Wcdma Channels ( Detailed ) CHAPTER ONE/THREE

WCDMA Channels

Logical Channels are not actually channels but rather they can be defined as different tasks performed by the network and the UE at different times.They describe the type of information to be transferred.
Logical channels are categorized into control channels and traffic channels.Control channels carry the control plane information.Traffic Channels carry the user plane information.
Transport channels describes how the logical channels will be transferred, or it can be defined as how and with what characteristics data is transferred over the interface.
It comes into two groups:
·       Dedicated Channels
·       Common Channels
Radio Frame Structure

In WCDMA the information is spread over 5MHz band (Wide Bandwidth). There are separate radio channels for both uplink and downlink of 5MHz.

One radio frame consist of 38400 chips and 15 slots. The duration of the radio frame is 10ms which have ship rate of 3.84 Mcps. Each slots in the radio frame comprises of 2650 chips.

Physical Channel

In uplink and downlink direction, each slot in a radio frame is defined with a code or a set of codes that carries a set of common and dedicated channels called Physical Channel.
The information rate of the channel varies from 15 kbps to 1920 kbps for downlink and 30 kbps to 1920 kbps corresponding to available Spreading Factor. In Downlink spreading factor from 256 to 4 and in downlink from 512 to 4.
Physical channels are the transmission media providing platform (radio) through which information is going to be transferred.Physical Channels means different kinds of bandwidths allocated for different purposes, its actually is the physical existence of the Uu interface between UE domain and Access domain. Physical channel are defined by specific carrier freq, scrambling and channelization code.

Logical Channel

Broadcast Control Channel (BCCH):
It broadcast system control information for all mobiles in a cell like Code values in the cell, neighbors information, allowed power levels in downlink direction.
Paging Control Channel (PCCH):
It transfer paging information in downlink direction.  Network page the mobile to discover the UE location or UE in a cell connected state.
Common Control Channel (CCCH):
Network may have certain tasks which are or may be common to UE in the cell. Its being used in both direction downlink & uplink. The CCCH is also used when UE is accessing a new cell after cell reselection.
Dedicated Control Channel (DCCH)
When there is dedicated/active connection means RRC connection between the network and the mobile, the control information transferred using DCCH. Its a bi-directional channel.
Dedicated Traffic Channel (DTCH):
It is used transfer the user data between the network and the UE in both uplink and downlink directions.
Common Traffic Channel (CTCH):
It is used to transfer the data from one point to all mobiles or a specified group of mobiles.

Transport Channels

Transport channels describes how the logical channels to be transferred.
Transport channel is defined by how and with what characteristics data is transferred over the interface.
It comes into two groups:
·       Dedicated Channels
·       Common Channels
Transport Channels
Dedicated Traffic Channels:
Dedicated Channel (DCH):
It is the channel used to send dedicated control and user data between UE and the network in both uplink & downlink directions
Enhanced Dedicated Channel (E-DCH):
It is a enhanced uplink transport channel.
Common Transport Channel :
Random Access Channel (RACH):
It is used to send a control information from UE in the uplink direction. Also may carry short user packets.

Common Packet Channel (CPCH):

Its an uplink shared channel used for packet data.  Several UE can use the same channel for data transfer.

Forward Access Channel (FACH):

It is a downlink common channel used to send small amounts of control and user data
Downlink Shared Channel (DSCH):
It is common channel used to send dedicated control and user data.
Broadcast Channel (BCH):
It broadcast system information in the downlink direction for all mobile stations in a cell.
Paging Channel (PCH):
It’s a downlink common channel used to send paging notification messages.
Dedicated Channel (DCH)
It is a channel that is used to send dedicated control and user data between the UE and the network in both directions.

Physical channels:

In Uplink there are both dedicated and common physical channels.
Dedicated Channels:
·       Dedicated Physical Data Channel (DPDCH)
·       Dedicated Physical Control Channel (DPCCH)
Common Channels:
·       Physical Random Access Channe
·       Physical Common Packet Channel

Dedicated Physical Control Channel

For each radio link, there is one DPCCH, it carries control information like
·       Pilot bits (used to support channel estimation at rake receiver)
·       Transmit Power Control (TPC) commands (Used power control)
·       Feedback information (FBI) needed with transmit diversity
·       Transport Format Combination Indicator (TFCI)
·       Spreading factor is always 256.

