Architecture of Bts

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Base transceiver station

A base transceiver station (BTS) is a piece of equipment that facilitates wireless communicationbetween 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. Thenetwork can be that of any of the wireless communication technologies like GSM, CDMA, WLL,

WAN, WiFi, WiMAX etc. BTS is also referred to as RBS (radio base station), Node B (in 3GNetworks) or simply BS (base station). For discussion of the LTE standard the abbreviation eNB

for enhanced Node B is widely used.

BTS in Mobile Communication

Though the term BTS can be applicable to any of the wireless communication standards, it isgenerally and commonly associated with mobile communication technologies like GSM and

CDMA. In this regard, a BTS forms part of thebase station subsystem (BSS) developments forsystem management. It may also have equipment forencrypting and decrypting communications,spectrum filtering tools (band pass filters) etc. antennas may also be considered as components ofBTS 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 ofthe cell (in the case of sectorised base stations). A BTS is controlled by a parentbase stationcontrollervia the Base station Control Function (BCF). The BCF is implemented as a discreteunit or even incorporated in a TRX in compact base stations. The BCF provides an Operationsand 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 basicstructure and functions of the BTS remains the same regardless of the wireless technologies.

General Architecture

A BTS in general has the following units:

TRX : transceivero Quite widely referred to as DRX (driver receiver)o Basically does transmission and reception of signalso Also does sending and reception of signals to/from higher network entities (like

base station controllerin mobile telephony)

PA : power amplifiero Amplifies the signal from DRX for transmission through antennao May be integrated with DRX

Combiner


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o Combines feeds from several DRXs so that they could be sent out through asingle antenna

o For reduction of number of antenna used Duplexer

o For separating sending and receiving signals to/from antennao Does sending and receiving signals through the same antenna ports (cables to

antenna)

Antennao Antenna is also considered a part of BTS

Alarm extension systemo Collects working status alarms of various units in BTS and extends them to

operations and maintenance (O&M) monitoring stations

Control Functiono Does the control of BTSo Manages the various units of BTSo Has the software for functioning of BTSo On-the-spot configurations, status changes, software upgrades etc. done through

the control function

Baseband receiver unit (BBxx)o Frequency hopping, signal DSP,

Important terms regarding a mobile BTS

Diversity techniques

In order to improve the quality of received signal, often two receiving antennas are used, placedat an equal distance to an uneven multiple of a quarter of wavelength (for 900 MHz thewavelength 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 horizontallyor 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 otherdiversity techniques like frequency/timediversity, antenna pattern diversity, polarization diversity etc.

Splitting

The process of creating more coverage and capacity in a wireless system by having more than onecell 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


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A cell is subdivided to a sure number of fields, every one of which is illuminated from anantenna directive (or panel), that is an antenna that does not illuminate in all the directions, butconcentrates the flow of power within a particular area of the cell, known as sector. Every fieldcan 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 thereare 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 aunidirectional antenna, which radiates in all directions. Bisectored cells are also implementedwith the antennas serving sectors of 180 separation to one another.

Base station subsystemBase station subsystem

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A typical GSM Base Station

The Base Station Subsystem (BSS) is the section of a traditional cellular telephone networkwhich is responsible for handling traffic and signaling between a mobile phone and the networkswitching subsystem. The BSS carries out transcoding of speech channels, allocation of radiochannels to mobile phones,paging, quality management oftransmission and reception over the

air interface and many other tasks related to the radio network.

Contents

[hide]

1 Base Transceiver Stationo 1.1 Sectorisation

2 Base Station Controllero 2.1 Transcoder

3 Packet Control Unit


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4 BSS interfaces 5 See also 6 References

[edit] Base Transceiver Station

Two GSM base station antennas disguised as trees in Dublin, Ireland.Main article:Base Transceiver Station

Thebase transceiver station, or BTS, contains the equipment for transmitting and receiving ofradio signals (transceivers), antennas, and equipment forencrypting and decryptingcommunications with the Base Station Controller (BSC). Typically a BTS for anything other thanapicocell 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 iscontrolled by a parent BSC 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 BCFprovides 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 functions of a BTS vary depending on the cellular technology used and the cellular telephoneprovider. There are vendors in which the BTS is a plain transceiver which receives informationfrom the MS (mobile station) through the Um (air interface) and then converts it to a TDM("PCM") based interface, the Abis interface, and sends it towards the BSC. There are vendors

which build their BTSs so the information is preprocessed, target cell lists are generated and evenintracell handover (HO) can be fully handled. The advantage in this case is less load on theexpensive Abis interface.

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 forEDGE-enabled networks it is GMSK and 8-PSK.

Antenna combiners are implemented to use the same antenna for several TRXs (carriers), the

more TRXs are combined the greater the combiner loss will be. Up to 8:1 combiners are found inmicro and pico cells only.

Frequency hopping is often used to increase overall BTS performance; this involves the rapidswitching of voice traffic between TRXs in a sector. A hopping sequence is followed by theTRXs and handsets using the sector. Several hopping sequences are available, and the sequencein use for a particular cell is continually broadcast by that cell so that it is known to the handsets.


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A TRX transmits and receives according to the GSM standards, which specify eight TDMAtimeslots per radio frequency. A TRX may lose some of this capacity as some information isrequired to bebroadcast to handsets in the area that the BTS serves. This information allows thehandsets to identify the network and gain access to it. This signalling makes use of a channel

known as the BCCH (Broadcast Control Channel).

