Speech processing through gsm physical layer
GSM physical layer is nothing but the modules through which speech will pass through before they are transmitted in the air. These modules are depicted in the figure below.
This page on GSM tutorial covers GSM speech processing modules at layer-1 i.e. Physical layer.
Fig.4 GSM Physical Layer
These modules are speech coding, channel coding, interleaving, ciphering, burst assembly, modulation. Speech coding block uses 13kbps RELP (Residually Excited Linear Predictive coder). Channel coding block uses convolution coding of rate 1/2 with constraint length of 5. Interleaving block does diagonal interleaving, after 456 encoded bits in 20ms duration are broken into 57 bits sub-blocks.
There will be about total 8 sub blocks of 57 bits each. Ciphering block uses A3 and A5 encryption algorithms. Encryption is changed call by call to enhance privacy. Burst assembly block frames the burst as required by GSM frame structure. The same is modulated and Gaussian filtered. Modulation block minimizes the occupied BW using GMSK modulation with BT of 0.3. For more details on GSM Physical layer refer our page in articles section with link mentioned below in related links.