Telecom Techniques Guide


How Voice Signal Processing in GSM

How Voice Signal Processing in GSM

The radio channel is quite different from the wired channel. First, the radio channel has a distinct time-change characteristic. The radio channel is exposed to the air, so it is vulnerable to the  interferences in the air. The signal is influenced by various interferences, multi-path fading and shadow fading, so the error bit ratio is rather high.

Voice Signal Processing

To solve the problems mentioned above, a series of forward and backward(uplink & downlink) transmission techniques are applied. The original subscriber data or signaling data are transformed before being carried by the radio waves. And at the other end of the transmission, a reverse transforming will be done.

This can provide necessary protection to the transmitting signal. The transformation methods roughly include the channel coding/decoding, interleaving/de-interleaving, burst formatting, encryption/decryption, and modulation/demodulation.

For the voice, to pass an analog-to-digital converter is actually a sampling process in the rate of 8KHz, after quantification each 125μs contains 13bit of code stream; then speech coding is performed  with every 20ms as a segment and the code transmission rate is reduced to 13Kbit/s, which becomes 22.8Kbit/s after the channel coding; then the voice becomes a code stream at 33.8kbit/s after code interleaving, encryption and burst formatting and is transmitted finally. The processing at the terminal is just the reverse of the above procedures.

Voice Signal Processing in GSM – Summary

Voice signal processing in GSM is a fundamental aspect of mobile communication technology. It starts with the conversion of analog voice signals into digital format through Analog-to-Digital Conversion (ADC), employing a standard 13-bit PCM technique to ensure high-quality representation. The next step involves speech coding, which compresses the digital voice signal using the Regular Pulse Excitation – Long-Term Prediction (RPE-LTP) algorithm. This compression conserves bandwidth and network resources efficiently.

Channel coding, specifically Convolutional Coding, is employed to make the digital voice signal more robust against transmission errors, ensuring error-free communication. Afterward, modulation, using Gaussian Minimum Shift Keying (GMSK), converts the digital signal into radio waves for transmission, optimizing spectral efficiency and minimizing interference.

At the receiving end, the signal is demodulated, channel decoded, and speech decoded to reconstruct the original voice signal. This comprehensive voice signal processing system ensures the clarity and reliability of voice calls in GSM, making it a cornerstone of mobile communication worldwide.