[Huawei] GSM Interference Analysis Training

HUAWEI TECHNOLOGIES CO., LTD. All rights reserved

www.huawei.com

Internal

GSM Interference Analysis

HUAWEI TECHNOLOGIES CO., LTD. All rights reserved Page 2

To introduce affection brought by

interference to the GSM system, fault

location method and troubleshooting

method, this course hereby lists out

the possible interference sources, and

offers description of cases to facilitate

fault location and troubleshooting.


Upon completion this course, you will be

able to:

Understand the cause of interference

and master the method of locating the

interference source;

Master the method of overcoming

interference.


Chapter 1 OverviewChapter 1 Overview

Chapter 2 Location and Clearance of Chapter 2 Location and Clearance of

Interference Interference

Chapter 3 Case AnalysisChapter 3 Case Analysis


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

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

There are Level 4~Level 5 interference band in TCH measurement function , and the measurement value is more than 1.

Congestion rate is comparatively high.

The call drop rate is higher than other cells.

The handover success rate is low.

Through Drive Test, it is found that:

− It is difficult to handover .

− The Rx level is high, but the quality is bad.

Through tracing the Abis interface signaling with signaling analyzer (MA10/K1205), it is found that the bit error rate is higher than other cells.


Classification of Interference Sources

Natural Noise

Atmosphere noise

Galaxy noise

Solar noise (quiet period)

Man-made Noise

Interference of ignition systems of vehicles or other engines

Interference of electronic communication system

Interference of power line

Interference of scientific research, medical and household

appliances


Main Interference Sources Affecting Mobile Communication Internal interference

TRX fault: if the performance of TRX is reduced due to manufacture cause or application, which will cause self-excitation of TRX amplification circuit resulting in interference.

CDU or divider fault: as active amplifier has been adopted for the divider and divider module of CDU, self-excitation might be caused when fault occurs.

Spurious emission and inter-modulation: if out-band spurious emission index of BTS TRX or amplifier exceeds the limit, or isolation between TX and RX of the duplexer in CDU is not enough, all these will form interference on the receiving channel. Inter-modulation may also occur in passive equipment such as passive antenna and feeder.

Improper frequency planning:

− Co-frequency interference

− Adjacent frequency interference


Main Interference Sources Affecting Mobile Communication

Repeater interference

As the installation of repeater is non standard, causing insufficient

isolation between the donor antenna and the subscriber antenna, so

self-excitation is caused. This affects normal working of BTS that the

repeater belongs to.

For the repeater adopting wideband non-linear amplifier, the inter-

modulation index far exceeds the requirement of the protocol. If the

power is comparatively high, the inter-modulation component will be

large, and this will cause interference to the BTS around.


Main Interference Sources Affecting Mobile Communication

External interference

Interference of other communication equipment with high power:

Radar station: From 70s~80s of the 20th century, the frequency used by the

decimeter wave radar was similar to that of GSM, and its transmitting power

was very high, which generally reached tens and hundreds of kilowatts, so

the out-band spurious emission is comparatively large. Thus, it easily

causes interference to the BTS.

Analog BTS: The frequency band used by the analog mobile BTS overlaps

with the GSM frequency band in certain segment.

Communication equipment at same frequency band: As the types of

communication equipment are so many, some manufacturers maybe adopt

the frequency band but doesn’t comply with the current communication

standards. As the GSM frequency band is occupied by the equipments,

interference will be caused within the GSM system coverage area.



Chapter 2 Location and Clearance of Interference Chapter 2 Location and Clearance of Interference



Discovering Interference via OMC Traffic Measurement

Discovering potential interference via traffic statistics result

Check the “Average TCH busy time (second)” in TCH measurement

function of each cell, the reason is that this index can show the TCH

mean occupied time (s”), which is usually called “TCH mean holding

time” in the BSC of other manufacturer”, within the measurement

period. If it is found that the Average TCH busy time (second) of

certain cell is comparatively short (such as less than 10s), then

maybe there is strong interference in the cell, causing that

handover/call drop happens due to bad quality after TCH channel

occupied to MS.

Certainly, if hardware fault occurs in certain TRX (non-BCCH or

non-SDCCH carrier) of a cell, the case mentioned above will also

appear.


