GSM RNO Subject-Network Interference and Solutions_R2.0

Network Solutions R2.0

Interference

and

Network Interference and Solutions

Internal Use Only▲

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I



Revision Revision History Product Version

Document Version

Serial Number

Reason for Revision

R1.0

First published

R2.0

1. The TaAllowed parameter related to overshooting is added. 2. The networking optimization analysis analysis (adopting co-BCCH, multi-TRX cascading, and cell layer-dividing techniques) is added. 3. The interference between GSM and CDMA networks is introduced. 4. The contents about MR is aded.

Author Date

Document Docume nt Version

Prepared by

Reviewed by

2009-3-19

R1.0

Su Shaoli

Gan Wenjun

2011-3-22

R2.0

Xie Jin

Zheng Hao



Approved by

II


Intended audience:

Proposal: SEQ 1


GSM Network Optimization Engineers

Before reading this document, you had better have the following knowledge and skills. Knowledge Knowled ge and skills

Null

Reference material materia l Null

2 3

Follow-up document: After reading this document, you may need the following information. SEQ 1

Reference Reference material Null

Information Null

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III



About This Document Summary Chapter 1

GSM Frequency Allocation

Description Introduces GSM frequency allocation.

2 Phenomena and Classification of Interference

Introduces interference phenomena and classification and relevant causes.

3 Flow of Handling Interference Problem

Introduces the flow to handle interference problems.

4 Analytical Analytical Methods Methods of Interference Problem

Introduces analytical methods of interference problems.

5

Introduces typical cases to handle GSM interference.

Typical Cases



IV



TABLE OF CONTENTS 1

GSM Frequency Allocation.......................................................................... 1

2 2.1 2.2 2.2.1 2.2.2 2.2.3 2.3 2.3.1 2.3.2

Phenomena Phenomena and Classification Classification of Interference Interference ................... ..................... ........... ............. ... 2 Phenomena Phenomena and Classification Classification of Interference Interference ................... ..................... ........... .................. ........ 2 Internal Interference ...................................................................................... 2 Interference Interference due to Unreasonable Unreasonable Frequency Planning ...................... ........... ..................... ............. ... 2 Interference Interference due to Skip-Zone Skip-Zone Coverage ..................... .......... ...................... ...................... ...................... ............. .. 2 Interference Interference due to Equipment Problem........... Problem...................... ..................... ..................... ...................... ............... .... 3 External Interference ..................................................................................... 4 Interference Interference due to Unreasonable Setting of the Repeater ...... ...................... ........... ............. .. 4 Interference Interference Caused by Insufficient Insufficient CDMA Antenna Isolation ......... ................. ........... ...... 4

3

Flow Flow of Handling Handling Interference Interference Problem............. Problem........................ ..................... ..................... ..................... .......... 6

4 4.1 4.1.1 4.1.2 4.1.3 4.2 4.2.1 4.2.2 4.2.3 4.2.4 4.3 4.3.1 4.3.2 4.4 4.4.1 4.4.2 4.4.3 4.5 4.6 4.7 4.7.1 4.7.2 4.7.3 4.7.4 5 5.1

Analytical Analytical Methods of Interference Interference Problem ..................... .......... ...................... ...................... ............... .... 8 Statistical Statistical Analysis of Network Network Performance Indicators ................ ..... ....................... ................... ....... 8 Statistics of Interference Band ....................................................................... 8 Statistics Statistics of Handover Handover due to UL/DL Interferen Interference ce ...................... ........... ..................... ..................... ............. 8 Collection Collection of UL/DL RQ Samples During Speeches Speeches .................... ......... ...................... .................... ......... 8 Parameter Parameter Checking and Analysis Analysis ...................... ........... ...................... ...................... ...................... ..................... ............ 9 Checking Checking of Parameters Parameters Related to Transmitting Transmitting Power ..................... .......... ...................... ............. .. 9 Checking Checking of Parameters Parameters Related to Skip-Zone Coverage ...................... ........... ................... ........ 10 Checking Checking Engineering Engineering Parameters of Antennas ...................... ........... ..................... ..................... ............. 10 Checking Checking Frequency Planning Parameters Parameters ...................... ........... ..................... ..................... ................... ........ 11 Investiga Investigation tion of Hardware Fault ..................... .......... ...................... ..................... ..................... ...................... ............... .... 11 Analysis Analysis of OMCR W arning ...................... ........... ...................... ...................... ...................... ..................... .................. ........ 11 Checking Checking of Latent Equipment Fault Fault ..................... .......... ...................... ..................... ..................... ................... ........ 12 Networking Optimization .............................................................................. 12 Adopting the the Co-BCCH Co-BCCH Technique Technique ..................... .......... ...................... ...................... ...................... .................... ......... 12 Multi-TRX Cascad Cascading ing Technique ...................... ........... ...................... ...................... ...................... ..................... ............ 13 Cell Layer-Dividing Technique ..................................................................... 13 MR Frequency Replanning .......................................................................... 13 Drive Test Test and Call Call Quality Test Test ...................... ........... ..................... ..................... ...................... ..................... ............. ... 14 Analytical Analytical Method Method of External External Interference Interference ..................... .......... ...................... ...................... .................... ......... 15 Repeater Repeater Checking Checking..................... .......... ...................... ..................... ..................... ...................... ...................... ...................... ........... 15 CDMA Antenna Antenna Troubleshooting Troubleshooting ...................... ........... ...................... ...................... ...................... ..................... ............ 15 Confirm External External Interference With SITEMASTER SITEMASTER ...................... ........... ..................... .................. ........ 16 Confirm External External Interference Interference With NetTek Analyzer Analyzer ...................... ........... ...................... ............... .... 17 Typical Cases ............................................................................................ 20 Interference Interference Existing Existing in a Cell ...................... ........... ..................... ..................... ...................... ...................... ................. ...... 20



