WCDMA Channels & Channel Switching

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WCDMA Channels & Channel switching:

WCDMA Channels & Channel switching

WCDMA Protocol stack OR WCDMA OSI model:

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 Protocol – Set 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 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 will be 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 and timing.

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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. Synchronization 6. 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 the current 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 layer – It is the upper neighbor of the physical layer. 3GPP divides it into the radio link control (RLC) and the medium 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 the layer 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 is exchanged 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 UE’s with basic cell and network parameters(eg. Frequency lists , code lists etc). Paging control channel(PCCH) : Point to multipoint ,DL It transports paging information. All the UE’s 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/DL It 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 UE and 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 UE’s TRAFFIC CHANNELS:

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Characterizes “HOW” data is transported over the air interface . Each transport channel is accompanied by the Transport format indicator(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 layers for 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 ( D edicated Ch annel) 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 ( B roadcast Ch annel) Downlink; Transports BCCH to UE; Always transmitted using the same transport format in entire cell FACH ( F orward A ccess Ch annel) Downlink; Transports small amount of data from BCCH/CCCH/DCCH/CTCH to either a specific UE or over the entire cell PCH ( P aging Ch annel) Downlink; Transports data from PCCH to UE over entire cell RACH ( R andom A ccess Ch annel) Uplink; Used by UE for initial access to UTRAN CPCH ( C ommon P acket Ch annel) Uplink; Used by UE to send packet data to UTRAN DSCH ( D ownlink S hared Ch annel) 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 4 provide a DSCH with 2 MBPS. Dedicated Transport Channel Common Transport Channel

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Mapping Logical To Transport Channels PCCH DCCH CCCH CTCH BCCH DTCH BCH PCH CPCH RACH FACH DCH DSCH In The UE In The Node B PCCH DCCH CCCH CTCH BCCH DTCH BCH PCH CPCH RACH FACH DCH DSCH

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Physical Channels : 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 DCH BCH PHYSICAL P-CCPCH S-CCPCH PICH PCPCH AP-AICH CSICH CD/CA-ICH PRACH AICH PDSCH DPDCH DPCCH P/S - SCH

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Channel 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 Carries Paging data to UE for incoming call 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 and downlink. Channel Switching only applies to interactive RAB packet data(browsing , email file download etc) traffic because : It belongs to the Interactive and Background 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 down switch is triggered. Admission Control: down switch when resources needs to be freed up for new users. Congestion Control: down switch when needed to solve a congestion 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 64/128 to CELL_DCH 64/384 CELL_DCH 64/64 to CELL_DCH 64/128 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 either UE 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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