By Shivendra Verma

Physical channels(Code,frequency,etc)

OSI reference model

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

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 link-This layer transforms the physical layer from a raw transmission facility to a reliable link.. Layer 1: Physical - This is the level of the actual hardware. It defines the physical characteristics such as connections, voltage levels and timing.

For WCDMA the lower 3 layers are most important:Layer 1: Physical layer- This layer is responsible for transporting dataacross 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

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 channels2. Selection of the transport format based on current needs3. Data encryption4. Error protection as a function of the desired QOS5. Acknowledgement of data blocks and requests for retransmission in case of errors

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

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.The channels carried are categorized into three: logical, transport and physical channels. The logical channels define the way in which the data will be transferred, the transport channel along with the logical channel again defines the way in which the data is transferred, the physical channel carries the payload data and govern the physical characteristics of the signal.There are 3 different types of channels in WCDMA

WCDMA CHANNELS :

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.

Broadcast control channel ( BCCH) : Point to multipoint ,DLIt is a broadcasting channel that supplies all the UEs 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 UEs are required to regularly read the information on the BCCH and on the PCCH Dedicated control channel (DCCH) : Point to point ,UL/DLThis channel is used to carry dedicated control information in both directions. 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 This bi-directional channel is used to transfer control information. Control channels:

Dedicated traffic channel (DTCH): Point to point ,UL/DLDTCH 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. This is a bidirectional channel used to carry user data or traffic.Common traffic channel (CTCH) : Point to multipoint ,DL It transfers dedicated user information for all , or a group of specified UEsTRAFFIC CHANNELS:

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).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. They are all unidirectional.

Transport channels are divided into common channels and dedicated channels.

Transport Channels:

Dedicated Transport Channel Common Transport Channel

DCH (Dedicated Channel)This is used to transfer data to a particular UE. Each UE has its own DCH in each direction

BCH (Broadcast Channel)This channel broadcasts information to the UEs in the cell to enable them to identify the network and the 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 cellPCH (Paging Channel)Downlink; Transports data from PCCH to UE over entire cell. This channel carries messages that alert the UE to incoming calls, SMS messages, data sessions or required maintenance such as re-registration. RACH (Random Access Channel)This channel carries requests for service from UEs trying to access the systemCPCH (Common Packet Channel)This channel provides additional capability beyond that of the RACH and for fast power control.DSCH (Downlink Shared Channel)This channel can be shared by several users and is used for data that is "bursty" in nature such as that obtained from web browsing etc.

Mapping Logical To Transport Channels

Physical Channels Overview

ChannelNameUplink/ DownlinkDescriptionCPICHCommon CPICH ChannelDownlinkReference channel for estimation of received signal strength & quality; Fixed bit pattern which is stored in Physical layer; Determines coverage of a cellP-CCPCHPrimary Common Control Physical ChannelDownlinkThis channel continuously broadcasts system identification and access control information.S-CCPCHSecondary Common Control Physical ChannelDownlink This channel carries the Forward Access Channel (FACH) providing control information, and the Paging Channel (PACH) with messages for UEs that are registered on the network.PRACHPhysical Random Access ChannelUplinkUsed 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

PCPCHPhysical Common Packet ChannelUplinkUsed by UE for packet data transfer involving small quantitiesDPDCHDedicated Physical Data ChannelUplink / DownlinkUsed for dedicated mode transfer of user data (from DCH) & associated control data (from DCCH) to & from UEDPCCHDedicated Physical Control ChannelUplink / DownlinkUsed for dedicated signaling to and from UE; To ensure coherent detection it contains both necessary pilot symbols & commands for fast power controlP/S - SCHPrimary / Secondary Synchronization ChannelDownlinkUsed for WCDMA cell search and timing synchronization; Never spread or scrambledPDSCHPhysical Downlink Shared ChannelDownlinkThis channel shares control information to UEs within the coverage area of the node B.

PICHPaging Indication ChannelDownlinkPaging messages are indicated to the intended UE by use of bit masks; On positive identification, UE starts reading S-CCPCH to find pageAICHAcquisition Indication ChannelDownlinkDetermines whether system can be accessed or not; Uses simple bit sequences for operationAP-AICHAccess Preamble - AICHDownlinkUTRAN uses this to confirm reception of acquisition preamble sent by the UE on the PCPCHCSICHCPICH Status Indicator ChannelDownlinkChannel informs UE about current availability of CPCH uplink transport channel for packet data useCD/CA - ICHCollision Detection / Channel Assignment Indicator ChannelDownlinkUTRAN uses this channel to inform UEs about any packet collisions on the uplink & also about the current assignment of the packet data channel.

Mapping Transport To Physical ChannelsPHYSICALP-CCPCHS-CCPCHPICHPCPCHAP-AICHCSICHCD/CA-ICHPRACHAICHPDSCHDPDCHDPCCHP/S - SCH

Thank you!

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