GSM Optimization and Troubleshooting

Introduction

Mobile Network-A living Organism

• A radio mobile network may be compared to a living organism. • Like all living systems, the mobile network has a period of gestation before its birth.

– This period estimates the financial validity and profitability of the mobile network to deploy. If the gestation period concludes on the decision to deploy the network, the network starts with the deployment of the first base stations.

• At the very first stage of its life, the network aims at extending its cover to handle all communications on the given area.

• Then, as a second step, it grows at every local or global extension based on service quality criteria.

• Each of these extension steps are called new deployments. • Regarding economical criteria, a good (cheap) deployment must involve a minimal set of

base stations while performing an optimal radio coverage e.g. as larger as possible for a given signal quality threshold.


• Each of the deployment and the optimization phases of radio mobile network may be considered has a stage, or period.

• We focus in this course on Optimization stages and of their relationships inside an optimization problem: the optimal location and parameterization of base stations.


• With the rapid growth of the wireless industry, GSM (Global System for Mobile communications) networks are rolling out and expanding at a high rate. The industry is also becoming intensely competitive.

• In this environment, high quality of service is a competitive advantage for a service provider. Quality of service can be characterized by such factors as contiguity of coverage, accessibility to the network, speech quality and number of dropped calls.


• The primary tool used by most service providers to solve network problems is a drive-test system.

• A conventional drive-test system is comprised of a test mobile phone, software to control and log data from the phone, and a Global Positioning System (GPS) receiver for position information.

• A test mobile gives a customer’s view of the network, but can only indicate the type of problem that exists. It can-not show the cause of the problem. These limitations are over-come if a GSM receiver is integrated with the phone if a GSM receiver is integrated with the phone.

The Optimization Process


Optimization Process

Optimization is an important step in the life cycle of a wireless network. Drive-testing is the first step in the process, with the goal of collecting measurement data as a function of location.

Once the data has been collected over the desired RF coverage area, it is output to post-processing software

Engineers can use the collection and post-processing software to identify the causes of RF coverage or interference problems and determine how these problems can be solved

When the problems, causes and solutions have been identified, steps are performed to solve the problems


Optimization Plan Cluster Definition

Drive test/NOC Measurement

Report Analysis

Is the KPI met?

Cluster sign off Report and

summary of optimization

Is there need of New Drive Test?

Route Definition Cluster Site Readiness

Verification

Drive test Measurement

Report

Drive test Measurement

Cluster Analysis

Next ClusterOverall Network

Optimization

No

Yes

Yes

No

NOC Measurements


• Drive test and optimization activities generally are started six weeks before network launch date at each market.

• Time reserved for optimization at each market is five to six weeks. • Actual cluster optimization can be started when 80% of sites has been integrated at each

cluster. • Pre-work activities as cluster- and route definitions and getting the drive test equipment to

market should be done as early as possible. • If it is necessary to start optimization earlier, less than 80% of the sites integrated, clusters

or drive routes may be changed or modified to make optimization happen sooner

• All Operator Specific


• Key Performance Parameters• Operator specifies the KPI which the optimization team has to achieve for acceptable

network performance.


• Key Performance Indicators• Call Success Rate, TCH Drop Call Ratio (%)

– Acceptable CSR is down to 95 % over network. Excellent value is CSR 98% or bigger. CSR = 100% - DCR%.• Downlink Quality (%)

– The GSM Quality is categorized in 8 different classes based on received Bit Error Ratio (BER). In the quality measurements the measured samples are cumulatively shared in different classes.

– Acceptable cumulative Downlink RX Quality is classes 0…5 of 95% of the time. 99 % of the time is excellent value.

• Call Setup Success Ratio– Within the coverage area when requested by user, ratio of successful SDCCH reservations and made call attempts.

Acceptable CSSR value in general is 95% and excellent value is 99%.

• Coverage (% of time), Downlink Level– Within the coverage area the received downlink signal is measured to define if the recommended signal threshold is

reached. Threshold is dependent on the used frequency, and must be terminated to 50 Ohms. Acceptable Downlink Link level's value of time in general is 95% and excellent value is 99%.

• Handover Success Ratio– Within the coverage area, ratio of successful handovers and handover attempts. Acceptable HOSR value in general

is 95% and excellent value is 99%.


• Before any drive tests and optimization is performed the site readiness have to be checked. Every site goes through the acceptance verification.

• The network analysis and optimization is done by clusters. • A cluster is set of adjacent sites which have already been defined. Sites belong to the same

BSC. • Cluster are created using apprx. 10-30 adjacent sites (BTSs) or 30-90 sectors depending of

area geography. • Mapinfo software is used to create clusters.


• The sites belonging to one optimization cluster have been accepted by implementation before any drive test or optimization activities are started in that specific cluster. The following items has to be performed at the BTS:

– Antenna and cable installation checked – Cable Sweeps– Link Budget Verification– BTS checked – BTS parameters checked (adjacencies, frequencies, handovers, timeslots, BTS

TRX power, Cell ID,LAC, BSIC), done from NMS.• Acceptance verification will be performed prior to optimization on per site basis. As sites

come on air, they will be tested in order to verify site functionality and confirm that site data fill is correct.

• The evaluation will include successful call origination and termination, intra-site handovers, frequency and BSIC verification.

• Radio network planner provides default parameter list to implementation teams.


• Dominance areas of the each cell and frequency plan are needed. This information is is provided planning process.

• Drive routes are designed by using e.g. Mapinfo software.Printouts of the each drive route are generated.

• Drive routes are defined carefully before drive tests are started. Once drive route is defined the same route is used all the way during optimization process. Following things have to take into account while planning the drive routes

• Max. duration of the drive test of the each cluster is 4 hrs. Still enough calls (>=200) have to be generated during each drive tests to have reliable results

• Route design have to include all the sectors/cells of the cluster• Handovers are measured in both ways (clockwise and anticlockwise) if possible At least all

the major roads are drive tested in the cluster


• Drive test are performed in one person teams. The driver, local people should be used for that job, does the actual driving and checks also the measurement system works properly during drive tests. The real live site information, including azimuth and downtilt is checked visually during drive test if possible.

• Drive test system includes– Measurement software and laptop– Measurement rack, power supply and connection cable to the laptop– GPS system connected to the laptop– GSM phones:Two phones in call generator mode, call length 90 seconds and duration between

calls 20 seconds. The third phone is making a call until it drops, call duration max. possible and duration between calls is 15 seconds. Third one is only for coverage purposes.

