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)

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

Base Station System (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.


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).

Capacity and Frequency Re-use

It is the number of frequencies in a cell that determines the cell’s capacity.
However, a cellular network can overcome this constraint and maximize the number of subscribers that it can service by using frequency re-use.
Each company with a license to operate a mobile network is allocated a limited number of frequencies.
Depending on the traffic load and the availability of frequencies, a cell may have one or more frequencies allocated to it.
It is important when allocating frequencies that interference is avoided.
Frequency re-use means that two radio channels within the same network can use exactly the same pair of frequencies, provided that there is a sufficient geographical distance.
The tighter frequency re-use plan, the greater the capacity potential of the networkInterference can be caused by a variety of factors.
A common factor is the use of similar frequencies close to each other. The higher interference, the lower call quality.
To provide coverage to all the subscribers, frequencies must be reused many times at different geographical locations in order to provide a network with sufficient capacity.
The same frequencies can not be re-used in neighboring cells as they would interfere with each other so special patterns of frequency usage are determined during the planning of the network.

These frequency re-use patterns ensure that any frequencies being re-used are located at a sufficient distance apart to ensure that there is little interference between them.
The term “frequency re-use distance” is used to describe the distance between two identical frequencies in a re-use pattern.
The lower frequency re-use distance, the more capacity will be available in the network.

Cell Characteristics

The Basic Union In The System
defined as the area where radio coverage is given by one base station.
A cell has one or several frequencies, depending on traffic load.
Frequencies are reused, but not used in neighboring cells due to interference.
Each served by its own antenna
Served by base station consisting of transmitter, receiver, and control unit
Use multiple low-power transmitters
Cell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM)
Pico cells covering building interiors, Micro cells covering selected outdoor areas, and Macro cells for more wider area

CELLS

A cell may be defined as an area of radio coverage from one BTS antenna system.
It is the smallest building block in a mobile network and is the reason why mobile networks are often referred to as cellular networks.
Typically, cells are represented graphically by hexagons.

SOLUTIONS TO TRANSMISSION PROBLEMS

Some solutions to the problems described in previous sections are

1. Channel Coding
2. Interleaving
3. Diversity
   

Although many of these do not entirely solve all problems on the radio transmission path, they do play an important part in maintaining call quality for as long as possible

Channel Coding
In digital transmission, the quality of the transmitted signal is often expressed in terms of how many of the received bits are incorrect.
This is called Bit Error Rate (BER).
BER defines the percentage of the total number of received bits which are incorrectly detected.
This percentage should be as low as possible. It is not possible to reduce the percentage to zero because the transmission path is constantly changing.
This means that there must be an allowance for a certain amount of errors and at the same time an ability to restore the information, or at least detect errors so the incorrect information bits are not interpreted as correct.
Channel coding is used to detect and correct errors in a received bit stream.

Mobile Radio Propagation Effects

Signal strength must be strong enough between base station and mobile unit to maintain signal quality at the receiver must not be so strong as to create too much interference with channels in another cell. Attenuation due to rain presence of raindrops can severely degrade the reliability and performance of communication links trees near subscriber sites can lead to multipath fading Loss of strength of signal due to absorption by trees, plants, parks (greenary) is called vegetation losses