60118029 hsdpa-parameter-description

RAN

HSDPA
Parameter Description

Issue 02

Date 2009-06-30

Copyright © Huawei Technologies Co., Ltd. 2009. All rights reserved.
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Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd

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HSDPA Contents

Contents

1 Introduction to This Document...............................................................................................1-1
1.1 Scope.............................................................................................................................................................1-1
1.2 Intended Audience.........................................................................................................................................1-1
1.3 Change History..............................................................................................................................................1-1

2 Overview of HSDPA .................................................................................................................2-1
2.1 General Principles of HSDPA .......................................................................................................................2-1
2.2 HSDPA Channels ..........................................................................................................................................2-2
2.2.1 HS-DSCH and HS-PDSCH .................................................................................................................2-3
2.2.2 HS-SCCH.............................................................................................................................................2-3
2.2.3 HS-DPCCH..........................................................................................................................................2-3
2.2.4 DPCCH and DPCH/F-DPCH...............................................................................................................2-4
2.3 Impact of HSDPA on NEs .............................................................................................................................2-4
2.4 HSDPA Functions .........................................................................................................................................2-4
2.4.1 HSDPA Control Plane Functions .........................................................................................................2-4
2.4.2 HSDPA User Plane Functions ..............................................................................................................2-6

3 Control Plane ..............................................................................................................................3-1
3.1 Bearer Mapping.............................................................................................................................................3-1
3.2 Access Control ..............................................................................................................................................3-2
3.3 Mobility Management ...................................................................................................................................3-2
3.4 Channel Switching ........................................................................................................................................3-3
3.5 Load Control .................................................................................................................................................3-5
3.6 Power Resource Management .......................................................................................................................3-5
3.7 Code Resource Management.........................................................................................................................3-6
3.7.1 HS-SCCH Code Resource Management..............................................................................................3-6
3.7.2 HS-PDSCH Code Resource Management ...........................................................................................3-7
3.7.3 RNC-Controlled Static Code Allocation ..............................................................................................3-7
3.7.4 RNC-Controlled Dynamic Code Allocation ........................................................................................3-7
3.7.5 NodeB-Controlled Dynamic Code Allocation .....................................................................................3-9
3.7.6 Dynamic Code Tree Reshuffling........................................................................................................3-10

4 User Plane ....................................................................................................................................4-1
4.1 Flow Control and Congestion Control ..........................................................................................................4-1

Issue 02 (2009-06-30) Huawei Proprietary and Confidential iii

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Contents HSDPA

4.1.1 Flow Control ........................................................................................................................................4-2
4.1.2 Congestion Control ..............................................................................................................................4-3
4.2 RLC and MAC-d...........................................................................................................................................4-3
4.2.1 RLC......................................................................................................................................................4-3
4.2.2 MAC-d .................................................................................................................................................4-4
4.3 MAC-hs Scheduling......................................................................................................................................4-4
4.3.1 Determining the Candidate Set ............................................................................................................4-4
4.3.2 Calculating Priorities ...........................................................................................................................4-5
4.3.3 Comparison of Four Algorithms ..........................................................................................................4-8
4.4 HARQ ...........................................................................................................................................................4-9
4.4.1 HARQ Retransmission Principles........................................................................................................4-9
4.4.2 Soft Combining During HARQ .........................................................................................................4-10
4.4.3 Preamble and Postamble ....................................................................................................................4-10
4.5 TFRC Selection ...........................................................................................................................................4-11
4.5.1 Basic Procedure of TFRC Selection...................................................................................................4-11
4.5.2 Determining the TBSmax .....................................................................................................................4-11
4.5.3 Determining the TBSused, Modulation Scheme, Power, and Codes....................................................4-13
4.5.4 Determining the Number of MAC-d PDUs .......................................................................................4-14

5 QoS and Diff-Serv Management ............................................................................................5-1
5.1 QoS Management..........................................................................................................................................5-1
5.2 Diff-Serv Management..................................................................................................................................5-3
5.2.1 SPI Weight Description........................................................................................................................5-3
5.2.2 Differentiated Services Based on Service Types..................................................................................5-4
5.2.3 Differentiated Services Based on User Priorities .................................................................................5-4
5.3 QoS Parameter Mapping and Configuration .................................................................................................5-5

6 Parameters ...................................................................................................................................6-1
7 Counters .......................................................................................................................................7-1
8 Glossary .......................................................................................................................................8-1
9 Reference Documents ...............................................................................................................9-1

iv Huawei Proprietary and Confidential Issue 02 (2009-06-30)

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HSDPA 1 Introduction to This Document

1 Introduction to This Document

1.1 Scope
This document describes the HSDPA functional area. It provides an overview of the main
functions and goes into details regarding HSDPA control and user plane functions.

1.2 Intended Audience
It is assumed that users of this document are familiar with WCDMA basics and have a
working knowledge of 3G telecommunication.
This document is intended for:
System operators who need a general understanding of HSDPA
Personnel working on Huawei products or systems

1.3 Change History
This section provides information on the changes in different document versions.
There are two types of changes, which are defined as follows:
Feature change: refers to the change in the HSDPA feature.
Editorial change: refers to the change in wording or the addition of the information that
was not described in the earlier version.

Document Issues
The document issues are as follows:
02 (2009-06-30)
01 (2009-03-30)
Draft (2009-03-10)
Draft (2009-01-15)

Issue 02 (2009-06-30) Huawei Proprietary and Confidential 1-1

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1 Introduction to This Document HSDPA

02 (2009-06-30)
This is the document for the second commercial release of RAN11.0.
Compared with 01 (2009-03-30) of RAN11.0, this issue incorporates the changes described in
the following table.

Change Type Change Description Parameter Change

Feature change None. None.
Editorial change The description of MAC-hs Scheduling is The deleted parameters
optimized. For details, see section 4.3 are as follows:
MAC-hs Scheduling. MaxDchVoipHarqRt
MaxDchAmrHarqRt
The added parameters
are as follows:
8KRSCLMT
16KRSCLMT
32KRSCLMT
64KRSCLMT
128KRSCLMT
256KRSCLMT
384KRSCLMT
The description of QoS and Diff-Serv The added parameters
Management is optimized. For details, see are as follows:
section 5.2 Diff-Serv Management and 5.3 SingalUlMBR
QoS Parameter Mapping and Configuration.
SingalDlMBR
StreamUlMBR
StreamDlMBR
ConverUlMBR
ConverDlMBR
ARP1Priority
ARP2Priority
ARP3Priority
ARP4Priority
ARP5Priority
ARP6Priority
ARP7Priority
ARP8Priority
ARP9Priority
ARP10Priority
ARP11Priority
ARP12Priority
ARP13Priority

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HSDPA 1 Introduction to This Document

ARP14Priority
TrafficClass
THP
USERPRIORITY
UlGBR
DlGBR
SPI
FACTOR
HappyBR
THPClass
The structure of the document is adjusted. None.

