EN V4.0.1 ( )

Transcription

1 European Standard (Teleommuniations series) Digital ellular teleommuniations system (Phase 2); Substitution and muting of lost frames for Enhaned Full Rate (EFR) speeh traffi hannels () GLOBAL SYSTEM FOR MOBILE COMMUNICATIONS R European Teleommuniations Standards Institute

2 2 Referene DEN/SMG P (bj0200o.pdf) Keywords Digital ellular teleommuniations system, Global System for Mobile ommuniations (GSM) ETSI Seretariat Postal address F Sophia Antipolis Cedex - FRANCE Offie address 650 Route des Luioles - Sophia Antipolis Valbonne - FRANCE Tel.: Fax: Siret N NAF 742 C Assoiation à but non luratif enregistrée à la Sous-Préfeture de Grasse (06) N 7803/88 X.400 = fr; a=atlas; p=etsi; s=seretariat Internet seretariat@etsi.fr Copyright Notifiation No part may be reprodued exept as authorized by written permission. The opyright and the foregoing restrition extend to reprodution in all media. European Teleommuniations Standards Institute All rights reserved.

3 3 Contents Intelletual Property Rights...4 Foreword Sope Normative referenes Definitions and abbreviations Definitions Abbreviations General Requirements Error detetion Lost speeh frames First lost SID frame Subsequent lost SID frames Example solution Example solution for substitution and muting of lost speeh frames Example solution for substitution and muting of lost SID frames... 9 History...10

4 4 Intelletual Property Rights IPRs essential or potentially essential to the present doument may have been delared to ETSI. The information pertaining to these essential IPRs, if any, is publily available for ETSI members and non-members, and an be found in ETR 314: "Intelletual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to ETSI in respet of ETSI standards", whih is available free of harge from the ETSI Seretariat. Latest updates are available on the ETSI Web server ( Pursuant to the ETSI Interim IPR Poliy, no investigation, inluding IPR searhes, has been arried out by ETSI. No guarantee an be given as to the existene of other IPRs not referened in ETR 314 (or the updates on whih are, or may be, or may beome, essential to the present doument. Foreword This European Standard (Teleommuniations series) has been produed by the Speial Mobile Group (SMG) Tehnial Committee of the European Teleommuniations Standards Institute (ETSI. This EN defines a frame substitution and muting proedure whih is used by the Reeive (RX) Disontinuous Transmission (DTX) handler when one or more lost speeh or Silene Desriptor (SID) frames are reeived from the Radio Sub System (RSS) within the digital ellular teleommuniations system. This EN orresponds to GSM tehnial speifiation, GSM 06.61, version National transposition dates Date of adoption of this EN: 19 Deember 1997 Date of latest announement of this EN (doa): 31 Marh 1998 Date of latest publiation of new National Standard or endorsement of this EN (dop/e): 30 September 1998 Date of withdrawal of any onfliting National Standard (dow): 30 September 1998

5 5 1 Sope This EN defines a frame substitution and muting proedure whih shall be used by the Reeive (RX) Disontinuous Transmission (DTX) handler when one or more lost speeh or Silene Desriptor (SID) frames are reeived from the Radio Sub System (RSS). The requirements of this EN are mandatory for implementation in all GSM Base Station Systems (BSS)s and Mobile Stations (MS)s apable of supporting the enhaned Full Rate speeh traffi hannel. It is not mandatory to follow the bit exat implementation outlined in this EN and the orresponding C-soure ode. 2 Normative referenes This EN inorporates by dated and undated referene, provisions from other publiations. These normative referenes are ited at the appropriate plaes in the text and the publiations are listed hereafter. For dated referenes, subsequent amendments to or revisions of any of these publiations apply to this EN only when inorporated in it by amendment or revision. For undated referenes, the latest edition of the publiation referred to applies. [1] GSM (ETS ): "Digital ellular teleommuniations system (Phase 2); Channel oding". [2] GSM (EN ): "Digital ellular teleommuniations system; Enhaned Full Rate (EFR) speeh transoding". [3] GSM (EN ): "Digital ellular teleommuniations system; Disontinuous transmission (DTX) for Enhaned Full Rate (EFR) speeh traffi hannels". [4] GSM (ETS ): "Digital ellular teleommuniations system (Phase 2+); Inband ontrol of remote transoders and rate adaptors for Enhaned Full Rate (EFR) and full rate traffi hannels". 3 Definitions and abbreviations 3.1 Definitions For the purposes of this EN, the following definition applies: 5-point median operation: Consists of sorting the 5 elements belonging to the set for whih the median operation is to be performed in an asending order aording to their values, and seleting the third largest value of the sorted set as the median value. Further definitions of terms used in this EN an be found in GSM (EN ) [2], GSM (EN ) [3], GSM (ETS )[1] and GSM (ETS ) [4]. 3.2 Abbreviations For the purposes of this EN, the following abbreviations apply: BFI BSI_Abis PrevBFI RSS RX DTX SID CRC CCU Bad Frame Indiation from Radio Sub System Bad Sub-blok Indiation obtained from A-bis CRC heks Bad Frame Indiation of Previous frame Radio Sub System Reeive Disontinuous transmission SIlene Desriptor frame Cyli Redundany Chek Channel Coding Unit

