Santhakumar Raja

The orbits of satellite can be rotated around the earth through various paths. These paths are called orbits of the satellite. These orbits are used to cover the specific application areas. Orbits of Satellite When the satellite is moving in the orbits, it stays in position because the centripetal force on the satellite balances the

Fig shows the block diagram of satellite communication system. The satellite consists of transmitters as well as receivers. They are also called as transponders. The satellite receives signals from earth station transmitter, processes them (filtering, amplification) and transmits towards earth station receiver. The terrestrial communication system transmits or receives signals to users. It acts as

Principle : The narrow beam of radio waves is aimed at particular part of space. The same channel is reused over the another narrow beam aimed at another part of the space. This division of space in different directions of base station through highly directional beams is called Space Division Multiple Access (SDMA). As shown

Because of the development in communications and computer fields, the applications involving multiple users are also growing. For example, a cellular telephone network involve large number of mobile subscribers which communicate with each other through their base stations. Other example is satellite networks where transmission between multiple users is through satellites. The multiple users share

In the direct sequence spread spectrum modulation, the pseudo-noise sequence of large bandwidth is multiplied with the narrowband data signal. Therefore the output signal is spreaded over the complete output bandwidth at every instant. The ability to overcome jamming is determined by the processing gain of system. We know that processing Gain = N; length

In the previous subsection we studied the baseband transmission of direct sequence spread spectrum modulated signal. For long distance transmission or transmission over a satellite channel, BPSK can be used. DS-SS BPSK Transmitter Fig. shows the transmitter of Direct Sequence Spread Spectrum with BPSK. The multiplier multiplies the two signals b(t) and c(t). The output

The pseudo-noise (or pseudo random) sequence is a noise like high frequency signal. This signal is binary in nature. Thus it looks like pulses. The sequence is not completely random but it is generated by a well defined logic. The same logic is used at transmitter and receiver. Since the sequence is generated by a

Necessity of Secure Communication There are some applications where it is necessary for the system to resist external interference and to make it difficult for unauthorized receivers to receive the message being transmitted. This type of communication is called secure communication such that noise interference and unwanted receivers should not detect the message. Such communication

A convolutional coding is done by combining the fixed number of input bits. The input bits are stored in the fixed length shift register and they are combined with the help of mod-2 adders. This operation is equivalent to binary convolution and hence it is called convolutional coding. This concept is illustrated with the help