Part7 GSM Interference Analysis and Optimization

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1 © Nokia Siemens Networks

NSN Internal Document

GSM Interference Analysis/JP/NNPO/ 1st to 3rd July 2010 @ VF RoB

GSM Interference Analysisand Optimization

NPO Refresher CourseJuly, 1st to 3rd 2010

Vodafone MS – RoB

Jignesh Parmar

[email protected]

Nokia Siemens Networks

National NPO, Ahmedabad, India






What is interference?

• Unwanted signal to the receiver

– Internal System Interference

Resulting from tight frequency re-use

Multipath

Hardware Fault

Spurious transmission

Repeater malfunction

– External Interference

Radar

Other Communication system

Jammer






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 manufacturecause or application, which will cause self-excitation of TRX

amplification circuit resulting in interference. – Spurious emission and inter-modulation: if out-band spurious emission

index of BTS TRX or amplifier exceeds the limit, or isolation betweenTX and RX antennas is not enough, all these will form interference onthe receiving channel. Inter-modulation may also occur in passiveequipment such as passive antenna and feeder.

– Improper frequency planning

– Co-frequency interference

– Adjacent frequency interference







Communication• Repeater interference

– If the installation of repeater is non standard, causing insufficient

isolation between the donor antenna and the serving antenna, so self-excitation is caused. This affects normal working of BTS that therepeater 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 belarge, and this will cause interference to the BTS around.

– If the repeater is malfunctioning due to some fault it will go inoscillation mode results in interference.







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 itstransmitting power was very high, which generally reached tens andhundreds of kilowatts, so the out-band spurious emission iscomparatively large. Thus, it easily causes interference to the BTS.

– Analog BTS: The frequency band used by the analog mobile BTSoverlaps with the GSM frequency band in certain segment.

– Communication equipment at same frequency band: As the types ofcommunication equipment are so many, some manufacturers maybeadopt the frequency band but doesn’t comply with the current

communication standards. As the GSM frequency band is occupied bythe equipments, interference will be caused within the GSM systemcoverage area.






Impact of Interference

• When there is interference in the network, the subscribersusually 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 subscribercalls fixed telephone subscriber, call drop occurs after “du, du , du” isheard.

– The conversation cannot be carried on smoothly, and call drop oftenhappens.

– When interference exists in the network, from the result of trafficstatistic, 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, it isfound that the bit error rate is higher than other cells.






Cellular network

• partial overlap of cells

• only a few frequencies per cell

• frequency re-use distance

1

1

2

2

4

4

5

5

6

6 7

7

3

3

re-use distance

Carrier to Interference Ratio

Interference and reuse/reuse distance






Receiver Sensitivity Level impacts Interference &

Noise

Level[dBm

]

-80

-60

-40

-20

0

20

-100

Bandwidth

-97 dBm

min.sensitivity

GSM (BTS)

-88 dBm min.

sensitivity

GSM (BTS)

picoBTS

-104 dBm min.

sensitivity GSM (BTS)

normalBTS micro

BTS

C/N

C/N C/N

The necessaryCNR increases






Carrier to Interference Ratio Requirements

Following GSM 05.05:

Reference interference ratio for all BTS and MS types:

• For co-channel interference: C/Ic = 9 dB

• For (first) adjacent channel interference: C/Ia1 = - 9 dB

• For (second) adjacent channel interference: C/Ia2 = - 41 dB

• For (third) adjacent channel interference: C/Ia3

= -49 dB

At these values, the so called reference interference performance in terms of(maximum) frame erasure rate, bit error rate or residual bit error rate must be metfor the different type of channels in different specified propagation conditions (TU3

no FH, TU3 ideal FH, TU50 no FH, TU50 ideal FH, RA250 no FH).






