chapter10 special topic

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Wireless Network Planning Table of contents

Table of Contents

Chapter 1 Special Topic ...............................................................................................................11.1 The Problem of Coverage .................................................................................................1

1.1.1 Equipment Configuration ........................................................................................1

1.1.2 Base Station Site Selection ....................................................................................1

1.1.3 Antenna Engineering Design and Installation .........................................................3

1.1.4 Antenna feeder, combiner (divider), CDU connections ...........................................6

1.1.5 Parameters settings and others ..............................................................................6

1.2 TCH Congestion ................................................................................................................9

1.2.1 The Causes of High TCH Congestion Ratio ...........................................................9

1.2.2 Positioning Methods of High TCH Congestion Ratio ............................................101.3 Voice Prompt................................................................................................................... 13

1.3.1 Paging Strategy ....................................................................................................13

1.3.2 Paging procedure .................................................................................................14

1.3.3 Analysis on the Problem Subscriber Out of the Service Area ............................15

1.3.4 Supplementary Notes ...........................................................................................17

1.4 The Problem of Signal Fluctuation ..................................................................................18

1.4.1 Examine the Stableness of the Base Stations Transmission Power ....................18

1.4.2 Cell Reselection (in idle state) or Handover (in conversation state) ......................19

1.4.3 Power Control and DTX ........................................................................................191.4.4 TRX Down ............................................................................................................21

1.4.5 Interferences .........................................................................................................21

1.4.6 Cell Congestion ....................................................................................................21

1.4.7 Multipath Effect..................................................................................................... 22

1.4.8 Other causes ........................................................................................................22

1.5 Other Main Problems ......................................................................................................24

1.5.1 Poor Voice Quality ................................................................................................24

1.5.2 Failure to attach the network ................................................................................24

1.5.3 Slow to attach the network ....................................................................................241.5.4 Access slow ..........................................................................................................24

1.5.5 unilateral connection ............................................................................................24

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Wireless network Planning Chapter 10 Special Topic

Chapter 1 Special Topic

1.1 The Problem of Coverage

At present there are more or less problem of coverage in the network. The problem ofcoverage concerns a large number of aspects. This section analyses the problem ofcoverage that is available on-line from the perspective of base station selection,carrier configuration, antenna feeder organization, antenna installation, installationquality and parameter setting, etc.

1.1.1 Equipment Configuration

I. Combiner Configuration

Solution recommendations under normal circumstances:

For cells with less than, or equal to 2 TRXs, use EDU.

For cells with less than, or equal to 4 TRXs, use CDU combiner.

For cells with more than, or equal to 5 TRXs, consider using cavity combiner or four-in-one (SCU) combiner.

II. Tower Amplification

For some areas or bands whose uplink signal quality needs to be strengthened,consider designing tower amplification.

III. Antenna

Using high gain antenna can improve the base stations coverage capacity to acertain extent.

When doing network planning, first consider the network quality from the perspectiveof network coverage and select appropriate antenna. Under normal circumstancesthe 900M directional antenna gain shall be in the range of 15-17dBi; the 1800Mdirectional antenna gain shall be in the range of 15-18dBi. For city areas thedirectional antenna with horizontal half power angle of 65 degrees is preferable.

When high gain antenna is used, the problem of Shadow right under the towershould be taken into account. Preferably zero-point filling antenna should be used. Ifthe omni-antenna is built over high mountains, the problem of Shadow right underthe tower should be taken into account too. Preferably omni-antenna with built-in lowincidence should be used.

1.1.2 Base Station Site Selection

I. Bad base station location and antenna installation will lead to the problemof coverage.

When there is a possibility that bad station location and antenna installation may lead

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of the directional antenna points to the area where emphatic coverage is desired anddescribe the coverage differences between the omni-antenna and directionalantenna.

1.1.3 Antenna Engineering Design and Installation

I. The problem of the directional antenna installation

Generally, the base stations uplink and downlink signals are balanced. Bad antennainstallation may leave the mobile phone subscribers such impression that the basestations uplink coverage capacity is weak.

(1) The transmitting antenna is installed inverse.

Transmitting feeder and the cells antenna were mistakenly connected or BTS set topjumper was mistakenly connected.

Possible phenomenon clue for fault discovery:

The mobile phone is somewhere in the cell but cannot call out (thereceiving antenna is in the other direction, the uplink is bad);

Unilateral call connection;

From a distance of the cell, drop-off happens frequently (infrequentnear the base station);

Drop-off after frequent handovers (the handovers are found to bemostly caused by weak uplink signal strength or bad uplink quality);

The field intensity distribution of the BCCH frequency band between the

adjacent sectors were found to be displaced during the drive test; Unexpected severe interferences with the adjacent frequencies.

