HSDPA fast facts 1

HSDPA fast facts 1

HSDPA basics:

• Based on shared channel transmission in the time domain (and code domain).
• 2 ms TTI which enables faster and more frequent adaptation to the varying channel conditions of a time dispersive channel.
• Rate controlled which is implemented by rapid adjustment of coding rate and modulation scheme.
• Channel dependent scheduling: since the instantaneous channel conditions vary independently between the active users, it is possible to only schedule users that have good channel conditions for each scheduling moment. This allows for a high network system throughput.
• For fast rate control and channel adaptation, some L1-2 functionality has been moved closer to the air interface, to the node B.
• A new sub-layer, Mac-hs, which is responsible for scheduling, rate control and hybrid-ARQ has been introduced and resides in the nodeB

High-Speed Downlink Shared Channel (HS-DSCH)

• HS-DSCH is the transport channel used to support shared channel transmission and the other HSDPA features mentioned above
• 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 HS-DSCH which gives a more or less constant transmission power.
• Unlike the R99 DCH there is no need to handle issues like DTX or compressed mode with the HS-DSCH. However, actual data transmissions and signaling are indeed suspended during the compressed mode gaps.

Control signaling channels

• HSDPA downlink control signaling is carried on the High Speed Shared control Channel (HS-SCCH) which is transmitted in parallel to HS-DSCH.
• ACK-NACK’s for each TTI that the UE has been scheduled in is sent in 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.
• Fractional Dedicated Physical Channel (F-DPCH) containing power control commands for the uplink transmissions are sent in the downlink.
• Since downlink scheduling takes place in the nodeB, HSDPA does not support macro-diversity or soft handover.

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.
• 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.

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).
• Due to the shorter TTI in HSDPA the channel conditions are more static during the length of one TTI which improves the accuracy of the status reports.
• 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 tranport format.
• A one-bit new data indicator is used 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