08 Voice Channel Coding Wk v3.1

BSSPAR- Voice & Channel Coding (FR, HR, EFR, AMR)

Training DocumentBSSPAR CTXX 12 S10.5 12-2002

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Voice & Channel Coding

The information in this document is subject to change without notice and describes only the product defined in the introduction of this documentation. This document is intended for the use of Nokia's customers only for the purposes of the agreement under which the document is submitted, and no part of it may be reproduced or transmitted in any form or means without the prior written permission of Nokia. The document has been prepared to be used by professional and properly trained personnel, and the customer assumes full responsibility when using it. Nokia welcomes customer comments as part of the process of continuous development and improvement of the documentation.

The information or statements given in this document concerning the suitability, capacity, or performance of the mentioned hardware or software products cannot be considered binding but shall be defined in the agreement made between Nokia and the customer. However, Nokia has made all reasonable efforts to ensure that the instructions contained in the document are adequate and free of material errors and omissions. Nokia will, if necessary, explain issues which may not be covered by the document.

Nokia's liability for any errors in the document is limited to the documentary correction of errors. NOKIA WILL NOT BE RESPONSIBLE IN ANY EVENT FOR ERRORS IN THIS DOCUMENT OR FOR ANY DAMAGES, INCIDENTAL OR CONSEQUENTIAL (INCLUDING MONETARY LOSSES), that might arise from the use of this document or the information in it.

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Copyright © Nokia Oyj 2007. All rights reserved.

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Contents

8. Module Objectives..........................................................48.1 Introduction.....................................................................48.2 Full Rate..........................................................................48.3 Half Rate.........................................................................58.3.1 Traffic channel allocation................................................78.3.2 Channel rate control in handovers..................................98.4 Enhanced Full Rate Codec...........................................128.5 Adaptive Multi Rate.......................................................158.5.1 Generic AMR description..............................................168.5.2 Link adaptation..............................................................188.5.2.1 Codec mode adaptation................................................188.5.2.2 Channel mode adaptation.............................................21

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8. Module Objectives

At the end of the module, the participant will be able to:

Explain the difference between full rate (FR), half rate (HR), dual rate (DR), and adaptive multi-rate (AMR) channel coding

List the parameters that are used for allocation of HR and FR channels by the BSC

Discuss the parameters that are used to control channel rate changes during handover

Name the parameters associated with AMR channel allocation and handovers

10.1 Introduction

Due to the limited bandwidth available on the air-interface, voice signals are digitally coded and compressed before transmission. Compression of speech to remove redundant information from the encoded voice signal can result in loss of quality. Thus a compromise is needed between the degree of compression, the perceived quality of the speech arising from the compression, and the capacity and complexity of the overall system. Other factors taken into consideration in the choice of voice coding techniques, were end-to-end delay, complexity of the compression, DC power requirements, susceptibility to transmission errors, fading, etc.

The selection of the GSM codec was based on a number of proposals and extensive testing in various languages and operating conditions. Regular-Pulse Excitation - Linear Predictive Coding (RPE-LPC) was chosen for use in GSM. This codec produces a bit rate 13 kbps (FR).

10.2 Full Rate

In Full Rate coding, speech is divided into 20 ms segments and each of these segments is converted into a 260 bit pattern. The output of the speech codec is 13 kbps. Every 260 bits of the speech codec is ordered into 3 groups of 50, 132, and 78 bits based on their importance.

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The 50 bit groups are called Class 1A bits and are very important bits. Transmission errors within these bits are catastrophic to speech intelligibility and have error detection 3 CRC bits and 4 tail bits added to them.

The 132 bit groups are called Class 1B bits and are not parity checked. However

Class 1A and 1B bits are convolutionally coded for forward error correction using a half rate convolution encoder resulting in 378 bits (= 2* (132 +50 +3 +4)).

The last group of 78 bits are the least sensitive bits and not protected in any way. This uncoded 78 bits are added to the 378 convolution encoded bits resulting in 456 bits. The resulting 456-bit block is then transmitted using an interleaving scheme where eight frames are used to transmit a block that forms a burst.

The effective speech rate in full rate coding is 13 kbps. The user data rate is 456/20 ms = 22.8 kbps which includes 13 kbps of speech plus 9.8 kbps of parity, channel coding and tail bits.

