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GSM (Groupe Spéciale Mobile)



WCDMA - Logical Channels


WCDMA mainly follows the ITU Recommendation M.1035 in the definitions of logical channels. The following logical channels are defined for WCDMA.


Control Channels

BCCH: Broadcast Control Channel carries system and cell specific channels;

PCH: Paging Channel for messages to the mobiles in the paging area;

FACH: Forward Access Channel for messages from the base station to the mobile in one cell.


In addition to the two Control Channels there are also two Dedicated Channels;

DCCH : Dedicated Control Channel, this covers two channels : stand-alone dedicated control channel (SDCCH), and the Associated Control Channel (ACCH);

DTCH : Dedicated Traffic Channel for point to point data transmission in the uplink and downlink.

Uplink Physical Channels

There are two dedicated channels and one common channel for the uplink. User data is transmitted on one of these channels, the Dedicated Physical Data Channel (DPDCH), as is the control information. The Random Access Channel is a common access channel.


The Dedicated Physical Control Channel (DPCCH) is needed to transmit pilot symbols for coherent reception, power signaling bits, and rate information for rate detection. The two basic solutions for multiplexing physical control and data channels are time multiplexing and code multiplexing. A combined IQ and code multiplexing solution (dual-channel QPSK) is used in the WCDMA uplink and to avoid Electromagnetic Compatibility (EMC) problems with Discontinuous Transmission (DTX).

There is a major drawback of a time multiplexed control channel, that is where there are EMC problems that arise when DTX is used for data transmission. Figure 31 shows an example of DTX during normal speech transmission. During periods when there is silence there is no need for information bits to be transmitted. This results in pulsed transmission of the of the control data that has to be transmitted.















Figure 31 Pulsed Transmission when time multiplexed control channel
(Adapted from Holma, H. et al 2000)

Downlink Physcialy Channels

There are three physical channels in the downlink; the primary and secondary Common Control Physical Channels (CCPCH) carry the downlink for the common control logical channels (BCCH, PCH and FACH); the SCH provides timing information and in WCDMA is used for handover measurements by the Mobile Station (MS).


The dedicated channels (DPDCH and DPCCH) are multiplexed, however this means that there is an EMC problem caused by discontinuous transmission, but this is not considered a significant problem to the downlink, for two reasons :

  • There are signals to several users transmitted in parallel and at the same time.
  • Base Stations are not so close to other electrical equipment.