Calculating CTFC (Calculated Transport Format Combination)

Date: 27/11/2004

Introduction

Whenever new transport channels are established then RNC has to inform both the NodeB and the UE about the permitted transport format combinations. The Calculated Transport Format Combination (CTFC) is a tool for efficient signalling of transport format combinations.

Let I be the number of transport channels that are included in the transport format combination. Each transport channel TrCH_i, i = 1, 2, …, I, has L_i transport formats, i.e. the transport format indicator TFI_i can take L_i values,TFI_i ε {0,1,2,...,L_i-1}

Pi =

i-1 ∏ j = 0

Lj , where i=1,2,...,I and L0=1

Let TFC(TFI₁, TFI₂, …, TFI_I) be the transport format combination for which TrCH₁ has transport format TFI₁, TrCH₂ has transport format TFI₂, etc. The corresponding CTFC(TFI₁, TFI₂, …, TFI_I) is then computed as:

CTFC =

l ∑ i = 1

TFIi•Pi

For FACH and PCH transport channels, "TrCH₁" corresponds to the transport channel listed at the first position in IE "FACH/PCH information" in IE "Secondary CCPCH System Information", "TrCH₂" corresponds to the transport channel listed at the second position in IE "FACH/PCH information" and so on.

For all other transport channels in FDD and for all configured transport channels of the same transport channel type (i.e. DCH, DSCH, USCH) in TDD, "TrCH₁" corresponds to the transport channel having the lowest transport channel identity in the transport format combination mapped to the TFCI field. "TrCH₂" corresponds to the transport channel having the next lowest transport channel identity, and so on.

Examples

To understand the above mathematical equations better, lets take couple of examples

AMR Codec

This example is specified in [2] in more detail. Also for details about the conversational speech CS RAB in this example see section 6.10.3.4.1.4 in [4].

In case of a voice call being setup, the network will make use of an AMR codec. In the AMR codec, there are 3 transport channels to support class A, B and C bits. Also one transport channel is required to support signalling. Hence we can represent the transport formats as shown in the table below:

The permitted transport format combinations are in table below:

The CTFC for the permitted combinations will now be calculated as follows:

CTFC for TFCI 1 = (0) + (0*3) + (0*3*2) + (0*3*2*2) = 0

CTFC for TFCI 2 = (1) + (0*3) + (0*3*2) + (0*3*2*2) = 1

CTFC for TFCI 3 = (2) + (1*3) + (1*3*2) + (0*3*2*2) = 11

CTFC for TFCI 4 = (0) + (0*3) + (0*3*2) + (1*3*2*2) = 12

CTFC for TFCI 5 = (1) + (0*3) + (0*3*2) + (1*3*2*2) = 13

CTFC for TFCI 6 = (2) + (1*3) + (1*3*2) + (1*3*2*2) = 23

The following CTFC will now be sent in the Radio Bearer Setup message: CTFC_6,0, CTFC_6,1, CTFC_6,11, CTFC_6,12, CTFC_6,13 and CTFC_6,23. Six bits will be required to send the CTFC from which the table of transport formats can be contructed by the UE.

PS Data Transfer

Now we take an example of 128Kbps Interactive or background PS RAB with signalling. We need only one transport channel for the PS RAB and one transport channel for signalling. The transport formats are shown in the table below:

The permitted transport format combinations are in table below:

The CTFC for the permitted combinations will now be calculated as follows:

CTFC for TFCI 1 = (0) + (0*5) = 0

CTFC for TFCI 2 = (1) + (0*5) = 1

CTFC for TFCI 3 = (2) + (0*5) = 2

CTFC for TFCI 4 = (3) + (0*5) = 3

CTFC for TFCI 5 = (4) + (0*5) = 4

CTFC for TFCI 6 = (0) + (1*5) = 5

CTFC for TFCI 7 = (1) + (1*5) = 6

CTFC for TFCI 8 = (2) + (1*5) = 7

CTFC for TFCI 9 = (3) + (1*5) = 8

CTFC for TFCI 10 = (4) + (1*5) = 9

The following CTFC will now be sent in the Radio Bearer Setup message: CTFC_4,0, CTFC_4,1, CTFC_4,2, CTFC_4,3, CTFC_4,4, CTFC_4,5, CTFC_4,6, CTFC_4,7, CTFC_4,8, CTFC_4,8, CTFC_4,9 and CTFC_4,10. Four bits will be required to send the CTFC from which the table of transport formats can be contructed by the UE.

References

[1] 3GPP TS 25.331: Radio Resource Control (RRC) protocol specification

[2] Convergence Technologies for 3G Networks - J Bannister et al

[3] TSGR#17(99)b33: TFCI mapping - Ericsson

[4] 3GPP TS 34.108: Common test environments for User Equipment (UE) conformance testing