10.3 Half Rate

Half Rate is a feature designed to maximise the spectrum efficiency by almost doubling the amount of radio resources as compared the use of the full rate traffic resources only.

Half Rate support can be summarised as follows:

HR is a Phase 2 GSM feature Ph 1 MS do not support HR (i.e. full rate only)

HR utilises 8 kbits/s signalling on BTS-BSC (A-bis) interface to maximise capacity

Can also be used on BSC-transcoder (A-ter) interface if BSS and MS both support HR

Requires modifications to the TCH channel allocation procedure in BSC

FR TCH always allocated for emergency calls (MS HR capabilities not known at this point of set-up)

Introduction of HR increases the load in measurement reporting when 16 kbits/s signalling on the A-bis is used need (i) 32 kbits/s or (ii) use of BTSMeasAver (BMA)(BTS) pre-processing of measurement reports in BTS prior to sending to BSC.

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When using HR with FH, random hopping should be used to maximise FH benefits

Each radio timeslot of the BTS TRX can be configured to be a full rate, half rate or dual rate TCH resource. In the last case, the BSC will be able to allocate an idle radio timeslot either for half rate or full rate coding dynamically on a call by call basis.

BSCF/H

DX 200

MSC

BTSFR

BTSF/H

BTSF/H

16 kbit/sor

8 kbits/s16kbit/s

16 kbit/sor

2*8kbit/s

TCSM2

HR

FR

Ph2 MSs can provideinformation on initial channeltype requirements, during MOC,if NECI = ON @ BTS

BSC needs to optimise TCH allocationbased on penetration of MSs which doand do not support HR, whilst co-operating with other NEs which may havedifferent HR capabilities

Figure 8-1. Half Rate Support - Overview

Full rate speech and data is coded and transferred by using 16 kbit/s channels in BSS. With the half rate coding 8 kbit/s transmission can be used on the A-bis interface. The A- interface will support different types of transcoders capable of full rate coding or half rate coding or both. The utilisation of a Half Rate (HR) and a Full Rate (FR) coding and corresponding transmission bit rates in Nokia system is introduced in the following picture:

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Figure 8-2. Channel Coding Rates in BSS

It will be possible to introduce the half rate coding to existing full rate GSM/DCS network gradually. The BSS is able to co-operate with both old Full Rate mobiles and with new mobiles which support both Full and Half Rate.

10.3.1 Traffic channel allocation

In those cases where MSC does not uniquely determine the channel type, the BSC decides on the channel rate of the TCH to be allocated. The A interface circuit allocated by the MSC must enable the BSC to allocate a radio channel of requested type, the actual A interface circuit pool configuration determines how BSC should select the channel rate. In those cases when the A interface circuit which the MSC has allocated belongs to a pool which does not support the type of TCH, BSC wishes to allocate, then BSC can request the MSC to switch the circuit to the appropriate pool.

The TCH channel rate is determined by the following factors

Initial channel type requirement received from an MS

- MS sends indication of whether HR or FR channel is required as part of establishment

- cause indication (Ph2 MSs in MOC when network supports new establishment causes, NECI on) in channel request message

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On establishing RR connection, MS notifies network of its HR capabilities on main DCCH using Bearer Capability IE, which contains;

- FR only (Ph1 MS) or FR/HR (Ph2 & preferred rate)

- Permitted Speech Version Indicator (supported speech codecs in order of preference)

Information received by BSC in Channel Type IE in Assignment or Handover Request message (in handover case information on source TCH can also be used in TCh channel type decision)

Most cases (Ph2 MSs) MSC can leave decision of TCh type to BSC (MSC just states preference)

BSC decision depends on speech codec rates supported by BTS

The following apply to TCH Configuration on BTS-BSC Interface

BTS can contain 3 types of TCH resources (configurable at BSC by operator)

Permanent FR TCH - serves only FR traffic channels (each RTSL = 1 logical FR TCh channel)

Permanent HR TCH - serves only HR traffic channels (each RTSL = 2 logical HR TCH channels)

Dual Rate (DR) TCH - can serve both FR and HR (idle DR RTSL = either 1 FR TCH or 2 HR TCH)(FR or HR TCHs can be dynamically configured from idle DR resources on call basis)

The following apply to circuit configuration on BSC-MSC Interface

When allocating an A interface circuit, MSC simultaneously chooses transcoder to be used

An A interface circuit pool is a group of A interface circuits that have the same transcoding capabilities

DX200 BSC supports 10 speech-codec specific circuit pools (including EFR)

A interface circuit allocated by MSC must enable BSC to allocate resource of the requested type (in mismatch situation, BSC sends appropriate failure message to MSC)

The type of traffic channel - half rate or full rate - to be allocated can be determined according to the actual traffic load of a cell. Full rate TCHs are allocated from the BTS until the number of free full rate resources are reduced below a particular lower limit, the half rate resources shall then be allocated. When the number of the free full rate resources of the

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cell increases above a particular upper limit, full rate TCHs can be allocated again.