V



FIGURES Figure 3-1

Flow of handling handling interference.......... interference..................... ...................... ...................... ...................... ..................... ..................... ............6 .6

Figure 4-1

Correspo Corresponding nding relation between between RxQual RxQual and and BER ...................... ........... ..................... ..................... ............9 .9

Figure 4-2

Drive test..................... .......... ...................... ...................... ...................... ..................... ..................... ...................... ...................... ................. ...... 14

Figure 4-3

Correspo Corresponding nding relation relation between SQI and call call quality ...................... ........... ...................... ................. ...... 15

Figure 4-4

Using SiteMaste SiteMasterr to confirm external external interference......... interference......... ...................... ........... ..................... ............. ... 16

Figure 4-5 4-5

Analysis Analysis of SiteMaster SiteMaster frequency frequency spectrum spectrum ...................... ........... ..................... ..................... ..................... .......... 17

Figure 4-6

Connection Connection to divider divider output output port port ...................... ........... ...................... ...................... ...................... ..................... ............. ... 17

Figure 4-7

YBT250 test graph I ...................... ........... ..................... ..................... ...................... ...................... ...................... ...................... ........... 18

Figure 4-8

YBT250 test graph II ..................... .......... ...................... ..................... ..................... ...................... ...................... ...................... ........... 18

Figure 5-1

Connection Connection diagram diagram of common common CDU..................... .......... ...................... ..................... ..................... ................... ........ 20

Figure 5-2

Interference Interference wave form graph graph I ...................... ........... ..................... ..................... ...................... ...................... ................. ...... 21

Figure 5-3

Scatter Scatter graph graph of interference interference time time I ..................... .......... ...................... ...................... ...................... ..................... ............ 22

Figure 5-4

Connection Connection graph with CDMA CDMA used used ...................... ........... ..................... ..................... ...................... ..................... .......... 22

Figure 5-5 W ave ave form form of of interference interference II ...................... ........... ...................... ...................... ...................... ..................... .................... .......... 23 Figure 5-6

Scatter Scatter graph graph of interference interference time time II ...................... ........... ..................... ..................... ...................... ..................... .......... 23

Figure 5-7 5-7

Connection Connection graph of of IRCDU+CDMA IRCDU+CDMA wave wave filter filter ..................... .......... ...................... ...................... ............... .... 24

Figure 5-8 W ave ave form form of interference interference III ..................... .......... ...................... ...................... ...................... ..................... .................... .......... 25 Figure 5-9

Scatter Scatter graph graph of interference interference time time III ..................... .......... ...................... ..................... ..................... ..................... .......... 25

TABLES Table 1-1

GSM Frequency Frequency Allocation Allocation ...................... ........... ..................... ..................... ...................... ..................... ..................... ................1 .....1

Table 4-1

Correspondin Corresponding g Relation Relation Between Between C/I and Call Call Quality ..................... .......... ...................... ................. ...... 15



VI



1

GSM Frequency Frequency Allocation Allocation GSM frequency includes EGSM/PGSM/DCS1800, whose allocation is shown below.

Table 1-1

GSM Frequency Allocation

Frequency Band

UL Frequency

DL Frequency

Duplex Interval

Band Width

Carrier Frequency Interval

EGSM+GS M900

880 MHz~915 MHz

925 MHz~960 MHz

45 MHz

35 MHz

200 kHz

DCS1800

1710 MHz~1785 MHz

1805 MHz~1880 MHz

95 MHz

75 MHz

200 kHz



1



2

Phenomena and Classificatio Classif ication n of Interference

2.1

Phenomena Phenomena and Classification Classification of Interferen Interferen ce If interference exists in a cell, the following phenomena may appear: poor speech quality, on-and-off speech, metallic ring/noise, call drop and unable to establish calls, which can be complained by subscribers or detected in DT; changes on indicators, like sudden deterioration in call drop rate, handover success rate, traffic volume, congestion rate and interference band, can also reflect interference in a cell. Interference in GSM system falls into internal interference and external interference, which is subdivided into UL interference and DL interference. Internal interference refers to unreasonable frequency planning or system hardware fault, which can result in decrease in service quality; external interference refers to unknown signal sources, which seriously interferes the network signal from outside and causes decrease in service quality.

