07-12-2012, 02:05 PM
DIGITAL MODULATION AND DEMODULATION TECHNIQUES.
DIGITAL MODULATION.ppt (Size: 1.02 MB / Downloads: 45)
The technique of superimposing the message signal on the carrier is known as modulation
That is, modulation is the process by which a property or parameter of one signal (in this case the carrier) is varied in proportion to the second signal (in this case the message signal)
That is, modulation is the process by which a property or parameter of one signal (in this case the carrier) is varied in proportion to the second signal (in this case the message signal) Modulation is performed at the transmitter, and the reverse operation (demodulation /detection) is performed at the receiving end
Why Modulate?
Coupling EM wave into space - antenna size α wavelength λ
λ = c / f
For speech signal f = 3 kHz λ = 105 m
Antenna size without modulation λ = 105m = 60 miles
Practically unrealizable
Hence, efficient antenna of realistic physical size is needed for radio communication system
Information signal must conform to the limitation of its channel
(channel matching)
Reduce the effect of interference, e.g. Spread Spectrum
Place signals at desired frequency band for signal processing purposes such as filtering, amplification, multiplexing
Used to map digital information sequence into waveforms
Classification of Modulation Techniques
Modulation Techniques can be broadly classified as
Digital versus Analog Modulation
Base band versus Band pass (Pass band) Modulation
Binary versus M-ary Modulation
Memoryless Modulation versus Modulation with memory
Linear versus Nonlinear Modulation
Constant envelope versus Non-constant envelope Modulation
Power efficient versus Bandwidth efficient Modulation
ADVANTAGES OF DIGITAL COMMUNICATION
Can withstand noise since we are interested in 2 values either ‘1’ &’0’not the shape of received signal
Regenerative repeaters along transmission path can detect signal & retransmit noise free signal
Hardware implementation is flexible (use of microprocessor ,digital switching ,LSI & MSI )
Can be coded to yield low error rates
More efficient in relating the SNR & BW
Digital hardware cost is very less
Factors Affecting Choice of Modulation
Signal-to-noise ratio (SNR)
Probability of error or Bit Error Rate (BER)
Power Efficiency, ηp
Power efficiency is a measure of how much received
power is needed to achieve a specified BER(
inversely proportional to BER
As BER increases, ηp decreases since transmitted
power is “wasted” on more bad data
Bandwidth Efficiency (or Spectral Efficiency), ηB
Defined as the ratio of the bit rate to the channel
bandwidth