Download - WCDMA Channels Channel Switch

8/8/2019 WCDMA Channels Channel Switch

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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

ProtocolProtocol Set of certain basic rulesSet of certain basic rules WCDMA protocol stack is based on OSI (open system interconnection)WCDMA protocol stack is based on OSI (open system interconnection)

reference modelreference model

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

Each of this layer performs certain tasks on the data to make it travelEach 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,from one device to another over a network.

These tasks at each layer,prepare it for the next layer.prepare it for the next layer.

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

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

messages , application layer interfaces with themessages , application layer interfaces with the operating systemoperating system..

-- L

ayer 6: PresentationL

ayer 6: Presentation--

Layer 6 takes the data provided by theLayer 6 takes the data provided by theApplication layer and converts it into a standard format that theApplication layer and converts it into a standard format that the

other layers can understand.other layers can understand.

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

communication with the receiving device.communication with the receiving device.


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Layer 4: Transport - This layer maintains flow control of 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: Network This layer determines the way that the data willbe sent to the recipient. Logical protocols, routing and addressing are

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:For WCDMA the lower 3 layers are most important:

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

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

are :are :

1. Forward error correction(FEC)1. Forward error correction(FEC)

2. Interleaving2. Interleaving

3. Transport channel multiplexing3. Transport channel multiplexing

4. Channel quality measurements4. Channel quality measurements

5. Synchronization5. Synchronization6. Spreading , scrambling, modulation6. 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 CodePower

Idle / Connected Periodic Or Upon Request

RSSI Received Signal StrengthIndicator

Connected Periodic Or Upon Request

TrCh BLER Estimate of BLER on RxTransport 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 layerLayer 2 : Data link layer It is the upper neighbor of the physicalIt is the upper neighbor of the physicallayer.layer.3GPP3GPP divides it into the radio link control (RLC) and thedivides it into the radio link control (RLC) and the

medium access control(MAC) layer. It delivers data block by block tomedium access control(MAC) layer. It delivers data block by block to

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

In the other direction, layer 2 communicates with the adjoiningIn the other direction, layer 2 communicates with the adjoiningnetwork layer vianetwork layer via Logical channels.Logical channels.

The RLC and MAC layers have the following essential functions:The RLC and MAC layers have the following essential functions:

-- 1. Mapping of logical channels on transport channels1. Mapping of logical channels on transport channels

-- 2. Selection of the transport format based on current needs2. Selection of the transport format based on current needs

-- 3. Data encryption3. Data encryption

-- 4. Error protection as a function of the desired QOS4. Error protection as a function of the desired QOS

-- 5. Acknowledgement of data blocks and requests for retransmission in case of5. Acknowledgement of data blocks and requests for retransmission in case of

errorserrors


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Layer 3 : Network Layer(RRC Layer):Layer 3 : Network Layer(RRC Layer): RRC layer is directly above theRRC layer is directly above the

layer 2 and is responsible for radio resource control. The main RRClayer 2 and is responsible for radio resource control. The main RRC

tasks include :tasks include :

1. Broadcasting of system information1. Broadcasting of system information

2. Establishment and clear down a RRC connection2. Establishment and clear down a RRC connection

3. Management of radio connections3. Management of radio connections

4. Cell selection in IDLE mode4. Cell selection in IDLE mode

5. Querying and evaluation of physical layer measurements5. Querying and evaluation of physical layer measurements


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Channels are simply a tool for specifying and describing the variousChannels are simply a tool for specifying and describing the various

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

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

There are 3 different types of channels in WCDMAThere are 3 different types of channels in WCDMA

1.1.Logical channels :Logical channels : Used for communication between Layer 3Used for communication between Layer 3

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

content ,logical channels are characterized intocontent ,logical channels are characterized into traffic & Controltraffic & Control

channels.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 TrafficChannel (CTCH)

LOGICAL CHANNELS


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

It is a broadcasting channel that supplies all the UEs with basic cellIt is a broadcasting channel that supplies all the UEs with basic cell

and network parameters(eg. Frequency lists , code lists etc).and network parameters(eg. Frequency lists , code lists etc).

