Speech Coding

GSM TECHNICAL DETAILS:

The GSM network can be divided into three broad parts

* The subscriber carries the mobile station
* The base station subsystem controls the radio link with the mobile station
* The network subsystem performs the switching of calls between the mobile users and other mobile and fixed network users

SPEECH CODING:

GSM is a digital system, so speech, which is inherently analog, has to be digitised.

The GSM group studied several speech coding algorithms on the basis of subjective speech quality and complexity (which is related to cost, processing delay and power consumption once implemented) before arriving at the choice of a Regular Pulse Excited - Linear Predictive Coder (RPE-LPC) with a long term predictor loop. Basically, information from previous samples, which does not change very quickly, is used to predict the current sample.

The coefficients of the linear combination of the previous samples, plus an encoded form of the residual, the difference between the predicted and actual sample, represent the signal. Speech is divided into 20 (ms) samples, each of which is encoded as 260 bits, giving a total bit rate of 13kbps (kilobits per second). This is the so-called full-rate speech coding. Recently, an enhanced full-rate (EFR) speech coding algorithm has been implemented by some North American GSM1900 operators. This is said to provide improved speech quality using the existing 13kbps bit rate.