2.2

Internal Interference Internal interference is mainly caused by the following factors: unreasonable frequency planning, skip-zone coverage, and equipment hardware problem.

2.2.1

Interference due to Unreasonable Frequency Planning If frequency and adjacent cell relation are set unreasonable in network planning because of planning tools or human mistakes, interference will be reflected in too large DL_RxQuality, MS unable to access into network, poor speech quality, and call drop.

2.2.2

Interference Interference due to Skip-Zone Coverage If engineering parameters and network parameters are not set correct in planning, the actual cell coverage can greatly exceed requirement; too large coverage will increase interference. Setting of engineering parameters: Engineering parameters mainly consist of antenna parameters. Antennas differentiate from each other in terms of antenna gain, horizontal beamwidth, vertical beamwidth, front-to-back ratio, etc., and they are suitable for different types of landforms and network coverage. Therefore, it ’s very important to choose the suitable antenna in accordance



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with the specific coverage requirements. Any deviation of antenna down-tilt in planning or mishandling in installation regardless of planning data will cause cell coverage to exceed the actual coverage needs, which will result in interference to other cells and influence network service quality. Therefore, when interference exists in network, checking antenna parameters is a must. Setting of network parameters: Network parameters include: minimum access level, BTS transmission power, MS max transmission power, handover thresholds, etc.. Improper setting of these parameters will result in skip-zone coverage problem and interference as well.

2.2.3

Interference due to Equipment Problem Deterioration of antenna performance: Antennas belong to passive device, and it ’s not easy to be broken, but once it is damaged or its performance deteriorates, poor speech quality will be resulted. Header problem: GSM RF signal is micro wave signal. Poor contact between any of these parts TRX —CDU—feeder cable —antenna will cause too large VSWR and increase in inter-modulation and interference. Inverse connection of antenna: This is a common problem, which will cause dramatic discrepancy between the actual cell frequency and that set in planning; co-channel and adjacent-channel interference, call drop and handover problem will be resulted too. For network with fewer frequencies, influence of inverse connection on network quality can be much more remarkable. TRX problem: If TRX performance decreases during operation because of problems in production, TRX may enlarge circuit self-excitation, which will cause problems like stronger interference, shrunk coverage and difficult access. Clock failure: Large deviation on BTS clock will lead to two results. On one hand, it ’ll make it difficult for MS to access BTSs, thus result in MS handover failure or make MS unable to reside in cells under the BTS; on the other hand, it will make the BTS unable to decode the MS signals, leading to error code. What we need to note is that clock failure doesn ’t actually bring about interference, however, increased transmission error code will cause decrease in speech quality. CDU/divider fault: Because active amplifier is used in CDU divider, self-excitation self-excitation is easy to be caused when a problem occurs. Spurious signals and intermodulation: If the out-band spurious signals in TRX or power amplifier exceed standards, or the isolation of transmit-receive of the duplexer in CDU is too small, interference to receive channel will be caused. Intermodulation among passive devices like antenna and feeder cables will be resulted as well.



3


2.3


External Interference External interference refers to interferences caused by wide-band repeater, CDMA system (trailing signal), or signal jammer, but not due to equipment problem or unreasonable frequency planning. This kind of interference is difficult to detect without specific devices.

2.3.1

Interference due to Unreasonable Setting of the Repeater Unreasonable setting of repeater can lead to interference to surrounding signals. In order to save investment and increase coverage range, the small BTSs in towns usually adopt repeater to amplify signals. However, currently the most widely used repeater is 900MHz wideband amplifier, which directly amplifies received signals and then transmits them; besides, BTS and repeater are connected with radio method, and there are usually some problems in repeater planning and site selection, interference to signals around is easy to be resulted. Repeater interference falls into two types:

2.3.2

1.

If the installation of repeater is not up to standard, there may not be enough insulation between the donor antenna and the subscriber antenna, and self-excitement is easy to be formed, thus the BTS performance will be affected.

2.

As for for repeater which adopts wideband wideband nonlinear amplifier, ampl ifier, its intermodulation indicator is far larger than that requested in the protocol. If the power is high and the intermodulation quantity is large, interference to surrounding BTSs is easy to be resulted.

Interference Interference Caused by Insufficient Insufficient CDMA Antenna Isolation The CDMA and GSM networks mainly locate at the 800-M frequency band and 900-M frequency band and the interference always appears. The interference between the networks has become an important factor affecting the network quality. The major kinds of interference existing between the CDMA and GSM networks are listed as follows. 

