8.5.16 Configuration of CTCH occasions

25.3313GPPProtocol specificationRadio Resource Control (RRC)Release 17TS

The CTCH, carrying CBS data is mapped onto only one S-CCPCH. If more than one CTCH is defined, the first CTCH that is configured in the list of S-CCPCHs is the one that is used for CBS data. If the UE is in connected mode it shall ignore any CTCH configuration contained in System Information Block type 6 and use the CTCH configuration contained in System Information Block type 5 or System Information Block type 5bis.

The CTCH occasions are identified by the first radio frame of the TTI that can contain CTCH data. The CTCH occasions are fixed on the system frame number cycle 0 .. 4095 (i.e. no modulo calculation) and thus repeated cyclically.

The CTCH occasions are determined by a set of parameters.

MTTI: number of radio frames within the TTI of the FACH used for CTCH

N: period of CTCH allocation on S-CCPCH, integer number of radio frames,
MTTI £ N £ 256, where N is a multiple of MTTI (see [27] and [31]).

MaxSFN: maximum system frame number = 4095 (see [10]).

K: CBS frame offset, integer number of radio frames 0 £ K £ N-1 where K is a multiple of MTTI .

The CTCH occasions are calculated as follows:

SFN = (K + m N), m = 0, 1,…, M, with M chosen that K+MN £ MaxSFN.

The parameters N and K are broadcast as system information.

If the IE "Period of BMC scheduling messages (P)" is included in System Information Block type 5 or System Information Block type 5bis then BMC Schedule Messages are transmitted periodically every P CTCH occasions. One BMC Schedule Message may be segmented across more than one CTCH occasion. The CTCH occasions that contain the start of the BMC Schedule Messages are calculated as follows:

SFN = (K + m P N), where m is an integer and 0 £ SFN £ MaxSFN

If the IE "Period of BMC scheduling messages (P)" is not included in system information block type 5 or System Information Block type 5bis then BMC Schedule Messages could be transmitted in any CTCH occasion.