2.5 Incorrect Jumper Connection of BTS....................................................................14 2.2.6 TRX Fault.............................................................................................................14 2.2.7 Clock Unlocking....................................................................................................16 2.2.8 Summary..............................................................................................................17 2.3 Intra-network Interference...............................................................................................18 2.3.1 Co-channel Interference.......................................................................................18 2.3.2 Adjacent-channel Interference..............................................................................20 2.3.3 Interference Caused by Over-coverage................................................................22 2.3.4 Interference Caused by Aggressive Reuse..........................................................23 2.4 Repeater Interference.....................................................................................................23 2.5 Off-network Interference.................................................................................................24 2.5.1 Microwave Interference........................................................................................24 2.5.2 High-power Broadcasting Station Interference.....................................................25 2.6 Other Phenomena Causing Interference.........................................................................26 2.7 False Interference...........................................................................................................27 Chapter 3 Anti-interference Measures......................................................................................29 Chapter 4 Interference Test Tools.............................................................................................30 4.1 Brief introduction to Spectrum Analyzer..........................................................................30 4.2 Directional Antenna.........................................................................................................30 Chapter 5 Way to Test Interference...........................................................................................31 5.1 The way to Test Internal Interference..............................................................................31 i

GSM Interference Analysis Table of Contents

5.2 The way to Test External Interference............................................................................31 5.3 The way to Search External Interference Sources..........................................................32

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Course Description Introduction to Course To introduce affection brought by interference to the GSM system, fault location method and troubleshooting method, this course hereby lists out the possible interference sources, and offers description of cases to facilitate fault location and troubleshooting. The main contents of the course are as follows: overview, classification of interference source, judgement method, location and troubleshooting, anti-interference 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 interference source; Master the method of overcoming interference.

Relevant Materials Interference

Starwiarski

GSM Principle and Network Optimization

Han Binjie

Mobile Communication Engineering

Lu Er’Rui

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Chapter 1 Overview Frequency resource is a kind of rare resource. In GSM system, frequency reuse is very necessary for GSM system to provide enough capacity. Frequency reuse means that same frequency can be used simultaneously in several cells when the distance between them is far enough. The distance between the cells allocating the same frequency is called the reuse distance. And the ratio between the reuse distance and the cell radius is called co-frequency interference factor. For certain frequency resource, the network capacity will be larger if the frequency reuse is more tight, but the interference will be stronger because the reuse distance becomes nearer. The interference caused by frequency reuse is called internal network interference (it is also called internal system interference). Besides, the GSM network may be affected by interference from other communication systems. Judging from the aspects such as conversation quality, call drop, handover and congestion, 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 becoming the principal task of network planning and optimization. This document herein provides a systemic description of interference source, interference location and troubleshooting 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 the following phenomenon: 



 

During conversation, the subscriber usually cannot hear the voice , and the background noise is too loud. When fixed telephone subscriber calls MS subscriber, or MS subscriber calls fixed 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, there are some characteristic as following. 1. There are Level 4~Level 5 interference band in TCH measurement function , and the measurement value is more than 1. 2. Congestion rate is comparatively high (As there is interference in SDCCH channel, 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.

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Interference Sources 1.2 Classification of Interference Sources The interference sources of mobile communication system/noise are mainly classed as 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 = 10 log

Ta To

Wherein, To=2900K. 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 above 100MHz, as the radio noise of galaxy is lower than the thermal noise of typical receiver, the interference can be ignored as well. Thus, natural noises (atmosphere

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noise, galaxy noise and solar noise) are unnecessarily considered for the mobile systems above 450MHz, 800MHz, 900MHz, 1800MHz and 2000MHz[3]. The affection of sunspot fastigium on mobile communication is temporarily unknown, but scientists believe that sunspot fastigium has great affection on the electric power and communication. According to the research of National Bureau of Standards (NBS), it is found that the man-made noise is one of the main interference sources. Among those man-made interference/noise sources, certain interference is uncontrollable, such as ignition interference of vehicle engine, electric power interference and industrial electric equipment interference. However, certain interference, such as interference between communication equipment and internal network interference, which can be overcome by reasonable network planning and system optimization. The latter is main research field of this document.