The noise interference interferenc e is caused by the spurious wave wave out of the band regulated by the CDMA BTS B TS (or repeater). repeater). The noise int erference erference can lead to the signal-to-noise s ignal-to-noise ratio decrease of the receiving system of the GSM BTS and then the calling quality of the GSM system deteriorates. In the actual condition, this kind of interference is the most common and serious one.



The congestion interference is cause by the strong strong TRX TRX power power of the CDMA system, short distance between the antennas, and the non-linearity of the filter of the receiver. This kind of interference can lead to receiver suppression supp ression out of the bandpass and then the saturation happens and the receiver cannot work normally.



4





Because the CDMA system uses multiple mult iple carrier frequencies and the system system is non-linear, the inter-modulation product locates at the UL frequency band of the neighbor GSM system and the signal-to-noise ratio decreases. Then the inter-modulation interference happens.



5


3


Flow of Handling Handling Interference Problem General flow of handling interference is shown as follows.

Figure 3-1

Flow of handling handlin g interference poorspeechquality;on-andoffspeech;metallicnoise;call droporunabletoestablish calls.

checkifitiscausedby newsites(incl. repeater),thorough changeoffrequency 1 orconfiguration parameters

Judgeinterferencerange: 1.aregionorseveralcells; 2.allTRXsofsomecells; 3.onlysomeTRXs.

CheckVSWR/ antenna/divider/duplexer andotherhardware; checkpowerandother 2 relatedparameters; checkifrepeater interferenceexists.

3 Checkfrequencyplanning data,findouttheinterfering frequency;makechanges accordinglyandcheck again. Interference exists

Interference exists

Checkhardware fault,focusonTRX; makechanges accordinglyand checkagain. Interference exists

Interference exists

Checkwithfrequency spectrometerto eliminateexternal interference.

Complete

1. When interference exists, subscribers will complain com plain about poor speech quality, which can be detected by DT; speech will be on and off, and there is metal noise during speech; it ’s unable to establish calls and call drops are easy to happen. 2.

Check indicators like BER, RxQual statistics, statist ics, idle interference band, statistics statist ics of handovers due to UL/DL interference, etc.. Carry out DT/CQT to confirm the cells and frequencies being interfered, when it ’s necessa n ecessary. ry.

3.

When interference exists in several cells of an area:



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First find out if any sites (incl. repeaters) are added recently, if all the frequencies are re-planned or any changes on settings of parameters are made; if there are no changes on network, we can deduce that the interference is probably due to external factors, such as interferences from CDMA system (trailing signal), signal jammer, etc.; as for internal interference interference caused by changes on network network configuration, we can restore the configuration parameters or re-plan them; as for external interference, we can use devices to investigate and locate problems. 4.

When interference exists in all carriers of a cell: It’s

recommended to check VSWR, antenna, divider and duplexer, etc.; check whether power parameter/skip-zone coverage parameter/antenna parameters are set correct; check whether repeater is installed and whether its setting is reasonable. If interference still exists after the investigation, use frequency scanning meter to further locate the source and eliminate the interference finally.

5.

If interference just exists in some carriers: We recommend checking of frequency planning data to locate the carriers being interfered; check power parameter and engineering parameters of antenna; observe OMCR fault warning, check hardware like carriers, antenna, divider, duplexer, etc., focus on checking of carriers. If interference still exists after these procedures, use frequency scanning meter to further locate the source and eliminate the interference finally.



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4


Analytical Analytical Methods of Interference Problem For interference, we can investigate and locate and solve the problems through the following methods.

4.1

Statistical Statistical Analysis of Network Performance Indicators

4.1.1

Statistics of Interference Interfer ence Band When TCHs are in idle status, UL noise/interference is constantly being monitored by BTS, and the measurement result will be analyzed, and interference level will be sent to BSC in 6 levels. The levels can be divided at OMCR, whose default values are 10, 15, 20, 25, 63 ( – –100 dBm, –95 dBm, –90 dBm, –85 dBm and –47 dBm). Through adjustment on the boundary of interference band, we can find out the severity of interference. Interference band of cell level is counted in basic measurement, and that of TRX level is counted in TRX measurement. measurement.

4.1.2

Statistics of Handover due to UL/DL Interference Interferenc e We can judge whether interference exists through statistics of handover caused by UL/DL interference. interference.

4.1.3

Collection of UL/DL RQ Samples During Speeches RxQual is an indicator to reflect speech quality, which is based on error rate and falls into 8 grades (0~7). In basic measurement, speech quality of all grades (0~7)UL/DL is counted into RQ sample statistics, which clearly reflects the situation when subscribers are influenced during speeches.