Discovering interference via handover data

When certain cell initiates handover, if the average receiving quality

(uplink) is 4 (this is true when there is no frequency hopping, if there

is, it should be 5), and the mean receiving level is 25(-85dBm), then

it is possibly caused by uplink interference.

When certain cell initiates handover, if the times of receiving quality

level above 5 is more than that below 4, then there may be uplink

interference as well.

If the times of certain cell attempts to initiate handover (uplink/downlink

quality) is more than 10% of total handover attempt times, then there

may be interference in the cell. The two indices are all related to quality

handover threshold and interference handover threshold within the cell

parameters.


Discovering potential interference via call drop index

If the call drop times of certain cell is rather higher the the other with

same traffic load, and the main cause of call drop is owning to

connection fault, then it is possibly caused by interference.

If the average receiving level during call drop is comparatively high

(25), while the average receiving quality level is 6, then the cell

should be listed into the interference source.


Discovering potential interference via interference band

BTS will utilize an idle TS in a frame to scan the uplink frequencies of the

frequencies used by TRX, and then make measurement to the level 5

interference band. The default setting of interference bands in BSC of

Huawei is as follows: 110, 105, 98, 90, 87 and 85 (unit: -dBm)

Compared with other measurement indices, the measurement index of

interference band can reflect the cell interference situation more directly,

but it can only reflect whether there is interference in the uplink.

If the values of interference band 4 and interference band 5 are

comparatively large (1), then, there may be co-frequency interference

in the cell. If the measurement values mainly distribute in interference

band 1 and interference band 2, then the possibility of interference will

be small. However, if there is comparatively high value in band 3, then

attention should be paid to this.


OMC Alarm and Subscriber Complaint

Subscriber complaint is also important clew for finding the potential

interference. Information which should be collected from user complaint

includes MS number, MS model, called number and fault phenomenon

of calling side and fault phenomenon of called side and particular fault

location, etc. If the alarm information is more detailed, it will be more

easily to find out the network problems.

When there is interference in the network, the direct feelings of the

subscriber may be: heavy noise, both parties or either party cannot hear

each other clearly, call drops and call cannot be put through, etc. Thus,

when many subscribers within the same area complain the same

problem, then work should be done to check whether there is

interference in the area.


Discovering Interference via Drive Test

there are two Drive Test methods available: idle mode test and dedicated

mode test.

Under the idle mode, the test equipment can measure the signal level of both

serving cell and adjacent cell. In addition, the equipment can also perform

frequency scanning test to the specified frequency or frequency band.

Under dedicated mode test, the test equipment can measure the signal levels,

receiving qualities, power control registrations and time advance, etc. of both

the serving cell and adjacent cells. When high level (30) and low quality

(Rx_Qual6) remain in certain section, it can be concluded that interference

exists in the section. Further, part test equipment can directly display the

frame elimination rate (FER). Generally when the FER 25%, subscribers can

feel the discontinuous voice, that is to say, interference exists in this section of

highway.


Recommended Procedures for Location and Clearance of Interference

Determine Interference Cell according to Key Performance Index (KPI)

Check Alarm of OMC

Frequency Planning Check

Check Parameter Setting of Cell

Drive Test

Interference Clearance


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

the spectrum of the receiving signals. The receiving signal resolution bandwidth (RBW): namely the minimum signal bandwidth

the Spectrum Analyzer can recognize. The smaller the parameter is, the higher the receiving sensitivity of instrument.

Input frequency: the frequency range the Spectrum Analyzer can receive. Sensitivity: generally the minimum receiving level with 1Hz signal bandwidth is defined as

receiving sensitivity of the Spectrum Analyzer. The receiving sensitivity of HP85 series Spectrum Analyzer can be below -142 dBm.

Video Filter Bandwidth (VBW): it refers to the bandwidth of the intermediate filter after Frequency mixing of the Spectrum Analyzer. The smaller the bandwidth is, the smoother the curve is;

Central frequency (F0): it refers to the central frequency of the spectrum that the Spectrum Analyzer can test;

Bandwidth (SPAN): it refers to the spectrum span the Spectrum Analyzer can test; Input signal attenuation (ATT): when there is large signal input, it is required to make

attenuation on signal properly. The Spectrum Analyzer itself may produce large number of inter-modulation components without attenuation. So it will influence the veracity of the testing result.


