Base Transceiver Station (BTS) is a basic element of the GSM mobile phone system, more commonly known as GSM relay antenna (to differentiate UMTS antenna relay, Node B).
Basically, it is composed mainly of an interface element with the controlling station (BSC), a transmitter / receiver (transceiver TRX) and antenna: it thus forms a cell (based mesh network ).
It is responsible for the radio link with the mobile stations.
The main roles are a BTS:
- activation and deactivation of a radio channel;
- time division multiplexing (TDMA) and frequency hopping (Frequency Hopping)
- encrypting the content to be transmitted (for the confidentiality of wireless communication);
- channel coding, encryption frames, modulation, demodulation and decoding of radio signal (protection against transmission errors, interference, noise ….);
- monitoring binding;
- monitoring of field levels and received signal quality (required for handover);
- control the transmission power (to limit the power which is enough to not disturb neighboring cells too).
TRU (Transmission Reception Unit) is a transceiver that operates a pair of carrier frequencies (one uplink, a downlink channel). It can multiplex up to 8 simultaneous calls on a TRX through the TDMA multiple access technique.
In theory, the maximum capacity of a BTS is 12 TRX. Thus, it can handle up to 96 simultaneous calls. But this limit is never reached in practice.
In rural areas, the role of the BTS is to ensure coverage. It is generally limited to one or two TRX if the operator provides a TRX tire.
In urban areas, the BTS must provide coverage but also a consistent traffic flow. It can be equipped with two to nine TRX.
To pass more traffic, operators prefer to increase the number of BTS rather than increasing the number of TRX by BTS. Thus, interference between channels using the same frequencies are limited.
Type of antenna
We can distinguish the omnidirectional antennas (which emit at 360 °) bi-sector antennas (180 ° antenna) or tri-sector (120 ° antenna) are the most common because they allow an optimized mesh geographical area (see Article mobile network) and limit interference between channels using the same frequency.
A classic BTS can transmit up to 35 miles up (in the frequency band of 900 MHz). We talk about macro-cell to a radius between 2 km and 35 km and small cell (small-cell) for a radius between 500 m and 2 km.
Micro BTS are designed for urban areas and define micro-cells (less than 500 m radius), the Femtocells have a range of 10 to 20 m (typically used in closed rooms).
The GSM standard requires a minimum sensitivity of -104 dBm for GSM 900 and DCS 1800. It also defines several classes according to the maximum electrical power before coupling: