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GSM Interference Analysis
GSM Interference AnalysisTable of Contents Table of Contents Course Description ................................................................................. 1 Introduction to Course………………………………………………………… 1 Course Objective ……………………………………………………………… 1 Relevant Materials .....................................................................................1 Chapter 1 Overview ................................................................................... 2 1.1 Affection of Interference on Network .....................................................2 Interferences Sources ............................................................................... 3 1.2 Classification of Interference Sources ...................................................3 1.2.1 Natural Noise ......................................................................................3 1.2.2 Man made Noise .................................................................................3 1.3 Main Interference Sources Affecting Mobile Communication ................4 Discovery of Interference ......................................................................... 6 1.4 Discovering Interference via OMC Traffic Measurement........... ............6 1.5 OMC Alarm and Subscriber Complaint.................................................. 8 1.6 Discovering Interference via Drive Test..................................................8 Chapter 2 Location and Clearance of Interference Source ...................9 2.1 Recommended Procedures for Location and Clearance of Interference……………………………………………………………………….9 2.1.1 Determine Interference Cell according to Key Performance Index (KPI) .............................................................................................................9 2.1.2 Check Alarm of OMC ..........................................................................9 2.1.3 Frequency Planning Check .................................................................9
2.1.4 Check Parameter Setting of Cell.................................................................10 2.1.5 Drive Test..........................................................................................10 2.1.6 Interference Clearance .....................................................................10 2.2 Location and Clearance of Hardware Fault..................................................... 10 2.2.1 Antenna Performance Degradation ..................................................10 2.2.2 Water Seepage of Antenna or Feeder ..............................................11 2.2.3 Jumper Connector Fault....................................................................12 2.2.4 Antenna Connected Inversely ...........................................................12 2.2.5 Incorrect Jumper Connection of BTS ................................................14 2.2.6 TRX Fault...........................................................................................14 2.2.7 Clock Unlocking ................................................................................16 2.2.8 Summary ..........................................................................................17 2.3 Intra-network Interference ....................................................................18 2.3.1 Co channel Interference ...................................................................18 2.3.2 Adjacent-channel Interference ..........................................................20 2.3.3 Interference Caused by Over-coverage ...........................................22 2.3.4 Interference Caused by Aggressive Reuse ......................................23 2.4 Repeater Interference ..........................................................................23 2.5 Off-network Interference ......................................................................24 2.5.1 Microwave Interference ....................................................................24 2.5.2 High power Broadcasting Station Interference ................................25 2.6 Other Phenomena Causing Interference .............................................26 2.7 False Interference ............................................................................... 27 Chapter 3 Anti-interference Measures ...................................................29 Chapter 4 Interference Test Tools ..........................................................30 4.1 Brief introduction to Spectrum Analyzer ..............................................30 4.2 Directional Antenna .............................................................................30 Chapter 5 Way to Test Interference .......................................................31 5.1 The way to Test Internal Interference .................................................31 i
Course Description Introduction to Course To introduce affection brought by interference to the GSM system, fault locationm e t h o d a n d t r o u b l e s h o o t i n g m e t h o d , t h i s c o u r s e h e r e b y l i s t s o u t t h e p o s s i b l e interference sources, and offers description of cases to facilitate fault location and troubleshooting. The main contents of the course a r e a s f o l l o w s : o v e r v i e w , classification of interference source, judgment 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 i n a
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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 andthe cell radius is called cofrequency 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 (iti s a l s o c a l l e d i n t e r n a l s y s t e m i n t e r f e r e n c e ) . B e s i d e s , t h e G S M n e t w o r k m a y b e 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 becomingt h e p r i n c i p a l t a s k o f n e t w o r k p l a n n i n g a n d o p t i m i z a t i o n . T h i s d o c u m e n t h e r e i n provides a systemic description of interfere nce source, interference location andtroubleshooting on the basis of experience of experts.
1.1 Affection of Interference on Network When there is interference in the network, the subscribers usually encounter the following phenomenon: During conversation, the subscriber usually cannot hear the voice and the back ground 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.T h e r e a r e L e v e l 4 ~ L e v e l 5 i n t e r f e r e n c e b a n d i n T C H m e a s u r e m e n t 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 signalin g a n a l y z e r (MA10/K1205), it is found that the bit error rate is higher than other cells
Interference Sources 1.2 Classification of Interference Sources The interference sources of mobile communication system/noise are mainly classes as follows:
1.2.1 Natural Noise Atmosphere noise Galaxy noise Solar noise (quiet period)
1.2.2 Man-made Noise Interference of ignition systems of vehicles or other engines Interference of electronic communication system; Interference of power line Interference of scientific research, medical and household appliances; Research data of ITT, America on the above noise/interference is shown as follows
In the figure, Ta denotes the noise temperature, Fa denotes equivalent noise factor, and the relation between the two is as follows: Fa= 10log Ta/To Wherein, To=290 0
K.From the research data of ITT, it can be viewed that the atmosphere noise and solar n o i s e i s c o m p a r a t i v e l y l o w i n 30~1000MHz, which can be ignored. And above100MHz, as the radio noise of galaxy is lower t h a n t h e t h e r m a l n o i s e o f t y p i c a l 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 b e l i e v e t h a t s u n s p o t f a s t i g i u m h a s g r e a t a f f e c t i o n o n t h e e l e c t r i c p o w e r a n d co mmunication.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 ignitioninterference of vehicle engine, electric power inte rference and industrial electric equipment interference. However, certain interference, such as interference between communication equipment and internal network interference, which can be overcome by reasonable, network planning and system optimization. The latter is main research field of this document.
1.3 Main Interference Sources Affecting Mobile Communication In mobile communication system, when BTS receives the s i g n a l s f r o m t h e M S comparatively far away, it will encounter interference from the ambient communicationequipment,and BTS or MS of the same system [3] , as shown in the figure below
Figure 1.2 Schematic diagram of mobile communication interference The main interference of this part is as follows: Hardware fault: TRX fault: if the performance of TRX is reduced due to manufacture cause or application, which will cause self-excitation of TRX amplification circuit resultingin 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 theduplexer in CDU is not enough, all these will form interference on the receivingchannel. Inter-modulation may also occur in passive equipment such as passiveantenna and feeder. Internal network interference: Improper frequency planning: Co-frequency interference Adjacent frequency interferenceRepeater interference:It is commonly adopted that coverage of BTS is extended with repeater in the earlys t a g e n e t w o r k c o n s t r u c t i o n . D u e t o i t s o w n c h a r a c t e r i s t i c s , i t w i l l e a s i l y c a u s e interference to the BTS if it is improperly used. For repeater, there are mainly twointerference modes: As the installation of repeater is non standard, causing insufficient isolationbetween the donor antenna and the subscriber antenna, so self-excitation iscaused. This affects normal working of BTS that the repeater belongs to. For the repeater adopting wideband non-linear amplifier, the inter-modulationindex far exceeds the requirement of the protocol. If the power is comparativelyh i g h , t h e i n t e r modulation component will be large, and this wi l l c a u s e interference to the BTS around. Interference of other communication equipment with high power: Radar station: From 70s~80s of the 20 thecentury, the frequency used by thedecimeter wave radar was similar to that of GSM, and its transmitting power wasvery high, which generally reached tens and hundreds of kilowatts, so the out b a n d
