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AMR by Qasim

Apr 10, 2018

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    Basics

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    Existing Speech CODECS

    Currently there are two speech coding schemes

    used in GSM.

    FR - Full Rate (The original coding scheme)

    EFR - Enhanced Full Rate ( An enhancement of FR)

    EFR offers better quality speech.

    Most modern mobiles are EFR capable.

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    Why change from what weve got ?

    The traditional fixed coding schemes (FR or EFR) use afixed amount of bandwidth - some dedicated to encode thespeech and the rest used to offer data protection - ForwardError Coding, Checksums etc.

    When poor RF conditions exist, there may be errors in thespeech data which cannot be corrected or recovered. Theerrors will be heard as an interruption in the speech.

    With EFR or FR, we can only achieve a best effort with thecoding scheme and protection available - AMR offers someimprovements.

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    What is AMR

    Adaptive Multi-rate (or AMR) Codec in GSM is a new variablerate voice coding algorithm/technique that increases capacityand improves voice quality while reducing cost.

    AMR allows multiple different coding schemes to be used.

    The higher speech bandwidth schemes offer good speechquality - with lower error protection. The lower speech

    bandwidth schemes offer slightly lower speech quality - with

    much higher error protection.

    4.75kbps 5.15kbps 5.9kbps 6.7kbps 7.4kbps 7.95kbps 10.2kbps 12.2kbps

    Better Protection

    Better Speech Quality

    EFR FR

    13kbps

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    PRINCIPLE

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    Principle of AMR

    In EFR the vocoder operates at fixed rate of 13 Kbps

    with 22.8 Kbps global rate irrespective of radio

    condition (C/I)

    In AMR there are Multiple Codecs with multiplemodes offering variable levels of protection and source

    coding depending on radio condition

    We can easily understand from the following slides.

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    AMR HR AMR FR

    Global Rate : 11.4 kb/sGlobal Rate : 22.8 kb/s

    Good C/I

    Bad C/I

    Bad C/I

    Good C/IG

    lobal Rate: 22.8 Kbps

    EFR

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    Principle Contd.

    When the transmission is good, a high rate

    vocoder is chosen and the number of bits

    dedicated to the channel protection is low.

    In case of degraded radio conditions, the

    vocoder rate is decreased, in order to provide

    a better channel protection and allow a better

    voice quality

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    AMR FR and AMR HR

    AMR can operate in FR or HR - this is called Channel Mode

    Within HR or FR domain there is a set of Voice Codec- Codec

    Mode

    AMR FR provides better tolerance to interference and thus extra

    coverage and VQ. AMR HR provides extra capacity by allowing two users in a TS.

    Source coding

    Channel coding

    AMR HR AMR FR

    Global Rate : 11.4 kb/s

    Global Rate : 22.8 kb/sGood C/I

    Bad C/I

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    AMR Channel and Speech CodecChannel

    mode

    Channel

    codec

    Mode

    Source coding

    bit-rate, speech

    Net bit-rate,

    in-band

    channel

    Channel

    coding

    bit-rate,speech

    Channel

    coding

    bit-rate, in-band

    CH0-FS 12.20kbit /s (GSMEFR) 0.10 kbi t/ s 10.20 kbi t/ s 0.30 kbi t/ s

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

    CH2-FS 7.95 kbit /s 0.10 kbit /s 14.45 kbit /s 0.30 kbit /s

    TCH/FR CH3-FS 7.40 kbi t/ s (IS-641) 0.10 kbi t/ s 15.00 kbi t/ s 0.30 kbi t/ s

    CH4-FS 6.70 kbit /s 0.10 kbit /s 15.70 kbit /s 0.30 kbit /s

    CH5-FS 5.90 kbit /s 0.10 kbit /s 16.50 kbit /s 0.30 kbit /s

    CH6-FS 5.15 kbit /s 0.10 kbit /s 17.25 kbit /s 0.30 kbit /s

    CH7-FS 4.75 kbit /s 0.10 kbit /s 17.65 kbit /s 0.30 kbit /s

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

    TCH/HR CH9-HS 7.40 kbi t/ s (IS-641) 0.10 kbi t/ s 3.80 kbi t/s 0.10 kbi t/ s

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

    CH11-HS 5.90 kbit /s 0.10 kbit /s 5.30 kbit /s 0.10 kbit /s

    CH12-HS 5.15 kbit /s 0.10 kbit /s 6.05 kbit /s 0.10 kbit /s

    CH13-HS 4.75 kbit /s 0.10 kbit /s 6.45 kbit /s 0.10 kbit /s

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

    supported by Nokia BSS10.

