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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OMF000403 GSM Interference Analysis ISSUE1.0

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



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



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


3) 4)

5) 6)


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)


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.

14



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


1)

2) 3)

4)


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

16



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.

17



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<12dB or C/Ia<-6dB. The adoption of aggressive frequency reuse will also increase the probability of interference.

2.3.1 Co-channel Interference Since frequency reuse in the GSM is unavoidable, co-channel interference will be easily caused when the reuse distance between two cells using the same frequency is too small compared with the cell radius. The frequency reuse in many cases, according to the experience, should be avoided.

Figure 1.3 Cellular cell

For A~D BTSs shown in the above figure, suppose that frequency N is assigned for cell A-3, then frequency N cannot be assigned to A1, A2, B1, B2, B3, C1, C2, C3, D1, D2, and D3; frequency N±1 cannot be assigned to A1, A2, A3, B1, C2, D1, and D2 (without frequency hopping). The case of co-channel interference is scarce, there are only one earlier case and one case in laboratory for reference besides the above case of co-channel interference that the antenna is connected inversely. Fault phenomenon: The co-channel interference of Huawei early 2.0 BTS (O2) in certain area leads to high call drop rate and poor voice quality. Serious interruption with occasional strong noise (whizz in general) occurs. It is after the BTS’s normal running for a certain time that the call drop occurs. Located in a little town (Du city) on the border of the city, the BTS is surrounded by the BTSs of the other manufacturer. Fault location process

1) 2)

The frequencies assigned for the BTS are 64, and 92 (64 is of BCCH frequency). In the optimizing test, the receiving quality (quality level is less than 3) is continuously good as the downlink signal level is -95dBm in the direction away from Huanggang and Du City. In the direction from Du City to Huanggang, the receiving quality is also good when the receiving level is more than -70dBm.

18


3)

4)

5) 6)


Then move forward until to the place where TA=5, the receiving quality is sometimes good, and sometimes more than 5 in about 1 minute when the receiving level is about -75dBm. And network-drop occurs frequently when Idlemode test is made at this place. It is suspected that downlink interference may exist on BCCH frequency. Carry out continuous conversation test with one test MS and scan test for 64# frequency with another MS. The test carried out again in the section from Du City to Huanggang reveals that the signal strength of 64# frequency is already less than -100dBm near the Huanggang, and call drop already had occured. But the signal strength of 64# frequency rises up to -65dBm and disappears after a duration of 100 seconds when entering the downtown area of Huanggang. So it can be judged that the co-channel interference may be from the TCH frequency of cell nearby. Carry out scan test for this frequency after arriving at the hotel, the signal level still remains high, but the conversation is not implemented on this frequency. The next day, carry out designated scan test at the place with the strongest signal of this frequency in the street of Huanggang, and test in Idle mode with another test MS. From the system message, it can be confirmed that 64# frequency is assigned to the TCH in HG08 cell with BCCH frequency 45, and the conversation is actually established once on 64# frequency in multiple times of conversation tests. The interference disappears after the application is made to the customer to modify the frequency. At the same time, the customer should adjust the adjacent cell relation of peer equipment. Carry out test again at the place where the original co-channel interference exists after the frequency is modified, the call drop and network drop disappear, and conversation quality Rx_Qual<3. It indicates that the problem of co-channel interference (downlink) is solved.

The GSM is a duplex system, so the interface may occur on both the uplink and downlink frequencies. Current Drive Test tools are unable to measure the co-channel interference directly, the equipment such as SAFCO, and ANT with so-called C/I test function actually test the C/A. And in addition, the Drive Test equipment can only measure the downlink frequency and nothing for the uplink frequency. The co-channel interference in downlink frequency can be confirmed by using current Drive Test equipment to measure it indirectly. First, lock the test MS in the cell of the service area to carry out Drive Test of 2~3 hours in continuous conversation mode. If the receiving signal in some areas is a relatively high (e.g., -85dBm) while the receiving quality is very low (e.g., RxQual>4), it is quite possible that co-channel interference exists in this channel. The interference of uplink can be judged with the help of interference band statistics data in traffic measurement Case 0008119 Fault phenomenon: It is found in the training equipment room some day: it is hard for the MS to access the network in BTS30 under BSC2. Even it accesses the network narrowly, it will drop soon . Test MS shows that the signal exists intermittently and the receiving level is very high (about -50dBm). Fault location process

1)

2) 3)