Dedicated Physical Data Channel

·       Each radio link may have no DPDCHs or several DPDCHs.

·       Spreading factor for the DPDCH can be between 256 and 4.

·       DPCCH & DPDCH are time – multiplexed in downlink but uplink these are I/Q modulation.

Physical Random Access Channel (PRACH)

·       It carries the Random Access Channel (RACH transport Channel). UE uses when physical random access procedure is initiated.

RACH Preamble Part

Each preamble is of length 4096 chips and consist of 256 repetitions of a signature of length 16 chips.

RACH Message Part

The 10 ms message part spilt into 15 slots each length of 2560 ships. Each slot consist of data part onto which RACH transport part is mapped and control part that carries layer 1 control information. The 20ms message consists of two consecutive 10ms message part radio frames.

Physical Common Packet Channel (PCPCH)

It carries Common Packet Channel (CPCH Transport Channel). The access transmission consists of Access Preambles (AP), one collision Detection Preamble (CDP), a DPCCH Power Control Preamble (PCP) and message. Check out the Structure from ECE book.

Downlink Dedicated Channel

In downlink there is only one dedicated channel DPCH. The control data and user data are time multiplexed with one DPCH.

The length of frame is 10 ms divided into 15 slots

The no. of bits in the data part depends on the spreading factor between 512 and 4 correspondence of 15kbps to 1,920kbps.

DPCCH + DPDCH

Common Pilot Channel

It has pre-defined bit sequence transmitted with a bit rate of 30 Kbps (Fixed Scrambling code 256)

ü  CPICH is divided into pilot channels:

1.     Primary Common Pilot Channel (P-CPICH)

2.     Secondary Common Pilot Channel (S-CPICH)

Reason for low bit rate as it is transmitted with high power to reduce the interference. It is phase reference, must be transmitted in whole cell.

Primary Common Pilot channel (P-CPICH)

It has following characteristics:

·       The same channelization code is always used

·       The P-CPICH is scrambled by primary scrambling code

·       There is only one P-CPICH per cell

·       The P-CPICH broadcast over the entire cell

Secondary Common Pilot Channel (S-CPICH)

It has following characteristics:

·       An arbitrary channelization code is used SF256

·       A S-CPICH may scrambled with primary or secondary scrambling code

·       There may be zero, one or several S-CPICH per cell

·       A S-CPICH transmitted over the entire cell or part of the cell

Primary Common Control Physical Channel (P-CCPCH)

·       It has fixed bit rate of 30 Kbps (Fixed Scrambling code 256)

·       There are no TPC, Pilot bits and first 256 chips of the slot are used for the primary and secondary synchronization Channel. This used to carry the FACH and the PCH.

P-CCPCH + SCH

Secondary Common Control Physical Channel (S-CCPCH)

It has variable bit rate from 30kbps to 1,920 kbps. There are data bits, pilot bits and optional TFCI bits. It is transmitted only to smaller are of the cell.

The only difference between the P-CCPCH and S-CCPCH the transport channel mapped on this can only have fixed predefined transport format combination and in case S-CCPCH support multiple transport format combination using TFCI

Synchronization Channel

This channel used in the cell search procedure. It is divided into the Primary Synchronization and Secondary Synchronization Channel

Primary Synchronization Channel

It carries primary synchronization codes transmitted in each slot of a radio frame. It is same for cells in the network.

Secondary Synchronization Channel

It consist of the secondary Synchronization codes (SSC). System selects the SSC to be transmitted in a slot, based on the scrambling code group and the slot number.

Physical Downlink Shared Channel (PDSCH)

It is used for data transmission by one or-several simultaneous users in the downlink direction.

CD/CA-ICH

The collision Detection Assignment Indicator Channel is used in the CPCH Access Procedure

CSICH

The CPCH Status Indicator Channel (CSICH) is associated with a CPCH. This is uses the free spaces that occurs in the AICH. It is used to inform the UE about CPCH existence and configuration. The duration of CSICH is 20ms. Each frame consist of 15 slots. The length of slot is 40 bits. Each slot has a part that is no transmitted and a status indicator part that includes 8 status indicator bits.

Acquisition Indicator Channel (AICH)

1:It is used in a random access procedure by the network to indicate that the RACH preamble was detected.

2:It has 15 consecutive access slots.

3:Length of access slot is 5120 chips

4:Each access slot is further divided into 32 Acquisition indicators and one part has no transmission.