Satellite Communication

1. Baseband Digital Data Transmission
Digital Transmission system does not require modulation. That is, this system is intended
To be transmitted at baseband; i.e., along a coaxial cable or a twisted pair of wires, which
is what a telephone channel consists of. In the next section we discuss digital transmission
where modulation is involved.
As we saw earlier, the power spectral density out of the encoder has infinite bandwidth.
Due to the infinite bandwidth, it is not possible to transmit this signal over practical channels
such as the airwaves, or a telephone or satellite channel, since the bandwidth of these
systems is limited.

2. In order to design a practical communications system, we must limit the bandwidth of
the transmitted signal. This may be done by inserting a transmit (Tx) filter, with
frequency response denoted by G(f) at the transmitter.

3. The block that follows the TX filter in Fig. 1 is the channel, whose frequency
response is denoted H(f). The channel is the medium over which the digital signal
is transmittede.g. a telephone wire, a fibre optic cable, the airwaves, the coaxial
cable etc.

4. In the case of a telephone wire for example, the frequency response of the channel
rolls off at a rate proportional to 1/f. In most other cases, the frequency response of
the channel is flat, except for the case of the airwaves, when multipath is present.

5. The effect of thermal and other noise sources is to add a noise process to the
receiver input, as shown in Fig. 1. Even though the noise has a very low power
level, it can nevertheless significantly impact the performance of the digital
transmission system. This is because the signal level is comparable to that of the
noise in a typical receiver system.

6. Since the noise is a Gaussian process, there is a finite probability that the noise in
any bit interval is large enough that it could cause a transmitted bit to appear on the
wrong side of the threshold in the decision device. The receiver will then detect the
wrong value for that bit, resulting in a bit error. We wish to minimize the probability
of occurrence of bit errors.

7. The next block in the transmission system of Fig. 1 is the receiver (Rx) filter. This,
like the Tx filter, also has a cutoff of approximately 1/2T
3Hz. The purpose of this filter is to reduce the noise power present at the decision
device of the receiver, thereby reducing the probability of a bit error and improving
performance.

8. After the receiver filter, the signal is sampled at the rate of 1/T samples per second.
The samples are then fed to a decision device, which is essentially a 1-bit
analogue–to–digital convertor. If y(nT) > K, where K is a threshold value, then the
corresponding output bit ˆb(nT) = 1; otherwise ˆb(nT) = 0. The threshold value K is
typically set to zero.

9. THANK YOU