– During each drive measurements system generates at least 200 GSM calls to produce correct and reliable status of the current network quality

• During drive test driver checks failures of the measurement system and problems of the network (e.g. dropped calls). Driver makes notes if something fails during drive tests and report afterwards

Optimization Process• All the measurement files from the drive tests are saved and processed with the post

processing tools

Weekly NOC Statistics

Drive Test Measurement per drive


• Cluster Analysis Process• Process for measurement analysis is following

– Make the drive test analysis– Study the handovers between the cells– Study the level and quality values– Study the drive test statistics

KPI DL Quality criteria shown below will be applied to the system drive test results:

– RXLEV –98 dBm or better in 95% of the samples

– RXQUAL < 5 in 95 % of the samples

Uplink quality will be monitored using NMS call tracing feature with the drive test mobiles and will target the same thresholds:

– RXLEV –98 dBm or better in 95% of the samples

– RXQUAL <5 in 95 % of the samples


• From these studies RF planner comes conclusion to execute needed modifications to the network.

– The first thing is to identify if the problem detected is a hardware or a software problem.– Hardware problem can be for example tilting, azimuth, cables, timeslots, frequencies.– Software problem can be for example RX-level, RX-quality, handover failures, call setup– problems or drop calls.

• If everything is performing according to predefined network criteria planner signs off the cluster optimization.

• To perform inter-cluster optimization at least two adjacent clusters are having common border.The purpose of inter-cluster verification is to check that the performance of the system (mainly handovers) is good in the border area between the clusters. The process itself is exactly the same as for a single cluster.

GSM –Cell selection and reselection process • MS operates in two modes: idle mode and dedicated mode. • In the idle mode, MS monitors the broadcast channels in order to "hear" if it is being paged.

It also measures other BTSs' BCCH carrier and decides whether it should camp on another cell.

• This is called cell reselection and the reselection algorithm used in GSM .• In dedicated mode (i.e. during a call), changing cell is called a handover (HO). • For the purpose of cell selection and reselection, the MS shall be capable

of detecting and synchronizing to a BCCH carrier and read the BCCH data at reference sensitivity level and reference interference levels.

• These levels are specified in 3GPP TS 05.05. • For the purposes of cell selection and reselection, the MS is required to

maintain an average of received signal levels for all monitored frequencies. These quantities termed the "received level averages" (RLA_C), shall be unweighted averages of the received signal levels measured in dBm

GSM –Cell selection and reselection process • Cell selection• Cell selection is performed immediately after MS is switched on. If MS is located in the

same cell it in which it was previously was switched off, the SIM card should have the local BCCH frequency stored in memory and MS should find network quite expeditiously. If MS has moved to another cell since it was turned off, it enters a cell selection procedure

• The MS shall not use the discontinuous reception (DRX) mode of operation (i.e., powering itself down when it is not expecting paging messages from the network) while performing cell selection algorithms. However, use of powering down is permitted at all other times in idle mode.

• For the cell selection, the MS shall be able to select the correct (fourth strongest) cell and be able to respond to paging on that cell within 30 seconds of switch on, when the three strongest cells are not suitable.

GSM –Cell selection and reselection process • Cell selection• Measurements for normal cell selection• This type of measurements is performed by an MS which has no prior knowledge of which

RF channels are BCCH carriers. • The MS shall search all RF channels within its bands of operation, take readings of

received RF signal level on each RF channel, and calculate the RLA_C for each. • The averaging is based on at least five measurement samples per RF carrier spread over 3

to 5 s, the measurement samples from the different RF carriers being spread evenly during this period.

• A multi band MS shall search all channels within its bands of operation as specified above.

GSM –Cell selection and reselection process • Cell selection• Measurements for normal cell selection• The number of channels searched will be the sum of channels on each band of operation.• BCCH carriers can be identified by, for example, searching for frequency correction bursts. • On finding a BCCH carrier, the MS shall attempt to synchronize to it and read the BCCH

data. The maximum time allowed for synchronization to a BCCH carrier is 0.5 s, and the maximum time allowed to read the BCCH data, when being synchronized to a BCCH carrier, is 1.9 s or equal to the scheduling period for the BCCH data, whichever is greater (see 3GPP TS 05.02).

• The MS is allowed to camp on a cell and access the cell after decoding all relevant BCCH data.

GSM –Cell selection and reselection process • Cell selection• Measurements for stored list cell selection• The MS may include optional storage of BCCH carrier information when switched off

– For example, the MS may store the BCCH carriers in use by the PLMN selected when it was last active in network. – The BCCH list may include BCCH carriers from more than one band in a multi band operation PLMN.

• A MS may also store BCCH carriers for more than one PLMN which it has selected previously (e.g. at national borders or when more than one PLMN serves a country), in which case the BCCH carrier lists must be kept quite separate.

• The stored BCCH carrier information used by the MS may be derived by a variety of different methods. – The MS may use the BA_RANGE information element, which, if transmitted in the channel release message

indicates ranges of carriers which include the BCCH carriers in use over a wide area or even the whole PLMN.– It should be noted that the BA(BCCH) list might only contain carriers in use in the vicinity of the cell on which it

was broadcast, and therefore might not be appropriate if the MS is switched off and moved to a new location.• The BA_RANGE information element contains the Number of Ranges parameter (defined as NR) as

well as NR sets of parameters RANGEi_LOWER and RANGEi_HIGHER.

GSM –Cell selection and reselection process • Cell selection• Measurements for stored list cell selection• The MS should interpret these to mean that all the BCCH carriers of the network have

ARFCNs in the following ranges:

Range1 = ARFCN(RANGE1_LOWER) to ARFCN(RANGE1_HIGHER);Range2 = ARFCN(RANGE2_LOWER) to ARFCN(RANGE2_HIGHER);RangeNR = ARFCN(RANGENR_LOWER) to ARFCN(RANGENR_HIGHER).

• If RANGEi_LOWER is greater than RANGEi_HIGHER, the range shall be considered cyclic and encompasses carriers with ARFCN from range RANGEi_LOWER to 1 023 and from 0 to RANGEi_HIGHER.

• If RANGEi_LOWER equals RANGEi_HIGHER then the range shall only consist of the carrier whose ARFCN is RANGEi_LOWER.

GSM –Cell selection and reselection process • Cell selection• Measurements for stored list cell selection• If an MS includes a stored BCCH carrier list of the selected PLMN it shall perform the

same measurements as in normal cell selection except that only the BCCH carriers in the list need to be measured.

• If stored list cell selection is not successful, normal cell selection shall take place.• Since information concerning a number of channels is already known to the MS, it may

assign high priority to measurements on the strongest carriers from which it has not previously made attempts to obtain BCCH information, and omit repeated measurements on the known ones.