Carrier to Interference Ratio Requirements

Distance

-80

-60

-40

-20

0

20

-100

Level

[dBm]

Distance

CIR CoCH

+ 9 dB

R x L E

V R x L

E V

Quality (Co Ch related on reuse distance; cs-service)

Server cell

Neighbour cell

Co-channel interference: 9 dB






Adjacent Channel Interference

Distance

-80

-60

-40

-20

0

20

-100

Adjacent channelInterference BTS and MS

Level[dBm] R

x L E V

f

1

+ 9 dB

Distance

R x L

E V

f 2

- 9 dB

Serving cell

CIR AdCH

Neighbourcell

R x L E V

f 2 m e a

s u r e d a t

f 1

CIR CoCH

Quality limitof servingcell

Adjacent channel related quality (CS)







Adjacent Ch. Interference; cs -service

200 kHz

F 1

-80

-60

-40

-20

0

-100

PWRLevel[dBm

]

CIR AdCH

+ 9 dB

Frequencyrange

200 kHz

F 2

10

30 dB21 dB






Carrier to Interference Ratio : Exercise

• Carrier = - 79 dBm

• Interference = - 70 dBm

• Carrier to Interference Ratio (C/I) = - 9 dB

• Interfering signal is 9 dB higher than Wanted Signal

• Carrier = - 80 dBm• Interference = - 104 dBm

• Carrier to Interference Ratio (C/I) = 24 dB

• Interfering signal is 24 dB lower than Wanted Signal







200 KHz

30 dB

9 dB

fC - 100KHz

fC + 100KHz

Surface below black curvefrom fC - 100 KHz to fC + 100

KHz -> serving power

Surface below red curvefrom fC - 100 KHz to fC +100 KHz -> interferingadjacent power

Must be at least 9dBsmaller than surface belowblack curve






Locating Interference






Discovering Interference via OMC Traffic

MeasurementDiscovering potential interference via traffic statistics result

• Check the “Average TCH busy time (second)” in TCHmeasurement function of each cell, the reason is that thisindex can show the TCH mean occupied time (s”), which isusually called “TCH mean holding time” in the BSC of othermanufacturer”, within the measurement period. If it is foundthat the Average TCH busy time (second) of certain cell is

comparatively short (such as less than 10s), then maybethere is strong interference in the cell, causing thathandover/call drop happens due to bad quality after TCHchannel occupied to MS.

• Certainly, if hardware fault occurs in certain TRX (non-BCCHor non-SDCCH carrier) of a cell, the case mentioned abovewill also appear.






Quality

Interference analysis DL /UL

Rx Quality x Rx Level

Coverage Problem:

Bad quality and

Low Rx Level

Interference Problem:Bad quality and

High Rx Level

Good Quality

High Rv Level

HW Problem:

Bad Quality

for all Rx Levels

NWD report 204 model

HW Problem

All samples below -100dBm

CL10 <-100dBm

Same level – quality distributionfor both UL and DL








Discovering interference via handover data

• When certain cell initiates handover, if the average receivingquality (uplink) is ≥ 4 (this is true when there is no frequencyhopping, if there is, it should be ≥5), and the mean receiving

level is ≥25(-85dBm), then it is possibly caused by uplinkinterference.

• When certain cell initiates handover, if the times of receivingquality 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 handoverattempt times, then there may be interference in the cell. Thetwo indices are all related to quality handover threshold andinterference 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 other

with same traffic load, and the main cause of call drop isowning 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 makemeasurement 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 measurementindex of interference band can reflect the cell interferencesituation more directly, but it can only reflect whether there isinterference in the uplink.

• If the values of interference band 4 and interference band 5are comparatively large (≥1), then, there may be co-frequencyinterference in the cell. If the measurement values mainly

distribute in interference band 1 and interference band 2, thenthe possibility of interference will be small. However, if thereis comparatively high value in band 3, then attention shouldbe paid to this.






OMC Alarm and Subscriber Complaint

• Subscriber complaint is also important clew for finding thepotential interference. Information which should be collectedfrom user complaint includes MS number, MS model, called

number and fault phenomenon of calling side and faultphenomenon of called side and particular fault location, etc. Ifthe alarm information is more detailed, it will be more easily tofind out the network problems.

