09 gsmp&o b-en-gsm network interference & solutions -word--201009

GSM Network Interference & Solutions

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Contents

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

2 Phenomena & Classification of Interference ........................................................................................... 3

2.1 Phenomena & Classification of Interference..................................................................................... 3

2.2 Internal interference .......................................................................................................................... 3

2.2.1 Interference due to unreasonable frequency planning............................................................ 3

2.2.2 Interference due to skip-zone coverage.................................................................................. 3

2.2.3 Interference due to equipment problem.................................................................................. 4

2.3 External interference ......................................................................................................................... 5

2.3.1 Interference due to unreasonable setting of repeater.............................................................. 5

3 Flow of Handling Interference Problem .................................................................................................. 7

4 Analytical Methods of Interference Problem......................................................................................... 11

4.1 Statistical analysis of network performance indicators ................................................................... 11

4.1.1 Statistics of interference band .............................................................................................. 11

4.1.2 Statistics of handover due to UL/DL interference................................................................ 11

4.1.3 Collection of UL/DL RQ samples during speeches ............................................................. 11

4.2 Parameter checking and analysis .................................................................................................... 12

4.2.1 Checking of parameters related to transmitting power......................................................... 12

4.2.2 Checking frequency planning parameters ............................................................................ 14

4.3 Investigation of hardware fault ....................................................................................................... 14

4.3.1 Analysis of OMCR warning................................................................................................. 14

4.3.2 Checking of latent equipment fault ...................................................................................... 15

4.4 Drive Test and Call Quality Test ..................................................................................................... 15

4.5 Analytical method of external interference ..................................................................................... 17

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4.5.1 Repeater checking.................................................................................................................17

4.5.2 Confirm external interference with SITEMASTER .............................................................17

4.5.3 Confirm external interference with NetTek Analyzer...........................................................19

5 Typical cases ..............................................................................................................................................21

5.1 Interference exists in a cell ..............................................................................................................21

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1 GSM Frequency Allocation

GSM frequency includes EGSM/PGSM/DCS1800, whose allocation is shown

in Table 1-1.

Table 1-1 GSM frequency allocation

Frequency band UL frequency DL frequency Duplex interval Band width

Carrier

frequency

interval

EGSM+GSM900 880MHz ~915MHz 925MHz~960MHz 45MHz 35MHz 200kHz

DCS1800 1710MHz~1785MHz 1805MHz~1880MHz 95MHz 75MHz 200kHz

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2 Phenomena & Classification of Interference

2.1 Phenomena & 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 interfere the

network signal from outside and cause decrease in service quality.

2.2 Internal interference

Internal interference is mainly caused by the following factors: unreasonable

frequency planning, skip-zone coverage, 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.


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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 mishandle 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: antenna belongs 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 as a result.

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

problem in production, TRX may enlarge circuit self-excitation, which will

cause problems like stronger interference, shrunk coverage and difficult

Chapter 2 Phenomena & Classification of Interference

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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 makes the BTS unable to decode the MS signal, leading to error code. What

we need to note is that clock failure doesn’t actually bring 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 problem occurs.

Spurious signals & 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 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:


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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.

3 Flow of Handling Interference Problem

General flow of handling interference is shown in Fig 3-1:

Fig 3-1 Flow of handling interference

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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

Chapter 3 Flow of Handling Interference Problem

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interference finally.

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4 Analytical Methods of Interference Problem

For interference, we can investigate and locate and solve the problem 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 （-100dBm, -95dBm,

-90dBm, -85dBm and -47dBm）. 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.


Fig 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, etc. 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

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Chapter 4 Analytical Methods of Interference Problem

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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.

Checking of parameters related to skip-zone coverage

In network planning, if engineering parameters and network parameters are

not set correct, 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).

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 it 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.

Checking engineering parameters of antenna

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


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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.

4.2.2 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 receive

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.


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When antenna insulation isn’t up to 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 problem:

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 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 Fig 4-2.

Fig 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 in Fig 4-3.

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Fig 4-3 Corresponding relation between SQI and call quality

4.5 Analytical method of external interference

4.5.1 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.

4.5.2 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. ( refer to Fig 4-4)

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Fig 4-4 Use 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 890～915MHz, 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

Fig 4-5, persistent strong level exists within the bandwidth of 20MHz, we can

conclude that serious UL interference exists.

Fig 4-5 Analysis of SiteMaster frequency spectrum

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4.5.3 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.

4.5.3.1 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 in Fig 4-6:

Fig 4-6 Connection to divider output port

CDU

YBT 250

Feeder

Antenna

4.5.3.2 Oscillogram of interference

Fig 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.

Fig 4-7 YBT250 test graph I

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4.5.3.3 Time scatter graph of interference

Common frequency spectrometer possesses no ability to record continuously,

but those produced by TEK provide an output function. See Fig 4-8:

Fig 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 -73dBm. 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, colour spectrum=strength).

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5 Typical cases

5.1 Interference exists 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 -85dbm,

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】

Used interference tester YBT250 to test and eliminate interference.

Analysis of interference source in YBT250 test –just connected to common CDU

Test connection graph:

Fig 5-1 Connection diagram of common CDU

Test result

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Antenna

Common

CDU

YBT 250

Feeder


From Fig 5-2, we can see that CDMA wave form was strong when wave filter wasn’t

used, the peak value reached about -35dbm (average about -60dbm), which was close

to GSM UL wave band and could cause UL interference to GSM network.

Average wave form of YBT250 test

Fig 5-2 Interference wave form graph I

From the figure above we can see that when wave filter wasn’t used, the wave form of

both CDMA and GSM background noise was strong, thus interference occurred.

Three dimensional graph of interference tested by YBT250

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Chapter 5 Typical cases

Fig 5-3 Scatter graph of interference time I

From Fig 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


Fig 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 -100dbm, but it still couldn’t be

eliminated, thus CDMA frequency band still caused interference to the marginal area of

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Antenna Common

CDU

CDMA wave filter

Feeder

YBT 250


GSM UL.

Refer to Fig 5-5:

Fig 5-5 Wave form of interference II

From Fig 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

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Fig 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


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

Test result:

In Fig 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 -104dbm. This kind of filtering effect can help completely avoid CDMA

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Antenna CDMA wave

filter

IRCDU

YBT 250


network interfering GSM UL network.

The test result is shown in Fig 5-8:

Fig 5-8 Wave form of interference III

Fig 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:

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Fig 5-9 Scatter graph of interference time III

From Fig 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

improved obviously.

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09 gsmp&o b-en-gsm network interference & solutions -word--201009