GSM BSS Network KPI (MOS) Optimization Manual

INTERNAL

Product Name GSM BSS Product Version V00R01

Confidentiality Level INTERNAL Total 36 pages

GSM BSS Network KPI (MOS) Optimization Manual

For internal use only

Prepared by Reviewed by Reviewed by Granted by

GSM&UMTS Network Performance Research Department

Dong Xuan

Date Date Date Date

2008-2-21 yyyy-mm-dd yyyy-mm-dd yyyy-mm-dd

Huawei Technologies Co., Ltd. All rights reserved

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Revision Record
Date 2008-1-21 2008-3-20 Revision Version 0.9 1.0 Change Description Draft completed. The document is modified according to review comments. Author Dong Xuan Wang Fei

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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 MOS PESQ PSQM PAMS Expansion Mean Opinion Score Perceptual Evaluation of Speech Quality Perceptual Speech Quality Measurement 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 13 1.3.5 UMG14 2 Factors That Affect the MOS in GSM.........................................................................................15 2.1 Introduction to GSM Speech Acoustic Principles................................................................15 2.2 Impact of Field Intensity and C/I on the Speech Quality.....................................................16 2.3 Impact of Handover on the Speech Quality..........................................................................16 2.4 Impact of DTX on the Speech Quality..................................................................................17 2.5 Impact of Speed (Frequency Deviation) on the Speech Quality..........................................17 2.6 Impact of Speech Coding Rate on the Speech Quality.........................................................18 2.7 Impact of Transmission Quality on the Speech Quality.......................................................18 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......................................................................21 3.2.1 Consistency Check and Sample Check.................................................................21 3.2.2 Um Interface Check................................................................................................22 3.2.3 BTS Check.............................................................................................................25 3.2.4 Abis Transmission Check.......................................................................................26 3.2.5 BSC Check.............................................................................................................26 3.2.6 A Interface Transmission Check.............................................................................27 3.2.7 MGW Check...........................................................................................................27 3.2.8 Miscellaneous (Comparison of MOS Before and After Network Replacement).....27 4 Test Methods and Suggestions.....................................................................................................29 4.1 Test Tool Selection and Test Suggestions............................................................................29 4.2 Suggestions on the Test of the Existing Network.................................................................29 5 MOS Cases...................................................................................................................................30 5.1 Differences Between Speech Signal Process and Signaling Process...................................30 5.1.1 GSM Speech Signal Process.................................................................................30 5.1.2 Signaling Process...................................................................................................30 5.2 Identified MOS Problems......................................................................................................31
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6 Feedback on MOS or Speech Problems.......................................................................................33 6.1 Test Requirements.................................................................................................................33 6.2 Requirements for Configuration Data in Existing Network.................................................35

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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.......................................18 Table 1Mapping between speech sample and MOS.........................................................................21 Table 1Impact of TFO on the improvement of speech quality (GSM Rec. 06.85)..........................26 Table 1Identified MOS problems.....................................................................................................31 Table 1Network configuration parameters to be provided...............................................................35

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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.........................................................................11 Figure 1Typical MOS test process....................................................................................................13 Figure 1Speech processing on the MS side......................................................................................13 Figure 1Speech processing on the BTS side.....................................................................................13 Figure 1Handling process in the GTCS............................................................................................14 Figure 1Codec cascading..................................................................................................................15 Figure 1Fault location flow...............................................................................................................21 Figure 1Speech data transmission on the Um interface (schematic drawing)..................................23 Figure 1BSC6000 speech signal process..........................................................................................30

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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 Very good Good Average Poor Very poor

Score 5 4 3 2 1

Listening Effect Scale The listener can be totally relaxed without paying attention. The listener should pay some attention. The listener should pay close attention. The listener should pay very close attention. The listener cannot understand even with very close attention.

Although the formal subjective listening test is the most reliable evaluation method and the network performance and any coding/decoding algorithm can be evaluated, the test result varies from listener to listener. In addition, the factors such as the listening
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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 been commercially used. Figure 1 shows the PESQ process.
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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 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:

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y = 0.999 +
5 4.5 4 3.5 Mapped P.862 3 2.5 2 1.5 1 0.5 0 –1

4.999 − 0.999 1 + e −1.4945 *x +4.6607

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.

