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View Full Version : Calculating CTFC (Calculated Transport Format Combination)



mansoor2
2008-10-03, 11:02 AM
<i>Introduction</i>
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<sub>i</sub>, i = 1, 2, …, I, has L<sub>i</sub> transport formats, i.e. the transport format indicator TFI<sub>i</sub> can take L<sub>i</sub> values,TFI<sub>i</sub> ε {0,1,2,...,L<sub>i</sub>-1}
<br>
<table cellpadding="0" cellspacing="0">
<tbody><tr>
<td>P<sub>i</sub> = </td>
<td align="center">i-1
<br><big><big><big>∏</big></big></big><small>
<br>j<small> </small>=<small> </small>0</small></td>
<td>L<sub>j</sub><sup> </sup>, where i=1,2,...,I and L<sub>0</sub>=1</td>
</tr>
</tbody></table>

Let TFC(TFI<sub>1</sub>, TFI<sub>2</sub>, …, TFI<sub>I</sub>) be the transport format combination for which TrCH<sub>1</sub> has transport format TFI<sub>1</sub>, TrCH<sub>2</sub> has transport format TFI<sub>2</sub>, etc. The corresponding CTFC(TFI<sub>1</sub>, TFI<sub>2</sub>, …, TFI<sub>I</sub>) is then computed as:

<table cellpadding="0" cellspacing="0">
<tbody><tr>
<td>CTFC = </td>
<td align="center">l
<br><big><big><big>∑</big></big></big><small>
<br>i<small> </small>=<small> </small>1</small></td>
<td>TFI<sub>i</sub>•P<sub>i</sub><sup> </sup></td>
</tr>
</tbody></table>

For FACH and PCH transport channels, "TrCH<sub>1</sub>"
corresponds to the transport channel listed at the first position in IE
"FACH/PCH information" in IE "Secondary CCPCH System Information", "TrCH<sub>2</sub>" 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, "TrCH<sub>1</sub>" corresponds to the
transport channel having the lowest transport channel identity in the
transport format combination mapped to the TFCI field. "TrCH<sub>2</sub>" corresponds to the transport channel having the next lowest transport channel identity, and so on.

<i>Examples</i>
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:
<br>
<center>
<table border="1">
<tbody><tr><td> </td><td colspan="3">AMR</td><td>Signalling</td></tr>
<tr><td> </td><td>Class A</td><td>Class B</td><td>Class C</td><td><br></td></tr>
<tr><td> </td><td>TrCH1</td><td>TrCH2</td><td>TrCH3</td><td>TrCH4</td></tr>
<tr><td>L</td><td>3</td><td>2</td><td>2</td><td>2</td></tr>
<tr><td>TFI</td><td><br></td><td><br></td><td><br></td><td><br></td></tr>
<tr><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td></tr>
<tr><td>1</td><td>81</td><td>103</td><td>60</td><td>148</td></tr>
<tr><td>2</td><td>39</td><td>-</td><td>-</td><td>-</td></tr>
</tbody></table>
</center>

The permitted transport format combinations are in table below:
<br>
<center>
<table border="1">
<tbody><tr><td>TFCI</td><td>TFI<sub>1</sub></td><td>TFI<sub>2</sub></td><td>TFI<sub>3</sub></td><td>TFI<sub>4</sub></td><td>Explanation</td></tr>
<tr><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>DTX</td></tr>
<tr><td>2</td><td>1</td><td>0</td><td>0</td><td>0</td><td>SID</td></tr>
<tr><td>3</td><td>2</td><td>1</td><td>1</td><td>0</td><td>12.2 Kbps</td></tr>
<tr><td>4</td><td>0</td><td>0</td><td>0</td><td>1</td><td>Signalling</td></tr>
<tr><td>5</td><td>1</td><td>0</td><td>0</td><td>1</td><td>SID + Signalling</td></tr>
<tr><td>6</td><td>2</td><td>1</td><td>1</td><td>1</td><td>12.2kbps + signalling</td></tr>
</tbody></table>
</center>

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<sub>6,0</sub>, CTFC<sub>6,1</sub>, CTFC<sub>6,11</sub>, CTFC<sub>6,12</sub>, CTFC<sub>6,13</sub> and CTFC<sub>6,23</sub>. 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:
<br>
<center>
<table border="1">
<tbody><tr><td> </td><td>PS RAB 128 Kbps</td><td>Signallling</td></tr>
<tr><td> </td><td>TrCH1</td><td>TrCH2</td></tr>
<tr><td>L</td><td>5</td><td>2</td></tr>
<tr><td>TFI</td><td><br></td><td><br></td></tr>
<tr><td><br></td><td>0*336</td><td>0</td></tr>
<tr><td><br></td><td>1*336</td><td>148</td></tr>
<tr><td><br></td><td>2*336</td><td>-</td></tr>
<tr><td><br></td><td>4*336</td><td>-</td></tr>
<tr><td><br></td><td>8*336</td><td>-</td></tr>
</tbody></table>
</center>

The permitted transport format combinations are in table below:
<br>
<center>
<table border="1">
<tbody><tr><td>TFCI</td><td>TFI<sub>1</sub></td><td>TFI<sub>2</sub></td></tr><tr>
</tr><tr><td>1</td><td>0</td><td>0</td></tr><tr>
</tr><tr><td>2</td><td>1</td><td>0</td></tr><tr>
</tr><tr><td>3</td><td>2</td><td>0</td></tr><tr>
</tr><tr><td>4</td><td>3</td><td>0</td></tr><tr>
</tr><tr><td>5</td><td>4</td><td>0</td></tr><tr>
</tr><tr><td>6</td><td>0</td><td>1</td></tr><tr>
</tr><tr><td>7</td><td>1</td><td>1</td></tr><tr>
</tr><tr><td>8</td><td>2</td><td>1</td></tr><tr>
</tr><tr><td>9</td><td>3</td><td>1</td></tr><tr>
</tr><tr><td>10</td><td>4</td><td>1</td></tr><tr>
</tr></tbody></table>
</center>

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<sub>4,0</sub>, CTFC<sub>4,1</sub>, CTFC<sub>4,2</sub>, CTFC<sub>4,3</sub>, CTFC<sub>4,4</sub>, CTFC<sub>4,5</sub>, CTFC<sub>4,6</sub>, CTFC<sub>4,7</sub>, CTFC<sub>4,8</sub>, CTFC<sub>4,8</sub>, CTFC<sub>4,9</sub> and CTFC<sub>4,10</sub>. Four bits will be required to send the CTFC from which the table of transport formats can be contructed by the UE.