The correctness of the data configured by the trainees cannot be ensured in the training equipment room. So load standard data (which is tested to ensure that call can be connected) for BSC2, the fault still exists, so the data problem can be excluded. For the fault phenomenon, it is quite possible that fault occurs in the carrier board. The fault still exists after replacing the TRX. Check each process related to the voice for the BTS: TMU, DIP switch, 19


4)

5) 6)

7)

8)


backplane cable, etc. No problem is found, so the BTS is all right. Then focus is turned to the BSC. a BTS20 instead of BTS30 and change the data into BTS20 (in this way, overall check of BSC can be performed).And the call is set up easily , so the BSC is all right. At this time, it seems that there is no way out, but restore the BTS30, however, the MS can call again after power-on loading. It seems that the problem is solved but the cause is still not located. Careful consideration from the beginning to end reveals that all data of BSC2 and BTS30 are not modified, but BTS20 is switched off during BTS30 rollback. Think about the phenomenon of the test MS: the signal exists intermittently and the receiving level is very high (about -50dBm), it is possible that BTS20 works interference on BTS30? Since BTS20 is in BSC1, check the data of BSC1. It is found that the frequency of BTS20 is set to be the same with that of BTS30. And the BSC2 data (BTS30), configured by the trainee, is loaded in BSC1 after it is changed into BTS20, leading to the same frequency of BTS20 and BTS30. The BTS30 is connected with the attenuator, and the signal is far weaker than that of BTS20, so the MS cannot access the BTS30, while can access the BTS20 . In this way, you tend to think that it is the fault of BTS30, but the truth is hidden herein. The problem is solved after the frequency of BTS30 is modified.

2.3.2 Adjacent-channel Interference For the cells to which the adjacent channel cannot be allocated, please refer to the section supra. Case 0003451 Fault description: the customer reports that the call drop often occurs in the office during optimization of a certain area. The traffic statistic data and Drive Test data are normal, The BTS distribution and topology map of frequency planning are shown as follows:

20



In the figure, red stands for BCCH frequencies, while black for TCH frequencies. Fault location process

1)

2) 3)

4)

5)

Thorough test reveals that the level of channel 112 is up to -73dBm in the position of call drop. First we think the test is incorrect, but several tests indicates that channel 112 exists from BTS A to B, and the level of which is even up to -70dBm at some places. When the MS occupies the channel 111, call drop occurs due to interference of channel 112. Test MS indicats that channel 112 is BCCH channel in D3 cell by distinguishing CGI. Surveying in BTS D reveals that antenna of D3 cell is installed on a platform at the top of the building, while 8m away there is one house in glass structure which is 4m lower than the platform. Testing near the antenna. The transmitting signal of antenna is about -26dBm, while the strength of test signal near the glass is unexpectedly up to -14dBm. The signal by complete reflection of glass and leads to overlap ,then cause interference and call drop. It is recommended that the customer should change the installation position of the antenna.To meet the emergency: exchange frequency 111 of BTS A with frequency 114; increase the downtilt angle of antenna in A3 cell; adjust the direction angle of frequency 113 in C1 cell to avoid the interference of frequency 114 after the exchange. The test is all right after the modification. Frequency 113 of BTS C will work no interference for frequency 114, and the call drop disappears.

Case 0004034 Fault phenomenon: The call drop ratio is universally high and even up to about 15% in busy hour after several BTSs are cut over. And it is difficult for the call to be set up during on-site test. There is no alarm message in OMC system Fault location process

1)

All BTSs are connected to the same BSC, and call drop occurs after cutover of 21


2)

3)

4)

5)


new BTSs. The transmission quality is good, and TRX test is carried out for the above BTS with call drop and this indicates that each TRX is all right. No fault is found by checking the data and carrying out the test for 32BIE port corresponding to the BTSs. From the above analysis, the TRX fault, BSC hardware fault, A interface circuit fault and transmission fault can be excluded. Analysis of traffic measurement result reveals that serious interference occurs in each cell of the above several BTS. Most cells have measurement values in interference band 4 and 5, and the number of channels falling into interference band 5 in several cells is up to 7. So it is sure that the interference in the above several cells is quite serious. It is found that there are many adjacent channels and the frequency planning is irrational after checking the frequency configuration of above BTSs and the adjacent cells. Especially, the area where the above BTSs are located, is newly added, and interference exists among them. And they also have interference in between them and the surrounding running BTSs. Call drop disappears after adjusting and loading the frequency configuration of this area.