8


Figure 4-1


Corresponding relation relation between RxQual and BER

4.2

Parameter Checking and Analysis

4.2.1

Checking of Parameters Related to Transmitting Power Unreasonable setting of transmitting parameters like MsTxPwrMaxCch, PwrReduction, BsTxPwrMi, and so on may lead to interference. If MsTxMaxCCH (the max power level of control channels) is set too large, serious adjacent channel interference may be caused to the serving cell by MSs around the BTS, which impedes MSs under the cell to establish calls and affects speech quality; if it ’s set too small, it will be hard for MSs at boundaries of the cell to seize channels and the external interference can be more serious. PwrReduction refers to the static power class of TRX. In addition to the TRX transmitting power stipulated by PwrReduction, a static power control shall also be imposed, which means an extra restriction on the base of max transmitting power, then we will get the real max transmitting power of TRX (Pn), which can actually be used by TRX in the cell. Dynamic power control functions on the base of max transmitting power (Pn) obtained after static power control. Minimum BS power level (BsTxPwrMin): When BTS communicates with MS, its transmitting power is controlled by network. Network sets BTS power through power command. BTS output power must be the transmitting power stated by power command. When BSC is under power control, BsTxPwrMin is the minimum transmitting power to be used by BTSs in the cell, and the max power level of BTS is Pn.



9


4.2.2


Checking of Parameters Related to Skip-Z one Coverage In network planning, if engineering parameters and network parameters are not set correctly, too large coverage can be resulted; hence the interference seriousness will greatly increase. Incorrect setting of parameters like MS minimum receive level, BTS transmitting power, MS max transmitting power, handover thresholds, etc. can lead to skip-zone coverage and interference. 

RxLevAccessMin RxLevAccess Min (minimum receive level allowed to access): In order to prevent MS MS from accessing into network when its receive signal level is rather low (access into network at low receive signal level can not guarantee normal speeches), which causes unsatisfactory communication quality and wastes radio resource of network, it is stipulated in GSM system that when MS accesses into network, its receive level must be larger than a certain threshold, the minimum receive level allowed to access (RxLevAccessMin). (RxLevAccessMin).



Static power level (the PwrReduction parameter): PwrReduction parameter): The static power control should be added on the basis of the TRX transmission power level regulated by the PwrClasss PwrClasss parameter. Then the maximum transmission power Pn that can be actually used by this TRX can be gotten. Dynamic power control is made on the basis of the maximum transmission power Pn after the static power control.



MS max power level (MsTxPwrMax): When MS communicates with with BTS, its transmitting power is controlled by network. Network sets MS power through power command, which is transmitted on SACCH (SACCH has two head bytes, one of which is for power control; the other is for Time Advance). MS must extract the head byte for power control from the UL SACCH, and adopt the transmitting power stipulated by power control as output power. If MS is not able to output the power stipulated, then the power power outputs shall be the closest to th e stipulated. stipulated. When BS C is in power control, MsTxPwrMax is the max transmitting power to be used by MSs in the cell area.



TA allowed (the TaAllowed parameter): TaAllowed parameter): This parameter indicates the maximum TA allowed to be accessed in this cell. During the user access, if the access delay required by the channel is larger than the allowed TA value of the cell, the user access will be prohibited. The engineers can control coverage range of the users that can be accessed of the cell through reducing the value of the TaAllowed parameter, so as to avoid false signal access.

4.2.3

Checking Engineering Engineerin g Parameters of Antennas Engineering parameters mainly refer to those related to antenna. Signals of different types of antenna vary in terms of gains, horizontal lobe, vertical lobe, and front and back ratio, etc.; with these different features they suit for different areas and network coverage. Therefore, it ’s essential to choose suitable antenna according to specific coverage requirements. If there is deviation in antenna down-tilt during planning, or if equipment installation is not up to standard according to planning data, it may result in real cell



10



coverage larger than the actual needs, which may interfere with other cells and affect network service quality. Therefore, when interference occurs, checking antenna parameters is a must.

4.2.4

Checking Frequency Planning Parameters As for the cell with possible interference, interference, check frequency frequency planning planning of the cell and its neighbor cells. Find out distribution of BTSs and each cell ’s azimuth angle, draw a topological diagram and mark BCCH/TCH frequencies and BSIC; compare the planned frequencies frequencies with those actually configured configured in BSC, check whether whether discrepancy exists. exists. For boundary areas, it ’s hard to get frequencies plan of external areas. In order to precisely locate the interference in marginal networks, we can block co-channel cells in the network; meanwhile, make tracing test with DT devices at areas with emergence of large DL_RxQuality. If co-channel interference does exist, the DL_RxQuality value shall become smaller after the blocking of co-channel cells, thus we can adjust the cell ’s frequencies to eliminate the interference. According to topological topological diagram of frequency frequency planning, we can deduce if possible co-channel/adjacent-channel interference exists in the network.