Figure 1 Overall speech quality prediction of P.563

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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 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.
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1.3.4.1 FTC Processing on Speech Coding/decoding is performed on the speech signals and rate adaptation is performed on the data signals so that the communication between a GSM subscriber and a PSTN subscriber is realized and the transparent transmission on the SS7 signaling over the A interface is implemented.

Figure 1 Handling process in the GTCS

1.3.4.2 FTC Loopback In a loopback, a message is transmitted by a transmission device or transmission channel and then is received by the same to check the health of the hardware and the settings of the software parameters. The FTC loopback is one of the most commonly used method for locating the transmission problems and for checking whether the settings of the trunk parameters are accurate.

1.3.5 UMG
The UMG performs the coding/decoding conversion. Different coding/decoding algorithms have different impacts on the speech quality. If the communication is performed between different networks, if the MSs use different coding/decoding algorithms, or if the same coding/decoding uses different rates to perform communications, the coding/decoding conversion is required. Generally, the UMG8900 coding/decoding algorithm uses the codec cascading to perform speech conversions. As
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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.

Figure 1 Codec cascading

2 Factors That Affect the MOS in GSM

D ecoder A

The MOS is affected by many factors, such as the background noise, mute suppression, low-rate coder, frame error rate, echo, mobile terminal (MS). Here, the frame error rate pertains to the frame handling strategy (handling of frame loss during signaling transmission), frame stealing, bit error, handover, and number of online subscribers (congestion degree). During the speech propagation, several NEs participate in the speech handling: MS, BTS, TC, and MGW. The following paragraphs describe the impact of each NE on the speech quality.

2.1 Introduction to GSM Speech Acoustic Principles
In a radio network, the basic processing of speech data involves source sampling, source coding, framing, Um-interface radio transmission, internal NE processing, handover, terrestrial transmission, and source decoding at the receive end. A fault in any segment of the speech transmission will result in bit errors, thus leading to poor speech quality. For the wireless communication system, the speech quality is significantly affected by

the Um interface, that is, the radio transmission part. An intrinsic characteristic of radio transmission is time-variant fading and interference. Even for a normally functioning network, the radio transmission characteristics are changing from time to time. For a
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radio network, the radio transmission has a great impact on the speech quality. A speech signal is transmitted to the BSS system over the Um interface. Then, the signal is transmitted within the BSS system through the standard and non-standard interfaces. The process requires the transmission lines to be stable and the port BER to be lower than the predefined threshold. If a transmission alarm is generated, the related speech transmission lines should be checked. If the speech quality is poor, a port BER test should be conducted.

2.2 Impact of Field Intensity and C/I on the Speech Quality
For the wireless communication system, the speech quality is significantly affected by the Um interface, that is, the radio transmission part. An intrinsic characteristic of the radio transmission is time-variant fading and interference. Even for a normally functioning network, the radio transmission characteristics are changing from time to time. For a radio network, the radio transmission has a great impact on the speech quality. If the changes in the signal field intensity do not cause the BER/FER to be greater than zero, the RXQUAL remains zero. In this case, the speech quality is not affected theoretically. If the changes in the signal filed intensity cause the BER/FER to be greater than zero (equivalently some interference exists), the C/I and the field intensity have a great impact on the MOS. Both the in-network interference and the out-network interference may affect the C/I and the receive quality and degrade the demodulation capability of the BTS. This will lead to continuous bit errors and faulty parsing of speech frames. Thus, frame loss may occur, causing adverse effect on the speech quality.

2.3 Impact of Handover on the Speech Quality
The GSM network uses hard handovers, so a handover from a source channel to a target channel definitely causes loss of downlink speech frames on the Abis interface. Therefore, audio discontinuity caused by handovers is inevitable during a call. Hence, the handover parameters should be properly set to avoid frequent handovers. In addition, the audio discontinuity caused by handovers should be minimized to improve the speech quality.
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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 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. 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. 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.