This type of error is easy to discover in the network. It can be found by checkinginstallation or by analyzing the field intensity distribution of the BCCH frequency bandduring the drive test.

(2) Wrong receiving antenna installation:

This problem, which cannot be found by analyzing the field intensity distribution of themain BCCH frequency band in the downlink, can cause bad uplink in the current cell.After excluding the possibility that it is caused by transmitting antenna, check if the jumper or feeder of the receiving antenna are wrongly installed, also review themeasurement report to see if the uplink level is too low, or if there are any call-out orhandover problems, power control triggered by the uplink signal, or irrationalhandover ratio, etc.

II. The problem of shadow adjacent to the coverage area in the directional cell

During the installation of the directional antenna, caution must be taken to avoid thecreation of large area of shadow in its coverage area. Shadow usually comes intobeing as a result of huge hindrance near the base station, such as skyscrapers, ormountains. Caution must be taken to circumvent such hindrance during theinstallation. When the directional antenna is installed at the surface of the top of thebuilding, caution must be taken so that the wave beam is not hindered by the edge ofthe building, and that the antenna is not installed near the edge, thus reducing or

extirpating the shadow. Due to the complexity of the antenna surface, when theantenna must be installed away from the edge of the building, it must be installed high

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M S _ T X P W R _ M

M S _ T X P W R ( n )

H OB T S 1 B T S 2

R X L E V _ N C E L L ( n )

M s O p t L e v

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I. Static Power Setting

To reduce the interference with the adjacent frequencies, lower the height of theantenna, increase the down-tilt, or decreasing the base stations transmitting power.

Usually, decreasing the transmitting power can also worsen the indoor coverage. Sowhat are commonly adopted arefalling down the height of the antenna and increasingthe down-tilt.

II. RACH Parameters Setting

To control the uplink access (call, paging response), and to balance between thecoverage and call drop ratio, Huawei adds the parameter RACH access threshold inthe BTS3.X base station. The parameter can go upward from 110dBm, andeffectively controls the mobile phones uplink access.

III. Cell Selection and Reselection Parameters

C1 and C2 decide the cell selection and reselection. RXLEV_ACCESS_MIN iscommonly set between -100dBm to 105dBm. Setting the parameter too great willmake it difficult for the mobile phone to select the cell, which, when viewed from thesubscribers perspective, is the mobile phones call drop and no coverage; Setting theparameter too small will lead to the situation that the mobile phone can receive thesignal but cannot call out.

During the parameter setting, also note the differences of sensitivity and maximumtransmitting power between the 900M and 1800M networks, ignoring which maycause the unbalance between the uplink and downlink.

IV. Other Parameters and Phenomena

The setting of the random access error threshold can also restrict the mobile phonesaccess. Its value, which is usually greater than 180, is set subject to the radioenvironments bottom noise and statistical result. The other parameters settings thathave effect on the mobile phone attached, access, handover, and connection are:BSIC, NCC_PERMITTED, CELL_BAR_ACCESS, CELL_BAR_QUALIFY, radio linkfailure timer, the number of mobile phones maximum retransfer, the number of senddistribution time slots (the number of extended distribution time slots), the number ofSACCH multi-frames, the number of maximum physical retransfer, radio linkconnection timer, location update and paging-related parameters (including LACdistribution), handover-related parameters, power control-related parameters, flowcontrol-related parameters, roaming permit-related parameters, TRX power settingsof different CDU channels, etc.

When the base stations upper clock is bad, MCK (TMU) is failure, propagation isunstable, or the carrier frequency board is failure (no warning, sometimes stableinterferences), the mobile phones accessing, call and handover (usuallyasynchronous handover) will be affected.

When the system information is wrong or is not delivered correctly, the mobilephones accessing, call or handover will be affected.

When the frequency interference is severe, the mobile phones accessing, call andhandover will be affected.

When the channel is congested the mobile phones handover and call access will beaffected.

Multipath effect can lead to signal fluctuation

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During the cell reselection and frequent location border update, the signal of themobile phone will fluctuate.

During the connection, direct re-try and load handover caused by congestion can leadto strong fluctuation of the signal; Priority handover (for layered network) and edge

handover (the edge handover threshold is set too low and there is no PBGThandover) can also lead to strong fluctuation of the signal. If the power control andhandover parameters are not set properly during the signal fluctuation, the fluctuationwill be strengthened until call drop happens.