spurious emission is comparatively large. Thus i t e a s i l y c a u s e s interference to the BTS. Analog BTS: The frequency band used by the analog mobile BTS overlaps withthe GSM frequency band in certain segment. Communication equipment at same frequen c y b a n d : A s t h e t y p e s o f communication equipment are so many, some manufacturers maybe adopt thefrequency band but doesn’t comply with the current communication standards.As the GSM frequency band is occupied by the equipments, interference will becaused within the GSM system coverage area Discovery of Interference To solve the interference problem and improve the conversation quality, find out theinterference first, and the n use proper means for locating the interference, and at lasteliminating or reducing interference.The methods available for discovering the interference in GSM system are as follows:OMC traffic measurement, OMC alarm, Drive Test and subscriber complaint, etc. Asthe puissant tools, special test equipment such as signaling analyzer and spectrumanalyzer are commonly unused in this stage. 1.4 Discovering Interference via OMC Traffic Measurement After a network on srvice, following traffic measurement tasks should be registered todiscover the problems in time: TCH measuremen functiont, SDCCH measurementfunction and handover measurement function. After task registration, check the trafficstatus of various cells, handover and traffic measurement indices related to cellquality to discover the cells with potential interference.What needs to be mentioned is that only the existence of the potential interferencec a n b e j u d g e d a c c o r d i n g to the checking r e s u l t s . To f i n d o u t w h e t h e r t h e r e i s 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 eachcell, the reason is that this index can show the TCH mean occupied time (s”), which isusually called “TCH mean holding time” in the BSC of other manufacturer”, within themeasurement period. If it is found that the Average TCH busy time (second) of certain cell is
comparatively short (such as less than 10s), then maybe there is stronginterference in the cell, causing that handover/call drop happens due to bad qualityafter TCH channel occupied to MS.Certainly, if hardware fault occurs in certain TRX (non-BCCH or non-SDCCH carrier)of a cell, the case mentioned above will also appear. 2. Discovering interference via handover data The handover measurement data reflects the mobility of the subscribers within thec e l l u n d e r m e a s u r e m e n t . G e n e r a l l y, w e c a n d i v i d e t h e h a n d o v e r d a t a i n t o t w o categories for analysis, intra-cell handover and inter-cell handover. Inter-cell handover There are many reasons of MS triggering handover The handover measurementindices mainly used for judging whether there is interference are as follows: times of attempt to initiate handover (downlink quality), times of attempt to initiate handover (uplink quality), times of receiving quality level (0~7) when initiating handover (uplink)and average receiving quality when initiating handover (uplink).When certain cell initiates handover, if the average receiving quality (uplink) is ≥ 4(this is true when there is no frequency hopping, if there is, it should be ≥ 5), and them e a n r e c e i v i n g l e v e l i s ≥ 25(-85dBm), then it is possibly caused by u p l i n k interference.When certain cell initiates handover, if the times of receiving quality level above 5 ismore than that below 4, then there may be uplink interference as well.If the times of certain cell attempts to initiate handover (uplink/downlink quality) is
more than 10% of total handover attempt times, then there may be interference in thecell. The two indices are all related to quality
handover threshold and 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-cellhandover is caused by uplink/downlink quality, and the proportion taken up by totalintra-cell handover times among the total inter-cell handover times is comparativelyhigher than that of others, then there may be interference in the cell.The handover measurement index is closely related to the setting of cell parameters.T h e r e d u c i n g o f h a n d o v e r j u d g m e n t t h r e s h o l d a n d P / N d u r a t i o n c a n m a k e t h e handover more sensitive, and cause more handovers. And on the contrary, handover t i m e s w i l l b e r e d u c e d . Too f e w h a n d o v e r t i m e s m a y b e n o g o o d t o t h e n e t w o r k performance at whiles, and affect the handover success rate directly. However, toom a n y h a n d o v e r i s n o g o o d e i t h e r . A s f o r t h e hard handover attribute of GSM,h a n d o v e r i s a l s o t h e m ain cause of call drop. According to the online data measurement, the proportion that one handover per c o n v e r s a t i o n w i l l b e comparatively reasonable. 3. Discovering potential interference via call drop index Call drop is one of the network faults un-acceptable for t h e s u b s c r i b e r s . T h e 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 meanuplink/downlink quality in case of SDCCH/TCH call drop.If the call drop times of certain cell is rather higher the the other with same traffic load,and the main cause of call drop is owning to connection fault, then it is possibly caused by interference.If the average receiving level during call drop is comparatively high ( ≥ 25), while thea v e r a g e r e c e i v i n g q u a l i t y l e v e l i s ≥ 6 , t h e n t h e c e l l s h o u l d b e l i s t e d i n t o t h e 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 frequenciesused by TRX, and then make measurement to the level
5 interference band. Thedefault setting of interference bands in BSC of Huawei is as follows: 110, 105, 98, 90,87 and 85 (unit: -dBm), corresponding to the following interference bands in trafficmeasurement: Table 2.1 Interference bandI
Compared with other measurement indices, the measurement index of interferenceband 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 measurementvalues mainly distribute in interference band 1 and interference band 2, then thepossibility of interference will be small. However, if there is comparatively high valuein band 3, then attention should be paid to this
Note: As Huawei’s measurement method of interference band is based on cell, thev a l u e o f i n t e r f e r e n c e b a n d o f c e r t a i n T R X t h a t s u f f e r s s e r i o u s c o - f r e q u e n c y interference will be comparatively small in large site type (S8/8/8). The reason is thatthis value has been averaged by the values of other seven TRX without interference.Thus, the values cannot reflect the actual interference condition. 1.5 OMC Alarm and Subscriber Complaint OMC alarm subsystem can report the hardware fault of BTS side. Before starting tolocate the interference source, analysis must be performed to the
alarm informationfirst. Before implementation of any optimization works, it is wise to eliminate hardwarefault first.What needs to be mentioned is that the interference, whether from MS or other BTS,cannot be judged via the alarm information of alarm subsystem.Subscriber complaint is also important clew for finding the potential interference. Information which should be collected from user complaint includes MS number, MSmodel, called number and fault phenomenon of calling side and fault phenomenon of called side and particular fault location, etc. If the alarm information is more detailed,it will be more easily to find out the network problems.T h e d e s c r i p t i o n o f s u b s c r i b e r c o m p l a i n t i s c o m p a r a t i v e l y f u z z y. H o w e v e r , i t i s reasonable if the subscriber cannot tell you where the interference is, the reason ist h a t t h e y d o n ’ t k n o w m u c h a b o u t t h e c e l l u l a r n e t w o r k a c t u a l l y. W h e n t h e r e i s 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 cannotbe put through, etc. Thus, when many subscribers within the same area complain thesame problem, then work should be done to check whether there is interference inthe area. 1.6 Discovering Interference via Drive Test D r i v e Tes t i s a m e t h o d t h a t i s m o s t c o m m o n l y u s e d t o f i n d o u t p r o b l e m s . T h e 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 actualimplementation, there are two Drive Test methods available: idle mode test andspecial mode test.Under the idle mode, the test equipment can measure the signal level of both servingc e l l a n d a d j a c e n t c e l l . I n a d d i t i o n , t h e e q u i p m e n t c a n a l s o p e r f o r m f r e q u e n c y 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, receivingqualities, power control registrations and time advance, etc. of both the serving celland adjacent cell. When high level ( ≥ 30) and low quality (Rx_Qual ≥ 6) remain incertain section of highway, then, it can be concluded that interference exists in thesection. Further, part test equipment can directly display the frame elimination rate(FER). Generally when the FER ≥
25%, subscribers can feel the discontinuous voice,that is to say, interference exists in this section of highway (FER measurement of ANTis inaccurate.) Chapter 2 Location and Clearance of InterferenceSource The most important process during optimization is how to locate the fault in thenetwork which requires much experience. The above section only reveals that 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. 2.1 Recommended Procedures for Location and Clearanceof Interference 2.1.1 Determine Interference Cell according to Key Performance Index (KPI) The sudden deterioration of such indices as call drop rate, handover success rate,traffic, congestion rate, interference band indicates that interference exists in the cell.At this time, check the log of the operation on the cells is required. Check whether BTS hardware is added or modified, and data is modified recently, or whether theoccurrence of the interference is associated with these operations in terms of time.I f n o d a t a m o d i f i c a t i o n i s p e r f o r m e d i n t h i s d u r a t i o n , i t c a n b e s u r e t h a t t h e interference comes from the hardware or external of the network. It is recommendedt o c h e c k t h e h a r d w a r e f i r s t , i f i n t e r f e r e n c e s t i l l e x i s t s a f t e r t h e h a r d w a r e f a u l t i s excluded, then check whether external interference exists (the method for checkingexternal interference, please refer to the chapter infra). 2.1.2 Check Alarm of OMC Sometimes, the high call drop rate, low handover success rate and high congestionrate may be relevant with the equipment fault, check of OMC alarm record will sparemuch time spent in judgment and analysis. Similarly, the association of alarm recordwith the deterioration of these indices in terms of time is analyzed here.It should be noted that most alarms of OMC aim at the hardware fault of the such asTRX no power output, etc. For most potential fault in optimization such as TRX or CDU receiving performance degradation, self-excitation, etc., the alarm informationcannot be reported. (Compared with digital components, the analog components suchas microwave and so on are more difficult to detect)