Directional Antenna

Directional antenna is used for searching interference sources.

The stronger the directionality of antenna is, the higher the

antenna gain is. And the ability to search will become better. So

the logarithm-period antenna with broad frequency band is the

best choice. This kind of antenna has broad frequency band,

high antenna gain and strong directionality.


The way to Test Internal Interference

Set the Spectrum Analyzer to proper state:

For 900M BTS: f0=902MHz,SPAN=30MHz,ATT=0,RBW=30kHz,VBW=30kHz;

For 1800M BTS: f0=1715MHz,SPAN=10MHz,ATT=0,RBW=30kHz,VBW=30kHz.

Screw out the connector of output port of CDU divider, then connect the output signal of the divider to the Spectrum Analyzer to carry out a test. If the fractional frequency spectrum level is less than –80dBm, it shows that there is no internal interference; if more than –80dBm, it shows that CDU or TRX inside Base Station are under interference or self-excitation.

If internal interference exists, further make sure that it belongs to CDU or TRX. At first confirm TRX carrier board, cut down the cable via which TRX is connected to divider, and use the Spectrum Analyzer to test the main or diversity connector of TRX. If the fractional frequency spectrum level is less than –80dBm, it shows that TRX is normal, otherwise it is required to change carrier board.

The three steps above aim at interference measure for the uplink frequency band. If there is suspect that interference exists in the downlink frequency band, please follow steps below.

Check interference of transmission band. First, set the Spectrum Analyzer in transmission frequency band of the BTS. Due to the large ouput power of BTS, attenuation should be made on the input signal. Generally ATT is set as 40dB, then the tx_test signal of CDU should be imported to the Spectrum Analyzer to be observed to make sure weather interference signal is generated.


The way to Test External Interference When we are sure that interference is caused by the external cause, first we should

confirm the location of interference source and the spectrum distribution state.

First, set the Spectrum Analyzer to proper state.

Choose output port of divider of cell under interference.

Screw out the selected connector, then use Coaxial Cable to import the output signal of divider to the Spectrum Analyzer;

View the spectrum distribution state of the Spectrum Analyzer, and find out the abnormal interference signal. The way to calculate the level of interference signal is as follows:

− Antenna port interference level = interference level tested by the Spectrum Analyzer – 15dB Tower Top Amplifier Gain + 3dB cable loss – 7dB divider gain.

− The maximum interference level at antenna port without influence on system = -108dBm sensitivity – 9dB co-channel interference protection= -117dBm


The way to Search External Interference Sources

In the cell under interference, select a test point without building obstruction.

Set the Spectrum Analyzer, and connect the directional antenna.

If there is rotatable platform, the antenna can be placed on it, and make the wave

beam of the antenna point to the front, and the antenna with vertical polarization

should be placed vertically; if there is no interferent signal, one can raise the

antenna over head with hands. Rotate the antenna slowly, and at the same time

view the change of signal of the Spectrum Analyzer. Once there exists abnormal

signal, fix the orientation of the antenna immediately and change uptilt of the

antenna to make the receiving signal to the strongest.

Analyze the signal spectrum distribution carefully, and confirm that it is interference

signal, record the signal strength and record the azimuth and downtilt of antenna

wave beam.

Find new test point along with the direction of antenna wave beam, then return to

step 2 to carry out a test till interference source is found.



Chapter 2 Location and Clearance Chapter 2 Location and Clearance

of Interference of Interference



Antenna Performance Degradation

Fault description: There are 5 BTSs for a certain network in a

county configured as S4/4/4 and 6/6/6, the BTS type includes

BTS20 and BTS30. The interference band 5 in TCH

measurement function of some cells is over 15, and there is no

alarm information in OMC


Antenna Performance Degradation Fault location process

Register the statistics task of interference band of 24 hours for the cell with

problems, it is found that interference band 5 mainly occurs in daylight, and in the

small hours near middle night, the interference band value is almost 0.

After opening the idle BURST of all BTSs and transmitting it in the early morning, it

is found that the interference band occurs. It disappears after transmitting is

stopped. It can be judged from this phenomenon that the interference comes from

internal network and has nothing to do with other equipment.

No frequency in the network and data are modified before the interference occurs,

accordingly, the interference is irrelevant with the frequency planning.

It can be seen from the above second and third points that the problem is relevant

with the BTS equipment.

