61607619 01 GSM BSS Network KPI MOS Optimization Manual

GSM BSS Network KPI (MOS) Optimization Manual INTERNAL

Product Name Confidentiality LevelGSM BSS INTERNAL

Product Version Total 36 pages V00R01

GSM BSS Network KPI (MOS) Optimization

Manual

For internal use only

Prepared by GSM&UMTS Network Performance Research Department

Dong Xuan

Date 2008-2-21

Reviewed by Date yyyy-mm-ddReviewed by Date yyyy-mm-ddGranted by Date yyyy-mm-dd

Huawei Technologies Co., Ltd.

All rights reserved

2011-08-04 Huawei Technologies Proprietary Page 1 of 36


Revision Record

Date Revision Version

Change Description Author

2008-1-21 0.9 Draft completed. Dong Xuan2008-3-20 1.0 The document is modified

according to review comments. Wang Fei



GSM BSS Network KPI (MOS) Optimization Manual

Key words: MOS, interference, BER, C/I, power control, DTX, frequency hopping,

PESQ, PSQM /PSQM+, PAMS

Abstract: With the development of the radio network, mobile operators become more

focused on end users experience instead of key performance indicators (KPIs). The

improvement of the end users experience and the improvement of the network capacity

are regarded as KPIs. Therefore, Huawei must pay close attention to the improvement of

the soft capability of the network quality as well as the fulfillment of KPIs. At present,

there are three methods of evaluating the speech quality: subjective evaluation, objective

evaluation, and estimation. Among the three methods, objective evaluation is the most

accurate. The PESQ algorithm defined by the ITU can objectively evaluate the speech

quality of the communication network. This document uses the mean opinion score

(MOS) to label the speech quality after objective evaluation.

This document describes the factors of MOS, the impact of each factor on the MOS, and

the methods of improving the network QoS and then the speech quality. It also describes

the attention points during the test of speech quality of the existing network and the

device capability value of the lab test. In addition, this document introduces the

differences between the speech test tools. The methods and principles of using the test

tools are omitted. This document serves as a reference to the acceptance of network

KPIs and the marketing bidding.

References: ITU-T P.800\ ITU-T P.830\ ITU-T P.861\ ITU-T P.862\ITU-T P.853

List of acronyms:

Acronym ExpansionMOS Mean Opinion ScorePESQ Perceptual Evaluation of Speech QualityPSQM Perceptual Speech Quality MeasurementPAMS Perceptual Analyse Measurement Sytem



Contents

1 Basic Principles of MOS..................................................................................................................81.1 Subjective Speech Quality Evaluation......................................................................................81.2 Objective Speech Quality Evaluation.....................................................................................9

1.2.1 PSQM (P.861) Recommendation or Algorithm.........................................................91.2.2 PESQ (P.862) Recommendation or Algorithm.........................................................91.2.3 P862.1 Recommendation (Mapping Function for Transforming)...........................101.2.4 P.563 Recommendation.........................................................................................11

1.3 Speech Processing of Involved NEs.....................................................................................121.3.2 MS 131.3.3 BTS 131.3.4 BSC 131.3.5 UMG142 Factors That Affect the MOS in GSM.........................................................................................15

2.1 Introduction to GSM Speech Acoustic Principles................................................................152.2 Impact of Field Intensity and C/I on the Speech Quality.....................................................162.3 Impact of Handover on the Speech Quality..........................................................................162.4 Impact of DTX on the Speech Quality..................................................................................172.5 Impact of Speed (Frequency Deviation) on the Speech Quality..........................................172.6 Impact of Speech Coding Rate on the Speech Quality.........................................................182.7 Impact of Transmission Quality on the Speech Quality.......................................................18

3 Method of Analyzing the Problem of Low MOS........................................................................193.1 Process of Analyzing the Problem of Low MOS..................................................................193.2 Method of Solving the Problem of Low MOS......................................................................21

3.2.1 Consistency Check and Sample Check.................................................................213.2.2 Um Interface Check................................................................................................223.2.3 BTS Check.............................................................................................................253.2.4 Abis Transmission Check.......................................................................................263.2.5 BSC Check.............................................................................................................263.2.6 A Interface Transmission Check.............................................................................273.2.7 MGW Check...........................................................................................................273.2.8 Miscellaneous (Comparison of MOS Before and After Network Replacement).....274 Test Methods and Suggestions.....................................................................................................29

4.1 Test Tool Selection and Test Suggestions............................................................................294.2 Suggestions on the Test of the Existing Network.................................................................29

5 MOS Cases...................................................................................................................................305.1 Differences Between Speech Signal Process and Signaling Process...................................30

5.1.1 GSM Speech Signal Process.................................................................................305.1.2 Signaling Process...................................................................................................30

5.2 Identified MOS Problems......................................................................................................31



6 Feedback on MOS or Speech Problems.......................................................................................336.1 Test Requirements.................................................................................................................336.2 Requirements for Configuration Data in Existing Network.................................................35



Tables

Table 1Relations between the quality grade, score, and listening effect scale...................................8Table 1Impact of DTX on the speech quality...................................................................................17Table 1Mapping between the speech coding scheme and the MOS value.......................................18Table 1Mapping between speech sample and MOS.........................................................................21Table 1Impact of TFO on the improvement of speech quality (GSM Rec. 06.85)..........................26Table 1Identified MOS problems.....................................................................................................31Table 1Network configuration parameters to be provided...............................................................35



Figures

Figure 1PESQ process.......................................................................................................................10Figure 1Mapping between P862 and P862.1....................................................................................11Figure 1Overall speech quality prediction of P.563.........................................................................11Figure 1Typical MOS test process....................................................................................................13Figure 1Speech processing on the MS side......................................................................................13Figure 1Speech processing on the BTS side.....................................................................................13Figure 1Handling process in the GTCS............................................................................................14Figure 1Codec cascading..................................................................................................................15Figure 1Fault location flow...............................................................................................................21Figure 1Speech data transmission on the Um interface (schematic drawing)..................................23Figure 1BSC6000 speech signal process..........................................................................................30



1 Basic Principles of MOS1.1 Subjective Speech Quality Evaluation

ITU-T Rec. P.830 defines a subjective evaluation method toward speech quality, that is,

MOS. In this method, different persons subjectively compare the original speech

materials and the system-processed speech materials and then obtain an opinion score.

