Basic Cellular Communications System
Basic Cellular Communications System
Cellular systems are widely used today to offer very efficient use of the available frequency spectrum. With billions of mobile phones in use around the globe today, it is necessary to re-use the available frequencies many times over without mutual interference of one cell phone to another, here frequency re-use that is at the very heart of cellular technology. Early radio telephones schemes used a single central transmitter to cover a wide area. These radio telephone systems suffered from the limited number of channels that were available.
Each channel needs to have a bandwidth of around 25 kHz to enable sufficient audio quality to be carried as well as enabling there to be a guard band between adjacent signals to ensure there are no undue levels of interference. Using this concept it is only possible to accommodate 40 users in a frequency band 1 MHz wide. Even of 100 MHz were allocated to the system this would only enable 4000 users to have access to the system. Today cellular systems have millions of subscribers and therefore a far more efficient method of using the available spectrum is needed.
The cell phone comprises analogue electronics as well as digital circuits ranging from processors to display and keypad electronics. A mobile phone typically consists of a single board, but within this there are a number of distinct functional areas, but designed to integrate to become a complete mobile phone:
Radio frequency - receiver and transmitter
Digital signal processing
Analogue / digital conversion
Control processor
SIM or USIM card
Power control and battery
Types of radio propagation
Free space propagation: Here the radio waves travel in free space, or away from other objects which influence the way in which they travel. It is only the distance from the source which affects the way in which the signal strength reduces. This type of radio propagation is encountered with radio communications systems including satellites where the signals travel up to the satellite from the ground and back down again. Typically there is little influence from elements such as the atmosphere, etc. . . . .
Ground wave propagation: When signals travel via the ground wave they are modified by the ground or terrain over which they travel. They also tend to follow the Earth's curvature. Signals heard on the medium wave band during the day use this form of RF propagation.
Ionospheric propagation: Here the radio signals are modified and influenced by a region high in the earth's atmosphere known as the ionosphere. This form of radio propagation is used by radio communications systems that transmit on the HF or short wave bands. Using this form of propagation, stations may be heard from the other side of the globe dependent upon many factors including the radio frequencies used, the time of day, and a variety of other factors. . . . .
Tropospheric propagation: Here the signals are influenced by the variations of refractive index in the troposphere just above the earth's surface. Tropospheric radio propagation is often the means by which signals at VHF and above are heard over extended distances.