HSDPA fast facts 2

HSDPA fast facts 2

Mobility

• The UE measurements reports are initiated and managed by the RNC.
• The RNC can order the UE to change it’s serving cell based on the measurements reports.

Change of serving HS-DSCH cell

• When change of HS-DSCH serving cell takes place between different node B's. The source node B will flush it's buffers and it's up to RLC retransmissions to recover the lost PDU's. However, with perfect timing of when to stop forwarding PDU's to the source cell retransmission can be completetely avoided. If both source and target cell belong to the same nodeB and it supports HARQ preservation, the buffer content at the time of the serving cell change will be transfered from the source cell to the target cell.
• Change in the serving HS-DSCH cell may be triggered by measurement event 1D

UE categories

The UE HSDPA categories describe the UE capabilities in terms of:

• Maximum number of HS-DSCH codes received
• Minimum inter-TTI interval
• Maximum transport block size
• Maximum number of schemes
• Supported modulation

Constellation rearrangement

• Due to the outline of the symbol constellation diagram for higher modulation degrees, certain symbols (and transmitted bits) have a shorter distance to some of the neighbors in the diagram which makes them more likely to be received in error.
• For turbo codes, systematic bits are of greater importance than parity bits.
• For these reasons, there is a gain in rearranging the symbol constellation between retransmissions with regards to both parity bits and bits that were previously received in error.

Channel Quality Indicator (CQI)

• Each of the 30 available, five-bit CQI values are directly mapped to a given transport block size, modulation scheme and number of channelization codes.
• For highest efficiency and utilization of the retransmission and error correcting coding schemes, the CQI value chosen should result in a block error rate (BLER) not exceeding 10%.
• Each step in the CQI value corresponds to an increase/decrease in the carrier-to-interference ratio by one dB.
• CQI values is not only based on measurements of the common pilot channel SIR and EcN0. Other factors include: multipath environment, terminal receiver type, ratio of interference of the own base station compared with others.

Downlink Control Signalling (HS-SCCH)

The following information is carried on the HS-SCCH:

• HS-DSCH transport format: channelization code set, modulation scheme and transport block size.
• HARQ information: HARQ process number, redundancy version and new data indicator.

The HS-SCCH information is split in two parts depending on how urgently the receiver needs the information:

• Part 1: channelization code set and modulation scheme for the HS-DSCH.
• Part 2: transport block size and HARQ parameters.

For identification needs, part 1 and 2 use different methods:

• part two contains a CRC which is also used for identification of the receiving UE
• part one uses a terminal specific masking operation which enables identification of the receiving UE

• HS-SCCH power control is recommended at every TTI and can be based on associated DPCCH power control commands and CQI's
• The highest number of different HS-SCCH’s that the UE has to monitor is 4. However, if data was received in the previous TTI and the current, there can be no change of HS-SCCH between TTI’s.

F-DPCH

• In essence, the F-DPCH is a slot format DPCH for Transmission Power Control (TPC) bits only which allows up to ten different users to share a single channelization code.
• The SRB can be mapped to the HS-DSCH using the F-DPCH

RRM for HSDPA

• At the RNC, the RRM algorithms include: resource allocation, admission control and mobility management.
• At the nodeB its link adaption, HS-SCCH power control and MAC-hs packet scheduler.