## 5.3 Packet random access channel (PRACH, CPRACH and MPRACH)

3GPP45.003GSM/EDGE Channel codingRelease 17TS

Two coding schemes are specified for access bursts on the packet switched channels. The packet access burst containing 8 information bits and the extended packet access burst containing 11 information bits. Only the 11 information bits access burst may be transmitted on the CPRACH.

### 5.3.1 Packet Access Burst

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

### 5.3.2 Extended Packet Access Burst

#### 5.3.2.1 Block constitution

The burst carrying the extended packet random access uplink message contains 11 information bits d(0),d(1),…,d(10).

#### 5.3.2.2 Parity bits

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

d(0)D16 +…+ d(10)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.

#### 5.3.2.3 Addition of BSIC

An MS that has not enabled PEO or EC operation shall use the six bit BSIC of the BTS to which the access burst is intended. The six bits of the BSIC, {b(0),b(1),…,b(5)}, 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(20)} by:

u(k) = d(k) for k = 0,1,…,10

u(k) = C(k‑11) for k = 11,12,…,16

u(k) = 0 for k = 17,18,19,20 (tail bits)

An MS that has enabled PEO or EC operation shall use the nine bit BSIC of the BTS to which the access burst is intended . The first six bits of the BSIC, {b(0),b(1),…,b(5)}, are added bitwise modulo 2 to the six parity bits while the last three bits of the BSIC {b(6),b(7),b(8)} are added bitwise modulo 2 to the three last information bits d(8), d(9) and d(10). The bitwise modulo 2 operation results in nine colour bits, C(0) to C(8), defined as C(k) = b(k) + p(k) (for k = 0 to 5) and C(k) = b(k) + d(k+2) (for k = 6 to 8) where:

b(0) = MSB of PLMN colour code

b(8) = LSB of Radio frequency colour code.

This defines {u(0),u(1),…, u(20)} by:

u(k) = d(k) for k = 0,1,…,7

u(k) = C(k-2) for k = 8,9,10

u(k) = C(k‑11) for k = 11,12,…,16

u(k) = 0 for k = 17,18,19,20 (tail bits)

#### 5.3.2.4 Convolutional code

The coded bits {c(0),c(1),…, c(41)} are obtained by the same convolutional code of rate ½ as for TCH/FS, defined by the polynomials:

G0 = 1 + D3 + D4

G1 = 1 + D + D3 + D4

and with:

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,…,20 ; u(k) = 0 for k < 0

The code is punctured in such a way that the following coded bits:

c(0), c(2), c(5), c(37), c(39), c(41) are not transmitted.

This results in a block of 36 coded bits, {e(0), e(1),…,e(35)}.