Observe the RXM test interface of CDU with the spectrum analyzer in peak hour in

the daylight, it can be seen that unstable strong broadband interference and rise of

back noise occur.



First replace all boards (TRX, CDU, FPU, HPA, and power board) of this BTS

(BTS20, with Tower Top Amplifier) one by one, at the same time observe the

spectrum signal of RXM test interface, it can be seen that interference exists all

the time. This indicates that the interference is relevant with the antenna feeder

(including divider, combiner, feeder, antenna, lightning arrester, Tower Top

Amplifier, jumper and connector) instead of the board.

Since the above BTS under test has the Tower Mount Amplifier, the antenna and

feeder check is inconvenient, replace another BTS30(S4/4/4) (dual-CDU, and

dual-polarization antenna) with interference and check the antenna feeder.

Since no interference exists in one of the cells while strong interference exists in

another two cells in the BTS, interchange the antenna and feeder (changing the

jumper at the top of the cabinet) of the cells which are with and without

interference in the BTS in the evening. Then send idle BURST, it is found that the

interference follows the antenna and feeder. This step helps further locate the fault

which should exist in antenna and feeder system.



The situation remains the same even after replacing lightning arrester of antenna

feeder and checking all jumper connectors. Then it can be sure that fault exists in the

feeder or antenna.

Replace the jumper (i.e., antenna) at the top of the tower, it is found that the

interference follows the antenna, so the feeder fault can be excluded while the

antenna fault is quite possible. (It should be noted that the external interference at

this step cannot be excluded because the actual installation place of the antenna

does not change, but the external interference has already been excluded in the

above step. )

Finally check the antenna. The strong interference disappears immediately after the

antenna is replaced on the tower by using the dual-polarization antenna. For further

verification, replace the antenna of one cell with strong interference in another BTS20

with a new one, and then the interference disappears, thus the problem is solved

here.


Antenna Connected Inversely

Fault description: the interference bands 4 and 5 often occur

in the traffic measurement after certain BTS is on service, the

inter-cell handover success rate is very low and the congestion

rate is up to 5%. There is no alarm in OMC.


Antenna Connected Inversely

Fault location process

Since the interference bands 4 and 5 occur along with low handover success rate and congestion, it is doubted that the interference causes the above phenomena.

Check the frequency planning first, no problem is found. The external interference becomes the chief consideration after the frequency planning problem is excluded. Change the original used frequency 9 into the far-away frequency 94 to avoid external interference, but situation si the same. Confirmation made with the operator’s branch office indicates that the BTS is remote and without any high-power radio equipment nearby. It looks as if the frequency planning or external interference should be excluded.

Since handover failure is involved at the same time, it is found that handover failure occurs between cells 1 and 3 according to the registration of outgoing/incoming cell handover performance measurement.


Antenna Connected Inversely Fault location process

The congestion analysis indicates that TCH assignment failure is usually

caused by uplink. After registering the traffic measurement of

uplink/downlink balance, it is found that the measurement item of

uplink/downlink balance for cell 1 and cell 3 focus on level 1 and 11. This

indicates that severe imbalance occurs between uplink and downlink.

The imbalance between uplink and downlink, in combination with much

handover failure in cell 1 and 3 turn the doubt to the antenna and feeder

which may be connected inversely.

On-site examination indicates that the antennas of cell 1/2/3 become

crossed pair which causes the transmitter antennas of cell 1 and 3 to stay

in the same cell, while the receiver antennas of them connect to another

cell. The interference band and congestion disappear and the handover is

all right after it is corrected.


Co-channel Interference

Fault phenomenon: The co-channel interference of Huawei

early 2.0 BTS (O2) in certain area leads to high call drop rate

and poor voice quality. Serious interruption with occasional

strong noise (whizz in general) occurs. It is after the BTS’s

normal running for a certain time that the call drop occurs.

Located in a little town (Du city) on the border of the city, the

BTS is surrounded by the BTSs of the other manufacturer.




The frequencies assigned for the BTS are 64, and 92 (64 is of BCCH

frequency).

