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3.1 Terms
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For the purposes of the present document, the terms given in TR 21.905 [1] and the following apply. A term defined in the present document takes precedence over the definition of the same term, if any, in TR 21.905 [1].
example: text used to clarify abstract rules by applying them literally.
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3.2 Symbols
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For the purposes of the present document, the following symbols apply:
<symbol> <Explanation>
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3.3 Abbreviations
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For the purposes of the present document, the abbreviations given in TR 21.905 [1] and the following apply. An abbreviation defined in the present document takes precedence over the definition of the same abbreviation, if any, in TR 21.905 [1].
<ABBREVIATION> <Expansion>
Annex A (informative):
Change history
Change history
Date
Meeting
TDoc
CR
Rev
Cat
Subject/Comment
New version
2025-08
SA5#162
Initial skeleton
V0.0.0
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1 Scope
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The present document …
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2 References
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The following documents contain provisions which, through reference in this text, constitute provisions of the present document.
- References are either specific (identified by date of publication, edition number, version number, etc.) or non‑specific.
- For a specific reference, subsequent revisions do not apply.
- For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.
[1] 3GPP TR 21.905: "Vocabulary for 3GPP Specifications".
…
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3 Definitions of terms, symbols and abbreviations
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3.1 Terms
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For the purposes of the present document, the terms given in 3GPP TR 21.905 [1] and the following apply. A term defined in the present document takes precedence over the definition of the same term, if any, in 3GPP TR 21.905 [1].
example: text used to clarify abstract rules by applying them literally.
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3.2 Symbols
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For the purposes of the present document, the following symbols apply:
<symbol> <Explanation>
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3.3 Abbreviations
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For the purposes of the present document, the abbreviations given in 3GPP TR 21.905 [1] and the following apply. An abbreviation defined in the present document takes precedence over the definition of the same abbreviation, if any, in 3GPP TR 21.905 [1].
<ABBREVIATION> <Expansion>
Annex <X>:
Change history
Change history
Date
Meeting
TDoc
CR
Rev
Cat
Subject/Comment
New version
2025-08
-
n/a
-
-
-
Initial skeleton
0.0.0
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1 Scope
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The present document …
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2 References
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The following documents contain provisions which, through reference in this text, constitute provisions of the present document.
- References are either specific (identified by date of publication, edition number, version number, etc.) or non‑specific.
- For a specific reference, subsequent revisions do not apply.
- For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.
[1] 3GPP TR 21.905: "Vocabulary for 3GPP Specifications".
…
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3 Definitions of terms, symbols and abbreviations
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3.1 Terms
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For the purposes of the present document, the terms given in TR 21.905 [1] and the following apply. A term defined in the present document takes precedence over the definition of the same term, if any, in TR 21.905 [1].
example: text used to clarify abstract rules by applying them literally.
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3.2 Symbols
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For the purposes of the present document, the following symbols apply:
<symbol> <Explanation>
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3.3 Abbreviations
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For the purposes of the present document, the abbreviations given in TR 21.905 [1] and the following apply. An abbreviation defined in the present document takes precedence over the definition of the same abbreviation, if any, in TR 21.905 [1].
<ABBREVIATION> <Expansion>
Annex <A> (informative):
Change history
Change history
Date
Meeting
TDoc
CR
Rev
Cat
Subject/Comment
New version
2024-8
SA5#162
Initial Skeleton
0.0.0
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1 Scope
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The present document is part of the Release 6 work item "FDD Enhanced Uplink".
The purpose of the present document is to help the TSG RAN WG3 group to specify the changes to existing Iub/Iur specifications, needed for the introduction of "Iub/Iur Congestion Control" measures for Release 6.
This work task belongs to the TSG RAN Building Block "FDD Enhanced Uplink: UTRAN Iub/Iur Protocol Aspects", and as such this document is expected to be completed within the Release 6 timeframe.
This document also includes 3.84 Mcps TDD Enhanced Uplink which is part of the Release 7 work item “3.84 Mcps Enhanced Uplink”.
This document also includes 7.68 Mcps TDD Enhanced Uplink which is part of the Release 7 work item “7.68 Mcps Enhanced Uplink”.
This document also includes 1.28 Mcps TDD Enhanced Uplink which is part of the Release 7 work item "1.28 Mcps Enhanced Uplink".
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2 References
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The following documents contain provisions which, through reference in this text, constitute provisions of the present document.
• References are either specific (identified by date of publication, edition number, version number, etc.) or non‑specific.
• For a specific reference, subsequent revisions do not apply.
• For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.
[1] void.
[2] void.
[3] void.
[4] void.
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3 Definitions, symbols and abbreviations
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3.1 Definitions
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For the purposes of the present document, the following terms and definitions apply:
E-DCH: Enhanced DCH, a new dedicated transport channel type or enhancements to an existing dedicated transport channel type.
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3.2 Symbols
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For the purposes of the present document, the following symbols apply:
void
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3.3 Abbreviations
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For the purposes of the present document, the following abbreviations apply:
CFN Connection Frame Number
DRT Delay Reference Time
FSN Frame Sequence Number
HSDPA High Speed Downlink Packet Access
RFN RNC Frame Number
RNL Radio Network Layer
SFN System Frame Number
TNL Transport Network Layer
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4 Background and introduction
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In RAN Plenary Meeting #27, it was agreed to create a Technical Report on the subject of "Iub/Iur congestion control (Rel-6)".
The technical objective of this TR is to improve the Congestion Handling performance of the UTRAN over the Iub and the Iur interfaces.
Any solution should take into account backwards compatibility aspects.
This work item is applicable to UTRA FDD only.
In a similar manner the work items for enhanced uplink for 3.84 Mcps TDD and 7.68 Mcps TDD also lead to the need to improve the Congestion Handling over the Iub and Iur interfaces.
In a similar manner the work item for enhanced uplink for 1.28 Mcps TDD also leads to the need to improve the Congestion Handling over the Iub and Iur interfaces.
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5 Requirements
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For Iub/Iur Congestion Controlled, the following requirements were agreed in RAN3:
• RNC shall have a means for detecting congestion.
• Receiving node shall have a means for notifying the source of congestion i.e. sending node, that congestion has occurred.
• Iub/Iur Congestion control for both HSDPA and Enhanced Uplink should – if possible – employ similar solutions.