FAMR12.2

HAMR5.9

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

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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 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
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retransmission of LAPD signaling, LAPD bad frame, and overload. These counters are used to monitor the transmission quality on the Abis interface. If too many bad frames are generated or if the signaling retransmission occurs frequently, the transmission quality is probably poor. From the perspective of principle, poor transmission quality is equivalent to the loss of some speech frames. If the speech frames are lost, the speech quality deteriorates greatly.

3 Method of Analyzing the Problem of Low MOS
3.1 Process of Analyzing the Problem of Low MOS
The MOS aims at an end-to-end communication. The communication involves many NEs and interfaces. The fault in any NE or interface will cause high BER, thus leading to low MOS. If the MOS is low, the involved NEs and interfaces should be checked in succession. Figure 9 shows the fault location flow.

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Start

W hether speech MOS problem exists

End

1. Test MS

W hether the test tool , test MS, and test sample have an impact on the MOS test ? W hether related to coverage or interference ?

Replace the test tool , test MS, or test sample Refer to the guide related to coverage or interference
Optimize the neighboring cell relations check the , configurations of the handover-related parameters , and reduce the number of handovers Check the full -rate/half-rate busy threshold and parameters related to AMR rate adjustment Check the related data and disable the DTX function to perform another testand , then check the software version

2. Um interface check

W hether too many handovers affect the MOS?
Whether the occupied channel is halfrate channel and whether the AMR coding rate is too low ? Whether the uplink/downlink DTX function is enabled and whether related to software version or hardware

3. BTS check

4. Abis interface check

Transmission bit error or intermittence on Abis interface W hether the TFO function is enabled

Check for intermittence alarms and bit errors on Abis interface
This function is valid for the call from one MS to another and can be used to improve the MOS This function is valid for the call from one MS to another under the same BSC and can be used to improve the MOS

5. BSC check

W hether the local switch function is enabled W hether intermittence occurs on A interface transmission ? W hether speech damage occurs between MGW? s W hether such factors

6. A interface check

Check for intermittence alarms and bit errors on A interface Check the coding scheme between UMGs Use the same route to perform test and eliminate the effect of Page 20 different factors of 36
Use the same test speed in the case of comparison test . The frequency deviation algorithm should be enabled for the BTS if test speed

7. MGW check

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8. Miscellaneous as test route are (comparison of MOS before and after consistent in the case network replacement Huawei ) Technologies Proprietary of comparison test ?

W hether test speed (frequency deviation ) has an impact on

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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

Network Type 900M 900M 900M 900M 900M 900M 900M
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Speech Sample French Italian Arabic Russian Japanese Greek Spanish

MOS 3.4 3.46 3.5 3.54 3.54 3.57 3.59
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900M 900M

German American English

3.61 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 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 Sub-stream C

CRC

Sub-stream B

Sub-stream C

1/2 coding

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 switching is used, the TC coding/decoding is not required if the transcoding function is
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implemented in the core network, thus improving the speech quality. When the BTS local switching is used, the TC coding/decoding is not required because the speech signals do not pass the BSC. This also improves the speech quality.

3.2.6 A Interface Transmission Check
The rules for checking the A interface transmission is similar to those for checking the Abis interface transmission. You can refer to the section Abis Transmission Check. To check the A interface transmission, you have two methods: first, query the BSC alarms (for example, the Loss of E1/T1 Signals alarm) to determine whether intermittence occurs on the A interface; second, use a BER tester to check whether bit errors occur on the A interface transmission.

3.2.7 MGW Check
If this problem does not occur when you use an MS to call another MS during the MOS test, you can skip this section. As is mentioned in section UMG, if the communication is performed between different networks, if the MSs use different coding/decoding algorithms, or if the same coding/decoding uses different rates to perform communications, the coding/decoding conversion is required. The inter-code conversion, however, may adversely affect the speech quality. Therefore, if you use an MS to call a fixed-line phone during the MOS test, you should check whether the deterioration of the speech quality is caused by the following: whether the route between the MS and the fixed-line phone passes through two UMGs and whether the two UMGs use the speech compression algorithm.

