## 4 Control Channels

05.033GPPChannel codingRelease 1999TS

## 4.1 Slow associated control channel (SACCH)

### 4.1.1 Block constitution

The message delivered to the encoder has a fixed size of 184 information bits {d(0),d(1),…,d(183)}. It is delivered on a burst mode.

### 4.1.2 Block code

a) Parity bits:

The block of 184 information bits is protected by 40 extra bits used for error correction and detection. These bits are added to the 184 bits according to a shortened binary cyclic code (FIRE code) using the generator polynomial:

g(D) = (D23 + 1)*(D17 + D3 + 1)

The encoding of the cyclic code is performed in a systematic form, which means that, in GF(2), the polynomial:

d(0)D223 + d(1)D222 +…+d(183)D40 + p(0)D39 + p(1)D38 +…+p(38)D + p(39)

where {p(0),p(1),…,p(39)} are the parity bits , when divided by g(D) yields a remainder equal to:

1 + D + D2 +…+ D39.

b) Tail bits

Four tail bits equal to 0 are added to the information and parity bits, the result being a block of 228 bits.

u(k) = d(k) for k= 0,1,…,183

u(k) = p(k‑184) for k = 184,185,…,223

u(k) = 0 for k = 224,225,226,227 (tail bits)

### 4.1.3 Convolutional encoder

This block of 228 bits is encoded with the 1/2 rate convolutional code (identical to the one used for TCH/FS) defined by the polynomials:

G0 = 1 + D3 + D4

G1 = 1 + D + D3 + D4

This results in a block of 456 coded bits: {c(0),c(1),…,c(455)} defined by:

c(2k) = u(k) + u(k‑3) + u(k‑4)

c(2k+1) = u(k) + u(k‑1) + u(k‑3) + u(k‑4) for k = 0,1,…,227 ; u(k) = 0 for k < 0

### 4.1.4 Interleaving

The coded bits are reordered and interleaved according to the following rule:

i(B,j) = c(n,k) for k = 0,1,…,455

n = 0,1,…,N,N+1,…

B = B0 + 4n + (k mod 4)

j = 2((49k) mod 57) + ((k mod 8) div 4)

See table 1. The result of the reordering of bits is the same as given for a TCH/FS (subclause 3.1.3) as can be seen from the evaluation of the bit number‑index j, distributing the 456 bits over 4 blocks on even numbered bits and 4 blocks on odd numbered bits. The resulting 4 blocks are built by putting blocks with even numbered bits and blocks with odd numbered bits together into one block.

The block of coded data is interleaved "block rectangular" where a new data block starts every 4th block and is distributed over 4 blocks.

### 4.1.5 Mapping on a Burst

The mapping is given by the rule:

e(B,j) = i(B,j) and e(B,59+j) = i(B,57+j) for j = 0,1,…,56

and

e(B,57) = hl(B) and e(B,58) = hu(B)

The two bits labelled hl(B) and hu(B) on burst number B are flags used for indication of control channel signalling. They are set to "1" for a SACCH.

## 4.2 Fast associated control channel at full rate (FACCH/F)

### 4.2.1 Block constitution

The message delivered to the encoder has a fixed size of 184 information bits. It is delivered on a burst mode.

### 4.2.2 Block code

The block encoding is done as specified for the SACCH in subclause 4.1.2.

### 4.2.3 Convolutional encoder

The convolutional encoding is done as specified for the SACCH in subclause 4.1.3.

### 4.2.4 Interleaving

The interleaving is done as specified for the TCH/FS in subclause 3.1.3.

### 4.2.5 Mapping on a Burst

A FACCH/F frame of 456 coded bits is mapped on 8 consecutive bursts as specified for the TCH/FS in subclause 3.1.4. As a FACCH is transmitted on bits which are stolen in a burst from the traffic channel, the even numbered bits in the first 4 bursts and the odd numbered bits of the last 4 bursts are stolen.

To indicate this to the receiving device the flags hl(B) and hu(B) have to be set according to the following rule:

hu(B) = 1 for the first 4 bursts (even numbered bits are stolen);

hl(B) = 1 for the last 4 bursts (odd numbered bits are stolen).

The consequences of this bitstealing by a FACCH/F is for a:

‑ speech channel (TCH/FS) and data channel (TCH/F2.4):

One full frame of data is stolen by the FACCH.

‑ Data channel (TCH/F14.4):

The bitstealing by a FACCH/F disturbs a maximum of 96 of the 456 coded bits generated from an input data block of 290 bits.

‑ Data channel (TCH/F9.6):

The bitstealing by a FACCH/F disturbs a maximum of 96 coded bits generated from an input frame of four data blocks. A maximum of 24 of the 114 coded bits resulting from one input data block of 60 bits may be disturbed.

