How is TFRI calculated in MAC-hs

TFRI(Transport Format and Resource Indicator) is signalled to the UE in the DL HS-SCCH channel. The UE uses this value to find out the Transport Block size in the upcoming transmission on HS-PDSCH channel.

The MAC-hs Scheduler determines the TBS, number of channelisation codes and the Modulation that is going to be used for the upcoming transmission to the UE on HS-PDSCH. This information has to be communicated to the UE, and is sent on the HS-SCCH channel.

TFRI = TBS index in table A - table B value

TBS index in table A :  This is the value Koi in the TBS table given in 25.321 Annex A(has index from 1 to 254).
table B value :  is the index in table B given below corresponding to the number of channelisation codes selected for the transmission and the Modulation to be used.

Table B    [Table 9.2.3.1 in 25.321]

Index	Modulation scheme	Number of channelization codes	*Koi
0	QPSK	1	1
1		2	40
2		3	63
3		4	79
4		5	92
5		6	102
6		7	111
7		8	118
8		9	125
9		10	131
10		11	136
11		12	141
12		13	145
13		14	150
14		15	153
15	16QAM	1	40
16		2	79
17		3	102
18		4	118
19		5	131
20		6	141
21		7	150
22		8	157
23		9	164
24		10	169
25		11	175
26		12	180
27		13	184
28		14	188
29		15	192

Example Calculation

If the output of MAC-hs scheduler for the given TTI is
TBS = 3704
number of channelisation codes = 4
Modulation = 16QAM

Then the TBS index from table A is 159
Koi from table B is = 118

Therefore, TFRI = 159-118 = 41

The HS-SCCH will signal a TFRI of 41 to the UE.

The UE at its end will calculate the TBS index by adding the Koi(it knows the modulation and number of codes).

TBS index = signalled TFRI + Koi

And from the TBS table it can determine the TBS.