In the optimizing test, the receiving quality (quality level is less than 3) is

continuously good as the downlink signal level is -95dBm in the direction

away from Huanggang and Du City. In the direction from Du City to

Huanggang, the receiving quality is also good when the receiving level is

more than -70dBm. Then move forward until to the place where TA=5,

the receiving quality is sometimes good, and sometimes more than 5 in

about 1 minute when the receiving level is about -75dBm. And network-

drop occurs frequently when Idle-mode test is made at this place. It is

suspected that downlink interference may exist on BCCH frequency.




Carry out continuous conversation test with one test MS and scan

test for 64# frequency with another MS. The test carried out again

in the section from Du City to Huanggang reveals that the signal

strength of 64# frequency is already less than -100dBm near the

Huanggang, and call drop already had occured. But the signal

strength of 64# frequency rises up to -65dBm and disappears

after a duration of 100 seconds when entering the downtown area

of Huanggang. So it can be judged that the co-channel

interference may be from the TCH frequency of cell nearby.




Carry out continuous conversation test with one test MS and scan test for 64#

frequency with another MS. The test carried out again in the section from Du City to

Huanggang reveals that the signal strength of 64# frequency is already less than -

100dBm near the Huanggang, and call drop already had occured. But the signal

strength of 64# frequency rises up to -65dBm and disappears after a duration of 100

seconds when entering the downtown area of Huanggang. So it can be judged that

the co-channel interference may be from the TCH frequency of cell nearby.

Carry out scan test for this frequency after arriving at the hotel, the signal level still

remains high, but the conversation is not implemented on this frequency. The next

day, carry out designated scan test at the place with the strongest signal of this

frequency in the street of Huanggang, and test in Idle mode with another test MS.

From the system message, it can be confirmed that 64# frequency is assigned to the

TCH in HG08 cell with BCCH frequency 45, and the conversation is actually

established once on 64# frequency in multiple times of conversation tests.




The interference disappears after the application is made to

the customer to modify the frequency. At the same time, the

customer should adjust the adjacent cell relation of peer

equipment.

Carry out test again at the place where the original co-

channel interference exists after the frequency is modified,

the call drop and network drop disappear, and conversation

quality Rx_Qual<3. It indicates that the problem of co-

channel interference (downlink) is solved.


Adjacent-channel Interference

Fault phenomenon:

The call drop ratio is universally high and even up to about

15% in busy hour after several BTSs are cut over. And it is

difficult for the call to be set up during on-site test. There is no

alarm message in OMC system


Adjacent-channel Interference Fault location process

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

cutover of new BTSs.

The transmission quality is good, and TRX test is carried out for the

above BTS with call drop and this indicates that each TRX is all right. No

fault is found by checking the data and carrying out the test for 32BIE

port corresponding to the BTSs. From the above analysis, the TRX fault,

BSC hardware fault, A interface circuit fault and transmission fault can be

excluded.

Analysis of traffic measurement result reveals that serious interference

occurs in each cell of the above several BTS. Most cells have

measurement values in interference band 4 and 5, and the number of

channels falling into interference band 5 in several cells is up to 7. So it

is sure that the interference in the above several cells is quite serious.


Adjacent-channel Interference


It is found that there are many adjacent channels and the

frequency planning is irrational after checking the frequency

configuration of above BTSs and the adjacent cells. Especially,

the area where the above BTSs are located, is newly added, and

interference exists among them. And they also have interference

in between them and the surrounding running BTSs.

Call drop disappears after adjusting and loading the frequency

configuration of this area.


Interference Caused by Over-coverage

Fault phenomenon: The hand over success rate of a certain GSM

network is low, the call drop rate is high and conversation quality is

poor. The hand over success rate is less than 80%, and the call drop

rate is more than 2%. It is found that there are many times of

downlink/uplink strength hand overs through view and analysis of

traffic measurement data. while there are many times of bad

downlink quality, and uplink strength among the times of

unsuccessful handover. The analysis of cause of call drop indicates

that the times of bad downlink quality are more than those of bad

uplink quality . There is no alarm message in OMC system


Interference Caused by Over-coverage Fault location process

From the result of traffic measurement, it can be judged that downlink interference may exist in the system or the coverage is not very good.

The actual result of Drive Test shows that the strength of outdoor signal can be up to -80dBm above in the downtown area, that is to say, the coverage is all right. But serious over-coverage exists. For example, the service cell used in the building where BTS A is located is cell B with the same BCCH frequency as cell A1, while cell B is 6 kilometers away from BTS A in the suburb. In this way, the problem exists in two aspects:

− 1. The signal of cell B forms co-channel interference which leads to poor downlink link quality in coverage area of cell 1 of BTS A. It is found that ”****” is displayed in the test MS when this cell is locked during the test.