3.2.8 Miscellaneous (Comparison of MOS Before and After Network Replacement)
In a network replacement project, if the MOS deviation occurs before and after the network replacement, the following factors should be considered: 3.2.8.1 Test Speed Generally, the drive speed should be stable (at about 30 km/h) during the test. If the drive speed is low, the test is equivalent to the fixed-point CQT test and thus the test result is high. In addition, if the drive speed is high (at more than 200 km/h), the generated frequency
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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 degrees cannot be compared directly. For example, China Unicom adopts a plan with
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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. In the case of outdoor antenna (vehicle body loss should be considered), it is recommended that Nokia6680 be used as the test MS.
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3. 4. 5. 6. 7. 8. 9.

The areas with good coverage and only a few handovers should be selected as the test routes. During the test, it is recommended that you use an MS to call a fixed-line phone. Thus, the MOS is high. The DTX function should be disabled. The drive speed during the drive test should not be too high. It is recommended that the idle hours be selected as the test time. Thus, the network C/I is high. During the test, it is recommended that the channels with good speech coding quality be occupied, for example, EFR and AMR full-rate channels. 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

5.1.2 Signaling Process
MS-BTS - GEIUB-GGNU-GXPUM -GGNU-GEIUT-GEIUT-GTNU-GEIUA –MSC…
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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 1

Problem In the case of FAMR/HAMR and FR, one frame is lost and then the frame is retransmitted. In case of frame loss during a handover, the smoothness handling performed

Problem Description The frame loss on the uplink during the FAMR/HAMR and FR speech leads to a sharp decrease in the MOS. The frame loss on the uplink during the EFR/HR speech leads to a sharp decrease in the MOS.
Huawei Technologies Proprietary

Related Product DPU(T C)

Affected Channel FAMR/HAM R/FR

Problem-Solved Version V9R8C01B048SP 01

2

DPU(T C)

EFR/HR

V9R8C01B048SP 01

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3

on the signals over the EFR/HR channels does not take effect. Random bit errors when TFO established 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 and the speech quality under EFR and HAMR obviously deteriorates. Damage is introduced on the TC side. The internal clock is slow. External interruption should be used to locate the period of 20 ms. SID_FIRST frame for FAMR

4

When the TFO is established, the MOS is lower than the expected value and there are random bit errors. 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. 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) DPU(T C)

EFR/FR/HR

V9R8C01B048SP 01 V9R8C01B048SP 01

HAMR7.4

5

DPU(T C)

EFR/HARM6. 7/HARM7.4

V9R8C01B048SP 01

6

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

DPU(T C)

All the speech channels

V9R8C01B048SP 01

7

DSP (BTS)

FAMR

V100R008C02B2 01 or V100R001C07B4 15 V100R008C02B2 01 or V100R001C07B4 15

8

SID_FIRST_INH frame for HAMR

DSP (BTS)

HARM

11

Frequent adjustment to downlink rate when uplink DTX enabled

DSP (BTS)

HARM

V100R008C02B2 01 or V100R001C07B4 15

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12

Reporting of HO_DET ahead of time during synchronous handover

13

One speech frame lost on old channel during asynchronous 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%. 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

DSP (BTS)

All the speech channels

V100R008C02B2 01 or V100R001C07B4 15

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.

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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 The MSs should be dialed mutually. The dialing, answering, and onhook dualband. 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
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reported. For different cities, the test should be conducted according to the different requirements mentioned in this chapter. The test reports should be archived. The dot information about the local e-map should be provided for the future network optimization of the areas with poor quality. During each test, the mean speed per hour should be recorded and archived. Dot statistics can be performed on the GPS.

6.2 Requirements for Configuration Data in Existing Network
The QoS of the existing Huawei network varies according to the economic development degree, network coverage, network user density, network density, network planning, frequency reuse degree, and external interference in the local area. Networks with different QoSs have different configurations and different configurations have different impacts on the network. For the R&D personnel to learn the existing network, the configurations of the existing network should be provided. Table 7 lists the network configuration parameters that should be provided.
Table 1 Network configuration parameters to be provided

Network Configuration 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
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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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