The cross-area coverage or coverage in the border areas for different services can allcause problems, leading to the subscribers huge increase of roaming bill. To solvethe problem, the coverage area should be controlled or enlarged during the planningand optimization phase to avoid mutual cell reselection or only single-direction cellreselection. Solve the problem by adjusting the height, down-tilt, and direction of theantenna, CRH and MS minimum access threshold, and BAI table.

The mobile phone subscriber having not sufficient understanding on the terrain,construction materials absorption loss, or multipath effect will tend to rise questions

on coverage.

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1.2 TCH Congestion

In 2001, the mobile phone companies in all the provinces presented five indexes asnetwork quality criteria. They are: long distance call success ratio, traffic call dropratio, traffic channel availability ratio, radio successful connection ratio, and worstcells ratio, the last two of which are directly influenced by TCH congestion ratio.

1.2.1 The Causes of High TCH Congestion Ratio

The causes for high TCH congestion ratio are many, among others, data configurationproblems, hardware problems and external interference. But in light of solution stepsand sequence, it is desirable to first check the software and hardware problems of theequipment per se, and then check other equipment-unrelated factors such as external

interference and constraints due to complex terrain

(1) Interface A trunk circuit data configuration errors.

(2) Carrier frequency board faults or unstable performance, leading to highcongestion ratio.

(3) Bad base station hardware installation causes unbalanced uplink and downlinklevel, leading to TCH congestion.

(a) Uplink branch: antennatower amplifierfeeder linedischarge arresterset

top connectordivider or CDUTRX board

Divider cascaded half-rigid cable connection error, leading to uplinklevel discrepancy;

Half-rigid cable distortion or loose connector, leading to the problem;

Data bus problem.

(b) Downlink branch: TRXHPAcombiner or CDUset top connectordischarge

arresterfeeder linetower amplifierantenna

The transmit branch has antenna feeder standing wave ratio warning;

Half-rigid cable distortion or loose connector;

Cell antenna connection error or TRX natural transmit channeldiscrepancy cause the discrepancy in the coverage direction and area

of main BCCH carrier and expansion carrier, leading to TCHcongestion.

(4) The repeater station is subordinated to the host cell. The repeater station doesntexpand as the cell does.

(5) Interferences leading to congestion

(6) Isolated station or complex terrain causes TCH assignment failure, leading tocongestion.

(7) Huge real traffic leading to congestion.

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1.2.2 Positioning Methods of High TCH Congestion Ratio

I. Remote Analysis of Congestion Ratio Cause

(1) By traffic statistics analysis

By conducting traffic statistics analysis on the cells TCH performance,check if TCH congestion is due to all-busy congestion. If it is, theproblem can be solved by network optimization which delegates thecells traffic to other cells, or by advising the operator to expand.

If the problem is not due to all-busy congestion, check if the congestionis related to interference, namely, check interference 1 to 5. Ifinterfered, the cells call drop ratio can be a little high.

Register the receiving performance measurement traffic statistics tasks.Query the traffic statistics result by object and see if the uplink/downlinkmeasurement report in the same TRX is balanced to decide if theuplink/downlink hardware branch is balanced. Query the traffic statisticsresult by time and see if there is any exceptions regarding themeasurement report number in the same cell to decide if thecongestion is related to that board.

(2) View the warning

Check the station warning to which the high-congestion-ratio cell belongs to see ifthere are any abnormal warning, such as VSWR warning, PCM synchronization lostwarning, uplink data bus warning, etc, and decide if the congestion is related to thatwarning, taking into account the traffic statistics.

(3) Remote maintenance terminal of the base station

Check if the software of the boards in the base station is uniform. The versionupgrade shall be subject to the notice from SUPPORT website.

Use the maintenance terminal of the base station to congest in turn the cell carrierboards TCH channels that have high congestion ratio to see if the high congestionratio is related to that cells carrier board. Solution principles:

If the fluctuation of the congestion ratio is related to the block of thecarrier boards channel, then very possibly that board is at fault. Checkif there is any co-channel interference. Check the hardwareperformance of the uplink, downlink, and the board.

If the congestion is not related to the carrier board, then the whole cell

may be interfered or affected by the terrain.(4) Use the Signaling Analyzer to diagnose the ABIS interface message.