01 (2009-03-30)
This is the document for the first commercial release of RAN11.0.
Compared with draft (2009-03-10), this issue incorporates the following changes:

Feature change None None
Editorial change The structure of the docuement is adjusted. None

Draft (2009-03-10)
This is the second draft of the document for RAN11.0.
Compared with draft (2009-01-15), draft (2009-03-10) optimizes the description.

Draft (2009-01-15)
This is the initial draft of the document for RAN11.0.
Compared with issue 03 (2008-11-30) of RAN10.0, draft (2009-01-15) incorporates the
following changes:

Change Change Description Parameter Change
Type

Feature The description of dynamic code tree The added parameters are as follows:
change reshuffling is added in section 3.7.6 CodeAdjForHsdpaUserNumThd
"Dynamic Code Tree Reshuffling."
CodeAdjForHsdpaSwitch
The description of setting the The added parameters are as follows:
maximum number of retransmissions


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1 Introduction to This Document HSDPA

Change Change Description Parameter Change
Type
on a service basis is added to section MaxDchVoipHarqRt
4.3.1 "Determining the Candidate MaxDchAmrHarqRt
Set."
MaxNonConverHarqRt
The description of HBR-based The added parameter is HappyBR.
resource allocation is added to section
4.3.2 "Calculating Priorities."
The description of a new resource The parameter RscAllocM is added
allocation method is added to section with a new value PowerCode_Bal.
4.5.3 "Determining the TBSused,
Modulation Scheme, Power, and
Codes."
Editorial The description of HSDPA is None
change rewritten for readability.
All the parameter names are replaced None
with the corresponding parameter IDs.


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HSDPA 2 Overview of HSDPA

2 Overview of HSDPA

2.1 General Principles of HSDPA
To meet the rapidly growing demands for data services on the mobile network, 3GPP Release
5 introduced HSDPA in 2005. HSDPA improves the downlink capacity, increases the user data
rate greatly, and reduces the transmission delay on the WCDMA network.
The characteristics of HSDPA are as follows:


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2 Overview of HSDPA HSDPA

Fast scheduling Fast scheduling introduced into the NodeB determines the UEs for
data transmission in each TTI (2 ms) and dynamically allocates
resources to these UEs. It improves the usage of system resources
and increases the system capacity.
For details about how Huawei RAN implements fast scheduling, see
section 4.3 "MAC-hs Scheduling."
Fast HARQ Fast hybrid automatic repeat request (HARQ) is used to rapidly
request the retransmission of erroneously received data.
Specifically, when the UE detects an erroneous data transmission, it
saves the received data and requests the NodeB to retransmit the
original data at the physical layer. Before decoding, the UE
performs soft combining of the saved data and the retransmitted
data. The combining makes full use of the data transmitted each
time and thus increases the decoding success rate. In addition, the
retransmission delay at the physical layer is reduced greatly,
compared with that at the RLC layer.
For details about how Huawei RAN implements fast HARQ, see
section 4.4 "HARQ."
Fast AMC To compensate for channel variations, the DCH performs power
control. To achieve this goal, HSDPA also performs fast adaptive
modulation and coding (AMC), that is, adjusts the modulation
scheme and coding rate in each TTI. AMC is based on the channel
quality indicator (CQI) reported by the UE, and its purpose is to
select an appropriate transmission rate so as to meet channel
conditions. When the channel conditions are good, 16QAM can be
used to provide higher transmission rates. When the channel
conditions are poor, QPSK can be used to ensure the transmission
quality.
For details about how Huawei RAN implements fast AMC, see
section 4.5 "TFRC Selection."

The MAC-hs, a new MAC sublayer, is introduced into the UE and NodeB to support HSDPA.

2.2 HSDPA Channels
To support the HSDPA technologies, 3GPP defines one transport channel (HS-DSCH) and
three physical channels (HS-PDSCH, HS-SCCH, and HS-DPCCH).
Figure 2-1 shows the physical channels of HSDPA in the shaded area.


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Figure 2-1 Physical channels of HSDPA

2.2.1 HS-DSCH and HS-PDSCH
HS-DSCH is a high speed downlink shared channel. Its TTI is fixed to 2 ms. It may be
mapped onto one or more HS-PDSCHs.
HS-PDSCH is a high speed physical downlink shared channel. Its spreading factor is fixed to
16. According to 3GPP TS 25.433, a maximum of 15 HS-PDSCHs can be used for
transmission at the same time. The number of HS-PDSCHs per cell is configurable.
Generally, the NodeB can use the HS-PDSCH codes only allocated by the RNC. The
NodeB-controlled dynamic code allocation, however, allows the NodeB to temporarily
allocate idle codes to the HS-PDSCH. "Dynamic Code Allocation Based on NodeB" is an
optional feature.
The use of 2 ms TTI reduces the round trip time (RTT) on the Uu interface and, together with
AMC, improves the tracking of channel variations. In addition, the use of 2 ms TTI enables
fast scheduling and resource allocation and thus improves the usage of transmission
resources.
In each TTI, HSDPA assigns the HS-PDSCHs onto which the HS-DSCH maps. More
HS-PDSCHs can provide higher transmission rates.
Unlike the DCH, the HS-DSCH cannot support soft handover. The reason is that this type of
handover requires different cells to use the same radio resource for sending the same data to
the UE, but the scheduling function can be performed only within the cell.

2.2.2 HS-SCCH
HS-SCCH is a high speed shared control channel. It carries the control information related to
the HS-DSCH. The control information includes the UE identity, HARQ-related information,
and information about transport format and resource combination (TFRC). For each
transmission of the HS-DSCH, one HS-SCCH is required to carry the related control
information. One cell can be configured with a maximum of four HS-SCCHs. The number of
HS-SCCHs determines the maximum number of UEs that can be scheduled simultaneously in
each TTI.

2.2.3 HS-DPCCH
HS-DPCCH is a high speed dedicated physical control channel. In the uplink, each HSDPA
UE must be configured with an HS-DPCCH. This channel is mainly used by the UE to report


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the CQI and whether a transport block is correctly received. The information about the
transport block is used for fast retransmission at the physical layer. The CQI is used for AMC
and scheduling to allocate Uu resources.

2.2.4 DPCCH and DPCH/F-DPCH
DPCCH is a dedicated physical control channel in the uplink. DPCH is a dedicated physical
channel in the downlink. F-DPCH is a fractional dedicated physical channel in the downlink.
The HSDPA UE must be configured with dedicated physical control channels in both the
uplink and the downlink. The uplink DPCCH is used for providing reference information
about the transmit power of HSDPA channels. In addition, it is used for closed-loop power
control by working with the DPCH or F-DPCH. In SRB over HSDPA mode, the downlink
channel can be established on the F-DPCH without the dedicated assisted DPCH. In this case,
a maximum of 10 UEs use an SF256 to transmit the TPC, thus saving a large amount of
downlink codes.

