19-11-2012, 05:03 PM
Modulation for Analog Communication
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Baseband Communications
• Signal strength attenuates with distance. Needs
repeaters to amplify the signals in stages
• Received signal is corrupted by noise
– R(t)=A S(t)+ n(t)
• Received signal quality depends on channel noise and
noise between repeaters accumulate
• To transmit a signal with bandwidth B, we need >=B Hz
in channel bandwidth
• If the signal is low-pass (0-B), must the channel operate
at 0-B range of frequency?
• How do we send multiple signals over the channel?
Why do we need “modulation”?
– A communication channel only operates at a certain frequency
range
• telephone cables, terrestrial (over the air broadcast), ethernet,
optical fiber, etc.
– Modulation translates a signal from its baseband to the operating
range of the channel
– By modulating different signals to different frequency bands,
they can be transmitted simultaneously over the same channel
frequency division multiplexing
Quadrature Amplitude Modulation
• With amplitude modulation: a signal with bandwidth B
needs 2B channel bandwidth
– This is called double sideband (DSB) AM
– Other techniques can reduce the bandwidth requirement
• Single sideband (SSB)
• Vestigial sideband (VSB)
• By using QAM, we can send 2 signals each with
bandwidth B over a channel bandwidth of 2B
– Equivalent to each signal with bandwidth B
Application of Modulation and FDM
• AM Radio (535KHz--1715KHz):
– Each radio station is assigned 10 KHz, to transmit a mono-channel
audio (bandlimited to 5KHz)
– Using Amplitude modulation to shift the baseband signal
• FM Radio (88MHz--108 MHz):
– Each radio station is assigned 200 KHz, to transmit a stereo audio.
– The left and right channels (each limited to 15KHz) are multiplexed into
a single baseband signal using amplitude modulation
– Using frequency modulation to shift the baseband signals
• TV broadcast (VHF: 54-88,174-216MHz, UHF:470-890MHz)
– Each station is assigned 6 MHz
– The three color components and the audio signal are multiplexed into a
single baseband signal
– Using vestigial sideband AM to shift the baseband signals.