103332667 HSDPA Principles

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Transcript of 103332667 HSDPA Principles

  • HSDPA Principles Seminar

    Corrado Carbone - RO/QoS South

  • 2

    Agenda

    3. Channel Structure

    2. Architecture

    1. Overview

    5. InterFrequency Mobility Principles

    6. Capacity Management

    4. Accessibility & Mobility Principles

    7. KPIs

  • 3

    Outline

    Lets get used with the main concepts of the HSDPA.

    HSDPA introduces new technologies in the UMTS world and consequently a new way to manage PS traffic.

    The most important impacts of it are on:

    Modulation

    Retransmission schemes

    Scheduling

    Usage of Power and Code

    This presentation reports an overview of the main issues to take in mind.

  • 4

    HSDPA Basic Principles

  • 5

    1 - Shared Channel Transmission (1/2)

    Shared-channel transmission implies that a certain amount of radio resources of a cell (codes and power) is seen as a common resource that is dynamically shared between users.

    The idea is that a part of the total downlink code resource is dynamically shared between a set of packet-data users, primarily in the time domain.

    The codes are allocated to a user only when they are actually to be used for transmission, leading to efficient code and power utilization.

    For P4 only 5 codes (SF = 16) will be available for the HSDPA feature and they will be shared on a time base.

    Channelization codes allocated

    for HS-DSCH transmission

    5 codes (example) SF=16

    SF=8

    SF=4

    SF=2

    SF=1

    User #1 User #2 User #3 User #4

    TTI

    Shared

    channelization

    codes

  • 6

    1 - Shared Channel Transmission (2/2)

    The Shared-channel transmission allows:

    Higher peak bit rate: all the resource can be allocated to a single user in case of low load.

    Better application performance being closer to the model TCP has being designed for.

    More efficient utilization of available code resources compared to the use of a dedicated channel, i.e. reduced risk for code-limited downlink.

    The Shared-channel transmission impacts:

    Scheduling become more complex

  • 7

    2 - Short 2 ms TTI (1/2)

    The Transmission Time Interval becomes extremely short in HSDPA; 2 ms compared to the 10 ms used by R99 high bit rate radio bearer.

    The HS channels are organised in sub-frame of 3 slots each; this means that the slot time 2/3 ms/slot is the same as for R99 slots (10/15 ms/slot).

    The scheduling and the link adaptation algorithms work at this frequency!

    10 ms

    20 ms

    40 ms

    80 ms

    Earlier releases

    2 ms

    Rel 5 (HS-DSCH)

    2 ms

  • 8

    2 - Short 2 ms TTI (2/2)

    The shorter TTI allows:

    Reduced air-interface delay: this is required by the the TCP at high data rates to Improved end-user performance

    The shorter TTI is necessary to benefit from other HSDPA features:

    Fast Link Adaptation

    Fast hybrid ARQ with soft combining

    Fast Channel-dependent Scheduling

    2 ms

  • 9

    3 - HSDPA Power Allocation

    HS-DSCH allocated power is decided by the RNC, prioritizing the DCH channel

    HS-DSCH adjusts the data rate to match the instantaneous radio conditions and the available transmission power in the RBS

    No closed loop power control is specified for HS-DSCH, unlike the DCH channel

    The system adjusts the data rate by

    varying the effective code rate

    changing the modulation scheme

    This leads to a higher efficiency in the usage of power.

    Dedicated channels (power controlled)

    Common channels

    Power usage with dedicated channels

    channels

    t

    Unused power

    Power

    HS-DSCH with dynamic power allocation t

    Dedicated channels (power controlled)

    Common channels

    HS-DSCH (rate controlled)

    To

    tal

    ce

    ll p

    ow

    er

    Power

    To

    tal

    ce

    ll p

    ow

    er

    3GPP Release 99 3GPP Release 5

  • 10

    Link Adaptation

    Available Power

    4 - Fast Link Adaptation (1/3)

    The target for the link adaptation is to select a TFRC (Transport Format and Resource Combination) resulting in transmitting an as large transport block as possible with a reasonable error probability.

    Channel Condition Coding

    Modulation

    TFC

    Bit

    Rate UE category

    Traffic (buffers state)

  • 11

    4 - Fast Link Adaptation (2/3)

    Adjust transmission parameters to match instantaneous channel conditions

    HSDPA: Adapt on 2 ms TTI basis the Rate (constant power)

    Adaptive coding

    Adaptive modulation (QPSK or 16QAM)

    Link adaptation is implemented by allowing the MAC-hs to set the TFRC (Transport Format and Resource Combination) independently for each 2 ms HS-DSCH TTI

    High data rate

    Low data rate

    feedback

  • 12

    4 - Fast Link Adaptation (3/3)

    In order to estimate current channel conditions, an estimate of the Channel Quality is reported by the UE to RBS (CQI).