2.3.3 Interference Caused by Over-coverage The objective of one rationally designed network is that each cell only covers the area around the BTS and the MS resides in the nearest cell. The over-coverage is that the service range of a certain cell is too wide that the signal level at an interval of more than 1 BTS is still strong to let the MS reselect or hand over to it. It is a phenomenon in which the actual cell service range deviates from the designed service range, leading to irrational traffic loading, interference, call drop, congestion, hand over failure, etc. Case 0005164 Fault phenomenon: The hand over success rate of a certain GSM network is low, the call drop rate is high and conversation quality is poor. The hand over success rate is less than 80%, and the call drop rate is more than 2%. It is found that there are many times of downlink/uplink strength hand overs through view and analysis of traffic measurement data. while there are many times of bad downlink quality, and uplink strength among the times of unsuccessful handover. The analysis of cause of call drop indicates that the times of bad downlink quality are more than those of bad uplink quality . There is no alarm message in OMC system Fault location process

1) 2)

From the result of traffic measurement, it can be judged that downlink interference may exist in the system or the coverage is not very good. The actual result of Drive Test shows that the strength of outdoor signal can be up to -80dBm above in the downtown area, that is to say, the coverage is all right. But serious over-coverage exists. For example, the service cell used in the building where BTS A is located is cell B with the same BCCH frequency as cell A1, while cell B is 6 kilometers away from BTS A in the suburb. In this way, the problem exists in two aspects: 1. The signal of cell B forms co-channel interference which leads to poor downlink link quality in coverage area of cell 1 of BTS A. It is found that ”****” is displayed in the test MS when this cell is locked during the test. 2. When cell B is selected as the service cell, its adjacent cell is only geographically adjacent to it, while the cell near the BTS A does not function as its adjacent cell. So when its signal is unavailable, the “effect of isolated island” will occur because the signal of its adjacent cell is poor, too. Then hand over fault and even call drop will easily occur. The on-site survey shows that the antenna of cell B is hung 50m above. The data provided by the customer reveals that the tilt angle is 5°, which is actually far less than 5°. 22


3)


The cause for bad network indices lies in over-coverage, so the basic way is adopted to lower the antenna and adjust downtilt of it to make actual coverage area consistent with planned coverage area. Temporarily this problem can only be solved by adjusting the network parameter. The following operations can be adopted: lower the power level of cell B and add the adjacent cell of cell B, at the same time increase the level threshold of candidate cell from 10 to 15. The network indices exhibit obvious improvement after the above check and modification are performed for all cells in the downtown area, moreover, the hand over success ratio rises up to 85%, while call drop ratio drops to 1.3%.

2.3.4 Interference Caused by Aggressive Reuse The capacity and quality are a pair of contradictions. In the downtown area, there are large number of subscribers, so the frequency planning technology of tight reuse should be adopted to satisfy the requirement of capacity. That is, the increase of capacity is traded by the sacrifice of quality. The adoption of tight reuse in the place where the layout of some BTSs is irrational will easily lead to collision of co-channel or adjacent channel. Case 0017397 Fault phenomenon: The interference of frequency hopping is serious and the conversation is intermittent on a bridge, this network adopts the mode of 1*3 frequency hopping. There is no alarm. Fault location process

1) 2)

The signals on the bridge are disorderly due to distribution of multiple BTSs around the river. So the interference of frequency hopping will easily occur when adopting frequency hopping. First, adjust the antenna to make one main cell on the bridge, adjust the antenna and transmission power of other cells to reduce the strength of their signals reaching the bridge. Then carry out the test. It is found that though the interference is reduced, yet the interference of frequency hopping still cannot be under control because all signal receiving levels reaching the bridge are very high due to the clearance around the bridge. Finally modify the frequency plan, and adopt the frequency hopping plan of A+B, i.e., adopt 1*3 frequency hopping for some cells, while 1*1 frequency hopping for other cells. The interference of 1*3 frequency hopping occurs in co-directional cell, while 1*1 frequency hopping averages the interference. A combination of them in some local areas is better for the dispersion of interference. The actual test result indicates that the conversation quality is improved significantly.

