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.
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)
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.
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.
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.
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.
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 modulation||Digital 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.
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
- Modulation : http://en.wikipedia.org/wiki/Modulation
- Amplitude Modulation : http://en.wikipedia.org/wiki/Amplitude_modulation
- Frequency Modulation : http://en.wikipedia.org/wiki/Frequency_modulation
- Phase Modulation : http://ironbark.xtelco.com.au/subjects/DC/lectures/7/