The parameter consists of two threshold values, lower limit, btsSpLoadDepTCHRate (FRL)(BTS)(0 .. 100) and upper limit, btsSpLoadDepTCHRate (FRL)(BTS)(0 .. 100). The former gives the limit value for the decreasing amount of free FR TCH resources from which HR TCHs shall be allocated; the latter determines the limit value for the increasing amount of free FR TCH resources when FR TCHs shall be allocated.

The limits shall be given as relative amount of free full rate TCH resources per working full rate TCH resources (%). The function can be deactivated by setting the lower limit higher than the upper limit (default value for the lower limit is 100% and for the upper limit 0%).

When the feature is activated and the lower limit is set to be higher than zero, last free full TCH resource will be split to two HR subchannels in TCH allocation. This makes it possible to determine the positive margin for the half rate TCH allocation in cells equipped only with one TRX without raising the lower limit value unnecessarily high.

While the value of the lower limit equals 0% then HR TCH resources are allocated for a speech or data call only if the MSC strictly requires a HR TCH regardless of the actual TCH configuration of the BTS.

Radio Resource Management of the BSC is able to optimise the allocation of the half rate traffic channel resources of the cell in such a way that they are primarily allocated from such radio timeslots where half rate call is already maintained.

10.3.2 Channel rate control in handovers

The channel rate can alter in handovers due to TCH resource reasons when only preferred channel rate without any channel rate change prohibition has initially been required in the radio resource request of MSC. The following apply to TCH rate control in handover:

Channel rate (FR to HR and visa versa) and speech codec version can change in inter and intracell handovers

Process controlled using parameters tchRateInternalHO (HRI)(BSC) and tchRateIntraCellHO (TRIH)(BTS)

Operator has the following alternatives;

- In the handover, the best candidate cell in which the same type of TCH and the same speech version can be used as in the source cell will be selected for a target cell. If such a cell does

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not exist, the other speech version can be used or finally the other type of TCH can be allocated from the best possible cell

- During the speech connection the call serving type of the traffic channel is allocated primarily and the original speech version is used. In the case of data connection, the TCH is allocated from the best cell in the candidate list, regardless of the type of the source TCH Channel rate and speech version changes are denied in total.

- The TCH is allocated from the best handover candidate cell in which the call can use the type of TCH and the speech version which the MSC has set to be the preferred ones in the initial radio resource request

- The TCH is allocated from the best BTS of the handover candidate list

The operator will be provided with the means to determine at the BSC site extra constraints for the channel rate changes in internal inter-cell and intra-cell handovers; following different alternatives are possible:

The source type of traffic channel shall be allocated primarily by the best candidate cell. If a suitable cell can not be found, then the other type of TCH can be allocated from the best possible cell.

During the speech connection, the source type of traffic channel shall be allocated primarily from the best cell. At the time of data connection, the TCH shall be allocated from the best cell of the candidate list regardless of the call-maintaining TCH type.

Channel rate changes are denied in total, i.e. the source type of channel is the only alternative in TCH allocation.

The TCH of the channel rate, which was determined to be preferred (initially in Assignment or Handover Request) by the MSC, shall be allocated primarily.

The TCH has to be allocated primarily from the best BTS of the handover candidate list regardless of the call-maintaining traffic channel type. This parameter value is not significant in intra-cell handovers; the same priority-setting method is applied for them as described in the first case above.