2.3 Impact of HSDPA on NEs
HSDPA has an impact on the RNC, NodeB, and UE.
On the control plane of the network side, the RNC processes the signaling about HSDPA cell
configuration, HS-DSCH related channel configuration, and mobility management. On the
user plane of the network side, the RLC layer and MAC-d of the RNC are unchanged. At the
NodeB, the MAC-hs is added to implement HSDPA scheduling, Uu resource allocation, AMC,
and Iub flow control. The MAC-hs implements these management functions in a short time.
Thus, it reduces both unnecessary delays and processing complexity caused by Iub message
exchange.
On the UE side, the MAC-hs is added between the MAC-d and the physical layer for data
reception. To support HSDPA, 3GPP defines 12 UE categories. These UEs support different
peak rates at the physical layer, ranging from 912 kbit/s to 14 Mbit/s. The UE of category 10
supports the highest rate. The UE of category 11 or 12 supports only the QPSK mode. For
details, see 3GPP TS 25.306. Huawei RAN supports all the UE categories.

2.4 HSDPA Functions
HSDPA functions are implemented on the HSDPA control plane and user plane.

2.4.1 HSDPA Control Plane Functions
The control plane is responsible for setting up and maintaining HS-DSCH connections and
managing cell resources.
Figure 2-2 shows the HSDPA control plane functions based on the service connection setup
and maintenance procedure.


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Figure 2-2 HSDPA control plane functions

The HSDPA control plane functions are described as follows:
Bearer mapping
The bearer mapping is used by the network side to configure the RAB during the setup
of a service connection in the cell. The network side then configures bearer channels for
the UE based on the requested service type, service rate, UE capability, and cell
capability.
For details, see section 3.1 "Bearer Mapping."
Access control
Access control, a sub-function of load control, checks whether the current resources of
the cell are sufficient for the service connection setup. If the resources are insufficient,
intelligent access control is triggered. If the resources are sufficient, the service
connection can be set up.
For details, see section 3.2 "Access Control."
Mobility management
For the established HS-DSCH connection, mobility management decides whether to
switch it to another cell for providing better services, based on the channel quality of the
UE.
For details, see section 3.3 "Mobility Management."
Channel switching
Channel switching is responsible for switching the transport channel among the
HS-DSCH, DCH, and FACH based on the requirements of mobility management or load
control.
For details, see section 3.4 "Channel Switching."
Load control
When the cell load increases, the load control function adjusts the resources configured
for the established radio connections to avoid cell overload.
For details, see section 3.5 "Load Control."
Resource management


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Resource management coordinates the power resource between the HS-DSCH and the
DCH and the code resource between the HS-SCCH and the HS-PDSCH. The downlink
power and codes are the bottleneck resources of the cell. Resource management can
increase the HSDPA capacity.
Power resource management reserves power for channels of different types and allocates
power for them. For details, see section 3.6 "Power Resource Management."
Code resource management allocates and reserves code resources for channels of
different types. In addition, it collects and reshuffles idle code resources.
For details, see section 3.7 "Code Resource Management."

2.4.2 HSDPA User Plane Functions
After the service is set up, the user plane is responsible for implementing data transmission.
Figure 2-3 shows the HSDPA user plane functions based on the data processing procedure.

Figure 2-3 HSDPA user plane functions

The service data carried on the HS-DSCH is passed to the RLC layer and MAC-d of the RNC
for processing and encapsulation. Then, the MAC-d PDU is formed and passed through the
Iub/Iur interface to the NodeB/RNC. To avoid congestion, the flow control and congestion
control functions control the traffic on the Iub/Iur interface through the HS-DSCH frame
protocol (3GPP TS 25.435).
After the MAC-d PDU is received by the NodeB, it is passed through the MAC-hs to the
physical layer and then sent out through the Uu interface. The MAC-hs provides MAC-hs
scheduling, TFRC selection, and HARQ. MAC-hs scheduling determines the HSDPA users in
the cell for data transmission. TFRC selection determines the transmission rates and Uu
resources to be allocated to the HSDPA UEs. HARQ is used to implement the hybrid
automatic repeat request function.


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HSDPA 3 Control Plane

3 Control Plane

This chapter consists of the following sections:
Bearer Mapping
Access Control
Mobility Management
Channel Switching
Load Control
Power Resource Management
Code Resource Management

3.1 Bearer Mapping
The HS-DSCH can carry services of multiple types and service combinations, as listed in
Table 3-1.

Table 3-1 Bearer mapping
CN Service Type Can Be Carried on Optional Feature?
Domain HS-DSCH?

- Signaling (SRB) Yes Yes
Feature name: SRB over HSDPA
CS Voice Yes Yes
Feature name: CS Voice over
HSPA/HSPA+
Videophone No No
Streaming No No
PS Conversational Yes Yes
Feature name: VoIP over
HSPA/HSPA+


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3 Control Plane HSDPA

CN Service Type Can Be Carried on Optional Feature?
Domain HS-DSCH?

Streaming Yes Yes
Feature name: Streaming Traffic
Class on HSDPA
Interactive Yes No
Background Yes No
IMS signaling Yes Yes
Feature name: IMS Signaling
over HSPA
MBMS PTP Yes Yes
Feature name: MBMS P2P over
HSDPA

During the service setup, the RNC selects appropriate channels based on the UE capability,
cell capability, and service parameters to optimize the use of cell resources and ensure the
QoS. Huawei RAN supports the setting of the types of RABs carried on the HS-DSCH
according to service requirements. For details, see the Radio Bearers Parameter Description.
Huawei supports bearer management of HSDPA over Iur. "HSDPA over Iur" is an optional
feature.

3.2 Access Control
Access control determines whether an HS-DSCH connection can be set up under the
precondition that the QoS is ensured. The determination is based on the status of cell
resources and the situation of Iub/Iur congestion. When the resources are insufficient, the
HS-DSCH is switched to the DCH and only the DCH connection is set up. When the
resources are sufficient, the DCH is switched to the HS-DSCH. The implementation of this
function requires the support of channel switching.
Access control allows the HSDPA UE to access an inter-frequency neighboring cell that has
the same coverage area as the source cell. The purpose is to achieve load balance between the
cells and improve HSDPA user experience. This is HSDPA directed retry decision (DRD), an
optional feature. For details, see the Load Control Parameter Description.

3.3 Mobility Management
The DCH supports soft handover, and therefore downlink data can be concurrently sent out
from all the cells in the active set in DCH transmission. In comparison, the HS-DSCH does
not support soft handover, and therefore downlink data can be sent out only from the
HS-DSCH serving cell and inter-cell handover has to be performed through the change of the
serving cell. Thus, HSDPA mobility management focuses on the change of the HS-DSCH
serving cell.


