GSM RNO Subject-Network Interference and Solutions_R2.0

Network Interference and Solutions

R2.0

Network Interference and Solutions Internal Use Only▲

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ZTE Confidential Proprietary © 2014 ZTE CORPORATION. All rights reserved. II

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 (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 Version Prepared by Reviewed by Approved by

2009-3-19 R1.0 Su Shaoli Gan Wenjun

2011-3-22 R2.0 Xie Jin Zheng Hao


ZTE Confidential Proprietary © 2014 ZTE CORPORATION. All rights reserved. III

Intended audience: GSM Network Optimization Engineers

Proposal: Before reading this document, you had better have the following knowledge and skills.

SEQ Knowledge and skills Reference material

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3

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

SEQ Reference material Information

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ZTE Confidential Proprietary © 2014 ZTE CORPORATION. All rights reserved. IV

About This Document

Summary

Chapter Description

1 GSM Frequency Allocation 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 Methods of Interference Problem

Introduces analytical methods of interference problems.

5 Typical Cases Introduces typical cases to handle GSM interference.


ZTE Confidential Proprietary © 2014 ZTE CORPORATION. All rights reserved. V

TABLE OF CONTENTS

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

2 Phenomena and Classification of Interference ........................................... 2 2.1 Phenomena and Classification of Interference ................................................ 2 2.2 Internal Interference ...................................................................................... 2 2.2.1 Interference due to Unreasonable Frequency Planning ................................... 2 2.2.2 Interference due to Skip-Zone Coverage ........................................................ 2 2.2.3 Interference due to Equipment Problem.......................................................... 3 2.3 External Interference ..................................................................................... 4 2.3.1 Interference due to Unreasonable Setting of the Repeater .............................. 4 2.3.2 Interference Caused by Insufficient CDMA Antenna Isolation .......................... 4

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

4 Analytical Methods of Interference Problem ............................................... 8 4.1 Statistical Analysis of Network Performance Indicators ................................... 8 4.1.1 Statistics of Interference Band ....................................................................... 8 4.1.2 Statistics of Handover due to UL/DL Interference ............................................ 8 4.1.3 Collection of UL/DL RQ Samples During Speeches ........................................ 8 4.2 Parameter Checking and Analysis .................................................................. 9 4.2.1 Checking of Parameters Related to Transmitting Power .................................. 9 4.2.2 Checking of Parameters Related to Skip-Zone Coverage .............................. 10 4.2.3 Checking Engineering Parameters of Antennas ............................................ 10 4.2.4 Checking Frequency Planning Parameters ................................................... 11 4.3 Investigation of Hardware Fault .................................................................... 11 4.3.1 Analysis of OMCR Warning ......................................................................... 11 4.3.2 Checking of Latent Equipment Fault ............................................................. 12 4.4 Networking Optimization .............................................................................. 12 4.4.1 Adopting the Co-BCCH Technique ............................................................... 12 4.4.2 Multi-TRX Cascading Technique .................................................................. 13 4.4.3 Cell Layer-Dividing Technique ..................................................................... 13 4.5 MR Frequency Replanning .......................................................................... 13 4.6 Drive Test and Call Quality Test ................................................................... 14 4.7 Analytical Method of External Interference .................................................... 15 4.7.1 Repeater Checking ...................................................................................... 15 4.7.2 CDMA Antenna Troubleshooting .................................................................. 15 4.7.3 Confirm External Interference With SITEMASTER ........................................ 16 4.7.4 Confirm External Interference With NetTek Analyzer ..................................... 17

5 Typical Cases ............................................................................................ 20 5.1 Interference Existing in a Cell ....................................................................... 20


ZTE Confidential Proprietary © 2014 ZTE CORPORATION. All rights reserved. VI

FIGURES

Figure 3-1 Flow of handling interference............................................................................6

Figure 4-1 Corresponding relation between RxQual and BER ............................................9

Figure 4-2 Drive test ....................................................................................................... 14

Figure 4-3 Corresponding relation between SQI and call quality ....................................... 15

Figure 4-4 Using SiteMaster to confirm external interference............................................ 16

Figure 4-5 Analysis of SiteMaster frequency spectrum ..................................................... 17

Figure 4-6 Connection to divider output port .................................................................... 17

Figure 4-7 YBT250 test graph I ....................................................................................... 18

Figure 4-8 YBT250 test graph II ...................................................................................... 18

Figure 5-1 Connection diagram of common CDU ............................................................. 20

Figure 5-2 Interference wave form graph I ....................................................................... 21

Figure 5-3 Scatter graph of interference time I ................................................................. 22

Figure 5-4 Connection graph with CDMA used ................................................................ 22

Figure 5-5 Wave form of interference II ........................................................................... 23

Figure 5-6 Scatter graph of interference time II ................................................................ 23

Figure 5-7 Connection graph of IRCDU+CDMA wave filter ............................................... 24

Figure 5-8 Wave form of interference III .......................................................................... 25

Figure 5-9 Scatter graph of interference time III ............................................................... 25

TABLES

Table 1-1 GSM Frequency Allocation ................................................................................1

Table 4-1 Corresponding Relation Between C/I and Call Quality ...................................... 15


ZTE Confidential Proprietary © 2014 ZTE CORPORATION. All rights reserved. 1

1 GSM Frequency 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+GSM900

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



2 Phenomena and Classification of Interference

2.1 Phenomena and Classification of Interference

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 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



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 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.