The MOS is obtained through the division of the total opinion scores by the number of

persons. The MOS reflects the opinion of a person about the speech quality, so the MOS

method is widely used. The MOS method uses an evaluation system of five quality

grades, each quality grade mapping to a score. In the MOS method, dozens of persons

are invited to listen in the same channel environment and to give a score. Then, a mean

score is obtained through statistical treatment. The scores vary largely from listener to

listener. Therefore, abundant listeners and speech materials and a fixed test environment

are required to obtain an accurate result.

Note that the opinion of a listener about the speech quality is generally related to the

listening effect of the listener. Therefore, the listening effect scale is introduced in this

method. Table 1 describes the relations between the quality grade, score, and listening

effect scale.

Table 1 Relations between the quality grade, score, and listening effect scaleQuality Grade Score Listening Effect ScaleVery good 5 The listener can be totally relaxed

without paying attention.Good 4 The listener should pay some

attention.Average 3 The listener should pay close

attention.Poor 2 The listener should pay very close

attention.Very poor 1 The listener cannot understand even

with very close attention.

Although the formal subjective listening test is the most reliable evaluation method and

the network performance and any coding/decoding algorithm can be evaluated, the test

result varies from listener to listener. In addition, the factors such as the listening



environment, listeners, and speech materials should be strictly controlled during the test.

As a result, this method consumes a lot of time and money. Therefore, several objective

evaluation methods, such as PSQM, PESQ, and P862.1, are introduced. For details

about the objective evaluation methods, see the next section.

1.2 Objective Speech Quality Evaluation

1.2.1 PSQM (P.861) Recommendation or Algorithm

The perceptual speech quality measurement (PSQM) recommendation or algorithm

introduces the system of five quality grades, with each grade further classified in the

form of percentages through the %PoW (Percent Poor or Worse) and %GoB (Percent

Good or Better) scales. Although the PSQM involves subclassification, it is still one of

the subjective evaluation methods. At present, someone uses a computer to generate a

wave file. Through the changes in the wave file before and after network transmission,

the quality grade is obtained to evaluate the speech quality. In 1996, the PSQM was

accepted as Recommendation P.861 by the ITU-T. In 1998, an optional system based on

measuring normalizing blocks (MNBs) was added to P.861 as an attachment.

1.2.2 PESQ (P.862) Recommendation or Algorithm

Jointly developed by British Telecom and KPN, the Perceptual Evaluation of Speech

Quality (PESQ) was accepted as ITU-T Recommendation P.862 in 2001. The PESQ

compares an original signal with a degraded signal and then provides an MOS. The

MOS is similar to the result of a subjective listening test. The PESQ is an intrusive test

algorithm. The algorithm is powerful enough to test both the performance of a network

element (NE) such as decoder and end-to-end speech quality. In addition, the algorithm

can give test results by degradation causes, such as codec distortion, error, packet loss,

delay, jitter, and filtering. The PESQ is the industrys best standard algorithm that has

been commercially used.

Figure 1 shows the PESQ process.



Figure 1 PESQ process

For both the PSQM and the PAMS, a speech reference signal should be transmitted on

the telephone network. At the other end of the network, the sample signal and the

received signal should be compared through the use of digit signal processing so that the

speech quality of the network can be estimated. The PESQ incorporates the advantages

of both the PSQM and the PAMS. It improves the VoIP and hybrid end-to-end

applications and modifies the MOS and MOS-LQ calculation methods. Initially, these

methods are used to measure the coding algorithm. Afterwards, they are also used to

measure the VoIP network system.

1.2.3 P862.1 Recommendation (Mapping Function for Transforming)

The perceptual evaluation of speech quality (PESQ) is a method of objectively

evaluating the speech quality of the communication network. It is developed on the basis

of the PSQM+ and PAMS. In February 2001, the PESQ was accepted as ITU-T

Recommendation P.862. Afterwards, P.862.1 (mapping function for transforming) was

added. Not an independent protocol, P.862.1 is only the mapping of P862. P.862.1

simulates the human ears perception of speech more exactly than P.862. Therefore,

P.862.1 is more comparable to a subjective listening test than P.862. The high scores

obtained according to P.862.1 are higher than those obtained according to P.862. The

low scores obtained according to P.862.1 are lower than those obtained according to

P.862. The watershed is at the score of 3.4. Therefore, according to P.862.1, the

percentage of MOSs above 3.4 should be increased to enhance end users experience.

The following is the formula to translate P.862 scores into P.862.1 scores:



7660.4*4945.11999.0999.4999.0++

+= xey

P.862.1_F1

00.5

11.5

22.5

33.5

44.5

5

1 0 1 2 3 4 5P.862

Map

ped

P.86

2

Figure 1 Mapping between P862 and P862.1

1.2.4 P.563 Recommendation

The P.563 Recommendation was prepared by the ITU in May 2004. As a single-end

objective measurement algorithm, P.563 can process only the received audio streams.

The MOSs obtained according to P.563 are spread more widely than those obtained

according to P.862. For an accurate result, several measurements should be performed

and the scores should be averaged. This method is not applicable to individual calls. If it

is used to measure the QoS of several calls, a reliable result can be obtained.

Figure 3 shows the overall speech quality prediction of P.563.

Figure 1 Overall speech quality prediction of P.563



1.3 Speech Processing of Involved NEs

This section introduces the speech processing of all the involved network elements

(NEs): MS, BTS, BSC, and UMG. Faulty speech processing of any one of the NEs will

affect the speech quality.

Accordingly, four transmission procedures are involved in the transmission of speech

signals. The transmission procedures are Um-interface transmission, Abis-interface

transmission, Ater-interface transmission, and A-interface transmission. Faults in any

one of the transmission procedures will lead to bit errors. Therefore, if a speech-related

problem occurs, the four NEs and the four transmission procedures should be

troubleshoot.

If the problem occurs on the Um interface, the transmission quality on the Um interface

should be optimized. If the problem occurs on the other interfaces, the fault should be

located on the basis of the bit error rate (BER). The BSC6000 can perform BER

detection.

