GSM Voice Quality Influence Factors Troubleshooting Methods and Tools Deliverables

8/10/2019 GSM Voice Quality Influence Factors Troubleshooting Methods and Tools Deliverables

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HUAWEI TECHNOLOGIES CO., LTD.

www.huawei.com

Huawei Confidential

Security Level:2015/1/6

Prepared by: Dong Xuan, Yang Chunjie

Reviewed by: Qi Haofeng

GSM Voice Quality:

Influence factors + Troubleshooting Methods and

Tools + Deliverables

July 30, 2010


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Abstract:

This document mainly discusses the main factors that affect the voice quality of

a GSM network, principles of improving the functions related to voice quality,

and suggested values of some key parameters. In addition, this document lists

the deliverables (see the attachments on page 33) that field engineers should

submit when reporting voice quality problems or evaluating the voice quality,

including drive test information, counter information, and guides to related

tools (see the operation guide). This document aims to quickly locate and solve

voice quality problems and to evaluate the voice quality and prevent voice

quality problems based on the collected information about the existing network.


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HUAWEI TECHNOLOGIES CO., LTD. Huawei Confidential Page 3

R&D Support

For voice quality problems, we can provide trainings and 7x24 hour technical support.

List of R&D support engineers

Name ID Phone

Dong Xuan 103945 See the phone book.

Yang Chunjie 119951 See the phone book.


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Contents

Statistics and Analysis of Factors Affecting the

Mean Opinion Score(MOS)

Voice-Related Key Parameters:

Quality Parameters

Coding Scheme Parameters

Handover Parameters


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HUAWEI TECHNOLOGIES CO., LTD. Huawei Confidential

Statistics and Analysis of Factors Affecting the MOS

Voice quality

Coding

schemes

Bit error (frame

erasure)HO

Directly-related

factors

Indirectly-

related factors

Traffic

Full/half

rate

High/low

codin

grate

AirInterface

Quality

TrafficBusyT

hreshold

RateadjustT

hreshold

F2HHO

Threshold

Thres

holdSelf-Adaptive

Vo

ice

version

Frequent HOs,

PingPong HOs,

and unreasonableHOs

ToolowPN

Rule

Inappropriate

Neighborin

gCell

TooSmallHO

Hysteresis

FrameTheftofPhysical

Messag

es

Parameters,

algorithms, and

optimizing

strategies

The channel is normal.

Interference

Algorithms

Engineering

netw

ork

optimization

3.5-Generatio

nPower

Control

D

T

X

V

A

D

Anti-Interferenc

esolution

Intermodu

lation

Interference

Quick

Troublesh

ooter

TOPOptim

ization

HO

OptimizationPacket

Calldrop

Lon

gCallDro

pTimer

CoBCCHResident

Strateg

y

DiscardedP

ackets

Compens

ation

The voice quality is mainly related

to three factors: coding scheme, bit

error, and handover (HO). The

coding factor benefits the voice

quality. The bit error and handover

factors, however, damage the voice

quality.

To optimize the voice quality, you

need to select reasonable codingschemes and reduce the effect of

the bit error rate (BER) and

handovers on the voice quality.

The prerequisite is that the channel

is normal.

The methods for improving the call

drop rate and handover success

rate usually damage the voice

quality and the experience of

subscribers. Therefore, strategies

that optimize the voice quality may

affect the call drop rate and

handover success rate.

Page 5


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Good voice quality firstly depends on good network quality. The quality of the entire network,

however, should be evaluated by counters. For details, see the following table.

For more detailed feedback information, see the attachment Voice Data Feedback Checklist.

Feedback Information About Counter Data Related to

Voice Problems


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Voice services are

key services in aGSM network. The

quality of voice

services is

determined by many

factors. To report

MOS problems or

voice problems, youneed to report all

factors related to

voice during the

drive test.

Feedback Information About Drive Test Data Related to Voice

Problems


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Typical Case 1:

In a site, it is required that the average value of MS-MS MOS should be

greater than 3.50 according to the drive test result of the entire network after the

migration. The value, however, is only 3.35 according to the drive test result.

Therefore, the value of MOS fails to reach the standard.

According to the statistics and analysis of the drive test information, the ratio of half rate channels

reaches 75%. This is the main factor that affects the overall MOS. The details are as follows:

Through parameter configuration and counter

analysis, it is discovered that the problem is

caused by that the values of TCH Traffic Busy

Threshold of many cells are set to be too

small (30%). After adjusting the values of this

parameter and performing another drive test,the ratio of occupied half rate channels is

reduced to 47% and the value of MOS

reaches 3.52, which exceeds the acceptance

standard.


