8.2.2 FEC error protection

26.2263GPPCellular text telephone modemGeneral descriptionRelease 17TS

The net bits are protected against transmission errors by means of an r=1/4 convolutional coder with the constraint length K=5. The structure of the encoder is depicted in Figure 3. The net bit‑stream (with net bits that are either 0 or 1) serves as input signal for four FIR filters with the impulse responses

g₁ = { 1, 0, 1, 0, 1 }

g₂ = { 1, 0, 1, 1, 1 }

g₃ = { 1, 1, 0, 1, 1 }

g₄ = { 1, 1, 1, 1, 1 }

The convolution of the filters is executed using mod‑2 arithmetics, i.e. the output signals of the filters are given by

,

where the mod‑2 operation denotes the remainder of a division by two. The output gross bit‑stream is obtained by merging the four output signals by means of the rotating switch in Figure 3, so that each net bit is mapped to four gross bits according to

.

The convolutional encoder is set to its initial state (i.e. all filter states are filled with zeros and the rotating switch is set to the u₁‑position) each time that a new burst is initiated (see Section 8.2.3, "Burst Structure"). During a running burst the convolutional encoder generates the gross bit‑stream as described before. At the end of a burst, the convolutional encoder is flushed by inserting K–1=4 zero-valued tail bits resulting in (K–1)/r=16 additional gross bits at the encoder’s output.

Figure 3 – Structure of the convolutional encoder