1.3 Main Interference Sources Affecting Mobile Communication In mobile communication system, when BTS receives the signals from the MS comparatively far away, it will encounter interference from the ambient communication equipment,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 self-excitation of TRX amplification circuit resulting in interference. CDU or divider fault: as active amplifier has been adopted for the divider and divider module of CDU, self-excitation 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 the duplexer in CDU is not enough, all these will form interference on the receiving channel. Inter-modulation may also occur in passive equipment such as passive antenna and feeder. Internal network interference: Improper frequency planning: Co-frequency interference Adjacent frequency interference

Repeater interference: It is commonly adopted that coverage of BTS is extended with repeater in the early stage network construction. Due to its own characteristics, it will easily cause 4

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interference to the BTS if it is improperly used. For repeater, there are mainly two interference modes: 



 





As the installation of repeater is non standard, causing insufficient isolation between the donor antenna and the subscriber antenna, so self-excitation is caused. This affects normal working of BTS that the repeater belongs to. For the repeater adopting wideband non-linear amplifier, the inter-modulation index far exceeds the requirement of the protocol. If the power is comparatively high, the inter-modulation component will be large, and this will cause interference to the BTS around. Interference of other communication equipment with high power: Radar station: From 70s~80s of the 20th century, the frequency used by the decimeter wave radar was similar to that of GSM, and its transmitting power was very high, which generally reached tens and hundreds of kilowatts, so the outband spurious emission is comparatively large. Thus, it easily causes interference to the BTS. Analog BTS: The frequency band used by the analog mobile BTS overlaps with the 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 the frequency band but doesn’t comply with the current communication standards. As the GSM frequency band is occupied by the equipments, interference will be caused within the GSM system coverage area.

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Discovery of Interference To solve the interference problem and improve the conversation quality, find out the interference first, and then use proper means for locating the interference, and at last eliminating 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. As the puissant tools, special test equipment such as signaling analyzer and spectrum analyzer 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 to discover the problems in time: TCH measuremen functiont, SDCCH measurement function and handover measurement function. After task registration, check the traffic status of various cells, handover and traffic measurement indices related to cell quality to discover the cells with potential interference. What needs to be mentioned is that only the existence of the potential interference can be judged according to the checking results. To find out whether there is interference 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 each cell, the reason is that this index can show the TCH mean occupied time (s”), which is usually called “TCH mean holding time” in the BSC of other manufacturer”, within the measurement 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 strong interference in the cell, causing that handover/call drop happens due to bad quality after 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 the cell under measurement. Generally, we can divide the handover data into two categories for analysis, intra-cell handover and inter-cell handover. 

Inter-cell handover

There are many reasons of MS triggering handover The handover measurement indices 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 the mean receiving level is ≥ 25(-85dBm), then it is possibly caused by uplink interference. When certain cell initiates handover, if the times of receiving quality level above 5 is more 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

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more than 10% of total handover attempt times, then there may be interference in the cell. The two indices are all related to quality handover threshold and interference handover 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-cell handover is caused by uplink/downlink quality, and the proportion taken up by total intra-cell handover times among the total inter-cell handover times is comparatively higher 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 the handover more sensitive, and cause more handovers. And on the contrary, handover times will be reduced. Too few handover times maybe no good to the network performance at whiles, and affect the handover success rate directly. However, too many 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 data measurement, the proportion that one handover per conversation will be comparatively reasonable.

3. Discovering potential interference via call drop index Call drop is one of the network faults un-acceptable for the subscribers. The measurement indices related to call drop are as follows: SDCCH/TCH call drop times, wireless link broken times when SDCCH/TCH occupied (connection failed) and mean uplink/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 possibly caused by interference. If the average receiving level during call drop is comparatively high (≥ 25), while the average receiving quality level is ≥ 6, then the cell should be listed into the interference source.

