GSM BSS Network KPI (MOS) Optimization Manual
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GSM BSS Network KPI (MOS) Optimization
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Dong
Date
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Revision Record
Date
Revision
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Author
Version
2008-1-21
0.9
Draft completed.
Dong Xuan
2008-3-20
1.0
The document is modified
Wang Fei
according to review comments.
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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
Expansion
MOS
Mean Opinion Score
PESQ
Perceptual Evaluation of Speech Quality
PSQM
Perceptual Speech Quality Measurement
PAMS
Perceptual Analyse Measurement Sytem
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Contents
1 Basic Principles of MOS..................................................................................................................8
1.1 Subjective Speech Quality Evaluation......................................................................................8
1.2 Objective Speech Quality Evaluation.....................................................................................9
1.2.1 PSQM (P.861) Recommendation or Algorithm.........................................................9
1.2.2 PESQ (P.862) Recommendation or Algorithm.........................................................9
1.2.3 P862.1 Recommendation (Mapping Function for Transforming)...........................10
1.2.4 P.563 Recommendation.........................................................................................11
1.3 Speech Processing of Involved NEs.....................................................................................12
1.3.2 MS 13
1.3.3 BTS 13
1.3.4 BSC 14
1.3.5 UMG15
2 Factors That Affect the MOS in GSM.........................................................................................15
2.1 Introduction to GSM Speech Acoustic Principles................................................................16
2.2 Impact of Field Intensity and C/I on the Speech Quality.....................................................16
2.3 Impact of Handover on the Speech Quality..........................................................................17
2.4 Impact of DTX on the Speech Quality..................................................................................17
2.5 Impact of Speed (Frequency Deviation) on the Speech Quality..........................................18
2.6 Impact of Speech Coding Rate on the Speech Quality.........................................................18
2.7 Impact of Transmission Quality on the Speech Quality.......................................................19
3 Method of Analyzing the Problem of Low MOS........................................................................19
3.1 Process of Analyzing the Problem of Low MOS..................................................................19
3.2 Method of Solving the Problem of Low MOS......................................................................22
3.2.1 Consistency Check and Sample Check.................................................................22
3.2.2 Um Interface Check................................................................................................23
3.2.3 BTS Check.............................................................................................................26
3.2.4 Abis Transmission Check.......................................................................................27
3.2.5 BSC Check.............................................................................................................27
3.2.6 A Interface Transmission Check.............................................................................28
3.2.7 MGW Check...........................................................................................................28
3.2.8 Miscellaneous (Comparison of MOS Before and After Network Replacement).....28
4 Test Methods and Suggestions.....................................................................................................30
4.1 Test Tool Selection and Test Suggestions............................................................................30
4.2 Suggestions on the Test of the Existing Network.................................................................30
5 MOS Cases...................................................................................................................................31
5.1 Differences Between Speech Signal Process and Signaling Process...................................31
5.1.1 GSM Speech Signal Process.................................................................................31
5.1.2 Signaling Process...................................................................................................32
5.2 Identified MOS Problems......................................................................................................32
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6 Feedback on MOS or Speech Problems.......................................................................................35
6.1 Test Requirements.................................................................................................................35
6.2 Requirements for Configuration Data in Existing Network.................................................36
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Tables
Table 1Relations between the quality grade, score, and listening effect scale...................................8
Table 1Impact of DTX on the speech quality...................................................................................17
Table 1Mapping between the speech coding scheme and the MOS value.......................................19
Table 1Mapping between speech sample and MOS.........................................................................22
Table 1Impact of TFO on the improvement of speech quality (GSM Rec. 06.85)..........................27
Table 1Identified MOS problems.....................................................................................................32
Table 1Network configuration parameters to be provided...............................................................37
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Figures
Figure 1PESQ process.......................................................................................................................10
Figure 1Mapping between P862 and P862.1....................................................................................11
Figure 1Overall speech quality prediction of P.563.........................................................................12
Figure 1Typical MOS test process....................................................................................................13
Figure 1Speech processing on the MS side......................................................................................13
Figure 1Speech processing on the BTS side.....................................................................................14
Figure 1Handling process in the GTCS............................................................................................15
Figure 1Codec cascading..................................................................................................................15
Figure 1Fault location flow...............................................................................................................22
Figure 1Speech data transmission on the Um interface (schematic drawing)..................................24
Figure 1BSC6000 speech signal process..........................................................................................31
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1 Basic Principles of MOS
1.1
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 scale
Quality Grade
Score
Listening Effect Scale
Very good
5
The listener can be totally relaxed
without paying attention.
Good
4
The listener
attention.
should
pay
some
Average
3
The listener
attention.
should
pay
close
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
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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 industry’s best standard algorithm that has
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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 ear’s 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
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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:
y = 0.999 +
4.999 − 0.999
1 + e −1.4945*x +4.6607
5
4.5
4
Mapped P.862
3.5
3
2.5
2
1.5
1
0.5
0
–1
0
1
2
P.862
3
4
5
P.862.1_F1
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.
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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.
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Figure 1 Typical MOS test process
1.3.2 MS
Figure 5 shows the speech processing on the MS side.
Session
processing
A/D
and
conversions
D/A
Speech
coding/decoding, DTX
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
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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.
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.
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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
PCM
Encoder A
Decoder B
Figure 1 Codec cascading
2 Factors That Affect the MOS in GSM
The MOS is affected by many factors, such as the background noise, mute suppression,
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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
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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 Quality
If 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
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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.
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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
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 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
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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
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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.
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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 MOS
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Network
Type
Speech
Sample
MOS
900M
French
3.4
900M
Italian
3.46
900M
Arabic
3.5
900M
Russian
3.54
900M
Japanese
3.54
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900M
Greek
3.57
900M
Spanish
3.59
900M
German
3.61
900M
American
English
3.64
INTERNAL
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.
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20ms speech frame
Sub-stream A Sub-stream B
Sub-stream
A
CRC
Sub-stream C
Sub-stream B
1/2 coding
Sub-stream C
Sub-stream C
TDMA frame
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
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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.
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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.
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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
.39
FR
.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
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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.
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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 innetwork 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
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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 Issues
According 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.
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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.
6.
The DTX function should be disabled.
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 Cases
5.1
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
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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
The frame loss on the uplink
during the FAMR/HAMR and FR
speech leads to a sharp decrease
DPU(T
C)
FAMR/HAM
R/FR
V9R8C01B048SP
01
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and then the frame
is retransmitted.
in the MOS.
2
In case of frame loss
during a handover,
the
smoothness
handling performed
on the signals over
the
EFR/HR
channels does not
take effect.
The frame loss on the uplink
during the EFR/HR speech leads
to a sharp decrease in the MOS.
DPU(T
C)
EFR/HR
V9R8C01B048SP
01
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(T
C)
EFR/FR/HR
V9R8C01B048SP
01
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(T
C)
HAMR7.4
V9R8C01B048SP
01
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(T
C)
EFR/HARM6.
7/HARM7.4
V9R8C01B048SP
01
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(T
C)
All the speech
channels
V9R8C01B048SP
01
7
SID_FIRST
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
V100R008C02B2
01
or
V100R001C07B4
15
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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
V100R008C02B2
01
or
V100R001C07B4
15
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
V100R008C02B2
01
or
V100R001C07B4
15
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)
All the speech
channels
V100R008C02B2
01
or
V100R001C07B4
15
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)
All the speech
channels
V100R008C02B2
01
or
V100R001C07B4
15
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6 Feedback on MOS or Speech Problems
To 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.
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 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
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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 provided
Network 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
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