Page 58 - From GMS to LTE

44 From GSM to LTE-Advanced Pro and 5G

Voice Speech coder Channel Interleaver Cipherer Modulator
(FR, EFR, AMR) coder

TRAU BTS

Mobile station

Voice Speech decoder Channel De- De- De-
(FR, EFR, AMR) decoder interleaver cipherer modulator

Figure 1.35 Transmission path in the downlink direction between the network and the mobile device.

Figure 1.36 GSM channel coder for full‐rate
260 speech frames.
Class Ia Class Ib Class II
78
50 132
CRC Filler
50 3 132 4

Convolutional coder

378 78
456 Bits = 4 Bursts

calculates two output bits. For the computation of the output bits, the coder uses not
only the current bit but also the information about the values of the previous bits. For
each input bit, two output bits are calculated. This mathematical algorithm is also called
a HR convolutional coder.
The remaining 78 bits of the original 260‐bit data frame belong to the third class,
which is called class II. These are not protected by a checksum and no redundancy is
added for them. Errors that occur during the transmission of these bits can neither be
detected nor corrected.
As has been shown, the channel coder uses the 260‐bit input frame to generate 456
bits on the output side. As a burst on the air interface can carry exactly 114 bits, four
bursts are necessary to carry the frame. As the bursts of a TCH are transmitted every
4.6152 milliseconds, the time it takes to transmit the frame over the air interface is
about 20 milliseconds. To get to exactly 20 milliseconds, the empty burst and the burst
used for the SACCH per 26‐burst multiframe has to be included in the calculation.