4. Discovering potential interference via interference band BTS will utilize an idle TS in a frame to scan the uplink frequencies of the frequencies used by TRX, and then make measurement to the level 5 interference band. The default 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 traffic measurement: Table 2.1 Interference band Interference band Interference band one Interference band 2 Interference band 3 Interference band 4 Interference band 5

Level scope (-dBm) -105 ~ -98 -97 ~ -90 -89 ~ -87 -86 ~ -85 ≥ -84

Compared with other measurement indices, the measurement index of interference band can reflect the cell interference situation more directly, but it can only reflect whether 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 measurement values mainly distribute in interference band 1 and interference band 2, then the possibility of interference will be small. However, if there is comparatively high value in band 3, then attention should be paid to this.

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Note: As Huawei’s measurement method of interference band is based on cell, the value of interference band of certain TRX that suffers serious co-frequency interference will be comparatively small in large site type (S8/8/8). The reason is that this 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 to locate the interference source, analysis must be performed to the alarm information first. Before implementation of any optimization works, it is wise to eliminate hardware fault 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, MS model, 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 is reasonable if the subscriber cannot tell you where the interference is, the reason is that they don’t know much about the cellular network actually. When there is interference 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 cannot be put through, etc. Thus, when many subscribers within the same area complain the same problem, then work should be done to check whether there is interference in the area.

1.6 Discovering Interference via Drive Test Drive Test is a method that is most commonly used to find out problems. The suspicion 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 actual implementation, there are two Drive Test methods available: idle mode test and special mode test. Under the idle mode, the test equipment can measure the signal level of both serving cell and adjacent cell. In addition, the equipment can also perform frequency scanning 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, receiving qualities, power control registrations and time advance, etc. of both the serving cell and adjacent cell. When high level (≥ 30) and low quality (Rx_Qual≥ 6) remain in certain section of highway, then, it can be concluded that interference exists in the section. 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 ANT is inaccurate.).

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Chapter 2 Location and Clearance of Interference Source The most important process during optimization is how to locate the fault in the network which requires much experience. The above section only reveals that the interference may exist in the network, but the causes of interference vary such as cofrequency 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 Clearance of 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 the occurrence 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 the interference comes from the hardware or external of the network. It is recommended to check the hardware first, if interference still exists after the hardware fault is excluded, then check whether external interference exists (the method for checking external 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 congestion rate may be relevant with the equipment fault, check of OMC alarm record will spare much time spent in judgment and analysis. Similarly, the association of alarm record with 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 as TRX no power output, etc. For most potential fault in optimization such as TRX or CDU receiving performance degradation, self-excitation, etc., the alarm information cannot be reported. (Compared with digital components, the analog components such as 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 and cells around. First you should be very clear about the BTS location distribution and azimuth of each cell, then make a drawing of the topology map, and mark the BCCH/TCH frequency and BSIC. At the same time, make a comparison between the planned frequency and actually configured frequency in BSC to check whether there is discrepancy. Generally you can judge whether there is co-frequency and adjacent frequency interference according to accurate topology map of frequency planning.

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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 cell relation. If CRO is set too larger than the neighbor cells, MS is drawn in this cell with in idle mode even the actual Rx level lower than the cells around, so once C/I cannot be more 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 to handover to the cell with better signal level and quality, this will also lead to interference. 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 more dangerous if the value is too small, the frequent handover not only increases the call drop rate, but also the system load, even leads to disastrous result, i.e., BSC breakdown.