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

It transports paging information.It transports paging information.

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

BCCH and on the PCCHBCCH and on the PCCH

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

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

during an active connectionduring an active connection

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

Control channels:


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Dedicated traffic channel (DTCH): Point to point ,UL/DLDedicated traffic channel (DTCH): Point to point ,UL/DLDTCH is used to transport user data from Node B to a specific UEDTCH is used to transport user data from Node B to a specific UE

and vice versa. This means it is dedicated to a specific subscriberand vice versa. This means it is dedicated to a specific subscriber

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

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

UEsUEs

TRAFFIC CHANNELS:


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Characterizes HOW data is transported over the air interface . EachCharacterizes HOW data is transported over the air interface . Each

transport channel is accompanied by the Transport formattransport channel is accompanied by the Transport format

indicator(TFI). The physical layer combines the TFI information ofindicator(TFI). The physical layer combines the TFI information of

different transport channels to the Transport format combinationdifferent transport channels to the Transport format combination

indicator(TFCI).indicator(TFCI).

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

Organize & Pack data from different services from the higher layersOrganize & Pack data from different services from the higher layers

for suitable transportationfor suitable transportation

Unpack incoming data and sort for delivery to upper layersUnpack incoming data and sort for delivery to upper layers

Offers flexible data speeds and channel encodingOffers flexible data speeds and channel encoding Efficient usage ofEfficient usage of

radio resourcesradio resources

Transport Channels:


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DCH (Dedicated Channel) Bi-directional channel for transporting both dedicated user and

control 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 mappedto 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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Physical Channels :

By varying the SF, physical layer matches transmission rateto 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


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Channe

l

Name Uplink/Downlink

Description Spreading

Factor /

Scrambling

Code

CPICH Common CPICH Channel Downl ink Reference channel for estimation of received signal strength & quali ty; 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) Carries Paging data to UE for incoming call

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 cal l 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 quantit ies

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 Downl ink 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 mul tiplexed

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 Downl ink UTRAN uses th is to conf irm reception of acquisi tion 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 -

ICHCollision Detection / ChannelAssignment Indicator Channel

Downlink UTRAN uses this channel to inform UEs about any packet collisions on theuplink & 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 and

downlink.

Channel Switching only applies to interactive RAB packet data(browsing ,

email file download etc) trafficbecause :

It belongs to the Interactive and BackgroundQuality 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 betweendifferent 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 algorithmChannel switching algorithm

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

Admission Control:Admission Control: down switch when resources needs to bedown switch when resources needs to be

freed up for new users.

freed up for new users.

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

Triggers of Channel Switching


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

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

direction.direction.

--

Suitable for carrying common control information and are shared by all users inSuitable for carrying common control information and are shared by all users inthe cell.the cell.

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

CELL_DCHCELL_DCH--

Characterized by allocation of Dedicated Transport Channels to the UE.

Characterized by allocation of Dedicated Transport Channels to the UE.

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

high bit rate traffic.high bit rate traffic.

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

RAB State Transitions


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

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



CELL_DCH to CELL_FACHCELL_DCH to CELL_FACH

CELL_FACH to CELL_DCH 64/64 kbpsCELL_FACH to CELL_DCH 64/64 kbps

CELL_FACH to Idle ModeCELL_FACH to Idle Mode

Single RAB State Transitions


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Channel Switching Algorithms determine for each interactive RABChannel Switching Algorithms determine for each interactive RAB

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

to another.to another.

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

Channel Switching Algorithm consists of the following subChannel Switching Algorithm consists of the following sub--

algorithmsalgorithms

-- Common to Dedicated EvaluationCommon to Dedicated Evaluation-- Common to Idle EvaluationCommon to Idle Evaluation

-- Dedicated to Common EvaluationDedicated to Common Evaluation

-- Dedicated to Dedicated EvaluationDedicated to Dedicated Evaluation

-- Coverage Triggered Down Switch EvaluationCoverage Triggered Down Switch Evaluation

Channel Switching Algorithms


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