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Eurostar Networks Limited deals with satellite receivers and dish antennas. I was given a briefing about the basic principles involved in satellite communication .The various bandwidths associated with this form of communication is listed below: VHF (very high frequency) Lower band 48.25 to 224.25MHZ ----- band 1 UHF (Ultra high frequency) 421 to 827 MHZ The other frequency bands which are classified according to usage and the types of antennas and receivers are given below: S band C band Q band or KU band S band This band of frequencies is generally used for general-purpose satellite communications. For example, the Doordarshan relays on its signals in this band of frequencies. C band This band of frequencies range from 3.4 to 4.2 GHz Satellite televisions and other forms of communications mostly utilize this band of frequencies. KU band This band of frequencies range from 10.75 to 12.75 GHz Cable TV channels are received in this band .This method is becoming popular nowadays. As we know, the signals that we receive are nothing but modulated signals .The carrier signals are selected such that the carrier frequencies lie between 3.4 to 4.2 GHz. Analog modulations The Amplitude Modulation The amplitude of the carrier is modulated in accordance with the message. The frequency modulation The frequency of the carrier is changed in accordance with the instantaneous value of the message. There are many other kinds of modulations like phase modulation, PAM, PTM, PPM, etc Another kind of modulation called pulse code modulation is used, where the signal is sampled and quantized and then modulated. One drawback of analog modulation is that only one message can be modulated using one carrier. Therefore, to transmit more than one signal at a time we need a larger bandwidth. To overcome this we use another technique called digital modulation. Digital modulation First, we sample the message signal and round it off, that are, to quantize it with 28 divisions. For correctness of the signal, we further use another 28 divisions to represent the correct values. One major difference between analog and digital modulations is that in analog we have to transmit the signal in sequence. However, in digital it is not necessary. What we mean here is that in analog modulation the values of the modulated signal as shown in the following diagram is transmitted only in the sequence 1 2 3….. However, in case of digital modulation this sequence is not necessary. So due to its flexibility digital modulation is generally being used for satellite communication purposes. However, digital modulation is not as simple as that of analog modulation. In this technique, we have to process the message signal in various steps. These steps will be clearly explained in the forthcoming text with a detailed diagram. Let us understand digital modulations with a few diagrams. The basic processes involved in the digital modulation are formatting, encoding, encrypting, channel encoding, multiplexing, pulse modulation, band pass modulation, frequencies spreading, multiple access and then transmitting . All these processes are done in the reverse manner to get the original signal back. This is shown in the block diagram given below . The formatting is done so that date of different types namely text data, digital data and analog data are converted in such a form so that they can be easy to the modulated in the pulse modulator. After this formatting a process, called encoding is adopted. After the stage a process called encrypting is done. By this process, we can ensure some security of the signals. This is because we as sign or encrypt the signal with a separate set of codes.
Approximate Word count = 2365
Approximate Pages = 9.5
(250 words per page double spaced)
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