3G UMTS Data Channels: physical; logical; transport

- in order to carry the required data across the radio access network, data is carried in various channels. These are split into three groups: physical channels; logical channels; transport channels.

There are many 3G UMTS channels that are used within the UMTS system. The data carried by the UMTS / WCDMA transmissions is organised into frames, slots and channels.

In this way all the payload data as well as the control and status data can be carried in an efficient manner.

3G UMTS channel structures

3G UMTS uses CDMA techniques (as WCDMA) as its multiple access technology, but it additionally uses time division techniques with a slot and frame structure to provide the full channel structure.

A channel is divided into 10 ms frames, each of which has fifteen time slots each of 666 microseconds length. On the downlink the time is further subdivided so that the time slots contain fields that contain either user data or control messages.

On the uplink dual channel modulation is used so that both data and control are transmitted simultaneously. Here the control elements contain a pilot signal, Transport Format Combination Identifier (TFCI), FeedBack Information (FBI) and Transmission Power Control (TPC).

The channels carried are categorised into three:

  • Logical Channels:   The logical channels define the way in which the data will be transferred
  • Transport Channels:   The 3G transport channels along with the logical channel again defines the way in which the data is transferred
  • Physical channels:   The physical channels carry the payload data and govern the physical characteristics of the signal.

The channels are organised such that the logical channels are related to what is transported, whereas the physical layer transport channels deal with how, and with what characteristics. The MAC layer provides data transfer services on logical channels. A set of logical channel types is defined for different kinds of data transfer services.

3G UMTS Logical Channels:

The 3G logical channels include:

  • Broadcast Control Channel (BCCH)   (downlink). This channel broadcasts information to UEs relevant to the cell, such as radio channels of neighbouring cells, etc.
  • Paging Control Channel (PCCH)   (downlink). This channel is associated with the PICH and is used for paging messages and notification information.
  • Dedicated Control Channel (DCCH)   (up and downlinks) This channel is used to carry dedicated control information in both directions.
  • Common Control Channel (CCCH)   (up and downlinks). This bi-directional channel is used to transfer control information.
  • Shared Channel Control Channel (SHCCH)   (bi-directional). This channel is bi-directional and only found in the TDD form of WCDMA / UMTS, where it is used to transport shared channel control information.
  • Dedicated Traffic Channel (DTCH)   (up and downlinks). This is a bidirectional channel used to carry user data or traffic.
  • Common Traffic Channel (CTCH)   (downlink) A unidirectional channel used to transfer dedicated user information to a group of UEs.

3G UMTS Transport Channels:

The 3G UMTS transport channels include:

  • Dedicated Transport Channel (DCH)   (up and downlink). This is used to transfer data to a particular UE. Each UE has its own DCH in each direction.
  • Broadcast Channel (BCH)   (downlink). This channel broadcasts information to the UEs in the cell to enable them to identify the network and the cell.
  • Forward Access Channel (FACH)   (down link). This is channel carries data or information to the UEs that are registered on the system. There may be more than one FACH per cell as they may carry packet data.
  • Paging Channel (PCH)   (downlink). This channel carries messages that alert the UE to incoming calls, SMS messages, data sessions or required maintenance such as re-registration.
  • Random Access Channel (RACH)   (uplink). This channel carries requests for service from UEs trying to access the system
  • Uplink Common Packet Channel (CPCH)   (uplink). This channel provides additional capability beyond that of the RACH and for fast power control.
  • Downlink Shared Channel (DSCH)   (downlink).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.

3G UMTS Physical Channels:

The 3G UMTS physical channels include:

  • Primary Common Control Physical Channel (PCCPCH)   (downlink). This channel continuously broadcasts system identification and access control information.
  • Secondary Common Control Physical Channel (SCCPCH)   (downlink) 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.
  • Physical Random Access Channel (PRACH)   (uplink). This channel enables the UE to transmit random access bursts in an attempt to access a network.
  • Dedicated Physical Data Channel (DPDCH)   (up and downlink). This channel is used to transfer user data.
  • Dedicated Physical Control Channel (DPCCH)   (up and downlink). This channel carries control information to and from the UE. In both directions the channel carries pilot bits and the Transport Format Combination Identifier (TFCI). The downlink channel also includes the Transmit Power Control and FeedBack Information (FBI) bits.
  • Physical Downlink Shared Channel (PDSCH)   (downlink). This channel shares control information to UEs within the coverage area of the node B.
  • Physical Common Packet Channel (PCPCH)   This channel is specifically intended to carry packet data. In operation the UE monitors the system to check if it is busy, and if not it then transmits a brief access burst. This is retransmitted if no acknowledgement is gained with a slight increase in power each time. Once the node B acknowledges the request, the data is transmitted on the channel.
  • Synchronisation Channel (SCH)   The synchronisation channel is used in allowing UEs to synchronise with the network.
  • Common Pilot Channel (CPICH)   This channel is transmitted by every node B so that the UEs are able estimate the timing for signal demodulation. Additionally they can be used as a beacon for the UE to determine the best cell with which to communicate.
  • Acquisition Indicator Channel (AICH)   The AICH is used to inform a UE about the Data Channel (DCH) it can use to communicate with the node B. This channel assignment occurs as a result of a successful random access service request from the UE.
  • Paging Indication Channel (PICH)   This channel provides the information to the UE to be able to operate its sleep mode to conserve its battery when listening on the Paging Channel (PCH). As the UE needs to know when to monitor the PCH, data is provided on the PICH to assign a UE a paging repetition ratio to enable it to determine how often it needs to 'wake up' and listen to the PCH.
  • CPCH Status Indication Channel (CSICH)   This channel, which only appears in the downlink carries the status of the CPCH and may also be used to carry some intermittent, or "bursty" data. It works in a similar fashion to PICH.
  • Collision Detection/Channel Assignment Indication Channel (CD/CA-ICH)   This channel, present in the downlink is used to indicate whether the channel assignment is active or inactive to the UE.

By using the logical, physical and transport channels it is possible to carry the data for the control and payload in a structured manner and provide efficient effective communications. The 3G UMTS channels are thus an essential element of the overall system.

By Ian Poole

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