2.4 Repeater Interference Repeater really exhibits a little convenience, so customers often make use of it. The repeater is also one of the main interference sources. Case 0017086 Fault phenomenon: subscribers of a certain network complain that they cannot occupy channel in some area for conversation since some day, or noise is heavy after occupancy of channel though the signal of MS is strong at this time. There are two directional BTSs in this area and both of them are BTS30 with the version 05.0529. The azimuth of the first cell directs at north. The BTS in this area works normally and the network indices conform to requirement before the complaint. Both BSC and MSC are devices of Huawei and two BTSs are connected in star mode. The traffic measurement indices show that traffic of both BTSs decreases obviously after occurrence of this problem, especially in the first and third cells. Although the signal of

23



channel is very strong, the quality of voice is poor. Then it can be seen from traffic measurement that the interference band of these four cells is in class three, four, and five, and 95% of channels are under interference. In addition, interference of different classes also exists in other cells. So subscribers complained strongly. And there is no alarm message in OMC system Fault location process

1) 2)

3)

4)

The feedback of subscribers shows the possible causes as follows: 1. Problem occurs in transmission and leads to error code; 2. Problem occurs in antenna feeder; 3. Fault exists in TMU; 4. Internal or external interference may exist. The traffic measurement console shows the possible reasons as follows: 1. There may exist strong uplink interference signal in the north lean to west in this area. This leads to interference of different levels in the first, second and third cells, especially in the first and third cells; It is found that it is difficult to put through the call in the first and third cells through on-site dial test. Although the call is put through, the quality of voice is very poor, and the voice is intermittent seriously with strong interference. If MS subscriber calls fixed telephone subscriber in this area, it is hard for fixed telephone subscriber to hear the voice clearly, instead, MS subscriber can hear fixed telephone subscriber clearly. This also proves the above analysis that the interference may be external (It can be judged from this point that interference only exists in uplink.) Carry out test on site with antenna feeder analyzer. No problem of BTS itself is found, and the situation remains the same after TMU is replaced. Therefore, we ask customers whether there are such newly constructed equipment as microwave station, repeater, etc. surrounding the BTS. They told that they didn’t set up them. It is said that China Mobile sets up a repeater in the area, which is located about two kilometers away in the north lean to the west of both BTSs of Huawei, and when it is activated, the problem will occur in Huawei BTS. Then customers negotiated with China Mobile several times. Finally China Mobile carried out on-site test with Huawei engineers together, and found that only if the repeater is switched off, the interference band and call become normal along with the recovery of Huawei BTS, if the repeater is activated, problem will occur soon in Huawei BTS, i.e. call cannot be put through, or interference is strong even after call is put through. The traffic measurement relevant with interference band in two different cases was handed on to China Mobile, and they approved our viewpoint. Finally the problem is solved with the coordination of China Mobile.

2.5 Off-network Interference External interference sources are TV station, high-power broadcasting station, microwave, radar, high-voltage power line, analogue BTS, CDMA, GSM BTS of another operater, etc.

2.5.1 Microwave Interference Case 0001084 Fault phenomenon: it is found that call drop rate in the second and third cell of a certain BTS (S2/2/2) in traffic measurement increases abruptly. Call drop rate is up to about 20% at some time. Fault location process

1)

View of BSC traffic measurement shows that idle TCH number in interference band in this BTS begins to increase in interference band 3-5 at about 8:30, in interference band 4, 5 at 10:00, and in interference band 1 at about 22:00. It can 24



be judged from the above phenomenon that interference exists. Since this BTS operates well before, the problem of frequency planning can be excluded. 3) Perform power-off restart and replace board for BTS. But interference still exists, so the possibility of TRX self-excitation can be excluded. 4) TRX management information reveals that interference exists in four boards of the second and third cell in this BTS, and the possibility of damage of the four boards at the same time is little, so the problem of TRX can be excluded. TRX board is replaced for caution, but interference remains 5) View of all BSC traffic measurement data shows that interference of different levels exists in the cells of all BTSs nearby, which is facing the same direction as the second and third cells in this BTS. And sometimes SDCCH channels in the cells with serious interference are all occupied at the same time, while the occupancy ratio of SDCCH at the same time is very little according to the amount of subscriber. So it can be sure that external interference exists in uplink, but the interference may be relevant with direction instead of frequency. 6) To further locate, jumpers of the first and third cell are interchanged on the rack top. As a result, it is found that interference occurs in the first cell, but interference disappears in the third cell, so this has proved the above judgement. 7) Since interference is not relevant with frequency, BTS interference may be caused by high-power signal sent into BTS system. 8) Measurement on BTS divider output port with spectrum analyzer shows that high-power signal exists on 904MHz frequency (5M away from the used frequency), and this signal level come up to about –25dbm in BTS with serious interference, while in other BTS it is about –50dbm. So it can be judged that this signal has impact on BTS. 9) After frequency scanning around BTS with spectrum analyzer, it is found that a microwave antenna outputting high-power signal is at a frequency of 904. 10) When the microwave equipment is switched off and TRX management information is traced, the interference disappeared.