Figure 4 takes the DSLA as an example to illustrate a typical MOS test process.



Figure 1 Typical MOS test process

1.3.2 MS

Figure 5 shows the speech processing on the MS side.

Figure 1 Speech processing on the MS side

1.3.3 BTS

On the BTS side, the TMU performs speech exchange with the BSC, and the DSP

performs speech coding/decoding. Figure 6 shows the speech processing on the BTS

side.

Figure 1 Speech processing on the BTS side

1.3.4 BSC

The BSC modules other than the GTCS perform transparent transmission on the speech

signals. Instead of participating in the speech coding/decoding, these modules are only

responsible for the establishment of the speech channel, wiring, and speech connection.

For the transparent transmission process, see the BSC6000 speech process figure.


Session processing

A/D and D/A conversions

Speech coding/decoding, DTX


1.3.4.1 FTC Processing on Speech

Coding/decoding is performed on the speech signals and rate adaptation is performed on

the data signals so that the communication between a GSM subscriber and a PSTN

subscriber is realized and the transparent transmission on the SS7 signaling over the A

interface is implemented.

Figure 1 Handling process in the GTCS

1.3.4.2 FTC Loopback

In a loopback, a message is transmitted by a transmission device or transmission channel

and then is received by the same to check the health of the hardware and the settings of

the software parameters. The FTC loopback is one of the most commonly used method

for locating the transmission problems and for checking whether the settings of the trunk

parameters are accurate.

1.3.5 UMG

The UMG performs the coding/decoding conversion. Different coding/decoding

algorithms have different impacts on the speech quality. If the communication is

performed between different networks, if the MSs use different coding/decoding

algorithms, or if the same coding/decoding uses different rates to perform

communications, the coding/decoding conversion is required. Generally, the UMG8900

coding/decoding algorithm uses the codec cascading to perform speech conversions. As



shown in Figure 8, codec A is cascaded with codec B. First, the compressed code stream

is restored to the PCM linear code through the corresponding decoder. Then, the PCM

linear code is encoded through another coding/decoding algorithm. The codecs involve

lots of redundancy operations, so the speech quality is degraded to some extent.

Decoder A Encoder B

Encoder A Decoder B

PCM

Figure 1 Codec cascading

2 Factors That Affect the MOS in GSMThe MOS is affected by many factors, such as the background noise, mute suppression,

low-rate coder, frame error rate, echo, mobile terminal (MS). Here, the frame error rate

pertains to the frame handling strategy (handling of frame loss during signaling

transmission), frame stealing, bit error, handover, and number of online subscribers

(congestion degree). During the speech propagation, several NEs participate in the

speech handling: MS, BTS, TC, and MGW. The following paragraphs describe the

impact of each NE on the speech quality.

2.1 Introduction to GSM Speech Acoustic Principles

In a radio network, the basic processing of speech data involves source sampling, source

coding, framing, Um-interface radio transmission, internal NE processing, handover,

terrestrial transmission, and source decoding at the receive end.

A fault in any segment of the speech transmission will result in bit errors, thus leading to

poor speech quality.

For the wireless communication system, the speech quality is significantly affected by

the Um interface, that is, the radio transmission part. An intrinsic characteristic of radio

transmission is time-variant fading and interference. Even for a normally functioning

network, the radio transmission characteristics are changing from time to time. For a



radio network, the radio transmission has a great impact on the speech quality. A speech

signal is transmitted to the BSS system over the Um interface. Then, the signal is

transmitted within the BSS system through the standard and non-standard interfaces.

The process requires the transmission lines to be stable and the port BER to be lower

than the predefined threshold. If a transmission alarm is generated, the related speech

transmission lines should be checked. If the speech quality is poor, a port BER test

should be conducted.

2.2 Impact of Field Intensity and C/I on the Speech Quality

For the wireless communication system, the speech quality is significantly affected by

the Um interface, that is, the radio transmission part. An intrinsic characteristic of the

radio transmission is time-variant fading and interference. Even for a normally

functioning network, the radio transmission characteristics are changing from time to

time. For a radio network, the radio transmission has a great impact on the speech

quality.

If the changes in the signal field intensity do not cause the BER/FER to be greater than

zero, the RXQUAL remains zero. In this case, the speech quality is not affected

theoretically. If the changes in the signal filed intensity cause the BER/FER to be

greater than zero (equivalently some interference exists), the C/I and the field intensity

have a great impact on the MOS.

Both the in-network interference and the out-network interference may affect the C/I

and the receive quality and degrade the demodulation capability of the BTS. This will

lead to continuous bit errors and faulty parsing of speech frames. Thus, frame loss may

occur, causing adverse effect on the speech quality.

2.3 Impact of Handover on the Speech Quality

The GSM network uses hard handovers, so a handover from a source channel to a target

channel definitely causes loss of downlink speech frames on the Abis interface.

Therefore, audio discontinuity caused by handovers is inevitable during a call. Hence,

the handover parameters should be properly set to avoid frequent handovers. In addition,

the audio discontinuity caused by handovers should be minimized to improve the speech

quality.



2.4 Impact of DTX on the Speech QualityIf the DTX is enabled for a radio network, comfort noise and voice activity detection (VAD) are introduced. Affected by the background noise and system noise, the VAD cannot be totally exact. This definitely leads to the clipping of speech signals. Thus, the loss of speech frames and the distortion of speech may occur, and the speech quality and MOS test may be greatly affected. When the Comarco device marks a speech score, the statistics on the clipping are collected. Generally, the value of the clipping has a positive correlation with the clipped portion of speech. Therefore, if the intrusive algorithm is used, the MOS is definitely low.

Table 2 describes the result of the lab test.

Table 1 Impact of DTX on the speech quality

Impact of DTX on the Speech Quality

FR 1. If the uplink DTX of the FR is enabled, the PESQ decreases by about 0.053 on average. Varying from sample to sample, the decrease of PESQ ranges from 0.03 to 0.08. 2. If the downlink DTX of the FR is enabled, the PESQ decreases by about 0.054 on average. Varying from sample to sample, the decrease of PESQ ranges from 0.02 to 0.12.

