WCDMA Air interface channel


Briefly describe UMTS air interface channel types and their functions.
There are 3 types of channels across air interface – physical channel, transport channel and logical channel:
·         Physical Channel: carries data between physical layers of UE and NodeB.
·         Transport Channel: carries data between physical layer and MAC layer.
·         Logical Channel: carries data between MAC layer and RRC layer.
1.      Logical Channel:
·         Control channel:   BCCH, PCCH, CCCH, DCCH.
·         Traffic channel:    DTCH, CTCH.
2. Transport Channel:
·         Common control channel:  BCH, FACH, PCH, RACH, CPCH.
·         Dedicated channel: DCH, DSCH.
3.      Physical Channel:
·         Common control channel: P-CCPCH, S-CCPCH, P-SCH, S-SCH, CPICH,        AICH, PICH, PDSCH, PRACH, PCPCH, CD/CA-ICH.
·         Dedicated channel:         DPDCH, DPCCH.

Broadcast Control Channel (BCCH):
 It broadcast system control information for all mobiles in a cell like Code values in the cell, neighbor information, allowed power levels in downlink direction.
Paging Control Channel (PCCH):
   It transfers 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. It’s being used in both direction DL & UL. 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. It’s 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.
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
Random Access Channel (RACH):
It is used to send control information from UE in the uplink direction. Also may carry short user packets.
Forward Access Channel (FACH):
It is a downlink common channel used to send small amounts of 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

(Paging Indicator Channel) PICH:
 It is used to carry paging and notification message to all UEs and it is always associated with SCCPCH
 (Acquisition Indicator channel) AICH:
It is used in random access procedure by network to indicate that the RACH preamble was detected
(Synchronization channel) SCH:
It is downlink channel. It is used to cell search procedure and synchronization with network. There is two type of SCH primary & secondary
 (Physical Random access channel) PRACH:
It is uplink channel and Initial message when UE want gain access to network and transfer small amount of data
(common Pilot channel) CPICH:
It is downlink channel and used to continuously sending scrambling code to cell. It is also aids channel estimation for cell re-selection and Handover for UEs. There are two types of CPICH- Primary and secondary.
 (Primary-common control physical channel) P-CCPCH:
It is downlink channel and used to carry Synchronization channel (SCH) and BCH. It has fixed rate of 30 Kbps.
 (Secondary- common control physical channel) S-CCPCH:
It is downlink channel and used to carry PCH and FACH. It has variable bit rate from 30kbps to 1,920 kbps.
Dedicated Channels:
Dedicated Physical Data Channel (DPDCH)
Dedicated Physical Control Channel (DPCCH)
Dedicated Physical Control Channel (DPCCH)
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 (DPDCH)
 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.

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