    In high-error conditions more bits are used for error correction to obtain error robust

    coding, while in good transmission conditions a lower amount of bits is needed for

    sufficient error protection and more bits can therefore be allocated for source coding

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    CODE

    C RATE

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    AMR Codec Rates

    Up to 4 codec modes are allowed in AMR at any one time - This isknown as the Active Codec set.

    The Active codec set may contain something like :-

    12.2, 7.95, 5.9 and 4.75 kbps.

    AMR switches between the 4 rates - independently on the Uplink and

    the Downlink - based on the prevailing RF conditions on each link.

    Under good RF conditions - a Rate of 12.2 kbps may be used which issimilar to EFR. Under poor conditions 4.75 kbps may be used - offeringlower quality speech - but higher protection against errors.

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    C C C K

    [> ] 10.2

    [ ] 6.70

    [ ] 5.90

    [< ] 4.75

    [> ] 6.70

    [ ] 5.90[< ] 4.75

    FR

    HR

    Best performing codecs

    Codecs providing the most continuity Codecs with most efficient implementation

    Codecs with most capacity benefits

    Available AMR CODECs

    C C R~ M R

    10.2

    6.70

    5.90

    4.75

    6.70

    5.90

    4.75

    R

    HR

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    AMR Codec Rate Switching

    AMR will switch modes within the active codec set, based on C/I

    and RxQual calculations.

    C/I

    12.2

    kbps

    7.95

    kbps

    5.9 kbps

    4.75

    kbps

    Hysterisis

    Hysterisis

    Hysterisis

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    Adaptation : principle Principle : the MS and the BTS evaluate the C/I of each frame, compare this

    value to a set of thresholds and deduce an appropriate codec mode.

    C/IAMR FR

    t

    4.75

    5.9

    6.7

    10.2

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    ADAPTATION

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    Codec Mode Adaptation

    At call setup, the mobile (MS) is assigned an AMR Half Rate or Full Rate channel

    based on

    y Mobile capabilities

    y Network capabilities

    y Radio conditions

    Mobile is given information about

    y Codec mode set

    y Initial codec mode

    y Thresholds and hysteresis

    During the communication

    y Mobile and network compare assigned codec to C/I

    y Requested and applied codec info is exchanged

    y Mobile is told which codec to use for uplink

    y Network makes decision on which codec to use for downlink and MS sends its

    recommendation based on DL measurements

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    Codec Mode Adaptation

    The purpose of AMR codec mode adaptation is to provide the "best" compromisebetween data rate of codec mode and channel protection, according to the link

    quality. This adaptation is done for uplink and downlink and there is no

    interdependence between the 2 links, but both sets of codec have to be identical.

    Each 40ms, according to the requested codec mode and the applied codec mode,

    the BTS :

    increases by one step the rate of the codec mode, if the requested codec

    mode (CMR) is greater than the applied codec mode,

    decreases by one step the rate of the codec mode, if the requested codec

    mode (CMR) is lower than the applied codec mode,

    keeps the same codec mode, if the requested codec mode (CMR) is equal tothe applied codec mode.

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    For each mobile, the following set of parameters has to be defined : one threshold per codec mode and per link (4+4 in FR and 3+3 in HR

    channel for UL and DL),

    one hysteresis (the same value is used for each codec mode, but one for

    FR and another one for HR channel).

    These parameters are linked to a set of factors, some of them being

    determined by the BTS (frequency hopping, MS speed), others being

    network dependent (environment profile).

    The operator using one parameter (AMRAdaptationSet on bts object)

    and the BSS using the TS configuration and the MS speed applies theappropriate set.

    Codec Mode Adaptation

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    PARAMETER

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    Basic Parameter AMR

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    Parameter Description

    AMR FR and AMR HR are activated by changing BSC exchange properties. Transcoder pools for

    AMR FR and AMR HR must also be defined in order for AMR to work. Note if AMR HR shall beactivated the feature Half Rate Channels must have been purchased by the operator.

    AMRFRSUPPORT

    Indicates if AMR FR is turned ON or OFF and also which full rate codec set that shall be used

    within the BSC.

    AMRHRSUPPORT

    Indicates if AMR HR is turned ON or OFF and also which half rate codec set that shall be used

    within the BSC.

    Codec sets 3 and 4 for both FR and HR channels can be defined per BSC using the command

    RLADC. The following parameters are set using this command:

    SET :Indicates which of the codec sets for a specific channel rate that shall be modified.