[edit] Sectorisation

Base Transceiver Station Antenna in Paris

Further information: Sector antenna

By using directional antennas on a base station, each pointing in different directions, it is possibleto sectorise the base station so that several different cells are served from the same location.Typically these directional antennas have a beamwidth of 65 to 85 degrees. This increases thetraffic capacity of the base station (each frequency can carry eight voice channels) whilst notgreatly increasing the interference caused to neighboring cells (in any given direction, only a

small number of frequencies are being broadcast). Typically two antennas are used per sector, atspacing of ten or more wavelengths apart. This allows the operator to overcome the effects offading due to physical phenomena such as multipath reception. Some amplification of thereceived signal as it leaves the antenna is often used to preserve the balance between uplink anddownlink signal.

[edit] Base Station Controller

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 toBTS (except in the case of an inter-BSC handover in which case control is in part theresponsibility of the anchor MSC). A key function of the BSC is to act as a concentratorwhere

many different low capacity connections to BTSs (with relatively low utilisation) become reducedto a smaller number of connections towards the Mobile Switching Center (MSC) (with a highlevel of utilisation). Overall, this means that networks are often structured to have many BSCs

distributed into regions near their BTSs which are then connected to large centralised MSC sites.


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The BSC is undoubtedly the most robust element in the BSS as it is not only a BTS controllerbut, for some vendors, a full switching center, as well as an SS7 node with connections to theMSC and SGSN (when using GPRS). It also provides all the required data to the OperationSupport Subsystem (OSS) as well as to the performance measuring centers.

A BSC is often based on a distributed computing architecture, with redundancy applied to critical

functional units to ensure availability in the event of fault conditions. Redundancy often extendsbeyond the BSC equipment itself and is commonly used in the power supplies and in the

transmission equipment providing the A-ter interface to PCU.

The databases for all the sites, including information such as carrier frequencies, frequencyhopping lists, power reduction levels, receiving levels for cell border calculation, are stored in theBSC. This data is obtained directly from radio planning engineering which involves modelling of

the signal propagation as well as traffic projections.

[edit] Transcoder

The transcoder is responsible fortranscoding the voice channel coding between the coding usedin the mobile network, and the coding used by the world's terrestrial circuit-switched network, the

Public Switched Telephone Network. Specifically, GSM uses a Regular Pulse Excited-LongTerm Prediction (RPE-LPC) coder for voice data between the mobile device and the Base

Station Subsystem, but Pulse Code Modulation (A-law oru-law standardized in ITU G.711)upstream of the BSS. RPE-LPC coding results in a data rate for voice of 13 kbit/s where standardPCM coding results in 64 kbit/s. Because of this change in data ratefor the same voice call, the

transcoder also has a buffering function so that PCM 8-bit words can be recoded to constructGSM 20 ms traffic blocks.

Although Transcoding (compressing/decompressing) functionality is defined as a Base Station

function by the relevant standards, there are several vendors which have implemented the solutionoutside of the Base Station Controller. Some vendors have implemented it in a stand-alone rackusing a proprietary interface. In Siemens' andNokia's architecture, the Transcoder is anidentifiable separate sub-system which will normally be co-located with the MSC. In some ofEricsson's systems it is integrated to the MSC rather than the BSC. The reason for these designs

is that if the compression of voice channels is done at the site of the MSC, the number of fixedtransmission links between the BSS and MSC can be reduced, decreasing network infrastructurecosts.

This subsystem is also referred to as the TRAU(Transcoder and Rate Adaptation Unit). Somenetworks use 32 kbit/s ADPCM on the terrestrial side of the network instead of 64 kbit/s PCMand the TRAUconverts accordingly. When the traffic is not voice but data such as fax or email,

the TRAUenables its Rate Adaptation Unit function to give compatibility between the BSS andMSC data rates.

[edit] Packet Control Unit

The Packet Control Unit (PCU) is a late addition to the GSM standard. It performs some of the

processing tasks of the BSC, but for packet data. The allocation of channels between voice anddata is controlled by the base station, but once a channel is allocated to the PCU, the PCU takes

full control over that channel.


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The PCU can be built into the base station, built into the BSC or even, in some proposedarchitectures, it can be at the SGSN site. In most of the cases, the PCU is a separate nodecommunicating extensively with the BSC on the radio side and the SGSN on the Gb side.

[edit] BSS interfaces

Image of the GSM network, showing the BSS interfaces to the MS, NSS and GPRS CoreNetwork

Um The air interface between the MS (Mobile Station) and the BTS. This interfaceuses LAPDm protocol for signaling, to conduct call control, measurement reporting,Handover, Power control, Authentication, Authorization, Location Update and so on.Traffic and Signaling are sent in bursts of 0.577 ms at intervals of 4.615 ms, to form data

blocks each 20 ms.

Abis The interface between the Base Transceiver Station and Base Station Controller.Generally carried by a DS-1, ES-1, or E1 TDM circuit. Uses TDM subchannels for traffic(TCH), LAPD protocol for BTS supervision and telecom signaling, and carries

synchronization from the BSC to the BTS and MS.

A The interface between the BSC and Mobile Switching Center. It is used for carryingTraffic channels and the BSSAP user part of the SS7 stack. Although there are usually

transcoding units between BSC and MSC, the signaling communication takes placebetween these two ending points and the transcoder unit doesn't touch the SS7

information, only the voice or CS data are transcoded or rate adapted.

Ater The interface between the Base Station Controller and Transcoder. It is aproprietary interface whose name depends on the vendor (for example Ater by Nokia), it

carries the A interface information from the BSC leaving it untouched. Gb Connects the BSS to the Serving GPRS Support Node (SGSN) in the GPRS Core

Network.

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Architecture of Bts