Page 57 - From GMS to LTE

Global System for Mobile Communications (GSM) 43

Signal forming
Voice, Determination of Determination of
PCM coded filter parameters filter parameters Coding of
20 ms, for the vocal for the period the ‘rest
160 values, tract filter filter signal’
1280 bit

36 Bit 36 Bit 188 Bit
Sender
Frame (260 bits) for transmission every 20 milliseconds
Receiver

Signal forming
Voice,
PCM coded Vocal tract Excitation
20 ms, filter Period filter signal
160 values,
1280 bit

Figure 1.34 Complete transmission chain with the transmitter and receiver of the GSM FR codec.

1.7.6 Channel Coder and Interleaver in the BTS
When a 260‐bit data frame from the TRAU arrives at the base station every 20 milliseconds,
it is further processed before being sent over the air as shown in Figure 1.35. In the
reverse direction, the tasks are performed in the mobile device.
In the first step, the voice frames are processed in the channel coder unit, which adds
error detection and correction information to the data stream. This step is very important
as the transmission over the air interface is prone to frequent transmission errors due to
the constantly changing radio environment. Furthermore, the compressed voice infor-
mation is very sensitive and even a few bits that might be changed while the frame is
transmitted over the air interface create an audible distortion. To prevent this, the chan-
nel coder separates the 260 bits of a voice data frame into three different classes as
shown in Figure 1.36.
Fifty of the 260 bits of a speech frame are class Ia bits and are extremely important for
the overall reproduction of the voice signal at the receiver side. Such bits are, for exam-
ple, the higher order bits of the filter parameters. To enable the receiver to verify the
correct transmission of those bits, a three‐bit cyclic redundancy check (CRC) check-
sum is calculated and added to the data stream. If the receiver cannot recreate the
checksum with the received bits later on, the frame is discarded.
The other 132 bits of the frame are also quite important and are thus put into class Ib.
However, no checksum is calculated for them. To generate the exact amount of bits that
are necessary to fill a GSM burst, four filler bits are inserted. Afterward, the class Ia bits,
checksum, class Ib bits and the four filler bits are treated by a convolutional coder that
adds redundancy to the data stream. For each input bit, the convolutional decoder