GSM –Cell selection and reselection process • Cell Reselection• Phase 1• Cell reselection is performed as MS traverses through a network in idle mode. MS

continuously keeps list of the six strongest BCCH carriers.• From the radio propagation point of view it is desirable that MS camps to a cell with the

lowest path loss. • The most favorable cell is indicated by the so called C1 parameter for a MS of phase 1,or

by C2 for a MS of phase 2 capabilities. • The parameter C2 is essentially an improved version of C1. C1 is evaluated separately for

each cell and it is defined according to the criterion

GSM –Cell selection and reselection process • Cell Reselection• Phase 1

The path loss criterion parameter C1 used for cell selection and reselection is defined by:C1 = (A - Max(B,0))whereA = RLA_C - RXLEV_ACCESS_MINB = MS_TXPWR_MAX_CCH – P (In dBm)– The path loss criterion (3GPP TS 03.22) is satisfied if C1 > 0 – The received average level (AV_RXLEV) is found by averaging RXLEV samples over a period of

3-5 seconds. RX_ACCESS_MIN is a cell dependent parameter dictating the minimum allowed RXLEV for an MS to access that cell. MS_TXPWR_MAX_CCH is the maximum TX power an MS may use when accessing the system (using RACH). P is the maximum RF output power of the MS, usually 33 dBm for a handheld GSM900 and 30 dBm for a handheld GSM1800 MS. Often the latter term in C1 equals 0 and equation can be simplified to:

C1 = A = AV_RXLEV – RX_ACCESS_MIN– For example, if the minimum allowed level to gain access to a cell is –100dBm and the

received average level at the cell’s BCCH frequency is –80 dBm, MS calculates C1 as +20 for that particular cell. MS camps to the cell with the highest C1 value.


– There is an exception to the standard procedure described above. When MS evaluates C1 values for cells belonging to a different Location Area (LA), it subtracts a parameter called CELL_RESELECT_HYSTERESIS from the C1 value, which means that those cells are given a negative offset.

– The reason for this is that changing LA requires a Location Update (LU) procedure that consumes network signaling capacity. Thus, by assigning a negative offset to C1, unnecessary LUs caused by slow fading can be reduced. MS receives information of the cell dependent CELL_RESELECT_HYSTERESIS values through BCCH


In class 3 DCS 1 800 MS Parameter

B = MS_TXPWR_MAX_CCH + POWER OFFSET – P

RXLEV_ACCESS_MIN = Minimum received signal level at the MS required for access to thesystem.MS_TXPWR_MAX_CCH = Maximum TX power level an MS may use when accessing the systemuntil otherwise commanded.

POWER OFFSET = The power offset to be used in conjunction with the MS_TXPWR_MAX_CCH parameter by the class 3 DCS 1 800 MS.

P = Maximum RF output power of the MS. All values are expressed in dBm.

GSM –Cell selection and reselection process • Cell Reselection• Phase 2• Cell Reselection Criterion C2 is defined as

C2 = C1 +CELL_RESELECT_OFFSET-TEMPORARY_OFFSETWhen timer T<PENALTY_TIME andC2 = C1 + CELL_RESELECT_OFFSET otherwise

• T is a timer implemented for each cell in the list of strongest carriers (6 Stronger). T shall be started from zero at the time the cell is placed by the MS on the list of strongest carriers, except when the previous serving cell is placed on the list of strongest carriers at cell reselection. In this, case, T shall be set to the value of PENALTY_TIME (i.e. expired).


C1

PENALITY_TIME

C2

TE

MPO

RA

RY

_OFF

SET

CE

LL

_RE

SEL

EC

T_O

FFSE

T

T

GSM –Cell selection and reselection process • Cell Reselection• Phase 2• CELL_RESELECT_OFFSET applies an offset to the C2 reselection criterion for that cell.

– CELL_RESELECT_OFFSET may be used to give different priorities to different bands when multiband operation is used.

• TEMPORARY_OFFSET applies a negative offset to C2 for the duration of PENALTY_TIME after the timer T has started for that cell.

• PENALTY_TIME is the duration for which TEMPORARY_OFFSET applies. CELL_RESELECT_OFFSET, TEMPORARY_OFFSET, PENALTY_TIME and CELL_BAR_QUALIFY are optionally broadcast on the BCCH of the cell. If not broadcast, the default values are CELL_BAR_QUALIFY = 0, and C2 = C1.

• These parameters are used to ensure that the MS is camped on the cell with which it has the highest probability of successful communication on uplink and downlink.

GSM –Cell selection and reselection process • Cell Reselection• Phase 2• C2 criterion C2 is applied in hierarchical cell structures to keep fast moving MS in an upper

layer and the slow moving MS in micro cells. It is assumed that a fast moving MS passes through the microcell before PENALITY_TIME is reached. This efficiently prevents unnecessary Lus and thus saves network signaling capacity.

• A parameter called CELL_RESELECT_PARAM_IND informs MS about which reselection criterion is used in the cell. It is broadcast on the BCCH.

GSM –Cell selection and reselection process • Cell Reselection• Phase 2• The signal strength threshold criterion parameter C4 is used to determine whether

prioritised LSA cell reselection shall apply and is defined by:

C4 = A - PRIO_THR

• Where A is defined earlier and PRIO_THR is the signal threshold for applying LSA reselection. PRIO_THR is broadcast on the BCCH. If the idle mode support is disabled for the LSA or if the cell does not belong to any LSA to which the MS is subscribed or if no PRIO_THR parameter is broadcast, PRIO_THR shall be set to ∞.

GSM –Cell selection and reselection process

GSM – Radio link measurements • Radio link measurements are used in the handover and RF power control processes.• Signal Level

– The received signal level is employed as a criterion in the RF power control and handover processes.

– The R.M.S received signal level at the receiver input shall be measured by the MS and the BSS over the full range of -110 dBm to -48 dBm with an absolute accuracy of ±4 dB from -110 dBm to -70 dBm under normal conditions and ±6 dB over the full range under both normal and extreme conditions.

– The R.M.S received signal level at the receiver input shall be measured by the MS above -48 dBm up to -38 dBm with an absolute accuracy of ± 9 dB under both normal and extreme conditions.

– For each channel, the measured parameters (RXLEV) shall be the average of the received signal level measurement samples in dBm taken on that channel within the reporting period of length one SACCH multiframe

GSM – Radio link measurements • Radio link measurements are used in the handover and RF power control processes.• Signal Level

– Range of parameter– The measured signal level shall be mapped to an RXLEV value between 0 and 63, as follows:

RXLEV 0 = less than -110 dBm + SCALE.RXLEV 1 = -110 dBm + SCALE to -109 dBm + SCALERXLEV 2 = -109 dBm + SCALE to -108 dBm + SCALE.::RXLEV 62 = -49 dBm + SCALE to -48 dBm + SCALE.RXLEV 63 = greater than -48 dBm + SCALE.where SCALE is an offset that is used only in the ENHANCED MEASUREMENT REPORT message,

otherwise it is set to 0.