2.1.3 Frequency Planning Check If one cell is doubted with interference, check the frequency planning of this cell andcells around. First you should be very clear about the BTS location distribution anda z i m u t h o f e a c h c e l l , t h e n m a k e a d r a w i n g o f t h e t o p o l o g y m a p , a n d m a r k t h e BCCH/TCH frequency and BSIC. At the same time, make a comparison between theplanned frequency and actually configured frequency in BSC to check whether thereis discrepancy.Generally you can judge whether there is cofrequency and adjacent frequencyinterference according to accurate topology map of frequency planning 2.1.4 Check Parameter Setting of Cell Some cell parameters have impact on the interference such as CRO, handover threshold, handover judgment/statistics duration (P/N c riteria), and adjacent cellrelation.If CRO is set too larger than the neighbor cells, MS is drawn in this cell with in idlemode even the actual Rx level lower than the cells around, so once C/I cannot bemore than the threshold 12dB in case of conversation, interference will emerge.If configuration of adjacent cell is omitted in adjacent cell relation, MS will fail toh a n d o v e r t o t h e cell with better signal level and q u a l i t y, t h i s w i l l a l s o l e a d t o interference. Slight interference may also occur in case of large handover threshold,a n d P / N criteria as well as handover difficulty between cells. But i t i s f a r m o r e dangerous if the value is too small, the frequent handover not only increases the calld r o p r a t e , b u t a l s o t h e s y s t e m l o a d , e v e n l e a d s t o d i s a s t r o u s r e s u l t , i . e . , B S C bre akdown. 2.1.5 Drive Test Drive Test is an effective way to locate the interference. The method is the same asproblem location in 3.3. The difference is that the cell with interference is highlightedduring the test when locate the interference. 2.1.6 Interference Clearance Adjustments should be made respectively according to the above location results. For details, please refer to the technical cases. Finally the effect of interference clearancewill be evaluated according to KPI, and Drive Test result.The specific methods for interference location and clearance are described in
form of cases as follows, at the same time the case ID is provided for the convenience of consultation. 2.2 Location and Clearance of Hardware Fault When a certain cell is doubted with interference through the above analysis, it isrequired to check whether the BTS first where the cell is located works normally or not. Check whether there is antenna feeder alarm, TRX alarm, BTS clock alarm andso on remotely; check whether there is antenna damage, water seepage, feeder (including jumper) damage, water seepage, CDU fault, TRX fault, incorrect BTS jumper connection, system clock unlocked. 2.2.1 Antenna Performance Degradation As one of the passive devices, the possibility of antenna damage is very little, but thereal antenna damage or performance degradation will lead to poor communicationquality.Case 0010761. Fault description: There are 5 BTSs for a certain network in a county configured asS4/4/4 and 6/6/6, the BTS type includes BTS20 and BTS30. The interference band 5i n T C H m e a s u r e m e n t f u n c t i o n o f s o m e c e l l s i s o v e r 1 5 , a n d t h e r e i s n o a l a r m information in OMC Fault location process 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 inthe small hours near middle night, the interference band value is almost 0. 2) After opening the idle BURST of all BTSs and transmitting it in the early morning,it is found that the interference band occurs. It disappears after transmitting isstopped. It can be judged from this phenomenon that the interference comes from internal network and has nothing to do with other equipment. 3) No frequency in the network and data are modified before the interferenceoccurs, accordingly, the interference is irrelevant with the frequency planning. 4) It can be seen from the above second and third points t h a t t h e p r o b l e m i s 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 andrise of back noise occur. 6) First replace all boards (TRX, CDU, FPU, HPA, and power board) of this BTS(BTS20, with Tower Top Amplifier) one by one, at the same time observe thespectrum signal of RXM test interface, it can be seen that interference exists allthe time. This indicates that the interference is relevant with the antenna feeder (including divider, combiner, feeder, antenna, lightning arre ster, Tower TopAmplifier, jumper and connector) instead of the board. 7) Since the above BTS under test has the Tower Mount Amplifier, the antenna andfeeder check is inconvenient, replace another BTS30(S4/4/4) (dual-CDU, anddualpolarization antenna) with interference and check the antenna feeder. 8) Since no interference exists in one of the cells while strong interference exists inanother two cells in the BTS, interchange the antenna and feeder (changing the j u m p e r a t t h e t o p o f t h e c a b i n e t ) o f t h e c e l l s w h i c h a r e w i t h a n d w i t h o u t interference in the BTS in the evening. Then send idle BURST, it is found thatthe interference follows the antenna and feeder. This step helps further locatethe fault which should exist in antenna and feeder system. 9) The situation remains the same even after replacing lightning arrester of antennafeeder and checking all jumper connectors. Then it can be sure that fault existsin the feeder or antenna. 10) Replace the jumper (i.e., antenna) at the top of the tower, it is found that theinterference follows the antenna, so the feeder fault can be excluded while theantenna fault is quite possible. (It should be noted that the external interferenceat this step cannot be excluded because the actual installation place of thea n t e n n a d o e s n o t c h a n g e , b u t t h e e x t e r n a l i n t e r f e r e n c e h a s a l r e a d y b e e n 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 inanother BTS20 with a new one, and then the interference disappears, thus theproblem is solved here.Sometimes the alarm cannot be reported to the OMC alarm console after the antennais damaged. While the antenna damage will lead to the degradation of the radiationperformance and inter-modulation. And the inter-modulation product is fed back to thereceiving channel of the BTS, generating interference and effect the conversation quality. Similar case also includes case 0017185. 2.2.2 Water Seepage of Antenna or Feeder The water seepage of antenna and feeder will change the media structure and bringhigh loss of radio signals. Fault cases in the on service network indicate that thepossibility for water seepage of antenna and feeder is less than that of antenna damage, while the former often leads to the reduction of service range. And no casethat interference is caused by water seepage of antenna and feeder till now
2.2.3 Jumper Connector Fault The RF signal of GSM falls into the UHF signal, if loose contact exists in any sectionf r o m T R X , C D U , f e e d e r to antenna will lead to high VSWR, increase of intermodulation and interference.Case 0015118 Fault description: the type of certain BTS is BTS2.0,site configuration is S242, MS isdifficult to access cell 2, and also call drop often occurs during conversation. It isfound that the interference band 4 and 5 appear in cell 2 interference band of the BTSby viewing the traffic measurement. Interference of different value occur on severalcarriers of this cell through signaling tracing, but interference band only occurs to i n t e r f e r e n c e band 2 and 3 several days later. Though t h e c o n v e r s a t i o n i s n o t affected, the interference does not disappear. there is no alarm message in OMCsystem Fault location process 1) Check carefully the frequency planning of the BTS. The possibility of internal co-frequency and adjacent frequency interference is excluded because the BTSs inthis area are not so dense and with loose frequency reuse. 2)
It is found that though interference spectrum occurs, but the interference signallevel is not too high when checking with spectrum analyzer. 3) The interference still exists after replacing carrier board, power amplificationb o a r d , p o w e r b o a r d , a n d C D U. Subsequently the interference occurs anddisappears a long with the operations of screwing up and screwing off t h e connector at the exit of CDU. 4) It is found that some scraps exist on the CDU output connector. Screw up theconnector after blowing off the scraps, and since then, interference never occurs.T h i s p r o b l e m b e l o n g s t o t h e soft fault. The scraps i n t h e c o n n e c t o r w i l l b r i n g unconspicuous interference, so exp erience is quite important here. On the other hand, this problem reminds us of the importance of checking the engineering qualitycarefully. 2.2.4 Antenna Connected Inversely It is quite usually that the antenna is connected inversely, which will lead to completedifference between the using frequency and the planned frequency. It will also lead toco-frequency and adjacent frequency interference, call drop, handover failure, etc. If the frequency resources for operator is a few, antenna connected inversely will affectthe network quality much more.Case0011108 Fault description: the interference bands 4 and 5 often occur in the tr a f f i c measurement after certain BTS is on service, the inter-cell handover success rate isvery low and the congestion rate is up to 5%. There is no alarm in OMC. Fault location process 1) Since the interference bands 4 and 5 occur along with low handover successr a t e a n d c o n g e s t i o n , i t i s d o u b t e d t h a t t h e i n t e r f e r e n c e c a u s e s t h e a b o v e phenomena. 2) Check the frequency planning first, no problem is f o u n d . T h e e x t e r n a l interference becomes the chief considera tion after the frequency planningproblem is excluded. Change the original used frequency 9 into the farawayf r e q u e n c y 9 4 t o a v o i d e x t e r n a l i n t e r f e r e n c e , b u