• The development of an Iub/Iur congestion control solution should bear in mind both the existing HSDPA and soon to be completed Enhanced Uplink features.
• Any solution should take into account backwards compatibility aspects.
6 Study areas
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6.1 Background information
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6.1.1 Introduction
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There are many types of congestion control mechanisms, the main groups are window based, rate based or combination of both. The method often used for congestion detection is the method based on the loss of packets. Other methods appropriate for congestion detection are: packet delay, average queue and rate difference.
Different congestion control algorithms might be used for IP network and ATM network respectively. It is known, that in IP network as a congestion control protocol mostly TCP is used, so to ensure fairness and other quality congestion control parameters, TCP like congestion control protocol should be used. TFRC protocol (TCP-Friendly Rate-based congestion Control protocol), as one of the many examples, which intends to compete fairly for bandwidth with TCP flows, could be named.
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6.1.2 Example 1: TFRC
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Congestion Factor depends on the congestion control algorithm, and on the congestion detection method. By detecting the loss of packets and using some method to derive RTT, transmit rate could be prepared according to transmit rate formula X = f(s, RTT, p) where s is the packet size in bytes/second, RTT – the round trip time in seconds, p is the loss event rate (based on the packet loss derived from the congestion detection).
Congestion Factor depends on the computed data rate X. The Credit, Interval and Repetition Period of FC Allocation message will be influenced by computed Congestion Factor in Congestion Control and the message Capacity Allocation with modified IEs will be sent to RNC.
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6.1.3 Example 2: "ABR like" congestion control
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"ABR like" congestion control has "additive increase, exponential decrease" type of algorithms. Different formulas exist for computing ACR (Allowed Cell Rate) for increase and for decrease. ACR i.e. current transmission rate in cell/s, should be computed in octets or in number of MAC-d PDUs. Then from the computed ACR, by a given HS-DSCH Interval, HS-DSCH Credits can be derived, because ACR is equal to Credits divided by Interval. Capacity Allocation message will be sent to RNC.
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6.2 Functional description
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6.2.1 Iub/Iur congestion detection
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The Node B scheduler decides on when and with which bit rate each and every UE is allowed to transmit in the cell. Each received MAC-es PDU is placed in a frame protocol data frame and sent to the SRNC (in some cases several PDUs are bundled into the same data frame). For each data frame, the Node B attach the following information:
• A reference time, that gives an indication on when the frame was sent.
• A sequence number, that gives an indication on which frame this is in relation to other data frames.
At the reception of the data frames the SRNC can do the following:
• With the use of the reference time, the SRNC can compare the relative reception time with the relative transmission time (the reference time included in the data frame). With that information the SRNC can detect if there is a delay build-up in the transmission path. A delay build-up is an indication on that frames are being queued due to overload in the transport network.
• With the use of the sequence number, the SRNC can detect a frame loss. A frame loss is an indication that packets have been lost in the transport network due to overload reasons.
This procedure is illustrated in Figure 1.
Figure 1: Iub/Iur Congestion Detection
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6.2.2 Iub/Iur congestion reduction
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When the RNC has detected that there is a congestion situation in the transport network, it needs to inform the Node B that this is the case. This is done by means of a frame protocol control frame, in which the Node B is informed about the congestion situation. This control frame will be called Congestion Indication. This is illustrated in Figure 2.
Figure 2: Iub/Iur Congestion Indication
As the RNC can detect congestion in two different ways, there exist no motivation why such information should not be communicated to the Node B. For that reason the Congestion Indication Control Frame can take the following values: "Congestion – detected by frame loss", "Congestion – detected by delay build-up", and "No congestion".
At the reception of the Congestion Indication control frame, the Node B should reduce the bit rate on the Iub interface. The exact algorithm the Node B should use is outside the scope of the specifications, but the specifications should address the expected behaviour of the Node B.
Such behaviour should include:
• At the reception of a congestion indication control frame indicating "congestion" the Node B should reduce the bit rate for at least the MAC-d flow on which the congestion indication control frame was received.
• At the reception of a congestion indication control frame indicating "no congestion" the Node B can gradually go back to normal operation.
• If the Node B has not received a congestion status control frame indicating congestion for the last X seconds, the Node B can gradually go back to normal operation. The value of the parameter X is configured by higher layers.
Editor's note: Whether the third bullet above should be included in the specifications is an open issue.
This level of specification of the Node B behaviour is sufficient, for the following reasons:
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1 The purpose of the congestion control, is not to act as a flow control but rather as an "emergency break" in order to keep the system at a stable state.
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2 The output bit rate from the node B depends on many things, for example radio interference, distance from mobile to Node B, available hardware resources etc. The Node B scheduler will need to take all that into consideration when assigning the bit rate to each mobile.
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3 Performance wise, to specify very detailed behaviour when the control frame is received is not possible due to the reasons in bullet 2.
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6.2.3 A similar solution for HSDPA
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It has been acknowledged that similar functionality shall also be introduced for HSDPA. Further it was expressed that such a solution should be as similar as possible to any solution for Enhanced Uplink. In this clause such functionality is proposed and analysed.
From a conceptual point of view, the reuse of the concept that the detection of Iub/Iur congestion is done by measuring a delay build-up, and/or by detecting frame loss (or lost number of bytes/bits) is proposed.
For Enhanced Uplink it was required to introduce a specific congestion indication control frame for informing the Node B about the congestion. This is not required in the case of HSDPA - a working flow control mechanism already exists. In order to minimize complexity, implementation and tuning efforts, the reuse of this mechanism for the purpose of congestion control is proposed.
As a result, the only required changes in the specifications would be to add support for the Node B to detect congestion situations. From the discussion on Enhanced Uplink, it is known that this mechanism should be based on the measuring of a delay build-up or by detecting some kind of sequence loss.
Time stamp for measuring delay build-up
For Enhanced Uplink a "time stamp" has already been agreed implicitly by the introduction of CFN and SFN for reordering purposes. The CFN and SFN fields can be used also for the purpose of detecting delay build-up and there is no need for any additional information.
For HSDPA, CFN and SFN are not used. Therefore, the introduction of a delay reference time tied to RFN is proposed. RFN is already defined and should not impose and additional complexity. The Node B can detect delay build-ups by noting the arrival time of subsequent Delay-Reference-Time (DRTs) and comparing them.