According to the call flow and TCH assignment failure statistics, use the SignalingAnalyzer to trace the ABIS interface message at every high-congestion-ratio cell. Thefigure shown here uses MA10 Signaling Analyzer, the detailed analysis of which is asfollows:

Analyze the assignment command Assignment CMD delivered in SDCCH, as the TEIvalue in figure 10-2, to determine the carrier board that SDCCH is in. The carrierboard that TCH is in can be determined by referring to the ARFCN radio frequencyband. Determine if the assignment failures are all in a TRX. Also analyze the causesof the assignment failures by focusing on the mobile phones TA value,

uplink/downlink level value, and uplink/downlink signal quality in the measurement

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carrier board whose levels are uneven, replace the board oruplink/downlink antenna feeder system to look for the causes.

Note: for frequency hopping cells, use command line parameters to change that cellto non-frequency-hopping cell for the convenience of local calling.

(4) Do the drive test by the network optimization software ANT-PLOT to see if there isany handover exceptions, downlink interference for any clues on the problem ofcongestion.

(5) Use the spectrum analyzer to look for the interference source.

(6) See if the terrain of the stations coverage area is complex.

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1.3 Voice Prompt

The voice prompt in GSM is controlled by the MSC of GSM network and broadcast tothe subscribers. MSC plays the pre-recorded voice prompt in the channelcorresponding to the voice card according to the different cause values and dataconfiguration of the message. Typical voice prompt problems can take the followingforms:

(1) When the called is in idle state, The subscriber youre calling is busy or Themobile phone youre calling has turned off is delivered.

(2) When the called is in idle state, The subscriber youre calling is out of the servicearea is delivered.

(3) When the called is in idle state, For the time being the subscriber youre calling

cannot be accessed is delivered.

The reasons that cause the above phenomena can be some of the following:Subscriber status management exception on the NSS side, roaming number fetchfailure, or no paging response.

In the following section we emphasize on the analysis of the problem Subscriber outof the service area.

1.3.1 Paging Strategy

The primary causes for the problem of subscriber out of the service area aretwofold: one is paging response time over; the other is no paging response. First lets

look at how the paging is done. The paging strategy can be divided into the followingthree parts as per MSC, BSC, and BTS.

I. MSC Paging Retransfer and Paging Mode Selection

MSC can page the same message for a maximum of four times. If the latest paging isnot responded, it will retransfer the paging message. The resend time interval is 3seconds, 3 seconds, 2 seconds, and 2 seconds respectively. Two seconds after thelast resend, i.e., twelve seconds after the first retransfer, if the paging is notresponded, MSC will regard it as time-over and deliver prompt tone of Subscriber outof the service area to the subscriber. The paging methods MSC can choose areTMSI and IMSI.

II. BSC Paging Group Calculation and Paging Message Transfer between theModules

After BSC receives the Paging Request delivered by MSC, it will calculate the paginggroup that the paging belongs to by the last three digits of IMSI, the cells CCCHchannel configuration, and the paging block configuration, and then deliver PagingCommand to that cell. Under multiple modules conditions, the Paging Commandneeds to be transferred between the modules.

III. BTS Paging Queue and Paging Combination

After BTS receives the Paging Command from BSC, it will put the paging in the

Paging Group Queue that the Paging Command designates, and send the paging

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1.3.3 Analysis on the Problem Subscriber Out of the Service Area

The reasons that cause Subscriber out of the service area are the time-out of themobile phones response to the paging or no response. From the flow of the paging

response report and analysis on the real cases, there are the following reasons thatcause this problem:

I. MSC subscriber status management

If MSC subscriber status management is down and the request cannot be deliveredto BSC, then MS will fail to receive the paging and to respond, leading to time-out. Inreal situations, there are cases when MSC didnt deliver the paging. There were alsosuch abnormalities as faulted called being regarded as caller and 3Tick releaseabnormalities. Such abnormalities are non-recurrent. And can be identified by beingtraced by the GSM subscriber interface of MSC Maintenance Station.

II. Receiver in the base station

If the receiver of the base station, including DSP, TRX, and antenna feeder, is down, itwill fail to detect the mobile phones access pulse or to establish connection with themobile phone within the time limit, the paging response (in essence the instruction toset up connection) cannot be reported to BSC, leading to paging response time out orno paging response. For example, we found that T2688 in somewhere is slow ingetting online. After the amplifier and the antenna feeder are discarded, the accessspeed can be 10 seconds faster. As this situation involves air interface andsometimes can be caused by the problems of MS, the diagnosis of it is difficult anddictates experts equipped with special devices.