• The network may request the MS to report serving cell and neighbour cell measurements with Enhanced Measurement Report message by the parameter REPORT_TYPE, provided that BSIC for all GSM neighbour cells has been sent to the MS . This reporting is referred as Enhanced Measurement Reporting.

GSM – Radio link measurements • Radio link measurements are used in the handover and RF power control processes.• Signal quality

– The received signal quality shall be measured by the MS and BSS in a manner that can be related to an equivalent average BER before channel decoding (i.e. chip error ratio), assessed over the reporting period of 1 SACCH block.

– For each channel, the measured parameters (RXQUAL) shall be the received signal quality, averaged on that channel over the reporting period of length one SACCH multiframe .

– In averaging, measurements made during previous reporting periods shall always be discarded.– When the quality is assessed over the full-set and sub-set of frames defined in subclause 8.4, eight

levels of RXQUAL are defined and shall be mapped to the equivalent BER before channel decoding as follows:

RXQUAL_0 BER < 0,2 % Assumed value = 0,14 %RXQUAL_1 0,2 % < BER < 0,4 % Assumed value = 0,28 %RXQUAL_2 0,4 % < BER < 0,8 % Assumed value = 0,57 %RXQUAL_3 0,8 % < BER < 1,6 % Assumed value = 1,13 %RXQUAL_4 1,6 % < BER < 3,2 % Assumed value = 2,26 %RXQUAL_5 3,2 % < BER < 6,4 % Assumed value = 4,53 %RXQUAL_6 6,4 % < BER < 12,8 %Assumed value = 9,05 %RXQUAL_7 12,8 %< BER Assumed value = 18,10 %.

GSM – Radio link measurements • Radio link measurements are used in the handover and RF power control processes.• Measurement Reporting• Measurement reporting for the MS on a TCH• For a TCH, the reporting period of length 104 TDMA frames (480 ms) is defined in terms

of TDMA frame numbers• When on a TCH, the MS shall assess during the reporting period and transmit to the BSS in

the next SACCH message block the following:– RXLEV for the BCCH carrier of the 6 cells with the highest RXLEV among those with known

and allowed NCC part of BSIC. For a multi band MS the number of cells, for each frequency band supported.

– RXLEV_FULL and RXQUAL_FULL: RXLEV and RXQUAL for the full set of TCH and SACCH TDMA frames. The full set of TDMA frames is either 100 (i.e. 104 - 4 idle) frames for a full rate TCH or 52 frames for a half-rate TCH.

– RXLEV_SUB and RXQUAL_SUB: RXLEV and RXQUAL for the subset of 4 SACCH frames and the SID TDMA frames/L2 fill frames. In case of data traffic channels TCH/F9.6, TCH/F4.8, TCH/H4.8 and TCH/H2.4

GSM – Radio link measurements • Radio link measurements are used in the handover and RF power control processes.• Measurement Reporting• Measurement reporting for the MS on a SDCCH • For a SDCCH, the reporting period of length 102 TDMA frames (470.8 ms) is defined in

terms of TDMA frame numbers (FN)• When on a SDCCH, the MS shall assess during the reporting period and transmit to the

BSS in the next SACCH message block the following:– RXLEV for the BCCH carrier of the 6 cells with the highest RXLEV among those with known

and allowed NCC part of BSIC. For a multi band MS the number of cells, for each frequency band supported .

– RXLEV and RXQUAL for the full set of 12 (8 SDCCH and 4 SACCH) frames within the reporting period. As DTX is not allowed on the SDCCH, -SUB values are set equal to the -FULL values in the SACCH message.

GSM – Radio link measurements • Radio link measurements are used in the handover and RF power control processes.• Measurement Reporting• Measurement reporting for the BSS• Unless otherwise specified by the operator, the BSS shall make the same RXLEV (full and

sub) and RXQUAL (full and sub) assessments as described for the MS for all TCH's and SDCCH's assigned to an MS, using the associated reporting periods

GSM – Radio link measurements • Radio link measurements are used in the handover and RF power control processes.• Measurement Reporting• Measurement reporting for the BSS• Extended measurement reporting• When on a TCH or SDCCH, the mobile station may receive an Extended Measurement

Order (EMO) message. The mobile station shall then, during one reporting period, perform received signal level measurements according to the frequency list contained in the EMO message

GSM – Radio link measurements • Radio link measurements are used in the handover and RF power control processes.• Measurement Reporting• Measurement reporting for the BSS• Enhanced Measurement Reporting• The network may request the MS to report serving cell and neighbour cell measurements

with Enhanced Measurement Report message by the parameter REPORT_TYPE, provided that BSIC for all GSM neighbour cells has been sent to the MS. T

• his reporting is referred as Enhanced Measurement Reporting. • If Enhanced Measurement Reporting is used, the BCCH carriers and corresponding valid

BSICs of the GSM neighbour cells are sent to the MS within System Information messages and MEASUREMENT INFORMATION message .

– The MEASUREMENT INFORMATION message also includes the parameters SERVING_BAND_REPORTING, MULTIBAND_REPORTING, XXX_MULTIRAT_REPORTING (XXX indicates other radio access technology/mode), XXX_REPORTING_THRESHOLD (XXX indicates GSM band or other radio access technology/mode), XXX_REPORTING_OFFSET (XXX indicates GSM band or other radio access technology/mode), REP_PRIORITY, REPORTING_RATE and NKNOWN_BSIC_REPORTING.

• Only GSM cells with the valid BSIC shall be reported unless otherwise stated

GSM – Handover • The handover algorithm decides when, why, how and to which cell the HO is made. Some

of the many aspects of HO are touched upon briefly in the following discussion. In the GSM system MS takes active part in HO process. This type of HO is called Mobile Assisted Handover (MAHO).

• Handover types• There are many different types of HOs. They can be enabled or disabled by using several

flags in the BSS• parameter database.• The different HO types, in the order of signaling complexity, are:• 1. Intracell HO• 2. Intra-BSS HO• 3. Intra-MSC HO• 4. Inter-MSC HO• Intracell HO can be executed whenever the co-channel interference is too high and some

other physical channel• in cell has less interference.

GSM – Handover • Handover types• There are many different types of HOs. They can be enabled or disabled by using several

flags in the BSS parameter database.• The different HO types, in the order of signaling complexity, are:

1. Intracell HO2. Intra-BSS HO3. Intra-MSC HO4. Inter-MSC HO

• Intracell HO can be executed whenever the co-channel interference is too high and some other physical channel in cell has less interference.

GSM – Handover • Handover causes• There are four causes for HO defined.