2)

2.5.2 High-power Broadcasting Station Interference Case 0000279 Fault phenomenon: the quality of MS call is very poor in the morning of the third day (Monday) after a BTS cutover, and it is hard to make calls 2 km away from the BTS. The voice is under a serious interference, and the complaint of subscriber increases obviously. Interference band is found large in traffic measurement of OMC. Fault location process

1) 2) 

When Drive Test is carried out around the BTS, it is found that MS’s RX signal level (downlink level) has little change (about –60dBm), but the signal quality RQ has too much change, it will jump from 0 to 7 and call drop occurs. The fault remains the same after the TRX corresponding to the frequency is replaced. It is suspected that fault is caused by external interference. Measurement for external electromagnetic environment with HP8595E spectrum analyzer shows that strong interference exists around the frequency of BTS. Through communicating with customer and the Radio Resource Management Committee, we get known that the interference is caused by signals transmitted by one message transmitter of some company, whose RF band is just adjacent to the planned GSM frequency. This transmitter usually transmits every Monday, and no such phenomenon is found in two days after BTS is activated.

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2.6 Other Phenomena Causing Interference Sometimes although no intra-network and ex-network interference exists, fault of some devices can also make conversation quality bad. Some cases below are analyzed detailedly. They have high reference value. Case 0015274 Fault phenomenon: BSC has six modules. The whole BSC is cascaded with 150 BTSs (BTS312, 750TRX in total). One month after the capacity expansion of BSC and MSC, customers report that there is serious voice quality problem in the whole network. There exists serious chink during conversation under many BTSs. We come to county A with serious complaint to make a dial test. Fault phenomenon is found as follows: there exists strong background noise. It mainly contains sound of metal, such as “ting, dang” and so on. One party or both parties can not hear each other clearly when the noise is serious (One party is in the range of coverage of this BTS and the other under a normal BTS. When fault occurs the subscriber under this BTS can not hear clearly, while the other can). When the voice channel of the failed BTS is occupied, all the voice channel of each cell may have sound of metal. But it occurs just at some time. The occurrence ratio is 10%. The signal of the MS is very good and there is no interference on the radio interface. The fault phenomenon occurs when the ringback tone is sent back. That is to say, so long as the voice channel is established, the fault will occur. Disturbance of different degrees will exist during normal conversation. When the fault is serious, one party (or both parties) can not hear clearly during conversation. The fault has a certain continuity in terms of time. Sometimes the probability of occurrence of metal chink is very high and sometimes fault phenomenon cannot be captured. There is no alarm message in OMC system Cause analysis: this case is about noise in voice problem. It is usually caused by error code. The possible cause for error code is the fault of boards, connectors or wiring through which the voice signal is sent. Besides, grounding, interference, clock also should be considered. Interference existing on radio link may also cause error. The clock that is unsynchronized will cause frame slip or frame loss. Different error codes have their certain regularity: as for error code on wiring, if it is A interface or upper, noise and voice have superimposed effect because PCM sample value is influenced. Therefore, the noise is regular and there is little fluctuation. If it is below A interface, error codes are also regular. However, because the compressed voice signal is influenced, it needs decoding before hearing. Therefore , there is great noise fluctuation. Some words are normal, while some words can not be recognized, such as bubbling sound, discontinuity, sound of metal and so on. As for frame slip or frame loss of clock, it has a time regularity. For example, one frame lost in several minutes. Therefore, noise occurs in time during conversation. Combine the description of fault phenomenon, make specific locating processing, refer to part of processing course. Fault location process

1)

2)

Check the BTS of county A: test transmission error code and find transmission is without error code; check TRX, FPU, combiner. There is no fault. All voice channel of each cell may bring about metal chink. So the possibility of occurrence of TRX board fault can be excluded. MCK board is normal. BTS clock state shows “slow pull-in or tracing”. The clock is normal. So the possibility of bit slip or frame loss caused by clock fault can be excluded. Trace Abis interface information, judge the receiving quality of radio interface is good without interference according to the measurement report. The result of traffic measurement also shows that there is no interference. Use Site master to test antenna feeder system. The Standing Wave Radio is normal. BTS earth is good. From the above analysis, it is sure that the problem has nothing to do with this BTS. Make detailed dial test in B city (22 BTS totalled downtown) and C city (10 BTS 26


