HSDPA enables much higher speed data transfer and one reason it can achieve it is because of new channels that have been added.
The three main channels are High Speed Downlink Shared Channel, HS-DSCH; High Speed Signalling Control Channel, HS-SCCH; and High Speed Dedicated Physical Control Channel, HS-DPCCH.
The use of these channels which are overlaid on top of the existing 3G UMTS channels provides the capability of the high speed data downloads.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:
- 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.
- Channel Quality Information which is used to provide instantaneous channel information to the scheduler.
These new channels, HS-DSCH, HS-SCCH, & HS-DPCCH provide the high speed data capability along with the other improvements added to HSDPA including higher order modulation adaptive HARQ and improved scheduling.
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