• When there is interference in the network, the direct feelingsof the subscriber may be: heavy noise, both parties or eitherparty cannot hear each other clearly, call drops and callcannot be put through, etc. Thus, when many subscribers

within the same area complain the same problem, then workshould be done to check whether there is interference in thearea.






Discovering Interference via Drive Test

There are two Drive Test methods available: idle mode test anddedicated 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 tothe specified frequency or frequency band.

• Under dedicated mode test, the test equipment can measure

the signal levels, receiving qualities, power controlregistrations and time advance, etc. of both the serving celland adjacent cells. When high level (≥30) and low quality(Rx_Qual≥6) remain in certain section, it can be concludedthat interference exists in the section. Further, part test

equipment can directly display the frame elimination rate(FER). Generally when the FER ≥25%, subscribers can feelthe discontinuous voice, that is to say, interference exists inthis section of highway.






Finding the Source of Interference






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 displaythe spectrum of the receiving signals.

• The receiving signal resolution bandwidth (RBW): namely the minimum signal bandwidth theSpectrum 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 asreceiving sensitivity of the Spectrum Analyzer. The receiving sensitivity of HP85 seriesSpectrum Analyzer can be below -142 dBm.

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

• Central frequency (F0): it refers to the central frequency of the spectrum that the SpectrumAnalyzer 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 makeattenuation on signal properly. The Spectrum Analyzer itself may produce large number ofinter-modulation components without attenuation. So it will influence the veracity of thetesting result.






Directional Antenna

• Directional antenna is used for searching interferencesources. The stronger the directionality of antenna is, thehigher the antenna gain is. And the ability to search will

become better. So the logarithm-period antenna with broadfrequency band is the best choice. This kind of antenna hasbroad frequency band, high antenna gain and strongdirectionality.






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, itshows 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 buildingobstruction.

• Set the Spectrum Analyzer, and connect the directional antenna.

• If there is rotatable platform, the antenna can be placed on it, and makethe wave beam of the antenna point to the front, and the antenna withvertical polarization should be placed vertically; if there is no interferentsignal, one can raise the antenna over head with hands. Rotate theantenna slowly, and at the same time view the change of signal of the

Spectrum Analyzer. Once there exists abnormal signal, fix the orientationof the antenna immediately and change uptilt of the antenna to make thereceiving signal to the strongest.

• Analyze the signal spectrum distribution carefully, and confirm that it isinterference signal, record the signal strength and record the azimuth anddowntilt of antenna wave beam.

• Find new test point along with the direction of antenna wave beam, thenreturn to step 2 to carry out a test till interference source is found.






Antenna Performance Degradation

Fault description: There are 5 BTSs for a certain network in acounty configured as S4/4/4 and 6/6/6, the BTS type includesBTS20 and BTS30. The interference band 5 in TCH

measurement function of some cells is over 15, and there is noalarm information in OMC







Fault location process

• Register the statistics task of interference band of 24 hours for the cellwith problems, it is found that interference band 5 mainly occurs indaylight, 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 aftertransmitting is stopped. It can be judged from this phenomenon that theinterference 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 frequencyplanning.

• 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 broadbandinterference and rise of back noise occur.








• First replace all boards (TRX, CDU, FPU, HPA, and power board) of thisBTS (BTS20, with Tower Top Amplifier) one by one, at the same timeobserve the spectrum signal of RXM test interface, it can be seen that

interference exists all the time. This indicates that the interference isrelevant 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 theantenna feeder.

• Since no interference exists in one of the cells while strong interferenceexists 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 withand without interference in the BTS in the evening. Then send idle BURST,it is found that the interference follows the antenna and feeder. This stephelps further locate the fault which should exist in antenna and feedersystem.








• The situation remains the same even after replacing lightning arrester ofantenna feeder and checking all jumper connectors. Then it can be surethat fault exists in the feeder or antenna.

• Replace the jumper (i.e., antenna) at the top of the tower, it is found thatthe interference follows the antenna, so the feeder fault can be excludedwhile the antenna fault is quite possible. (It should be noted that theexternal interference at this step cannot be excluded because the actual

installation place of the antenna does not change, but the externalinterference has already been excluded in the above step. )

• Finally check the antenna. The strong interference disappearsimmediately 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 thenthe interference disappears, thus the problem is solved here.