2.1.5 Drive Test Drive Test is an effective way to locate the interference. The method is the same as problem location in 3.3. The difference is that the cell with interference is highlighted during 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 clearance will 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 is required 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 and so 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 the real antenna damage or performance degradation will lead to poor communication quality. Case 0010761. Fault description: There are 5 BTSs for a certain network in a county configured as S4/4/4 and 6/6/6, the BTS type includes BTS20 and BTS30. The interference band 5 in TCH measurement function of some cells is over 15, and there is no alarm information in OMC Fault location process

1)

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

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problems, it is found that interference band 5 mainly occurs in daylight, and in the 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 is stopped. It can be judged from this phenomenon that the interference comes from internal network and has nothing to do with other equipment. 3) No frequency in the network and data are modified before the interference occurs, accordingly, the interference is irrelevant with the frequency planning. 4) It can be seen from the above second and third points that the problem is relevant 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 and rise 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 the spectrum signal of RXM test interface, it can be seen that interference exists all the time. This indicates that the interference is relevant with the antenna feeder (including divider, combiner, feeder, antenna, lightning arrester, Tower Top Amplifier, jumper and connector) instead of the board. 7) Since the above BTS under test has the Tower Mount Amplifier, the antenna and feeder check is inconvenient, replace another BTS30(S4/4/4) (dual-CDU, and dual-polarization antenna) with interference and check the antenna feeder. 8) Since no interference exists in one of the cells while strong interference exists in another 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 without interference in the BTS in the evening. Then send idle BURST, it is found that the interference follows the antenna and feeder. This step helps further locate the fault which should exist in antenna and feeder system. 9) The situation remains the same even after replacing lightning arrester of antenna feeder and checking all jumper connectors. Then it can be sure that fault exists in the feeder or antenna. 10) Replace the jumper (i.e., antenna) at the top of the tower, it is found that the interference follows the antenna, so the feeder fault can be excluded while the antenna fault is quite possible. (It should be noted that the external interference at this step cannot be excluded because the actual installation place of the antenna does not change, but the external interference has already been excluded 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 in another BTS20 with a new one, and then the interference disappears, thus the problem is solved here. Sometimes the alarm cannot be reported to the OMC alarm console after the antenna is damaged. While the antenna damage will lead to the degradation of the radiation performance and inter-modulation. And the inter-modulation product is fed back to the receiving channel of the BTS, generating interference and effect the conversation quality. 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 bring high loss of radio signals. Fault cases in the on service network indicate that the possibility for water seepage of antenna and feeder is less than that of antenna damage, while the former often leads to the reduction of service range. And no case that interference is caused by water seepage of antenna and feeder till now.

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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 section from 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 is difficult to access cell 2, and also call drop often occurs during conversation. It is found that the interference band 4 and 5 appear in cell 2 interference band of the BTS by viewing the traffic measurement. Interference of different value occur on several carriers of this cell through signaling tracing, but interference band only occurs to interference band 2 and 3 several days later. Though the conversation is not affected, the interference does not disappear. there is no alarm message in OMC system Fault location process

1) 2) 3)

4)

Check carefully the frequency planning of the BTS. The possibility of internal cofrequency and adjacent frequency interference is excluded because the BTSs in this area are not so dense and with loose frequency reuse. It is found that though interference spectrum occurs, but the interference signal level is not too high when checking with spectrum analyzer. The interference still exists after replacing carrier board, power amplification board, power board, and CDU. Subsequently the interference occurs and disappears along with the operations of screwing up and screwing off the connector at the exit of CDU. It is found that some scraps exist on the CDU output connector. Screw up the connector after blowing off the scraps, and since then, interference never occurs.

This problem belongs to the soft fault. The scraps in the connector will bring unconspicuous interference, so experience is quite important here. On the other hand, this problem reminds us of the importance of checking the engineering quality carefully.

