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 48 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

2022-10-11 Huawei Technologies Proprietary

Page 1 of 48


Revision RecordDate Revisi

onVersio

n

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


Page 2 of 48


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, mobileoperators 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 thethree 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 thelab 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 networkKPIs and the marketing bidding.


Page 3 of 48


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


Page 4 of 48


Contents1 Basic Principles of MOS......................................3

1.1 Subjective Speech Quality Evaluation....................31.2 Objective Speech Quality Evaluation......................3

1.2.1 PSQM (P.861) Recommendation or Algorithm..................31.2.2 PESQ (P.862) Recommendation or Algorithm..................31.2.3 P862.1 Recommendation (Mapping Function for Transforming). 31.2.4 P.563 Recommendation......................................3

1.3 Speech Processing of Involved NEs........................31.3.2 MS 31.3.3 BTS.......................................................31.3.4 BSC.......................................................31.3.5 UMG.......................................................32 Factors That Affect the MOS in GSM............................3

2.1 Introduction to GSM Speech Acoustic Principles...........32.2 Impact of Field Intensity and C/I on the Speech Quality. .32.3 Impact of Handover on the Speech Quality.................32.4 Impact of DTX on the Speech Quality......................32.5 Impact of Speed (Frequency Deviation) on the Speech

Quality.................................................32.6 Impact of Speech Coding Rate on the Speech Quality.......32.7 Impact of Transmission Quality on the Speech Quality.....3

3 Method of Analyzing the Problem of Low MOS....................33.1 Process of Analyzing the Problem of Low MOS..............33.2 Method of Solving the Problem of Low MOS.................3

3.2.1 Consistency Check and Sample Check........................33.2.2 Um Interface Check........................................33.2.3 BTS Check.................................................33.2.4 Abis Transmission Check...................................33.2.5 BSC Check.................................................33.2.6 A Interface Transmission Check............................33.2.7 MGW Check.................................................33.2.8 Miscellaneous (Comparison of MOS Before and After Network

Replacement)..............................................34 Test Methods and Suggestions..................................3

4.1 Test Tool Selection and Test Suggestions.................34.2 Suggestions on the Test of the Existing Network..........3

5 MOS Cases.....................................................3


Page 5 of 48


5.1 Differences Between Speech Signal Process and Signaling Process.................................................3

5.1.1 GSM Speech Signal Process.................................35.1.2 Signaling Process.........................................3

5.2 Identified MOS Problems..................................36 Feedback on MOS or Speech Problems............................3

6.1 Test Requirements........................................36.2 Requirements for Configuration Data in Existing Network. .3


Page 6 of 48


Tables

TABLE 1 RELATIONS BETWEEN THE QUALITY GRADE, SCORE, AND LISTENING EFFECT SCALE.............................................................3

TABLE 2 IMPACT OF DTX ON THE SPEECH QUALITY...........................3TABLE 3 MAPPING BETWEEN THE SPEECH CODING SCHEME AND THE MOS VALUE........3TABLE 4 MAPPING BETWEEN SPEECH SAMPLE AND MOS..........................3TABLE 5 IMPACT OF TFO ON THE IMPROVEMENT OF SPEECH QUALITY (GSM REC.

06.85).......................................................3TABLE 6 IDENTIFIED MOS PROBLEMS......................................3Table 7 Network configuration parameters to be provided.........3


Page 7 of 48


Figures

FIGURE 1 PESQ PROCESS..............................................3FIGURE 2 MAPPING BETWEEN P862 AND P862.1.............................3FIGURE 3 OVERALL SPEECH QUALITY PREDICTION OF P.563.....................3FIGURE 4 TYPICAL MOS TEST PROCESS....................................3FIGURE 5 SPEECH PROCESSING ON THE MS SIDE.............................3FIGURE 6 SPEECH PROCESSING ON THE BTS SIDE............................3FIGURE 7 HANDLING PROCESS IN THE GTCS................................3FIGURE 8 CODEC CASCADING............................................3FIGURE 9 FAULT LOCATION FLOW.........................................3FIGURE 10 SPEECH DATA TRANSMISSION ON THE UM INTERFACE (SCHEMATIC DRAWING)..3Figure 11 BSC6000 speech process................................3


Page 8 of 48


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 thespeech 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 channelenvironment 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 environmentare 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 thelistener. 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 scaleQualityGrade

Score Listening Effect Scale

Very good 5 The listener can betotally relaxed without


Page 9 of 48


paying attention.

Good 4 The listener should paysome attention.

Average 3 The listener should payclose attention.

Poor 2 The listener should payvery close attention.

Very poor 1The listener cannotunderstand even with veryclose 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 detailsabout 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


Page 10 of 48


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 industry’s best standard

algorithm that has been commercially used.