FAMR12.2 1. If the uplink DTX of the FAMR12.2 is enabled, the PESQ decreases by about 0.05 on average. Varying from sample to sample, the decrease of PESQ ranges from 0.01 to 0.33. 2. If the downlink DTX of the FAMR12.2 is enabled, the PESQ decreases by about 0.08 on average. Varying from sample to sample, the decrease of PESQ ranges from 0.02 to 0.20.

HAMR5.9 1. If the uplink DTX of the HAMR5.9 is enabled, the PESQ decreases by about 0.018 on average. Varying from sample to sample, the decrease of PESQ ranges from 0.01 to 0.07. 2. If the downlink DTX of the HAMR5.9 is enabled, the PESQ decreases by about 0.079 on average. Varying from sample to sample, the decrease of PESQ ranges from 0.05 to 0.11.

2.5 Impact of Speed (Frequency Deviation) on the Speech Quality

Generally, at a speed of 200 km/h, the BER increases and the speech quality deteriorates

because of multi-path interference. If the speed is increased to 400 to 500 km/h, a

certain frequency deviation occurs in the signals received by the BTS from the MS

because of the Doppler effect. The uplink and downlink frequency deviations may

accumulate to 1,320 Hz to 1,650 Hz. Thus, the BTS cannot correctly decode the signals

from the MS.

With the development of high-speed railways and maglev trains, mobile operators pay

increasing attention to the speech quality in high-speed scenarios. In 2007, Dongguan

Branch of China Mobile requested Huawei to optimize the speech quality for the

railways in Dongguan under the coverage of Huawei equipment. After optimizing the



speech quality, Huawei enabled the HQI (HQI indicates the percentage of quality levels

0-3 to quality levels 0-7 in the measurement report) to be 97.2%, which is the

competitors level. In addition, the highest HQI reached 98.5%. The percentage of SQIs

distributed between 20 and 30, however, is only 40% and that distributed between 16

and 20 is also only 40%. The distribution of the highest SQIs is sparser than that (about

90%) with the same speech quality at a low speed. Therefore, high speed greatly affects

the speech quality. Ensure that the speed is stable during acceptance tests or comparative

tests.

2.6 Impact of Speech Coding Rate on the Speech Quality

The speech coding schemes are HR, FR, EFR, and AMR.

Each speech coding scheme maps to an MOS. Table 3 lists the mapping between the

speech coding scheme and the MOS value.

Table 1 Mapping between the speech coding scheme and the MOS value

2.7 Impact of Transmission Quality on the Speech Quality

Generally, if the transmission quality is poor, the BER and the slip rate are high and the

transmission is intermittent. The statistics on OBJTYPE LAPD involve the



retransmission of LAPD signaling, LAPD bad frame, and overload. These counters are

used to monitor the transmission quality on the Abis interface. If too many bad frames

are generated or if the signaling retransmission occurs frequently, the transmission

quality is probably poor. From the perspective of principle, poor transmission quality is

equivalent to the loss of some speech frames. If the speech frames are lost, the speech

quality deteriorates greatly.

3 Method of Analyzing the Problem of Low MOS

3.1 Process of Analyzing the Problem of Low MOS

The MOS aims at an end-to-end communication. The communication involves many

NEs and interfaces. The fault in any NE or interface will cause high BER, thus leading

to low MOS. If the MOS is low, the involved NEs and interfaces should be checked in

succession.

Figure 9 shows the fault location flow.



2. Um interface check

3. BTS check

4. Abis interface check

5. BSC check

6. A interface check

7. MGW check

1. Test MS Replace the test tool, test MS, or test sample

Check for intermittence alarms and bit errors on

Abis interface

Check the related data and disable the DTX function to

perform another test, and then check the software

version

Refer to the guide related to coverage or

interference

8. Miscellaneous (comparison of MOS

before and after network replacement)

Optimize the neighboring cell relations, check the

configurations of the handover-related parameters,

and reduce the number of handovers

Check the full-rate/half-rate busy threshold and

parameters related to AMR rate adjustment

This function is valid for the call from one MS to another and can be used

to improve the MOSThis function is valid for the call from one MS to another under the same BSC and can be used to

improve the MOSCheck for intermittence alarms and bit errors on

A interface

Check the coding scheme between

UMGs

Use the same route to perform test and

eliminate the effect of different factors

Whether the test tool, test MS, and test

sample have an impact on the MOS test?

Whether related to coverage or interference?

Whether too many handovers affect the

MOS?

Whether the occupied channel is half-rate channel

and whether the AMR coding rate is too low?

Whether the uplink/downlink DTX function is enabled and

whether related to software version or hardware

Transmission bit error or intermittence on

Abis interface

Whether the TFO function is enabled

Whether the local switch function is

enabled

Whether intermittence occurs on A interface

transmission?

Whether speech damage occurs

between MGWs?

Whether such factors as test route are

consistent in the case of comparison test?

EndWhether speech MOS problem exists

Start

Whether test speed (frequency deviation)

has an impact on speech quality?

Use the same test speed in the case of comparison test.

The frequency deviation algorithm should be enabled

for the BTS if test speed reaches 200 KM/H.



Figure 1 Fault location flow

3.2 Method of Solving the Problem of Low MOS

3.2.1 Consistency Check and Sample Check

The consistency check involves the test devices, the MSs that serve the test devices, and

the grading standards adopted by the test devices. Different test devices adopt different

grading standards and are served by different MSs. These differences lead to various

combinations, which will definitely cause differences in the opinion scores. Even if the

same device uses different grading standards, the difference in the opinion scores is

large. For example, if you use the Comarco and DSLA to test the speech quality of the

same speech code, the MOS with the Comarco is lower than the MOS with the DSLA.

The Comarco and the DSLA adopt different grading standards, test samples, and test MSs.

If the test samples are different, the test results differ irrespective of whether the

environment (for example, shielded cabinet in non-interference environment), MS,

wireless equipment, core network equipment, and parameter setting are the same.

Therefore, the speech samples for the speech tests before and after the network

replacement must be the same. The following table lists the mapping between the speech

sample and the MOS. According to Table 4, the MOS varies according to the speech

sample. The tests of a large number of speech samples show that American English has

the highest MOS, German has the second highest MOS, and Spanish has the third

highest MOS.