1. Quality, RXQUAL too high2. Received level, RXLEV too low3. MS > BTS distance too large, maximum radius of a GSM cell is about

35 km4. Better cell, power budget for another cell is more favorable, i.e., path loss

is smaller• If the network is strictly noise limited (very low interference), RXLEV HO (or more

preferably power budget HO) should be the dominant reason for a HO. • In an interference limited network (i.e. urban area) power budget related HO should be the

overwhelming HO cause because this guarantees that MS expends as little RF power as possible (assuming that uplink power control is used) thus creating less interference and saving MS battery.

GSM – Handover • Handover measurements• During each SACCH multiframe the MS measures the following parameters:

1. RXQUAL, quality of reception, depends on BER.2. RXLEV, received power level from “home” BTS.3. RXLEV_NCELL(n), received power level from neighbor cells defined on home

cell BCCH.The measurement results are transmitted to BTS during the next SACCH multiframe for processing.

BTS carries out similar measurements in uplink, in addition to

4. MS_BS_DIST, distance between MS and BTS, evaluated from Timing Advance (TA)

5. Interference level, measured in unallocated time slots.

GSM – Handover • Measurement preprocessing• The information gathered by BTS and MS is preprocessed within BTS before making a

decision on HO. Several BSS parameters influence this algorithm. The measurement results are averaged and weighted.

• Discontinuous transmission (DTX) affects preprocessing because SID frames are considered less reliable. The averaging window size and the importance weight given to SID frames can be adjusted for each of the parameters.

• For instance, A_LEV_HO is the parameter that controls averaging window size for measured RXLEV values. A_LEV_HO = 10 means that the last ten measured RXLEV values are averaged for HO decision purposes.W_LEV_HO = 2 means that measured RXLEV values for normal speech frames are weighted by a factor of two, as compared to RXLEV measured for SID frames.

GSM – Handover • Handover decision

• Here XX = UL or DL (uplink or downlink)• MS_TX_PWR_MAX = Maximum allowed TX power of the MS in the serving cell• MS_TX_PWR_MAX(n) = Maximum allowed TX power of the MS in the adjacent cell n• P [dBm] = Maximum power capability of the MS

GSM – Handover • Handover decision• From the decision criteria listed it can be seen that HO due to quality or received level is

performed only if transmit power in DL and UL is on its maximum. This means that power control should function before HO.

• Example: L_RXLEV_UL_H = 20. MS_TXPWR = MS_TXPWR_MAX = 26 dBm (MS transmits maximum power allowed in the cell i.e.

MS power control has done all it can) P = 30 dBm (handheld GSM1800 MS TX power capability). RXLEV_UL = 15 (received averaged level). Now the received level is smaller than the threshold

L_RXLEV_UL_H by 5 steps and HO is initiated.

GSM – Handover • Handover decision• The Power budget is computed for all cells separately and it is expressed for cell n as

PBGT (n) = RX_LEV_NCELL (n) – (RXLEV_DL + PWR_C_D) + min (MS_TX_PWR_MAX,P) -min (MS_TX_PWR_MAX(n),P)

Here PWR_C_D is the averaged difference between the maximum downlink RF power BS_TXPWR_MAX, and the actual used

downlink power due to power control.

• In a simplified case the downlink power control is not used and MS_TXPWR_MAX is the same for all cells. Power budget can now be written as PBGT(n) =RX_LEV_NCELL (n) –RXLEV_DL

which portrays the path loss difference between cell n compared to serving cell, if the TX power of both BTS is the same.

GSM – Handover • Handover decision• In order to initiate a power budget HO to cell n, PBGT(n) must exceed PBGT of the serving

cell by at least HOMARGIN(n) which is also defined separately for each cell.• HOMARGIN assures that MS will not bounce back and forth between cells due to slow

fading or minor user movements. In other words, the condition

PBGT (n) > HOMARGIN (n) must be fulfilled in order to initiate a power budget HO to cell n.

GSM – Handover

No HO action due to

quality or level

Intercell HO due to

level

Intarcell HO due to

quality

Intercell HO due to

quality

RXQUAL

RXLEV0

7

63

L_RXQUAL_XX_H

L_RXLEV_XX_H

L_RXQUAL_XX_IH

GSM – Handover

RXLEV_MIN

L_RXLEV_XX_H

L_RXLEV_XX_IH

Receiver Sensitivity

GSM – Handover • In the figure, the limits for different RXLEV HO thresholds are presented. The innermost

limit indicates RXLEV_MIN, the minimum required RXLEV for a MS to enter the cell by HO. The outermost limit indicates the largest possible radius of the cell limited by the MS receiver sensitivity.

• It should be noted that the figure represents “sensible” threshold settings. One could adjust the parameters to any ridiculous value.

GSM – Handover • Speed sensitive HO algorithm• A subclass of the power budget HO is the speed sensitive power budget HO that is

employed in hierarchical cell structures. • A hierarchical cell structure consists of overlayer macro cells and embedded underlayer

micro cells. • This kind of architecture is often utilized in high traffic areas. • The network is interference limited and the power budget HO is the dominant HO

mechanism to in minimizing TX power.• Ideally, the macro cell layer serves fast moving mobiles, and pedestrian mobiles stay in the

micro cell layer. • This can be achieved by means of C2 cell reselection (idle mode) and the speed sensitive

HO algorithm (dedicated mode).• There are quite a few BSS parameters associated with hierarchical cell structures, priority

layers and ways to control them.

GSM – Handover • Speed sensitive HO algorithm• Speed sensitive HO is analogous to C2 cell reselection. It differs from the ordinary power

budget HO in that HO_MARGIN(n) is replaced by HO_MARGIN_TIME(n). Thus HO is initiated if the condition

PBGT (n) > HO_MARGIN_TIME (n) is fulfilled.• The time dependent handover margin is given by

HO_MARGIN_TIME (n) = HO_MARGIN (n) + HO_STATIC_OFFSET(n) when timer T < DELAY_TIME

• AndHO_MARGIN_TIME (n) = HO_MARGIN(n) + HO_STATIC_OFFSET(n) –

HO_DYNAMIC_OFFSET(n) when T > DELAY_TIME.• The parameter DELAY_TIME is measured in SACCH multiframes and the static and

dynamic offsets are measured in dBm. • By setting a large static offset, HO can be prevented during the runtime of the timer T for

that cell

GSM – Handover • HANDOVER Parameters

GSM – Power Control • Like frequency hopping and DTX, power control is a tool for reducing the interference in

the network. T• his can be understood easily if we consider a case of only one allowed transmit power for

MS. • Mobiles far from BTS will not produce unnecessary interference because they would have

to use more RF power to reach the quality target anyway. • But if mobiles near the BTS expend the same amount of power, most of this power will be

wasted and the overall power level in the network increases and “excess” interference is created.

• This situation is known as the near-far problem and it is even more harmful in CDMA systems.

GSM – Power Control • Power control can be used in both uplink and downlink. All MS have power control

capability; this is required by the specifications. It is up to the operator whether to use power control or not.