2.2.4 Antenna Connected Inversely It is quite usually that the antenna is connected inversely, which will lead to complete difference between the using frequency and the planned frequency. It will also lead to co-frequency and adjacent frequency interference, call drop, handover failure, etc. If the frequency resources for operator is a few, antenna connected inversely will affect the network quality much more. Case 0011108 Fault description: the interference bands 4 and 5 often occur in the traffic measurement after certain BTS is on service, the inter-cell handover success rate is very low and the congestion rate is up to 5%. There is no alarm in OMC. Fault location process

1) 2)

Since the interference bands 4 and 5 occur along with low handover success rate and congestion, it is doubted that the interference causes the above phenomena. Check the frequency planning first, no problem is found. The external interference becomes the chief consideration after the frequency planning problem is excluded. Change the original used frequency 9 into the far-away frequency 94 to avoid external interference, but situation si the same. Confirmation made with the operator’s branch office indicates that the BTS is 12

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3) 4)

5) 6)

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remote and without any high-power radio equipment nearby. It looks as if the frequency planning or external interference should be excluded. Since handover failure is involved at the same time, it is found that handover failure occurs between cells 1 and 3 according to the registration of outgoing/incoming cell handover performance measurement. The congestion analysis indicates that TCH assignment failure is usually caused by uplink. After registering the traffic measurement of uplink/downlink balance, it is found that the measurement item of uplink/downlink balance for cell 1 and cell 3 focus on level 1 and 11. This indicates that severe imbalance occurs between uplink and downlink. The imbalance between uplink and downlink, in combination with much handover failure in cell 1 and 3 turn the doubt to the antenna and feeder which may be connected inversely. On-site examination indicates that the antennas of cell 1/2/3 become crossed pair which causes the transmitter antennas of cell 1 and 3 to stay in the same cell, while the receiver antennas of them connect to another cell. The interference band and congestion disappear and the handover is all right after it is corrected.

Case 0005237 Fault description: the Drive Test for certain network reveals that the BCCH frequencies of several cells is different from the design, the adjacent cell relation is disordered with serious co-frequency interference. Moreover, with handover success rate affected, the conversation quality is poor, and the call drop rate is very high. On-site detection reveals that the connections of antenna feeder system in several BTSs are disordered. Verify each cell with the test MS, it is found that 3 sector cell of some BTSs are configured correctly but rotated 120 clockwise, and crossed pair exists between two cells of some BTSs, causing the TX/RX of two cells to cover the same area in the same direction. Fault location process

1) 2) 3)

4)

Make clear the frequency of several cells according to the networking plan design, and locate the erro with the test MS on site. Two methods can be adopted to correct the connection error and verify the accuracy. Method 1: there is one length flag every other 1m on 7/8 feeders, through which the successive two length flags on 7/8 feeders corresponding to each antenna in each cell at the tower top can be observed and recorded. So the increase or decrease of flag can be judged in case of cabling of each feeder from the tower to the equipment room, then check this flag at the side of lightning arrester of indoors antenna. The length for each feeder from the tower top to equipment room is basically the same, so the length flags at the two top and at the side of lightning arrester can be used to judge to which cell does the feeder belong. Correct the connection at the lightning arrester of the antenna after clear judgment is obtained. Finally correct the incorrect flag. Method 2: the feeder flag of some BTSs may be blurry due to scratch in construction, or no method is available to judge whether the feeder connection is correct. In this case, Drive Test can be adopted for judgment. The judgment for TX/RX feeder of one cell is easy and whether the connection is correct can be known from the results of Drive Test. But RXD transmits no signal, it is unable to know whether the antenna feeder is correctly connected through the Drive Test. At this time, switch off the HPA of all carriers in the cell, and connect RXD feeder to TX/RX output of the CDU where the main BCCH of cell is located, then switch on the HPA for Drive Test. If correct frequency is received in the specified cell, the connection of RXD feeder in this cell is correct, otherwise, it is incorrect. From the Drive Test, the cell to which the RXD feeder belongs can be obtained.

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5)

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Correct the connection at the lightning arrester of antenna and attach new flag again after making a clear judgment of all feeders. Verification with Drive Test is required after correction.