Sequence Number for detecting frame/data loss.
Furthermore, some kind of sequence number added to the data frame is required - in order to allow the receiver to detect when a frame has been lost. There are two possible options, a frame sequence number (FSN) or a quantum sequence number (QSN). The pros and cons with those has been discussed and it has been concluded that for Enhanced Uplink the usage of a 4 bit field (FSN) would be sufficient.
The HSDPA solution should be as similar as possible – if possible – to that for Enhanced Uplink. A 4 bit FSN would fit into the spare bits of today's data frame, while an introduction of a 12-16 bit (minimum) QSN would require to make use of the spare extension mechanism, adding a minimum of three octets to the data frame. Considering that data frames are not bundled for HSDPA, results in a general smaller frame, as well as a lower standard deviation of the frame size, the extra overhead with QSN is motivated.
The usage of Congestion Indication Control Frame
For Enhanced Uplink the usage of a control frame for indicating that there is a congestion situation is proposed. Such a solution would be possible to apply also for HSDPA. There is however an important difference in the functional split between HSDPA and Enhanced Uplink. HSDPA already has a flow control mechanism in order not to overflow the Node B buffers. For that reason the easiest (both specification wise and implementation wise) will be to reuse the mechanism for flow control. For that reason, only the need to specify the means for the Node B to detect a congestion situation, i.e. DRT and FSN, is required.
Conclusion
The outlined solutions for HSDPA and Enhanced Uplink are functionality wise similar, congestion detection is done by observing a time stamp and a sequence number.
Although it would be nice to have exactly the same coding of the detection and notification for both HSDPA and Enhanced Uplink, smaller differences can be accepted if that leads to more efficient coding, and implementation, saving overhead. The most obvious case is the time stamp, CFN and SFN, already exists for Enhanced Uplink, but it cannot be inserted into the HSDPA user data header. As there is no CFN and SFN defined for HSDPA, using a time stamp linked to the RFN is proposed.
There is a possibility to have the exact same coding of the sequence number: A 4 bit FSN fits into both the HSDPA and the Enhanced Uplink user data frame headers.
For the notification message a control frame for Enhanced Uplink is proposed and the reuse of the existing flow control mechanism for HSDPA.
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6.2.4 Handling of the Iur
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Two philosophies can be distinguished for the handling of the Iub traffic, referred to as the "Iub pipe" and the "Iub cloud".
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6.2.4.1 Iub pipe philosophy
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Figure 3: "Iub pipe" philosophy
With the "Iub pipe", logic the CRNC enforces the traffic limit injected on the Iub interface in the DL, so it is able to instantaneously detect any congestion situation. The advantage of this approach is that there is no need for using any new congestion mechanisms in the Node B, because the congestion detection is instantaneous – the only place it can occur is at the "pipe" entry. The drawback of the "pipe" logic is that in some scenarios it may require complex configuration of TNL topologies in the CRNC in order to leverage statistical multiplexing.
If the "pipe" logic on the Iub is to be preserved, when the HS-DSCH connection extends across the Iur interface it is important to note that the HS-DSCH Flow Control should be terminated in the DRNC. This case is depicted in Figure 4.
Figure 4: FC termination in DRNC and "pipe" on Iub
As illustrated in Figure 4, there are two separate Flow Control loops exerted on both Iub and Iur.
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6.2.4.2 Iub Cloud philosophy
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Figure 5: "Iub cloud" philosophy
With the "Iub cloud" logic, the traffic injected by the RNC is less tightly controlled i.e. the RNC is likely to inject too much traffic in the network, thus yielding a congestion situation. This approach should allow for statistical multiplexing in some scenarios without complex configuration of TNL topology in the CRNC. However, with this approach a new congestion control mechanisms become a necessity.
This "Iub cloud" logic can easily be extended to the handling of the Iur as shown in figure 6:
Figure 6: "Iub cloud" philosophy extended to the Iur
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6.2.4.3 Co-existence of the two philosophies
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In the situation where the HS-DSCH connection extends across the Iur interface it is important to note that – should it be employed – the HS-DSCH flow control may be terminated in the DRNC.
In this scenario, two separate flow control loops would then be employed on both Iub and Iur.
If, in the "Iub pipe" logic, a DRNC detects congestion, it will either buffer or discard the excess data. In either case, a CC-enabled NodeB (i.e. a Node B with the "Iub cloud" logic) would detect the congestion situation as well.
In order to avoid race conditions between the two competing mechanisms (Congestion Control in the Node B and Congestion Control in the CRNC), it is thus proposed to introduce the possibility to turn off Congestion Control in the Node B via Control Plane mechanisms. By doing so, congestion will be detected and handled in only one place in the network.
There are two possible solutions to achieve that result:
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1 The DRNC makes the decision to use Congestion Control and indicates to the Node B – via Control Plane - not to perform Congestion Control (e.g. using the Physical Shared Channel Reconfiguration procedure).
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2 The DRNC makes the decision and indicates to the SRNC that it shall include the user plane protocol extensions that are used by the Node B to detect congestion (namely the timestamp and the Frame Sequence Number) by introducing a new User Plane Congestion Field Inclusion IE in the HS-DSCH FDD/TDD Information Response IEs.
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This would allow the DRNC to indicate to the SRNC if User Plane fields destined to be used for Congestion detection by the Node B are to be included or not in the HS-DSCH Data Frames. If not included, Congestion detection and Congestion Control will not be employed by the Node B.
This second approach is preferred as it allows to save bandwidth on the UTRAN interfaces and it really allows not to perform any Congestion Control in the Node B as no information is available.
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6.3 Impacts on Iub/Iur control plane protocols
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TS 25.423
• a new User Plane Congestion Field Inclusion IE in the HS-DSCH FDD/TDD Information Response IEs.
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6.4 Impacts on Iub/Iur user plane protocols
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TS 25.427
• EDCH data frame: Introduction of a 4 bit Frame Sequence Number (FSN) field.
• EDCH data frame: Clarification that CFN and SFN can be used for dynamic delay measurements.
• Introduction of a Congestion Status control frame.
• Specification of desired behaviour when Node B receives the Congestion Status control frame.
TS 25.425 and TS 25.435
• HS-DSCH data frame: Introduction of a 4 bit Frame Sequence Number (FSN) field.