‑ Data channel (TCH/F4.8):

The bit stealing by FACCH/F disturbs a maximum of 96 coded bits generated from an input frame of two data blocks. A maximum of 48 of the 228 coded bits resulting from one input data block of 60 bits may be disturbed.

NOTE: In the case of consecutive stolen frames, a number of bursts will have both the even and the odd bits stolen and both flags hu(B) and hl(B) must be set to 1.

## 4.3 Fast associated control channel at half rate (FACCH/H)

### 4.3.1 Block constitution

The message delivered to the encoder has a fixed size of 184 information bits. It is delivered on a burst mode.

### 4.3.2 Block code

The block encoding is done as specified for the SACCH in subclause 4.1.2.

### 4.3.3 Convolutional encoder

The convolutional encoding is done as specified for the SACCH in subclause 4.1.3.

### 4.3.4 Interleaving

The coded bits are reordered and interleaved according to the following rule:

i(B,j) = c(n,k) for k = 0,1,…,455

n = 0,1,…,N,N+1,…

B = B0 + 4n + (k mod 8) ‑ 4((k mod 8) div 6)

j = 2((49k) mod 57) + ((k mod 8) div 4)

See table 1. The result of the reordering of bits is the same as given for a TCH/FS (subclause 3.1.3) as can be seen from the evaluation of the bit number‑index j, distributing the 456 bits over 4 blocks on even numbered bits and 4 blocks on odd numbered bits. The 2 last blocks with even numbered bits and the 2 last blocks with odd numbered bits are put together into 2 full middle blocks.

The block of coded data is interleaved "block diagonal" where a new data block starts every 4th block and is distributed over 6 blocks.

### 4.3.5 Mapping on a Burst

A FACCH/H frame of 456 coded bits is mapped on 6 consecutive bursts by the rule:

e(B,j) = i(B,j) and e(B,59+j) = i(B,57+j) for j = 0,1,…,56

and

e(B,57) = hl(B) and e(B,58) = hu(B)

As a FACCH/H is transmitted on bits which are stolen from the traffic channel, the even numbered bits of the first 2 bursts, all bits of the middle 2 bursts and the odd numbered bits of the last 2 bursts are stolen.

To indicate this to the receiving device the flags hl(B) and hu(B) have to be set according to the following rule:

hu(B) = 1 for the first 2 bursts (even numbered bits are stolen)

hu(B) = 1 and hl(B) = 1 for the middle 2 bursts (all bits are stolen)

hl(B) = 1 for the last 2 bursts (odd numbered bits are stolen)

The consequences of this bitstealing by a FACCH/H is for a:

‑ speech channel (TCH/HS):

two full consecutive speech frames are stolen by a FACCH/H.

‑ data channel (TCH/H4.8):

The bitstealing by FACCH/H disturbs a maximum of 96 coded bits generated from an input frame of four data blocks. A maximum of 24 out of the 114 coded bits resulting from one input data block of 60 bits may be disturbed.

‑ data channel (TCH/H2.4):

The bitstealing by FACCH/H disturbs a maximum of 96 coded bits generated from an input frame of four data blocks. A maximum of 24 out of the 114 coded bits resulting from one input data block of 36 bits may be disturbed.

NOTE: In the case of consecutive stolen frames, two overlapping bursts will have both the even and the odd numbered bits stolen and both flags hu(B) and hl(B) must be set to 1.

## 4.4 Broadcast control, Paging, Access grant, Notification and Cell broadcast channels (BCCH, PCH, AGCH, NCH, CBCH), CTS Paging and Access grant channels (CTSPCH, CTSAGCH)

The coding scheme used for the broadcast control , paging, access grant, notification and cell broadcast messages is the same as for the SACCH messages, specified in subclause 4.1. In CTS, the coding scheme used for the paging and access grant messages is also the same as for the SACCH messages, specified in subclause 4.1.

## 4.5 Stand‑alone dedicated control channel (SDCCH)

The coding scheme used for the dedicated control channel messages is the same as for SACCH messages, specified in subclause 4.1.

## 4.6 Random access channel (RACH)

The burst carrying the random access uplink message has a different structure. It contains 8 information bits
d(0),d(1),…,d(7).

Six parity bits p(0),p(1),…,p(5) are defined in such a way that in GF(2) the binary polynomial:

d(0)D13 +…+ d(7)D6 + p(0)D5 +…+ p(5), when divided by D6 + D5 + D3 + D2 + D + 1 yields a remainder equal to D5 + D4 + D3 + D2 + D + 1.

The six bits of the BSIC, {B(0),B(1),…,B(5)}, of the BS to which the Random Access is intended, are added bitwise modulo 2 to the six parity bits, {p(0),p(1),…,p(5)}. This results in six colour bits, C(0) to C(5) defined as C(k) = b(k) + p(k) (k = 0 to 5) where:

b(0) = MSB of PLMN colour code

b(5) = LSB of BS colour code.

