Base Transceiver Station BTS

Base Transceiver Station (BTS) is the equipment which facilitates the wireless communication between user equipments and the network.

The term BTS is generally and commonly associated with mobile communication technologies like GSM and CDMA. A BTS forms part of the Base Station Subsystem (BSS) and has the equipments (transceivers) for transmitting and receiving of radio signals, signal processors, signal paths, signal amplifiers, and equipments for system management. It may also have equipments for encrypting and decrypting communications, spectrum filtering tools (band pass filters) etc. Antennas may also be considered as components of BTS in general sense as they facilitate the functioning of BTS. A BTS is controlled by a parent Base Station Controller via the Base station Control Function (BCF).The BCF provides an Operations and Maintenance (O&M) connection to the Network management system (NMS), and manages operational states of each TRX, as well as software handling and alarm collection.

The BTSs are equipped with radios that are able to modulate layer 1 of interface Um; for GSM 2G+ the modulation type is GMSK, while for EDGE-enabled networks it is GMSK and 8-PSK.

The Base Station Controller (BSC) provides, classically, the intelligence behind the BTSs. Typically a BSC has 10s or even 100s of BTSs under its control. The BSC handles allocation of radio channels, receives measurements from the mobile phones, controls handovers from BTS to BTS. Networks are often structured to have many BSCs distributed into regions near their BTSs which are then connected to large centralised MSC sites.

Although the Transcoding (compressing/decompressing) function is as standard defined as a BSC function, there are several vendors which have implemented the solution in a stand-alone rack using a proprietary interface. This subsystem is also referred to as the TRAU (Transcoder and Rate Adaptation Unit). The transcoding function converts the voice channel coding between the GSM (Regular Pulse Excited-Long Term Prediction, also known as RPE-LPC) coder and the CCITT standard PCM (G.711 A-law or u-law). Since the PCM coding is 64 kbit/s and the GSM coding is 13 kbit/s, this also involves a buffering function so that PCM 8-bit words can be recoded to construct GSM 20 ms traffic blocks, to compress voice channels from the 64 kbit/s PCM standard to the 13 kbit/s rate used on the air interface. Some networks use 32 kbit/s ADPCM on the terrestrial side of the network instead of 64 kbit/s PCM and the TRAU converts accordingly. When the traffic is not voice but data such as fax or email, the TRAU enables its Rate Adaptation Unit function to give compatibility between the BSS data rates and the MSC capability.

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