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Upper limit for free FR TCHs- btsSpLoadDepTCHRate BTSlevel- btsLoadDepTChRate BSC level

Lower limit for free FR TCHs- btsSpLoadDepTCHRate BTSlevel- btsLoadDepTChRate BSC level

Allocation of FRTCHs

Allocation of HRTCHs

Allocation of FRTCHs

• Free FR TCHs based onratio of available toworking FR TCHs

• Process is disabled bysetting lower limit >upper limit

• Enabling load analysisat BTS level utomaticallydisables it at a BSC evel

• To disable at BTS levelmust be disabled at bothBTS and BSC level

Figure 8-3. TCH Allocation based on Cell Load

The interaction between cell load based radio channel allocation and the channel rate change control is:

When the TCH is going to be allocated for internal handover and the channel rate changes are denied in total either by the MSC or by the BSC, then, neither of them will be allowed due to the cell-load-dependent resource conditions.

When the TCH is going to be allocated for internal handovers and the source type of TCH is determined to be the preferred alternative, then a half rate channel shall be allocated for a call which already is going on a half rate channel regardless of the cell-load-dependent resource conditions.

Although the channel rate control is valid at BSS level for all internal handovers, they can be readjusted for intra-cell handovers in each BTS. In external handovers, these parameters are not possible to apply because the source channel type and mode information is not transferred from the source BSS to the target BSS of the handover.

The parameter tchRateInternalHo (HRI)(BSC)(1 .. 5) which controls the channel rate in internal handovers at BSS level belongs to the BSC object class.

By default, the call-maintaining type of TCH shall be allocated in internal handovers.

The parameter tchrateIntraCellHo (HRI)(BSC) controlling the Intra-cell handovers is adjustable BTS-specifically when necessary.

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The default value of the parameter is that the TCH channel rate shall be determined according to the BSS level parameter also in intra-cell handovers.

halfRateSupport (TX level) Y / N

btsSpLoadDepTCHRate Lower Limit 0….100% Upper Limit 0….100%

btsLoadDepTCHRate Lower Limit 0…100% Upper Limit 0…100%

tchRateInternalHO 1…5

tchRateIntraCellHO 1…5

BTS Level

BSC Level

Parameter Value

Figure 8-4. Half Rate Support – Parameter Summary

10.4 Enhanced Full Rate Codec

This feature introduces a new full rate speech coder to the BSS. Enhanced Full Rate (EFR) speech coder uses the existing GSM/DCS full rate channel coding but gives a considerably better performance in all channel conditions.

The tests indicate that in good channel conditions qualities better or comparable to ADPCM can be offered with this codec. TCSM2 will be SW upgradable to EFR.

From the Air interface dimensioning point of view, no special attention has to be paid to EFR channels. A-interface and transcoders have to be dimensioned according to each type of pools.

EFR can coexist with previous HR/FR ‘dual-codec’.

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MSC

FR

FR/HR/EFR

F/H/EDX200BSC

BTSF/EFR

BTSF/EFR

Dual Rate Mobile(EFR channel in use)

EFR Mobile ?

Dual Rate Mobile(FR channel in use)

FR Mobile

A-interface A-ter A-bis

16 kbit/s

16 kbit/s

16 kbit/s 16 kbit/s

1)

2)

Figure 8-5. Enhanced Full Rate (EFR) in BSS

The basic information of a codec type comes from MSC. If MSC does not know anything about the types of codecs supported on per circuit group over the A-interface then all A-interface channels must be equipped by TCSM2.

The BSC will make the final decision about the codec based upon input from the MSC, the BSS speech coder capabilities and possibly radio channel configurations and availability (FR/HR).

In the case of handover, the BSC has the option to change the speech coder at the time of handover. For intra-BSC handover, the BSC uses the previously stored information from the MSC/VLR about the speech coder preferences in the selection process.

BSC forwards the information of codec type to BTS in the channel activation message. BTS uses this information to configure activated RTSL to support either conventional full rate codec or EFR. Inband signalling between transcoder and BTS is used to control the transcoder codec selection on a call basis.

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Parameters related to Half / Full Rate

BSC: lower limit FR TCH resourcesbtsLoadDepTCHRate

(HRU)(BSC)(0..100)

upper limit FR TCH resources btsLoadDepTCHRate

(HRU)(BSC)(0..100)

BTS: lower limit FR TCH resources btsLoadDepTCHRate

(HRU)(BSC)(0..100)

upper limit FR TCH resources btsLoadDepTCHRate

(HRU)(BSC)(0..100)

tch rate intra cell ho tchrateIntraCellHo

(TRIH)(BTS)(0..4)

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10.5 Adaptive Multi Rate

Adaptive Multi Rate Codec (AMR) introduces a new set of codecs and adaptive algorithm for codec changes and thus can provide significantly better speech quality and more capacity.