• HS-DSCH data frame: Introduction of a 16 bit Delay Reference Time (DRT) field.
TS 25.435
• Usage of 2 of 4 previously spare bits within Capacity Allocation Procedure payload for indication of congestion in TNL. Used for Congestion Indication for HSDPA only.
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6.5 Open issues
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• Thus far Iub/Iur Congestion Control has been considered for HSDPA and Enhanced Uplink only. Could any final solution be applicable for UL and DL DCH?
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6.6 Backwards compatibility
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void
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7 Agreements and associated contributions
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1 The development of an Iub/Iur Congestion control solution should bear in mind both the E-DCH and HSDPA features.
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2 Iub/Iur Congestion control for both HSDPA and Enhanced Uplink should – if possible – employ similar solutions.
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3 The RNC remains the entity in charge of the Congestion Control function.
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4 NodeB behaviour when receiving the congestion indication shall be specified.
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5 The detection algorithm will not be specified in the TR. (However example algorithms may be given in an annex.)
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6 Congestion indication should be signalled via the user plane.
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7 Signalling of Congestion via the user plane will also include varying levels of congestion severity.
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8 Congestion Detection will be performed on a per flow basis.
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9 Within the E-DCH data frame (user plane), congestion detection will be based upon a time reference or a sequence number.
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10 For the handling of Iub/Iur Congestion due to HSDPA, the CRNC decides whether all or none of the HS-DSCH MAC-d Flows of a context are subject to Congestion Control.
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11 A "counter" field be attached to EVERY E-DCH data frame.
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12 The "counter" field within the E-DCH frame will take the form of a "frame sequence number" (FSN).
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13 Different levels of congestion shall be indicated by "No congestion", "TNL Congestion – detected by delay build-up", "TNL Congestion – detected by frame loss".
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14 The resulting behaviour following the signalling of Congestion Indication will not be defined – this is an implementation matter.
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15 For impacts upon RNL xxxAP Signalling protocols, please refer to CR 1080 against TS 25.423. This CR allows a CRNC to decide whether a particular E-DCH flow is subject to congestion control at flow setup.
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16 Regarding the possibility of an Iub/Iur Congestion Control solution incorporating Rate Adaptation, this functionality was discussed, but a solution was not found, nor foreseen as possible at this time.
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17 With respect to Softhandover, no issues have been found concerning the relationship/interaction with E-DCH Congestion Control.
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8 Specification impact and associated Change Requests
This clause is intended to list the affected specifications and the related agreed Change Requests. It also lists the possible new specifications that may be needed for the completion of the Work Task.
CR Title
Impacted Specification
CR implemented against version:
CR Number
Transport Network Congestion Detection and Control
TS 25.427
V6.2.0
109
Transport Network Congestion Detection and Control
TS 25.425
V6.1.0
99
Transport Network Congestion Detection and Control
TS 25.435
V6.1.0
142
Congestion Indication for HSDPA
TS 25.435
V6.3.0
143
Congestion control for HSDPA
TS 25.423
V6.5.0
1080
For 3.84 Mcps TDD the concepts added for FDD in the above CRs are also added in the general CR for TDD that add the feature to the specifications. For 7.68 Mcps TDD the concepts added for FDD in the above CRs are also added in the general CR for TDD that add the feature to the specifications. For 1.28 Mcps TDD the concepts added for FDD in the above CRs are also added in the general CR for TDD that add the feature to the specifications.
Annex A:
Change history
Change history
Date
TSG #
TSG Doc.
CR
Rev
Subject/Comment
Old
New
06/2005
TSG-RAN#28
RP-050231
Presentation of TR for information
-
1.0.0
09/2005
TSG-RAN#29
RP-050436
Presentation of TR for approval
1.0.0
2.0.0
09/2005
TSG-RAN#29
RP-050436
TR approved at TSG-RAN#29 and placed under change control
2.0.0
6.0.0
09/2006
TSG-RAN#33
RP-060507
3
1
Removal of erroneous References from TR 25.902 Iub/Iur Congestion Control
6.0.0
6.1.0
09/2006
TSG-RAN#33
RP-060511
2
Introduction of 3.84 Mcps and 7.68Mcps TDD Enhanced Uplink
6.1.0
7.0.0
03/2007
TSG-RAN#35
RP-070062
4
Introduction of 1.28 Mcps TDD Enhanced Uplink
7.0.0
7.1.0
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1 Scope
|
This present document is for the 3GPP Release 6 Work Item "Network Assisted Cell Change – Network Side Aspects.".
The purpose of the present document is to aid TSG RAN WG3 to standardise the signalling of relevant GERAN information during cell re-selection across the relevant UTRAN interfaces.
This document is intended to gather all information in order to compare the solutions and to draw a conclusion on the way forward.
This document is a 'living' document, i.e. it is permanently updated and presented to TSG-RAN meetings.
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2 References
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The following documents contain provisions which, through reference in this text, constitute provisions of the present document.
• References are either specific (identified by date of publication, edition number, version number, etc.) or non‑specific.
• For a specific reference, subsequent revisions do not apply.
• For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.
[1] 3GPP TS 44.060: "3rd Generation Partnership Project; Technical Specification Group GSM/EDGE Radio Access Network; General Packet Radio Service (GPRS); Mobile Station (MS) - Base Station System (BSS) interface; Radio Link Control/Medium Access Control (RLC/MAC) protocol".
[2] 3GPP TS 44.901: "3rd Generation Partnership Project:; Technical Specification Group GSM/EDGE Radio Access Network; External Network Assisted Cell Change".
[3] 3GPP TR 21.905: "Vocabulary for 3GPP Specifications".
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3 Definitions, symbols and abbreviations
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3.1 Definitions
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For the purposes of the present document, the following terms and definitions apply.
Local RNC: the local RNC(s) to a given cell or BSS is/are the RNC(s) with cells which are neighbouring to the GERAN cell or BSC.
Remote RNC: an RNC is remote to a given GERAN cell or BSS if none of its cells are neighbours of the GERAN cell or BSS.
