22-09-2016, 09:29 AM
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• Modulation is the process of encoding information from a
message source in a manner suitable for transmission
• In general it involves translating a baseband signal (source
signal) to a modulated signal signal at a higher frequency
Motivations:
– Reduce antenna size: the size of an antenna is proportional to the
signal wavelength. By increasing the carrier frequency, the
wavelength decreases.
– Allow to share the spectrum: see frequency-division multiplexing
Digital vs Analog Modulation
• Analog modulation uses mainly AM (Amplitude) and FM
(Frequency Modulations)
• Modern mobile communication systems use digital
modulation techniques
– e.g. sophisticated and efficient schemes such as Phase
modulation...
• Digital modulation offers many advantages:
– greater noise immunity and robustness to channel impairments
– easier multiplexing
– greater security (encryption)
– more flexibility
Factors that influence the
choice of Digital Modulation (2)
• Bandwidth efficiency:
– Problem: increasing the data rate implies decreasing the pulse
width of the digital symbol, which increases the bandwidth of the
signal.
– Bandwidth efficiency describes how efficiently the allocated bandwidth is used
– Defined as the ratio of the throughput data rate per Hertz (bps/
Hz)
– Fundamental upper bound:
C/B = log2(1+S/N)
where C is the channel capacity (bps), B the bandwidth (Hz) and S/
N the signal-to-noise ratio.