t s i t u a t i o n s i t h e s a m e . 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 thefrequency planning or external interference should be excluded. 3) Since handover failure is involved at the same time, it is found that handover f a i l u r e o c c u r s b e t w e e n c e l l s 1 a n d 3 a c c o r d i n g t o t h e r e g i s t r a t i o n o f outgoing/incoming cell handover performance measurement. 4) The congestion analysis indicates that TCH assignment failure is usually causedby uplink. After registering the traffic measurement of uplink/downlink balance, itis found that the measurement item of uplink/downlink balance for cell 1 and cell3 focus on level 1 and 11. This indicates that severe imbalance occurs betweenuplink 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 beconnected inversely. 6) On-site examination indicates that the antennas of cell 1/2/3 become crossedpair which causes the transmitter antennas of cell 1 and 3 to stay in the samec e l l , w h i l e t h e r e c e i v e r a n t e n n a s o f t h e m c o n n e c t t o a n o t h e r c e l l . T h e interference band and congestion disappear and the handover is all right after itis corrected.Case 0005237 Fault description : t h e D r i v e Tes t f o r c e r t a i n n e t wo r k r e v e a l s t h a t t h e B C C H frequencies of several cells is different from the design, the adjacent cell relation isdisordered with serious co-frequency interference. Moreover, with handover successrate 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 severalBTSs 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 thesame area in the same direction.
Fault location process 1) Make clear the frequency of several cells according to the networking plandesign, and locate the erro with the test MS on site. 2) Two methods can be adopted to correct the connection error and verify theaccuracy. 3) Method 1: there is one length flag every other 1m on 7/8 feeders, through whichthe successive two length flags on 7/8 feeders corresponding to each antenna ineach 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 equipmentroom 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.C o r r e c t t h e c o n n e c t i o n a t t h e l i g h t n i n g a r r e s t e r o f t h e a n t e n n a a f t e r c l e a r judgment is obtained. Finally correct the incorrect flag. 4) Method 2: the feeder flag of some BTSs may be b l u r r y d u e t o s c r a t c h i n construction, or no method is available to judge whether the feeder connection iscorrect. 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 beknown from the results of Drive Test. But RXD transmits no signal, it is unable toknow 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 switchon 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 flagagain after making a clear judgment of all feeders. 5) Verification with Drive Test is required after correction. 2.2.5 Incorrect Jumper Connection of BTS
There are many jumpers from BTS TRX to the antenna, the confusion of which willlead to high call drop rate.Case 0015303. Fault description : a newly constructed BTS is of S333 type and dual - C D U configuration with the version as 05.0529. The subscriber complains that it is difficultto make MOC or MTC in the areas covered by this BTS since the on service. In thetraffic 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 dataerror, and hardware fault. The location will focus on these 3 aspects. 2) The dial test carried out by maintenance engineer of the operator indicates thatthis fault phenomenon occurs in cell 3. The “tick” always occurs in the MS andthe 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 isvery high, i.e., 50%, and moreover, all causes are radio link loss connections( e r r o r i n d i c a t i o n ) . B u t T C H a s s i g n m e n t i s n o r m a l , s o i t c a n b e j u d g e d t h a t 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 bando f t h i s c e l l i s n o r m a l , s o t h e i m p a c t o f i n t e r f e r e n c e o n S D c a l l d r o p c a n b e 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 neworkplanning data shows that there is no error. 6) It is found that there are 3 carriers in 3 cells of thi s B T S w i t h d u a l - C D U configuration, the input of two carriers, which should be connected to thecombiner input end of CDU, is connected to TX-COMB and TX-DUP e n d s instead. The fault does not exist after the connection is corrected. 2.2.6 TRX Fault
As the core component of BTS, the fault of T R X w i l l l e a d t o t h e i n c r e a s e o f interference, decrease of coverage, access difficulty, etc.Case 0011519 Fault description: the subscriber reports that the conversation quality in the areanear certain BTS in certain city is poor and with call drop. From the cell measurementfunction report, it can be found that the average idle TCH number of interferenceband 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 thesame time it is found that the number of TCH assignment failure of this cell is up to50, and the call drop rate is about 10%. Since this site is on service for two years, andrunning 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 viarigid RF cable, while the last 2 are connected to the “2-in-1” combiner. The duplex TMA is installed in the antenna and feeder system. No alarm information has beengenerated recently
Fault location process 1) On-site dial test and analysis of traffic measurement indicate that this is aninterference case. Generally, the number of uplink TCH, TCH congestion andc a l l d r o p r a t e w i l l i n c r e a s e w h e n i n t e r f e r e n c e e x i s t s . B u t f u r t h e r l o c a t i o n i s necessary to determine whether it is external interference. 2) For external interference, the interference source can be located through thesignal strength within uplink band with spectrum analyzer. 3) The cause of internal interference is basically the same with that of adjacentfrequency interference. But the following will also lead to internal interference:the performance degradation of RF devices such as a ntenna, Tower MountAmplifier, lightning arrester, combiner/divider, TRX board, and so on, as well ast h e
loose contact of RF cable. This problem can be solved b y r e p l a c i n g corresponding boards and devices. 4) The co-frequency/adjacent frequency interference can be excluded because thefrequency has already been rationally planed by the optimization engineers. Theexternal 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 isgenerated within the network. 5) Check the connection of antenna and feeder, no such ant e n n a c o n n e c t e d reversely or loose contacted. 6) Then replace the duplex TMA of cell 2 with normal Tower Mount Amplifier, theinterference band has no change, it indicates that the interference is not causedby Tower Mount Amplifier. 7) Replace related devices in cell 2 with normal SPL, combiner and RF cable, theinterference band has no change, too. So it can be sure that the interference isnot caused by the above devices. So the location focuses on the antenna andsuch boards as TRX, FPU, HPA, etc. Since replacement of antenna is quitetroublesome, the board should be located first. A valuable phenomenon is foundwhen 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 thecongestion rate and call drop rate are lowered obviously after BT10, and BT11are blocked at the same time. From this phenomenon, it can be sure that theinterference 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, TR X11 andcorresponding boards in the cabinet of cell 3 are interchanged. Then observe thecell performance measurement report. After TRX10, TRX11 and TRX16, TRX17are 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 TCHoccupation fault 1, and interference band 3, 4, and 5 0. This indicates that thefaulty 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 indicatesthat the interference band 3, 4, and 5 of cell 3 become 0, the congestion rate andcall 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 lockingdial 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 isusually up to 10%. The careful view of traffic measurement (TCH measurementfunction) reveals that about 7 TCHs in this cell often stay in interference band 4~5, sothe interference maybe exist. Fault location process
1) To locate the frequency with interference, register the traffic measurement of thecell (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 (frequency28), indicating that the interference is from the frequency. 2) Connect the antenna and feeder of cell 2 with spectrum analyzer, set the centralfrequency is frequency 28 (895.6MHZ). No external interference exists throughobserving the level value. So the interference may come from the equipment. 3) Interchange the carrier board with interference in cell 2 with normal carrier boardin cell 3, then observe the traffic measurement. It is found that the number of idleTCH 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 befound that the radio resource indication of all timeslots in this carrier
reportsinterference band 5. This helps further locate that the carrier board is theinterference source. 5) The problem is solved after this faulty carrier board is replaced.The selfexcitation of carrier board or the invalidation of devices will bring stronginterference for the receiving device. So large amount of assignment fault leads tohigh congestion rate of the cell.Case 0007646 Fault description: The interference of a cell always falls into interference band 4 and5 through the analysis of traffic measurement of certain network, and carrier 28 isunder the interference via tracing and analysis. Fault location process The interference source usually comes from radio commu n i c a t i o n s y s t e m a s following. 1) Carry out a test for the electromagnetic environment of the BTS with spectrumanalyzer. Set the start scan frequency to 895MHz and end scan frequency to8 9 6 M H z f o r t h e s p e c t r u m a n a l y z e r , t h e n c a r r y o u t t h e t e s t i n d i f f e r e n t t i m e segments. No continuous interference signal falls into this band, this indicatesthat the interference maybe come from within the system and may be caused bythe 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 thatthe 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 themto the two carriers free from interference. Then register the traffic measurementof 5 minutes for tracing and observation, it can be found that the interferenceoccurs to frequency where the carrier board with mark is located.This problem is also caused by TRX self-excitation. 2.2.7 Clock Unlocking The clock for the digital system similar to GSM is just like the nervous system of theequipment, so the clock unlocking of BTS will cause the BTS to “go mad”. On onehand, the large deviation of BTS clock will bring difficulty for the MS to be locked inthe frequency of the BTS, and lead to MS handover failure or unavailability of residingin the cell of the BTS. On the
