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

RF Propagation

Basic propagation models
Reflection
Diffraction
Scattering

• Reflection

Reflection occurs when a propagating electromagnetic wave is incident upon an object which has very large dimensions when compared to the wavelength of the propagating wave
Reflection occurs from the surface of the earth and from buildings and walls

• Diffraction

Diffraction occurs when the radio path between the transmitter and receiver is obstructed by a surface that has sharp irregularities (edges)
Building edges, roof tops

• Scattering

Scattering occurs when the medium through which the wave travels consist of objects with dimensions that are small compared to the wavelength .
Rough surfaces , Sign posts
Essentially, the radio waves interact with the physical environment along each of these paths.
There are typically (unless you are in free-space) many paths from the transmitter to the receiver.
Each path is called a multipath.

• Fading

Fading occurs when there is more than one transmission path to the MS or BTS, and therefore more than one signal is arriving at the receiver.
This may be due to buildings or mountains, either close to or far from the receiving device.

• Rayleigh Fading

This occurs when a signal takes more than one path between the MS and BTS antennas.
In this case, the signal is not received on a line of sight path directly from the Tx antenna.
Rather, it is reflected off buildings, for example, and is received from several different indirect paths.

TRANSMISSION PROBLEMS

Many problems may occur during the transmission of a radio signal.
Some of the most common problems are described below.
Path Loss
Shadowing
Fading


PATH LOSS
Path loss occurs when the received signal becomes weaker and weaker due to increasing distance between MS and BTS, even if there are no obstacles between the transmitting (Tx) and receiving (Rx) antenna.
The path loss problem seldom leads to a dropped call because before the problem becomes extreme, a new transmission path is established via another BTS.

Shadowing
Shadowing occurs when there are physical obstacles including hills and buildings between the BTS and the MS.
The obstacles create a shadowing effect which can decrease the received signal strength.
When the MS moves, the signal strength fluctuates depending on the obstacles between the MS and BTS
A signal influenced by fading varies in signal strength.
Drops in strength are called fading dips.

RADIO FREQUENCY

The term Radio Frequency (RF or rf) refers to the electromagnetic field that is generated when an alternating current is input to an antenna. This field, also called an RF field or radio wave
This can be used for wireless broadcasting and communications and many other purposes over a significant portion of the electromagnetic radiation spectrum
In vacuum, all electromagnetic waves travel at the speed of light: c = 3x108 m/sec.