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Typical Case 2:

In a site, it is required that more than 95% MS-PSTN MOS values should be greater than 2.7 in the

drive test of the entire network after the migration. According to the drive test result, only 90% MS-PSTNMOS values are greater than 2.7. Therefore, the MOS value fails to reach the standard.

A: According to the analysis, no known voice problem exists in the BSS after the migration. The reason that the

MOS value of the existing network fails to reach the standard is that the quality over the air interface is low,

many handovers occur, and the ratio of half rate channels is high.

B. After optimizing the concentric handover parameters, adjusting the Assign Optimum Layerand the Pref.

Subcell in HO of Intra-BSCparameters and their thresholds, and optimizing the number of handovers

reasonably, the ratio between traffic and number of successful handovers rises from 68.4 to 71.9. In addition,

the ratio between number of MOS values and number of handovers in the drive test rises from 1.86 to 2.76.

Therefore, the ratio of MOS values that are grater than 2.7 of the entire network rises about 4%.

C. After optimizing cells one by one and expanding the capacity of busy cells, the ratio of half rate channels in the

test is reduced from 46% to about 35% . This improves the overall MOS.

D. After optimizing problem sites one by one and take optimization measures at a low carrier-to-interference ratio,

the ratio of air interfaces whose quality is at level 0 to level 4 rises 2%.

After taking a series of optimization measures, the MOS value in the drive test is improved obviously. The ratio of

MOS values that are greater than 2.7 rises about 10% and reaches over 95%.


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Contents

Statistics and Analysis of Factors Affecting the

MOS

Voice-Related Key Parameters

Quality Parameters


Handover Parameters


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Voice-Related Key Parameters Summary of Quality

Parameters


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Quality Parameters:Parameters Related to Improving the

Voice Quality at a Low C/I

Description of the Function of AMR Error Frame Filter:This function mainly improves the decoding capability of the BTS at a low

C/I. This function improves the voice quality by adopting different filter

algorithms for different speech versions and coding schemes.

Suggested Parameter Settings:

BTS software parameter 53: 231.

Description of the Function of C/I Filter

This function improves the estimation precision of the C/I and thus

improving the voice quality by adopting the most suitable speech coding

schemes.


BTS software parameter 53: 231.


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Description of the Function of Co-Channel and Adjacent-Channel Filter:

It is recommended to set a narrow (180 k) filter in the neighboring frequency interference scenario

and a broad (260 k) filter in the co-channel interference scenario. This function improves the

capability of the BTS in suppressing adjacent-channel and co-channel interference signals

and the decoding performance of the BTS.


BTS software parameter 53: 231 in the co-channel interference scenario, 39 in the

adjacent-channel interference scenario, and 103 in other scenarios.

Description of the Enhanced Interference Cancellation Combining (EICC) Function:

Among signals received by dual antennas, the interference is related to both the

space (between antennas) and the time. The EICC function considers both the

relation between interference and space and the relation between interference

and time. In this way, it suppresses interference more effectively and improves

the voice quality.


STIRC Allowed:Yeswhen serious interference exists.

Quality Parameters:Parameters Related to Improving

the Voice Quality at a Low C/I


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Quality Parameters:User Experience-Related Parameters

VQE: mainly includes four sub-features, such as AEC, ALC, ANR, and ANC.

Description of the Acoustic Echo Cancellation (AEC) Function

This function cancels the acoustic echo generated during the call. It determines

whether the signals input by the local and remote ends are echo to the local end. If

the signals received by the local end are echo from the remote end, this functionattenuates the echo and replaces the echo with comfortable noise. If the signals

received by the local end are voice of the speaker, this function keeps the voice

unchanged.



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Description of the Automatic Noise Compensation (ANC) Function:

This function compensates the noise automatically. It evaluates the level of

the background noise at the local end and the voice level at the remote end.

When the background nose at the local end is great, this function turns up

the volume of the voice input by the remote end. This improves the signal-to-

noise ratio between the voice at the remote end and the background noise at

the local end. Therefore, the hearer at the local end can hear the voice of the

speaker at the remote end clearly.


Quality Parameters:User Experience-Related Parameters


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Quality Parameters:Parameters Related to Power Control

Parameters Related to Power Control: III Power Control Algorithm Switch

III Power Control Optimization Algorithm Switch

Basic Principle:

Power control: When the uplink and downlink signals are strong, reduce the

uplink and downlink transmitting power to reduce the interference of the entire

network.

Remarks: The principle of 3.5-generation power control algorithm is advanced in

the industry. This algorithm implements power control based on the quality.

Suggested Parameter Settings: Currently, 3.5-generation power control algorithm is widely promoted globally. It

is required to enable both III power control algorithm switch and III power control

optimization algorithm switch. The 3rd-generation power control algorithm,

however, is not recommended.


