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Speech Quality Factors

Speech Quality Factors


Introduction
The intent of this section is to describe speech quality factors in Nortel system. In GSM there are four different speech codecs that can be used, two full rate codecs (Full Rate and Enhanced Full Rate), one half rate codec (Half Rate) and one multi-rate codec (AMR). The upper limit of the speech quality is set by these four codecs. The speech codec quality is fixed and the focus is thus on the speech degradation factors. The following areas have been identified.

Speech Levels:
The speech levels affect speaker level, distortion and echo canceller performance.

Echo control:
Poor network echo cancelling results in disturbing echo for calls to the PSTN and PLMN with full duplex hands-free mobiles. Echo effect can also be caused by acoustic coupling in mobiles.

Radio Network and Radio Network features:
High bit error rate as a result of a low carrier/noise (C/N) and/or carrier/interference (C/I) ratio causes interruptions or degradation of the speech. The speech degradation because of radio environment factors can be minimized by using the radio network features Discontinuous transmission (DTX), Power control and Frequency hopping. The use of the radio network feature DTX affect the speech quality by removing the true background. The handover procedure creates a short speech interruption.

Transmission Network:
High bit error rate (BER) causes degradation of the speech. The speech quality is sensitive to BER on the A-bis and A-ter interface.

Mobile stations:
The implementation of audio functions in the mobile stations affects the speech quality, i.e. speaker, microphone and speech processing. The mobile station implementations is not further discussed in this document.


Feature Description
Nortel has no specific feature to supervise Speech quality measurements. In order to do so, several counters were introduced to measure RxQualDL , RxQualUL and FER UP. Traditionally RxQual has been used to measure the speech quality. RxQual is a part of the GSM standard measurement report sent from the mobile to the base stations and is therefore readily available. Unfortunately, RxQual does not do a very good job in evolved GSM systems, e.g systems with frequency hopping, AMR speech codec. RxQual is then not correlated to true network quality.

In attempt to improve speech quality measurement, the Frame Erasure Rate (FER) was introduced; Mobile GSM operators are using frequency hopping. Frequency hopping is a technique, which improves the speech quality of the users in the system and/or increases the capacity in a GSM system.

By using frequency hopping a frequency and interference diversity gain is achieved. Both gains are due to the coding and interleaving in GSM. So the gains appear after the de-interleaving and de-coding and thus in the speech quality.

The speech quality in a GSM system, which uses frequency hopping, is best described by the Frame Erasure Rate (FER). This FER measure is built on a CRC check in the speech frame. If a Frame Erasure is found normally the previous frame is repeated. Since a frame only lasts about 20 ms, a single frame erasure does not degrade the speech quality much. However if more frame erasures follow in a row, the speech quality might seriously degrade by this technique. So not only the amount of frame erasures, but also the way they are distributed might have influence on the speech quality.


Error measures comparison: FER, RBERxx, BERÃ…RXQUAL:
The definitions of different error ratios go hand in hand with channel coding and burst formatting which is different for each logical channel. For example, speech frames are encoded differently from signalling frames.
Convolution coding, along with interleaving, is used extensively in GSM. The 50 first bits of speech frame (Figure 2) are considered especially important for the speech quality and no errors are allowed in these bits.


Error ratios used in conjunction with GSM speech channels:
•Frame Erasure Rate, FER, is defined as the amount of swept speech frames (260 bits each) divided by the amount of transmitted speech frames. The speech frame is swept if even one of its most important 50 bits is observed not to be correct. The three parity bits following the 50 class Ia bits are used for error detection.
•Bit Error Rate, BER, is the ratio of erroneously received bits to all received bits. It is important to notice that BER is evaluated before channel decoding, i.e. after equalizer. BER is used for defining the RXQUAL value according to Table 1.
•Residual Bit Error Rate, RBER, is the ratio of erroneous bits to all bits after frame erasure. It is estimated separately for class Ib and class II bits. Example: After frame erasures, a thousand frames have been passed to the speech decoder, with a total of 5000 bit errors in class II bits. Thus, the estimated RBERII is 5000/780006.4%.

According to some research the most important error measure in speech applications is FER even though RXQUAL value is used as a handover criterion . RXQUAL does not take into account channel coding; even if RXQUAL is bad the speech quality may still be satisfactory due to channel coding and the fact that speech is not very sensitive to errors in bits other than class Ia.



SOURCES:

Observation Counter Dictionary. Nortel V15.1.1

Mouly M., Pautet M., "The GSM System for Mobile Communications", published by the authors, 1992

Redl, Siegmund M., Weber, Matthias K., Oliphant, Malcolm W., "An introduction to GSM� Artech House, 1995

Mansikkaviita, J., Talvo, M.,"Johdatus solukkopuhelintekniikkaan" (in Finnish),
Opetushallitus, Helsinki 1998

Carg, V.J., Wilkes, J.E., Principles and applications of GSM, Prentice-Hall Inc., Upper Saddle River 1999

Posted by: Christie Ingram

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