GSM Mobile Network – mobile network of 2nd generation
Structure and Functionality of Mobile Station

In the following two chapters the structure and operating principles of the MS and its coupling to the BS will be described.

The MS is composed of transmitting and receiving parts, a control microprocessor, SIM card and other accessories (handset, keyboard, display, etc.).

Block scheme of mobile station

The analogue voice signal on microphone output is digitized by A/D converter, which operates on the principle of PCM (8 kHz sampling rate, the sample of 13 bits, linear quantization, signal transmission speed vp = 8000 × 13 = 104 kbit/s). The next block, COD-S (CODer of Source), carries out speech synthesis on the principle of vocoder, also called as source coding and that is performed by RPE (Regular Pulse Excitation) excitation method, thereby reducing the transmission rate to a value vp = 13 kbit/s.

In block COD-C (CODer of Channel) is implemented channel coding. This process ensures reduced risk of error generation in speech and protect digital signal against errors originating in the transmission. Additionally, there is interleaving, which purpose is to increase the signal robustness against bursts of errors. On the receiving side, the signal is inverted by regulation adopted interleaved sequence and subsequently it is transformed to the original sequence. If the several consecutive bits are received with errors (burst errors), these may be repaired by using implemented security processes.

In addition, part of the signal, which contains user information, is still subjected to the encryption process in order to prevent direct monitoring of the communications. After of all these modifications, transmission rate of one radio channel is vp = 22.8 kbit/s.

Afterward, the following sequence of integration of signal into TDMA frame is processed in GMSK-MOD (Gaussian Minimum Shift Keying MODulator) and the overall transmission rate is then vp = 270,833 kbit/s (physical layer frames with length of 156.25 bits are transmitted every 0,577 ms).

After processing in other circuits of the Radio Transmitter (RT), the signal is radiated by the antenna of the MS. In terms of transmission power, the MSs are divided into five classes ranging from 0,1 to 20 W.

The signal transmitted from the antenna to the Radio Receiver (RR) block, is further processed in the demodulator (DEM) and both decoders (DECODing-C, DECODing-S). The D/A converter then transforms digital signal to analogue signal. Finally, the analogue signal is transferred into the handset.