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Content for  TR 25.704  Word version:  12.0.0

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1  ScopeWord‑p. 6

The present document captures the studies for [2] about the need to introduce mechanisms in UMTS to provide additional broadcast capacity, considering:
  • the currently existing deployments and their future evolutions,
  • the broadcast load expected from different releases, including Rel-11 and estimate for Rel-12,
  • the impact on system performance and end-user experience in case of increased system information scheduling latency.

2  References

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.
TR 21.905: "Vocabulary for 3GPP Specifications".
RP-131386, SI description for "Study on Enhanced Broadcast of System Information", RAN #61, Porto, Portugal, Sep. 2013.
R4-136915, Way forward on the minimum number of carriers a UE should be able to monitor for UTRA and for E-UTRA, TeliaSonera, SoftBank Mobile, KT Corp, NTT DOCOMO, Deutsche Telekom, Qualcomm, RAN4#69.

3  Definitions, symbols and abbreviations

3.1  Definitions

For the purposes of the present document, the terms and definitions given in TR 21.905 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.

3.2  Symbols

For the purposes of the present document, the following symbols apply:

3.3  AbbreviationsWord‑p. 7

For the purposes of the present document, the abbreviations given in TR 21.905 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.
Access Class Barring
Broadcast CHannel
Domain Specific Access Control
Extended Access Barring
Galileo and Additional Navigation Satellite Systems
Multi-Operator Core Network
Paging Permission with Access Control
Radio Link Failure

4  Introduction

The BCH channel has a fixed/constant throughput (12.3 kbps), i.e. broadcasts a 246 bits BCH transport block every 20 ms TTI. The BCH transport block carries a SYSTEM INFORMATION message which includes the SFN (11 bits). A SYSTEM INFORMATION message may include one or more segments. The maximum size of a segment depends on the segment type, but is in the order of 28 octets.
There are different factors determining the BCH load, but the main factors are the SIB configuration of the SIBs (e.g. repetition period, scheduling, segmentation and concatenation), the number of supported features (e.g. common E-DCH) and the amount of information broadcasted (e.g. number of neighbors relations, use of explicit configuration instead of mandatory defaults).
The aim of the study item is to investigate the need for additional BCH capacity, i.e. evaluate the scenarios where the BCH capacity limit is reached.

5  Mechanisms to handle System Information capacity

5.1  Longer SIB repetition period

When the SIB repetition period is increased more SIBs can be broadcasted. But increasing the SIB repetition period goes at the expense of a longer System Information (SI) acquisition latency which could have a negative impact on some use cases, as explained in chapter 6.
The MIB is broadcast with a fixed interval of 80 ms, i.e. every 4 BCH transport blocks. SIB7, which contains information that needs to be updated frequently (UL interference), is typically scheduled quite often (e.g. 80 - 160 ms). Scheduling block 1 and/or 2 could be also broadcast with a high frequency to decrease SI acquisition latency. MIB, scheduling blocks, and SIB7, which need to be send with a high frequency, require a comparatively high share of the BCH capacity. Thus the BCH capacity gain does not scale linearly with the repetition period of a single SIB.

5.2  Segmentation and concatenation

If the System Information Block (SIB) size is too large, i.e., greater than 226 bits, it needs to be segmented into several SYSTEM INFORMATION messages. A SIB can be segmented up to maximum 16 segments (4-bit SEG_COUNT). The position of subsequent segments of a SIB is indicated by an offset SIB_OFF. The position of the first segment of a SIB is indicated by an offset SIB_POS. The counters and offsets associated with segmentation take some of the BCH capacity.
Furthermore concatenation can be used to transmit two or more (smaller) SIBs in a single SYSTEM INFORMATION message to optimally use the available BCH capacity. Concatenation thus enables more efficient use of the available BCH capacity. However given the sizes of the SIBs to be broadcasted, and the possible concatenation and scheduling options, a small percentage of the BCH may remain un-used.

6  Impact of BCH load on use casesWord‑p. 8

In this chapter the impact of longer SIB repetition periods is described. It is noted that the introduction of additional BCH capacity does not remove the impact of longer repetition periods, but may reduce the impact.

6.1  The impact of longer repetition periods on use cases

A longer SIB repetition period increases the SI acquisition latency. A longer SI acquisition latency has a negative effect on use cases where if the UE needs to acquire the complete system information again, e.g. CS Fallback, cell re-selection, RRC state change, cell update in case of RLC unrecoverable error or RLF, and redirection. These use cases are explained in more detail in the following paragraphs.
Reception failure of a single segment of a large SIB, broadcasted with a long repetition period (e.g. neighbor cell information), further delays the SI acquisition latency.
It is up to UE implementation to store the system information of one or more cells for later usage. In case the UE has stored system information for a cell (based on Cell Identity), the SI acquisition latency would be largely reduced, but the UE needs to read the MIB to check if any of the stored SIBs has changed. Furthermore it may need to check SIB3 for the Cell Identity of the cell. In case one or more SIBs have changed, the UE needs to re-acquire those. The UE may also have to re-acquire a timer based SIB, which timer has expired, or SIBs that were added. The UE shall delete any stored system information after 6 hours. The SI acquisition latency is impacted in the following use cases when the UE (re-)selects a cell for which it does not have up to date system information stored (i.e. expired or changed).
Impact on legacy UE
When the SIB repetition period is increased due to the introduction of a new feature, then legacy UEs are also affected, when the repetition period of a "legacy" SIB is increased. Typically a SIB type supports more than one feature.
CS fallback from LTE
When the UE is re-directed to UTRA due to CS fallback, UE needs to acquire the system information of the selected cell which would impact the CS fallback performance because of system information acquisition latency. Deferred Measurement Control Reading (DMCR) can be used to avoid the acquisition latency of SIBs that are under DMCR control. Furthermore the network may signal UTRAN system information in advance in a system information container in the connection release in LTE. Thus these two enhancements speed up CS fallback from LTE by avoiding SI acquisition. However the UE needs to acquire the SIBs that are not under deferred measurement control.
Cell Reselection
In case the UE does not have stored system information of the target cell, or the system information has expired or changed, the UE has to re-acquire the system information after cell re-selection. During SI acquisition for inter-frequency and iRAT cells, the UE cannot be reached, i.e. no DL data or paging can be conveyed to the UE. Furthermore UL access is delayed until the system information has been acquired and uplink access is configured. However, DMCR feature can be used to alleviate the access delay in RRC connection request and cell update case via allowing UE perform random access before acquiring complete SIBs.
RRC state switch from CELL_DCH to CELL_FACH or CELL_PCH/URA_PCH
This case is similar to cell re-selection, i.e. when the UE is reconfigured from CELL_DCH to CELL_FACH or CELL_PCH/URA_PCH, the UE needs to re-acquire SI as part of the reconfiguration from CELL_DCH. This delays the reconfiguration procedure. However, when a UE transits from CELL_DCH to CELL_FCH/CELL_PCH/URA_PCH with no user plane data to transmit or receive, then no noticeable user plan impact is foreseen.
Call re-establishment
In case of RLF or RLC unrecoverable error in CELL_DCH state, the UE performs CELL UPDATE procedure to recover. In such case if UE has no stored SIB information, the UE needs to re-acquire system information of the new cell leading to longer voice and/or data interruption.
RRC Connection Reject/Release with redirection
In case of RRC Connection Reject/Release with re-direction, the UE may need to read the system information of the new target cell before the call can be established. This leads to longer call setup time.

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