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BTS

BSS The Base Station System (BSS) is

responsible for all the radio related functions in the system, such as:

Radio communication with the mobile units

Handover of calls in progress between cells

Management of all radio network resources and cell configuration data.

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BSS – Base Station Subsystem

A BSS consists of the following elements:

• One or more BTSs (base transceiver station)

• One BSC (base station controller)

• One TRAU (transcoding rate and adaptation unit).

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BTS–Base Transceiver Station BTS provides physical connection of an

MS to the network in the form of Air Interface.

On the other side BTS connected to BSC thru Abis-interface.

Cabinet size is reduced substantially from 1991 to current.

Functionality and basic structure largely unchanged.

A BTS cabinet can have up to 16 TRX (GSM recommendation)

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BTS Block Diagram

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Architecture – Transmitter /Receiver Module

• TRX – Most important module of BTS from signal processing point of view

• Consists of a low frequency part for signal processing and a high frequency part for modulation/demodulation.

• Both parts are connected by a separate or integrated Frequency hopping unit.

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BTS - TRX

• All other parts of BTS are associated with TRX and perform auxiliary or admin tasks.

• A TRX with integrated hopping serves the following tasks:

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Function LF HF

Channel coding and decoding #

Interleaving and ordering again #

Encryption and decryption (ciphering) #

Slow frequency hopping #

Burst formatting #

TRAU frame formatting and conversion in direction to/from the BSC, setup of the LAPD connection to the BSC #

Tasks of a TRX With Integrated Frequency Hopping

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TRX Function LF HF

GMSK modulation of all downlink data # #

GMSK demodulation of all received MS # #signals

Creation and transmission of the broadcast # # common control channel (BCCH)on channel 0 of the BCCH-TRX

Measurement of signal strength and quality # # for active connections

Interference measurements # #(idle channel measurements) on free channels and forwarding of the results to the BSC

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Operations and Maintenance Module

O&M module consists of one central unit for administering all other parts of BTS.

Connected directly to BSC with a specially assigned O&M channel.

Ensures commands from BSC/MSc are processed directly into the BTS and report results.

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O&M Module

Typically contains system and operation software for TRX (can be upgraded/reloaded without consulting BSC).

O&M module provides a human-machine interface (HMI), which allows for local control of the BTS.

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Clock Module Part of O&M module – clock generation

and distribution. Reference clock is derived from PCM

signal on the Abis- Interface. BTS Internal clock generation

mandatory. Especially needed during testing in

standalone environment (without connection to BSC or PCM clock unavailable due to link failure).

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Clock Module Cost Saving because of deriving clock

from PCM signal, much cheaper clock generators can be used.

Less maintenance –synchronize with the clock coming from the PCM link.

Clock deviations to be considered especially in call handling (handover situations)

Precision of 0.05 parts per million. All TRX of a BTS to use same clock signal.

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Input and Output Filters I/P and O/P filters are used to limit

bandwidth of received and transmitted signals.

Input filter is non adjustable wideband filter that lets pass all 900(or 1800/1900) frequencies in uplink direction.

In contrast, remote controllable filters or wideband filters are used for downlink direction that limits the bandwidth of output signal to 200khz.

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BTS - Standard configuration

Different BTS configurations, depending on the load, have to be considered to provide optimum radio coverage of an area.

All BTS are assigned a unique Cell Identity CIs

A number of BTS form a Location area [LA]. Figure shows location areas with 1, 3 and 5

BTSs.

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Standard Configuration

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Location Area Identity (LAI) The Location Area Identity (LAI) is a temporary

network identity, which is also required for routing.

The LAI contains the following:

LAC Location Area Code, the maximum length of LAC is 16 bits, enabling 65,536 different location areas to be defined in one PLMN

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Cell Global Identity (CGI) The Cell Global Identity (CGI) is used for

identifying individual cells within a LA. Cell identification is achieved by adding

a Cell Identity (CI) to the LAI components.

The CI has a maximum length of 16 bits. The CGI consists of:

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Umbrella Configuration

One BTS with high transmission power and an antenna installed high above the ground that serves as an “umbrella” for a number of BTSs with low transmission power and small diameters.

Such configuration appears to make no sense at first, because the frequency of umbrella cell cannot be reused in all the cells of that area due to interference.

The umbrella cell configuration still has its merits in certain situations and therefore result in relief from load and an improvement of the network.

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Umbrella Configuration

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Sectorized BTS Configuration in which BTSs antennas

cover only an area of 120 or 180 degrees.

this configuration is used mostly in highly populated areas.

Sectorization provides frequency reuse and reduces reuse distance.

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Frequency Hopping can be of two types

Baseband hopping: this involves hopping between frequencies on different transceivers in a cell

Synthesizer hopping: this involves hopping from frequency to frequency on the same transceiver in a cell

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BTS Functions

Radio resources Signal processing Signaling link management Synchronization Local maintenance handling

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Radio Resources An BTS’s main function is to provide

connection with the MSs over the air interface. This includes the following tasks:

Configuration and system start: site configuration involves loading of software from the BSC and setting parameters prior to system startup, including: Transmitter and receiver frequencies Maximum output power Base Station Identity Code (BSIC)

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Radio Resources contd.., Radio transmission: to transmit several

frequencies using the same antenna. Transmission power is controlled from

the BSC. Radio reception: in addition to

reception of traffic on the physical channels, a primary BTS function the detection of channel requests from MSs (e.g. when a call is being made).

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Signal Processing An BTS is responsible for the

processing of signals before transmission and after reception. This includes:

Ciphering & deciphering using the ciphering key

Channel coding, decoding, interleaving, de-interleaving

Modulation & Demodulation

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BTS Functions contd.., Signaling Link Management An BTS manages the signaling link

between the BSC and MS, applying the appropriate protocols to the information being sent.

Synchronization Timing information is extracted from the

PCM-links from the BSC and is sent to a timing module within the BTS.

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BTS Functions contd.., Local Maintenance Handling An BTS enables operation and

maintenance functions to be carried out locally at the BTS site, without BSC connection.

In this way, field technicians can maintain BTS equipment and software on site.

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