3)

4)

5) 6)

7)


in total in downtown) and find that some BTSs have the same fault phenomenon. Analyze the commonness of each failed BTS and find that these BTS are all under BM4. Make sure whether all BTS under BM4 have this case. Choose other BTSs under BM4 to make dial test and find every BTS has this problem. All BTSs under other five BM modules have no this case through dial test. The problem of metal chink is related to BM4 through above location, and the problem exists in one certain process of BM4 “voice signal transmission path”. Transmission path of voice signal: MS—— radio link (including antenna feeder system)—— BTS—(E1 line)— BTS_DDF—— trunk transmission—— BSC_DDF—(E1 line)—32BIE—(HW line)—GNET——GOPT—(optic fiber)— GFBI——GCTN——E3M—(E1 line or transmission device)—MSM——FTC—— MSC. Adopt locating method of analizing section by section. At first analyze and see whether there is something wrong with swithing network board (GNET and GCTN) and fiber interface circuit: check the connection and connector between AM and BM4. They are normal. Use absolute alcohol to clean the fiber connector, but the fault remains the same. Exchange the third and fourth FBIs (including FBC) corresponding to BM3 and BM4 with the first and second FBI board corresponding to BM1 and BM2, but the fault remains the same. Change BM4 GOPT, but the fault remains the same. The possibility of existence of circuit error code in the part “OPT—(fiber) —FBI” can be excluded. Swap BM4 GNET board, the problem still exists; swap GCTN board, the problem is still; exchange GNET board with BM3, the problem is still; exchange E3M board with BM3, the problem is still. The same fault exists in all BTS under the whole BM4. Error code should not exist on HW Lines between 32BIE and NET board. That is because it is impossible that all HW Lines between 32BIE and GNET board are not good. Check the connection line of BM4 from E3M to MSM carefully, and find that the connector of E3M is normal; check one connector of MSM and find that the quality of the connector of trunk lines behind one MSM (board No. is 3) is bad. Deformation and invagination occur on the inner pin of one connector. The result is that the connector cannot connect the connector of MSM board on backplane well. The cause lies in here. When the subscriber occupied TCSM under BM4 sound of metal may occur. The number of MSM of the failed TCSM is 3, and the number of FTC is 84, 85, 86, 87. A interface circuit CIC is 2336~2463. Redo the trunk connector, the fault is eliminated. During the conversation under all BM4 there is no sound of metal.

When MSC assigns circuit, the circuits in the module that the signaling link belongs to will be selected with higher priority. If the signaling of the MS call is transmitted through module 4 of MSC, MSC will assign the circuit of module 4 first (the corresponding CIC is from 2240 to 2815). The failed circuit (CIC is from 2336 to 2463) corresponds to module 4 of MSC. So if the signaling of call is transmitted through module 4 of MSC, the rate for being assigned to the failed circuit is very high. As for BSC, BTS of module 4 selects A interface link first to transmit signaling. There are three links on module 4 of BSC, Two of which correspond to module 4 of MSC. So if the MS is used under module 4 BTS, there is a possibility of 2/3 to be assigned to the circuit of MSC module 4. 128 of MSC module 4 circuits has fault (corresponding to BSC 4 module) (the proportion of failed circuit of MSC module 4 is 128/576) and it may bring about problems of voice quality. The highest rate of occurrence of fault (all have fault when 128 circuits is busy) is about 2/3 * 128/576 = 15 %. In fact the fault of dial test under BTS is 10 % or so.

2.7 False Interference Sometimes we made great efforts to check, but only find that it is false alarm in the

27



end. In fact the interference is false. Indeed it is better to have no interference than have it. Fault phenomenon: the reconstruction of some office found that the interference band of an S6/6/6 BTS20 in a county is very high. Theinterference band of two cells in it changed from 5 to 7. Fault location process

1)

2)

3) 4)

5)

6)

