WCDMA Channels & Channel Switching

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  • 8/11/2019 WCDMA Channels & Channel Switching


    WCDMA Channels &

    Channel switching

  • 8/11/2019 WCDMA Channels & Channel Switching


    WCDMA Protocol stack OR WCDMA OSI model

    Physical channels


    Logical channels

    (What is transmitted)

    Transport channels

    (How it is transmitted)

    Radio Resource Control (RRC)

    Physical Layer

    Medium Access Control (MAC)


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    OSI reference model

    ProtocolSet of certain basic rules

    WCDMA protocol stack is based on OSI (open system interconnection)

    reference model

    OSI reference model ,developed by ISO, is a set of seven layers.

    Each of this layer performs certain tasks on the data to make it travel

    from one device to another over a network. These tasks at each layer,

    prepare it for the next layer.

    - Layer 7: Application- This is the layer that actually interacts with

    the user. For example if the user chooses to transfer files or read

    messages , application layer interfaces with the operating system.

    - Layer 6: Presentation- Layer 6 takes the data provided by the

    Application layer and converts it into a standard format that the

    other layers can understand.

    - Layer 5: Session- Layer 5 establishes, maintains and ends

    communication with the receiving device.

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    Layer 4: Transport- This layer maintains flow controlof data ,means that if data is coming from more than one application ,

    transport layer integrates it into a single stream for the physical

    network. It also provides error checking and recovery of data

    between the devices.

    Layer 3: NetworkThis layer determines the way that the data will

    be sent to the recipient. Logical protocols, routingand addressingare

    handled here.

    Layer 2: Data- This layer assigns the physical protocol to the data.

    Also, defines the type of network and the packet sequencing.

    Layer 1: Physical- This is the level of the actual hardware. It defines

    the physical characteristics such as connections, voltage levels andtiming.

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    For WCDMA the lower 3 layers are most important:

    Layer 1: Physical layer- This layer is responsible for transporting data

    across the available physical path (radio link). Main jobs of physical layer

    are :

    1. Forward error correction(FEC)

    2. Interleaving

    3. Transport channel multiplexing

    4. Channel quality measurements

    5. Synchronization6. Spreading , scrambling, modulation

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    According to the 3GPP WCDMA specs, the physical layer has 2 interfaces to

    the higher layers-

    1. To the data link layer (which is situated directly above the

    physical layer),with which data is exchanged in the form of transport

    blocks on transport channels. The transport blocks contain the user

    data and control data which are exchanged via the radio channel

    between the base station and UE.

    2. The physical layer also has a direct communications interface to

    the part of the layer which is responsible for managing the radio

    resources i.e. the RRC(Radio resource control).Using this interface ,

    the RRC directly controls the work of the physical layer. And vice

    versa, the physical layer directly transmits information about thecurrent channel quality to the RRC, based on which RRC undertake

    suitable measures in case the connection quality drops.

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    Some important measurements which physical layer must perform and report to

    RRC are :

    Measurement Explanation Mode Evaluation

    CPICH Ec/No Energy per chip/ RSSI Idle / Connected Periodic Or Upon Request

    CPICH RSCP Received Signal Code


    Idle / Connected Periodic Or Upon Request

    RSSI Received Signal Strength


    Connected Periodic Or Upon Request

    TrCh BLER Estimate of BLER on Rx

    Transport channel

    Connected Periodic Or Upon Request

    UE TX Pwr UE Transmit power Connected Periodic Or Upon Request

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    Layer 2 : Data link layerIt is the upper neighbor of the physical

    layer. 3GPP divides it into the radio link control (RLC) and themedium access control(MAC) layer. It delivers data block by block to

    layer 1 via transport channels (or fetches the data when receiving).

    In the other direction, layer 2 communicates with the adjoining

    network layer via Logical channels.

    The RLC and MAC layers have the following essential functions:

    - 1. Mapping of logical channels on transport channels

    - 2. Selection of the transport format based on current needs

    - 3. Data encryption

    - 4. Error protection as a function of the desired QOS

    - 5. Acknowledgement of data blocks and requests for retransmission in case of


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    Layer 3 : Network Layer(RRC Layer): RRC layer is directly above thelayer 2 and is responsible for radio resource control. The main RRC

    tasks include :

    1. Broadcasting of system information

    2. Establishment and clear down a RRC connection

    3. Management of radio connections

    4. Cell selection in IDLE mode

    5. Querying and evaluation of physical layer measurements

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    Channels are simply a tool for specifying and describing the various

    communication and data related services. Various type of data isexchanged between UE and base station over the radio channel. To

    characterize this different type of data , we refer to different channels.

    There are 3 different types of channels in WCDMA

    1. Logical channels : Used for communication between Layer 3

    (RRC) & Layer 2 (RLC/MAC). Depending on the information

    content ,logical channels are characterized into traffic & Control



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    Control Channel (Signaling)

    Broadcast Control Channel (BCCH)

    Paging Control Channel (PCCH)

    Dedicated Control Channel (DCCH)

    Common Control Channel (CCCH)

    Traffic Channel

    (User Data)

    Dedicated Traffic


    Common Traffic

    Channel (CTCH)


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    Broadcast control channel ( BCCH) : Point to multipoint ,DL

    It is a broadcasting channel that supplies all the UEs with basic celland network parameters(eg. Frequency lists , code lists etc).

    Paging control channel(PCCH) : Point to multipoint ,DL

    It transports paging information.

    All the UEs are required to regularly read the information on the

    BCCH and on the PCCH

    Dedicated control channel (DCCH) : Point to point ,UL/DL

    DCCH is provided in parallel to a DTCH for point to point signaling

    during an active connection

    Common control channel ( CCCH) : Point to multipoint ,UL/DLIt enables access by a UE to the UTRAN

    Control channels:

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    Dedicated traffic channel (DTCH): Point to point ,UL/DL

    DTCH is used to transport user data from Node B to a specific UEand vice versa. This means it is dedicated to a specific subscriber

    Common traffic channel (CTCH) : Point to multipoint ,DL

    It transfers dedicated user information for all , or a group of specified



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    Characterizes HOW data is transported over the air interface . Each

    transport channel is accompanied by the Transport formatindicator(TFI). The physical layer combines the TFI information of

    different transport channels to the Transport format combination


    It is not necessary to transmit the TFCI for fixed data rates.

    Organize & Pack data from different services from the higher layersfor suitable transportation

    Unpack incoming data and sort for delivery to upper layers

    Offers flexible data speeds and channel encoding Efficient usage

    of radio resources

    Transport Channels:

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    DCH (Dedicated Channel) Bi-directional channel for transporting both dedicated user andcontrol data; carries DTCH & DCCH.DCH is characterized by fast

    data rate change on a frame by frame basis.

    BCH (Broadcast Channel) Downlink; Transports BCCH to UE; Always transmitted using the

    same transport format in entire cell

    FACH (Forward Access Channel) Downlink; Transports small amount of data from

    BCCH/CCCH/DCCH/CTCH to either a specific UE or over the

    entire cell

    PCH (Paging Channel) Dow