    Based on the channel conditions and the available power, the network will select the Transport Format to have the maximum throughput achievable

    High data rate

    Low data rate

    feedback

  • 13

    Scheduling = which UE to transmit to at a given time instant

    There is a main tradeoff to choose between:

    fairness vs. cell throughput

    5 - Fast Channel-dependent Scheduling (1/2)

    Every user has the

    same rights to

    access the resource

    The user with better

    radio condition

    transmit more

    User1 User2 User3 User4

    time 2 ms 2 ms

  • 14

    5 - Fast Channel-dependent Scheduling (1/2)

    2 opposite strategies are:

    Round Robin: radio resources are allocated to communication links on a sequential basis.

    Proportional Fair: transmit at fading peaks. This may lead to large variations in data rate between users.

    high data rate

    low data rate

    Time

    #2 #1 #2 #2 #1 #1 #1

    User 2

    User 1

    Scheduled

    user

  • 15

    6 - Fast Hybrid ARQ with Soft Combining (1/2)

    HSDPA introduces a new retransmission level under the RLC scheme in the RNC.

    This new level allows rapid retransmissions of erroneous data:

    Hybrid ARQ protocol terminated in RBS short RTT (typical example: 12 ms)

    Soft combining in UE of multiple transmission attempts reduced error rates for retransmissions

    P1,1

    P1,1

    P1,2

    P1,2

    P2,1

    P2,1

    P2,2

    P2,2

    P3,1

    P1,1 P2,1 P3,1

    + +

    Transmitter

    Receiver

  • 16

    6 - Fast Hybrid ARQ with Soft Combining (2/2)

    A fundamental difference between conventional ARQ (used in RLC) and HARQ is that:

    in the latter case received data blocks that cannot be correctly decoded are not discarded but buffered

    They are soft combined with later received retransmissions of the same set of information bits.

    Finally, decoding is applied to the combined signal.

  • 17

    7 - UE capabilities

    The UE capabilities are divided into a number of parameters:

    Total RLC AM and MAC-hs buffer size

    Maximum number of HS-DSCH transport channel bits received within a HS-DSCH TTI

    Support of HS-PDSCH Yes/No

    Maximum number of HS-DSCH codes received

    Total number of soft channel bits in HS-DSCH

    Minimum inter-TTI interval in HS-DSCH

    Supporting 16QAM

    These physical layer UE capabilities can be translated in a limit on the requirements for 3 different UE resources:

    the de-spreading resource (codes decoded in parallel)

    the soft buffer memory used by the hybrid ARQ functionality

    the turbo decoding speed (the maximum number of transport channel bits received within an HS-DSCH TTI and the minimum inter-TTI interval).

  • 18

    Throughput level: UE type cat 12

    There are several levels for throughput calculation: lets clarify!

    The biggest MAC-HS transport block size is 3440 including HS header and padding bits:

    X 10 = + + = 3440 bits

    RLC SDU

    = 320 RLC

    head = 16

    MAC- HS SDU

    head = 3360

    MAC- HS

    head = 21

    Padding

    bits = 59

    That means the DSCH max scheduled bit rate could be 1720 kb/s: That is including headers, padding and every type of retransmission

    This is the level used by the RBS counters and Couei!

    This corresponds to a max RAB bit rate of 1600 kb/s =320*10/2

    In reality considering at least the HS retransmissions at this level the maximum bit rate could not be higher

    than 1600*0.9 =1440 bit/s

  • 19

    Agenda

    3. Channel Structure

    2. Architecture

    1. Overview

    5. InterFrequency Mobility Principles

    6. Capacity Management

    4. Accessibility & Mobility Principles

    7. KPIs

  • 20

    Protocol stacks (1/4)

    UE RBS SRNC

    PHY

    AAL2

    ATM

    FP MAC-hs

    L1

    RLC

    User Data

    PHY PHY

    AAL2

    ATM

    AAL5

    ATM

    GTP-U

    UDP/IP FP

    RLC

    MAC

    CN

    PHY

    AAL5

    ATM

    GTP-U

    UDP/IP

    User Data

    Uu Iub Iu

    The figure shows the R99 protocol stack.

    Note in particular that MAC is a protocol between the RNC and the

    UE

    MAC

    L1

  • 21