III. The differences of the mobile phones fake accessing and accessing mode

According to the protocol, whenever the MS is performing open/close, it must updateits position and do the IMSI detachment according to the systems requirement. Somemobile phones, mostly Ericsson, dont perform the above operations. If the mobilephone fails to update its position when attempting open, it will display the messagethat it has already been attached. But what happens is that there is no messagereported and the subscriber status doesnt change. Dialing that subscriber will receivethe message that it is close. Another example is that IMSI is not detached when themobile phone switches off, so the subscriber status is still attached. Paging is stilldelivered to that subscriber when it is called. Out of service area message will comeout when the time is out.

Different mobile phones have different accessing mode and speed after the drop-off,causing some to have long time in accessing, leading to the problem of Subscriber

out of the service area. Under the protocols of GSM 02 and 03, after the call drop themobile phone follows the principles of ASAP (as soon as possible) and energy savingto access the network. The mobile phone will attempt to access the network inreceiving signals strength order. 900M mobile phones search 30 frequency bands;1800M mobile phones search 40 BCCH frequency bands; Double frequency mobilephones search 70 frequency bands. After the failed attempt, the mobile phone willdecide when to start next time according to its own algorithm. Mobile phones ofdifferent models have different algorithms. For example, some models of Motorolamobile phones can set accessing network frequency. Under the low speed accessingmode, it takes 50 minutes to access the network after the mobile phone drops off thenetwork and re-enter the coverage area. In addition mobile phones of differentmanufacturers have different accessing mode. Some mobile phones, after severalfailures in finding the network, will stop doing so for a long time to come. So, this

problem is related to the mobile phone itself and can be solved by the switch on and

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switch off of the mobile phone.

IV. Uplink / Downlink Unbalance of Radio Link

The radio signal can be classified into uplink and downlink according to the

propagation direction. Ideally the uplink-downlink are at balance, that is to say, at anyarea the base station and the mobile phone can receive (or not receive) the othersides signal simultaneously. The uncertainty of the radio signal propagation and thediscrepancy of the actual circumstances dictate that complete balance within thewhole network is impossible. So in the network there must be some areas where hedownlink signal can cover but the uplink signal cannot. In these areas, the subscribercan receive message sent by the network, while the network cannot receive themessage reported by the subscriber, including paging response. Therefore in theseareas it is very common for the Subscriber out of the service area message to occur.To solve such problem of Subscriber out of the service area, adjust the radioparameters such as RACH access threshold, random access error threshold, MSminimum access level, and RSSI check to optimize the balance. In particular, notethat the displacements of the measured level value in the measurement report for

different versions of base stations are different.

V. SDCCH Congestion

After the mobile phone receives the paging command, it will send channel request tothe network side. If there is no SDCCH channel available, or if the procedure to set upSDCCH channel fails, the paging response cannot be delivered to the network side,thus the problem of Subscriber out of the service area occurs. The causes for theSDCCH congestion are SDCCH all busy, random radio link failure, etc. For theSDCCH all busy situation, adjust that cells coverage area to reduce the SDCCHcongestion. For congestion caused by other reasons, such as random radio linkfailure or surface link failure, the solution is subject to real situation.

VI. PCH Overload

The paging message in the network is random. Due to the restrictions on thestructure of the radio channels and the limitations on the capability to send the pagingcommand, it is possible that some of the paging groups are overloaded, leading tofailure to send the paging messages out timely, and the resend paging cannot beresponded within the valid time, thus causing the problem of Subscriber out of theservice area.

This situation can be improved by modifying CCCH configuration parameters thenumber of frames in the same paging, the number of access grant reserved block,and CCCH channel allocation. Reducing the number of access grant reservedblock to a proper extent will increase the number of PCH sub channels, thusincreasing the capacity of the paging channels; Reducing the number of frames inthe same paging will improve the frequency of sending the paging message;Increasing the number of CCCH channels in the cell can significantly improve thesystems paging capacity, but at the same time reducing the number of TCHconfigurations. Thus this method is seldom used. If PCH is severely overloaded, thesize of the location area needs to be reduced, thus lowering the flow of the pagingmessage.

VII. Mobile Phone Quality

When the mobile phones radio frequency module is down, the antenna loosens, orthe battery is down, its receiving sensitivity goes down, the uplink signal quality

worsens, and the capability to receive the paging command and access the networklowers. Mobile phones with such problems are prone to have the phenomenon of

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Subscriber out of the service area. The evaluation on the quality of the mobilephone sometimes requires special devices. The problems of the mobile phone cantake the following forms:

(1) Bad power supply leads to insufficient uplink transmitting power and uplink access

failure.