Table 1 Mapping between speech sample and MOSNetwork Type

Speech Sample

MOS

900M French 3.4900M Italian 3.46900M Arabic 3.5900M Russian 3.54900M Japanese 3.54900M Greek 3.57900M Spanish 3.59



900M German 3.61900M American

English3.64

3.2.2 Um Interface Check

The GSM speech codes use the Un-equal Error Protection (UEP) mechanism. Figure 10

shows the data transmission and clipping.

The differences between the speech data transmission on the air interface of GSM and

that of WCDMA/CDMA2000 are as follows:

Cyclic redundancy check (CRC): For the GSM, the CRC of the full-rate TCH checks

only three bits. The error check capability of the GSM is far weaker than that of the

CDMA2000 and WCDMA. For the GSM, the CRC of the enhanced full-rate TCH

checks ten bits. The error check capability of the GSM is close to that of the 3G.

Error correction coding: For the GSM, sub-stream C does not have error correction

coding, so the error probability is large.

Power control: The GSM does not have fast power control. Therefore, the burst fading

or interference cannot be resisted and the errors in the radio transmission cannot be

reduced quickly. Power control improves the speech quality by reducing the BER and

FER.



Figure 1 Speech data transmission on the Um interface (schematic

drawing)

Like the CDMA2000, the GSM also uses the frame stealing method to transmit some

signaling. The frame stealing method has an impact on the speech quality. If continuous

frame stealing occurs, the speech quality is greatly affected.

In the GSM system, if the full-rate speech coding is used, the CRC of sub-stream A

checks only three bits and the error check capability is limited. The errors that cannot be

detected through the CRC also affect the speech quality. Hence, the speech quality can be

reflected only when the measurement of the remaining bit error rate (RBER) is

performed.

The RBER cannot be measured, but the GSM system provides an alternative method,

that is, to measure the demodulation BER. In other words, first, perform error correction

on the demodulation result; second, encode the obtained result; third, compare the

demodulation result with the encoded result. Thus, the BER in the radio transmission can

be reflected indirectly. The standard measuring value that corresponds to BER is


20ms speech frame

Sub-stream A Sub-stream B Sub-stream C

Sub-stream A Sub-stream B Sub-stream CCRC

1/2 coding Sub-stream C

TDMA frame


RXQUAL. Therefore, for high speech quality, the BER must be reduced and the

receive quality on the Um interface must be improved.

For the enhanced full rate (EFR), the statistics of FER can basically reflect the speech

quality because the 10-bit CRC is used.

From the perspective of the Um interface, the factors that affect the speech quality are

sub-stream A, BER (or RXQual), and frame stealing. Only RxQual, however, can solve

the problem of poor speech quality through network optimization.

3.2.2.2 Coverage- and Interference-Related Problem Check

If the network coverage is poor, it is definite that many areas in the network have poor

receive quality. Therefore, the speech quality is affected.

The interference leads to an increase of BER on the radio link. The increase may exceed

the demodulation capacity of the BTS so that speech frames cannot be identified. Thus,

the speech frames may be lost and thus the speech is discontinuous.

To solve the two types of problems, refer to the corresponding guide:

G-Guide to Eliminating Interference - 20050311-A-1.0

G-Guide to Analyzing Network Coverage - 20020430-A-1.0

3.2.2.3 Low MOS due to Handovers

Low MOS is caused by not only frequent handovers but also the following factors.

1. The GSM network uses hard handovers, so a handover from a source channel to a

target channel definitely causes loss of downlink speech frames on the Abis interface. As

a consequence, audio discontinuity caused by handovers is inevitable during a call.

Therefore, the handover-related parameters must be checked to avoid frequent

handovers.

2. The handover is not reasonable. For example, a call is handed over to a cell with poor

quality because of configurations, and thus the MOS is low.

3. The parameter settings are improper, so the handover is slow. If the QoS of the

serving cell is poor for a long time, the speech call cannot be handed over to a better

neighboring cell in time. Thus, the speech quality is always poor, leading to low MOS,

handover failure, and call drops.

4. Some networks disable the bad quality handover, so the MOS is low.

5. The intra-cell handover is configured as asynchronous handover, so the connection on

the Um interface is long, leading to low MOS.



3.2.2.4 Occupation Ratios of Half Rate and Low AMR Rate

All the MOS tests using the PESQ algorithm adopt intrusive speech scores, which are

process values. If the existing network has several types of speech coding, the conduct

of speech quality DT test or CQT test leads to channel handovers and AMR speech

coding rate handovers. Several types of speech coding may be involved in the speech

grading process. Therefore, the network speech quality test is performed on different

types of speech coding. The speech quality test value of the high coding rate is low, and

the speech quality test value of the low coding rate is high. When the transmission

quality on the Um interface is stable, the MOS is low if the occupation ratio of the half

rate is high. Therefore, the full rate and the high AMR rate coding are recommended.

3.2.3 BTS Check

3.2.3.1 Software Version Check

Check for the version-related problems that have been detected.

The old BTS uses a too early version and is incompatible with the new BTS, so the

speech problems occur.

3.2.3.2 Whether the Uplink and Downlink DTX Function Is Enabled

DTX means VAD and silent frames. Replacing the speech with silent frames is a kind of distortion, which brings about difficulties for all the perceptual models to predict the MOS. Generally, the 50ms clipping (VAD) at the front end and rear end does not have a great impact on the subjective impression. In the case of clipping during the speech, however, replacing the speech with silent frames after the packet loss significantly affects the subjective impression. If 50 ms is lost, the MOS is decreased by one. For the PESQ, each 50ms clipping generally leads to the decrease in the MOS of about 0.5, irrespective of the location. The VAD cannot be 100% correct, so the speech quality definitely deteriorates if the uplink and downlink DTX function is enabled during the MOS test.

3.2.3.3 Hardware Factors

The audio discontinuity caused by BTS hardware fault affects the MOS. Bugs in the speech processing part of the hardware also affect the speech quality. You are advised to confirm with the R&D personnel that no identified problems exist in the version.