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3.2 Symbols
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For the purposes of the present document, the following symbols apply:
Gb Interface between the BSS and the 2G SGSN
Gn Interface between two GSNs in the same PLMN
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3.3 Abbreviations
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Applicable abbreviations can be found in [3]. For the purposes of the present document, the following abbreviations apply:
BSSGP Base Station Subsystem GPRS Protocol
DRNC Drift RNC
GERAN Gsm/Edge Radio Access Network
NACC Network Assisted Cell Change
PSI Packet System Information
RAN Radio Access Network
RIM Ran Information Management
RNC Radio Network Controller
SI System Information
SRNC Serving RNC
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4 Introduction
|
At the 3GPP TSG RAN #19 meeting, the Work Item Description on "Network Assisted Cell Change from UTRAN to GERAN – Network Aspects" was approved.
In today's GPRS networks (without NACC), cell re-selection can causes a service interruption in the region of 4 – 8 seconds, which obviously has an impact on the user experience. Similar interruption times can be expected in mixed UMTS and GPRS networks, during UE cell re-selection from UTRAN to GERAN.
Consequences of this: e.g. TCP applications may time-out at cell change and suffer from the slow-start mechanism, streaming applications may stop at cell change due to client buffer depletion. All such problems will lead to an unacceptable user experience.
This "Network Assisted Cell Change" feature has already been introduced in the GERAN specifications and the appropriate changes have been to the RLC/MAC protocol [1] within Release 4. Additional enhancements were approved in Release 5 in order to exchange (Packet) System Information between BSSs, so that NACC can work across BSS boundaries.
Currently, there are procedures defined on the Gb and Gn interfaces to enable signalling of GERAN SI/PSI between BSSs. This RAN Information Management (RIM) mechanism was defined initially for the use of NACC, although in a manner that could be extended for applications other than NACC. It consists of the following messages:
- RAN INFORMATION REQUEST - from Source BSS to Target BSS – requests GERAN SI/PSI.
- RAN INFORMATION – from target BSS to source BSS – analogous to the Information Exchange over Iur and includes GERAN SI/PSI for one or more GERAN cells.
- RAN INFORMATION ACKNOWLEDGE – from Source BSS to Target BSS.
- RAN INFORMATION ERROR - to inform about e.g. message syntax errors.
In Release 5, TSG RAN approved the provision of the GERAN (P)SI messages in the CELL CHANGE ORDER FROM UTRAN message. In order for this feature to work successfully, a standardised method is required to signal relevant GERAN information across the relevant UTRAN interfaces.
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5 Requirements
|
The standardisation of NACC from UTRAN to GERAN shall meet the following requirements:
1) The impact to the Gb and Gn interfaces shall be minimised.
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6 Study Areas
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6.1 UTRAN NACC signalling architecture
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6.1.1 General
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Three possible mechanisms have been identified to gain access to the GERAN SI/PSI at the SRNC, whilst minimising the impacts on the existing Gb/Gn procedures:
1) The (P)SI is stored by the SRNC.
2) The (P)SI is stored by the local RNC
3) O&M-based distribution of (P)SI.
These solutions are explained in the following sub-clauses.
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6.1.2 Solution 1: (P)SI stored by the SRNC
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6.1.2.1 General description
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This solution is based on the serving RNC directly requesting and receiving the SI/PSI from the target BSS and it is depicted in figure 1.
1) The SRNC receives a measurement report from the UE and decides to move the UE to GERAN.
NOTE: The SRNC could request the info earlier on receiving GERAN n_cell info from DRNC.
2) The SRNC triggers a REQUEST to the SGSN.
3) The SGSN then uses existing RIM procedure to forward the request to the BSS.
4) The BSS uses existing RIM procedure towards SGSN to pass the GERAN SI/PSI back to the SRNC via SGSN either "on-demand" (i.e. single report) or on an "on-modification" basis (i.e. multiple reports).
5) The SGSN then relays this information to the SRNC via the Iu interface.
6) If multiple reports are used, the SRNC could terminate the reporting using a procedure TERMINATION/END message.
NOTE: The measurement report from the UE is a connection oriented procedure, whereas the RAN Information Request procedure is connectionless. It was noted for further study that currently the RNC does not have the functionality to deal with this situation.
Figure 1: Signalling diagram for Solution 1 of GERAN SI/PSI Retrieval
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6.1.2.2 Analysis of the solution
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Pros:
1) No additional Iur load generated.
2) No additional Iur implementation required.
3) Synchronised update of SI/PSI is possible using "on-modification" measurement reporting.
Cons:
1) Generally more SI/PSI stored in each RNC than in other solutions.
2) Additional load on the SGSN due to signalling path of RIM procedures.
3) Additional load on the BSS due to a potentially high number of measurement contexts being required (for each different SRNC).
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6.1.3 Solution 2: (P)SI stored by the local RNC
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6.1.3.1 General description
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This solution is based on the local RNC requesting SI/PSI from the BSS, and receiving it on an "on-modification" basis. This procedure is depicted in figure 2.
1) After installation and configuration of the GERAN neighbouring cell lists in the local RNC, a REQUEST message is sent to the SGSN requesting GERAN SI/PSI for the GERAN cells that are configured in the local RNC neighbouring cell list.
2) The SGSN then uses existing RIM procedure to forward the request to the BSS.
3) BSS uses existing RIM procedure towards SGSN to pass the GERAN SI/PSI back to the (D)RNC via SGSN "on-modification".
4) The SGSN would then relay this information to DRNC via the Iu interface.
5) The GERAN SI/PSI is transferred using existing RNSAP procedures over the Iur interface towards the SRNC when it requires it.
Figure 2: Signalling diagram for Solution 2 of GERAN SI/PSI Retrieval
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6.1.3.2 Analysis of the solution
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Pros:
1) Generally less SI/PSI stored in each RNC than in other solutions.
2) Synchronised update of SI/PSI is possible using "on-modification" measurement reporting.
3) Impact on SGSN load is minimised.
Cons:
1) More Iur signalling than SRNC terminated solution.
2) Additional load on the DRNC due to potentially high number of measurement contexts being created (for each different SRNC).
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6.1.4 Solution 3: O&M-based distribution of (P)SI
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6.1.4.1 General description
|
This solution is based on the operator using O&M to update the stored SI/PSI in the neighbouring GERAN cell list of the RNC every time it is modified by O&M in the GERAN cell. This is depicted in figure 3.
1) On installation and initial configuration by O&M of the GERAN neighbouring cell lists in the (C)RNC, the GERAN SI/PSI is included in the information sent to the RNC.