2.2.5 Incorrect Jumper Connection of BTS There are many jumpers from BTS TRX to the antenna, the confusion of which will lead to high call drop rate. Case 0015303. Fault description: a newly constructed BTS is of S333 type and dual-CDU configuration with the version as 05.0529. The subscriber complains that it is difficult to make MOC or MTC in the areas covered by this BTS since the on service. In the traffic measurement, the SDCCH call drop rate of one cell in this BTS is up to 50%. There is no alarm in OMC Fault location process

1) 2)

3)

4)

5) 6)

The causes for the above phenomenon may include: interference, BSC data error, and hardware fault. The location will focus on these 3 aspects. The dial test carried out by maintenance engineer of the operator indicates that this fault phenomenon occurs in cell 3. The “tick” always occurs in the MS and the MS returns to the idle mode when it initiates a call, moreover, only one in four and five times of calling is successful. From the traffic measurement, it is found that SDCCH call drop rate of this cell is very high, i.e., 50%, and moreover, all causes are radio link loss connections (error indication). But TCH assignment is normal, so it can be judged that SDCCH call drop is the reason why the MS is difficult to make MOCor MTC. Further analysis of the traffic measurement indicates that the interference band of this cell is normal, so the impact of interference on SD call drop can be excluded. Considering that this BTS is a newly on service, the check should base on data and BTS hardware. A careful check of the data of this BTS including hardware data and nework planning data shows that there is no error. It is found that there are 3 carriers in 3 cells of this BTS with dual-CDU configuration, the input of two carriers, which should be connected to the combiner input end of CDU, is connected to TX-COMB and TX-DUP ends instead. The fault does not exist after the connection is corrected.

2.2.6 TRX Fault As the core component of BTS, the fault of TRX will lead to the increase of interference, decrease of coverage, access difficulty, etc. Case 0011519 Fault description: the subscriber reports that the conversation quality in the area near certain BTS in certain city is poor and with call drop. From the cell measurement function report, it can be found that the average idle TCH number of interference band 1 is 11.44 when BTS cell 2 is busy; that of interference band 2 is 32.27; that of interference band 3 and 4 is 0 all the time; that of interference band 5 is 7.2. At the same time it is found that the number of TCH assignment failure of this cell is up to 50, and the call drop rate is about 10%. Since this site is on service for two years, and running always stably . The BTS type is BTS20(M900) with the configuration of S6/6/6, and the first 4 TRXs of each cell are connected to the “4-in-1” combiner via rigid RF cable, while the last 2 are connected to the “2-in-1” combiner. The duplex TMA is installed in the antenna and feeder system. No alarm information has been generated recently.

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Fault location process

1)