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:

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 repeater which adopts wideband nonlinear amplifier, 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.

2.3.2 Interference Caused by 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 is caused by the spurious wave out of the band regulated by

the CDMA BTS (or repeater). The noise interference can lead to the signal-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 TRX 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 out of the

bandpass and then the saturation happens and the receiver cannot work normally.



Because the CDMA system uses multiple carrier frequencies and the 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.



3 Flow of Handling Interference Problem

General flow of handling interference is shown as follows.

Figure 3-1 Flow of handling interference

Judge interference range:

1. a region or several cells;

2. all TRXs of some cells;

3. only some TRXs.

phenomena of interference:

poor speech quality; on-and-

off speech; metallic noise; call

drop or unable to establish

calls.

check if it is caused by

new sites (incl.

repeater), thorough

change of frequency

or configuration

parameters

Check VSWR/

antenna/divider/duplexer

and other hardware;

check power and other

related parameters;

check if repeater

interference exists.

Check frequency planning

data, find out the interfering

frequency; make changes

accordingly and check

again.

1

3

2

Check hardware

fault, focus on TRX;

make changes

accordingly and

check again.

Interference

exists

Check with frequency

spectrometer to

eliminate external

interference.

Interference

exists

Complete

Interference

exists

Interference

exists

1.

When interference exists, subscribers will complain 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, idle interference band, statistics of

handovers due to UL/DL interference, etc.. Carry out DT/CQT to confirm the cells

and frequencies being interfered, when it’s necessary.

3. When interference exists in several cells of an area:



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 caused by changes on 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.



4 Analytical Methods of Interference Problem

For interference, we can investigate and locate and solve the problems through the

following methods.

4.1 Statistical Analysis of Network Performance

Indicators

4.1.1 Statistics of Interference 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.

4.1.2 Statistics of Handover due to UL/DL Interference

We can judge whether interference exists through statistics of handover caused by

UL/DL 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.



Figure 4-1 Corresponding 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.



4.2.2 Checking of Parameters Related to Skip-Zone 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 (minimum receive level allowed to access): In order to prevent 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).

Static power level (the PwrReduction parameter): The static power control should

be added on the basis of the TRX transmission power level regulated by the

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 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 outputs shall be the closest to the stipulated. When BSC

is in power control, MsTxPwrMax is the max transmitting power to be used by MSs

in the cell area.

TA allowed (the 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 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



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, check frequency 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 with those actually configured in BSC, check whether discrepancy 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 diagram of frequency planning, we can deduce if possible

co-channel/adjacent-channel interference exists in the network.

4.3 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.



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.



4.4.2 Multi-TRX 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 Technique

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

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



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 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 4-1 for corresponding relation between co-channel C/I and call quality.



Table 4-1 Corresponding Relation Between C/I and Call Quality

RxQual 0 1 2 3 4 5 6 7

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 between SQI and call quality

4.7 Analytical Method of External Interference

4.7.1 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

Antenna isolation



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 isolation degree should be kept. Because the 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 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 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

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.



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 Analysis of SiteMaster frequency spectrum

4.7.4 Confirm External Interference 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

Antenna

CDU

YBT 250

Feeder



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 YBT250 test graph I

4.7.4.3 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



After a certain period of test, we can see 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).



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 we thought it was caused by poor signal. After on-site test, we found 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 statistics at OMCR showed that the rank of idle channel interference band

was high.

Problem handling

Use interference tester YBT250 to test and eliminate interference.

Conduct analysis of interference source in YBT250 test by connecting it to common CDU.

Test connection graph is as follows.

Figure 5-1 Connection diagram of common CDU

Test result

Antenna

Common CDU

YBT 250

Feeder



From Figure 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 form of YBT250 test is as follows.

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.



Figure 5-3 Scatter graph of interference time I

From Figure 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 source in YBT250 test –connected to common CDU+ CDMA

wave filter

Test connection graph is as follows.

Figure 5-4 Connection graph with CDMA used

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.

Antenn

a

Common

CDU

YBT 250

Feeder

CDMA wave

filter



Figure 5-5 Wave form of interference II

From Figure 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 graph of interference time II



This graph illustrates that when wave filter was adopted, the UL interference in GSM

frequency band clearly became less.

Analysis of interference source in YBT250 test –connected to IRCDU+ CDMA wave filter

Test connection graph

Figure 5-7 Connection graph of IRCDU+CDMA wave filter

Test result:

In Figure 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.

Antenna CDMA wave

filter

YBT 250

IRCDU



Figure 5-8 Wave form of interference 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 graph of interference time III



From Figure 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.

Documents

GSM RNO Subject-Network Interference and Solutions_R2.0