13 BSSPAR-Voice & Channel Coding/ V3 © NOKIA 2006


7.4

5.9

5.15 4.75

12.2

7.95

5.9

4.75

HO

HO

Current Full Rate & AMR Half Rate

AMR Full Rate

Base Station Cell Coverage

HO

Adaptive Multi RateOverview

• New GSM standard based voice coding in phone and network. Implementation in radio network and phone is needed to get benefit out.

• AMR has Full Rate and Half Rate options. When network has high load it uses dynamically AMR Half Rate code and accommodates two calls to onetime slot.

• AMR increases network voice capacity because AMR half rate has same radio performance as normal full rate.

Network chooses optimum AMR Speech and Channel coding

0

5

10

15

20

25

FR12.2

FR10.2

FR7.95

FR 7.4

FR 6.7

FR 5.9

FR5.15

FR4.75

HR7.95

HR 7.4

HR 6.7

HR 5.9

HR5.15

HR4.75

AMR codec mode

Ch

an

ne

l bit

-ra

te (

kb

it/s

)

Channel coding

Speech coding

Figure 8-6 Adaptive Multi Rate

With AMR we can achieve very good speech quality in full rate (FR) mode even in low C/I conditions or increase the speech capacity by using the half rate (HR) mode and still maintain the quality level of current FR calls. Optimal interworking with power control and handover algorithms together with enhanced quality measurements (FER Measurement feature) will provide full benefits and interworking with prior Nokia top-of-the-world capacity features including Intelligent Frequency Hopping (IFH).

The Adaptive Multi Rate (AMR) codec consists of a family of codecs (source and channel codecs with different trade-off bit-rates) operating in the GSM FR and HR channels.

The idea behind the AMR codec concept is that it is capable of adapting its operation optimally according to the prevailing channel conditions.

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10.5.1 Generic AMR description

AMR consists of 8 different speech codec modes with total of 14 channel codec modes, which are listed, in the following Table:

Channel

Code

Channel

codec

code

Source coding

bit-rate, speech

Net bit-

rate, in-

band

channel

Channel

coding

bit-rate,

speech

Channel

coding

bit-rate, in-

band

CH0-FS 12.20 kbit/s (GSMEFR)

0.10 kbit/s 10.20 kbit/s 0.30 kbit/s

CH1-FS 10.20 kbit/s 0.10 kbit/s 12.20 kbit/s 0.30 kbit/s


TCH/FR CH3-FS 7.40 kbit/s (IS-641)






CH8-HS 7.95 kbit/s (*) 0.10 kbit/s 3.25 kbit/s 0.10 kbit/s

TCH/HR CH9-HS 7.40 kbit/s (IS-641)


CH10-HS 6.70 kbit/s 0.10 kbit/s 4.50 kbit/s 0.10 kbit/s




Table 1. Channel and speech codec modes for AMR

(*) Requires 16 kbit/s TRAU. Therefore it is not seen as a feasible codec mode and will not be supported by Nokia BSS10.

Codec mode adaptation for AMR is based on received channel quality estimation in both MS and BTS, followed by a decision on the most appropriate speech and channel codec mode to apply at a given time. In high-error conditions more bits are used for error correction to obtain error robust coding, while in good transmission conditions only low amount of bits is needed for sufficient error protection and more bits can therefore be allocated for source coding.

An inband-signalling channel is defined for AMR that enables the MS and the BTS to exchange messages on applied or requested speech

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and channel codec modes. The above mentioned selected speech codec mode is then sent, by using the inband signalling channel, to the transmitting side, where it is applied for the other link. BTS commands the MS to apply a particular speech codec mode in the uplink, but MS can only request BTS to apply a particular speech codec mode in the downlink because BTS has an option to override the MS's request.

MS shall support all speech codec modes, although only a set of up to 4 speech codec modes is used during a call. BSC supports all of speech codec modes, except 7.95 kbit/s on HR channel, and it has one default set for each channel mode. The default codec sets include also a default set of decision thresholds and hysteresis. The initial codec mode and codec set with thresholds and hysteresis are transferred between network elements and MS by using the existing layer 3 signalling, i.e. the basic principles for EFR are reused. Only a few add-ons are needed.