Antenna Connected Inversely

Fault description: the interference bands 4 and 5 often occurin the traffic measurement after certain BTS is on service, theinter-cell handover success rate is very low and the congestion

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



37 © Nokia Siemens NetworksNSN Internal Document

GSM Interference Analysis/JP/NNPO/ 1st to 3rd July 2010 @ VF RoB Page 37



• 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.







• The congestion analysis indicates that TCH assignment failure is usuallycaused by uplink. After registering the traffic measurement ofuplink/downlink balance, it is found that the measurement item ofuplink/downlink balance for cell 1 and cell 3 focus on level 1 and 11. Thisindicates that severe imbalance occurs between uplink and downlink.

• The imbalance between uplink and downlink, in combination with muchhandover 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 stayin the same cell, while the receiver antennas of them connect to anothercell. The interference band and congestion disappear and the handover is

all right after it is corrected.




GSM Interference Analysis/JP/NNPO/ 1st to 3rd July 2010 @ VF RoB Page 39

Co-channel Interference

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. Seriousinterruption 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 alittle town (Du city) on the border of the city, the BTS is surrounded by the BTSsof the other manufacturer.

C h l I f







• The frequencies assigned for the BTS are 64, and 92 (64 is ofBCCH frequency).

• In the optimizing test, the receiving quality (quality level isless than 3) is continuously good as the downlink signal levelis -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 morethan 5 in about 1 minute when the receiving level is about -

75dBm. And network-drop occurs frequently when Idle-modetest is made at this place. It is suspected that downlinkinterference may exist on BCCH frequency.



C h l I t f







• Carry out continuous conversation test with one test MS and scan test for64# frequency with another MS. The test carried out again in the sectionfrom Du City to Huanggang reveals that the signal strength of 64#frequency is already less than -100dBm near the Huanggang, and calldrop already had occured. But the signal strength of 64# frequency risesup to -65dBm and disappears after a duration of 100 seconds whenentering the downtown area of Huanggang. So it can be judged that theco-channel interference may be from the TCH frequency of cell nearby.

• Carry out scan test for this frequency after arriving at the hotel, the signallevel still remains high, but the conversation is not implemented on thisfrequency. The next day, carry out designated scan test at the place withthe 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 beconfirmed that 64# frequency is assigned to the TCH in HG08 cell withBCCH frequency 45, and the conversation is actually established once on64# frequency in multiple times of conversation tests.

Co channel Interference







• The interference disappears after the application is made tothe customer to modify the frequency. At the same time, the

customer should adjust the adjacent cell relation of peerequipment.

• 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 conversationquality Rx_Qual<3. It indicates that the problem of co-channelinterference (downlink) is solved.



Adjacent channel Interference





Adjacent-channel Interference


• All BTSs are connected to the same BSC, and call dropoccurs after cutover of new BTSs.

• The transmission quality is good, and TRX test is carried outfor the above BTS with call drop and this indicates that eachTRX is all right. No fault is found by checking the data andcarrying 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 seriousinterference 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 interferenceband 5 in several cells is up to 7. So it is sure that theinterference in the above several cells is quite serious.

Adjacent channel Interference





Adjacent-channel Interference


• It is found that there are many adjacent channels and thefrequency planning is irrational after checking the frequency

configuration of above BTSs and the adjacent cells.Especially, the area where the above BTSs are located, isnewly added, and interference exists among them. And theyalso 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 certainGSM network is low, the call drop rate is high and conversationquality 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 aremany times of downlink/uplink strength hand overs throughview and analysis of traffic measurement data. while there aremany times of bad downlink quality, and uplink strength among

the times of unsuccessful handover. The analysis of cause ofcall drop indicates that the times of bad downlink quality aremore than those of bad uplink quality . There is no alarmmessage in OMC system








• 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-coverageexists. 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 cellis 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 showsthat 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, sothe basic way is adopted to lower the antenna and adjust

downtilt of it to make actual coverage area consistent withplanned coverage area. Temporarily this problem can only besolved by adjusting the network parameter. The followingoperations can be adopted: lower the power level of cell B

and add the adjacent cell of cell B, at the same time increasethe level threshold of candidate cell from 10 to 15. Thenetwork indices exhibit obvious improvement after the abovecheck and modification are performed for all cells in the

downtown area, moreover, the hand over success ratio risesup to 85%, while call drop ratio drops to 1.3%.

