A base transceiver station or cell site (BTS) is a piece of equipment that facilitates wireless communication between user equipment (UE) and a network. UEs are devices like mobile phones (handsets), WLL phones, computers with wireless internet connectivity, WiFi and WiMAX gadgets etc. The network can be that of any of the wireless communication technologies like GSM, CDMA, WLL, WAN, WiFi, WiMAX etc. BTS is also referred to as the radio base station (RBS), node B (in 3G Networks) or, simply, the base station (BS). For discussion of the LTE standard the abbreviation eNB for enhanced node B is widely used.
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[edit] BTS in Mobile Communication
A GSM BTS network is made up of three subsystems: • The Mobile Station (MS) • The Base Station subsystem (BSS) – comprising a BSC and several BTSs • The Network and Switching Subsystem (NSS) – comprising an MSC and associated registers.
Though the term BTS can be applicable to any of the wireless communication standards, it is generally and commonly associated with mobile communication technologies like GSM and CDMA. In this regard, a BTS forms part of the base station subsystem (BSS) developments for system management. It may also have equipment 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. Typically a BTS will have several transceivers (TRXs) which allow it to serve several different frequencies and different sectors of the cell (in the case of sectorised base stations). A BTS is controlled by a parent base station controller via the base station control function (BCF). The BCF is implemented as a discrete unit or even incorporated in a TRX in compact base stations. 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 basic structure and functions of the BTS remains the same regardless of the wireless technologies.
[edit] General Architecture
A BTS in general has the following units:
- Transceiver (TRX)
- Quite widely referred to as the driver receiver (DRX). Basically does transmission and reception of signals. Also does sending and reception of signals to/from higher network entities (like the base station controller in mobile telephony)
- Power amplifier (PA)
- Amplifies the signal from DRX for transmission through antenna; may be integrated with DRX.
- Combiner
- Combines feeds from several DRXs so that they could be sent out through a single antenna. Allows for a reduction in the number of antenna used.
- Duplexer
- For separating sending and receiving signals to/from antenna. Does sending and receiving signals through the same antenna ports (cables to antenna).
- Antenna
- This is also considered a part of the BTS.
- Alarm extension system
- Collects working status alarms of various units in the BTS and extends them to operations and maintenance (O&M) monitoring stations.
- Control function
- Control and manages the various units of BTS including any software. On-the-spot configurations, status changes, software upgrades, etc. are done through the control function.
- Baseband receiver unit (BBxx)
- Frequency hopping, signal DSP, etc..
[edit] Important terms regarding a mobile BTS
- Diversity techniques
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In order to improve the quality of received signal, often two receiving antennas are used, placed at an equal distance to an uneven multiple of a quarter of wavelength (for 900 MHz the wavelength it is 30 cm). This technique, famous as antenna diversity or diversity in the space, concurs to resolve the problems connected to the fading. The antennas can be spaced horizontally or vertically; in the first case though a greater facility of installation is required, advanced performance is obtained.
Other than antenna or space diversity, there are other diversity techniques like frequency/time diversity, antenna pattern diversity, polarization diversity, etc..
- Splitting
- The process of creating more coverage and capacity in a wireless system by having more than one cell site cover a particular amount of geography. Each cell site covers a smaller area, with lower power MHz and thus offers the ability to reuse frequencies more times in a larger geographic coverage area, such as a city or MTA.
- Sectoring
- A cell is subdivided to a sure number of fields, every one of which “is illuminated” from an antenna directive (or panel), that is an antenna that “does not illuminate” in all the directions, but concentrates the flow of power within a particular area of the cell, known as sector. Every field can therefore be considered like one new cell. By using directional antennas, the co-channel interference is reduced. A typical structure is the trisector, also known as clover, in which there are 3 sectors, each one served by separate antennas. Every sector has a separate direction of tracking of 120° with respect to the adjacent ones. If not sectorised, the cell will be served by an omnidirectional antenna, which radiates in all directions. Bisectored cells are also implemented with the antennas serving sectors of 180° separation to one another.
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