4.3

Investigation Investigation of Hardware Fault

4.3.1

Analysis of OMCR Warning Both BTS transmitting and receiving of signals are performed through antenna-feeder system, therefore, installation quality and performance of the system will have direct influence on not only speech quality, but radio signal coverage and transceiver ’s performance. When there is fault with antenna transmitting system, transmitting signal will experience loss and BTS coverage will be affected. If the fault is rather serious, BTS will shut the transceivers connected with it. When there is fault with antenna receiving system, the signals it receives from MS will become weak. If MS receives signal within the BTS coverage is strong, it will be hard for the MS to seize radio channel of the BTS, and speech quality will be affected and even call drops can be resulted. When antenna insulation is not in accordance with the standards, transmitting signal from one transmitter may invade into another transmitter, and inter-modulate with its transmitting signal, and the two signals will create a new combined frequency signal, which will be transmitted along with normal signals. In this case, interference to receiver will be inevitably resulted. Therefore, up-to-standard installation of antenna-feeder system is the precondition for ensuring speech quality. Besides, antenna-feeder system is the base for good error control. When checking hardware faults, first look at warning analysis at OMCR, focus on checking whether fault warnings or VSWR warnings exist.



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4.3.2


Checking of Latent Equipment Fault BTS wireless problems are mainly caused by defective UL unit parts. The following procedures can be adopted to judge whether defective UL unit parts is cause of problems. Block the two inputs of TRX, observe UL interference band; if interference band class is 0, it’s proved that TRX hasn ’t brought UL interference. Input the two stimulations of TRX without connecting them to power amplifier, observe UL interference band; if interference band class is 0, it means external interference doesn ’t exist. If serious UL interference exists even though there is no stimulation imposed on power amplifier, disconnect the rack top feeder cables, and observe UL interference band; if the interference isn ’t fading at all, then we can conclude that the problem is with the divider unit. If the UL interference disappears when the rack top feeder cables are disconnected, we can infer that the problem has nothing to do with equipment.

4.4

Networking Optimization

4.4.1

Adopting the Co-BCCH Technique In the double frequency co-cell mode, the inner circle and outer circle share one BCCH and one BCCH can be saved. At the same time, the handover algorithm based on the path loss/TA is used to make the inner circle (Subcell 2) absorb the traffic of the outer circle (Subcell 1) better. Through the Subcell 2 TRX number increase and usage of tighter frequency reuse degree, the system capacity can be enhanced. The frequency reuse coefficients of inner circle and outer circle are different and the frequency is reused more tightly in the inner circle. In the outer circle, the frequency reuse mode 4 × 3, 5 × 3, or even lower frequency reuse mode is usually adopted in the outer circle; the frequency reuse mode 4 × 3, 3 × 3, or even higher frequency reuse mode or high-load frequency hopping moe is adopted in the inner circle. For the urban dense coverage area, the co-BCCH technique can be adopted, so as to reduce the interference of the outer circle and then reduce the interference in the whole network.



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4.4.2


Multi-TRX Cascading Cascading Technique In the dense urban area, because there are plenty of users in the business buildings, shopping malls, and parking lots and the penetration loss of the buildings is large, the coverage signal quality is poor. In this condition, the indoor and outdoor coverage should be considered at the same time. The traditional methods include adopting the repeater or leakage cable and they can bring interference to the outdoor sites and affect the network performance and user sensitivity degree. The multi-TRX cascading is adopted. With the advantage of remote RF, the RRUs are installed at the rooftops, underground parking lots, and places between the stories. Then the outdoor coverage is completed and the indoor blind area is considered. What is more, the interference brought by the outdoor site is reduced due to the pointed coverage.

4.4.3

Cell Layer-Dividing Layer-Dividi ng Technique Techniqu e In order to alleviate the network congestion and utilize the capacity of networks of all the layers in the urban area more effectively, ZTE introduces the cell layer-dividing technique. In ZTE iBSC, every cell can set its neighbor cell as the undefined layer cell, upper layer cell, the same layer cell, or lower layer cell and the traffic can be diverted under control through layer control parameters and different handover algorithms. The cell layer dividing structure includes the pico cells, micro cells, and macro cells. The pico cell layer servers for the indoor users; the micro cell layer absorbs the traffic of a specific area or area with high density; macro cell layer is used to satisfy the outdoor user requirement in the urban areas or villages. The antenna of micro cell is installed lower than the rooftop of the building or in the indoor area and that of macro cell is stalled on the rooftop of building with medium height to provide consecutive coverage. The engineers can reduce the network interference through reducing the antenna height. Cell layer-dividing technique can control the traffic flow direction more properly and the congestion. One network can have the cell structure with two or three layers. The high layer cells are the cells with large coverage range and low layer cells are the cells with limited coverage range. If the capacity of the low layer cells is used fully, the capacity pressure of high layer cells can be alleviated. If the initial configuration is improper, the high layer cell configuration should be lowered.