On-site dial test and analysis of traffic measurement indicate that this is an interference case. Generally, the number of uplink TCH, TCH congestion and call drop rate will increase when interference exists. But further location is necessary to determine whether it is external interference. 2) For external interference, the interference source can be located through the signal strength within uplink band with spectrum analyzer. 3) The cause of internal interference is basically the same with that of adjacent frequency interference. But the following will also lead to internal interference: the performance degradation of RF devices such as antenna, Tower Mount Amplifier, lightning arrester, combiner/divider, TRX board, and so on, as well as the loose contact of RF cable. This problem can be solved by replacing corresponding boards and devices. 4) The co-frequency/adjacent frequency interference can be excluded because the frequency has already been rationally planed by the optimization engineers. The external interference is suspected. Then carry out a test for the signal strength of whole uplink band in the air under the tower with spectrum analyzer, and no stable signal of more than -100dB is found. This indicates that the interference is generated within the network. 5) Check the connection of antenna and feeder, no such antenna connected reversely or loose contacted. 6) Then replace the duplex TMA of cell 2 with normal Tower Mount Amplifier, the interference band has no change, it indicates that the interference is not caused by Tower Mount Amplifier. 7) Replace related devices in cell 2 with normal SPL, combiner and RF cable, the interference band has no change, too. So it can be sure that the interference is not caused by the above devices. So the location focuses on the antenna and such boards as TRX, FPU, HPA, etc. Since replacement of antenna is quite troublesome, the board should be located first. A valuable phenomenon is found when interference is located through blocking the baseband channel. That is, the number of idle TCH in interference band 5 of the cell becomes 0 and the congestion rate and call drop rate are lowered obviously after BT10, and BT11 are blocked at the same time. From this phenomenon, it can be sure that the interference is caused by one or several boards among TRX10, TRX11, FPU10, FPU11, HPA10, and HPA11. 8) To further locate, the FPU10, FPU11, HPA10, HPA11, TRX10, TRX11 and corresponding boards in the cabinet of cell 3 are interchanged. Then observe the cell performance measurement report. After TRX10, TRX11 and TRX16, TRX17 are interchanged, it is found that the interference band 5 of cell 3 becomes 5.2. While both congestion rate and call drop rate of cell 2 become 0, number of TCH occupation fault 1, and interference band 3, 4, and 5 0. This indicates that the faulty board is just in TRX16, and TRX17. 9) Replace TRX16, and TRX17 respectively and observe the traffic measurement. The traffic measurement obtained some time after TRX17 is replaced indicates that the interference band 3, 4, and 5 of cell 3 become 0, the congestion rate and call drop rate 0, too. It can be known that the interference is caused by selfexcitation of TRX17 internal RF circuit and performance degradation of devices. 10) No call drop occurs and the voice is distinct when performing frequency locking dial test on each cell’s BCCH frequency. Case 0005940 Fault description: congestion rate of 2 cells in BTS (S6/6/6)2 in certain county is usually up to 10%. The careful view of traffic measurement (TCH measurement function) reveals that about 7 TCHs in this cell often stay in interference band 4~5, so the interference maybe exist. Fault location process 15

OMF000403 GSM Interference Analysis ISSUE1.0

1)

2) 3)

4)

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To locate the frequency with interference, register the traffic measurement of the cell (a period of 15 minutes) first. Block the carriers one by one. The number of TCH in interference band 4 ~5 becomes 0 when blocking carrier 8 (frequency 28), indicating that the interference is from the frequency. Connect the antenna and feeder of cell 2 with spectrum analyzer, set the central frequency is frequency 28 (895.6MHZ). No external interference exists through observing the level value. So the interference may come from the equipment. Interchange the carrier board with interference in cell 2 with normal carrier board in cell 3, then observe the traffic measurement. It is found that the number of idle TCH in interference band 5 of cell 3 becomes 0, while that in cell 2 becomes 0, too. So it can be sure that the carrier board fault leads to the interference. Use Maintenance Console—GSM Interface Tracing—ABIS Interface Tracing, and set the filtering condition to TRX management message. Then it can be found that the radio resource indication of all timeslots in this carrier reports interference band 5. This helps further locate that the carrier board is the interference source. The problem is solved after this faulty carrier board is replaced.

5) The self-excitation of carrier board or the invalidation of devices will bring strong interference for the receiving device. So large amount of assignment fault leads to high congestion rate of the cell. Case 0007646 Fault description: The interference of a cell always falls into interference band 4 and 5 through the analysis of traffic measurement of certain network, and carrier 28 is under the interference via tracing and analysis. Fault location process The interference source usually comes from radio communication system as following. 1)

2)

3)

Carry out a test for the electromagnetic environment of the BTS with spectrum analyzer. Set the start scan frequency to 895MHz and end scan frequency to 896MHz for the spectrum analyzer, then carry out the test in different time segments. No continuous interference signal falls into this band, this indicates that the interference maybe come from within the system and may be caused by the fault of certain part in the equipment. Replace the carrier board with interference and power amplifier board to another cell at the same time, and register the traffic measurement of 5 minutes. The interference disappears from original cell and occurs in new cell, indicating that the interference may be caused by self-excitation of carrier board or power amplifier board. Mark the carrier board and power amplifier board respectively, and replace them to the two carriers free from interference. Then register the traffic measurement of 5 minutes for tracing and observation, it can be found that the interference occurs to frequency where the carrier board with mark is located.