− 2. When cell B is selected as the service cell, its adjacent cell is only geographically adjacent to it, while the cell near the BTS A does not function as its adjacent cell. So when its signal is unavailable, the “effect of isolated island” will occur because the signal of its adjacent cell is poor, too. Then hand over fault and even call drop will easily occur. The on-site survey shows that the antenna of cell B is hung 50m above. The data provided by the customer reveals that the tilt angle is 5°, which is actually far less than 5°.


Interference Caused by Over-coverage


The cause for bad network indices lies in over-coverage, so the

basic way is adopted to lower the antenna and adjust downtilt of it

to make actual coverage area consistent with planned coverage

area. Temporarily this problem can only be solved by adjusting the

network parameter. The following operations can be adopted: lower

the power level of cell B and add the adjacent cell of cell B, at the

same time increase the level threshold of candidate cell from 10 to

15. The network indices exhibit obvious improvement after the

above check and modification are performed for all cells in the

downtown area, moreover, the hand over success ratio rises up to

85%, while call drop ratio drops to 1.3%.


Repeater Interference Fault phenomenon: subscribers of a certain network complain that they

cannot occupy channel in some area for conversation since some day, or

noise is heavy after occupancy of channel though the signal of MS is strong

at this time. There are two directional BTSs in this area and both of them are

BTS30 with the version 05.0529. The azimuth of the first cell directs at north.

The BTS in this area works normally and the network indices conform to

requirement before the complaint. Both BSC and MSC are devices of

Huawei and two BTSs are connected in star mode. The traffic measurement

indices show that traffic of both BTSs decreases obviously after occurrence

of this problem, especially in the first and third cells. Although the signal of

channel is very strong, the quality of voice is poor. Then it can be seen from

traffic measurement that the interference band of these four cells is in class

three, four, and five, and 95% of channels are under interference. In addition,

interference of different classes also exists in other cells. So subscribers

complained strongly. And there is no alarm message in OMC system


Repeater Interference Fault location process

The feedback of subscribers shows the possible causes as follows: 1. Problem

occurs in transmission and leads to error code; 2. Problem occurs in antenna

feeder; 3. Fault exists in TMU; 4. Internal or external interference may exist.

The traffic measurement console shows the possible reasons as follows: 1. There

may exist strong uplink interference signal in the north lean to west in this area.

This leads to interference of different levels in the first, second and third cells,

especially in the first and third cells;

It is found that it is difficult to put through the call in the first and third cells through

on-site dial test. Although the call is put through, the quality of voice is very poor,

and the voice is intermittent seriously with strong interference. If MS subscriber

calls fixed telephone subscriber in this area, it is hard for fixed telephone subscriber

to hear the voice clearly, instead, MS subscriber can hear fixed telephone

subscriber clearly. This also proves the above analysis that the interference may be

external (It can be judged from this point that interference only exists in uplink.)


Repeater Interference Fault location process

Carry out test on site with antenna feeder analyzer. No problem of BTS itself is

found, and the situation remains the same after TMU is replaced. Therefore, we ask

customers whether there are such newly constructed equipment as microwave

station, repeater, etc. surrounding the BTS. They told that they didn’t set up them.

It is said that China Mobile sets up a repeater in the area, which is located about

two kilometers away in the north lean to the west of both BTSs of Huawei, and when

it is activated, the problem will occur in Huawei BTS. Then customers negotiated

with China Mobile several times. Finally China Mobile carried out on-site test with

Huawei engineers together, and found that only if the repeater is switched off, the

interference band and call become normal along with the recovery of Huawei BTS, if

the repeater is activated, problem will occur soon in Huawei BTS, i.e. call cannot be

put through, or interference is strong even after call is put through. The traffic

measurement relevant with interference band in two different cases was handed on

to China Mobile, and they approved our viewpoint.

Finally the problem is solved with the coordination of China Mobile.


Microwave Interference

Fault phenomenon: it is found that call drop rate in the second

and third cell of a certain BTS (S2/2/2) in traffic measurement

increases abruptly. Call drop rate is up to about 20% at some

time.




