SIB 11 calculation - UMTS

[zlatan]

Anyone can help with how to calculate SIB 11 space in order to limit the total number of neighbors allowed?

Thanks

[hari_2208]

hope below helps !!!!

In RRC Idle mode, CELL_FACH, CELL_PCH and URA_PCH, UE read the set of neighbouring cells from SIB 11 and potentially from SIB 12. The BCH transport channel is used to broadcast system information using a fixed transport block size of 246 bits and a Transmission Time Interval
(TTI) of 20 ms. A single transport block is sent during each TTI so the corresponding bit rate is 12.3 kbps.

The RLC and MAC layers do not add any overheads, allowing the RRC layer to use the complete set of 246 bits. The RRC layer is responsible for segmentation when an ASN.1 encoded SIB message cannot be accommodated by a single transport block. The RRC layer includes its own header information which occupies 24 bits and leaves a maximum of 222 bits for each segment of the ASN.1 encoded SIB message (the RRC header is smaller and a maximum of 226 bits are available when segmentation is not necessary and a complete SIB can be accommodated by a single transport block). The header added by the RRC layer includes the System Frame Number (SFN), the SIB type and segmentation information (when segmentation is used).

3GPP TS 25.331 specifies that a maximum of 16 segments can be used to transfer a single ASN.1 encoded SIB. This corresponds to a maximum of 3552 bits (444 bytes). 3552 bits is not sufficient to transfer information regarding 96 cells.

This means that it is necessary to limit the number of neighbouring cells included within SIB 11 and SIB 12. The precise number of neighbouring cells which can be included depends upon the quantity of information associated with each neighbour, i.e. as the quantity of information increases the number of neighbours which can be included decreases.

The release 6 version of the 3GPP specifications resolve this issue by introducing SIB 11bis. SIB 11bis can be used to accommodate any neighbours which have been excluded from SIB 11 as a result of
its capacity limitation.It is mandatory for SIB 11 to include information regarding the serving cell. The quantity of information signalled for the serving cell is usually less than the quantity of information signalled for non-serving cells.

Header
Header - 24 bits
optional - 8 bits
optional - 8 bits

If any inter-frequency neighbours are included an additional 10 bits of header information is included to specify whether or not the UE should delete any previously stored inter-frequency neighbours and
the number of inter-frequency neighbours included. Similarly, if any inter-system neighbours are included an additional 8 bits of header information is included to specify whether or not the UE should delete any previously stored inter-system neighbours and the number of inter-system neighbours included (the header for the inter-system neighbours is 2 bits
smaller than the header for the inter-frequency neighbours because there are two less optional information elements and so two fewer flags are required).

The information within this header includes whether or not SIB 12 is broadcast, whether or not HCS is used, whether the quality measure is CPICH Ec/Io or CPICH RSCP, whether or not any inter-frequency or inter-system neighbours are included, whether or not the UE should delete any previously stored intra-frequency neighbours and the number of intra-frequency neighbours included.

If HCS is enabled then the quantity of information associated with each neighbour can increase. HCS priorities and temporary measurement offsets can be broadcast for each neighbouring cell. If the ASN.1 encoded neighbour list exceeds the capacity of SIB 11 prior to the introduction of SIB 11bis, the RNC implementation may limit the number of neighbours broadcast as part of the system information, but allow longer neighbour lists to be used within dedicated Measurement Control messages in CELL_DCH.

INTRA-FREQ
serving cell - 23 bits
non-serving cell - 48-55 bits
INTER-FREQ
per neighbour - upto 67 bits
INTER-RAT
per neighbour - upt0 63 bits

[topoffenders]

 The length of SIB11 cannot exceed 3552 bits and this is insufficient to accommodate information regarding all 95 cells. The total length of SIB11 is variable and depends on:
 total number of neighbor cell relations defined in SIB11
 combination of IAF, IEF and GSM cell relations in SIB11
 number of different WCDMA carriers for IEF neighbors
 parameter settings per cell relation (i.e. setting of the parameters qRxLevMin, qQualMin, qOffset1sn, qOffset2sn, maxTxPowerUl)
 If the total length of SIB11 is exceeded, some neighbor relations needs to be removed