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For the UE with the HS-DSCH service, the best cell in the active set acts as the HS-DSCH
serving cell. When the best cell changes, the UE disconnects the HS-DSCH from the source
cell and attempts to set up a new HS-DSCH connection with the new best cell. For details, see
the Handover Parameter Description. By changing the HS-DSCH switching threshold, you
can modify the conditions for triggering the change of the best cell. Lowering this threshold
can increase both the handover frequency and the sensitivity of HS-DSCH switching to signal
variations in the serving cell. Raising this threshold can reduce the handover frequency but
may increase the probability of the HS-DSCH service being discontinuous or even dropping
on the cell edge. For the HS-DSCH service, Huawei supports inter-cell intra-frequency
handover, inter-cell inter-frequency handover, and inter-RAT handover.
Mobility management may trigger the switching from the HS-DSCH to the DCH. If the UE
with the HS-DSCH service cannot set up the HS-DSCH connection with the target cell, the
channel switching function, together with mobility management, switches the HS-DSCH to
the DCH. When the HS-DSCH connection is available, the channel switching function
switches the DCH back to the HS-DSCH. When the HSDPA user returns from the DCH cell
to the HSDPA cell, the DCH is set up to ensure successful handover. A certain period later
after the handover, the channel switching function switches the DCH to the HS-DSCH. For
details, see section 3.4 "Channel Switching."
"HSDPA over Iur" is an optional feature.

3.4 Channel Switching
After the HS-DSCH is introduced, the UE can stay in a new state, CELL_DCH (with
HS-DSCH). Thus, there are additional transitions between CELL_DCH (with HS-DSCH) and
CELL_FACH and transitions between CELL_DCH (with HS-DSCH) and CELL_DCH even
when both the cell and the UE support the HS-DSCH, as shown in Figure 3-1.

Figure 3-1 UE state transition

Table 3-2 lists new state transition and new channel switching.

Table 3-2 New state transition and new channel switching
New State Transition New Channel Switching

CELL_DCH (with HS-DSCH) <-> CELL_FACH HS-DSCH <-> FACH
CELL_DCH (with HS-DSCH) <-> CELL_DCH HS-DSCH <-> DCH


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Here, the switching between HS-DSCH and FACH can be triggered by traffic volume, which
is similar to the switching between DCH and FACH.
When the cell load is too high, load control may also trigger the switching from the
HS-DSCH to the FACH to relieve congestion. For details, see the Load Control Parameter
Description. When the cell load becomes low, channel switching aids load control in
attempting to switch the transport channel back to the HS-DSCH. For details, see the Rate
Control Parameter Description.
As the HS-DSCH is introduced later, it is inevitable that some cells support the HS-DSCH but
others do not. This is also the case with UEs. When a service is set up, the channel switching
function selects an appropriate bearer channel based on the cell capability and UE capability
to ensure the QoS while efficiently using the cell resources. When the user is moving, the
channel switching function adjusts the channel type based on the UE capability to ensure
service continuity while improving user experience.

Figure 3-2 Relations between channel switching and other functions

Triggers for switching from the HS-DSCH to the DCH are as follows:
The HS-DSCH is selected during the service setup but neither the resources of the
serving cell nor the resources of the inter-frequency same-coverage neighboring cell are
sufficient. In such a case, the HS-DSCH is switched to the DCH.
The HS-DSCH serving cell changes. The UE attempts to set up a new HS-DSCH
connection with the new best cell. In such a case, the possible scenarios are as follows:
− If the new best cell does not support the HS-DSCH, the UE cannot set up the
HS-DSCH connection. In this case, the HS-DSCH is switched to the DCH.
− If the new best cell supports the HS-DSCH but a new HS-DSCH connection cannot
be set up because the resources are insufficient, the DCH connection is set up and the
HS-DSCH is switched to this DCH.
The user moves from a cell supporting the DCH but not supporting the HS-DSCH to a
cell supporting the HS-DSCH. In this case, the DCH connection is also set up because
the DCH supports soft handover, which can increase the inter-cell handover success rate.
In one of the cases described previously, the DCH connection is set up in a cell supporting the
HS-DSCH or in an inter-frequency same-coverage neighboring cell supporting the HS-DSCH.
Then, the DCH is switched to the HS-DSCH by either of the following mechanisms:


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Channel switching based on timer
After the DCH connection is set up, this mechanism periodically attempts to switch the
DCH to the HS-DSCH.
Channel switching based on traffic volume
When the traffic volume of the UE increases and the RNC receives an event 4A report,
this mechanism attempts to switch the DCH to the HS-DSCH. For details on the event
4A report, see the Rate Control Parameter Description.

3.5 Load Control
When the cell is congested, load control selects some users (including HSDPA users) for
congestion relief. The selection is based on the integrated priority, which considers the
allocation retention priority (ARP), traffic class (TC), traffic handling priority (THP), and
bearer type. When the cell load is high, the basic congestion control selects some HSDPA
users for handover to an inter-frequency same-coverage neighboring cell or an inter-RAT
neighboring cell with lower load. When the cell load is too high, the overload congestion
control selects some HSDPA BE services for the switching to a common channel or releases
some HSDPA services. For details, see the Load Control Parameter Description.

3.6 Power Resource Management
Power resource management determines the transmit power of the HS-PDSCH, HS-SCCH,
and HS-DPCCH.
Generally, an HSDPA cell has the same coverage as the corresponding R99 cell. To improve
the resource usage in this case, the downlink power resources of HSDPA can be dynamically
allocated as follows:
1. The downlink power resources are first reserved for common physical channels and
allocated to the DPCH. The remaining power resources are available for HSPA,
including HSUPA and HSDPA.
2. The HSPA power resources are first allocated to the HSUPA downlink control channels,
including the E-AGCH, E-RGCH, and E-HICH. The remaining power resources are
available for HSDPA.
3. The HSDPA power resources are first allocated to the downlink control channel
HS-SCCH. For details, see the Power Control Parameter Description. The remaining
power resources are allocated to the traffic channel HS-PDSCH.
For details on power resource allocation, see section 4.5 "TFRC Selection."
Figure 3-3 shows the dynamic HSDPA power resource allocation.


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Figure 3-3 Dynamic HSDPA power resource allocation

Every TTI, the NodeB detects the power usage of R99 channels to determine the power
available for HSPA. To reserve the power for R99 power control itself, the power margin
PwrMgn needs to be set on the NodeB side. In addition, the power allocated to HSPA must
not exceed the maximum permissible power HspaPower, which can be set on the RNC side.
For details on uplink HS-DPCCH power control, see the Power Control Parameter
Description.
"HSDPA over Iur" is an optional feature.

3.7 Code Resource Management
Code resource management allocates code resources to the HS-SCCH and HS-PDSCH.
The NodeB supports HS-DSCH transmissions to multiple users in parallel in a TTI. If more
than one HS-PDSCH code can be allocated by the NodeB, then code multiplexing can be used
to allocate the codes to multiple users so as to improve resource usage and system throughput.
"Time and HS-PDSCH Code Multiplexing" is an optional feature.