• The TX power can be controlled by BSS parameters. This is done much in the same way as for HO. For instance, a RXQUAL threshold for power increase can be set. If RXQUAL falls under the set threshold level, MS (or BTS) is ordered to increase the TX power by an amount that is defined by parameters

GSM – Power Control • Example: • L_RX_QUAL_UL_P = 5, U_RX_QUAL_UL_P = 3, POW_INCR_STEP_SIZE = 4dB,

POW_RED_STEP_SIZE = 2 dB. • The downlink power control is disabled. Assume we have interference causing the

averaged RXQUAL at MS (which is reported to BTS on SACCH) to rise to 5. • BTS commands MS on SACCH to increase the TX power by one step, i.e., 4dB. The C/I

value increases by 4 dB as well and we presume that RXQUAL falls to 3.• This should trigger the upper threshold and BTS commands MS to lower its TX power by

one step, i.e., 2 dB. • Now RXQUAL falls to the “deadband” region which in this case is RXQUAL = 4 and no

further power control commands are issued for a while. • The example is totally fictious and its purpose is to clarify the power control process.• Note that the HO thresholds and power control thresholds have similar parameter names.

The difference is in the last letter, H and P, respectively.• The power control and HO threshold limits should usually be set in such a way that the

power control acts before HO.•

GSM – Power Control

Power Decreased

Good Quality

RXQUAL

RXLEV0

7

63

L_RXLEV_XX_P

L_RXQUAL_XX_IH

Power Decreased

Good Level

Power Increased

Bad Quality

Power Increased Bad Level

Dead Band

U_RXLEV_XX_P

2 X POW_RED_STEP_SIZE

U_RXQUAL_XX_P

L_RXQUAL_XX_P

GSM – Power Control • Example: • L_RX_QUAL_UL_P = 5, U_RX_QUAL_UL_P = 3, POW_INCR_STEP_SIZE = 4dB,

POW_RED_STEP_SIZE = 2 dB. • The downlink power control is disabled. Assume we have interference causing the

averaged RXQUAL at MS (which is reported to BTS on SACCH) to rise to 5. • BTS commands MS on SACCH to increase the TX power by one step, i.e., 4dB. The C/I

value increases by 4 dB as well and we presume that RXQUAL falls to 3.• This should trigger the upper threshold and BTS commands MS to lower its TX power by

one step, i.e., 2 dB. • Now RXQUAL falls to the “deadband” region which in this case is RXQUAL = 4 and no

further power control commands are issued for a while. • The example is totally fictious and its purpose is to clarify the power control process.• Note that the HO thresholds and power control thresholds have similar parameter names.

The difference is in the last letter, H and P, respectively.• The power control and HO threshold limits should usually be set in such a way that the

power control acts before HO.•

GSM Troubleshooting

Causes for GSM Network Congestion and Call Loss• Contiguous Coverage Plot • According to GSM specifications for generating RXLEV measurements, a

mobile should take a running average of five samples spread over a period of 5 seconds or the duration of 5 consecutive paging blocks of that mobile, whichever is larger.

• The duration of these 5 consecutive paging blocks is decided by the BCCH parameter BS_PA_MFRMS, which informs the mobile of the number of multiframes after which the same paging block is repeated. This can range from 1 to 9 multiframes.

• This means a mobile’s paging block can occur at intervals ranging from 470 ms to 2.1 s. Therefore, 5 consecutive paging blocks for that mobile will occur over a period ranging from 2.35 s to 10.5 s.

• According to GSM specifications, then, the average will be taken over a period ranging from 5s to 10.5 s.

Causes for GSM Network Congestion and Call Loss• Contiguous Coverage Plot • If we drive at a nominal speed of 40 km/h, we will cover 55 m in 5 s and

110 m in 10 s. Over any averaging period between 5 and 10 s, the mobile will take only 5 samples.

• If the averaging period is 5 s, then 1 sample is taken every 5.5 m. If the period is 10 s, then 1 sample is taken every 22.5 m.

• With this much distance between samples, there is a possibility that there will be many areas in which the mobile has not made measurements.

• Since coverage holes due to normal log fading at the edges of the cells can be found in the range of 5 to 50 m, a normal mobile phone’s RXLEV measurement can miss these holes.

• So, even though the coverage plot produced by a mobile phone may indicate contiguous coverage, there may actually be many holes, resulting in patchy coverage, frequent handovers and dropped calls.

• However, if we use a GSM receiver that can synchronize itself with the BCCH carrier, make carrier power measurements and perform a continuous RXLEV measurement at a specified distance of 1 to 5 m, we can get a true picture of coverage contiguity over the network’s drive area

Causes for GSM Network Congestion and Call Loss

What Causes poor RXQUAL?• Quality of the received signal (RXQUAL) is a key parameter for evaluating network

performance. RXQUAL is the Bit Error Rate (BER) derived from the 26 bits midamble on the TDMA burst.

• RXQUAL levels characterize speech quality and dropped calls, where 0 indicates the highest quality and 7 the worst. If we are doing a drive-test in a trouble zone with a phone, we can easily locate poor quality spots by monitoring RXQUAL. However, we may want to identify the cause of poor RXQUAL.

• RXQUAL can be poor because of poor RXLEV (coverage), low carrier-to-noise ratio (C/N), co-channel interference, adjacent channel interference or multipath. A phone-based system will report RXLEV, but will not provide adequate information about the other potential problems.

• If RXQUAL is poor and RXLEV is good, then it is generally assumed that the cause is interference. However, interference can exist in several forms, including co-channel, adjacent channel, multipath and external.

What Causes poor RXQUAL?• How do we determine which types of interference are present? • There are several traditional ways of isolating different forms of interference.

– For example, if RXQUAL improves when all the co-channel cells are switched off, then co-channel interference is present. Adjacent channel interference can be measured using a spectrum analyzer, but at present it is difficult to isolate multipath.

• Conventional methods are time-consuming and do not characterize interference over the entire coverage area of the network.

• Measurement tools come with interference analyzer which can do real-time measurements of co-channel interference, adjacent channel interference and multipath.

• The results from the tool can be drawn on the drive route through map info with specific information on the co channel and the adjacent channel interference data points.

TCH- TCH Interference• A GSM cell has more than one carrier to handle subscriber capacity

require-ments. Only one of the available carriers will be the BCH, which will be on con-tinuously; the remaining TCH carriers will only turn on for specific timeslots when a call is initiated on that channel.

• During peak hours, the activity on the TCH carriers will be at a maximum, whereas activity may be zero during off-peak hours.

• TCH carriers are also reused, and hence can contribute to co-channel interference, although this interference will not always be present; it will only be present when these TCH carriers have call activity (not necessarily during peak hours).