(2) Faulted mobile phone software leads to the mobile phones exceptional dead-endand failure to respond to the paging message.

(3) Problems in the radio frequency part of the mobile phone will lead to failure toreceive, unstable transmission, or high frequency error within some frequency band.

VIII. Non-comprehensive Coverage

In areas where the radio signal coverage is not good, usually indoors, the signalquality is likely to be bad, and the call drop is not uncommon .As the subscriber statusin the VLR has not changed, when that subscriber is being called, the network sidecan deliver paging message normally, but the subscriber cannot receive the paging

message and cannot respond to the paging message. Or because of the low qualityof the signal, the paging response cannot reach the network side. It is normal underthis circumstance that Subscriber out of the service area occurs. For suchSubscriber out of the service area phenomenon, the quality can be improved byincreasing the number of the base stations and perfecting the coverage.

There is a significant percentage of complaints concerning the problem of Subscriberout of the service area that is due to bad coverage.

1.3.4 Supplementary Notes

In the wireless network, because of the radio channel congestion or radio signal

coverage, it is certain that the problem of No paging response occurs.

In some areas where the coverage is bad, bad signal quality will lead to the failure todeliver the paging or paging response to the peer side, causing the phenomenonSubscriber out of the service area.

For example, due to the temporary congestion of the SDCCH channel, the MS cannotbe assigned an SDCCH channel, and the paging response cannot be reported, inwhich case the first dial may receive the prompt tone of Subscriber out of the servicearea and the second dial may succeed. As the paging sub channel of the cell wherethe subscriber is in is fixed, so the chance for that sub channel overload increasessignificantly. Calling that subscriber many times will be very likely to encounter thephenomenon of Subscriber out of the service area. These phenomena are all

normal no-paging-response.It is an exception only when the attempt to call an idle state subscriber with goodsignal continuously for a long time receives Subscriber out of the service area.

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1.4 The Problem of Signal Fluctuation

The causes that can lead to the mobile phones signal fluctuation are:

(1) Fluctuation in the base stations transmission power

(2) Cell reselection (in idle state) or handover (in conversation state)

(3) Power control, DTX

(4) TRX faults

(5) Interference

(6) Huge traffic, leading to channel congestion

(7) Multipath effect

1.4.1 Examine the Stableness of the Base Stations Transmission Power

(1) Directly measure the base stations output power

(a) Use Spectrum Analyzer 8594, connect to the HPAs amplifier interface that sendsBCCH through 30dB attenuator.

Spectrum

analyzer 8594

30dB

attenuatorHPA

Figure 10-3 Directly measure the base stations output power

(b) Connect the base 13M clock and CMD57 reference clock input by the clock lineand synchronize the equipment to the frequency band to test every timeslots power,frequency error, and phase error.

If the test result shows that every timeslots frequency error and phase error conformto the standards, and that base stations 13M clock works stably, then the possibilitythat signal fluctuation is due to the 13M clocks fluctuation can be excluded.

After ensuring the output power stability, lets check the installation quality of the

antenna feeder to see if there is any instability in the standing waves. If theinstallation quality is good, then the possibility that the signal fluctuation is due to theequipments output power instability can be excluded.

(2) Within the visual distance of the antenna (about 1km away. The purpose is to testthe influence of the multipath propagation), test the base stations transmitting level tosee if it is stable.

(a) Use the spectrum analyzer 8594 and do the test by feeding the signal receivedthrough omni-mini antenna to the spectrum analyzer directly, as shown in thefollowing figure:

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spectrum

analyzer 8594

Omni-mini

antenna

Figure 10-4 Use the mini-antenna to test the base stations transmittinglevel

(b) Use the mobile phone SAGEM and the test software MobileShow to do the test atthe place that is within visual distance and is 1 km away. Preserve and test the signalwave by the laptop computer to see weather the transmitting level is stable.

By the above test, we can determine if the output power at the amplifier interface isstable. The shortcoming of it is that it cannot obtain the concrete value of the outputpower in the amplifier interface.

It seldom happens that the output power of the base station is unstable.

1.4.2 Cell Reselection (in idle state) or Handover (in conversation state)

Both cell reselection and handover can lead to the fluctuation of the signal. To find thecauses, use the following testing methods:

(1) Watch and test the mobile phone to see if the cell reselection or handover occuras the signal level changes;

(2) Use the test mobile phone SAGEM and software MobileShow to see if the cellreselection and handover occur as the signal fluctuates.

