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					       OMF000403
GSM Interference Analysis
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OMF000403 GSM Interference Analysis
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                                              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
Chapter 2 Interference Sources ...................................................................................................... 3
   2.1 Classification of Interference Sources .................................................................................. 3
        2.1.1 Natural Noise.............................................................................................................. 3
        2.1.2 Man-made Noise ........................................................................................................ 3
   2.2 Main Interference Sources Affecting Mobile Communication ............................................... 4
Chapter 3 Discovery of Interference............................................................................................... 6
   3.1 Discovering Interference via OMC Traffic Measurement...................................................... 6
   3.2 OMC Alarm and Subscriber Complaint ................................................................................ 8
   3.3 Discovering Interference via Drive Test ................................................................................ 8
Chapter 4 Location and Clearance of Interference Source .......................................................... 9
   4.1 Recommended Procedures for Location and Clearance of Interference ............................. 9
        4.1.1 Determine Interference Cell according to Key Performance Index (KPI) .................. 9
        4.1.2 Check Alarm of OMC ................................................................................................. 9
        4.1.3 Frequency Planning Check ........................................................................................ 9
        4.1.4 Check Parameter Setting of Cell .............................................................................. 10
        4.1.5 Drive Test ................................................................................................................. 10
        4.1.6 Interference Clearance ............................................................................................. 10
   4.2 Location and Clearance of Hardware Fault ........................................................................ 10
        4.2.1 Antenna Performance Degradation.......................................................................... 10
        4.2.2 Water Seepage of Antenna or Feeder ..................................................................... 11
        4.2.3 Jumper Connector Fault........................................................................................... 12
        4.2.4 Antenna Connected Inversely .................................................................................. 12
        4.2.5 Incorrect Jumper Connection of BTS ....................................................................... 14
        4.2.6 TRX Fault ................................................................................................................. 14
        4.2.7 Clock Unlocking........................................................................................................ 16
        4.2.8 Summary .................................................................................................................. 17
   4.3 Intra-network Interference................................................................................................... 18
        4.3.1 Co-channel Interference........................................................................................... 18
        4.3.2 Adjacent-channel Interference ................................................................................. 20
        4.3.3 Interference Caused by Over-coverage ................................................................... 22
        4.3.4 Interference Caused by Aggressive Reuse ............................................................. 23
   4.4 Repeater Interference ......................................................................................................... 23
   4.5 Off-network Interference ..................................................................................................... 24
        4.5.1 Microwave Interference ............................................................................................ 24
        4.5.2 High-power Broadcasting Station Interference ........................................................ 25
   4.6 Other Phenomena Causing Interference ............................................................................ 26
   4.7 False Interference ............................................................................................................... 28
Chapter 5 Anti-interference Measures ......................................................................................... 29
Chapter 6 Interference Test Tools ................................................................................................ 30
   6.1 Brief introduction to Spectrum Analyzer ............................................................................. 30
   6.2 Directional Antenna ............................................................................................................ 30
Chapter 7 Way to Test Interference .............................................................................................. 31
   7.1 The way to Test Internal Interference ................................................................................. 31


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    7.2 The way to Test External Interference ................................................................................ 31
    7.3 The way to Search External Interference Sources ............................................................. 32




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                             Course Description
Introduction to Course
            To introduce affection brought by interference to the GSM system, fault location
            method and troubleshooting method, this course hereby lists out the possible              Comment [z1]: Enumerates
            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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                  Chapter 2 Interference Sources
2.1 Classification of Interference Sources
            The interference sources of mobile communication system/noise are mainly classed
            as follows:

2.1.1 Natural Noise
                 Atmosphere noise
                 Galaxy noise
                 Solar noise (quiet period)

2.1.2 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 2-1 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


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            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
            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.

2.2 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.




                                                                                                Sche
                  matic 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:

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            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
            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 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, interference will be
                 caused within the GSM system coverage area.




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             Chapter 3 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.

3.1 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.



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            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.
                 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 3-1 Interference band
              Interference band                                 Level scope (-dBm)
              Interference band one                             -105 ~ -98
              Interference band 2                               -97 ~ -90
              Interference band 3                               -89 ~ -87
              Interference band 4                               -86 ~ -85
              Interference band 5                               ≥ -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


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            possibility of interference will be small. However, if there is comparatively high value
            in band 3, then attention should be paid to this.
            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.

3.2 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.

3.3 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 4 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 co-
            frequency or adjacent frequency interference in the system, spurious interference of
            high-power transmitting equipment around, the self-excitation of the transmitter, etc.

4.1 Recommended Procedures for Location and Clearance
of Interference
4.1.1 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).

4.1.2 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)

4.1.3 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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4.1.4 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.

4.1.5 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.

4.1.6 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.

4.2 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.

4.2.1 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



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            1)  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.
            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.

4.2.2 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



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            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.
            For case of water seepage of feeder, please refer to case 0009057.

4.2.3 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 inter-
            modulation 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)   Check carefully the frequency planning of the BTS. The possibility of internal co-
                 frequency and adjacent frequency interference is excluded because the BTSs in
                 this area are not so dense and with loose frequency reuse.
            2)   It is found that though interference spectrum occurs, but the interference signal
                 level is not too high when checking with spectrum analyzer.
            3)   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.
            4)   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.

