Monday, January 3, 2011

Frequency Concepts in GSM

      An MS communicates with a BTS by transmitting or receiving radio waves, which consist of electromagnetic energy. The frequency of a radio wave is the number of times that the wave
oscillates per second. Frequency is measured in Hertz (Hz), where 1 Hz indicates one oscillation per second. Radio frequencies are used for many applications in the world today.
Some common uses include :
                           • Television  --------------> 300 MHz approx.
                           •  FM Radio  --------------> 100 MHz approx.
                           •  Police radios  ----------> Country dependent.
                           •  Mobile networks  ------> 300 - 2000 MHz approx

      The frequencies used by mobile networks varies according to the standard being used2. An operator applies for the available frequencies or, as in the United States, the operator bids for frequency bands at an auction.







Wavelength

      There are many different types of electromagnetic waves. These electromagnetic waves can be described by a sinusoidal function, which is characterized by wavelength. Wavelength (λ) is the length of one complete oscillation and is measured in meters (m). Frequency and wavelength are related via the speed of propagation, which for radio waves is the speed of light (3x108 m/s).
     The wavelength of a frequency can be determined by using the following formula:
                                   Wavelength = Speed / Frequency

 Thus, for GSM 900 the wavelength is:
                            Wavelength = ( 3 x 108m/s ) / 900 MHz
                           Wavelength = ( 300,000,000 m/s )  / ( 900,000,000 )
                           Wavelength = 0.33 m (or 33 cm)
     From this formula it can be determined that the higher the frequency, the shorter the wavelength. Lower frequencies, with longer wavelengths, are better suited to transmission over large distances, because they bounce on the surface of the earth and in the atmosphere.
Television and FM radio are examples of applications, which use lower frequencies.


      Higher frequencies, with shorter wavelengths, are better suited to transmission over small distances, because they are sensitive to such problems as obstacles in the line of the transmission path. Higher frequencies are suited to small areas of coverage , where the receiver is relatively close to the transmitter.
      The frequencies used by mobile systems compromise between the large-coverage advantages offered by lower frequencies and the closeness-to-the-receiver advantages offered by use of higher frequencies.

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