By the above tests we can decide if the signal level fluctuation is caused by the cellreselection or handover. This phenomenon normally happens at the edge of the cell.But if the network has serious trans-cell coverage,, frequent cell reselection andhandover can also arise, leading to the fluctuation of the signal.

1.4.3 Power Control and DTX

When the power control or DTX is down, the receiving level in the mobile phonefluctuates. If the threshold value for the uplink power control is set too low, the uplinksignal from the mobile phone to the base station will be weak, This, together with thenormal fluctuation in the radio space, will give rise to handover. During handover, ifthe adjacent signals quality is not strong to maintain the conversation, call drop will

arise. Naturally the Level Indicator in the mobile phone will go down to one scale orzero scale from the previous full scale.

Phenomenon:

The Level Indicator points to full scale, suddenly the peers voice at the other end ofthe line cannot be heard in the middle of the conversation; the Level Indicator in themobile phone points to one scale or zero scale. Call drop happens. After severalseconds the conversation is on again.

Procedure and Analysis:

The Level Indicator pointing to full scale means that mobile phone is in good receivingquality. The level value at this time is about -75dBm. If the downlink level exceeds thisvalue, the level fluctuation cannot be displayed in the mobile phone. So suchfluctuation is not easy to be seen when the signal is strong. So when the networks

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parameter, and test the modifications.

1.4.4 TRX Down

Phenomenon:

The mobile phones Receiving Signal Indicator points to the full scale. When thesubscriber dials the number and press , the mobile phone drops out within afew seconds, at the same time the Indicator lowers to one scale or zero scale. Thisphenomenon happens intermittently.

Cause and Analysis:

After key is pressed, BCCH assigns a TCH to the mobile phone throughSDCCH. When the TRX where the TCH is located is down, such as no power outputor output too small, then the TCH will fail to set up the communication, leading todrop-off, and the Indicator lowering from full scale to 1 scale or 0 scale. After a fewseconds it will return to the Wait Status, and the Indicator returns to full scale. As the

TCHs assignment is dynamic, that is to say, the first subscriber occupies the firstcarrier frequency, the first time slot; the second subscriber occupies the first carrierfrequency, the second time slot, and so on. After eight subscribers calls, (or onesubscribers eight consecutive calls), TCH will switch to the second carrier frequency,thus leading to normal conversation after several calls. As the number of callers islarge, the interval for the examiner to encounter such fault may vary. And the morethe number of carrier frequencies in the cell, the more difficult for it to discover thecause. On the other hand, there is no fault warning when the carrier frequency is notbeing used, but the warning of Power Too Small after the assignment.

Solution:

See if there is any warning in the TRX at the background when the mobile phone is

having call test. If yes, replace that TRX with the adjacent carrier frequency amplifieror TRX, decide where the fault is and substitute the standby board for the at-faultboard. Furthermore, this method can be one of the effective ways to determine ifthere are any faults in the carrier frequency channels at the cell. At the openingphase, use two mobile phones and finish continuous testing on all the carrierfrequencies at the cell within several minutes.

1.4.5 Interferences

Interferences can lead to signal fluctuations. Interferences can cause the timeout forthe DCS counter (90/the multiframes between the same paging) in the mobile phonesat the current service area, thus leading to cell reselection, which when reflected on

the mobile phone side, is the fluctuation of signals.

1.4.6 Cell Congestion

First let us explain the problem by analyzing the signals: After a call is initiated by themobile phone, first SDCCH channel is assigned, after the corresponding signaling isfinished, a TCH will be assigned to the mobile phone for its use by the network. Whatis the normal procedure is that BSC activates a TCH channel by sending to BTS inthe current cell a signal activation command, and then the channel is assigned to themobile phone. But what actually happens is that (in times when the current cell iscongested) the network gives the mobile phone a command to switch to other cells,whose signal is weaker than the current cell. As a result, the level goes down

immediately, leading to the fluctuations of the signal. It is possible that after the

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penalty time the mobile phone switches to other cell again, whose signal is stronger,in which case the level indicated in the mobile phone goes up. Or after hooking themobile phone is in IDLE state and receives the BCCH signal from the cell where themobile phone is in, which reflected in the Indicator is the going up of the signalstrength. Therefore the essential cause of this problem is too much traffic, causing

congestion in the cell.