This defines {u(0),u(1),…, u(17)} by:

u(k) = d(k) for k = 0,1,…,7

u(k) = C(k‑8) for k = 8,9,…,13

u(k) = 0 for k = 14,15,16,17 (tail bits)

The bits {e(0),e(1),…, e(35)} are obtained by the same convolutional code of rate 1/2 as for TCH/FS, defined by the polynomials:

G0 = 1 + D3 + D4

G1 = 1 + D + D3 + D4

and with:

e(2k) = u(k) + u(k‑3) + u(k‑4)

e(2k+1) = u(k) + u(k‑1) + u(k‑3) + u(k‑4) for k = 0,1,…,17 ; u(k) = 0 for k < 0

## 4.7 Synchronization channel (SCH), Compact synchronization channel (CSCH), CTS Beacon and Access request channels (CTSBCH-SB, CTSARCH)

The burst carrying the synchronization information on the downlink BCCH, the downlink CPBCCH for Compact, and in CTS the information of the CTSBCH-SB and the access request message of the CTSARCH, has a different structure. It contains 25 information bits {d(0),d(1),…, d(24)}, 10 parity bits {p(0),p(1),…, p(9)} and 4 tail bits. The precise ordering of the information bits is given in GSM 04.08.

The ten parity bits {p(0),p(1),,…,p(9)} are defined in such a way that in GF(2) the binary polynomial:

d(0)D34 +…+ d(24)D10 + p(0)D9 +…+ p(9), when divided by:

D10 + D8 + D6 + D5 + D4 + D2 + 1, yields a remainder equal to:

D9 + D8 + D7 + D6 + D5 + D4 + D3 + D2 + D+ 1.

Thus the encoded bits {u(0),u(1),…,u(38)} are:

u(k) = d(k) for k = 0,1,…,24

u(k) = p(k‑25) for k = 25,26,…,34

u(k) = 0 for k = 35,36,37,38 (tail bits)

The bits {e(0),e(1),…, e(77)} are obtained by the same convolutional code of rate 1/2 as for TCH/FS, defined by the polynomials:

G0 = 1 + D3 + D4

G1 = 1 + D + D3 + D4

and with:

e(2k) = u(k) + u(k‑3) + u(k‑4)

e(2k+1) = u(k) + u(k‑1) + u(k‑3) + u(k‑4) for k = 0,1,….,77 ; u(k) = 0 for k < 0

## 4.8 Access Burst on circuit switched channels other than RACH

The encoding of this burst is as defined in subclause 4.6 for the random access channel (RACH). The BSIC used shall be the BSIC of the BTS to which the burst is intended.

## 4.9 Access Bursts for uplink access on a channel used for VGCS

The encoding of this burst is as defined in subclause 4.5 for the RACH. The BSIC used by the Mobile Station shall be the BSIC indicated by network signalling, or if not thus provided, the last received BSIC on the SCH of the current cell.

## 4.10 Fast associated control channel at ECSD E-TCH/F (E-FACCH/F)

### 4.10.1 Block constitution

The message delivered to the encoder has a fixed size of 184 information bits. It is delivered on a burst mode.

### 4.10.2 Block code

The block encoding is done as specified for the SACCH in subclause 4.1.2.

### 4.10.3 Convolutional encoder

The convolutional encoding is done as specified for the SACCH in subclause 4.1.3.

### 4.10.4 Interleaving

The interleaving is done as specified for the SACCH in subclause 4.1.4.

### 4.10.5 Mapping on a Burst

A E-FACCH/F frame of 456 coded bits is mapped on 4 full consecutive bursts. As a E-FACCH/F is transmitted on bits, which are stolen in a burst from the ECSD traffic channel, the four full bursts are stolen.

The mapping on is given by the rule:

e(B,j)=i(B,j) and e(B,59+j)=i(B,57+j) for j=0,1,…,56

and

e(B,57)=hl(B) and e(B,58)=hu(B).

To indicate to the receiving device the flags hl(B) and hu(B) have to be set according to the following rule:

hu(B)=1 and hl(B)=1 for the all 4 bursts (4 full bursts are stolen).

The consequences of this bitstealing by a E-FACCH/F is for a:

– Data channel (E-TCH/F43.2)

The bitstealing by a E-FACCH/F disturbs a maximum of 288 of the 1368 coded bits generated from an input data block of 870 bits.

– Data channel (E-TCH/F32)

The bitstealing by a E-FACCH/F disturbs 464 of the 1392 coded bits generated from an input data block of 640 bits.

– Data channel (E-TCH/F28.8)

The bitstealing by a E-FACCH/F disturbs a maximum of 288 of the 1368 coded bits generated from an input data block of 580 bits.