other hand, it will cause the BTS to be unable to decodet h e s i g n a l c o r r e c t l y. I t s h o u l d b e n o t e d t h a t t h e c l o c k o u t - o f - l o c k w i l l n o t b r i n g interference, but the increase of error code in transmission will lead to the decrease of Fault description: The customer adopts transmission timeslot multiplexer to save thet r a n s m i s s i o n r e s o u r c e . A f t e r b e i n g u s e d f o r s o m e t i m e , t h e s l o t m u l t i p l e x e r i s dam aged because of the water seepage into the equipment room. After it is replaced,a l l B T S 1 3 M c l o c k s u n d e r t h e c o m b i n e r a r e o u t – o f - l o c k , a n d i n t e r r u p t i o n a n d cacophony occur during conversation. The inter-BTS handover is unavailable andcall drop rate rises up. Many alarms of 13M clock out-of-clock occur in OMC alarmconsole. 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 transmissionquality 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 nottransmission equipment only but the sharing equipment relevant with theseBTSs. 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 isfound), so BSC clock problem can be excluded. So emphasis should be laid onthe 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 withthe BSC. 6) Disconnect the BTS under the slot multiplexer, and connect it with the BSCdirectly via the jumper, skipping the slot multiplexer. It is
observed that the BTSclock changes from pull-in status to tracing status. After removing the jumper andrestoring 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 corre c t l y defined. 8) Check of grounding of slot multiplexer shows that the power grounding is allright, but the protection grounding is not connected. All BTS clocks can trace theupper-level clock normally after connecting the protection grounding, so theproblem is solved..The diversity of transmission connection from the BSC to the BTS will easily lead toBTS fault because of the problem of certain process in the connection. Detailedu n d e r s t a n d i n g o f t h e n e t w o r k i s q u i t e n e c e s s a r y f or solving the fault, and faultphenomenon can be used to judge the possible causes. Especially the poor grounding or contact will generate potential difference which will le a d t o t h e degradation of transmission quality and affect the stable running of overall network. 2.2.8 Summary The fault of any one of the TRX, CDU, feeder, antenna, jumper, and connector willlead to interference and call drop. This is also proved by large amount of relatedcases. So check and clearance of BTS hardware fault should be performed first asi n t e r f e r e n c e i s f o u n d . A n d i n a d d i t i o n , B T S c l o c k o u t o f - l o c k w i l l a l s o l e a d t o interference and call drop.The hardware fault, in most cases, can be easily located and handled by exchangingthe board and analyzing the traffic measurement data. Certainly the adoption of spectrum analyzer will help locate the problem faster. In the case that interferenceoccurs suddenly in some cells during the running of network data to which no datamodification is made, emphasis should be laid on the clearance of hardware fault
The above interference related to the BTS, is mainl y c a u s e d b y t h i r d o r d e r intermodulation, belongs to the uplink interference and will be directly reflected in theinterference band. 2.3 Intra-network Interference GSM intra-network interference mainly comes from cochannel/adjacent channeli n t e r f e r e c e s . I t i s u n a v o i d a b l e w h e n C / I < 1 2 d B o r C / I a < - 6 d B . T h e a d o p t i o n o f aggressive frequency reuse will also increase the probability of interference. 2.3.1 Co-channel Interference Since frequency reuse in the GSM is unavoidable, co-channel interference will beeasily caused when the reuse distance between two cells using the same frequencyis too small compared with the cell radius. The frequency reuse in many cases, according to the experience, should be avoided
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 ando n e c a s e i n l a b o r a t o r y f o r r e f e r e n c e b e s i d e s t h e
a b o v e c a s e o f c o - c h a n n e l interference that the antenna is connected inversely. Fault phenomenon: The co-channel interference of Huawei early 2.0 BTS (O2) incertain area leads to high call drop rate and poor voice quality. Serious interruptionwith occasional strong noise (whizz in general) occurs. It is after the BTS’s normalrunning for a certain time that the call drop occurs. Located in a little town (Du city)o n t h e b o r d e r o f t h e c i t y , t h e B T S i s s u r r o u n d e d b y t h e B T S s o f t h e o t h e r manufacturer. Fault location process 1) The frequencies assigned for the BTS are 64, and 92 (64 is of BCCH frequency). 2) In the optimizing test, the receiving q u a l i t y ( q u a l i t y l e v e l i s l e s s t h a n 3 ) i s continuously good as the downlink signal level is -95dBm in the direction awayfrom Huanggang and Du City. In the direction from Du City to Huanggang, thereceiving quality is also good when the receiving level is more than -70dBm. T h e n m o v e f o r wa r d u n t i l t o t h e p l a c e w h e r e TA= 5 , t h e receiving quality issometimes good, and sometimes more t h a n 5 i n a b o u t 1 m i n u t e w h e n t h e 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 mayexist 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 Cityto Huanggang reveals that the signal strength of 64# frequency is already lessthan -100dBm near the Huanggang, and call drop already had occured. But thesignal strength of 64# frequency rises up to -65dBm and disappears after a duration of 100 seconds when entering the downtown area of Huanggang. So itcan be judged that the cochannel interference may be from the TCH frequencyof cell nearby. 4)
Carry out scan test for this frequency after arriving at the hotel, the signal levelstill remains high, but the conversation is not implemented on this frequency. Thenext 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 testM S . F r o m t h e s y s t e m m e s s a g e , i t c a n b e confirmed that 64# frequency isa s s i g n e d t o t h e T C H i n H G 0 8 c e l l w i t h B C C H f r e q u e n c y 4 5 , a n d t h e 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 tomodify the frequency. At the same time, the customer should adjust the adjacentcell relation of peer equipment. 6) Carry out test again at the place where the original co-channel interferenceexists after the frequency is modified, the call drop and network drop disappear,and conversation quality Rx_Qual4), it is quite possible that co-channelinterference exists in this channel.The interference of uplink can be judged with the help of interference band statisticsdata in traffic measurementCase 0008119 Fault phenomenon: It is found in the training equipment room some day: it is hardfor the MS to access the network in BTS30 under BSC2. Even it accesses then e t w o r k n a r r o w l y , i t w i l l d r o p s o o n . T e s t M S s h o w s t h a t t h e s i g n a l e x i s t s 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 thetraining equipment room. So load standard data (which is tested to ensure thatcall can be connected) for BSC2, the fault still exists, so the data problem can beexcluded. 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 BT S: 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 thedata into BTS20 (in this way, overall check of BSC can be performed).And thecall 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. Carefulconsideration from the beginning to end reveals that all data of BSC2 and BTS30are not modified, but BTS20 is switched off during BTS30 rollback. Think aboutthe phenomenon of the test MS: the signal exists intermittently and the receivinglevel is very high (about -50dBm), it is possible that BTS20 works interference onBTS30? 7) Since BTS20 is in BSC1, check the data of BSC1. It is found that the frequencyof 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 connectedwith the attenuator, and the signal is far weaker than that of BTS20, so the MScannot access the BTS30, while can
access the BTS20 . In this way, you tend tothink 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. 2.3.2 Adjacent-channel Interference For the cells to which the adjacent channel cannot be allocated, please refer to thesection supra.Case 0003451Fault 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 arenormal, The BTS distribution and topology map of frequency planning are shown asfollows:
In the figure, red stands for BCCH frequencies, while black for TCH frequencies. Fault location process 1) T h o r o u g h t e s t r e v e a l s t h a t t h e l e v e l o f c h a n n e l 1 12 i s u p t o - 7 3 d B m i n t h e position of call drop. First we think the test is incorrect, but several tests indicatesthat 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 distinguishingCGI. 3) Surveying in BTS D reveals that antenna of D3 cell is installed on a platform atthe top of the building, while 8m away there is one house in glass structure whichis 4m lower than the platform. Testing near the antenna. The transmitting signalof 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 leadsto 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 withfrequency 114; increase the downtilt angle of antenna in A3 cell; adjust thedirection angle of frequency 113 in C1 cell to avoid the interference of frequency114 after the exchange. 5) The test is all right after the modification. Frequency 113 of BTS C will work nointerference for frequency 114, and the call drop disappears.Case 0004034Fault 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 upduring on-site test. There is no alarm message in OMC system Fault location process 1) All BTSs are connected to the same BSC, and call drop occurs after cutover of new BTSs.