4.2.4 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)   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.
            2)   Check the frequency planning first, no problem is found. The external
                 interference becomes the chief consideration after the frequency planning


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                 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
                 remote and without any high-power radio equipment nearby. It looks as if the
                 frequency planning or external interference should be excluded.
            3)   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.
            4)   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.
            5)   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.
            6)   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)   Make clear the frequency of several cells according to the networking plan
                 design, and locate the erro with the test MS on site.
            2)   Two methods can be adopted to correct the connection error and verify the
                 accuracy.
            3)   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.
            4)   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


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

4.2.5 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)   The causes for the above phenomenon may include: interference, BSC data
                 error, and hardware fault. The location will focus on these 3 aspects.
            2)   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.
            3)   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.
            4)   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.
            5)   A careful check of the data of this BTS including hardware data and nework
                 planning data shows that there is no error.
            6)   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.

4.2.6 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


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            TMA is installed in the antenna and feeder system. No alarm information has been
            generated recently.
             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 self-
                excitation 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


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            function) reveals that about 7 TCHs in this cell often stay in interference band 4~5, so
            the interference maybe exist.
             Fault location process
            1)   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.
            2)   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.
            3)   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.
            4)   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.
            5)   The problem is solved after this faulty carrier board is replaced.
            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)   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.
            2)   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.
            3)   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.

4.2.7 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


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            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
            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)   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.
            2)   The check of alarm box shows no transmission alarm.
            3)   Check the transmission of each BTS and test, The BER is not too high.
            4)   Check BSC clock, the clock is in normal tracing status.
            5)   Check of DDF shows that grounding is good and DDF combines grounding with
                 the BSC.
            6)   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.
            7)   Check of the program in slot multiplexer shows that all ports are correctly defined.
            8)   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.

4.2.8 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


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            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.
            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.

4.3 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.

     4.3.1 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 4-1 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)   The frequencies assigned for the BTS are 64, and 92 (64 is of BCCH frequency).



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            2)   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.
            3)   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.
            4)   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.
            5)   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.
            6)   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)   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


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                 call can be connected) for BSC2, the fault still exists, so the data problem can be
                 excluded.
            2)   For the fault phenomenon, it is quite possible that fault occurs in the carrier
                 board. The fault still exists after replacing the TRX.
            3)   Check each process related to the voice for the BTS: TMU, DIP switch,
                 backplane cable, etc. No problem is found, so the BTS is all right.
            4)   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.
            5)   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.
            6)   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?
            7)   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.
            8)   The problem is solved after the frequency of BTS30 is modified.



     4.3.2 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:




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            In the figure, red stands for BCCH frequencies, while black for TCH frequencies.
            Fault location process
            1)   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.
            2)    Test MS indicats that channel 112 is BCCH channel in D3 cell by distinguishing
                 CGI.
            3)   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.
            4)   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.
            5)   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


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            1)   All BTSs are connected to the same BSC, and call drop occurs after cutover of
                 new BTSs.
            2)   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.
            3)   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.
            4)   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.
            5)   Call drop disappears after adjusting and loading the frequency configuration of
                 this area.

     4.3.3 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)   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.
            2)   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


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                 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°.
            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%.

     4.3.4 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)   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.
            2)   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.

4.4 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


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            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
            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)   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.
            2)   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;
            3)   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.)
            4)   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.

4.5 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.

4.5.1 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


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            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
                be judged from the above phenomenon that interference exists.
            2) 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.

     4.5.2 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)   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.
            2)   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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4.6 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)   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.


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                 From the above analysis, it is sure that the problem has nothing to do with this
                 BTS.
            2)   Make detailed dial test in B city (22 BTS totalled downtown) and C city (10 BTS
                 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”.
            3)   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.
            4)   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.
            5)   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.
            6)   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.
            7)   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.



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4.7 False Interference
            Sometimes we made great efforts to check, but only find that it is false alarm in the
            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)   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.
            2)   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.
            3)   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.
            4)   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.
            5)   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.
            6)   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 5 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 6 Interference Test Tools
6.1 Brief introduction to Spectrum Analyzer
            1)   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;
            2)   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:
            3)   Input frequency: the frequency range the Spectrum Analyzer can receive. This
                 index determines the frequency range of interference signal that the Analyser
                 can test;
            4)   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.
            5)   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;
            6)   Central frequency (F0): it refers to the central frequency of the spectrum that the
                 Spectrum Analyzer can test;
            7)   Bandwidth (SPAN): it refers to the spectrum span the Spectrum Analyzer can
                 test;
            8)   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                   Operational frequency   Sensitivity (1Hz)   Minimum resolution
                                      band                                        bandwidth
              HP8591E                 30Hz-1.8GHz             145dBm              30Hz
              HP8594E                 30Hz-2.9GHz             142dBm              30Hz
              HP8595E                 30Hz-6.5GHz             142dBm              30Hz
              HP8561E                 30Hz-6.5GHz             145dBm              1Hz


6.2 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 7 Way to Test Interference
7.1 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.

7.2 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)    First, set the Spectrum Analyzer to proper state, please refer to the chapter
                  supra.
            2)    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.
            3)    Screw out the selected connector, then use Coaxial Cable to import the output
                  signal of divider to the Spectrum Analyzer;
            4)    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:


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                 (1) The maximum interference level at antenna port without influence on system
                 = -108dBm sensitivity – 9dB co-channel interference protection= -117dBm.               Comment [oxq2]: 这里的编号是否有误?
                 (2) The maximum interference level at divider output port without influence on
                 system = -117dBm+15-3+7 = -98dBm

7.3 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)   In the cell under interference, select a test point without building obstruction.
            2)   Set the Spectrum Analyzer, and connect the directional antenna.
            3)   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.
            4)   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.
            5)   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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