Information can be sent from A to B as an electromagnetic signal, in either an analog or digital form. The difference between the two is that :
Analog is continuous signal with intensity varying over time.
Digital is discrete signal, switching between two different states over time.
We shall cover digital wave modulation here . For reading about Analog Wave Modulation go here Wave Modulation – analog.
Data is typically sent as a packet that contains one or more bytes.
The “time to send” Ts= bits in packet / bits sent per packet .
The propagation delay or Tp = distance in metre / velocity in metre per second .
The most fundamental digital modulation techniques are based on keying:
PSK (phase-shift keying):
a finite number of phases are used.
FSK (frequency-shift keying):
a finite number of frequencies are used.
ASK (amplitude-shift keying):
a finite number of amplitudes are used.
QAM (quadrature amplitude modulation):
A finite number of at least two phases and at least two amplitudes are used.An inphase signal (or I, with one example being a cosine waveform) and a quadrature phase signal (or Q, with an example being a sine wave) are amplitude modulated with a finite number of amplitudes, and then summed. It can be seen as a two-channel system, each channel using ASK. The resulting signal is equivalent to a combination of PSK and ASK.
Representation of a signal modulated by a digital modulation scheme such as quadrature amplitude modulation or phase-shift keying. It displays the signal as a two-dimensional scatter diagram in the complex plane at symbol sampling instants.
Basic Diagram of QAM Transmitter and sender