3.7.1 HS-SCCH Code Resource Management
Each HS-SCCH uses an SF128 code. The number of HS-SCCHs determines the maximum
number of HSDPA users that can be scheduled simultaneously in a TTI. Generally, the
number of HS-SCCHs depends on the traffic characteristics of the cell. The default number is
4, which is specified by the parameter HsScchCodeNum on the RNC side. If the default
setting is used, the HS-PDSCH can use only 14 SF16 codes. To enable the HS-PDSCH to use
15 SF16 codes, you are advised to configure 2 HS-SCCHs.


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3.7.2 HS-PDSCH Code Resource Management
The DPCH and the HS-PDSCH coexist in a cell. Therefore, sharing the cell code resources
between them to improve the resource usage is of critical importance in HSDPA code resource
management.
Huawei supports both RNC-level and NodeB-level code resource management.
RNC-controlled static or dynamic code allocation is enabled through the parameter
AllocCodeMode. NodeB-controlled dynamic code allocation is enabled through the
parameter DynCodeSw. For details, see the following sections.
The dynamic code allocation controlled by the NodeB is more flexible than that controlled by
the RNC. It shortens the response time and saves the Iub signaling used for code reallocation.
Huawei recommends the following code allocation modes, where the first mode is preferred:
Configure the RNC to use static code allocation and the NodeB to use dynamic code
allocation.
If the NodeB does not support dynamic code allocation, configure the RNC to use
dynamic code allocation.
If not all the NodeBs controlled by an RNC support dynamic code allocation, the
RNC-controlled dynamic code allocation is recommended. In this case, the NodeB-controlled
dynamic code allocation can also be enabled for those supporting NodeBs.

3.7.3 RNC-Controlled Static Code Allocation
If the RNC-controlled static code allocation is used, the number of reserved HS-PDSCH
codes is specified by the parameter HsPdschCodeNum on the RNC side. Based on the
number, the RNC reserves codes for the HS-PDSCH. The DPCH, HS-SCCH, and common
channels use the other codes. The parameter HsPdschCodeNum can be set on the basis of the
traffic characteristics of the cell. If there are more HSDPA users and the traffic is high, the
parameter value can be increased. If there are more DCH users and the HSDPA traffic is low,
the parameter value can be decreased. A maximum of 15 codes can be allocated to the
HS-PDSCH.
Figure 3-4 shows the RNC-controlled static code allocation.

Figure 3-4 RNC-controlled static code allocation

3.7.4 RNC-Controlled Dynamic Code Allocation
If the RNC-controlled dynamic code allocation is used, the minimum number of available
HS-PDSCH codes is specified by the parameter HsPdschMinCodeNum on the RNC side.
The purpose of this setting is to prevent too many DCH users from being admitted and to
ensure the basic data transmission of the HS-PDSCH. In addition, the maximum number of
available HS-PDSCH codes is specified by the parameter HsPdschMaxCodeNum. The


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purpose of this setting is to prevent too many codes from being allocated for the HS-PDSCH
and to prevent DCH users from preempting codes during admission.
The number of codes that can be shared between HS-PDSCH and DPCH is equal to the value
of HsPdschMaxCodeNum minus the value of HsPdschMinCodeNum, as shown in Figure
3-5. When a code that can be shared is idle, it can be allocated to the HS-PDSCH if the idle
code is adjacent to the allocated HS-PDSCH codes.

Figure 3-5 RNC-controlled dynamic code allocation

Adding an HS-PDSCH Code
Figure 3-6 shows how to add an HS-PDSCH code. The solid dots represent the allocated
codes, and the circles represent the idle codes.

Figure 3-6 Adding an HS-PDSCH code

After a DCH RL is released or reconfigured (for example, because the spreading factor
becomes larger), the RNC adds an HS-PDSCH code if the following conditions are met:
The code adjacent to the allocated HS-PDSCH codes is idle.
After the code is added, the minimum spreading factor of the remaining codes is smaller
than or equal to the value of CellLdrSfResThd.
The parameter CellLdrSfResThd set on the RNC side is used to reserve codes for new users,
to avoid congestion due to code insufficiency, and to avoid unnecessary reshuffling of the
code tree.


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Releasing an HS-PDSCH Codes
Figure 3-7 shows how to release an HS-PDSCH code. The solid dots represent the allocated
codes, and the circles represent the idle codes.

Figure 3-7 Releasing an HS-PDSCH code

If idle DPCH codes are insufficient when a DCH RL is set up, added, or reconfigured (for
example, because the spreading factor becomes smaller), the RNC preempts HS-PDSCH
codes in the shared codes for the DPCH. In addition, if the minimum spreading factor of idle
DPCH codes is greater than the value of CellLdrSfResThd, the RNC can also reallocate
some HS-PDSCH codes to the DPCH. The reallocated code number must be the smallest one
of the available shared codes.

3.7.5 NodeB-Controlled Dynamic Code Allocation
Generally, the NodeB can use the HS-PDSCH codes only allocated by the RNC. The
NodeB-controlled dynamic code allocation, however, allows the NodeB to temporarily
allocate idle codes to the HS-PDSCH.

Figure 3-8 NodeB-controlled dynamic code allocation

Every TTI, the NodeB detects the SF16 codes that are not allocated to the HS-PDSCH. If
such an SF16 code or any of its subcodes is allocated by the RNC to the DCH or a common
channel, this SF16 code is regarded as occupied. Otherwise, it is regarded as unoccupied.
Therefore, the available HS-PDSCH codes include the codes reserved by the RNC and the
idle codes adjacent to the allocated HS-PDSCH codes. Every time the RNC allocates or
release HS-PDSCH codes, it notifies the NodeB through Iub signaling and the NodeB
performs the corresponding processes.
For example, the RNC reserves the SF16 codes numbered 11 to 15 for the HS-PDSCH and
those numbered 0 to 5 for the DCH and common channels in a TTI. Thus, the HS-PDSCH can
use the codes numbered 6 to 15 in this TTI.


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If the setup of an RL requires a DPCH code that is already allocated by the NodeB to the
HS-PDSCH, the NodeB releases this code and sends an NBAP message to the RNC,
indicating that the RL is set up successfully. Then, the DCH uses this code. After the DCH
releases it, the HS-PDSCH can use this code again.
"Dynamic Code Allocation Based on NodeB" is an optional feature.

3.7.6 Dynamic Code Tree Reshuffling
Regardless of whether dynamic code allocation is controlled by the RNC or the NodeB, the
number of continuous codes available for the HS-PDSCH shall be maximized. The dynamic
code tree reshuffling function can achieve this goal by reallocating DPCH codes.
When the minimum spreading factor of the remaining idle codes in a cell is greater than the
value of CellLdrSfResThd, the RNC reshuffles the codes used by the DPCH to provide more
continuous SF16 codes for HSDPA. This function can be enabled or disabled by the
parameter CodeAdjForHsdpaSwitch on the RNC side.
In addition, the threshold number of users that can be reshuffled needs to be specified by the
parameter CodeAdjForHsdpaUserNumThd. If the number of users on a subtree is smaller
than or equal to this parameter value, this subtree can be reshuffled. Otherwise, it cannot be
reshuffled. This parameter limits the number of users that can be reshuffled each time, to
prevent too many users from being reshuffled in a short time and thus to avoid affecting user
experience.

