92104358-Huawei-GSM-Interference-Analysis-Training.pdf

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GSM Interference Analysis www.huawei.com

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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.

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Upon completion this course, you will be able to:

 Understand the cause of interference and master the method of locating the interference source;  Master the method of overcoming interference.

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Chapter 1 Overview Chapter 2 Location and Clearance of Interference Chapter 3 Case Analysis

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Affection of Interference When there is interference in the network, the subscribers usually encounter the following phenomenon: 

The subscriber cannot hear the voice , and the background noise is too loud.

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

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

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When interference exists in the network, from the result of traffic statistic, there are some characteristic as following.  There are Level 4~Level 5 interference band in TCH measurement function , and the measurement value is more than 1.  Congestion rate is comparatively high.  The call drop rate is higher than other cells.  The handover success rate is low.

 Through Drive Test, it is found that: − It is difficult to handover . − The Rx level is high, but the quality is bad.  Through tracing the Abis interface signaling with signaling analyzer (MA10/K1205), it is found that the bit error rate is higher than other cells. HUAWEI TECHNOLOGIES CO., LTD.

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Classification of Interference Sources  Natural Noise

 Atmosphere noise  Galaxy noise  Solar noise (quiet period)  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

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Main Interference Sources Affecting Mobile Communication  Internal interference

 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.  Improper frequency planning: − Co-frequency interference

− Adjacent frequency interference HUAWEI TECHNOLOGIES CO., LTD.

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Main Interference Sources Affecting Mobile Communication  Repeater interference

 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.

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Main Interference Sources Affecting Mobile Communication  External interference

 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 out-band 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, 8 HUAWEI TECHNOLOGIES CO.,caused LTD. All rights interference will be within thereserved GSM systemPage coverage area.

Chapter 1 Overview Chapter 2 Location and Clearance of Interference Chapter 3 Case Analysis

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Discovering Interference via OMC Traffic Measurement  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. HUAWEI TECHNOLOGIES CO., LTD.

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Discovering interference via handover data  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 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.

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Discovering potential interference via call drop index  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.

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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)  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. HUAWEI TECHNOLOGIES CO., LTD.

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OMC Alarm and Subscriber Complaint  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.  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.

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Discovering Interference via Drive Test  there are two Drive Test methods available: idle mode test and dedicated

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.  Under dedicated 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 cells. When high level (30) and low quality (Rx_Qual6) remain in certain section, 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. HUAWEI TECHNOLOGIES CO., LTD.

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Recommended Procedures for Location and Clearance of Interference  Determine Interference Cell according to Key Performance Index (KPI)  Check Alarm of OMC  Frequency Planning Check

 Check Parameter Setting of Cell  Drive Test  Interference Clearance

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Brief introduction to Spectrum Analyzer  Spectrum Analyzer is a broadband signal receiver with high performance which can display

the spectrum of the receiving signals.  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.  Input frequency: the frequency range the Spectrum Analyzer can receive.  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. HUAWEI TECHNOLOGIES CO., LTD.

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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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The way to Test Internal Interference 

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.

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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.

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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.

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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.

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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.

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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.  First, set the Spectrum Analyzer to proper state.  Choose output port of divider of cell under interference.  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. − The maximum interference level at antenna port without influence on system = -108dBm sensitivity – 9dB co-channel interference protection= -117dBm HUAWEI TECHNOLOGIES CO., LTD.

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The way to Search External Interference Sources  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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Chapter 1 Overview Chapter 2 Location and Clearance of Interference Chapter 3 Case Analysis

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Antenna Performance Degradation  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

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Antenna Performance Degradation  Fault location process

 Register the statistics task of interference band of 24 hours for the cell with 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.  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.  No frequency in the network and data are modified before the interference occurs, accordingly, the interference is irrelevant with the frequency planning.  It can be seen from the above second and third points that the problem is relevant with the BTS equipment.  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. HUAWEI TECHNOLOGIES CO., LTD.

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Antenna Performance Degradation  Fault location process

 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.  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.  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. HUAWEI TECHNOLOGIES CO., LTD.

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Antenna Performance Degradation  Fault location process

 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.  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. )  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. HUAWEI TECHNOLOGIES CO., LTD.

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Antenna Connected Inversely  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.

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Antenna Connected Inversely  Fault location process

 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 faraway frequency 94 to avoid external interference, but situation si the same. Confirmation made with the operator’s branch office indicates that the BTS is 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. HUAWEI TECHNOLOGIES CO., LTD.

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Antenna Connected Inversely  Fault location process

 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. HUAWEI TECHNOLOGIES CO., LTD.

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Co-channel Interference  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.

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Co-channel Interference  Fault location process

 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. 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 Idle-mode test is made at this place. It is suspected that downlink interference may exist on BCCH frequency. HUAWEI TECHNOLOGIES CO., LTD.

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Co-channel Interference  Fault location process

 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.

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Co-channel Interference  Fault location process

 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.

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Co-channel Interference  Fault location process

 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 cochannel interference exists after the frequency is modified, the call drop and network drop disappear, and conversation quality Rx_Qual