Repeater Interference






Fault phenomenon: subscribers of a certain network complain that theycannot occupy channel in some area for conversation since some day, ornoise is heavy after occupancy of channel though the signal of MS is strongat 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 torequirement before the complaint. Both BSC and MSC are devices ofHuawei and two BTSs are connected in star mode. The traffic measurementindices show that traffic of both BTSs decreases obviously after occurrence

of this problem, especially in the first and third cells. Although the signal ofchannel is very strong, the quality of voice is poor. Then it can be seen fromtraffic measurement that the interference band of these four cells is in classthree, 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








• The feedback of subscribers shows the possible causes as follows: 1.Problem occurs in transmission and leads to error code; 2. Problemoccurs 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 westin this area. This leads to interference of different levels in the first, secondand 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 cellsthrough on-site dial test. Although the call is put through, the quality ofvoice is very poor, and the voice is intermittent seriously with stronginterference. 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 alsoproves the above analysis that the interference may be external (It can be judged from this point that interference only exists in uplink.)



Microwave Interference





Fault phenomenon: it is found that call drop rate in the secondand third cell of a certain BTS (S2/2/2) in traffic measurementincreases abruptly. Call drop rate is up to about 20% at some

time.







• View of BSC traffic measurement shows that idle TCH number ininterference band in this BTS begins to increase in interferenceband 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 abovephenomenon that interference exists.

• Since this BTS operates well before, the problem of frequencyplanning can be excluded.

• Perform power-off restart and replace board for BTS. Butinterference still exists, so the possibility of TRX self-excitation canbe excluded.

• TRX management information reveals that interference exists infour 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, sothe problem of TRX can be excluded. TRX board is replaced forcaution, but interference remains







• View of all BSC traffic measurement data shows that interferenceof different levels exists in the cells of all BTSs nearby, which isfacing the same direction as the second and third cells in this BTS.

And sometimes SDCCH channels in the cells with seriousinterference are all occupied at the same time, while the occupancyratio of SDCCH at the same time is very little according to theamount of subscriber. So it can be sure that external interferenceexists 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 inthe first cell, but interference disappears in the third cell, so this hasproved the above judgement.

• Since interference is not relevant with frequency, BTS interferencemay be caused by high-power signal sent into BTS system.







• Measurement on BTS divider output port with spectrumanalyzer shows that high-power signal exists on 904MHz

frequency (5M away from the used frequency), and this signallevel come up to about –25dbm in BTS with seriousinterference, while in other BTS it is about –50dbm. So it canbe 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-powersignal is at a frequency of 904.

• When the microwave equipment is switched off and TRX

management information is traced, the interferencedisappeared.

False Interference





Fault phenomenon: the reconstruction of some office found thatthe interference band of an S6/6/6 BTS20 in a county is veryhigh. Theinterference band of two cells in it changed from 5 to 7.

False Interference



58 © Nokia Siemens NetworksNSN Internal Document GSM Interference Analysis/JP/NNPO/ 1st to 3rd July 2010 @ VF RoB


• During the past interference test ofthis office, some cells withhigh interference band were located. This is because antennaintermodulation resulted in the interference of normal signal incase of large traffic and it made the interference band veryhigh. So change the antenna, but the interference band didnot fall down obviously after that.

• During the prior period, the frequency planning has been

checked several times, and the ad-frequency interference hasbeen avoided basically. At the same time the frequency thatmay cause third order intermodulation in the cell has beenadjusted. The problem of frequency planning has beenbasically excluded. It is unknown whether there is somethingwrong with Tower Top Amplifier, feeder or connector. But noproblem was found after checking the antenna feeder systemof the BTS several times.

