HSDPA Overview

HSDPAighpeedownlinkacketccess

HSDPA General Principles HSDPA was introduced in 3GPP Release 5 HSDPA is an optional feature in WCDMA RAN HSDPA includes the following key function: Fast link adaptationFast hybrid ARQ with soft combiningFast channel-dependent scheduling

3GPP Release 5 extends the specification of WCDMA with a new downlink transport channel for packet data:HS-DSCHighpeedownlinkharedhannelHSDPA General Principles (cont)5mHzCHANNELOVERHEADVOICETRAFFICUNUSEDHSDPA

Key Functions

Fast link adaptationFast adjustment of data rate every TTI(2ms)

Transmission power remains constant during a TTI

To further increase capacity and data rates 16-quadrature amplitude modulation (16QAM) may be used

Fast hybrid ARQ with soft combining The UE can rapidly request retransmission of erroneously received data

Prior to decoding the UE combines information from the original transmission with that of later retransmissions (soft combining)

Fast channel-dependent schedulingThe scheduler determines the UE to which the shared channel transmission is directed at any given moment

Channel-dependent scheduling means that the scheduler may consider instantaneous radio channel conditions

To enable fast link adaptation, fast hybrid ARQ, and fast scheduling, a new medium-access control sublayer (MAC-hs) has been placed in the RBS.

The MAC-d and RLC layers in the RNC are retained

Channel Structure

The channels introduced with HSDPA share the same carrier as other channels.No additional spectrum needs to be allocated to introduce HSDPA services.The transport channel HS-DSCH is mapped on one or several High-Speed Physical Downlink Shared Channels (HS-PDSCHs) which are simultaneously received by the UE.

3GPP standard allows up to 15 HS-PDSCs per cell with the spreading factor fixed at 16.In the initial release of UEs and WCDMA RAN, each cell has up to 5 HS-PDSCHs.Channelization codes from the shared code resource are dynamically allocated by the RBS every 2 ms.

Time multiplexing is the primary means of sharing common code resources among users.Code multiplexing is useful for providing small payloads or supporting UEs that cannot despread the full set of codes when the number of codes is high (>5).

The downlink High-Speed Shared Control Channel (HS-SCCH) carries control information from the MAC-hs in the RBS to the scheduled UE.In WCDMA RAN, the HS-DSCH is shared in the time domain.Since only the currently scheduled UE needs to receive the HS-SCCH, there is only one such channel configured in each cell.

An uplink channel, the High Speed Dedicated Physical Control Channel (HS-DPCH) must be set up for each UE that uses high-speed services.Channel quality reported by means of CQI is used by the scheduling and link adaptation functions explained further in HSDPA User Plane.

UEs that use high-speed services must always have a set of dedicated uplink and downlink channels.These dedicated channels use soft handover while HS-PDSCH and HS-SCCH do not.

Physical Channel StructureDPCCH, DPDCHDPCHHS-PDSCHHS-DPCCHHS-SCCHDPCHDPCCH, DPDCHUE

Iub Flow ControlThere is flow control between the RNC and the RBS, explained further in HSDPA User Plane.Iub flow control handles the trade-off between having enough data in the MAC-hs buffers to fully utilize the available air interface resources and keeping the MAC-hs buffers short to decrease the memory space.

SchedulingThe scheduler is part of the MAC-hs in the RBS, explained further in HSDPA User Plane.An increase in capacity can be obtained if the scheduler employs channel-dependent scheduling (proportional fair scheduling) instead of resources sequentially (round-robin scheduling).

SchedulingAs the load of a cell increases, the number of UEs queued for scheduling increases. This in turn raises the probability of scheduling UEs with good channel quality.There are two kinds of variations in channel quality, short-term and long-term.

For many packet data applications, relatively large short-term variation in service quality are acceptable, while long-term variations must be restricted.A practical scheduling strategy utilizes the short-term variations and maintains some degree of long-term fairness between the users.

Scheduling comparison from Parsippany Trial

Link Adaptation and Higher Order ModulationLink adaptation and higher order modulation can be combined to maximize the instantaneous use of the fading channelThe HS-DSCH adjusts the data rate to match the instantaneous radio conditions and the available transmission power in the RBS.

The system adjusts the data rate by varying the effective code rate, changing the modulation scheme, and changing the number of codes (TFRC selection, see HSDPA User Plane)Link Adaptation and Higher Order ModulationBesides QPSK, the HSDSCH can use 16QAM to provide higher data rates.