2) The GERAN SI/PSI is transferred using existing RNSAP procedures over the Iur interface towards the SRNC when it requires it.
NOTE: It was discussed that if the NM System is not updated if the BSS changes its system parameters, there can be periods of time where the SI/PSI held in the NM System are out of date. This issue is FFS.
Figure 3: Signalling diagram of Solution 3 of GERAN SI/PSI Retrieval
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6.1.4.2 Analysis of the solution
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Pros:
1) No direct signalling required between UTRAN and GERAN.
2) No impact on the SGSN.
Cons:
1) Maybe difficult to ensure that SI/PSI stored in UTRAN is always aligned with that in the GERAN cell.2) Extra impact on 3G NMS and R interface.
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6.1.5 Comparative analysis of the solutions
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The sub-clauses 6.1.2.2, 6.1.3.2 and 6.1.4.2 are summarised in Table 1.
Table 1: Comparative analysis of the proposed solutions.
Solution
Pros
Cons
1. (P)SI provided to the SRNC
• No additional Iur load generated.
• No additional Iur implementation required.
• Synchronised update of SI/PSI is possible using "on-modification" measurement reporting.
• Generally more SI/PSI stored in each RNC than in other solutions.
• Additional load on the SGSN due to signalling path of RIM procedures.
• Additional load on the BSS due to a potentially high number of measurement contexts being required (for each different SRNC).
2. (P)SI provided to the local RNC
• Generally less SI/PSI stored in each RNC than in other solutions.
• Synchronised update of SI/PSI is possible using "on-modification" measurement reporting.
• Impact on SGSN load is minimised.
• More Iur signalling than SRNC terminated solution.
• Additional load on the DRNC due to potentially high number of measurement contexts being created (for each different SRNC).
3. O&M distribution of (P)SI provided to
• No direct signalling required between UTRAN and GERAN.
• No impact on the SGSN.
• Maybe difficult to ensure that SI/PSI stored in UTRAN is always aligned with that in the GERAN cell.
• Extra impact on 3G NMS and R interface.
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6.2 UTRAN signalling procedures for NACC
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6.2.1 Iur signalling for GERAN SI/PSI transfer from the DRNC to the SRNC
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The transfer of SI/PSI over the Iur is relevant to solution 2, described in sub-clause 6.1.3. Two different solutions have been identified on the Iur interface for the transfer of (P)SI from the local DRNC to the SRNC.
Use the [RNSAP] RADIO LINK SETUP RESPONSE message.
- GERAN SI/PSI could be sent in the "GSM neighbouring cell information" IE for each of the GERAN neighbouring cells.
- It may be inefficient in terms of transmission load and delay increase due to the number of RADIO LINK SETUP RESPONSE messages containing the same information. In addition, there would be no way of knowing (without some kind of "GERAN cell id list" and "value tag" provision in the RADIO LINK Setup Request) which SI/PSI the SRNC already had and whether it was up-to-date.
- This is perhaps the most time critical of all RNSAP messages and the processing in the DRNC before sending this message should not be unduly complicated.
- This is related to Release 5 work on restricting neighbouring cell information on the Iur.
Use the [RNSAP] Information Exchange procedure.
- An Information Exchange procedure could be initiated from the SRNC towards the DRNC when the SRNC establishes its first radio link in the DRNS. The Report Characteristics would be set to "On-modification" (of GERAN SI/PSI messages).
- In Release 5, reporting is only allowed for a single cell.
- The transfer of GERAN SI/PSI information from the DRNC to the SRNC can be time critical i.e. the SRNC may wish to push the UE to GERAN almost immediately after RL establishment. Delays may occur in the SRNC acquiring the SI/PSI if multiple information reporting initiations, and multiple reports, are required.
NOTE: It needs to be analysed if the Information Exchange Object Type IE could contain a list of cells to enable a faster initial report and subsequent reports to the SRNC.
New RNSAP procedure.
- A new procedure could be initiated from the DRNC towards the SRNC after the SRNC establishes its first radio link in the DRNC.
- The DRNC already keeps a list of the GERAN neighbouring cells for each local UTRAN cell. In addition, the DRNC, as local RNC, also keeps the (P)SI of those GERAN cells.
- The local RNC would also be required to keep a list of other RNCs acting as SRNCs for UEs with radio links in local DRNC (i.e. DRNC) and whether they have an update copy of the (P)SI messages of the GERAN cells.
- This procedure would be initiated by the DRNC:
a) towards the SRNC of a UE when a new radio link is added/created to another UTRAN cells with new GERAN neighbouring cells for which the corresponding SRNC does not have the (P)SI;
b) towards one or more (S)RNCs when the local RNC receives an update from a GERAN BSS of the (P)SI for one or more GERAN cells.
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6.2.2 Use of RANAP or O&M for provision of GERAN SI/PSI to RNC
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The issue of whether to use O&M or whether to use RANAP signalling to inform the RNC of the GERAN SI/PSI is dependant on how often the GERAN SI/PSI would be changed, i.e. how often the operator optimises GERAN SI/PSI. If the O&M solution were chosen and if it is required to update the SI/PSI very frequently, there may be problems in synchronising the BSS/RNC provision of the GERAN SI/PSI between 2G and 3G O&M systems. This would be due to the fact that the operator would probably have to manually synchronise the carrying out of the respective procedures from their respective 2G/3G O&M systems.
GERAN SI/PSI information may evolve, which means that there may be a more dynamic change of this information. So in this case, O&M alignments would be more complex to organise.
6.2.3 Adaptation of GERAN RIM procedures for use across the Iu interface
6.2.3.1. General
In UMTS the information as to which BSC the target neighbouring GERAN cell belongs is not known at the RNC.
Whilst the present NACC RIM GERAN messages [2] can be used as a basis for inclusion into the relevant 3GPP/RAN3 specifications, the following areas for study are identified:
6.2.3.2. Message definition
A RANAP message similar to the GERAN "RAN INFORMATION REQUEST" message sent from the RNC should contain a source RNC-ID (instead of the source CGI as happens in GERAN), the destination (GERAN) CGI, plus a list of other (GERAN) neighbouring cells if whose (P)SI is requested.