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Description of the Counter Function:

Radio Link Timeout: This counter defines the time of radio ink connection failure for downlink links. The criterion isthat whether the SACCH measurement report can be correctly decoded.

SACCH Multi-Frames: This counter defines the time of radio link connection failure for uplink links. The criterion is

that whether the SACCH measurement report can be correctly decoded.


TC CRC Check:

According to GSM specifications, the BSC performs CRC check for each uplink data (TRAU frame) from the BTS.

If the TRAU frame fails to pass the CRC check, the BSC regards it as an invalid frame and smoothens it. This

avoids the noise caused by parameter transmission errors and improves the voice quality.


TC CRC Allowed: ON

Quality Parameters:Other Related Parameters


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Voice-Related Key Parameters:

Quality Parameters

Parameters related to improving the voice quality at a low carrier-to-interference ratio

Parameters related to user experience

Parameters related to power control

Other quality-related parameters


Speech versions

Parameters related to VQE

Parameters related to channel allocation

Handover Parameters

Handover-related parameters


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Voice-Related Key Parameters Summary of Coding Scheme

Parameters


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Description of the Enhance Packet Loss Concealment (EPLC) Function:

If packet loss occurs during the transmission of voice packets in the network, the voice quality may drop sharply.

This function ensures that the sound effect after the decoding is similar to that before the decoding by hiding

and reducing the effect of packet loss on the voice quality when decoding the received voice packets.


EPLC Switch: Off

Description of the AMR Adaptive Threshold Function:This function reduces the effect of incorrect estimation of the C/I or changes of the channel conditions with time on

the AMR performance. You do not need to estimate the value of this parameter. The value of this parameter at

various rates are adjusted automatically according to the fluctuation of the C/I. In this way, the AMR

performance will not be affected by incorrect estimation of the C/I or by changes of the channel conditions with

time.

Suggested Parameter Settings: AMR Uplink Adaptive Threshold Allowed: Yes

AMR Downlink Adaptive Threshold Allowed: No

Coding Scheme Parameters:Parameters Related to

Enhancement of Voice Quality


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Coding Scheme Parameters:Parameters Related to

Channel Allocation

Parameters Related to Channel Allocation:

Channel Allocate Strategy

Traffic Busy Threshold

Tch Traffic Busy Overlay/Underlay Threshold

Basic Principles:

The channel allocation strategy helps to allocate high-quality channels and to improve the voice quality.

A full-rate channel is better than a half-rate channel in improving the voice quality. Therefore, increasing

the ratio of full-rate channels helps to improve the overall MOS.

Note: When allocating channels, be sure to consider whether congestion exists in the cell. If no

congestion exists, you can set the traffic busy threshold to a greater value.


Channel Allocate Strategy : Quality preferred

Traffic Busy Threshold: 60

Tch Traffic Busy Overlay Threshold: 70

Tch Traffic Busy Underlay Threshold: 60


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Handover Parameters:Handover-Related Parameters

Handover-Related Parameters: HO Algorithm Selection Switch

PN Rule of PBGT

Timer for UL Data Forward

Basic Principle

When other KPIs are not affected, it is recommended to increase the PN of

PBGT handover and to delay triggering a handover. This improves the voice

quality.

Uplink data smooth timer: This timer delays releasing old channels and thus

reduce the number of lost frames during the handover.


HO Algorithm Selection Switch: HO I Algorithm

PBGT Valid Time (s): 5; PBGT Watch time (s): 4

Timer for UL Data Forward (ms): 180


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Description of the Intracell F-H HO Function:

For AMR calls, it is allowed to perform AMR full/half rate handover according to the C/I value of the current call.When the C/I value is great, you can switch full rate to half rate to increase the traffic capacity. When the C/I

value is small, you can switch half rate to full rate to improve the experience of subscribers.


Intracell F-H HO Allowed: Yes

TCH F-H Threshold: 20

Intracell F-H HO Stat. Time (s): 5

Intracell F-H HO Last Time (s): 4

Description of the CoBCCH Access Strategy Function:

To reduce unnecessary handovers between the UL and OL in a concentric cell and to avoid the damage to voice

caused by unnecessary handovers, you need to reasonably set the Assign Optimum Layer and Pref. Subcell in

HO of Intra-BSC parameters.


Assign Optimum Layer: System optimization

Subcell in HO of Intra-BSC : System optimization

Incoming-to-BSC HO Optimum Layer: Underlaid subcell

Handover Parameters:Handover-Related Parameters


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Documents

GSM Voice Quality Influence Factors Troubleshooting Methods and Tools Deliverables