Figure 1 shows the PESQ process.


Page 11 of 48


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 ear’s perception of speech more exactly


Page 12 of 48


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:

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,


Page 13 of 48


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 NEsThis 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.


Page 14 of 48


If the problem occurs on the Um interface, the transmissionquality on the Um interface should be optimized. If the problem occurs on the other interfaces, the fault should belocated 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.


Page 15 of 48


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 transparenttransmission process, see the BSC6000 speech process figure.


Page 16 of 48

Session processing

A/D and D/A conversions

Speech coding/decoding, DTX


1.3.4.1 FTC Processing on SpeechCoding/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 LoopbackIn 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.


Page 17 of 48


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 backgroundnoise, 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


Page 18 of 48


loss during signaling transmission), frame stealing, bit error, handover, and number of online subscribers (congestion degree). During the speech propagation, severalNEs 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 PrinciplesIn 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


Page 19 of 48


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, theradio transmission characteristics are changing from time to time. For a radio network, the radio transmission has agreat 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 adverseeffect on the speech quality.

2.3 Impact of Handover on the Speech QualityThe 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,


Page 20 of 48


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 minimizedto 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 ofthe clipping has a positive correlation with the clipped portion of speech. Therefore, if the intrusive algorithm is used, the MOSis 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 decreasesby about 0.053 on average. Varying from sample to sample, thedecrease of PESQ ranges from 0.03 to 0.08. 2. If the downlink DTX of the FR is enabled, the PESQdecreases by about 0.054 on average. Varying from sample tosample, 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 PESQdecreases by about 0.05 on average. Varying from sample tosample, the decrease of PESQ ranges from 0.01 to 0.33. 2. If the downlink DTX of the FAMR12.2 is enabled, the PESQdecreases by about 0.08 on average. Varying from sample tosample, 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 PESQdecreases by about 0.018 on average. Varying from sample tosample, the decrease of PESQ ranges from 0.01 to 0.07. 2. If the downlink DTX of the HAMR5.9 is enabled, the PESQ


Page 21 of 48


decreases by about 0.079 on average. Varying from sample tosample, the decrease of PESQ ranges from 0.05 to 0.11.

2.5 Impact of Speed (Frequency Deviation) on the SpeechQuality

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 competitor’s 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 stableduring acceptance tests or comparative tests.


Page 22 of 48


2.6 Impact of Speech Coding Rate on the Speech QualityThe 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 qualityis probably poor. From the perspective of principle, poor


Page 23 of 48


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 tolow MOS. If the MOS is low, the involved NEs and interfacesshould be checked in succession.

Figure 9 shows the fault location flow.


Page 24 of 48



Page 25 of 48


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, testsamples, 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


Page 26 of 48


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 MOSNetworkType

SpeechSample MOS

900M French 3.4900M Italian 3.46900M Arabic 3.5900M Russian 3.54

900MJapanese 3.54

900M Greek 3.57900M Spanish 3.59900M German 3.61

900M

AmericanEnglish 3.64

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


Page 27 of 48


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


Page 28 of 48

20ms speech frameSub-stream

ASub-stream B Sub-

stream C

Sub-stream A

Sub-stream B Sub-stream CCRC

1/2 coding Sub-stream C

TDMA frame


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 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.


Page 29 of 48


3.2.2.2 Coverage- and Interference-Related Problem CheckIf 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 HandoversLow 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 lossof 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


Page 30 of 48


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 RateAll 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 CheckCheck for the version-related problems that have been

detected.


Page 31 of 48


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 EnabledDTX 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 subjectiveimpression. 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 FactorsThe audio discontinuity caused by BTS hardware fault affects theMOS. Bugs in the speech processing part of the hardware alsoaffect the speech quality. You are advised to confirm with theR&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.


Page 32 of 48


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 EnabledDuring 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 beimproved. 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 EP2HR .85 .68 .39FR .53 .53 .35EFR .32 .46 .19

3.2.5.2 Whether Local Switch Is EnabledThe local switch consists of BSC local switch and BTS localswitch. For the BSC local switch, the calling MS and calledMS 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 functionis implemented in the core network, thus improving the


Page 33 of 48


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 Lossof 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-


Page 34 of 48


line phone passes through two UMGs and whether the two UMGsuse 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 SpeedGenerally, 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 testresult is high.

In addition, if the drive speed is high (at more than 200 km/h), the generated frequency deviation affects the speechquality. In this case, the BTS frequency deviation algorithm should be enabled to improve the speech quality.

