Basic
In telecommunication, for information transfer, idea is to use high frequency signal and then modify the characteristics of this signal to communicate the information. In other words, what we do is superimpose (low frequency) information signal onto (high frequency) carrier signal. At receiving end, this process is reversed.
Have a look at example diagram of "Analog modulation" below.
Top one is a information signal. In 2nd waveform, amplitude carrier signal is modified to reflect the value of information signal. In 3rd, frequency of carrier frequency is modified to so the same.
In similar way, we can modify the phase of carrier signal based on information signal. These techniques are known as amplitude modulation, frequency modulation, and phase modulation respectively.
As you may infer, the frequency difference between information signal and carrier signal need to be "sufficient" for receiver to "decode" the signals properly.
Digital modulation
In digital modulation, the input/information signal is in ones and zeros. In a way, digital modulation is "digital-to-analog" conversion !
There are numerous techniques:
Phase Shift Keying
Phase is given a value; in case of BPSK, two values (0 and 1) are separated by 180° phase; for QPSK, it would be 4 values (00, 01, 10, and 11) by 90° phase.
QPSK can be viewed as two BPSK signals ! One way to implement QPSK is split the actual data stream in even and odd bits. Then odd bits are given to modulate sine wave (like BPSK) and even bits to cosine wave. Resulting wave form is QPSK.
Below is an example of QPSK with information as "11".
In case of PSK, synchronisation is mandatory as absolute phases are given values. In DPSK (Differential PSK) rather than phase, phase "change" is used for indicating values.
Other variants of PSK are 8PSK, 16PSK, Offset PSK etc.
Frequency Shift Keying
FSK uses frequency change to indicate information. Due to the nature of FSK, it is still used in amateur radio. Another popular example is DTMF - used in today's keypad landline phones.
© Copyright Samir Amberkar 2010
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