2) The transmission quality is good, and TRX test is carried out for the above BTSwith call drop and this indicates that each TRX is all right. No fault is found bychecking the data and carrying out the test for 32BIE port corresponding to theBTSs. From the above analysis, the TRX fault, BSC hardware fault, A interfacecircuit fault and transmission fault can be excluded. 3) Analysis of traffic measurement result reveals that serious interference occurs ineach cell of the above several BTS. Most cells have measurement values ininterference band 4 and 5, and the number of channels falling into interferenceband 5 in several cells is up to 7. So it is sure that the interference in the aboveseveral cells is quite serious. 4) It is found that there are many adjacent channels and the frequency planning isirrational after checking the frequency configuration of above BTSs and theadjacent cells. Especially, the area where the above BTSs are located, is newlyadded, and interference exists among them. And they also have interference inbetween them and the surrounding running BTSs. 5) Call drop disappears after adjusting and loading the frequency configuration of this area. 2.3.3 Interference Caused by Over-coverage The objective of one rationally designed network is that each cell only covers the areaaround the BTS and the MS resides in the nearest cell. The over-coverage is that theservice range of a certain cell is too wide that the signal level at an interval of morethan 1 BTS is still strong to let the MS reselect or hand over to it. It is a phenomenonin 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 successrate is less than 80%, and the call drop rate is more than 2%. It is found that there aremany times of downlink/uplink strength hand overs through view and analysis of traffic measurement data. while there are many times of bad downlink quality, anduplink 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 baduplink quality . There is no alarm message in OMC system Fault location process 1) From the result of traffic measurement, it can be ju d g e d t h a t d o w n l i n k 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 beup to -80dBm above in the downtown area, that is to say, the coverage is allright. But serious over-coverage exists. For example, the service cell used in thebuilding where BTS A is located is cell B with the same BCCH frequency as cellA1, while cell B is 6 kilometers away from BTS A in the suburb. In this way, thep r o b l e m e x i s t s i n t w o a s p e c t s : 1 . T h e s i g n a l o f c e l l B f o r m s c o - c h a n n e l 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 lockedduring the test. 2. When cell B is selected as the service cell, its adjacent cell isonly geographically adjacent to it, while the cell near the BTS A does not functionas its adjacent cell. So when its signal is unavailable, the “effect of isolatedisland” will occur because the signal of its adjacent cell is poor, too. Then handover fault and even call drop will easily occur. The on-site survey shows that theantenna of cell B is hung 50m above. The data provided by the customer revealsthat 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 isadopted to lower the antenna and adjust downtilt of it to make actual coveragearea consistent with planned coverage area. Temporarily this problem can onlybe solved by adjusting the network parameter. The following operations can beadopted: lower the power level of cell B and add the adjacent cell of cell B, at thesame time increase the level threshold of candidate cell from 10 to 15. Then e t w o r k i n d i c e s e x h i b i t o b v i o u s i m p r o v e m e n t a f t e r t h e a b o v e c h e c k a n d modification are performed for all cells in the
downtown area, moreover, thehand over success ratio rises up to 85%, while call drop ratio drops to 1.3%. 2.3.4 Interference Caused by Aggressive Reuse The capacity and quality are a pair of contradictions. In the downtown area, there arelarge number of subscribers, so the frequency planning technology of tight reuseshould 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 irrationalwill 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 ado p t s t h e m o d e o f 1 * 3 frequency hopping. There is no alarm. Fault location process 1) The signals on the bridge are disorderly due to distribution of multiple BTSsaround the river. So the interference of frequency hopping will easily occur whenadopting frequency hopping. 2) First, adjust the antenna to make one main cell on the bridge, adjust the antennaand transmission power of other cells to reduce the strength of their signalsr e a c h i n g t h e b r i d g e . T h e n c a r r y o u t t h e t e s t . I t i s f o u n d t h a t t h o u g h t h e interference is reduced, yet the interference of frequency hopping still cannot beunder 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 hoppingfor 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 hoppingaverages the interference. A combination of them in some local areas is better f o r t h e d i s p e r s i o n o f i n t e r f e r e n c e . T h e a c t u a l t e s t r e s u l t i n d i c a t e s t h a t t h e conversation quality is improved significantly. 2.4 Repeater Interference Repeater really exhibits a little convenience, so customers often make use of it. Therepeater is also one of the main interference sources.Case 0017086 Fault phenomenon:
subscribers of a certain network complain that they cannotoccupy 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 twodirectional 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 andthe network indices conform to requirement before the complaint. Both BSC and MSCa r e devices of Huawei and two BTSs are connected in star mode. The trafficmeasurement indices show that traffic of both B T S s d e c r e a s e s o b v i o u s l y a f t e r 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 trafficmeasurement that the interference band of these four cells is in class three, four, andfive, and 95% of channels are under interference. In addition, interference of differentclasses also exists in other cells. So subscribers complained strongly. And there isno alarm message in OMC system Fault location process 1) The feedback of subscribers shows the possible causes as follows: 1. Problemoccurs in transmission and leads to error code; 2. Problem occurs in antennafeeder; 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 thisarea. This leads to interference of different levels in the first, second and thirdcells, 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 cellsthrough on-site dial test. Although the call is put through, the quality of voice isvery poor, and the voice is intermittent seriously with strong interference. If MSs u b s c r i b e r c a l l s f i x e d t e l e p h o n e s u b s c r i b e r i n t h i s a r e a , i t i s h a r d f o r f i x e d telephone subscriber to hear the voice clearly, instead, MS subscriber can hear fixed telephone subscriber clearly. This also proves the above analysis that theinterference may be external (It can be judged from this point that interferenceonly exists in uplink.) 4)
Carry out test on site with antenna feeder analyzer. No problem of BTS itself isfound, and the situation remains the same after TMU is replaced. Therefore, wea s k c u s t o m e r s w h e t h e r t h e r e a r e s u c h n e w l y c o n s t r u c t e d e q u i p m e n t a s microwave station, repeater, etc. surrounding the BTS. They told that they didn’tset up them. It is said that China Mobile sets up a repeater in the area, which islocated 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. Thencustomers negotiated with China Mobile several times. Finally China Mobilecarried out on-site test with Huawei engineers together, and found that only if therepeater is switched off, the interference band and call become normal alongwith 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 interferenceband in two different cases was handed on to China Mobile, and they approvedo u r v i e w p o i n t . F i n a l l y t h e p r o b l e m i s s o l v e d w i t h t h e c o o r d i n a t i o n o f C h i n a Mobile. 2.5 Off-network Interference External interference sources are TV station, high-power broadcasting station,microwave, radar, high-voltage power line, analogue BTS, CDMA, GSM BTS of another operater, etc. 2.5.1 Microwave Interference Case 0001084Fault phenomenon: it is found that call drop rate in the second and third cell of acertain BTS (S2/2/2) in traffic measurement increases abruptly. Call drop rate is up toabout 20% at some time. Fault location process 1) View of BSC traffic measurement shows that idle TCH number in interferenceband in this BTS begins to increase in interference band 3-5 at about 8:30, ininterference 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 beexcluded. 