View of BSC traffic measurement shows that idle TCH number in interference

band in this BTS begins to increase in interference band 3-5 at about 8:30, in

interference band 4, 5 at 10:00, and in interference band 1 at about 22:00. It

can be judged from the above phenomenon that interference exists.

Since this BTS operates well before, the problem of frequency planning can be

excluded.

Perform power-off restart and replace board for BTS. But interference still

exists, so the possibility of TRX self-excitation can be excluded.

TRX management information reveals that interference exists in four boards of

the second and third cell in this BTS, and the possibility of damage of the four

boards at the same time is little, so the problem of TRX can be excluded. TRX

board is replaced for caution, but interference remains


Microwave Interference Fault location process

View of all BSC traffic measurement data shows that interference of different

levels exists in the cells of all BTSs nearby, which is facing the same direction

as the second and third cells in this BTS. And sometimes SDCCH channels in

the cells with serious interference are all occupied at the same time, while the

occupancy ratio of SDCCH at the same time is very little according to the

amount of subscriber. So it can be sure that external interference exists in

uplink, but the interference may be relevant with direction instead of

frequency.

To further locate, jumpers of the first and third cell are interchanged on the

rack top. As a result, it is found that interference occurs in the first cell, but

interference disappears in the third cell, so this has proved the above

judgement.

Since interference is not relevant with frequency, BTS interference may be

caused by high-power signal sent into BTS system.




Measurement on BTS divider output port with spectrum analyzer shows

that high-power signal exists on 904MHz frequency (5M away from the

used frequency), and this signal level come up to about –25dbm in BTS

with serious interference, while in other BTS it is about –50dbm. So it

can be judged that this signal has impact on BTS.

After frequency scanning around BTS with spectrum analyzer, it is found

that a microwave antenna outputting high-power signal is at a frequency

of 904.

When the microwave equipment is switched off and TRX management

information is traced, the interference disappeared.


False Interference

Fault phenomenon: the reconstruction of some office found

that the interference band of an S6/6/6 BTS20 in a county is

very high. Theinterference band of two cells in it changed from

5 to 7.


False Interference Fault location process

During the past interference test ofthis office, some cells with high

interference band were located. This is because antenna intermodulation

resulted in the interference of normal signal in case of large traffic and it

made the interference band very high. So change the antenna, but the

interference band did not fall down obviously after that.

During the prior period, the frequency planning has been checked

several times, and the ad-frequency interference has been avoided

basically. At the same time the frequency that may cause third order

intermodulation in the cell has been adjusted. The problem of frequency

planning has been basically excluded. It is unknown whether there is

something wrong with Tower Top Amplifier, feeder or connector. But no

problem was found after checking the antenna feeder system of the BTS

several times.


False Interference Fault location process

Trace the state of interference band of each channel via Maintenance

Console and find that high interference band mainly focuses on the four TRXs

of the cell.

First set the frequency with less interference to the TRX with more

interference, and find that the interference band does not change. It shows

there is no relation to the frequency. Whether there is something wrong with

TRX board or not is unknown. However, the situation is unchanged after the

TRXs were interchanged.

Then confirm the TRX numbers, and find that the four carriers come from the

same divider. From the above process, the problem may be the divider. But it

is found that the interference band becomes high when the BTS is expanded

from S4/4/4 to S6/6/6. While the combiner/divider used to be normal. Maybe

the devices break down after some time. So we decided to change the

combiber/divider to have a try.


False Interference


The idea that four carriers with high interference band are connected to

the same second level divider before the divider is changed. The 7-dB

gain of the divider has been considered during interference judgement.

However, if the DIP switch is not moved to the right place while cascaded,

the interference signal will be amplified by 7dB. Simultaneously, higher

interference band will exist because the calculating error of 2.0 station

itself is up to 5dB. Check the DIP switch of the divider. It was really not

moved to “Off”. After it is corrected, view the traffic measurement of

interference band 4 and 5. They fall down to 0. And there is a little value

on the interference band three.


Overview

Location and Clearance of Interference

Case Analysis

SummarySummary

www.huawei.com

Thank You

[Huawei] GSM Interference Analysis Training

Documents

rxm test interface

tch measurement

spectrum distribution

call drop

broad frequency

continuous

cell initiates

tower top