TCH- TCH Interference• How do we measure the C/I for these reused TCH carriers?• One approach is to pick the suspected reuse interferer, set up calls on each timeslot

(eight calls), and then drive around in the interfering cell measuring the C/I. This is a tedious process. The easiest way to solve this measurement problem is to make delta (difference) measurements. As an example, consider two cells, Cell 1 and Cell 2.

• Cell 1 has a BCH carrier on ARFCN B1 and Cell 2 has a BCH carrier on ARFCN B2, while the TCH carriers in both Cell 1 and Cell 2 are on ARFCN T1. Instead of making the C/I measurement on T1, we can make a delta measurement of B2/B1, which is near to the C/I value of T1 when both the T1 carriers are on air, since the propagation loss for B2/B1 and T1 is nearly the same. . The delta measurement is the same as measuring the C/I on T1.

• Using post-processing software, we can also plot the C/I map for TCH-TCH interference

TCH- TCH Interference

Optimizing Handover Margin• The difference between the RXLEV of the server and that of the

neighbor can be recorded as delta value.• At some point on the drive-test route, the neighbor’s RXLEV will become

stronger than the server’s signal and this delta reading will become negative; when the delta exceeds the handover margin, a handover will occur.

• The value of the handover margin is set in the cell, but not broadcast on the air interface. By simultaneously monitoring RXQUAL during the handover, the value of the handover margin can be determined and a decision can be made whether that value is appropriate for the quality of service desired. A handover margin on the high side will result in a handover occurring after the user has experienced some deterioration in quality.

• High handover margins can result in poor reception and dropped calls, while very low values of handover margin can produce “Ping-Pong” effects as a mobile switches too often between cells

C2 Parameter, Reselection and BA Table• In the idle mode, the mobile always prefers to remain with or move to

the best serving cell. The best cell is decided on the basis of uplink and downlink path balance in the cells. This balance is calculated by GSM-defined C1 calculations. C1 calculations force the mobile to move to the strongest cell. In certain cases, such as macro-micro cell architecture, optimization may require that in certain areas the mobile not remain in the best cell, but instead remain in a cell depending on traffic loading. C2 parameters provide the option of adding fixed positive or negative offsets to the C1 calculation in each cell. So, although C1 might be better for a neighbor cell, the application of C2 parameters could delay reselec-tion. C2 parameters also allow the mobile to apply temporary offsets for a period known as penalty time, which helps reduce Ping-Pong effects.

C2 Parameter, Reselection and BA Table• A mobile does a cell selection when it turns on or comes out of a

coverage area.When the mobile has successfully camped onto a cell, it monitors the neighbor cell’s ARFCN (per the BA table, described below) and does C1 and/or C2 calculations for each of the neighbor and serving cell ARFCNs at regular intervals. C1 and C2 calculations inform the mobile of the uplink and downlink path balance in the cell, allowing it to select or reject the cell. Once the mobile finds a neighbor cell’s C1 and/or C2 (since C2 is optional) better than the C1 and/or C2 of the serving cell, it switches to the neighbor cell. This process of switching to another cell in the idle mode is known as cell reselection. Cell reselection is completely controlled by the mobile. The equivalent of cell reselection in the dedicated mode is handover, which is controlled by the network

C2 Parameter, Reselection and BA Table• A BCCH Allocation (BA) table or list is a set of ARFCNs broadcast to the

mobile in the idle and dedicated modes for monitoring as potential neighbor cells. In the idle mode, this list is broadcast on the BCCH in a System Information Type 2 message. The mobile decodes this message and monitors the ARFCNs listed in the table as idle mode neighbors. In the dedicated mode, a similarly formatted table is sent to the mobile on the Slow Associated Control Channel (SACCH) in a System Information Type 5 message. This dedicated mode table can contain the same list of ARFCNs as the idle mode table, or a different list.

Optimizing Neighbor Lists• In GSM, we can define several neighbors for a serving cell. Usually, we

want a handover to be made to the strongest neighbor, but in some cases frequent handovers to this best neighbor can result in congestion in the neighbor cell, affecting the users initiating calls from that cell. The situation can also occur in reverse, when a handover required to the best neighbor can result in a rejection due to unavailability of resources, causing the handover to be attempted to the next best neighbor, which can delay the process and deteriorate the quality further. Under certain circumstances, we may need to remove a potential neighbor from the neighbor list and provide alternatives. Usually, such decisions are made using demographic considerations. The BCH analyzer in the GSM receiver makes it easier to determine these alternative neighbors

Optimizing Neighbor Lists• The BCH analyzer can be used to create a list of all the possible BCH

carriers in the nearby vicinity and perform the RXLEV measurement (linked to the phone’s RXQUAL performance) on each of these carriers. When the RXQUAL reaches the handover decision threshold, we can determine the potential neighbors at that stage and set one of those as the optimum neighbor. This can also be done with the phone, but in this case you are limited to the BA list set in the network, which may not include good potential neighbors. For example, a cell that is a good neighbor because of propagation over water may not be set in the BA list. Also, the number of neighbors in the BA list is usually limited, because a large number reduces the measurement samples per neighbor and hence deteriorates the authenticity of handovers. The receiver gives a complete, independent view of all the BCH carriers available at a particular location where a handover is required, and its control software capabilities (including markers, delta markers and post processing) simplify the task of making neighbor settings.

Causes for GSM Network Congestion and Call Loss• Congestion and call loss caused by shortage of wireless resources:• The number of subscribers acceptable for a base station is limited by the base station's

capacity of carriers. • Taking a 6/6/6 directional base station for example, each base station cell's control channel

and voice channel configuration is 1BCCH+ 24SDCCH+44TCH. • Excluding the control channel, each cell permits 44 subscribers' simultaneous conversation

at most. • This is far from enough for a dense subscribers group. Therefore, many subscribers, when

calling in a "high traffic area", namely a highly centralized customers area, can't occupy the idle channel of the base station, even can't occupy the base station's dedicated control channel SDCCH, occurring a phenomenon of instantaneous no-signal of the handset during handover.

• The handover failure (SDCCH call loss) caused by lack of idle channel would result in marked increase of call loss. Any failure to repair the fault in time would also give rise to congestion.

Causes for GSM Network Congestion and Call Loss• Congestion and call loss resulted from co-channel interference:• GSM mobile cellular communication systems adopt frequency reuse technique to improve

spectrum efficiency, and increase system capacity. But co-channel interference is resulted the service area of the base station is continuously reduced by splitting of the cell and the same-frequency re-use coefficient increases, mass co-channel interference will replace man-made noise and other interference, becoming the main constraint to the cell system. Then the mobile radio environment will change from noise limited environment into interference limited environment. When the carrier-interference ratio subject to co-channel interference is smaller than a given value (Ericsson system empirical value approximates to 10dB), the communication quality of the handset will directly be affected, resulting in call loss or making it possible for handset users to set up normal call in case of severity.