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. TRXboard is replaced for caution, but interference remains 5) View of all BSC traffic measurement data shows that interference of differentlevels exists in the cells of all BTSs nearby, which is facing the same direction asthe second and third cells in this BTS. And sometimes SDCCH channels in thec e l l s w i t h s e r i o u s i n t e r f e r e n c e a r e a l l o c c u p i e d a t t h e s a m e t i m e , w h i l e t h e occupancy ratio of SDCCH at the same time is very little according to the amountof subscriber. So it can be sure that external interference exists in uplink, but theinterference may be relevant with direction instead of frequency. 6) To further locate, jumpers of the first and third cell are interchanged on the rackt o p . A s a r e s u l t , i t i s f o u n d t h a t i n t e r f e r e n c e o c c u r s i n t h e f i r s t c e l l , b u t interference disappears in the third cell, so this has proved the above judgement. 7) Since interference is not relevant with frequency, BTS interference may becaused by high-power signal sent into BTS system. 8) Measurement on BTS divider output port with spectrum analyzer shows thath i g h power signal exists on 904MHz frequency (5M away from t h e u s e d frequency), and this signal level come up to about –25dbm in BTS with seriousinterference, while in other BTS it is about –50dbm. So it can be judged that thissignal has impact on BTS. 9) After frequency scanning around BTS with spectrum analyzer, it is found that amicrowave antenna outputting high-power signal is at a frequency of 904.
10) When the microwave equipment is switched off and T R X m a n a g e m e n t information is traced, the interference disappeared. 2.5.2 High-power Broadcasting Station Interference Case 0000279 Fault phenomenon: the quality of MS call is very poor in the morning of the third day (Monday) after a BTS cutover, and it is hard to make calls 2 km away from theBTS. 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 signallevel (downlink level) has little change (about –60dBm), but the signal quality RQhas 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 isreplaced. It is suspected that fault is caused by external interference. Measurement for external electromagnetic environment with HP8595E spectrumanalyzer shows that strong interference exists around the frequency of BTS.Through communicating with customer and the Radio Resource ManagementCommittee, we get known that the interference is caused by signals transmittedby one message transmitter of some company, whose RF band is just adjacentto the planned GSM frequency. This transmitter usually transmits every Monday,and no such phenomenon is found in two days after BTS is activated
2.6 Other Phenomena Causing Interference Sometimes although no intra-network and ex-network interference exists, fault of s o m e d e v i c e s c a n a l s o m a k e c o n v e r s a t i o n q u a l i t y b a d .
S o m e c a s e s b e l o w a r e analyzed detailedly. They have high reference value.Case 0015274 Fault phenomenon: BSC has six modules. The whole BSC is cascaded with 150BTSs (BTS312, 750TRX in total). One month after the capacity expansion of BSCand MSC, customers report that there is serious voice quality problem in the wholenetwork. There exists serious chink during conversation under many BTSs. We cometo county A with serious complaint to make a dial test. Fault phenomenon is found asfollows: there exists strong background noise. It mainly contains sound of metal, suchas “ting, dang” and so on. One party or both parties can not hear each other clearlywhen the noise is serious (One party is in the range of coverage of this BTS and theother under a normal BTS. When fault occurs the subscriber under this BTS can noth e a r c l e a r l y, w h i l e t h e o t h e r c a n ) . W h e n t h e v o i c e c h a n n e l o f t h e f a i l e d B T S i s 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 andthere is no interference on the radio interface. The fault phenomenon occurs whent h e r i n g b a c k t o n e i s s e n t b a c k . T h a t i s t o s a y, s o l o n g a s t h e v o i c e c h a n n e l i s established, the fault will occur. Disturbance of different degrees will exist duringnormal 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 sometimesfault phenomenon cannot be captured. There is no alarm message in OMC systemCause analysis: this case is about noise in voice problem. It is usually caused byerror 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, clockalso 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 isinfluenced. Therefore, the noise is regular and there is little fluctuation. If it is below Ainterface, error codes are also regular. However, because the compressed voice signal is influenced, it needs decoding before hearing. Therefore , there is great noisefluctuation. Some words are normal, while some words can not
be recognized, suchas bubbling sound, discontinuity, sound of metal and so on. As for frame slip or frameloss 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 faultphenomenon, 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 isw i t h o u t error code; check TRX, FPU, combiner. There is no fault. All voicec h a n n e l o f e a c h c e l l m a y b r i n g a b o u t m e t a l c h i n k . S o t h e p o s s i b i l i t y o f 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 possibilityo f b i t s l i p o r fr ame loss caused b y clock fault can be excluded. Trace A b i s interface information, judge the receiving quality of radio interface is good withouti n t e r f e r e n c e a c c o r d i n g t o t h e m e a s u r e m e n t r e p o r t . T h e r e s u l t o f t r a f f i c measurement also shows that there is no interference. Use Site master to testantenna feeder system. The Standing Wave Radio is normal. BTS earth is good.From the above analysis, it is sure that the problem has nothing to do with thisBTS. 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 allunder 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. AllBTSs under other five BM modules have no this case through dial test. Thep r o b l e m o f m e t a l c h i n k i s r e l a t e d t o B M 4 t h r o u g h a b o v e l o c a t i o n , a n d t h e problem exists in one certain process of BM4 “voice signal transmission path”. 3) Transmission path of voice signal: MS—— radio link (including antenna feeder s y s t e m ) — — B T S — ( E 1 l i n e ) — B T S _ D D F — — t r u n k t r a n s m i s s i o n — — 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 seewhether there is something wrong with swithing network board (GNET andGCTN) and fiber interface circuit: check the connection and connector betweenA M a n d B M 4 . T h e y a r e n o r m a l . U s e a b s o l u t e a l c o h o l t o c l e a n t h e f i b e r 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 FBIboard corresponding to BM1 and BM2, but the fault remains the same. ChangeBM4 GOPT, but the fault remains the same. The possibility of existence of circuiterror code in the part “OPT—(fiber) —FBI” can be excluded. Swap BM4 GNETboard, the problem still exists; swap GCTN board, the problem is still; exchangeGNET board with BM3, the problem is still; exchange E3M board with BM3, theproblem is still. 5) The same fault exists in all BTS under the whole BM4. Error code should note x i s t o n H W L i n e s b e t w e e n 3 2 B I E a n d N E T b o a r d . T h a t i s b e c a u s e i t i s 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 theconnector of E3M is normal; check one connector of MSM and find that thequality 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. Theresult is that the connector cannot connect the connector of MSM board onbackplane 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 is3, 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 towill be selected with higher priority. If the signaling of the MS call is transmittedt h r o u g h m o d u l e 4 o f M S C , M S C w i l l a s s i g n t h e c i r c u i t o f m o d u l e 4 f i r s t ( t h e 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 throughmodule 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 thecircuit of MSC module 4. 128 of MSC module 4 circuits has fault (corresponding toBSC 4 module) (the proportion of failed circuit of MSC module 4 is 128/576) and itmay bring about problems of voice quality. The highest rate of occurrence of fault (allhave fault when 128 circuits is busy) is about 2/3 * 128/576 = 15 %. In fact the fault of dial test under BTS is 10 % or so. 2.7 False Interference Sometimes we made great efforts to check, but only find that it is false alarm in the end. In fact the interference is false. Indeed it is better to have no interference thanhave 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 cellsin it changed from 5 to 7. Fault location process 1) During the past interference test ofthis office, some cells with high interferenceband were located. This is because antenna intermodulation resulted in theinterference of normal signal in case of large traffic and it made the interferenceband 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 timethe frequency that may cause third order intermodulation in the cell has been adjusted. The problem of frequency planning has been basically excluded. It isunknown whether there is something wrong with Tower Top Amplifier, feeder or connector. But no problem was found after checking the antenna feeder systemof the BTS several times. 