December 2021

In TDMA, the time slots are available to the ground stations with full bandwidth. In FDMA, the frequency bands are available to the ground stations with continuous time availability. By combining these two multiple access techniques, a hybrid technique called Code Division Multiple Access (CDMA) can be formed. Frequency hopping can be used so that

The uplink and downlink carriers of the satellite are required to be modulated by the signal to be transmitted. The modulation can be analog or digital. In the earlier satellites analog modulations were dominant and therefore they were used in communication satellites. The analog voice and video transmission systems use frequency. modulation and frequency division

The communications satellites are used to carry telephone, television and data signals. The television and telephone signals are normally analog. The satellite earth station transmits these signals on the same link by multiplexing. Time division multiplexing (TDM) and frequency division (FDM) are the commonly used multiplexing techniques. FDM/FM for Telephone Systems Fig shows the block

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