3G HSUPA channels

- a tutorial, description or information about the basics of 3G UMTS HSUPA, High Speed Uplink Packet Access, an integral part of HSPA.

In order to provide the required data speeds and capabilities within HSUPA, further channels have been added to the basic 3G UMTS scheme that is used. These HSUPA channels provide additional signalling and data capabilities.

While HSUPA is effectively an uplink enhancement, channels have been added to both the uplink and the downlink. The reason for the downlink HSUPA channels is to provide the control, etc needed for the uplink data.

Uplink HSUPA channels

A variety of new channels have been introduced for HSUPA to enable the system to carry the high speed data. These new channels are:

  • E-DCH, the Enhanced Dedicated Channel:   This HSUPA uplink channel carries on block of data for each TTI (Transmission Time Interval). The E-DCH can be configured simultaneously with one or more DCHs. In this way high speed data transmission can occur at the same time and on the same UE as services that use the standard DCH.

    As a low latency (delay) is one of the key requirements for the high speed uplink a short TTI (Transmission Time Interval) of 2 ms is supported in addition to one of 10 ms. The short TTI allows for rapid adaptation of transmission parameters and it reduces the end-user delays.

    There is a balance to be determined for the TTI. It is found that the physical layer processing is proportional to the amount of data to be processed, and accordingly the shorter the TTI the lower the level of data per TTI. However for applications requiring relatively low data rates, the overheads required with a 2 ms TTI may be unduly high. In these circumstances a longer TTI is more appropriate.

    The E-DCH is mapped to a set of E-DCH Dedicated Physical Data Channels.
  • E-DPDCH (Enhanced Dedicated Physical Data Channel):   This HSUPA uplink channel carries uplink user data. Each UE can transmit up to four E-DPDCH channels at a spreading factor of SF256 to SF2. The number of E-DPDCHs s and their spreading factors are varied according to the instantaneous data rate required. As an example of a typical scenario, to achieve a 2 Mbps rate - the raw data rate of early devices - two SF2 E-DPDCHs were required.
  • E-DPCCH (Enhanced Dedicated Physical Control Channel):   This HSUPA channel carries the control data required by the Node B to decode the uplink channels including the E-DCH Transport Format Combination Indicator which indicates the block size, retransmission sequence number, etc.

Downlink HSUPA channels

A variety of new channels have been introduced for HSUPA to enable the system to carry the high speed data. These new channels are:

  • E-AGCH (Enhanced Absolute Grant Channel):   This HSUPA channel provides the absolute limit of the power resources, i.e. the serving grant, that the UE may use. The channel is used to send scheduling grants from the scheduler to the UE to control when and what data rate the UE should be used. The E-AGCH is only sent by one NodeB regardless of the number that the UE is communicating with. The NodeB used is the one that has the main responsibility for the scheduling operation. The E-AGCH is typically used for large changes in data rate.
  • E-RGCH (Enhanced Relative Grant Channel):   This channel is used to move the UE serving grant up, down or remain the same. This HSUPA channel is generally used for relatively small changes during an ongoing data transmission. Large changes are handled by the E-AGCH.
  • E-HICH (Enhanced DCH Hybrid ARQ Indicator Channel):   This HSUPA channel is used to provide the acknowledgement of the UE data received by the Node B.

By Ian Poole

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