The hybrid ARQ mechanism allows the UE to rapidly request retransmissions of erroneously received transport blocks (see HSDPA user plane.)Hybrid ARQ with Soft CombiningThe UE attempts to decode each transport block reporting to the RBS its success or failure 5 ms after the reception of the transport blocks.

During retransmission, the UW employs soft combining.Hybrid ARQ with Soft Combining1st Decoding in UE2nd Decoding in UEFinal Picture

The retransmission may be sent by Chase combining or by incremental redundancy.Hybrid ARQ with Soft CombiningChase combining is used in WCDMA RAN

The round-trip time for the MAC-hs protocol is about 12 ms, which is longer than TTI (2 ms).Hybrid ARQ with Soft CombiningTo avoid having to wait for ACK/NACK indicator from the UE before the next PDU is transmitted, the RBS supports multiple hybrid ARQ processes, see HSDPA User plane.

Compared with a 3GPP R99 UE, an HS-DSCH-enabled UE must contain processing capacity for the hybrid ARQ operation, multi-code processing, HS-SCCH reception, and HS-DPCCH signaling.User Equipment

Maximum Theoretical Data Rates

HD-DSCH categoryMax # of CodesMin TTI IntervalMax Transport Block SizeTotal # of soft channel bitsMax Modulation SchemeMax Frames per secondMax data rate (Mbps)Category 15372981920016QAM1671.2Category 25372982880016QAM1671.2Category 35272982880016QAM2501.8Category 45272983840016QAM2501.8Category 55172985760016QAM5003.6Category 65172986720016QAM5003.6Category 71011441111520016QAM5007.2Category 81011441113440016QAM5007.2Category 91512025117280016QAM50010.1Category 101512795217280016QAM50014.0Category 1152363014400QPSK2500.9Category 1251363028800QPSK5001.8

Connection Handling and MobilityIn the network it is possible to have cells where the HS-DSCH is enabled and other cells where it is disabled. Furthermore, there are UEs with and without HSDPA capability.At RAB establishment, the UE makes a HS-DSCH cell selection, (see Connection Handling)

Connection Handling and MobilityIf the HS-DSCH is disabled in the best cell but enabled in a cell that has a coverage relation to the best cell and if the UE is close enough to the RBS, the connection is set up in the latter cell.If this feature is not supported or if the HS-DSCH is disabled in the best cell, the connection is established on an interactive DCH.

Connection Handling and MobilityWhen a new cell in the active set becomes the best cell, or when the current serving HS-DSCH cell is to be removed from the active set for some reason, the UE makes a serving HS-DSCH cell change, (see Handover)

Connection Handling and MobilityThe bit rate of the downlink user data depends on the HS-DSCH UE category, the number of HS=PDSCH codes, and whether 16QAM is used.Theoretically, the max bit rate is 2.08 Mbps using QPSK and 4.32 Mbps using 16QAM for the initial release of UEs and WCDMA RAN, (see HSDPA User Plane)

Connection Handling and MobilityThere are two interactive RABs, with different uplink rates, that support HS-DSCH.An uplink rate of 64 kbps or 384 kbps is selected when the RAB is established, (see Connection Handling)

Connection Handling and MobilityThere is no channel switching between the uplink rates.In case of inactivity in the user data transmission uplink and downlink, the connection is released, (see Channel Switching)

Connection Handling and MobilityThe power of the HS-DSCH is give as an offset relative the DPCCH; there are different values depending on whether or not the UE is in soft handover.Repetition factors indicate how many times the CQI and ACK/NACK indicators are transmitted on the HS-DPCCH, (see Power Control)

Connection Handling and MobilityThe power of the HS-SCCH is configurable and given as an offset relative the P-CPICH.The ordinary power control of the dedicated channels is not affected by the introduction of the HS-DSCH.

Capacity ManagementThe admission control and congestion control functions control the load in the WCDMA cell with support from other radio network functions, (see Capacity Management)It is possible to set the load thresholds for when the non-HS-DSCH traffic should be rejected.

Capacity ManagementIf the total load of the cell becomes too high, the congestion control is able to release radio links in a predetermined priority order, to decrease load.

Load SharingThe inter-frequency load sharing function distributes the call accesses between cells, to even out the traffic load between cells on different frequencies coverage the same area, (see Load Sharing)

Questions and discussion

References[1]UE Radio Access Capabilities, 3GPP TS 25.306[2]HSDPA Overview WCDMA RAN User Description, Ericsson 91/1553-HSD 101 02/4 Uen A[3]Mastering HSDPA, Award Solutions, Inc.

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