Causes for GSM Network Congestion and Call Loss• Congestion and call loss as a result of adjacent channel interference: • It refers to the interference caused by falling of the adjacent channel power of the

interfering station into the pass-band of adjacent channel receiver. Adjacent channel interference will be caused by either the existence in the adjacent cell of channels adjacent to the operating channels in the local cell due to frequency planning reason, or the fact that the coverage of the base station cell is larger than design requirement for some reason. When the carrier-interference ratio in the adjacent channel is smaller than a given value (Ericsson system empirical value being 3dB), the communication quality of the handset will be directly affected, resulting in call loss or making it possible for handset users to set up normal call in case of severity.

Causes for GSM Network Congestion and Call Loss• Failure of calling due to too weak signal strength of handset:• Nowadays many shopping arcades and entertainment centers are decorated exteriorly

with large glass (this will cause strong reflection of electric waves), while many shielding materials are used for interior decoration (indoor penetration will affect the receiving level), both of which will reduce the indoor signal strength of handsets. And in urban areas, "high traffic areas" in particular, base stations are relatively close to each other and the maximum transmitting power of some busy base stations is artificially lowered in order to reduce the traffic by narrowing the coverage. So the handset's received signal is very weak in buildings. In BSC, a minimum threshold is set for handset users' received signal. When the signal is weaker than this threshold, normal conversation is impossible.

Causes for GSM Network Congestion and Call Loss• Call loss caused by irrational setting up of cell parameters:• As actual geographical environment is always somewhat different from the ideal model,

plus wave reflection and dispersion and other reasons, there are always some deficiencies in preliminary network design. Irrationality of partial cell parameters, especially adjacent cell relation and switching band setting up, is inevitable. When cells are actually adjacent but not set up or the switching bands are not rationally set up, call loss may occur.

Methods of Analysis of GSM Network Congestion and Call Loss

• Study of cell and traffic STS statistical results:• STS statistical results are used to analyze each cell's traffic, wireless channel congestion

rate, channel damage rate, switchover success rate and other indexes. Based on work practice, stress is laid on cells most serious with the foregoing problems, and analyses are made to find out the causes and solutions.

• Study of ANT measurement results:• ANT is a test tool combining TEMS test handset with digital map. ANT is applied for a

general survey of the wireless coverage and speech quality for the whole city. The aim is to get to know in time the signal strength, speech quality, the success of switchover, and check up whether the coverage of the cells transcends boundary, whether there exists co/adjacent channel interference

• The analyses of the STS statistical results and ANT statistical results will give the status of network quality and an all-round understanding of the areas and rough causes of the congestion and call loss, and provide a basis for carrying out network optimization.

Solutions to GSM Network Congestion and Call Loss

• Solutions to Wireless Resources Shortage:• 1) Readjustment of equipment configuration to reduce the congestion: • If it is found from STS statistical analysis that SDCCH congestion is high and

TCH congestion is not high, SDCCH congestion can be solved by increasing the quantity of SDCCH at the expense of TCH. In case of TCH congestion and non-congested SDCCH, reducing SDCCH and increasing TCH may be adopted, with limited effects, however.


• Solutions to Wireless Resources Shortage:• 2) Adjustment of traffic to reduce the congestion: As it is difficult to predict user

density distribution in construction planning and project design, and equipment allocation and use differs from reality. Traffic statement and ANT test results show that some equipment has high congestion rate with each line's traffic being high too, while some equipment are idle. So it is necessary to adjust the number of equipment for each cell based on the traffic. This is the best method to solve congestion, which has marked effect though such adjustment has limitations. Traffic balancing and traffic bifurcation can also be applied to balance and bifurcate high and low traffics in the adjacent cell. For a high traffic cell, such methods as adjusting of power, switchover parameters, antenna inclination and priority are applied to reduce its coverage and raise the power and priority in the adjacent low-traffic cell so as to attain the objective of balance and bifurcation.

•


• Solutions to Wireless Resources Shortage:• 3) For a really high-traffic cell, capacity expansion is the fundamental

solution. We can set up a microcellular base station in the cell with the highest traffic and increase the priority of that microcellular base station to bifurcate part of the high traffic to the microcellular base station.

Solutions to GSM Network Congestion and Call Loss• Solution for network quality deterioration caused by interference:• 1) For co/adjacent channel interference caused by improper frequency planning,

optimization adjustment should be made. CAD may be adopted for optimization adjustment of frequency allocation. The objective function of optimization is co/adjacent channel interference that should be minimized while changes in the original frequency allocation should be minimized. The statistics indicate that signal strength attenuates in accordance with the rule of 4th power exponent. Co-channel interference can be avoided if two cells using the same frequency group kept a distance long enough between them.

• 2) Decreasing base station transmitting power level and increasing antenna inclination can reduce interference with handsets in the co/adjacent channel cell. If we can judge a certain co/adjacent channel cell to see if it's a very strong co/adjacent channel interference source through tests of co/adjacent channel interference or analyses of OMC statistics, and find the coverage of that cell too large through field strength tests, then we may lower the transmitting power of base stations or increase the antenna inclination. This is a very good countermeasure for reducing the interference. The reduced power value and antenna inclination should depend on the right coverage.

Solutions to GSM Network Congestion and Call Loss• Solution for network quality deterioration caused by interference:• 2) Decreasing base station transmitting power level and increasing antenna inclination can

reduce interference with handsets in the co/adjacent channel cell. If we can judge a certain co/adjacent channel cell to see if it's a very strong co/adjacent channel interference source through tests of co/adjacent channel interference or analyses of OMC statistics, and find the coverage of that cell too large through field strength tests, then we may lower the transmitting power of base stations or increase the antenna inclination. This is a very good countermeasure for reducing the interference. The reduced power value and antenna inclination should depend on the right coverage.

Solutions to GSM Network Congestion and Call Loss• Solution for network quality problem caused by too weak indoor signal: • In urban areas, "high traffic areas" in particular, base stations are relatively close to each

other, and some busy base stations artificially lower their maximum transmitting power in order to reduce traffic by narrowing the coverage, thus making the handsets' received signal strength very weak in buildings and communications impossible. This kind of problem can only be solved through indoor coverage technique. "Indoor coverage" is to introduce outdoor signals into indoor or to build a base station indoor, send the signals uniformly to all indoor points via optical fiber or copper cables.

Solutions to GSM Network Congestion and Call Loss• Solution for irrational cell parameters set-up:• Using ANT test tools to conduct on-the-spot examination of GSM coverage and speech

quality, to check whether such parameters are rational as signal strength in a given area, power parameters, switchover parameters, and cell priority. And modifications will be made whenever irrationality is found.

GSM Optimization and Troubleshooting

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