1.3. General characteristics of the GSM standard

The GSM digital mobile cellular system is a second-generation cellular system (G2). The choice of digital technology in future generations of cellular systems is a fundamental and probably irreversible solution.

One of the most attractive aspects of digital transmission methods is that they are more effective in conditions of strong radio interference and provide higher system capacity compared to analog methods of the first generation of cellular systems.

The advantages of digital methods used in the GSM cellular mobile communication system are as follows.
- Digital speech coding with lower speeds. Low-speed speech coding, compatible with digital modulation techniques, allows multiple voice channels to be transmitted on a single carrier, thereby increasing spectrum utilization efficiency.
- Digital modulation, which allows to increase the efficiency of use of the frequency spectrum in comparison with analog methods.
- Flexible changeable bandwidth.
- Higher noise immunity.

Digital systems have higher characteristics than analog ones with strong co-channel (or internal) interference (CCI, Co-Channel Interference) and adjacent channel interference (ACI, Adjacent Channel Interference). This is one of the decisive reasons for adopting digital technology for the second and third generations of cellular systems. Digital systems are likely to function in conditions of much stronger co-channel interference, which allows designers to reduce cell size (for example, micro / pico cell) and distance between cells that reuse the same frequencies, and even simplify the structure of frequency reuse.

These parameters and these geometric changes increase the total capacity of cellular mobile networks.
- Reduced loss of capacity to the alarm.
- Improved access control and call transfer. For a fixed distribution of the frequency spectrum, a larger increase in capacity implies a corresponding reduction in cell size. This means that the load on the signaling channels increases as more frequent call transfer occurs. In each cell, the base station must process a greater number of requests for access and registration from the entire set of moving subscribers. These functions can be performed simply and quickly using digital methods.

In general, the GSM mobile communication system is designed for its use in the commercial sphere. It provides users with a wide range of services and applications for a variety of equipment for transmitting voice messages and data, call signals and alarms, and also provides the ability to connect to public telephone networks, data networks and integrated services digital networks.

Compared with other widely accepted standards of digital cellular mobile communication systems, the GSM standard provides [1.7]:
- the best energy performance;
- higher quality of communication;
- communication security and confidentiality.

Sufficiently high quality of received voice signals in the GSM standard is provided with a signal-to-noise ratio at the receiver input C / N = 9 dB (for the D-AMPSC / N standard = 1 dB), and the energy costs in real communication channels (with fading radio signals) are 6 ... 10 dB lower compared to the D-AMPS standard (USA).

The GSM standard, in addition, provides its users with a number of services that are not implemented (or not fully implemented) in other cellular communication standards. These include the following:
- the use of SIM-cards to provide access to the channel and communication services;
- encryption of transmitted messages;
- subscriber authentication and subscriber equipment identification using cryptographic algorithms;
- closed from listening to the radio interface;
- the use of SMS short message services (ShortMessageServices) transmitted via signaling channels;
- automatic roaming of subscribers of various GSM networks nationally and internationally;
- inter-network roaming of GSM subscribers with subscribers of DCS1800, PCS1900 networks, as well as with satellite personal communication networks (Globalstar, Inmarsat-P, Iridium).

In accordance with the recommendations of the CEP standard GSM digital pan-European cellular land mobile communication system provides for the operation of transmitters in two frequency bands:
- frequency band 890 ... 915 MHz - for transmitting messages from the mobile station to the base;
- frequency band 935 ... 960 MHz - for transmitting messages from the base station to the mobile.

When switching channels during a session, the difference between these frequencies is constant and equal to 45 MHz. The frequency spacing between adjacent communication channels is 200 kHz. Thus, in the 25 MHz frequency band allocated for reception / transmission, 124 communication channels are located. The GSM standard uses multiple access with time division (multiplexing) of TDMA channels (Time Division Multiple Access), which allows 8 voice channels to be placed simultaneously on one carrier frequency. A speech codec (encoder / decoder) [RPE-LTP (Regular Pulse Excited-Long Term Predictor) - linear prediction with excitation by a regular pulse sequence and long-term prediction] regular pulse excitation and conversion speed of 13 kbps or 6.5 kbps. Speech processing in the GSM standard is carried out within the framework of the adopted DTX (DiscontinuesTransmission) discontinuous voice transmission system, which ensures that the transmitter is turned on only when the user starts a conversation and disconnects it (the transmitter) during pauses and at the end of the conversation.

The DTX system controls the VAD (Voice Activity Detector) speech activity detector, which detects and extracts speech intervals with noise and noise without speech, even in cases where the noise level is commensurate with the level of the speech signal. Conventional and convolutional interleaved coding is used to protect against errors that occur in radio channels. Improving the efficiency of coding and interleaving at low speed of movement of mobile stations is achieved by slow switching of operating frequencies during a communication session (at a rate of 277 frequency jumps per second).

To counter the interference fading of received radio signals caused by multipath propagation of radio waves in city conditions, communication equipment uses equalizers that provide equalization (equalizing) of pulse signals with a standard deviation of the delay times of signals up to 16 μs.

The GSM equipment synchronization system is designed for compensation (up to 233 μs) of the absolute signal delay time. This corresponds to the maximum communication range (maximum cell radius) of 35 km. For modulation of a radio signal, a spectral-effective Gaussian frequency shift keying with a minimum frequency shift - GMSK (Gaussian Minimum Shift Keying) is used. The manipulation is called so because the sequence of information bits before the modulator passes through a low-pass filter with a Gaussian amplitude-frequency characteristic, which gives a significant reduction in the bandwidth of the emitted signal, and hence the mutual influence of the channels.

In the GSM standard, modulation is used with the value of the normalized band BT = 0.3 (where B is the filter bandwidth by level (-3 dB), Tb is the transmission duration of one bit).
GMSK modulation is characterized by the following positive properties:
- good enough noise immunity of the communication channel;
- a narrow frequency spectrum at the output of the transmitter power amplifier, which ensures a low level of out-of-band emission;
a constant level envelope that allows the use of power amplifiers operating in Class A mode in transmitters.

1.4. Technical characteristics of the standard GSM 900/1800

Operating frequency range:
- Transmission frequencies of mobile stations (MS) and base stations (BTS) stations (from mobile station to base station - Uplink):
■ GSM 900 - (890 ... 915) MHz;
■ GSM 1800 (DCS 1800) - (1710 ... 1785) MHz.
- Frequency of reception of mobile stations and transmission of base stations (from the base to the mobile station - Downlink):
■ GSM 900 - (935 ... 960) MHz;
■ GSM 1800 (DCS 1800) - (1805 ... 1880) MHz.
- Duplex frequency separation of reception and transmission:
■ GSM 900 - 45 MHz;
■ GSM 1800 —95 MHz.
- Equivalent frequency band per voice channel:
■ GSM 900 - 25 kHz;
■ GSM 1800 - 12.5 kHz.
- The bandwidth of the communication channel - 200 kHz.
- The maximum number of communication channels - 124.
- Number of voice channels per carrier:
■ GSM 900 - 8;
■ GSM 1800 - 16.
- The maximum number of channels organized in the base station:
■ GSM 900 - 16 .. .20.
- Access method - TDMA.
- Type of speech codec - RPE / LTP.
- The width of the pre-modulation Gaussian filter is 81.2 kHz.
- The speed of the speech codec conversion is 13 (6.5) kbps.
- The speed of information transfer in the radio channel - 270,833 kbit / s.
- Type of modulation - GMSK.
- The modulation index of the VL - 0.3.
- The number of jumps in frequency per second - 277 s-1.
- Cell radius - (0.5 ... 35) km.