6 6 median5 TRAU 5-point median operation Transoding Rate Adaptation Unit 4 General The purpose of frame substitution is to oneal the effet of lost frames. The purpose of muting the output in the ase of several lost frames is to indiate the breakdown of the hannel to the user and to avoid generating possible annoying sounds as a result from frame substitution proedure. The RSS indiates lost speeh or SID frames by setting its Bad Frame Indiation flag (BFI) based on its 3-bit and 8-bit CRCs and possibly other error detetion mehanisms. The TRAU alulates from the CRCs inserted by the CCU in the TRAU frames one BSI_Abis flag for every sub-blok of speeh parameters. If either one or more of these flags is set, the speeh deoder shall either perform frame substitution or subframe substitution. The example solution provided in lause 6 applies only for bad frame handling on a omplete speeh frame basis. However some parts ould be modified for substitution of bad sub-bloks. 5 Requirements 5.1 Error detetion An error is deteted and the BFI-flag is set-by the RSS aording to the priniple desribed in lause Lost speeh frames Normal deoding of lost speeh frames would result in very unpleasant noise effets. In order to improve the subjetive quality, lost speeh frames shall be substituted with either a repetition or an extrapolation of the previous good speeh frame(s). This substitution is done so that it will gradually derease the output level, resulting in silening of the output. Sublause 6.1 gives an example solution. 5.3 First lost SID frame A single lost SID frame shall be substituted by the last valid SID frame and the proedure for valid SID frames be applied as desribed in GSM (EN ) [3]. 5.4 Subsequent lost SID frames For the seond lost SID frame, a muting tehnique shall be used on the omfort noise that will gradually derease the output level (-3 db/frame), resulting in silening of the output of the deoder. For subsequent lost SID frames, the muting of the output shall be maintained. Sublause 6.2 gives an example solution. 6 Example solution The C-ode of the following example is embedded in the bit exat software of the enhaned full rate ode. 6.1 Example solution for substitution and muting of lost speeh frames This example solution for substitution and muting is based on a state mahine with seven states (figure 1).

7 7 The system starts in state 0. Eah time a bad frame is deteted, the state ounter is inremented by one and is saturated when it reahes 6. Eah time a good speeh frame is deteted, the state ounter is reset to zero, exept when we are in state 6, where we set the state ounter to 5. The state indiates the quality of the hannel: the bigger the state ounter, the worse the hannel quality is. The ontrol flow of the state mahine an be desribed with the following C-ode (BFI = bad frame indiator, State = state variable): if(bfi!= 0 ) State = State + 1; else if(state == 6) State = 5; else State = 0; if(state > 6 ) State = 6; In addition to this state mahine, the Bad Frame Flag from the previous frame is heked (PrevBFI). The proessing depends on the value of the State-variable. In states 0 and 5, the proessing depends also on the two flags BFI and PrevBFI. The proedure an be desribed as follows: STATE = 0 BFI=0 PrevBFI =0or1 STATE = 1 BFI=1 PrevBFI = 0 STATE = 2 BFI=1 PrevBFI = 1 STATE = 3 BFI=1 PrevBFI = 1 STATE = 4 BFI=1 PrevBFI = 1 STATE = 5 BFI=0or1 PrevBFI = 1 STATE = 6 BFI=1 PrevBFI =0or1 Bad frame Good frame (BFI=1) (BFI=0) BFI = 0, PrevBFI = 0, State = 0 Figure 1: State mahine for ontrolling the bad frame substitution No error is deteted in the reeived or in the previous reeived speeh frame. The reeived speeh parameters are used normally in the speeh synthesis. The urrent frame of speeh parameters is saved.

