HSDPA Overview

HSDPA

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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 adaptation

•Fast hybrid ARQ with soft combining

•Fast channel-dependent scheduling

3GPP Release 5 extends the specification of WCDMA with a new downlink transport channel for packet data:

HS-DSCH

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HSDPA General Principles (cont)

5mHz

CHANNEL

OVERHEAD

VOICE

TRAFFIC

UNUSEDHSDPA

Key Functions

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

Transmission power remains constant during a TTITransmission power remains constant during a TTI

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

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

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

Fast channel-dependent scheduling The scheduler determines the UE to which the The scheduler determines the UE to which the

shared channel transmission is directed at any shared channel transmission is directed at any given momentgiven moment

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

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

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

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 UE’s 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.

DPCCH, DPDCH

DPCHHS-PDSCH

HS-DPCCH

HS-SCCH

DPCH

DPCCH, DPDCH

UE

Physical Channel Structure

Iub Flow Control

There 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.

Scheduling

The 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).

Scheduling

As 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.

Scheduler Type vs Sector Throughput

0.000.200.40

0.600.801.001.20

1.401.60

Proportional Fair Round robin

Mbps

Scheduling comparison from Parsippany Trial

Link Adaptation and Higher Order Modulation

Link adaptation and higher order modulation can be combined to maximize the instantaneous use of the fading channel

The 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 Modulation

Besides 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 Combining

The 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.


1st Decoding in UE 2nd Decoding in UE Final Picture

The retransmission may be sent by Chase combining or by incremental redundancy.


Chase 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).


To 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 RatesHD-DSCH category

Max # of

Codes

Min TTI Interval

Max Transport

Block Size

Total # of soft

channel bits

Max Modulation Scheme

Max Frames per

second

Max data rate (Mbps)

Category 1 5 3 7298 19200 16QAM 167 1.2

Category 2 5 3 7298 28800 16QAM 167 1.2

Category 3 5 2 7298 28800 16QAM 250 1.8

Category 4 5 2 7298 38400 16QAM 250 1.8

Category 5 5 1 7298 57600 16QAM 500 3.6

Category 6 5 1 7298 67200 16QAM 500 3.6

Category 7 10 1 14411 115200 16QAM 500 7.2

Category 8 10 1 14411 134400 16QAM 500 7.2

Category 9 15 1 20251 172800 16QAM 500 10.1

Category 10 15 1 27952 172800 16QAM 500 14.0

Category 11 5 2 3630 14400 QPSK 250 0.9

Category 12 5 1 3630 28800 QPSK 500 1.8

Connection Handling and Mobility

In 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)


If 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.


When 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)


The 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)


There 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)


There 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)


The 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)


The 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 Management

The 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 Management

If 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 Sharing

The 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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HSDPA Overview

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