False Interference





• Trace the state of interference band of each channel viaMaintenance Console and find that high interference band mainlyfocuses on the four TRXs of the cell.

• First set the frequency with less interference to the TRX with moreinterference, and find that the interference band does not change. Itshows there is no relation to the frequency. Whether there issomething wrong with TRX board or not is unknown. However, thesituation is unchanged after the TRXs were interchanged.

• Then confirm the TRX numbers, and find that the four carrierscome from the same divider. From the above process, the problemmay be the divider. But it is found that the interference bandbecomes high when the BTS is expanded from S4/4/4 to S6/6/6.

While the combiner/divider used to be normal. Maybe the devicesbreak down after some time. So we decided to change thecombiber/divider to have a try.

False Interference





• The idea that four carriers with high interference band areconnected to the same second level divider before the divider

is changed. The 7-dB gain of the divider has been consideredduring interference judgement. However, if the DIP switch isnot moved to the right place while cascaded, the interferencesignal will be amplified by 7dB. Simultaneously, higher

interference band will exist because the calculating error of2.0 station itself is up to 5dB. Check the DIP switch of thedivider. It was really not moved to “Off”. After it is corrected,view the traffic measurement of interference band 4 and 5.They fall down to 0. And there is a little value on theinterference band three.

Content: Interference Analysis




• Statistics

– DL C/I and interference matrix

– UL receive level of unused time slots

• Drive Tests

DL - C/I and Interference Matrix (BCCH)






BTS-7

BTS-5BTS-11

51 multiframe of BTS-5



T51 start time0 235400

T 5 1 , B

T S - 5

T 5 1 , B

T S - 1 1

T 5 1 , B

T S - 7

Search for

interferers

by drive tests

BTS operating with the same frequencies can be distinguished from each other nevertheless, as the multiframes are not synchronized with each other. So e.g. the strong BCCH signals of nearby BTSs usually arewell separated in time.

• 51 multi frame -> Measurements of the BCCH

• 26 multi frame -> Measurements of the SACCH

DL - C/I and Interference Matrix (BCCH)






• Detection of interferers:The RX level and RX quality measurements can be performed on per TRX basis.

Non hopping network -> with affected TRX single affected frequency recognized

Hopping network -> affected MA list recognized onlyOften there are several nearby cells using the affected frequency or one of the

frequencies of the affected MA list

• Manual search for interferers:

The potentially interfering base stations have to be switched off temporarily - Very

time consuming. Especially in hopping networks hardly possible to find out theinterferers

• Search for interferers on basis of traffic:

If the interference level increases with the traffic in a potentially interfering cell, than

very probably this is indeed an interfering source

UL Receive Level of Unused Time Slots






Measurement of uplink receive level on idle channels = uplink interference

Averaging over interferenceAveragingProcess (AP) = 1..32 SACCHperiods

Classification into interference bands based oninterferenceAveragingProcess (BO1..BO4) = -110..-47 dBm

BSC tries to allocate TCH from best interference band (can be requested byMSC)If not available, BSC tries to take TCH from next band

0 71 2 3 4 5 6

rxLevUL = -75 dBm

BO5 –47 (fixed)

BO0 –110 (fixed)

BO4 -90

BO3 -95

BO1 -105BO2 -100

Interference






• Detection of interference:

•UL and DL RX level and RX quality statistics for busy TCH:

For a RX level guaranteeing a stable connection (for outdoor higher than about -90dBm) most of the RX quality measurements should indicate either quality 0 or 1

It is an indicator for interference if speech quality is bad and level high.

• UL RX level of un-used TCH:

The BTS can measure the RX level of unused time slots. Any signal detected there

indicates interference directly.

Drive TestsExample drive test






p

E ample dri e test

Drive Tests






Example drive test

Drive Test - Exercises






Case study

• Cell 11 is...

showing high drop rate in NMS

statistics. Customer complaints

point to a possible problem

area around a particular road

crossing.