The subsequent RAN INFORMATION message sent from the BSCBSS to SGSN would contain the source RNC-ID, in addition to the list of GERAN SI/PSI mapped to each target CGI.
Devising a RANAP message corresponding to the [GERAN] RAN INFORMATION message cannot be a direct correlation i.e. existing connectionless downlink RANAP messages do not include the RNC-ID, as the destination SCCP address is used to route the message on the Iu.
6.2.3.3. Format of RIM messages
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6.2.3.3.1 General
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What form should the BSS <=> 3G SGSN response RIM messages take?
Three options are analysed in this sub-clause:
1. As per existing agreements for inter-RAT signalling, the host source system - when inter-RAT signalling - adapts to the target system and thus constructs an appropriate RANAP message.
2. The GERAN system continues using with its existing RIM procedures in both directions, – i.e. no adaptation of messages to target system and this means the GERAN RIM message is translated at the CN. (The procedures would not be fully transparent, since the 3G SGSN must "look into" the message and translate the GERAN RIM message into the appropriate RANAP RIM message).
3. The GERAN continues using its existing RIM procedures in both directions, i.e. no adaptation of messages to target system. The GERAN RIM message is routed through the CN and terminated at the UTRAN. The 3G SGSN would need to place the contents of the GTP message on the Gn interface into a RANAP message on the Iu interface, without interpreting its contents.
These alternatives are depicted in Figure 4.
Principle
BSS RNC
RNC BSS
1. Source adapts to target
2. RIM translated at CN
Translation at: a) 2G SGSN, b) 3G SGSN
3. RIM ends at RNC
Figure 4: Alternatives for coding of RIM messages.
6.2.3.3.2. Format of RIM messages on the Iu interface
There are two possible options for the format of the messages sent on the Iu interface:
1. What form should the 3G SGSN <=> RNC RIM messages take?
a) A new RIM procedure is created in RANAP to transfer this NACC information/messages across the Iu interface to the source RNC;
b) OR, (corresponding to 2b) the RIM procedure messages are sent in a container inside messages of the existing Information Transfer procedure messages over the Iu interface. In this case, an INFORMATION TRANSFER REQUEST message would need to be created to contain the [GERAN] RAN INFORMATION REQUEST message.
When considering this question it should be noted that it is not seen as feasible to deconstruct the BSSMAP/RANAP message fully in the SGSN and construct a corresponding RANAP/BSSMAP message. This would have a higher impact on the SGSN implementation, with no extra gain, apart from the fact that the RNC-ID would not be transferred in the RANAP message.
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7 Agreements and associated Contributions
|
The main text of the document should start here, after the above clauses have been added.
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7.1 UTRAN NACC signalling architecture
|
The mechanism used to gain access to the GERAN SI/PSI at the SRNC is such that the (P)SI will be stored by the local RNC.
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7.2 Format of RIM messages
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GERAN does not adapt RIM messages to the target system and are routed via the CN without interpretation. The RNC alone needs to send and receive BSSGP messages within a container within the RANAP message.
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7.3 Exchange of Information over Iu
|
The transfer of RIM information over the Iu from UTRAN, will be performed using a new RANAP procedure – Direct Information Transfer. This generic Class 2 RANAP procedure has been designed such that it will transfer information from the RNC to the CN or vice versa, in unacknowledged mode – maintaining the previously agreed RIM principles.
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7.4 Exchange of Information over Iur
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The transfer of RIM information over the Iur between the SRNC and the DRNC will be performed using an existing RANAP R5 procedure – [RNSAP] Information Exchange – following an appropriate modification/addition to the procedure.
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8 Specification Impact & associated Change Requests
|
This section is intended to list the affected specifications and the related agreed Change Requests. It also lists the possible new specifications that may be needed for the completion of the Work Task.
8.1 TS 25.401 UTRAN Overall Description
8.1.1 Impacts
GERAN System Information Retrieval is introduced as an additional UTRAN function, as is RAN Information Management as an additional function related to radio resource management and control.
8.1.2 List of Change Requests
Refer to http://www.3gpp.org/ftp/tsg_ran/TSG_RAN/TSGR_24/Docs/ZIP/RP-040182.ZIP.
8.2 TS 25.410 UTRAN Iu Interface: General Aspects and Principles
8.2.1 Impacts
GERAN System Information Retrieval is introduced as an Iu Mobility Management function.
8.2.2 List of Change Requests
Refer to http://www.3gpp.org/ftp/tsg_ran/TSG_RAN/TSGR_24/Docs/ZIP/RP-040182.ZIP.
8.3 TS 25.413 UTRAN Iu interface RANAP signalling
8.3.1 Impacts
A generic Class 2 RANAP procedure (bi-directional i.e. UL and DL) – Direct Information Transfer - has been introduced in RANAP to enable the transfer of RIM-PDU for the NACC feature initially, and thereafter for future uses if/when they present themselves.
8.3.2 List of Change Requests
Refer to http://www.3gpp.org/ftp/tsg_ran/TSG_RAN/TSGR_24/Docs/ZIP/RP-040182.ZIP.
8.4 TS 25.420 UTRAN Iur interface general aspects and principles
8.4.1 Impacts
The exchange of information over the Iu of UTRAN and GERAN information has been included as a function of the Iur.
8.4.2 List of Change Requests
Refer to http://www.3gpp.org/ftp/tsg_ran/TSG_RAN/TSGR_24/Docs/ZIP/RP-040182.ZIP.
8.5 TS 25.423 UTRAN Iur interface RNSAP signalling
8.5.1 Impacts
In the case that the CRNC is not the SRNC, and the SRNC would like to request NACC information, the Information Exchange Procedure has been modified such that the SRNC can request NACC related data for one or several GSM cells.
8.5.2 List of Change Requests
Refer to http://www.3gpp.org/ftp/tsg_ran/TSG_RAN/TSGR_24/Docs/ZIP/RP-040182.ZIP.
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9 Project Plan
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9.1 Schedule
|
Date
Meeting
Scope
[expected] Input
[expected]Output
Sept 2003
RAN#21
RAN Approval
TR Approved
Mar 2004
RAN#23
RAN Approval
TR Approved
June 2004
RAN#24
RAN Approval
TR Approved
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9.2 Work Task Status
|
Planned Date
Milestone
Status
Annex A:
Change history
Change history
Date
TSG #
TSG Doc.