4.5

MR Frequency Replanning Replanning For GSM network, as the user increase and distribution change, through continuous capacity expansion and TRX number decrease, the site distribution and configuration have changed greatly and the frequency planning at the initial stage cannot meet the requirement of network development. In order to enhance the frequency utilization rate of



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the existing network and reduce the 900-M interference of the existing network, the engineers should make the frequency replanning in the whole network, so as to improve the network quality. The frequency replanning based on the MR is described as follows. With the measurement report reported by the users of mobile phones in the existing network, the engineers make statistics about the probability of intra-frequency interference and neighbor frequency interference existing between the two cells simultaneously in the existing network. Then, through the frequency replanning, the engineers reallocate the frequencies on the basis of the principle of minimizing the traffic loss of intra-frequency interference and neighbor frequency interference. Finally, the traffic loss caused by the intra-frequency interference and neighbor frequency interference is reduced and the performance in the whole network is enhanced.

4.6

Drive Test and Call Quality Test Drive test and call quality test are field test methods to reflect actual interference situation. In CQT, we can actually feel the speech quality at areas being interfered, and we can see call quality class on the test phone. If coverage level is good, while in the mean time speech keeps on and off with metallic noise or the speech quality class displayed on test phone remains high, we can deduce that interference exists. Drive test can effectively detect the location and degree of interference, which is convenient for analyzing the cause of interference. Refer to Figure to Figure 4-2.

Figure 4-2

Drive test

Different Drive Test software differs in parameters. For example, TEMS uses BER&C/A, SQI and C/I, while ANT Technologies uses RXQUAL&FER to illustrate interference. C/I: Refer to Table to Table 4-1 for corresponding relation between co-channel C/I and call quality.



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Table 4-1


Corresponding Relation Between C/I and and Call Quality

RxQual

0

1

2

3

4

5

6

7

C/I[dB] C/I[ dB]

23

19

17

15

13

11

8

4

SQI: SPEECH QUALITY INDEX is the comprehensive description of BER, FER and HANDOVER EVENT by TEMS. Corresponding relation between SQI and call quality is shown below.

Figure 4-3 Corresponding relation relation between SQI and call quality

4.7

Analytical Method of External Interference

4.7.1

Repeater Repeater Checking Check engineering parameters or consult with operators (companies) to find out if there is a repeater installed in the interfered area. If there is, carry out frequency sweep and make further observations; or propose closing the repeater and keep observing to see if the interference is solved.

4.7.2

CDMA Antenna Troubleshooting Troublesho oting 

Antenna isolation



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The main lobes of the two antenna should not be at the same direction when the antennas have the same height. Then the interference will be reduced. Usually, the deviation angle of the receving antenna should be larger than the half power angle of the main lobe of the transmission antenna. A certain vertical vertical isolation degree should be kept. Because the radiation radiation field intensities of the vertical polarized antennas used by the systems are mainly at the horizontal direction. Therefore, the vertical isolation requirement of the antenna of the neighbor system should be guaranteed. 

External External filter If the interference cannot be avoided with the space isolation of the antenna, the engineers should install the duplex filter with sound performance to enhance the out-of-band filter performance and reduce the intensity of the out-of-band noise signal.

4.7.3

Confirm External Interference Interferenc e With SITEMASTER SITEMASTER, which we are currently using, has the function of frequency scanning with low sensitivity, so it can not be directly used in interference analysis test. A Low-power amplifier is added to the front of SITEMASTER by its producer, which increase the frequency-sweep generator ’s ability to analyze interference, thus our cost to purchase it is increased and as well as its price. With the aim to utilize the SITEMASTER we currently possess in interference analysis, we can connect the input port of frequency-sweep generator to the output port of divider.

Figure 4-4

Using SiteMaster to confirm external interference interferenc e

For specific introduction of SiteMaster usage and operation, please refer to the attached manual. We can adjust the frequency sweep bandwidth of SiteMaster (referred to as SM hereafter) to 890~915 MHz, and observe the background noise in the UL frequency band.



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If persistent UL level exists in a certain frequency band, we should find out if UL interference exists or the background noise is too loud. For example, in the following figure, persistent strong level exists within the bandwidth of 20 MHz, we can conclude that serious UL interference exists.

Figure 4-5

4.7.4

Analysis of SiteMaster frequency frequency spectrum spectru m

Confirm External Interference Interferenc e With NetTek Analyzer Make UL interference analysis of GSM 900M UL frequency band with frequency spectrometer-NetTek Analyzer (TEK company). The model we usually use is YBT250.

4.7.4.1

Connection Method In order to obtain interference information with TEK frequency scanning meter, there are several methods of connecting equipment; one is to use its own test antenna, another is through connection to the output port of divider, as shown below.