The AMR system makes use of the inband signalling for the link adaptation, for each codec mode set there is an associated set of decision thresholds for mapping the channel quality measurements to the Mode Commands/Requests. The presents the summary of all AMR parameters. The detailed description of parameters can be found later in the training material. During the AMR session of this training we talk about the most important AMR parameters. If needed Nokia offers a special 1 day AMR course.



2G AMR Parameters2G AMR Parameter Relationships

AMRConflnhandovers1= Current Multirate Config Pref0= Target

AMRsetgradeEnableY= Downgrade & Upgrade

SlowAMRLAEnabledN= 40 ms (ETSI)Y= 480 ms (Nokia)

CODECModeSetCodec x4

InitCodecMode0= Second most robust1= Start mode

StartMode= 1 of 4 codecs to start

FRT1 and HRT1C/I threshold1

IHRFAMR HANDOVER(Intra cell HO-Channel Mode)

QDRF and QDRHThreshold DL RXQualAMR

ARLT= Radio Link Timeout for AMR calls

HRU (BSC) or FRU (BTS)

If Qual>IHRFthen FR->HR (Packing)

LINK ADAPTATION

PACK AND UNPACK

HO THRESHOLDS

CODEC INITIAL SELECTION

HRI=1=ACTUALTchRate InternalHO

12.2

7.4

5.9

4.75

GENERAL

CODEC HO SETTINGS

BTS LOAD

TRIH=0TCHRateIntra-cellHO

CHANNEL TYPE FOR INTRA and INTERNAL HOs

InitAMRchannelrate1= any rate2= AMR FR only

FRT2 and HRT2C/I threshold2

FRT3C/I threshold3

FRH1 and HRH1C/I threshold1

FRH2 and HRH2C/I threshold2

FRH3C/I threshold3

HRL (BSC) or FRL (BTS)

IHRHAMR HANDOVER(Intra cell HO-Channel Mode)

If IHRH<Qualthen HR->FR (Unpacking)

LDRF and LDRHPC Lower Threshold DLRXQual AMR

POWER CONTROL

LURF and LURHPC Lower Threshold ULRXQual AMR

UDRF and UDRHPC Upper Threshold DLRXQual AMR

UURF and UURHPC Upper Threshold ULRXQual AMRQURF and QURH

Threshold UL RXQualAMR Unpacking Level Threshold

for AMR HR

Lower Quality Limit to Prevent Interference and AMR Intracell HO

Lower Rx level Limit for Quality Based Intra Cell HO

Figure 8-7 2G AMR Parameters

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10.5.2 Link adaptation

Link adaptation is the capacity of AMR feature to vary the codec used according to the link conditions. Both networks, for uplink, and MS, for downlink, measure the radio conditions in each link and decide which codec to apply to each way. Two different types of link adaptation algorithms are defined: codec mode adaptation and channel mode adaptation. The channel mode adaptation algorithm decides whether speech can be handled by a full rate channel or by a half rate channel according to the link conditions, whereas for the channel selected, the codec mode adaptation algorithm decides which codec is the one that provides the best speech quality for the current radio conditions. Because each codec has a different channel protection and speech encoding performance, the idea of the codec mode adaptation is to select the codec that provides the best speech quality for the radio conditions that the receivers are submitted to.

10.5.2.1 Codec mode adaptation

Default AMR codec sets for FR and HR

The AMR set includes AMR codecs, their threshold and hysteresis values and initial codec mode definition. The basic AMR set for FR channel on BSC:



Definition of the Active Codec Set

ACS (Active Codec Set) which defines the codecsthat can be used in a BTS during a call

• amrConfigurationFr: codecModeSet (FRC)

• amrConfigurationHr: codecModeSet (HRC)

• Maximum of 4 codecscan be included in ACS, although it can be less (or even disable)

Values Range: 0..240 (0 or 1-4 values from these: 1, 2, 4, 8, 16, 32, 64 & 128) 0 = disabled 1 = 4.75 kbit/s 2 = 5.15 kbit/s 4 = 5.90 kbit/s 8 = 6.70 kbit/s 16 = 7.40 kbit/s 32 = 7.95 kbit/s 64 = 10.2 kbit/s 128 = 12.2 kbit/s

