Wave Modulation – analog and digital

This article touches upon wave modulation techniques used in physical layer that allows one or multiple low bandwidth signals to use or share a high frequency bandwidth. Note that post only covered the lowest layer of OSI model ie physical layer and not the data link layer which actually provides access technologies liek FDMA , TDMA , CDMA etc.

Amalog Modulation

Modulating a wave means changing one or more of its fundamental characteristics to encode information.

The general function for a sinusoid is

f(t) = A \sin(\omega t + \phi)

or

f(t) = A sin(omega t + phi)

we can see that this has 3 parameters that can be altered .

  • A – called the magnitude, or amplitude of the sinusoid.
  • omega – known as the frequency
  • phi – known as the phase angle.

Therefore there are three basic ways to modulate a carrier wave ( which is the modulated unaltered wave carrying some information) :

1. Amplitude Modulation

The amplitude of the carrier signal is modulated (changed) in proportion to the message signal while the frequency and phase are kept constant.

Amfm3-en-de

2. Frequency Modulation

The frequency of the carrier signal is modulated (changed) in proportion to the message signal while the amplitude and phase are kept constant.

Amfm3-en-de (1)

3. Phase Modulation

This encodes information as variations in the instantaneous phase of a carrier wave.It changes the phase angle of wave in direct proportion to the message signal.

fig_2010_07_08

The above discussed techniques are only applicable for analog modulation . For digital modulation got the next blog : Wave Modulation – digital.

Need for Modulation

1. Transmitting medium is acts like a bandpass filter

The transmitting channel ( air / wire o/ fibre ) is like a bandpass filter ie both the lowest frequency components and the highest frequency components are cut off beyond a range with transmission only being practical over some intermediate frequency range . In such a case our orignal wave is unusuable unless we shift our frequency up wthout changing the information contained . Thus modulation is the only solution for transmission.

2. Limits the size of Antenna from huge to small

A baseband (low frequency) signal has a long wavelength thus an anteena of the size of about 1/10 of wavelength is required to listen to it .This results in really large antenna sizes. However since modulation shifts the baseband signal up to a much higher frequency, which has much smaller wavelengths it allows the use of a much smaller antenna.


Digital Modulation

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 Wave modulationDigital Wave Modulation
continuous signal with intensity varying over time.discrete signal, switching between two different states over time.

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.

fig_2010_07_09

FSK (frequency-shift keying):

a finite number of frequencies are used.

fig_2010_07_05

ASK (amplitude-shift keying):

a finite number of amplitudes are used.

fig_2010_07_02

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.

Constellation Diagram

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.

modulation-constellation-bpsk
modulation-constellation-16qam
modulation-constellation-32qam
modulation-constellation-64qam

                

Basic Diagram of QAM Transmitter and sender

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Sources :


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