Download - WCDMA Channels & Channel Switching

8/11/2019 WCDMA Channels & Channel Switching

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WCDMA Channels &

Channel switching


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WCDMA Protocol stack OR WCDMA OSI model

Physical channels

(Code,frequency,etc)

Logical channels

(What is transmitted)

Transport channels

(How it is transmitted)

Radio Resource Control (RRC)

Physical Layer

Medium Access Control (MAC)

RLC RLC RLC


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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

Power

Idle / Connected Periodic Or Upon Request

RSSI Received Signal Strength

Indicator

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

errors


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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

channels.

WCDMA CHANNELS :


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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

(DTCH)

Common Traffic

Channel (CTCH)

LOGICAL CHANNELS


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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

UEs

TRAFFIC CHANNELS:


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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

indicator(TFCI).

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) Downlink; Transports data from PCCH to UE over entire cell

RACH (Random Access Channel) Uplink; Used by UE for initial access to UTRAN

CPCH (Common Packet Channel) Uplink; Used by UE to send packet data to UTRAN

DSCH (Downlink Shared Channel) With DSCH, user may be allocated different data rates,For example:

384kbps with SF 8and 192kbps with SF16.DSCH may be mapped

to a multicode case,Ex-3channelisation codes with spreading factor

Dedicated Transport Channel

Common Transport Channel


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Mapping Logical To Transport Channels

PCCH DCCH CCCH CTCHBCCH DTCH

BCH PCH CPCH RACH FACH DCHDSCH

In The UE

In The Node B

PCCH DCCH CCCH CTCHBCCH DTCH

BCH PCH CPCH RACH FACH DCHDSCH


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PhysicalChannels :

By varying the SF, physical layer matches transmission rate

to the current transport channel data.

Actual data transmission over the air


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Mapping Transport To Physical Channels

PCH CPCH RACH FACH DSCH DCHBCH

PHYSICAL

P-CCPCH S-CCPCH PICH PCPCH

AP-AICH

CSICH

CD/CA-ICH

PRACH

AICH

PDSCH DPDCH

DPCCHP/S - SCH

Ph i l Ch l O i


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Channe

l

Name Uplink/Downlink

Description Spreading

Factor /

Scrambling

Code

CPICH Common CPICH Channel Downlink Reference channel for estimation of received signal strength & quality; Fixed bit

pattern which is stored in Physical layer; Determines coverage of a cell

256 / 0

P-CCPCH Primary Common Control

Physical Channel

Downlink Responsible for transmitting BCCH logical channel within the cell; Every UE

knows this and can decode system information as soon as it acquires the SC

256 / 1

S-CCPCH Secondary Common Control

Physical Channel

Downlink 1) Carr ies Paging data to UE for incoming cal l

2) FACH transport channel also transmits small data or control

messages

Both PCH & FACH data can be multiplexed and sent together

Both Variable

PRACH Physical Random Access

Channel

Uplink Used by the UE for call setup; UE sends a preamble to Node B and on receipt,

Node B sends an immediate ACK or NACK on downlink AICH or AP-AICH

PCPCH Physical Common Packet

Channel

Uplink Used by UE for packet data transfer involving small quantities

DPDCH Dedicated Physical Data

Channel

Uplink /

Downlink

Used for dedicated mode transfer of user data (from DCH) & associated control

data (from DCCH) to & from UE

SF varies

between 4 &256

DPCCH Dedicated Physical Control

Channel

Uplink /

Downlink

Used for dedicated signaling to and from UE; To ensure coherent detection it

contains both necessary pilot symbols & commands for fast power control

SF 256

P/S - SCH Primary / Secondary

Synchronization Channel

Downlink Used for WCDMA cell search and timing synchronization; Never spread or

scrambled

N/A

PDSCH Physical Downlink Shared

Channel

Downlink Used by multiple UEs to get out of any bottlenecks in transmission capacity on

downlink; e.g.. Data from DSCH can be transmitted to various UEs multiplexed

together

PICH Paging Indication Channel Downlink Paging messages are indicated to the intended UE by use of bit masks; On

positive identification, UE starts reading S-CCPCH to find page

AICH Acquisition Indication Channel Downlink Determines whether system can be accessed or not; Uses simple bit sequences

for operation

AP-AICH Access Preamble - AICH Downlink UTRAN uses this to confirm reception of acquisition preamble sent by the UE on

the PCPCH

CSICH CPICH Status Indicator

Channel

Downlink Channel informs UE about current availability of CPCH uplink transport channel

for packet data use

CD/CA -

ICH

Collision Detection / Channel

Assignment Indicator Channel

Downlink UTRAN uses this channel to inform UEs about any packet collisions on the

uplink & also about the current assignment of the packet data channel.

Physical Channels Overview


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Channel Switching:

To optimize the use of radio resources allocated to the User Equipment (UE),

according to the amount of data that needs to be transmitted in uplink anddownlink.

Channel Switching only applies to interact ive RAB packet data(brows ing ,

email f i le downlo ad etc) traff ic because :

It belongs to the Interact ive and Backgroun d Quality of Service classes, which

1. has little or no quality of service(QOS) attribute requirements.

2. no guaranteed bit rates

3. and no packet delay requirements.

Large traffic variations : For example web browsing, E-mail, and file transfer.

After a web page has been downloaded and the user is reading the page, there

is very little data to transfer. Data requirement changes once the user requests

a new web page.

Consequently, it is not efficient to reserve dedicated resources continuously.


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Channel Switching is achieved by switching the interactive RAB users between

different transport channel types. Channel Switching only handles switching

between transport channels, and the logical channels are not affected.

Common transport channels: Whenever a user has a small amount of data to send

or receive. Maximum of 32 kbps is shared between the users in a cell.

Dedicated transport channel : When the traffic handled by the UE is large.

Supported bit rates are 64/64, 64/128, 64/384


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Channel switching algorithm

Soft Handover: when RL addition fails due to admission denial,and ongoing transmission is either 64/128kbps or 64/384kbps, a downswitch is triggered.

Admission Control:down switch when resources needs to befreed up for new users.

Congestion Control:down switch when needed to solve acongestion situation.

Triggers of Channel Switching


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CELL_FACH- UE is able to transmit control signals and data packets on the common

transport channel. RACH in the uplink direction and FACH for downlink

direction.

- Suitable for carrying common control information and are shared by all users in

the cell.

- A maximum of 32 kbps is available for user data transmission.

CELL_DCH- Characterized by allocation of Dedicated Transport Channels to the UE.

- Reserved for 1 user and provides closed-loop power control, well suited for

high bit rate traffic.

- For single interactive RAB, 64/64, 64/128, 64/384 kbps are available.

RAB State Transitions


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CELL_DCH to CELL_DCH- CELL_DCH 64/384 to CELL_DCH 64/128

- CELL_DCH 64/128 to CELL_DCH 64/64



CELL_DCH to CELL_FACH

CELL_FACH to CELL_DCH 64/64 kbps

CELL_FACH to Idle Mode

Single RAB State Transitions


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Channel Switching Algorithms determine for each interactive RAB

whether it is necessary to switch a UE from one transport channel

to another.

Channel Switching Algorithms receive measurements from eitherUE or RNC, depending on the behavior of UL and DL.

Channel Switching Algorithm consists of the following sub-

algorithms

- Common to Dedicated Evaluation- Common to Idle Evaluation

- Dedicated to Common Evaluation

- Dedicated to Dedicated Evaluation

- Coverage Triggered Down Switch Evaluation

Channel Switching Algorithms


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