HSDPA fast facts

Basics

HSDPA is based on shared channel transmission in the time and code domain by enabling channel dependent scheduling in the nodeB based on frequent and highly accurate channel status reports from the terminals. In this way, terminals that are reporting good channel conditions are favored in the following scheduling moments. This approach increases the overall system throughput. HSDPA also uses a 2 ms TTI which enables faster and more efficient adaption of the transmitted signal to the varying channel conditions of a time dispersive channel. Furthermore, by controlling the bit rate with regular updates of the code rate, transport block size and modulation scheme, the transmission power can be utilized close to the max at nearly all times which is an  improvement compared to earlier releases which employed power controlled schemes.. Finally, Hybrid Automatic Repeat Request (HARQ) is introduced which enables a fast retransmission scheme for the majority of all retransmissions on top of legacy, slow RLC retransmissions which now only serves as a safe and reliable backup scheme in the rare event that HARQ fails. A new sub-layer, mac-hs is introduced in the nodeB and is in charge of the scheduling, rate control and HARQ. By placing these features closer to the air interface, signalling between the nodeB and the RNC is minimized which reduces latency and improves the accuracy of the chosen transmission parameters.     

Key features

High Speed-Downlink Shared Channel (HS-DSCH)

• HS-DSCH is the transport channel used to support shared channel transmission, fast scheduling and 2ms TTI.
• HS-DSCH consists of a set of 16 channelization codes (each with spreading factor 16) which corresponds to 16 High Speed Physical Downlink Shared Channels (HS-PDSCH)
• To allow for code resources used for other purposes (R99 DCH) only 15 codes are available for HS-DSCH
• HS-DSCH resources can be shared in the code domain as well as in the time domain
• All transmission power that remains after serving all other channels is assigned to the rate-controlled HS-DSCH which gives a more or less constant transmission power.

High Speed-Shared Control Channel (HS-SCCH)

The HS-SCCH is transmitted in parallel with the HS-DSCH and carries the following information:

• 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.

High Speed-Dedicated Physical Control Channel (HS-DPCCH) and Fractional-Dedicated Physical Channel (F-DPCH)

• ACK-NACK’s for each TTI that the UE has been scheduled in is sent in the uplink on High Speed Dedicated Physical Control Channel (HS-DPCCH).
• Measurements on the downlink channel quality made by the UE are sent in the form of a Channel Quality Indicator (CQI) on the HS-DPCCH.
• HS-DPCCH has a fixed spreading factor of 256 and 2ms/3-slot structure. First slot is used for HARQ and remaining two for CQI’s.
• 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

 Scheduling:

• The scheduler in Mac-hs decides what part of the shared code and power resources should be assigned to a user in a certain TTI.
• Efficient scheduling relies on information about the instantaneous channel conditions which is conveyed by the CQI’s as well as buffer status information at the nodeB and negotitated QoS.
• The CQI value which is received by the node B is directly mapped to a transport block size, modulation scheme and number of channelization codes.
• Efficient scheduling also relies on buffer status information at the UE.
• Certain types of data such as RRC signaling is prioritized in the scheduling process.
• Since downlink scheduling takes place in the nodeB, HSDPA does not support macro-diversity or soft handover.

 Rate control

• The data rate is adjusted for every TTI by selecting the most appropriate modulation, transport block size and channel coding based on the instantaneous channel conditions.
• Although HSDPA is rate controlled and not power controlled, power can still change due to variations in power requirements for other downlink channels.

 HARQ

• HARQ functionality resides in both physical layer and MAC.
• HARQ introduces faster retransmissions compared to RLC since there is no signaling between nodeB and RNC and more frequent status reports (every TTI).
• For continuous transmission to a single UE, multiple HARQ-processes can operate in parallel.
• The number of parallel HARQ processes should match the roundtrip time between the UE and the nodeB.
• Due to the multiple HARQ processes, the order of the transmitted transport-blocks may become corrupted at the receiver and therefore a mechanism for putting them back in sequence is required before they are passed on to higher layers (RLC)
• In case a NAK is misinterpreted as an ACK by the transmitting HARQ entity, RLC will detect the error and request the necessary retransmission.
• For HSDPA, HARQ retransmissions are made in asynchronous and adaptive mode which means that they they can be sent at any time and with any transport format.
• A one-bit new data indicator is used in the transport blocks to distinguish between retransmissions and new data.
• ACK/NACK's are always sent at predefined intervals after receiving the transport block. In this way the UE always knows which HARQ process they belong to

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.
• 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 completely 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)

The CQI value can be seen as a measure of the downlink channel quality as perceived from the terminals side. It is transmitted in every reporting period to the nodeB which uses this information for scheduling of further downlink data transmissions. Due to the shorter TTI in HSDPA the channel conditions are more static or, have less time to change, during the length of one TTI which improves the accuracy of the reports.

·         Based on SIR-measurements on the Common Pilot Indicator Channel (CPICH)

·         CQI is sent on the HS-DPCCH together with the ACK/NACK’s

·         An increase of one step in the CQI value represents an increase of SIR of the CPICH by one db

·         Each 5 bit CQI value is directly mapped to a transport block size (TBS), number of channelization code and modulation degree. Depending on the capabilities of the receiver, these values may differ between receivers for each CQI value.

·         The nodeB uses these mapped values as an input to the scheduling algorithm. However, he scheduling algorithm for most implementations also considers other parameters such as buffer status and priority levels.

·         The CQI values range between 0(worst) and 30(highest).

·         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%. A too low BLER would lead to an under-utilization of the system.

·         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.

·         CQI values and their respective mappings to TBS, number of channelization codes and modulation degree for each HSDPA category can be found in 3GPP spec 25.214

·         CQI values and their respective mappings to TBS, number of channelization codes and modulation degree for each HSDPA category can be found in 3GPP spec 25.214