CR
Rev
Subject/Comment
Old
New
June 2004
TSG-RAN#24
RP-040186
Presentation of TR for information
-
1.0.0
June 2004
TSG-RAN#24
Approved at TSG RAN #24 and placed under Change Control
1.0.0
6.0.0
September 2004
TSG-RAN#25
RP-040307
001
-
Tidy Up CR for TR 25.901 (NACC)
6.0.0
6.1.0
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e77d3faff9be1de0a0fd3e5e46ddb1f5
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25.807
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1 Scope
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Some companies have shown their interest in the feasibility of a low output power FDD base station, because it would offer the following advantages:
1. It would not be necessary to use an expensive high power amplifier when a distribution system is connected after the node B. Distribution systems require low input powers. If a high power amplifier is used, the BS output signal must be attenuated before it is fed into the distribution system, thus the high power amplifier is not needed at all. Power consumption would be reduced, and this would also have positive environmental effects.
2. It would facilitate the sharing of infrastructures among operators, especially in locations where it is difficult to find sites, or where operators are forced by regulators to share infrastructures. A common distribution system could be connected to the low output power BS's from each operator.
3. It would increase the flexibility in radio network deployment, because it would allow the placement of one or several base stations in a centralised position with separate RF power amplifiers distributed closer to the subscriber positions. This would also reduce network interference.
A study item called "Low Output Powers for General Purpose FDD BSs" was created during TSG RAN plenary meeting #19, in order to study the feasibility of the low output power FDD base station. The main objectives of this study item are:
1) Identify the range of output powers to be considered. This SI must assess what must be understood as low output power.
2) Once the range of output powers to be considered has been delimited, it must be studied how the RAN specifications can be changed in order to allow that range of output powers.
This technical report will collect the results of the study item. It must be clearly understood that the purpose of this SI is not to create a new interface within the base station. Its purpose is just to enlarge the output power range of the current base station, in order to allow lower output power values. It must also be stated that a BS doesn't have necessarily to comply with the whole allowed output power range. A BS will only fulfil a subrange of the allowed output power range. The manufacturer must declare the maximum output power of the base station, and the power range supported by it. The only requirements for the BS output power range are those imposed by the dynamic range requirements in [2].
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e77d3faff9be1de0a0fd3e5e46ddb1f5
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25.807
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2 References
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The following documents contain provisions which, through reference in this text, constitute provisions of the present document.
• References are either specific (identified by date of publication, edition number, version number, etc.) or non‑specific.
• For a specific reference, subsequent revisions do not apply.
• For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.
[1] Tdoc RP-030198: "Low output power FDD Base Station".
[2] 3GPP TS 25.104 (V6.1.0): "BS Radio transmission and reception (FDD)".
[3] Tdoc R4-030744: "New approach to low output power", Telefonica.
[4] Tdoc R3-040128: "Clarification on the use of the downlink and uplink gain", Telefonica.
[5] Tdoc R3-040137: "Solutions for LOP", Telefonica.
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e77d3faff9be1de0a0fd3e5e46ddb1f5
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25.807
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3 Abbreviations
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For the purposes of the present document, the following abbreviations apply:
3G Third Generation
3GPP Third Generation Partnership Project
BS Base Station
CPICH Common Pilot Channel
DL Downlink
DPCH Dedicated Physical Channel
FDD Frequency Division Duplex
IE Information Element
NBAP Node B Application Part
P-CCPCH Primary-Common Control Physical Channel
Pmax Maximum Output Power
PRACH Physical Random Access Channel
RAN Radio Access Network
RF Radio Frequency
RNC Radio Network Controller
SI Study Item
TR Technical Report
TSG Technical Specification Group
UE User Equipment
UL Uplink
WG Working Group
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e77d3faff9be1de0a0fd3e5e46ddb1f5
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25.807
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4 Definition of low output power
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Maximum output power, Pmax, of the base station is the mean power level per carrier measured at the antenna connector in specified reference conditions, as defined in [2], section 6.2.1. The lowest maximum output power that can be set for a base station with the current specifications is 0 dBm. The individual channel codes in a carrier are assigned portions of the carrier maximum output power. The lowest output power that can be allocated to an individual channel code with the current specifications is -15 dBm (for the P-CCPCH) and -10 dBm (for all channels but for P-CCPCH).
After consulting several distributed systems manufacturers, they have quoted -20 dBm to +10 dBm as a useful range for the "maximum power" of the input signal to their equipments. Therefore, this range of output powers was initially taken as a definition of "low output power". If Pmax equals -20 dBm, this implies that an individual DPCH code can be radiated with down to -48 dBm (the dynamic power range for a DPCH, according to [2], section 6.4.2, is -3 dB to ‑28 dB). Some companies have argued that this power level could be below or near the noise level. Another company has argued that there are serious implementation problems if the codes are to be transmitted with powers of down to ‑48 dBm.
Because of these problems, the definition of low output power has been relaxed to 0 dBm. Let's analyse what requirements this imposes on the power of the individual codes.
According to [2], section 6.4.2, the power control dynamic range for a code channel is -3 dB to -28 dB. This implies that the power of a DPCH can range between -3 dBm and -28 dBm (assuming that the "maximum output power" of the carrier has been set to 0 dBm). According to [2], section 6.4.3, the total power dynamic range for a base station is 0 dB to -18 dB. This implies that the power of the CPICH can range between -3 dBm (this is the maximum allowed power for a single code channel) and -18 dBm (assuming a Pmax of 0 dBm). Finally, the power range for the P-CCPCH must be set. The current specifications allow a P-CCPCH power of down to -15 dBm. We think that this is more than enough for a realistic situation, because it allows transmitting the P-CCPCH with at least -15 dB relative to the maximum output power.
According to the previous reasoning, the next table summarises what must be understood as low output power:
Table 4.1: Definition of low output power
Power definition
Power range or value
Maximum output power (Pmax)
0 dBm
DPCH power
-3 dBm to -28 dBm
CPICH power
-3 dBm to -18 dBm
P-CCPCH power
-3 dBm to -15 dBm
Only the DPCH and CPICH power ranges are not allowed by the current specifications. So this study item must analyse what must be changed in order to allow these power ranges.
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e77d3faff9be1de0a0fd3e5e46ddb1f5
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25.807
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5 Solutions for getting low output power
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