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GSM Interference AnalysisTable of Contents

1

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

2.1.4 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

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 t r o u b l e s h o o t i n g . T h e m a i n c o n t e n t s o f t h e c o u r s e 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 t e r f e r e

n c e S t a r w i

a r s k i

GSM Principle

and Network Optimization Han Binjie Mobile Communication Engineering Lu Er’Rui

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 co-frequency interference factor. For certain co-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.

T h r o u g h t r a c i n g t h e A b i s i n t e r f a c e s i g n a l i n g w i t h s i g n a l i n 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

 Galaxy noise

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

3 0 ~ 1 0 0 0 M H z , w h i c h c a n b e i g n o r e d .

A n d a b o v e 1 0 0 M H z , a s t h e r a d i o n o i s e o f g a l a x y i s l o w e r 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

I n m o b i l e c o m m u n i c a t i o n s y s t e m , w h e n B T S r e c e i v e s t h e 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]

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

-m o d u l a t i o n c o -m p o n e n t w i l l b e l a r g e , a n d t h i s w i 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

s p u r i o u s e m i s s i o n i s c o m p a r a t i v e l y l a r g e . T h u s 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.

 C o m m u n i c a t i o n e q u i p m e n t a t s a m e f r e q u e n 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 t o t h e c h e c k i n g

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

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 ≥

2 5 ( - 8 5 d B m ) , t h e n i t i s p o s s i b l y c a u s e d b y

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 interferencehandover 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

h a r d h a n d o v e r a t t r i b u t e o f G S M , h a n d o v e r i s a l s o t h e m a i n c a u s e o f c a l l d r o p . A c c o r d i n g t o t h e o n l i n e d a t a m e a s u r e m e n t , t h e p r o p o r t i o n t h a t o n e h a n d o v e r p e r 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

C a l l d r o p i s o n e o f t h e n e t w o r k f a u l t s u n - a c c e p t a b l e f o r 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 reflectwhether 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 theinterference 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

co-frequency and adjacent co-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

c e l l w i t h b e t t e r s i g n a l l e v e l a n d

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

c r i t e r i a a s w e l l a s h a n d o v e r d i f f i c u l t y b e t w e e n c e l l s . B u t 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 comesfrom 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)

I t c a n b e s e e n f r o m t h e a b o v e s e c o n d a n d t h i r d p o i n t s 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, anddual-polarization 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 conversationquality. 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 antennadamage, 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

t o a n t e n n a w i l l l e a d t o h i g h V S W R , i n c r e a s e o f i n t e r -modulation 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

b a n d 2 a n d 3 s e v e r a l d a y s l a t e r . T h o u g h

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 . S u b s e q u e n t l y t h e i n t e r f e r e n c e o c c u r s a n d d i s a p p e a r s a l o n g w i t h t h e o p e r a t i o n s o f s c r e w i n g u p a n d s c r e w i n g o f f 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

s o f t f a u l t . T h e s c r a p s

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.Case 0011108

Fault description:

t h e i n t e r f e r e n c e b a n d s 4 a n d 5 o f t e n o c c u r i n t h e t r 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)

C h e c k t h e f r e q u e n c y p l a n n i n g f i r s t , n o p r o b l e m i s 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

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 T e s t f o r c e r t a i n n e t w o 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)

M e t h o d 2 : t h e f e e d e r f l a g o f s o m e B T S s m a y b e

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.

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 n e w l y c o n s t r u c t e d B T S i s o f S 3 3 3 t y p e a n d d u a l - 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 shouldbase 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)

I t i s f o u n d t h a t t h e r e a r e 3 c a r r i e r s i n 3 c e l l s o f t h i s B T S w i t h d u a l - C D U c o n f i g u r a t i o n , t h e i n p u t o f t w o c a r r i e r s , w h i c h s h o u l d b e c o n n e c t e d t o t h e c o m b i n e r i n p u t e n d o f C D U , i s c o n n e c t e d t o T X - C O M B a n d T X - D U P

e n d s instead. The fault does not exist after the connection is corrected.

A s t h e c o r e c o m p o n e n t o f B T S , t h e f a u l t o f

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

l o o s e c o n t a c t o f R F c a b l e . T h i s p r o b l e m c a n b e s o l v e d 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 nostable signal of more than -100dB is found. This indicates that the interference isgenerated within the network.

5)

C h e c k t h e c o n n e c t i o n o f a n t e n n a a n d f e e d e r , n o s u c h a n t 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.

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 self-excitation 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

T h e i n t e r f e r e n c e s o u r c e u s u a l l y c o m e s f r o m r a d i o c o m m u 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. Theinterference 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) C h e c k o f t h e p r o g r a m i n s l o t m u l t i p l e x e r s h o w s t h a t a l l p o r t s a r e c o r r e 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 o r s o l v i n g t h e f a u l t , a n d f a u l t p h e n o m e n o n c a n b e u s e d t o j u d g e t h e p o s s i b l e c a u s e s . E s p e c i a l l y t h e p o o r g r o u n d i n g o r c o n t a c t w i l l g e n e r a t e p o t e n t i a l d i f f e r e n c e w h i c h w i l l l e 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 dr-op.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

T h e a b o v e i n t e r f e r e n c e r e l a t e d t o t h e B T S , i s m a i n l 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

co-channel/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

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)

I n t h e o p t i m i z i n g t e s t , t h e r e c e i v i n g

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 w a 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 r e c e i v i n g q u a l i t y i s s o m e t i m e s g o o d , a n d s o m e t i m e s m o r e 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 aduration of 100 seconds when entering the downtown area of Huanggang. So itcan be judged that the co-channel interference may be from the TCH frequencyof cell nearby.

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 c o n f i r m e d t h a t 6 4 # f r e q u e n c y

i s a 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_Qual<3. It indicates that the problem of co-channelinterference (downlink) is solved.The GSM is a duplex system, so the interface may occur on both the uplink anddownlink frequencies. Current Drive Test tools are unable to measure the co-channelinterference directly, the equipment such as SAFCO, and ANT with so-called C/I testfunction actually test the C/A. And in addition, the Drive Test equipment can onlymeasure the downlink frequency and nothing for the uplink frequency.The co-channel interference in downlink frequency can be confirmed by using currentDrive Test equipment to measure it indirectly. First, lock the test MS in the cell of theservice area to carry out Drive Test of 2~3 hours in continuous conversation mode. If t h e r e c e i v i n g s i g n a l

i n s o m e a r e a s i s a r e l a t i v e l y h i g h ( e . g . , - 8 5 d B m ) w h i l e t h e receiving quality is very low (e.g., RxQual>4), 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).

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)

C h e c k e a c h p r o c e s s r e l a t e d t o t h e v o i c e f o r t h e B T S : T M U , D I P s w i t c h

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 112 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

times of bad downlink quality are more than those of baduplink quality . There is no alarm message in OMC system

Fault location process

1)

F r o m t h e r e s u l t o f t r a f f i c m e a s u r e m e n t , i t c a n b e j u 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