Range: 0..30 (0 or 1-4 values from these: 1, 2, 4, 8 & 16) 0 = disabled 1 = 4.75 kbit/s 2 = 5.15 kbit/s 4 = 5.90 kbit/s 8 = 6.70 kbit/s 16 = 7.40 kbit/s

Example (default) 1001 0101 = (4.75, 5.90, 7.40 & 12.2) 0001 0101 = (4.75, 5.90 & 7.40)

BTS AMR FR AMR-HRTalk Family AFS 475, AFS590, AFS740, AFS122 AHS475, AHS 590, AHS 740Metro & Ultrasite All codecs All exepct AHS790

Codecs supported in different BTS models

Figure 8-8 Definition of the Active Codec Set

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There are two link adaptation (LA) modes; the Standard specified fast LA and the Nokia proprietary slow LA. Fast LA BTS allows inband codec mode change on every other TCH frame, but in Nokia proprietary, slow LA BTS allows inband codec mode change only on SACCH frame interval. During both LA modes, BTS indicates the first and the last used codec during the last measurement interval and the average quality. The codec mode can be switched one up or one down at the time so that it is not possible to switch from the mode 12.2 kbit/s to 4.75 kbit/s when for example the modes 5.9 kbit/s and 7.4 kbit/s are included to the mode set. Also, it should be noted that codec changes do not take place immediately after the Codec Mode Command/Request is sent: there is a delay until a frame is received with the new codec.



Codec ModeAdaptation in AMR

• Codec Mode Adaptation is the capability of AMR feature to vary the codecused according to the link conditions

• Both network, for uplink, and MS, for downlink, measure the radio conditions in each link and take decisions on which codec should be applied toeach way AMR codec mode adaptation is done independently in UL and DL

• There are two modes; the ETSI specified fast LA and the Nokia proprietary slow LA

• slowAmrLaEnabled: if it is set to "N" (default) it is used ETSI fast LA; if it is set to "Y" it used Nokia slow LA

• With slow LA, BTS allows in-band codec mode changes only on the SACCH frame interval of 480 ms and this option give better flexibilitywith HO & PC algorithms

• Two different typesof link adaptation algorithms are defined:

• Codec Mode Adaptation

• Channel Mode Adaptation

Figure 8-9 Codec Mode Adaptation in AMR

Codec mode adaptation operates independently on the uplink and downlink. It is transparent to the channel allocation and operates without of it. Control depends mainly on measurements of the quality of the respective links. The parameters amrConfigurationFr:threshold and amrConfigurationFr: hysteresis define when to switch between codecs. The Figure 8-11 below illustrates the codec change procedure.

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• When deploying AMR the following parameters are important for the Link Adaptation:

•ACS(Active Codec Set) which defines the codecs that can be used ina BTS during a call

•Thresholdsused: Defines the CIR value to change the codec from a less robust codec to the immediate more robust one in the ACS

•Hysteresis: the values in dB to add to the thresholds in order to go from a robust codec to the immediate less robust one in the ACS.

Parameters for Codec Mode Adaptation

Figure 8-10 Parameters for codec mode adaptation



Codec mode adaptationThreshold and hysteresis

C/I estimation

FR4.75 (codec 1)

FR5.9 (codec 2)

FR7.4 (codec 3)

FR12.2 (codec 4)

4dB

FRT17dBFRT2

11dBFRT3

Codec mode

5dB 8dB 12dB

1dBFRH1

1dB

FRH2

1dBFRH3

Figure 8-11 Codec Mode Adaptation

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10.5.2.2 Channel mode adaptation

The channel mode (FR or HR) is switched to achieve the optimum balance between speech quality and capacity enhancements. The uplink and downlink use the same channel mode. The channel mode is selected by the network based on measurements of the quality of the uplink and downlink.



• Channel Mode Adaptation is an HO algorithm that aims at select the correct channel rate (FR or HR)

•The selection of the channel rate depends on 2 main factors: load and quality

Channel Mode Adaptation

loadload Good QualityGood Quality

CodecCodec

FRFR HRHR

HRHRFRFRBad Bad

QualityQuality

packingpacking

unpackingunpacking

Only if both AMR FR & HR

are active in the cell

Figure 8-12 Channel Mode Adaptation

Handovers between AMR FR and AMR HR

New RxQual HO thresholds are specified for AMR FR and AMR HR and they are taken into account when making intra-cell handovers between AMR FR and AMR HR:

intra HO threshold Rx qual for AMR FR

intra HO threshold Rx qual for AMR HR

Current Nx and Px values of RxQual thresholds are used.