    MODE :Specifies the codec modes that shall be included in the codec set. For Full Rate all 8

    codec modes can be used, expressed as a numeral between 1 and 8, and defined in ascending

    order.

    1 = 4.75 kbps

    2 = 5.15 kbps to8 = 12.2 kbps

    For Half Rate only the 5 lower codec modes can be used.

    1 = 4.75 kbps to

    5 = 7.4 kbps

    THR :Specifies the thresholds between the codec modes in the codec set, expressed as a

    numeral between 0 and 63, in steps of 0.5 dB.

    HYST :Specifies the hysteresis values for the threshold in the codec set. It is expressed as anumeral between 0 and 15, in steps of 0.5 dB.

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    EFF

    ECTS AND B

    ENE

    FIT

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    AMR effect on HO

    RXLEV and Power Budget HO parameters are identical for AMR and EFR

    AMR call would handover at the same point as an EFR call.

    Separate RXQUAL threshold settings for AMR

    Default set to worse values than EFR. (e.g. EFR =4, AMR = 5)

    With these default settings AMR calls would be expected to have fewer HO

    due to quality

    No difference in RXQUAL measurement method between EFR and AMR

    EFR call and AMR call in identical location should show identical RXQUAL

    measurements

    Packing/Unpacking

    Unpacking from HR to FR is always based on RX quality

    In congested cell with no available TS for unpacking, Inter-cell HO required

    based on RXQUAL.

    Improved robustness in AMR over EFR

    AMR better able to handle poor radio conditions - low RXLEV, poor RXQUAL,

    low C/I

    Optimization of separate AMR parameters is important to ensure no negative

    impact to HO. Different environments will need different parameter settings to

    optimise the performance. Other AMR parameters should have no negative

    impact in a network designed for EFR

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    AMR effect on KPIs

    The AMR feature itself will not impact the individual connections DCR, but it will

    affect the overall system DCR since the interference generated in the network is

    lower due to the AMR power control settings.

    Radio Link Timeout can be adapted to AMR in order for dropped calls to

    maintain the same correlation with voice quality degradation as with EFR

    Quality -> better perceived speech quality.

    Traditionally, GSM voice quality has been usually benchmarked based on BERmeasurements

    With Frequency Hopping and AMR, BER becomes increasingly meaningless and

    therefore alternative indicators are needed to benchmark the voice quality

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    AMR effect on Capacity & Coverage

    AMR provides a significant performance enhancement that can be translated into atradeoff between quality and capacity

    AMR feature impact and deployment strategy depends strongly upon the AMR capablemobile penetration

    AMR gains:

    y Quality -> AMR maintains good speech quality in the situation where the connectionfaces low C/I or low signal level. Also due to retransmissions schemes used by thesechannels the probability of signaling success maintain very high even for very degradedconditions

    y Capacity -> HR utilization doubles the hardware capacity of the cell since two half-rateconnections can be allocated to fill only one timeslot. Practically the gain is up to 150%higher capacity for the same quality

    y Coverage -> additional 3-4 dB effective coverage

    y Cost (HR hardware efficiency) -> 20-40% lower number of TRXs

    y

    Improved BCCH plan: tighter frequency reuse or better quality with same frequencyreuse, potentially releasing frequencies to be used on the non-BCCH layer

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    AMR Benefit

    AMR Half Rate nearly doubles capacity

    Increased Spectral Efficiency:AMR codec allows for more aggressivefractional frequency reuse solutions

    Improved Voice Quality:AMR FR has 4 - 6 dB of extra tolerance as

    compared to EFR. This can result in improved in-building penetration as well

    as better coverage at the cell-edge at least from a speech quality point of

    view*.

    Reduced Hardware Cost:AMR Half Rate modes increase capacity perradio without adding new equipment or sites.

    Capacity gain is the highest when both AMR FR and AMR HR modesare used in conjunction.

    AMR HR allows higher capacity by allowing 2 users per TS under good

    C/I conditions

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    What AMR brings

    to the operator & to the subscriber

    Even from here,

    I still can hear you!(AMR FR)

    Can you repeat ?

    (Standard HR)

    Hi, I am listening to you

    (Standard EFR)

    Hi, I am listening to you

    (AMR HR)

    Best voicequality

    Best indoorpenetration

    Double capacity onpart of the cell

    Standard

    Cell limit

    AMRExtended

    Cell limit

    Same voice qualityas Std EFR

    -

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    THANK YOU