1.4.7 Multipath Effect

Multipath effect is one of the main causes that lead to the signal fluctuation in thenetwork. The performance of the radio communications system is largely constrainedby the radio channel. The propagation path between the transmitter and receiver isvery complex, ranging from the simple visual distance propagation to encounteringsuch terrain as buildings, hills and trees. The form the electromagnetic wave takes topropagate is diverse. In the main, it can take the forms of reflection, diffraction, andscattering. Most cellular radio systems operate in the city. There is no visual pathbetween the transmitter and receiver, and whats more, the skyscrapers produce

strong scattering loss. Furthermore, as different objects have different multipathreflections, the electro-magnetic waves, after passing paths of different length,interfere to produce multi-path loss. At the same time the strength of the electro-magnetic waves attenuates as the distance between the transmitter and receiverincreases. The attenuation in the radio environment can be classified into fastattenuation and slow attenuation according to the estimation on the field intensity.

In skyscraper cities, as the height of the mobile antenna (mobile phone) is muchlower than that of the surrounding buildings, there is in most cases no visualpropagation between the mobile station to the base station, thus causing attenuation.Even if there is such a path for visual propagation, multipath effect may still arise dueto the reflection of the surface and the surrounding buildings. The incident wavesarrive in different directions and have different propagation delay. The signal received

by the mobile station at any position in the space consists of many signal waves,which have randomly distributed amplitude, phase, and incident angle. Thesemultipath components are combined in vectors by the receiver, thus giving rise to theattenuation and infidelity of the signal being received. Even when the mobile receiveris static, attenuation can still arise as the signal being received is affected by thepropagation medium (the air flow change), or by the moving objects where the radiochannel is in.

The method the base station uses to solve the fast-attenuation phenomenon is spacediversity (polarized diversity), i.e. host diversity antenna. The effectiveness of this hostdiversity receipt is guaranteed by the irrelevancy of the host diversity antenna receipt.By irrelevancy, it means the signal received by the host antenna doesnt have thesimultaneous attenuation feature with the signal received by the diversity antenna.

This requires that the gap of the host diversity antenna is 10 times greater than theradio signal wavelength (for GSM 900M the antenna gap must be greater than 4meters), or that the polarized diversity is used to guarantee the different attenuationfeatures of the signals received by the host diversity antenna. Whereas the singleantenna mobile station (mobile phone) is helpless before the fast-attenuation featureunder the wireless environment. So multipath will lead to the fluctuation of the mobilephones receiving level.

1.4.8 Other causes

(1) Poor antenna feeder connection will lead to ups and downs of the standing wave,which when reflected in the mobile phone, is the fluctuation of the signals

(2) Serious propagation flickers will lead to the amplification of the on/off in the base

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station.

(3) Some mobile phones will also have fluctuation of the level when doing locationarea updating. This is especially so when in the location area border of the city.

(4) The signal level after the assignment will fluctuate deeply due to the difference ofthe carrier combination method or the disaccord of transmission channel gain withinthe cell.

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1.5 Other Main Problems

1.5.1 Poor Voice Quality

(1) Hardware faults, poor propagation and bad grounding will lead to poor voicequality.

(2) Worse interference leads to poor voice quality, especially when the frequency isdensity reuse.

(3) Poor coverage, leading to poor receiving level and receiving quality.

1.5.2 Failure to attach the network

Cannot update the location areaThe CGI table of MSC fails to keep record of thecell, leading to location area update failure and network access failure

1.5.3 Slow to attach the network

Possibly related to the mobile phones access mode. Generally when the mobilephone is opened, it will try to access the network according to the information storedwhen the mobile phone was closed last time. In cases when the information is invalidor there is no stored information, the mobile phone will try to access the network byscanning for the strongest signal. As the scanning method may differ, so does theaccess speed. Furthermore as the mobile phone needs to decode the scannedfrequency band according to the level, when the cell has a low access priority, it canonly access the network after ensuring that there is no higher priority. This will alsocause the slow attach speed.

As the network access procedure includes authority verification on the mobile phone,the speed is also related to the performance, propagation and system processing ofthe cell. Notwithstanding the foregoing, many factors shall be considered in anintegrated way.

1.5.4 Access slow

Possible causes:

(1) Effect on the same number of paging multi-frames. The paging may be delayed for0-0.235N seconds. Certainly its setting is related to many a factor such as locationarea. Its parameter change shall be considered in an integrated way.

(2) Effect on the number of time slots during propagation extension. In areas wherecoverage is poor, when the first access request is not decoded successfully, resendmay be delayed for 55x0.477 to 1 second. (Subject to the parameters)

(3) Related to the system processing performance.

1.5.5 unilateral connection

The common cause may come from the equipment or transmission. This problem will

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