Contents
Modulation
In the similar way as the human speech quickly loses
its strength and becomes inaudible with the distance, so would
become the transmitted signals. Also, the uplink and downlink
signals may interfere with each other. This problem can be
overcome by amplifying the signal but it also amplifies the noise
that is present. The other solution would be to superimpose the
information to be transmitted onto a higher frequency carrier wave.
This process is commonly called as modulating the carrier. The
signal which is being modulated is called the base band signal.
The method used to modulate a signal is called Modulation. In
other words, the base band signal is encoded by the carrier signal.
It must however be noted that the initial modulation of the signal
happens at the transmitting station and the final demodulation of
the downlink signal occurs at the receiving station.
MODULATION TECHNIQUES
There are a variety of modulation techniques developed over the years, broadly they are classified into 6 categories, namely, amplitude, frequency, phase, delta, differential and pulse code. Here, we discuss the 2 most important of the above techniques, i.e. frequency and amplitude modulation.1. Amplitude Modulation :
In this technique, we vary the instantaneous amplitude of the signal by the combination with a high frequency carrier wave. The highest frequency of the modulating data is less than 10% of the carrier frequency. Instantaneous amplitude varies depending on the instantaneous amplitude of the modulating data.
2. Frequency Modulation :
In this technique we vary the instantaneous frequency of the wave.
Until now, frequency modulation methods are extensively used in all communication satellite systems. But this is changing quite rapidly as current satellites are replaced from their analog predecessors by employing digital transmission.