• Cell 11 is ...

covering part of a suburban city

area including a road crossing.

There are other cells in the

area (see sketch), but Cell 11 is

dominant in and around the

road crossing.

Cell 11(high calldrop rate)

Cell 11( dominant inroad crossing )







Information

• No frequency assignment problems on neighboring

cells

• Drive tests confirm that the road crossing is causing

drops on very many calls

• Cell 11 has signal level around –85dBm at the road

crossing

• Downlink RXQUAL values are showing worse quality

than other cells in the area

• Uplink RXQUAL is showing same characteristic as

other cells in the area (no problem)

High traffic ?

High interference

(fading) ? Cell 11( dominant inroad crossing )

- 85 dBm Call drops







Cause of the problem

Problem is caused by acompetitor site at the building in

the upper right side of the roadcrossing. The competitor cellprovides around –35 dBm at theroad crossing, which createssignal differences in "our"network of around 50dB

between the serving level andthe interfering level.

Competitor Cell


- 35 dBm

- 85 dBm







Solution

The only solution is to provide ahigher signal level at the road

crossing, so that the signaldifference gets lower than thecurrent 50 dB.

• The probable solution is to add anew site close to the roadcrossing.

• Plus frequency reallocations

Competitor

Cell


- 35 dBm

- 85 dBm

Adding

New Site - 40 dBm

Rx







GSM specs requires that amobile must overcome this kind

of interference when the signaldifference is around 40dB even if

the serving level is close to the

sensitivity level of the mobile.

There is generally no exactspecifications for this kind ofinterference, but it is commonlyknown, that this phenomenon is

happening.

Note: The competitor channelis not adjacent to the serving

channel, it might be separatedby several MHz!

200kHz

F 1

-80

-60

-40

-20

0

-100

Rx

Level[dBm]

CIR (competitor - server) max. 40

dB allowed

Frequencyrange

200kHz

F n

10


-35 dBm

Competitor

„ channel“

-85 dBm

New BTSDrive Test - Exercises






A new BTS with a 3+2+3 configuration has been implemented in the network during night-time.The implementation engineer standing next to the BTS has made successful test calls on alltimeslots of all TRX's.Using a test mobile a short drive test has been done at approx. 500mfrom the site and the following verifications have been done :

· The directions of all 3 sectors by looking at the BCCH frequencies

· The functionality of the handover relations between the sectors

· The uplink cabling of each TRX by making one outdoor test call on each TRX

The implementation engineer parks his car to make some phone calls to arrange the rest of hiswork. The first call is successful. Another call is also successful. The 3rd call he has to make

fails during the setup. Again he makes some phone calls to check the new site and get's thefollowing results :1. OK2. OK3. NOK4. OK

5. OK6. NOK7. OKAll the time he is standing in the area between sector 1 and 2 and making phone calls onsector 1.







Description of the actions

• The BSS Technician went on site for checking the hardware, and the operator's OMC

controller checked the alarms. Nothing was found. The frequency plan was checked, it was

clear that too many frequencies were involved here and that interference can't easily beavoided. Different parameter fine tuning trials were implemented, but they didn't improve

anything. Base band hopping was deactivated for some hours to have statistics per TRX.

But nothing came out of this test.

• Some frequencies were swapped and changed, but no real improvement was seen. The

problem should come from an external source or from a problem with the hardware itself

(intermodulation problem was proposed as it was seen on other cells already).

• The operator went back on site to measure the UL interference, and concluded that UL

interference is coming from MS of other cells. It was reported that the MS were coming from

the back.







Solution 1

Change the antenna

Solution 2

Move the sector

Cause of problem

It turned out that the side and back lobes fromthe antenna were picking-up UL signals fromMSs on cells behind

Solution

• Replace the antenna with one which hasless back and side lobes

• Move the antenna on the roof, so that thebuilding is a screen for the MS on the cells

behind and the back/side lobes. The MSsoperating on the cells behind (co-channels)

are not seen by the antenna's back/side

lobes anymore






Thank You

Documents

Part7 GSM Interference Analysis and Optimization