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  • LTE, Long Term EvolutionCarlo Vallati

    C. VallatiUniversity of Pisa, Italy

    LTE, Long Term EvolutionAn introduction

  • LTE, Long Term EvolutionCarlo Vallati

    Introduction LTE (Long Term Evolution) is a standard for wireless

    communication of high speed data for mobile phones and data terminals

    Developed by 3GPP (3rd Generation Partnership Project), a consortium of telecommunication associations formed for defining communication standards

  • LTE, Long Term EvolutionCarlo Vallati

    Introduction LTE is the last major step in mobile radio

    communications A long history of standards have been defined since

    1990

  • LTE, Long Term EvolutionCarlo Vallati

    Introduction Peak Data Rate: target 150 Mbps (downlink) and

    50 Mbps (uplink) (20 MHz spectrum allocation, 2x1 MIMO)

    Throughput: Downlink target is 3-4 times better than HSDPA. Uplink target is 2-3 times better than HSUPA

  • LTE, Long Term EvolutionCarlo Vallati

    Introduction Latency: The one-way transit time for a packet

    traveling from UE to EnB and vice versa shall be less than 5 ms

    Quality of Service: End-to-end Quality of Service (QoS) shall be supported

  • LTE, Long Term EvolutionCarlo Vallati

    Introduction Mobility: The system should be optimized for low

    mobile speed (0-15km/h), but higher mobile speeds shall be supported as well including high speed train environment as special case

    Interworking: Interruption time for shall be less than 300 ms for real time services and less than 500 ms for non real time services

  • LTE, Long Term EvolutionCarlo Vallati

    Architecture

    EPCEvolved Packet Core Network

    E-UTRANUMTS Terrestrial Radio Access Network

  • LTE, Long Term EvolutionCarlo Vallati

    ArchitectureProtocol stack

  • LTE, Long Term EvolutionCarlo Vallati

    PHY Layer

    BS controls and regulates the access to the shared wireless,

    both uplink and downlink

    Standard defines the way the shared wireless means is accessed

  • LTE, Long Term EvolutionCarlo Vallati

    PHY Layer Downlink/Uplink multiplexing:

    v in time (Time Division Duplexing TDD)v in frequency (Frequency Division Duplexing - FDD)

    TDD

    Freq

    uenc

    y

    TIME

    FDD

    Freq

    uenc

    yTIME

    DL

    UL

  • LTE, Long Term EvolutionCarlo Vallati

    PHY Layer The generic radio frame for TDD and FDD has a

    duration of 10ms and consists of 10 sub-frames or TTI with a duration of 1ms

    Each TTI can be assigned for either downlink or uplink transmission

    At the beginning of each TTI the eNodeB broadcasts the allocation of the resources

    1ms

    10 ms

    1ms 1ms 1ms 1ms 1ms 1ms 1ms 1ms 1ms

    Radio Frame

    TTI

  • LTE, Long Term EvolutionCarlo Vallati

    PHY Layer LTE uses OFDMA for the downlink.

    v OFDMA meets the LTE requirement for spectrum flexibility v enables cost-efficient solutions for wide carriers with high

    peak rates It is a well established technology, for example in

    standards such asv IEEE 802.11a/g v 802.16 (WiMax)v Digital Video Broadcast (DVB)

  • LTE, Long Term EvolutionCarlo Vallati

    PHY Layer The frequency spectrum is divided into multiple sub-

    carriers, which are orthogonal to each other

  • LTE, Long Term EvolutionCarlo Vallati

    PHY Layer Orthogonal Frequency-Division Multiple Access

    (OFDMA) is a multi-user version of the popular OFDM digital modulation scheme.

    Multiple access is achieved in OFDMA by assigning subsets of subcarriers to individual users over the time

    Freq

    uenc

    y D

    omai

    n

    Time Domain

    User 1

    User 2

    User 3

    OFDMA

  • LTE, Long Term EvolutionCarlo Vallati

    PHY Layer Resource Block

    Freq

    uenc

    y D

    omai

    n

    Time Domain

    180 kHz = 12 subcarriersSubcarrier spacing = 15 kHz

    1 slot = 0.5 ms =7 OFDM symbols

    1 subframe =1 ms= 1 TTI

  • LTE, Long Term EvolutionCarlo Vallati

    PHY Layer OFDMA time-frequency multiplexing

  • LTE, Long Term EvolutionCarlo Vallati

    PHY Layer From the perspective of the MAC layer, each slot is the

    basic allocation unit, called Resource Block (RB) A subset is dedicated to control signaling:

    v downlink scheduling information, transport format, resource allocation, and Hybrid-ARQ information

    v uplink scheduling grant

    Resource Block

  • LTE, Long Term EvolutionCarlo Vallati

    PHY Layer Link adaptation is already known from HSPA as

    Adaptive Modulation and Coding Modulation and coding for the data channel is not

    fixed, but it is adapted according to radio link quality Each UE provides a feedback about channel quality The amount of bits transmitted on a resource block

    depends on the modulation adopted

  • LTE, Long Term EvolutionCarlo Vallati

    PHY Layer The LTE uplink transmission scheme for FDD and

    TDD mode is based on SC-FDMA (Single Carrier Frequency Division Multiple Access)v SC-FDMA signal processing has some similarities with

