- HSDPA requires additional channels to provide the additional data transfer. These include HS-DSCH, HS-SCCH, HS-DPCCH.
A number of new channels were added to the downlink within HSDPA to provide the additional data capacity as well as the control required. The new HSDPA channels are used in the downlink in addition to the existing 3G UMTS channels.
High Speed Downlink Shared Channel, HS-DSCH
Th HS DSCH channel is the data transport channel that all active HSDPA users connected to the NodeB will use. The use of a shared channel is a key characteristic of HSDPA and being a common resource, the HS-DSCH is dynamically shared between users.
The HS-DSCH supports adaptive coding and modulation changing to adapt to the changing conditions within the system. The use of the 2ms TTI means that scheduling delays are reduced and it also enables fast tracking of the channel conditions allowing for the optimum use of the available resource.
It is worth noting that the HS-DSCH is not power controlled but rate controlled. This allows the remaining power, after the other required channels have been serviced to be used for the HS-DSCH, and this means that the overall power available is used efficiently.
High Speed Signalling Control Channel, HS-SCCH
This HSDPA channel is used to signal the scheduling to the users every 2 ms according to the TTI. The HS-SCCH channel carries three main elements of information:
- It carries the UE identity to allow specific addressing of individual UEs on the shared control channel.
- The HS-SCCH carries the Hybrid ARQ to enable the combining process to proceed.
- This channel carries the Transport Format and Resource Indicator (TFRI). This identifies the scheduled resource and its transmission format.
High Speed Dedicated Physical Control Channel, HS-DPCCH
The High Speed Dedicated Physical Control Channel, HS-DPCCH is an HSDPA channel used to provide feedback to the scheduler and it is located in the uplink. The HS-DPCCH channel carries the following information:
- Channel Quality Information which is used to provide instantaneous channel information to the scheduler.
- HARQ ACK/NAK information which is used to provide information back about the successful receipt and decoding of information and hence to request the resending information that has not been successfully received.
These channels, HS-DSCH, HS-SCCH, & HS-DPCCH are added to the existing 3G UMTS channels and provide the additional data capability and adaptivity required to enable the much faster download speeds provided by 3G HSDPA.
By Ian Poole
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