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Packing of AMR FR calls to AMR HR calls due to cell load

Spontaneous packing of AMR FR calls to AMR HR calls is triggered when the cell load is high enough, i.e. the number of free full rate resources reduces below the value of the parameter Lower limit for FR TCH resources (according to the BTS level parameter, if it contradicts with the BSC level parameter). Packing continues until the cell load is low enough, i.e. the number of free full rate resources increases above the value of the parameter Upper limit for FR TCH resources (according to the BTS level parameter, if it contradicts with the BSC level parameter). Spontaneous packing is triggered by any new channel allocation. BSC keeps record of the AMR FR, AMR HR calls per BTS, and corresponding counters are updated during channel allocations and releases.

After a new channel allocation, BSC makes a request to perform an intra-cell HO for N amount of calls. N is defined by BSC and it follows the principle that the number of free full rate resources increases by one compared to the situation before the new channel allocation. Packing request is done with a new unacknowledged procedure. BSC performs the ordered HOs for FR AMR calls, whose quality is above the intra HO threshold Rx qual for AMR FR and which uses the least robust codec mode.

A packing request is valid until it is overwritten by a new one. A packing request, which indicates the amount N as 0, is used to remove any pending packing requests.



Channel mode adaptation: Packing

• Packing between AMR FR and AMR HR is intra-cell handover

• Spontaneous packing of FR AMR calls to HR AMR calls is triggeredwhen the cell load is high enough, the number of free full rate resources reduces below the value of the parameter btsLoadDepTCHRate(HRL or FRL).

• Packing continues until the cell load is low enough, the number of free full rate resources increases above the value of the parameter btsLoadDepTCHRate(HRU or FRU).

Free FR TCHs

Time

Upper limit for free FR TCHsbtsLoadDepTCHRate(HRU)

Lower limit for free FR TCHsbtsLoadDepTCHRate(HRL)

No packing ofAMR FR calls

Packing ofAMR FR calls

No packing ofAMR FR calls

Figure 8-13 Channel Mode Adaptation: Packing

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Unpacking of AMR HR calls to AMR FR calls due to call quality

Spontaneous unpacking of AMR HR calls to AMR FR calls is triggered when the quality of a AMR HR call degrades below the intra HO threshold Rx qual for AMR HR. Cell load does not have an effect.

When Rx level is good, BSC performs intra-cell HOs for AMR HR calls one by one according to the new threshold. Otherwise inter-cell HOs are performed according to the current threshold parameters. See Figure 8-14 and Figure 8-15

AMR FR and AMR HR call counters of BSC are again updated during channel allocations and releases.



Channel mode adaptation:Unpackingcase 1

• HoThresholdsQualUL(quality HO) = amrHandoverHR(unpacking) -> call will unpack if there is no suitable neighbor

When RXQUALITY reaches the set QHO threshold and there is no target cell the call will unpack to FR.

If there is a suitable target cell the call hands offand the rate is based on IAC and load.

Figure 8-14 Channel Mode Adaptation: Unpacking, Case 1

In the Case 1 the threshold for quality HO and (UL/DL) and threshold for unpacking are set the same. When the threshold is reached but there is no suitable target cell to make a handover the call will be unpacked to FR. If there is a suitable target cell the call is handed over to the new cell. The rate is based on the initial channel rate and load.

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Channel mode adaptation:Unpackingcase 2


• HoThresholdsQualUL(quality HO) > amrHandoverHR(unpacking) -> RXQUALITY weakens slowly -> the call will unpack

RXQUALITY reaches the unpacking threshold first and triggers unpacking. The call goes from HR to FR. Later, RXQUALITY reaches QHO threshold and QHO is initiated.

If there is a suitable target cell the call hands offand the rate is based on IAC and load. Otherwise it remains in the old cell.

Figure 8-15 Channel Mode Adaptation: Unpacking, Case 2

In the Case 2 the threshold for quality HO (UL/DL) is higher than the threshold for unpacking. When the unpacking threshold has been reached the call goes from HR to FR. After this, if the quality HO threshold has been reached a handover to a new cell will be performed. The rate is based on the initial channel rate.

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08 Voice Channel Coding Wk v3.1