8 8 BFI = 0, PrevBFI = 1, State = 0 or 5 No error is deteted in the reeived speeh frame but the previous reeived speeh frame was bad. The LTP-gain and fixed odebook gain are limited below the values used for the last reeived good subframe: g p, ( 1) ( 1), > ( 1) g g g = g g g p p p p p p (1) where g p = urrent deoded LTP-gain, g p ( 1 ) = LTP-gain used for the last good subframe (BFI = 0), and g, ( 1) ( 1), > ( 1) g g g = g g g where g = urrent deoded fixed odebook-gain and g ( ) (BFI = 0). 1 = fixed odebook gain used for the last good subframe The rest of the reeived speeh parameters are used normally in the speeh synthesis. The urrent frame of speeh parameters is saved. BFI = 1, PrevBFI = 0 or 1, State = An error is deteted in the reeived speeh frame and the substitution and muting proedure is started. The LTP-gain and fixed odebook gain are replaed by attenuated values from the previous subframes: (2) g p p p p p P ( state) g ( 1), g ( 1) median5( g ( 1),..., g ( 5)) = p p p p p P ( state) median5( g ( 1),..., g ( 5)), g ( 1) > median5( g ( 1),..., g ( 5)) (3) where g p p p = urrent deoded LTP-gain, g ( ),..., g ( n) 1 = LTP-gains used for the last n subframes, median5() = 5-point median operation, P(state) = attenuation fator (P(1) = 0.98, P(2) = 0.98, P(3) = 0.8, P(4) = 0.3, P(5) = 0.2, P(6) = 0.2), state = state number, and g C ( state) g ( 1), g ( 1) median5( g ( 1),..., g ( 5)) = C ( state) median5( g ( 1),..., g ( 5)), g ( 1) > median5( g ( 1),..., g ( 5)) (4) where g = urrent deoded fixed odebook gain, g ( ),..., g ( n) 1 = fixed odebook gains used for the last n subframes, median5() = 5-point median operation, C(state) = attenuation fator (C(1) = 0.98, C(2) = 0.98, C(3) = 0.98, C(4) = 0.98, C(5) = 0.98, C(6) = 0.7), and state = state number. The higher the state value is, the more the gains are attenuated. Also the memory of the preditive fixed odebook gain is updated by using the average value of the past four values in the memory: 4 1 ener ener i ( 0) = ( ) 4 i= 1 The past LSFs are used by shifting their values towards their mean: () () α () α () lsf _ q1i = lsf _ q2i = past_ lsf _ q i + ( 1 ) mean_ lsf i, i = (6) (5) where α = 0.95, lsf_q1 and lsf_q2 are two sets of LSF-vetors for urrent frame, past_lsf_q is lsf_q2 from the previous frame, and mean_lsf is the average LSF-vetor. The LTP-lag values are replaed by the past value from the 4th subframe of the previous frame. The reeived fixed odebook exitation pulses from the erroneous frame are always used as suh.

9 9 6.2 Example solution for substitution and muting of lost SID frames The first lost SID frame is replaed by the last valid SID frame. For subsequent lost SID frames, the last valid SID frame is repeated, but the fixed odebook gain is dereased with a onstant value of -3 db in eah frame down to the minimum value of 0. This value is maintained if additional lost SID frames our.

10 10 History Doument history V4.0.0 August 1997 One-step Approval Proedure OAP 9750: to V4.0.1 Deember 1997 Publiation ISBN X Dépôt légal : Déembre 1997