3.2.4 Abis Transmission Check

The networks built by Huawei cover many parts of the world. The development levels of the basic communication and data communication vary from region to region. In addition, the cost of investing and leasing the transmission lines is high. Therefore, different regions use different transmission types: microwave transmission, circuit transmission, optical transmission, and satellite transmission. Here, the quality of microwave transmission is very prone to weather conditions. Different BERs of different transmission types definitely lead to different transmission quality. Therefore, different networks of different mobile operators should be compared on the basis of the same transmission type.

The alarms to be checked include Broken LAPD Link and Excessive Loss of E1/T1

Signals in an Hour.

In addition, the Monitoring the Port BER function of the BSC and BER tester (E7580A)

can be used to check whether the Abis interface has bit errors.

3.2.5 BSC Check

3.2.5.1 Whether the TFO and EC Functions Are Enabled

During a call from an MS to another, if the calling MS and called MS use the same

speech service type, the times of speech coding/decoding can be reduced by one through

in-band signaling negotiation. Thus, the speech quality can be improved. When the EC

function is enabled, the speech quality can be improved if the echo occurs during the

call. If there is no bit error, enabling the TFO function can improve the speech quality

by more than 0.25 score.

Table 1 Impact of TFO on the improvement of speech quality (GSM Rec. 06.85)DMOS EP0 EP1 EP2

HR .85 .68 .39FR .53 .53 .35

EFR .32 .46 .19

3.2.5.2 Whether Local Switch Is Enabled

The local switch consists of BSC local switch and BTS local switch. For the BSC local

switch, the calling MS and called MS should be served by the same BSC. Thus, the Ater

interface and local transmission resources are saved. For the BTS local switch, the

calling MS and called MS should be served by the same BTS or BTS group. Thus, the

Ater interface and Abis interface transmission resources are saved. When the BSC local

switching is used, the TC coding/decoding is not required if the transcoding function is



implemented in the core network, thus improving the speech quality. When the BTS

local switching is used, the TC coding/decoding is not required because the speech

signals do not pass the BSC. This also improves the speech quality.

3.2.6 A Interface Transmission Check

The rules for checking the A interface transmission is similar to those for checking the

Abis interface transmission. You can refer to the section Abis Transmission Check.

To check the A interface transmission, you have two methods: first, query the BSC

alarms (for example, the Loss of E1/T1 Signals alarm) to determine whether

intermittence occurs on the A interface; second, use a BER tester to check whether bit

errors occur on the A interface transmission.

3.2.7 MGW Check

If this problem does not occur when you use an MS to call another MS during the MOS

test, you can skip this section.

As is mentioned in section UMG, if the communication is performed between different

networks, if the MSs use different coding/decoding algorithms, or if the same

coding/decoding uses different rates to perform communications, the coding/decoding

conversion is required. The inter-code conversion, however, may adversely affect the

speech quality.

Therefore, if you use an MS to call a fixed-line phone during the MOS test, you should

check whether the deterioration of the speech quality is caused by the following:

whether the route between the MS and the fixed-line phone passes through two UMGs

and whether the two UMGs use the speech compression algorithm.

3.2.8 Miscellaneous (Comparison of MOS Before and After Network Replacement)

In a network replacement project, if the MOS deviation occurs before and after the

network replacement, the following factors should be considered:

3.2.8.1 Test Speed

Generally, the drive speed should be stable (at about 30 km/h) during the test. If the

drive speed is low, the test is equivalent to the fixed-point CQT test and thus the test

result is high.

In addition, if the drive speed is high (at more than 200 km/h), the generated frequency



deviation affects the speech quality. In this case, the BTS frequency deviation algorithm

should be enabled to improve the speech quality.

3.2.8.2 Test Route and Test Time

The DT test of speech quality objectively reflects the coverage and receive quality of a

network. In a network, it is definite that some areas have good speech quality and other

areas have poor speech quality. During the DT test of speech quality, the trunk coverage

lines of the target network should be tested completely and the important branch lines

should also be tested. A test route should not be tested repeatedly. If you test the areas

with good speech quality repeatedly, the speech quality in the DT test becomes high. If

you test the areas with poor speech quality repeatedly, the speech quality in the DT test

becomes low.

You should also check whether the test time is consistent. In different periods, the traffic

models of the existing network are different. The busy traffic hours in each day occur

regularly. Therefore, the congestion during traffic peaks is heavy, thus causing more in-

network interference. According to the statistics about the receive quality on the Um

interface, the receive quality deteriorates during busy hours and the corresponding SQI

decreases. Therefore, to ensure the test consistency, you are advised to choose the same

test period.

For example, Huawei has conducted comparison tests at 4:00 a.m. and 9:00 p.m (busy

hour) in Tieling. The results show that the QoS on the Um interface in the early morning

is very good and that during busy hours is very poor. Accordingly, the speech quality in

the early morning is good and that during busy hours is poor. Therefore, the same test

periods should be selected for the comparison test.

3.2.8.3 Frequency Reuse Degree

For mobile communications, frequency is the most important resource. With the rapid

development of mobile communications, the number of mobile subscribers increases

sharply. To meet the increasing capacity requirements, all the mobile operators try to

raise the frequency reuse degree within their own frequency bands. The increase of the

frequency reuse degree, however, definitely brings about large network interference. If

the frequency reuse degree is high, the interference is strong. Thus, the network quality

is poor and the speech quality is poor. This may adversely affect the user experience.

Therefore, the speech quality of the mobile operators with different frequency reuse

degrees cannot be compared directly. For example, China Unicom adopts a plan with



high frequency reuse degree to reach the same cell configuration of BTSs for China

Mobile, so the speech quality of China Unicom is definitely lower than that of China

Mobile. In a word, if the frequency reuse degree is high, the test MOS is low.

3.2.8.4 Engineering Installation Quality IssuesAccording to the experience, check that the connector (on the DDF) on each

transmission segment is properly connected and that there are no exposed stubs. For

optical transmission, check that optical connector is clean and that the transmission BER

is not high.

The poor engineering quality in the antenna system also causes the MOS to decrease.

The speech quality may deteriorate because of errors in engineering installation, for

example, loose connector, misconnection, or poor coverage.