During the past interference test ofthis office, some cells with high interference band were located. This is because antenna intermodulation resulted in the interference of normal signal in case of large traffic and it made the interference band very high. So change the antenna, but the interference band did not fall down obviously after that. During the prior period, the frequency planning has been checked several times, and the ad-frequency interference has been avoided basically. At the same time the frequency that may cause third order intermodulation in the cell has been adjusted. The problem of frequency planning has been basically excluded. It is unknown whether there is something wrong with Tower Top Amplifier, feeder or connector. But no problem was found after checking the antenna feeder system of the BTS several times. Trace the state of interference band of each channel via Maintenance Console and find that high interference band mainly focuses on the four TRXs of the cell. First set the frequency with less interference to the TRX with more interference, and find that the interference band does not change. It shows there is no relation to the frequency. Whether there is something wrong with TRX board or not is unknown. However, the situation is unchanged after the TRXs were interchanged. Then confirm the TRX numbers, and find that the four carriers come from the same divider. From the above process, the problem may be the divider. But it is found that the interference band becomes high when the BTS is expanded from S4/4/4 to S6/6/6. While the combiner/divider used to be normal. Maybe the devices break down after some time. So we decided to change the combiber/divider to have a try. The idea that four carriers with high interference band are connected to the same second level divider before the divider is changed. The 7-dB gain of the divider has been considered during interference judgement. However, if the DIP switch is not moved to the right place while cascaded, the interference signal will be amplified by 7dB. Simultaneously, higher interference band will exist because the calculating error of 2.0 station itself is up to 5dB. Check the DIP switch of the divider. It was really not moved to “Off”. After it is corrected, view the traffic measurement of interference band 4 and 5. They fall down to 0. And there is a little value on the interference band three.

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Chapter 3 Anti-interference Measures Anti-interference Measures of GSM are: Frequency Hopping, dynamic power control, DTX. While the aggressive reuse is adopted, frequency hopping and dynamic power control should be adopted as well. To adjust the obliquity, azimuth, height of antenna is also a main optimization way to reduce network interference. The aim for adjusting is to make the real service area of each cell approach to the designed service area to reduce over-coverage. the methods of adjusting antenna please refer to relative guidance book. These anti-interference measures mainly aim at intra-network interference. When there is strong external interference, these anti-interference measures can hardly take effect. By the way, remind everybody that BSC of Huawei has the function of frequency hopping, power control and DTX by default. However, for some equipment of some other vendors, every function should be purchased by the operators themselves, and the price is very high. The currently serving network equipments of many operators have no these functions. During optimization as Third Party or co-operation to equipment of other vendors, please pay attention to this matter.

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Chapter 4 Interference Test Tools 4.1 Brief introduction to Spectrum Analyzer 1)

2)

3) 4) 5) 6) 7) 8)

At present, the main tool Huawei has for testing interference signal is Spectrum Analyzer. It is a broadband signal receiver with high performance which can display the spectrum of the receiving signals. The receiving sensitivity of signal bandwidth of xHz equals to 1Hz sensitivity+10logx, e.g., the receiving sensitivity of GSM signal of 200kHz is: -142dBm+10log(200*1000)=-89dBm; The receiving signal resolution bandwidth (RBW): namely the minimum signal bandwidth the Spectrum Analyzer can recognize. The smaller the parameter is, the higher the receiving sensitivity of instrument. That is to say, the noise of the instrument itself is less; The Spectrum Analyzer of different types has different receiving frequency band and receiving sensitivity. It is very important to make use of it correctly. A brief introduction to several key indexes of the Spectrum Analyzer is made as follows: Input frequency: the frequency range the Spectrum Analyzer can receive. This index determines the frequency range of interference signal that the Analyser can test; Sensitivity: generally the minimum receiving level with 1Hz signal bandwidth is defined as receiving sensitivity of the Spectrum Analyzer. The receiving sensitivity of HP85 series Spectrum Analyzer can be below -142 dBm. Video Filter Bandwidth (VBW): it refers to the bandwidth of the intermediate filter after Frequency mixing of the Spectrum Analyzer. The smaller the bandwidth is, the smoother the curve is; Central frequency (F0): it refers to the central frequency of the spectrum that the Spectrum Analyzer can test; Bandwidth (SPAN): it refers to the spectrum span the Spectrum Analyzer can test; Input signal attenuation (ATT): when there is large signal input, it is required to make attenuation on signal properly. The Spectrum Analyzer itself may produce large number of inter-modulation components without attenuation. So it will influence the veracity of the testing result.