Figure 3-9 Dynamic code tree reshuffling


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HSDPA 4 User Plane

4 User Plane

This chapter consists of the following sections:
Flow Control and Congestion Control
RLC and MAC-d
MAC-hs Scheduling
HARQ
TFRC Selection

4.1 Flow Control and Congestion Control
HSDPA flow control and congestion control are used to control the HSDPA data flow on the
Iub and Iur interfaces. HSDPA data packets are sent through the Iub interface to the NodeB
and then through the Uu interface to the UE. Thus, congestion may occur on the Uu, Iub, or
Iur interface. Flow control is used to relieve Uu congestion, and congestion control is used to
relieve Iub/Iur congestion. The two types of control are implemented by the NodeB. HSDPA
flow control and congestion control are part of the HSDPA Iub frame protocol (3GPP TS
25.435). They are implemented for each MAC-hs queue through the Capacity Request
message sent by the RNC and the Capacity Allocation message sent by the NodeB.
Figure 4-1 shows the basic principles of flow control and congestion control.


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4 User Plane HSDPA

Figure 4-1 Basic principles of Iub flow control and congestion control

4.1.1 Flow Control
For each MAC-hs queue, flow control calculates the pre-allocated Iub bandwidth based on the
Uu transmission rate and the amount of data buffered in the NodeB. The Uu transmission rate
of the MAC-hs queue is determined by the scheduling algorithm. For each MAC-hs queue, if
the Iub transmission rate is higher than the Uu transmission rate, the data packets are buffered.
Too much data buffered in the NodeB leads to transmission delay and even packet loss.
Therefore, each MAC-hs queue should not have too much data buffered in the NodeB. On the
other hand, it should keep a certain amount of data to avoid wasting the Uu resources due to
no data to transmit.
The flow control procedure is as follows:
1. The NodeB measures the buffered data amount of each MAC-hs queue and the average
Uu transmission rate.
2. The NodeB estimates the buffering time based on the measurements.
3. The NodeB adjusts the Iub bandwidth pre-allocated to the MAC-hs queue.
The pre-allocated Iub bandwidth is adjusted as follows:
If the buffering time is too short, you can infer that the RNC slows down the data
transmission, that is, the Iub transmission rate is lower than the Uu transmission rate. In
such a case, the pre-allocated Iub bandwidth is adjusted to a value greater than the
average Uu transmission rate.
If the buffering time is appropriate, the pre-allocated Iub bandwidth is adjusted to the
average Uu transmission rate.
If the buffering time is too long, the pre-allocated Iub bandwidth is adjusted to a value
smaller than the average Uu transmission rate.


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HSDPA 4 User Plane

4.1.2 Congestion Control
The Iub bandwidth may be lower than the Uu bandwidth. If the RNC uses the Iub bandwidth
pre-allocated to each MAC-hs queue, the Iub bandwidth for HSDPA is insufficient. This may
lead to congestion and even packet loss.
The amount of data to be transmitted is sent by the RNC to each MAC-hs queue through the
Capacity Request message. Based on this amount and the total Iub bandwidth available for
HSDPA, the congestion control function adjusts the bandwidth pre-allocated to each MAC-hs
queue. Thus, congestion control ensures that the total bandwidth actually allocated to all the
MAC-hs queues is not higher than the total available Iub bandwidth.
The total Iub bandwidth available for HSDPA depends on the variations in HSDPA packet
delay and the situation of packet loss. HSDPA shares the bandwidth with the DCH and control
signaling, and the DCH and control signaling has higher priorities than HSDPA. Thus, when
the HSDPA packet delay or packet loss increases, you can infer that the number of DCHs or
the amount of control signaling increases. In such a case, the bandwidth available for HSDPA
decreases and the bandwidth actually allocated for HSDPA decreases.
For details on congestion control, see the Transmission Resource Management Parameter
Description.

For the Iur interface, flow control and congestion control are also applied. The control principles and
processing procedures are the same as those for the Iub interface.

4.2 RLC and MAC-d
4.2.1 RLC
One of the main purposes of HSDPA is to reduce latency by handling retransmissions at
NodeB level. Retransmissions, however, may still be triggered at the RLC layer of the RNC
under the following circumstances:
The NodeB misinterprets an NACK sent by the UE.
The number of HARQ retransmissions exceeds the maximum permissible number.
The data buffered in the NodeB is lost when the HS-DSCH serving cell changes.
Therefore, HARQ retransmission cannot totally replace RLC retransmission, which is
described in 3GPP TS 25.322. For services with high requirements for data transmission
reliability, Huawei recommends that the RLC acknowledged mode (AM) also be used to
ensure correct transmission on the Uu interface even when the services such as the BE service
are carried on HSDPA channels.
Before the introduction of HSDPA, the size of an RLC PDU is usually 336 bits, where 320
bits are for the payload and 16 bits for the RLC header. Without additional overhead, the
MAC PDU is of the same size as the RLC PDU. According to the 3GPP specifications, a
maximum of 2,047 RLC PDUs can be transmitted within an RLC window, and the RTT at the
RLC layer is about 100 ms (50 TTIs). In this condition, the maximum peak rate can only be
336 bits x (2047/50)/2 ms = 6.88 Mbit/s. To reach higher rates, an RLC PDU of 656 bits is
introduced, where 640 bits are for the payload and 16 bits for the RLC header. The RLC PDU
size can be set for each typical service. For high-speed services, the size is set to 656 bits by
default.


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4 User Plane HSDPA

4.2.2 MAC-d
The MAC-d functionality is unchanged after the introduction of HSDPA. The HS-DSCH
bearers are mapped onto MAC-d flows on the Iub/Iur interface. Each MAC-d flow has its
own priority queue.
The theoretical peak rate of HSDPA on the Uu interface is 14.4 Mbit/s. It is calculated on the
assumption that the chip rate of WCDMA is 3.84 Mcps, the spreading factor for HSDPA is
SF16, the maximum number of available codes is 15, and the gain of 16QAM is 4. Thus, the
rate is 3.84 Mcps/16 x 15 x 4 = 14.4 Mbit/s.
Limited by many factors, the theoretical peak rate of 14.4 Mbit/s is unreachable in actual
situations. The UE capability is one factor. For example, 3GPP specifies that the UE of
category 10 can use a maximum of 15 codes and receive a transport block with a maximum of
27,952 bits. For details, see 3GPP TS 25.306. Thus, the theoretical peak rate is 27952 bits/2
ms = 13.976 Mbit/s.
In addition, the RLC PDU size is fixed to 656 bits, and a transport block of 27,952 bits can
contain a maximum of 42 PDUs. Thus, the maximum RLC payload rate is (656 bits – 16 bits)
x 42/2 ms = 13.44 Mbit/s.
In practice, the radio channel quality, retransmission probability, and available power also
need to be considered. Therefore, the UE of category 10 cannot reach 13.44 Mbit/s at the RLC
layer in most tests.