4 Test Methods and Suggestions

4.1 Test Tool Selection and Test Suggestions

1. Normally, the test tools are selected according to the requirements of the mobile

operators. At present, China Mobile accepts the PESQ as the evaluation standard of the

existing network and Ding Li or Hua Xing as the test tool. The overseas mobile

operators use different evaluation standards and use such test tools as DSLA, Cormarco,

and QVOICE.

2. During the bidding, the acceptance standard, test tool, speech sample, acceptance area

(recommended to exclude the suburb areas with poor coverage), calling method, test

duration, test time, and test route are determined for the convenience of future

acceptance.

4.2 Suggestions on the Test of the Existing Network

1. It is recommended that you use short call samples as the test samples to avoid some

blind areas or poor-coverage areas. For the network that has good coverage and that

does not require frequent handovers, long call samples are recommended.

2. Both Nokia6680 and Samsung zx10 can be used as the test MSs. Note that

Nokia6680 does not support half rate and has outdoor antenna (no vehicle body

loss) and that Samsung zx10 supports half rate and does not have outdoor antenna.

In the case of outdoor antenna (vehicle body loss should be considered), it is

recommended that Nokia6680 be used as the test MS.



3. The areas with good coverage and only a few handovers should be selected as the

test routes.

4. During the test, it is recommended that you use an MS to call a fixed-line phone.

Thus, the MOS is high.

5. The DTX function should be disabled. 6. The drive speed during the drive test should not be too high.

7. It is recommended that the idle hours be selected as the test time. Thus, the network

C/I is high.

8. During the test, it is recommended that the channels with good speech coding

quality be occupied, for example, EFR and AMR full-rate channels.

9. The TFO function should be enabled if the version is correct. Note that the TFO

function is valid only for the call from an MS to another.

5 MOS Cases5.1 Differences Between Speech Signal Process and Signaling Process

5.1.1 GSM Speech Signal Process

MS-BTS - GEIUB-GTNU-GEIUT-GEIUT- GTNU-GDSUC-GTNU-GEIUA-MSC

MS

Figure 1 BSC6000 speech signal process

5.1.2 Signaling Process

MS-BTS - GEIUB-GGNU-GXPUM -GGNU-GEIUT-GEIUT-GTNU-GEIUA MSC



MS

Here, the internal BSC signaling process contains the signaling handling process on the

Ater interface, which is omitted in this document.

The previous process indicates that the speech signal process and the signaling process

are different in terms of the path. The measurement of KPIs is mainly performed at the

signaling measurement points in the calling process. The speech MOS indicates the

audio experience of the end user. The signaling process and the speech signal process

are different. Therefore, if the KPIs are good, the MOS is not definitely high. Good KPI

is only a necessary condition of high MOS. The speech MOS is closely related to the

transmission quality on the Um interface, interference, C/I, frame erase ratio (FER),

SQI, and SNR.

5.2 Identified MOS Problems

After the handling of MOS problems on the existing network and the crisis handling of

the speech MOS, some devices of Huawei that affect the MOS are detected. If the MOS

of the existing network is low and if the problem of low MOS cannot be solved after

optimization, you can refer to the Problem Description column in the following table to

check whether the version is incorrect.

Table 6 lists only the problem-solved versions. To check whether the onsite version is

correct, consult the product maintenance department.

Table 1 Identified MOS problems

Problem Number

Problem Problem Description Related Product

Affected Channel

Problem-Solved Version

1 In the case of FAMR/HAMR and FR, one frame is lost and then the frame is retransmitted.

The frame loss on the uplink during the FAMR/HAMR and FR speech leads to a sharp decrease in the MOS.

DPU(TC)

FAMR/HAMR/FR

V9R8C01B048SP01

2 In case of frame loss during a handover, the smoothness handling performed

The frame loss on the uplink during the EFR/HR speech leads to a sharp decrease in the MOS.

DPU(TC)

EFR/HR V9R8C01B048SP01



on the signals over the EFR/HR channels does not take effect.

3 Random bit errors when TFO established

When the TFO is established, the MOS is lower than the expected value and there are random bit errors.

DPU(TC)

EFR/FR/HR V9R8C01B048SP01

4 Permanent loss of one frame during handover to half rate and permanent loss of one frame during activation under HAMR 7.4k

The uplink DTX is enabled in the case of HAMR7.4. During the transition from non-speech to speech, the MOS is decreased by one frame.

DPU(TC)

HAMR7.4 V9R8C01B048SP01

5 The uplink DTX is enabled and the speech quality under EFR and HAMR obviously deteriorates. Damage is introduced on the TC side.

The uplink DTX is enabled in the case of EFR and HAMR. During the transition from non-speech to speech, the MOS is decreased by one frame.

DPU(TC)

EFR/HARM6.7/HARM7.4

V9R8C01B048SP01

6 The internal clock is slow. External interruption should be used to locate the period of 20 ms.

If a call is made repeatedly on the same channel, audio discontinuity occurs.

DPU(TC)

All the speech channels

V9R8C01B048SP01

7 SID_FIRST frame for FAMR

In the test speech sample, two SP frames contain the SID_FIRST frame. In this case, the BTS misinterprets and discards the first speech frame after the SID frame. Thus, the MOS decreases.

DSP (BTS)

FAMR V100R008C02B201 or V100R001C07B415

8 SID_FIRST_INH frame for HAMR

In the test speech sample, two SP frames contain the SID_FIRST_INH frame. In this case, the BTS reports the SID_FIRST_INH frame as the NO_SP frame. Thus, the TC misinterprets and discards the first speech frame after the NO_SP frame. As a result, the MOS decreases.

DSP (BTS)

HARM V100R008C02B201 or V100R001C07B415

11 Frequent adjustment to downlink rate when uplink DTX enabled

After the uplink DTX is enabled, the adjustment (adjustment is made when silent frames are transmitted and adjustment is not made when speech frames are transmitted) is made on the downlink coding in the case of half-rate AMR multirate set. If the DTX is disabled, however, a fixed rate is always occupied. Therefore, the adjustment is not caused by the C/I.