3) Trace the state of interference band of each channel via Maintenance Consoleand 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 relationto the frequency. Whether there is something wrong with TRX board or not isu n k n o w n . H o w e v e r , t h e s i t u a t i o n i s u n c h a n g e d a f t e r t h e T R X s w e r e interchanged. 5) Then confirm the TRX numbers, and find that the four carriers come from thesame divider. From the above process, the problem may be the divider. But it isfound that the interference band becomes high when the BTS is expanded fromS4/4/4 to S6/6/6. While the combiner/divider used to be normal. Maybe thed e v i c e s b r e a k d o w n a f t e r s o m e t i m e . S o w e d e c i d e d t o c h a n g e t h e combiber/divider to have a try. 6) The idea that four carriers with high interference band are connected to the samesecond level divider before the divider is changed. The 7-dB gain of the divider has been considered during interference judgement. However, if the DIP switchis not moved to the right place while cascaded, the interference signal will beamplified by 7dB. Simultaneously, higher interference band will exist because thecalculating error of 2.0 station itself is up to 5dB. Check the DIP switch of thedivider. 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 alittle value on the interference band three
Chapter 3 Anti-interference Measures Anti-interference Measures of GSM are: Frequency Hopping, dynamic power control,DTX. While the aggressive reuse is adopted, frequency hopping and dynamic power control should be adopted as well.To adjust the obliquity, azimuth, height of antenna is also a main optimization way toreduce network interference. The aim for adjusting is to make the real service area of e a c h c e l l a p p r o a c h t o t h e d e s i g n e d s e r v i c e a r e a t o r e d u c e o v e r - c o v e r a g e . t h e methods of adjusting antenna please refer to relative guidance book.These antiinterference measures mainly aim at intra-network interference. Whenthere is strong external interference, these anti-interference measures can hardly takeeffect.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 someother vendors, every function should be purchased by the operators themselves, andthe price is very high. The currently serving network equipments of many operatorsh a v e n o t h e s e f u n c t i o n s . D u r i n g o p t i m i z a t i o n a s T h i r d P a r t y o r c o - o p e r a t i o n t o equipment of other vendors, please pay attention to this matter Chapter 4 Interference Test Tools 4.1 Brief introduction to Spectrum Analyzer 1) At present, the main tool Huawei has for testing interference signal is SpectrumAnalyzer. It is a broadband signal receiver with high performance which candisplay the spectrum of the receiving signals. The receiving sensitivity of signalbandwidth of xHz equals to 1Hz sensitivity+10logx, e.g., the receiving sensitivityof GSM signal of 200kHz is: -142dBm+10log(200*1000)=-89dBm; 2) The receiving signal resolution bandwidth (RBW): namely the minimum signalbandwidth the Spectrum Analyzer can recognize. The smaller the parameter is,the higher the receiving sensitivity of instrument. That is to say, the noise of theinstrument itself is less; The Spectrum Analyzer of different types has differentreceiving frequency band and receiving sensitivity. It is very important to makeuse 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. Thisindex determines the frequency range of interference signal that the Analyser can test; 4) Sensitivity: generally the minimum receiving level with 1Hz signal bandwidth isd e f i n e d a s r e c e i v i n g s e n s i t i v i t y o f t h e S p e c t r u m A n a l y z e r . T h e r e c e i v i n g 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 theSpectrum Analyzer can test; 7) Bandwidth (SPAN): it refers to the spectrum span the Spectrum Analyzer cantest; 8) Input signal attenuation (ATT): when there is large signal input, it is required tomake attenuation on signal properly. The Spectrum Analyzer itself may producelarge number of intermodulation components without attenuation. So it willinfluence the veracity of the testing result.The main technical indices of interference test spectrum analyzer :
4.2 Directional Antenna Directional antenna is used for searching interference sources. The stronger thedirectionality of antenna is, the higher the antenna gain is. And the ability to searchwill become better. So the logarithm-period antenna with broad frequency band is thebest choice. This kind of antenna has broad frequency band, high antenna gain andstrong directionality.
Chapter 5 Way to Test Interference 5.1 The way to Test Internal Interference 1. Set the Spectrum Analyzer to proper state:For 900M BTS: f0=902MHz,SPAN=30MHz,ATT=0,RBW=30kHz,VBW=30kHz; For 1800M BTS: f0=1715MHz,SPAN=10MHz,ATT=0,RBW=30kHz,VBW=30kHz.2. Screw out the connector of output port of CDU divider, then connect the outputsignal of the divider to the Spectrum Analyzer to carry out a test. If the fractionalfrequency 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 areunder interference or self-excitation.3. If internal interference exists, further make sure that it belongs to CDU or TRX. Atfirst confirm TRX carrier board, cut down the cable via which TRX is connected todivider, 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 isnormal, 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 followsteps below.4. Check interference of transmission band. First, set the Spectrum Analyzer intransmission 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, thent h e tx_test signal of CDU should be imported to the S p e c t r u m A n a l y z e r t o b e observed to make sure weather interference signal is generated. 5.2 The way to Test External Interference When we are sure that interference is caused by the external cause, first we shouldconfirm the location of interference source and the spectrum distribution state. Aconvenient 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 influencethe normal operation of BTS, the free output port (with dummy load) is generallychosen, either the main or the diversity can be selected. 3)
Screw out the selected connector, then use Coaxial Cable to import the outputsignal of divider to the Spectrum Analyzer; 4) View the spectrum distribution state of the Spectrum Analyzer, and find out theabnormal interference signal. The way to calculate the level of interference signalis 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=-84dBmNote: Cable loss may vary with cable length.The standard for judging weather interference level has impact on the system isdescribed as follows
(1) The maximum interference level at antenna port without influence on system= -108dBm sensitivity – 9dB co-channel interference protection= -117dBm. (2) The maximum interference level at divider output port without influence onsystem = -117dBm+15-3+7 = -98dBm 5.3 The way to Search External Interference Sources The location of interference source can be examined via BTS divider output port. If itis required to find the more detailed location of interference source, walk out of equipment room, and use the directional antenna with strong directionality mentionedabove 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 thew a v e b e a m o f t h e antenna point to the f r o n t , a n d t h e a n t e n n a w i t h v e r t i c a l polarization should be placed vertically; if there is no interferent signal, one canraise the antenna over head with hands. Rotate the antenna slowly, and at thesame time view
the change of signal of the Spectrum Analyzer. Once there exists abnormal signal, fix the orientation of the antenna immediately and changeuptilt of the antenna to make the receiving signal to the strongest. 4) A n a l y z e t h e s i g n a l s p e c t r u m d i s t r i b u t i o n c a r e f u l l y, a n d c o nfirm that it isinterference signal, record the signal stren g t h a n d r e c o r d t h e a z i m u t h a n d downtilt of antenna wave beam. 5) Find new test point along with the direction of antenna wave beam, then return tostep 2 to carry out a test till interference source is found
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