Agilent Technologies Switch/Measure Manual de usuario descargar pdf (Pagina 27)

GSM speech coding

To transmit actual voice data would require more bandwidth than a system can

practically afford. Most digital communication systems use some sort of voice

compression. GSM is no exception. The encoding goal is to represent speech

with the least amount of bits.

Human speech consists of an excitation signal (pitch, loudness) a filter (tongue,

teeth), and redundancy (long vowels and pauses). The GSM speech encoder

produces 260 bits every 20 ms that represent the human voice. The output bits

are separated into groups by their importance at reproducing an acceptable

representation of speech. The most important bits are the Class Ia bits. These

bits contain filter coefficients, and receive the most protection during the chan-

nel coding process. Next in importance are the Class Ib bits, and least impor-

tant are the Class II bits. Class II bits do not receive additional protection from

the channel coder.

During channel decoding, the CRC must be successful or the entire frame is

discarded. Once CRC is verified Class Ib and Class II bits are checked for errors.

The RBER test then becomes bit errors in accepted frames over the total

number of bits in the accepted frames.

FER

The frame erasure rate (FER) indicates how many of the received frames are

bad. The channel decoder erases frames when the CRC fails. FER is performed

on both speech and signaling frames. When a speech frame is discarded, the

system will interpolate. When a signaling frame is discarded, the mobile station

is instructed to resend. In the GSM standards, it is common to see FER limits

multiplied by correctional value α. This correction factor allows the system

designer to trade off between FER and RBER. If the system is designed to

discard a large number of frames, then the FER will be higher (multiply by α)

and RBER should be significantly lower (divide by α). Values for α range

between 1 and 1.6.

Mic

Filter

300 Hz to 4 kHz

A/D 8K

13 bit

104 kbps

Speach

encoder

(compression)

13 kbps

Modulator

Speach

encoder

(protection)

Class Ia CRC Class Ib Class II

50 bits 3 bits 132 bits 78 bits

1 2 ... 22 23 24 25 26 27 28 29 30 31 32 ... 35 36

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