    OFDMA signal processing, so parameterization of downlink and uplink can be harmonized

    v OFDMA in uplink would consume too much transmission power for UEs

  • LTE, Long Term EvolutionCarlo Vallati

    PHY Layer In terms of resource allocation SC-FDMA is similar to

    OFDM signal, butv in SC-FDMA, only contiguous carriers can be allocated

    to the same user

    Freq

    uenc

    y D

    omai

    n

    Time Domain

    User 1

    User 2

    User 3

    OFDMA

    User 1Fr

    eque

    ncy

    Dom

    ain

    Time Domain

    User 1

    User 2

    SC-FDMA

    User 1User 3

    User 3

    User 3

    User 3

    User 3

  • LTE, Long Term EvolutionCarlo Vallati

    MAC Layer LTE MAC defines:

    v format of the messages exchanged in the LTE networkv sequence of control messages for service operations

    and data transmission

    MAC Header MAC PDU

  • LTE, Long Term EvolutionCarlo Vallati

    MAC Layer The connection point between PHY layer and MAC

    layer is represented by the Hybrid Automatic Repeat reQuest (HARQ)

    The HARQ is an stop-and-wait protocol with a maximum number of retransmission

    HARQ is a combination of forwarding error coding (FEC) and error detection (ED) using ARQ

    UE 1 UE 3

    Downlink TTI

    UPLINK TTI

    UE 2

    ACK NACK ACK

  • LTE, Long Term EvolutionCarlo Vallati

    MAC Layer In addition, HARQ can be used with soft combining Two transmissions cannot be independently decoded

    without error The receiver combine them to increase decoding

    probability Two possible mode: chase combining and

    incremental redundancy

    PACKET

    1st tx 2nd tx

  • LTE, Long Term EvolutionCarlo Vallati

    MAC Layer MAC header can contain one or more control

    elements (CE) CE carry control information directed to UE or NodeB Most important CEs are:

    v Channel Quality Indicator (CQI)v Buffer Status Report (BSR)

  • LTE, Long Term EvolutionCarlo Vallati

    MAC Layer Channel Quality Indicator (CQI)

    v a measurement of the communication quality of the wireless channel

    v high CQI value indicates high quality channelv computed using signal-to-noise ratio (SNR)v reported from UEs to NODEBs through the control

    channels or embedded into the data channel (control element into the MAC PDU header)

    CQI

    SNR

  • LTE, Long Term EvolutionCarlo Vallati

    MAC Layer Buffer Status Report (BSR)

    v contains quantized information on the amount of data waiting for transmission in the UE buffers

    v is sent into an uplink MAC PDU header control element (CE)

    BSR

    MAC SDUBUFFER

  • LTE, Long Term EvolutionCarlo Vallati

    MAC Layer LTE Scheduling Request (SR)

    v it allows the UE to request uplink-transmission resources from the eNB

    v The SR conveys a single bit of information, indicating that the UE has new data to transmit

    v The SR mechanism is one of two types: dedicated SR (D-SR) : an allocated resource block is used, random access-based SR (RA-SR), no uplink resource block

    are available, a random procedure shared with other Ues is executed

    SR

  • LTE, Long Term EvolutionCarlo Vallati

    MAC Layer LTE Random Access Procedure

    RA Preamble

    RA Message (Ue Identity, BSR,etc.)

    RA Response (timing advance, UL Grant, etc.)

    RA contention resolution (UL grant, DL assignment)

    Further uplink/downlink transmissions

  • LTE, Long Term EvolutionCarlo Vallati

    MAC Layer From the perspective of the MAC layer, each time slot is

    viewed as a contiguous list of Resource Blocks (RBs) Scheduling is done in the base station (eNodeB) Scheduler assigns resources in terms of Resource Blocks and

    Modulation to be used to encode them Fr

    eque

    ncy

    Dom

    ain

    Time Domain

    UE 16 RBs

    UE 212 RBs

    UE 38RBs

  • LTE, Long Term EvolutionCarlo Vallati

    MAC Layer Scheduling policy: opportunism vs fairness

    Freq

    uenc

    y D

    omai

    n

    Time Domain

    UE 3Fr

    eque

    ncy

    Dom

    ain

    Time Domain

    UE 2

    UE 1

    UE 3

    Fairness Opportunism

    Good Ch Cond

    Bad Ch Cond

    UE 4

    UE 1 UE 2

    UE 4

  • LTE, Long Term EvolutionCarlo Vallati

    RLC Layer RLC Layer is the interface between upper layers and

    MAC layers.

  • LTE, Long Term EvolutionCarlo Vallati

    RLC Lay

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