Figure 4-6

Connection to divider output port CDU

Feeder

Antenna

YBT 250



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4.7.4.2


Oscillogram of Interference Figure 4-7 is the output graph of an interference test analysis, which shows the frequency and strength of interference. This output is the average value of the test results of one minute. Persistent observation is needed for confirming if the interference continues.

Figure 4-7

4.7.4.3

YBT250 test graph I

Time Scatter Graph of Interference Common frequency spectrometer possesses no ability to record continuously, but those produced by TEK provide an output function. See Figure 4-8.

Figure 4-8

YBT250 test graph II



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After a certain period period of test, we can see from from the figure that at 909.780 there is a persistent UL signal of about –73 dBm. TEK frequency spectrometer features in three dimensional recording of time, frequency and signal, which is convenient for fixing the problem. The vertical bold red lines in the graph represent the time duration, signal level strength and frequency (vertical axis=time, horizontal axis=frequency, color spectrum=strength).



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5

Typical Cases

5.1

Interference Existing in a Cell


Problem description Since March 2005, an operator has received a lot of complaints about poor speech quality; sometimes calls even couldn ’t be setup; the caller could hear the counterpart, but could not be heard. Problem analysis At the beginning beginning we thought it was caused by poor signal. A fter on-site test, we f ound it wasn ’t coverage problem. For example, when the level tested by MS was –85 dBm, UL call problem occurred, which was displayed as on-and-off speech, silence, metallic noise and current noise, so we concluded that the problem was caused by interference. Performance Performance statistics at OMCR showed that the rank of idle channel int erference erference band was high. Problem handling Use interference tester YBT250 to test and eliminate interference. Conduct analysis analysis of interference source in YBT250 t est by connecting it to common CDU. Test connection graph is as follows.

Figure 5-1

Connection diagram of common CDU

Common CDU

Feeder

Antenna

YBT 250

Test result



20



From Figure From Figure 5-2, we 5-2, we can see that CDMA wave form was strong when wave filter wasn’t used, the peak value reached about –35 dBm (average about –60 dBm), which was close to GSM UL wave band and could cause UL interference to GSM network. Average wave wave form form of YBT250 test is as follow follows. s.

Figure 5-2

Interference wave form graph I

From the figure above we can see that when wave filter was not used, the wave form of both CDMA and GSM background noise was strong, thus interference occurred. Three dimensional graph of interference tested by YBT250 is as follows.



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Figure 5-3


Scatter Scatt er graph of interference time I

From Figure 5-3, 5-3, we can see when wave filter wasn ’t used, wave form of CDMA was strong, and that of GSM background noise on the right was high for a long period of time. Analysis of interference interference source in YBT250 YBT250 test –connected to common CDU+ CDMA wave filter Test connection graph is as follows.

Figure 5-4

Connection graph with CDMA used Common CDU

CDMA wave filter

Antenn a

Feeder YBT 250

Test result: In the test graph shown bellow, we can see that through common CDU and CDMA wave filter, CDMA wave form was reduced to around –100 dBm, but it still couldn ’t be eliminated, thus CDMA frequency band still caused interference to the marginal area of GSM UL. Refer to the following figure.



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Figure 5-5


W ave form of interference interfe rence II

From Figure 5-5, 5-5, we could see when CDMA wave filter was used, CDMA wave form obviously became weak, but that at some points was still strong, and the background noise in GSM frequency band became less as well. Three dimensional graph of interference tested by YBT250 is as follows.

Figure 5-6

Scatter Scatt er graph of interference time II



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This graph illustrates that when wave filter was adopted, the UL interference in GSM frequency band clearly became less. Analysis of interference interference source in YBT250 YBT250 test –connected to IRCDU+ CDMA wave filter Test connection graph

Figure 5-7

Connection graph of IRCDU+CDMA wave filter IRCDU

CDMA wave filter

Antenna

YBT 250

Test result: In In Figure 5-8, 5-8, the wave filtering effect of combination of IRCDU+CDMA is much better than that of other combinations. This combination can effectively filter CDMA waves to below –104 dBm. This kind of filtering effect can help completely avoid CDMA network interfering GSM UL network. The test result is shown as follows.



24


Figure 5-8


W ave form of interference interfe rence III

Figure 5-8 shows that when IRCDU+CDMA wave filter was adopted, CDMA waves can be thoroughly filtered out, and there was no interference to GSM network any more, and the background noise in GSM UL was reduced too. Three dimensional graph of interference tested by YBT250.

Figure 5-9

Scatter Scatt er graph of interference time III



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From Figure From Figure 5-9, we 5-9, we can see that the result of wave filtering was good and stable; during the test period, CDMA interference was almost eliminated. Summary: The interference source was from CDMA system. Through comparisons of tests above, we can see after IRCDU+CDMA wave filter was used, call quality is improved obviously.



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GSM RNO Subject-Network Interference and Solutions_R2.0

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