DSP (BTS)

HARM V100R008C02B201 or V100R001C07B415



12 Reporting of HO_DET ahead of time during synchronous handover

During the synchronous handover, the HO_DET is reported ahead of time. Thus, the uplink speech frames on the old channel are lost and the handover disruption is long. The occurrence possibility of this problem during the lab test is about 5%-10%.

DSP (BTS)


V100R008C02B201 or V100R001C07B415

13 One speech frame lost on old channel during asynchronous handover

During the intra-BSC asynchronous handover, one frame out of the uplink speech frames is lost. This problem occurs on the three types of MSs. The occurrence possibility of this problem during the lab test is about 30%-50%.

DSP (BTS)


V100R008C02B201 or V100R001C07B415

6 Feedback on MOS or Speech ProblemsTo better compare the network quality before and after the network replacement, a

comprehensive test should be conducted before the network replacement and the trunk

roads, important branch roads, and important public places in the original network must

be tested. A test report on the original network should be provided. The test report

should include the following contents: RxQual (including the mean values, peak values,

and mean square errors), SQI (including the mean values, peak values, and mean square

errors), C/I (including the mean values, peak values, and mean square errors), test route

and speed, and dotted output figure (the dotted contents should be provided on the basis

of the previous three counters).

6.1 Test Requirements

1. Test time and periods: The test must be conducted at 9:00-12:00 and 17:00-20:00 on

workdays (Monday through Friday).

2. The test routes must evenly cover the trunk roads in the urban areas without repeated

coverage. The round-the-city express ways, viaducts, and roads between the urban

areas and the air port must be tested.



3. In the urban areas, the test speed should equal the normal drive speed. No limitation

is set on the test speed.

4. Irrespective of the traffic, the city with a population of more than 500 thousand

should be tested for three days and the city with a population of more than 200

thousand should be tested for two days. The test should last six hours for each test

day.

5. Dialing requirements:

The test MSs should be located inside the vehicle and both the calling

MS and called MS should be connected to the test instruments. The GPS

receiver should be connected to conduct the test.

Both the GSM calling MS and called MS for the test should be of auto

dualband.

The MSs should be dialed mutually. The dialing, answering, and onhook

of the MSs should be automatic. Each call should last 180 seconds with a call

interval of 20 seconds. If call failure or call drop occurs, another call attempt

should be made after 20 seconds. The call interval is set according to the

requirements of the mobile operator.

6. Daemon data analysis: All the tests must use the same test instruments and Daemon

data processing software.

7. Normally, the test tools are selected according to the requirements of the mobile

operators. At present, China Mobile accepts the PESQ as the evaluation standard of

the existing network and Hua Xing as the test tool. The overseas mobile operators

use different evaluation standards and use such test tools as SwissQual, QVoice, and

Cormarco.

8. The evaluation of the Um interface on the existing network should be complete and

the statistics on RxQual, C/I, and SQI should be provided. The three counters should

have the mean values, peak values, mean square errors in different periods, and

distribution interval list of different values. During the test, the GPS should be

dotted and the log files of the TEMS test should be archived.

9. When the network of several cities is replaced, the speech problems should be



reported. For different cities, the test should be conducted according to the different

requirements mentioned in this chapter. The test reports should be archived. The dot

information about the local e-map should be provided for the future network

optimization of the areas with poor quality.

During each test, the mean speed per hour should be recorded and archived. Dot

statistics can be performed on the GPS.

6.2 Requirements for Configuration Data in Existing Network

The QoS of the existing Huawei network varies according to the economic

development degree, network coverage, network user density, network density,

network planning, frequency reuse degree, and external interference in the local

area. Networks with different QoSs have different configurations and different

configurations have different impacts on the network. For the R&D personnel to

learn the existing network, the configurations of the existing network should be

provided.

Table 7 lists the network configuration parameters that should be provided.

Table 1 Network configuration parameters to be providedNetwork Configuration Test Result

Uplink/downlink DTX

UL PC Allowed

DL PC Allowed

Radio frequency hopping

Baseband frequency hopping

Transmit diversity

TFO

EC

Whether the core network uses IP bearing

Transmission mode of each interface

Frequency resources

Configuration of main BTS models

Setting of the handover threshold



Setting of the power control threshold

Setting of the coding rate and the use proportion

RxQual in the drive test of the entire network

SQI in the drive test of the entire network

C/I in the drive test of the entire network


1 Basic Principles of MOS1.1 Subjective Speech Quality Evaluation1.2 Objective Speech Quality Evaluation1.2.1 PSQM (P.861) Recommendation or Algorithm1.2.2 PESQ (P.862) Recommendation or Algorithm1.2.3 P862.1 Recommendation (Mapping Function for Transforming)1.2.4 P.563 Recommendation

1.3 Speech Processing of Involved NEs1.3.2 MS1.3.3 BTS1.3.4 BSC1.3.5 UMG

2 Factors That Affect the MOS in GSM2.1 Introduction to GSM Speech Acoustic Principles2.2 Impact of Field Intensity and C/I on the Speech Quality2.3 Impact of Handover on the Speech Quality2.4 Impact of DTX on the Speech Quality2.5 Impact of Speed (Frequency Deviation) on the Speech Quality2.6 Impact of Speech Coding Rate on the Speech Quality2.7 Impact of Transmission Quality on the Speech Quality

3 Method of Analyzing the Problem of Low MOS3.1 Process of Analyzing the Problem of Low MOS3.2 Method of Solving the Problem of Low MOS3.2.1 Consistency Check and Sample Check3.2.2 Um Interface Check3.2.3 BTS Check3.2.4 Abis Transmission Check3.2.5 BSC Check3.2.6 A Interface Transmission Check3.2.7 MGW Check3.2.8 Miscellaneous (Comparison of MOS Before and After Network Replacement)

4 Test Methods and Suggestions4.1 Test Tool Selection and Test Suggestions4.2 Suggestions on the Test of the Existing Network

5 MOS Cases5.1 Differences Between Speech Signal Process and Signaling Process5.1.1 GSM Speech Signal Process5.1.2 Signaling Process

5.2 Identified MOS Problems

6 Feedback on MOS or Speech Problems6.1 Test Requirements6.2 Requirements for Configuration Data in Existing Network

Documents

61607619 01 GSM BSS Network KPI MOS Optimization Manual