4.3 MAC-hs Scheduling
With the limited Uu resources for HSDPA in a cell, the user expects to maximize the service
rate while the telecom operator expects to maximize the system capacity. MAC-hs scheduling
is used to coordinate the Uu resources, user experience, and system capacity. It is
implemented at the NodeB MAC-hs.
The scheduling algorithm consists of two steps. At first, the algorithm determines which
initial transmission queues or retransmission processes can be put into the candidate set for
scheduling. Then, the algorithm calculates their priorities based on factors such as the CQI,
user fairness, and differentiated services. If the algorithm is weighted more towards the
channel quality of the UE, the HSDPA cell can have a higher capacity but user fairness and
differentiated services may be affected. If the algorithm is weighted more towards user
fairness and differentiated services, the system capacity may be affected.
Huawei provides four scheduling algorithms: maximum C/I (MAXCI), round-robin (RR),
proportional fair (PF), and Enhanced Proportional Fair (EPF). The EPF algorithm is optional.

4.3.1 Determining the Candidate Set
The candidate for scheduling contains new data packets (hereinafter referred to as initial
transmission queues) or data packets to be retransmitted (hereinafter referred to as
retransmission processes), with the following exceptions:
If the UE starts the compressed mode, its data cannot be put into the candidate set during
the GAP.
If the UE category requires the UE to wait for several TTIs before it can be scheduled
again, its data cannot be put into the candidate set in this period. The UE of category 1 or
2 needs to wait for 3 TTIs, and the UE of category 3, 4, and 11 must wait for 2 TTIs.


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HSDPA 4 User Plane

If the number of retransmissions of a data packet reaches or exceeds the maximum
number, the data of this UE cannot be put into the candidate set. The data should be
discarded.
Huawei supports that the maximum number of retransmissions is set on a service basis:
− MaxNonConverHarqRt: the maximum number of non-conversational service
retransmissions in the CELL_DCH state
Other user data can be put into the candidate set.

4.3.2 Calculating Priorities
Four algorithms are available for calculating the priorities of data packets in the candidate set.
The scheduling policies vary according to the algorithms for calculating the priorities of data
packets. The algorithm to be used is specified by the parameter SM on the NodeB LMT.

MAXCI Algorithm
The retransmission processes unconditionally have higher priorities than the initial
transmission queues. The retransmission processes are sorted in first-in first-out (FIFO) mode.
The initial transmission queues are sorted in the CQI order. A higher CQI means a higher data
priority.
The MAXCI algorithm aims to maximize the system capacity but cannot ensure user fairness
and differentiated services.

RR Algorithm
transmission queues. The retransmission processes are sorted in FIFO mode. The initial
transmission queues are sorted in the order of the waiting time in the MAC-hs queue. A longer
waiting time means a higher data priority.
The RR algorithm aims to ensure user fairness but cannot provide differentiated services. Not
considering the CQI reported by the UE leads to lower system capacity.

PF Algorithm
transmission queues. The retransmission processes are sorted in FIFO mode. The initial
transmission queues are sorted in the order of R/r. Here, R represents the throughput
corresponding to the CQI reported by the UE, and r represents the throughput achieved by the
UE. A greater R/r value means a higher data priority.
The PF algorithm aims to make a tradeoff between system capacity and user fairness. It
provides the user with an average throughput that is proportional to the actual channel quality.
The system capacity provided by PF is between the system capacity provided by RR and that
provided by MAXCI.

EPF Algorithm
The EPF algorithm can meet the requirements of telecom operators related to user fairness
and differentiated services and also provide a high system capacity.


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4 User Plane HSDPA

Firstly, priorities are determined on the basis of service types. The EPF algorithm
distinguishes between delay-sensitive data and throughput-sensitive data based on the QoS
requirements.
The amount of delay-sensitive data is generally small. The transmission delay of
delay-sensitive data should be as short as possible. When the transmission delay reaches a
specified threshold, data packets are discarded. The delay-sensitive data includes the
following data:
SRB signaling
VoIP and AMR service data whose waiting time approaches the value of the discard
timer
The amount of a throughput-sensitive data is generally small. A higher transmission rate
brings greater user satisfaction. The throughput-sensitive data includes the following data:
BE service data
Streaming service data
IMS data
VoIP and AMR service data whose waiting time is far from the value of the discard timer
The EPF algorithm meets the basic QoS requirements of users. For delay-sensitive data, the
transmission delay must not exceed the maximum permissible delay. For throughput-sensitive
data, the transmission rate must not be lower than the GBR. Users require higher QoS for
delay-sensitive data. Therefore, the delay-sensitive data has a higher priority than the
throughput-sensitive data.
Secondly, for delay-sensitive data or throughput-sensitive data, the EPF algorithm
distinguishes between retransmission processes and initial transmission queues. The
retransmission processes unconditionally have higher priorities than the initial transmission
queues.
Thirdly, the priorities of the initial transmission queues are calculated for delay-sensitive data
or throughput-sensitive data. The following factors are considered: the waiting time, CQI
reported by the UE, throughput achieved by the UE, guaranteed bit rate (GBR), scheduling
priority indicator (SPI) weight, happy bit rate (HBR), and power consumed in the queue for a
certain period. The impacts of these factors on the priority calculation are as follows:
For the delay-sensitive data, a longer waiting time means a higher data priority.
For the throughput-sensitive data, a greater R/r value means a higher data priority. Here,
R represents the throughput corresponding to the CQI reported by the UE, and r
represents the throughput achieved by the UE.
The UEs with the rates lower than the GBR have higher priorities than those with the
rates already reaching the GBR.
A higher SPI weight means a higher data priority.
A larger difference between the actual rate and the HBR means a higher data priority.
When the resource limitation switch (RscLmSw) is on, the algorithm allocates the
lowest priority to a queue whose power consumption exceeds the threshold. RscLmSw
is used to prevent the users in areas with poor coverage from consuming too many cell
resources so that there is no decrease in system capacity. The ratio of the maximum
available power of a queue to the total power of the cell depends on the GBR, as listed in
Table 4-1.
By calculating the priority of each queue, the scheduling algorithm achieves the following:


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HSDPA 4 User Plane

When the system resources are sufficient to meet the basic QoS requirements of all users,
the transmission delay of delay-sensitive data is within the permissible range and the
transmission rate of throughput-sensitive data is not lower than the GBR. High-priority
users can obtain more resources for higher QoS.
When the system resources are insufficient to meet the basic QoS requirements of all
users, delay-sensitive data has higher priorities than throughput-sensitive data.
High-priority users can obtain more resources to ensure the basic QoS.
Fourthly, special processing is performed.
Differentiated services based on SPI weights are provided. Different services have
different service types, and different users have different user priorities. Therefore, the
scheduling function needs to consider these two factors to provide differentiated services.
SPI is a parameter specified on the basis of service types and users priorities. The
parameter SPIweight can be specified according to the SPI to provide differentiated
services. This parameter is specified on the RNC, and its value ranges from 0% to 100%.
The SPI weight affects the calculation of queue priorities. It is used to quantify the
differentiated services. If all the rates of throughput-sensitive services with different SPI
weights exceed or none of the rates exceeds their GBRs, the proportion of SPI weights
determines the proportion of rates among users. For example, for three
throughput-sensitive service users with the same channel quality, if their GBRs are not
configured and the proportion of SPI weights is 100:50:30, the proportion of actual rates
is close to 100:50:30.
Differentiated services based on SPI weights are optional.
Users with poor channel quality are prevented from consuming too many radio resources.
If a user in a poor-coverage area, for example, at the edge of a cell, has a high priority,
too many radio resources may be consumed to meet the QoS requirement. In this case,
the QoS of other users may be affected. To solve this problem, resource restriction
parameters such as 8KRSCLMT, 16KRSCLMT, 32KRSCLMT, 64KRSCLMT,
128KRSCLMT, 256KRSCLMT, and 384KRSCLMT are defined to restrict the
maximum power consumption of each user. They are configured on the NodeB
according to the GBRs.

Table 4-1 Default maximum ratios based on the GBR
GBR (kbit/s) Maximum Ratio

8 10%
16 10%
32 15%
64 15%
128 20%
256 25%
384 30%

The HBR is configured. The HBR determines the throughput expected by the user based
on a study on user experience. When the rate for a user reaches the HBR, the scheduling
probability for the user is decreased. Therefore, the scheduling probability of the users
with rates lower than the HBR is increased. In this way, more users can obtain satisfying


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4 User Plane HSDPA

services. The HBR is specified by the parameter HappyBR on the RNC side. The setting
can be based on user levels, including gold, silver, and copper.
For details on the parameters related to QoS management, such as the GBR, SPI, SPI weight,
and HBR, see section 5.3 "QoS Parameter Mapping and Configuration."
The EPF algorithm is optional.

4.3.3 Comparison of Four Algorithms
Table 4-2 lists the factors considered in the four scheduling algorithms.

Table 4-2 Factors considered in the four scheduling algorithms

Factor MAXCI RR PF EPF

Service type No No No Yes
Initial transmission or Yes Yes Yes Yes
retransmission
Maximum power No No No Yes
Waiting time No Yes No Yes
CQI Yes No Yes Yes
Actual throughput No No Yes Yes
SPI No No No Yes
GPR No No No Yes
HBR No No No Yes

Table 4-3 lists the effects of the four scheduling algorithms.

Table 4-3 Effects of the four scheduling algorithms
Item MAXCI RR PF EPF

System capacity Highest High Higher Higher
User fairness Not guaranteed Best Guaranteed Guaranteed
Differentiated Not guaranteed Not guaranteed Not guaranteed Guaranteed
services
Real-time services Not guaranteed Not guaranteed Not guaranteed Guaranteed


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HSDPA 4 User Plane

4.4 HARQ
The main purpose of introducing HARQ is to reduce the retransmission delay and improve
the retransmission efficiency. HARQ enables fast retransmission at the physical layer. Before
decoding, the UE combines the retransmitted data and the previously received data, thus
making full use of the data transmitted each time. In addition, HARQ can fine-tune the
effective rate to compensate for the errors made by TFRC section.

4.4.1 HARQ Retransmission Principles
The HARQ process of HSDPA involves only the NodeB and the UE, without involving the
RNC. After receiving a MAC-hs PDU sent by the NodeB, the UE performs a CRC check and
reports an ACK or NACK on the HS-DPCCH to the NodeB:
If the UE reports an ACK, the NodeB transmits the next new data.
If the UE reports an NACK, the NodeB retransmits the original data. After receiving the
data, the UE performs soft combining of this data and the data received before, decodes
the combined data, and then reports an ACK or NACK to the NodeB.
RLC retransmission on the DCH involves the RNC, and therefore the RTT is relatively long.
In comparison, HARQ involves only the physical layer and MAC-hs of the NodeB and those
of the UE, and therefore the RTT is reduced to only 6 TTIs.
After a transmission, the HARQ process must wait at least 10 ms before it can transmit the
next new data or retransmit the original data. Therefore, to improve transmission efficiency,
other HARQ processes can transmit data during the waiting time. A maximum of six HARQ
processes can be configured in each of the NodeB HARQ entity and the UE HARQ entity.
Note that not all UE categories support six HARQ processes. For example, the UEs of some
categories can receive data every one or two TTIs. Thus, only two or three HARQ processes
can be configured. The RAN can automatically choose the most appropriate configuration
based on UE capability.

Figure 4-2 HARQ retransmission principle


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4 User Plane HSDPA

4.4.2 Soft Combining During HARQ
Before decoding a MAC-hs PDU, the UE performs soft combining of all the data received
before to improve the utilization of Uu resources and thus increase the cell capacity. The size
of the UE buffer determines the number of coded bits or the size of transport blocks.
For HARQ retransmission between the NodeB and the UE, two combining strategies are
available. They are Chase Combining (CC) and Incremental Redundancy (IR). In the case of
CC, all retransmitted data is the same as previously transmitted data. In the case of IR, the
retransmitted data may be different from the previously transmitted data. In comparison, IR
has a higher gain than CC but requires more buffer space. CC can be regarded as a special
case of IR. The IR strategy is hard-coded in Huawei RAN.

4.4.3 Preamble and Postamble
If the HS-SCCH is received, the UE checks whether the HS-PDSCH is also correctly received
and then reports an ACK or NACK in the first slot of the HS-DPCCH subframe. If the
HS-SCCH is erroneously received, the UE does not report any information in the first slot of
the HS-DPCCH subframe. This type of transmission is called DTX. In the case of high
interference, the NodeB may demodulate DTX as ACK by mistake when demodulating the
HS-DPCCH. Thus, the lost data blocks cannot be retransmitted through HARQ retransmission,
and the reception can be ensured only through RLC retransmission. To meet the requirement
of the 3GPP specifications for a low DTX misjudgment probability, more power has to be
allocated for HS-DPCCH ACK/NACK.
To solve this problem, 3GPP TS 25.214 introduces preamble and postamble. When the NodeB
demodulates an HS-DPCCH ACK/NACK, it considers the subframe prior to and the subframe
next to the HS-DPCCH subframe in addition to the HS-DPCCH subframe itself. Thus, for a
certain DTX misjudgment probability, the introduction of preamble and postamble reduces
the power required by ACK/NACK, lower the downlink load level, and increase the uplink
capacity. "HS-DPCCH Preamble Support" is an optional feature.

Figure 4-3 HS-DPCCH preamble and postamble


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