3.2.8.2 Test Route and Test TimeThe DT test of speech quality objectively reflects the coverage and receive quality of a network. In a network, itis definite that some areas have good speech quality and other areas have poor speech quality. During the DT test ofspeech 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 notbe 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


Page 35 of 48


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 DegreeFor 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


Page 36 of 48


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


Page 37 of 48


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.


Page 38 of 48


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.


Page 39 of 48


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 ProblemsAfter 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

ProblemNumber

Problem Problem Description RelatedProdu

AffectedChannel

Problem-SolvedVersion


Page 40 of 48


ct

1

In the caseof FAMR/HAMRand FR, oneframe is lostand then theframe isretransmitted.

The frame loss on theuplink during theFAMR/HAMR and FRspeech leads to asharp decrease in theMOS.

DPU(TC)

FAMR/HAMR/FR

V9R8C01B048SP01

2

In case offrame lossduring ahandover, thesmoothnesshandlingperformed onthe signalsover theEFR/HRchannels doesnot takeeffect.

The frame loss on theuplink during theEFR/HR speech leads toa sharp decrease inthe MOS.

DPU(TC) EFR/HR V9R8C01B048S

P01

3

Random biterrors whenTFOestablished

When the TFO isestablished, the MOSis lower than theexpected value andthere are random biterrors.

DPU(TC) EFR/FR/HR V9R8C01B048S

P01

4

Permanentloss of oneframe duringhandover tohalf rate andpermanentloss of oneframe duringactivationunder HAMR7.4k

The uplink DTX isenabled in the case ofHAMR7.4. During thetransition from non-speech to speech, theMOS is decreased byone frame.

DPU(TC) HAMR7.4 V9R8C01B048S

P01

5 The uplinkDTX is

The uplink DTX isenabled in the case of

DPU(TC)

EFR/HARM6.7/

V9R8C01B048SP01


Page 41 of 48


enabled andthe speechquality underEFR and HAMRobviouslydeteriorates.Damage isintroduced onthe TC side.

EFR and HAMR. Duringthe transition fromnon-speech to speech,the MOS is decreasedby one frame.

HARM7.4

6

The internalclock isslow.Externalinterruptionshould beused tolocate theperiod of 20ms.

If a call is maderepeatedly on the samechannel, audiodiscontinuity occurs.

DPU(TC)

All thespeechchannels

V9R8C01B048SP01

7SID_FIRSTframe forFAMR

In the test speechsample, two SP framescontain the SID_FIRSTframe. In this case,the BTS misinterpretsand discards the firstspeech frame after theSID frame. Thus, theMOS decreases.

DSP(BTS) FAMR

V100R008C02B201 orV100R001C07B415

8 SID_FIRST_INHframe forHAMR

In the test speechsample, two SP framescontain theSID_FIRST_INH frame.In this case, the BTSreports theSID_FIRST_INH frame asthe NO_SP frame. Thus,the TC misinterpretsand discards the firstspeech frame after theNO_SP frame. As aresult, the MOS

DSP(BTS)

HARM V100R008C02B201 orV100R001C07B415


Page 42 of 48


decreases.

11

Frequentadjustment todownlink ratewhen uplinkDTX enabled

After the uplink DTXis enabled, theadjustment (adjustmentis made when silentframes are transmittedand adjustment is notmade when speechframes aretransmitted) is madeon the downlink codingin the case of half-rate AMR multirateset. If the DTX isdisabled, however, afixed rate is alwaysoccupied. Therefore,the adjustment is notcaused by the C/I.

DSP(BTS) HARM

V100R008C02B201 orV100R001C07B415

12

Reporting ofHO_DET aheadof timeduringsynchronoushandover

During the synchronoushandover, the HO_DETis reported ahead oftime. Thus, the uplinkspeech frames on theold channel are lostand the handoverdisruption is long.The occurrencepossibility of thisproblem during the labtest is about 5%-10%.

DSP(BTS)


V100R008C02B201 orV100R001C07B415

13 One speechframe lost onold channelduringasynchronoushandover

During the intra-BSCasynchronous handover,one frame out of theuplink speech framesis lost. This problemoccurs on the threetypes of MSs. Theoccurrence possibilityof this problem duringthe lab test is about

DSP(BTS)


V100R008C02B201 orV100R001C07B415


Page 43 of 48


30%-50%.

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 Requirements1. 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.


Page 44 of 48


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


Page 45 of 48


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 thelocal area. Networks with different QoSs have different


Page 46 of 48


configurations and different configurations have different impacts on the network. For the R&D personnel to learn the existing network, the configurations of theexisting 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


Page 47 of 48


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


Page 48 of 48

Date post:	10-Jan-2023
Category:	Documents
Upload:	mdurohtak
View:	0 times
Download:	0 times

01 GSM BSS Network KPI (MOS) Optimization Manual

Documents