The main technical indices of interference test spectrum analyzer: Model HP8591E HP8594E HP8595E HP8561E

Operational frequency band 30Hz-1.8GHz 30Hz-2.9GHz 30Hz-6.5GHz 30Hz-6.5GHz

Sensitivity (1Hz) 145dBm 142dBm 142dBm 145dBm

Minimum resolution bandwidth 30Hz 30Hz 30Hz 1Hz

4.2 Directional Antenna Directional antenna is used for searching interference sources. The stronger the directionality of antenna is, the higher the antenna gain is. And the ability to search will become better. So the logarithm-period antenna with broad frequency band is the best choice. This kind of antenna has broad frequency band, high antenna gain and strong directionality.

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Chapter 5 Way to Test Interference 5.1 The way to Test Internal Interference 1. Set the Spectrum Analyzer to proper state: For 900M BTS: f0=902MHz,SPAN=30MHz,ATT=0,RBW=30kHz,VBW=30kHz; For 1800M BTS: f0=1715MHz,SPAN=10MHz,ATT=0,RBW=30kHz,VBW=30kHz. 2. Screw out the connector of output port of CDU divider, then connect the output signal of the divider to the Spectrum Analyzer to carry out a test. If the fractional frequency spectrum level is less than –80dBm, it shows that there is no internal interference; if more than –80dBm, it shows that CDU or TRX inside Base Station are under interference or self-excitation. 3. If internal interference exists, further make sure that it belongs to CDU or TRX. At first confirm TRX carrier board, cut down the cable via which TRX is connected to divider, and use the Spectrum Analyzer to test the main or diversity connector of TRX. If the fractional frequency spectrum level is less than –80dBm, it shows that TRX is normal, otherwise it is required to change carrier board. The three steps above aim at interference measure for the uplink frequency band. If there is suspect that interference exists in the downlink frequency band, please follow steps below. 4. Check interference of transmission band. First, set the Spectrum Analyzer in transmission frequency band of the BTS. Due to the large ouput power of BTS, attenuation should be made on the input signal. Generally ATT is set as 40dB, then the tx_test signal of CDU should be imported to the Spectrum Analyzer to be observed to make sure weather interference signal is generated.

5.2 The way to Test External Interference When we are sure that interference is caused by the external cause, first we should confirm the location of interference source and the spectrum distribution state. A convenient way is to make use of the existing antenna feeder of Base Station and low-noise amplifier of Radio Frequency Front End to have a test.

1) 2) 3) 4)

First, set the Spectrum Analyzer to proper state, please refer to the chapter supra. Choose output port of divider of cell under interference. In order not to influence the normal operation of BTS, the free output port (with dummy load) is generally chosen, either the main or the diversity can be selected. Screw out the selected connector, then use Coaxial Cable to import the output signal of divider to the Spectrum Analyzer; View the spectrum distribution state of the Spectrum Analyzer, and find out the abnormal interference signal. The way to calculate the level of interference signal is as follows:

Antenna port interference level = interference level tested by the Spectrum Analyzer – 15dB Tower Top Amplifier Gain + 3dB cable loss – 7dB divider gain. For example: Antenna port interference level = - 65dBm-15+3-7=-84dBm Note: Cable loss may vary with cable length. The standard for judging weather interference level has impact on the system is described as follows: 31

OMF000403 GSM Interference Analysis ISSUE1.0 




(1) The maximum interference level at antenna port without influence on system = -108dBm sensitivity – 9dB co-channel interference protection= -117dBm. (2) The maximum interference level at divider output port without influence on system = -117dBm+15-3+7 = -98dBm

5.3 The way to Search External Interference Sources The location of interference source can be examined via BTS divider output port. If it is required to find the more detailed location of interference source, walk out of equipment room, and use the directional antenna with strong directionality mentioned above to make a search, The steps for searching are as follows:

1) 2) 3)

4) 5)

In the cell under interference, select a test point without building obstruction. Set the Spectrum Analyzer, and connect the directional antenna. If there is rotatable platform, the antenna can be placed on it, and make the wave beam of the antenna point to the front, and the antenna with vertical polarization should be placed vertically; if there is no interferent signal, one can raise the antenna over head with hands. Rotate the antenna slowly, and at the same time view the change of signal of the Spectrum Analyzer. Once there exists abnormal signal, fix the orientation of the antenna immediately and change uptilt of the antenna to make the receiving signal to the strongest. Analyze the signal spectrum distribution carefully, and confirm that it is interference signal, record the signal strength and record the azimuth and downtilt of antenna wave beam. Find new test point along with the direction of antenna wave beam, then return to step 2 to carry out a test till interference source is found.

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GSM Interference Analysis

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