This problem is also caused by TRX self-excitation.

2.2.7 Clock Unlocking The clock for the digital system similar to GSM is just like the nervous system of the equipment, so the clock unlocking of BTS will cause the BTS to “go mad”. On one hand, the large deviation of BTS clock will bring difficulty for the MS to be locked in the frequency of the BTS, and lead to MS handover failure or unavailability of residing in the cell of the BTS. On the other hand, it will cause the BTS to be unable to decode the signal correctly. It should be noted that the clock out-of-lock will not bring interference, but the increase of error code in transmission will lead to the decrease of

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voice quality. Case 0017590 Fault description: The customer adopts transmission timeslot multiplexer to save the transmission resource. After being used for some time, the slot multiplexer is damaged because of the water seepage into the equipment room. After it is replaced, all BTS 13M clocks under the combiner are out –of-lock, and interruption and cacophony occur during conversation. The inter-BTS handover is unavailable and call drop rate rises up. Many alarms of 13M clock out-of-clock occur in OMC alarm console. The BTS is in free oscillation status through query of the TMU status of BTS. Fault location process

1)

2) 3) 4) 5) 6)

7) 8)

Generally BTS clock out-of-lock is caused by the degradation of transmission quality and rise of BER. The clock out-of-lock of such a large configuration BTS, according to the former experience and the phenomenon of the problem, it is not transmission equipment only but the sharing equipment relevant with these BTSs. Since these BTSs are under different modules of the BSC, in addition, these modules also support the BTS of other cities (where no clock out-of-lock is found), so BSC clock problem can be excluded. So emphasis should be laid on the physical link from the BSC to inter-BTS Abis interface. The check of alarm box shows no transmission alarm. Check the transmission of each BTS and test, The BER is not too high. Check BSC clock, the clock is in normal tracing status. Check of DDF shows that grounding is good and DDF combines grounding with the BSC. Disconnect the BTS under the slot multiplexer, and connect it with the BSC directly via the jumper, skipping the slot multiplexer. It is observed that the BTS clock changes from pull-in status to tracing status. After removing the jumper and restoring the original connection, the BTS clock is out of lock. So the fault is relevant with the slot multiplexer. Check of the program in slot multiplexer shows that all ports are correctly defined. Check of grounding of slot multiplexer shows that the power grounding is all right, but the protection grounding is not connected. All BTS clocks can trace the upper-level clock normally after connecting the protection grounding, so the problem is solved..

The diversity of transmission connection from the BSC to the BTS will easily lead to BTS fault because of the problem of certain process in the connection. Detailed understanding of the network is quite necessary for solving the fault, and fault phenomenon can be used to judge the possible causes. Especially the poor grounding or contact will generate potential difference which will lead to the degradation of transmission quality and affect the stable running of overall network.

2.2.8 Summary The fault of any one of the TRX, CDU, feeder, antenna, jumper, and connector will lead to interference and call drop. This is also proved by large amount of related cases. So check and clearance of BTS hardware fault should be performed first as interference is found. And in addition, BTS clock out-of-lock will also lead to interference and call drop. The hardware fault, in most cases, can be easily located and handled by exchanging the board and analyzing the traffic measurement data. Certainly the adoption of spectrum analyzer will help locate the problem faster. In the case that interference occurs suddenly in some cells during the running of network data to which no data modification is made, emphasis should be laid on the clearance of hardware fault.

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The above interference related to the BTS, is mainly caused by third order intermodulation, belongs to the uplink interference and will be directly reflected in the interference band.

2.3 Intra-network Interference GSM intra-network interference mainly comes from co-channel/adjacent channel interfereces. It is unavoidable when C/I