Digital Modulation Digitization

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Digital ModulationDigitization
MCS.04
Maj JW Paul
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All the worlds an analog stage, and digital
circuits only play bit parts
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Review
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Review

• Why do we use Fourier Series
• What is the difference/advantages of AM/FM DSB-SC
  SSB
• FM Modulation index - why is there no AM
  modulation index?
• How does FM represent Amplitude?

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Review

• Filter Plot

• No ringing, but allows in other frequencies

f

• Ringing any sharp edge will cause significant
  harmonics

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Todays Class

• Digital Modulation
• Digitization

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Digital Modulation
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Definition

• Modulation
• process by which some characteristics of a
  carrier (sinusoidal wave) is varied in accordance
  with a modulating wave (data or analog message)
• can change amplitude, phase or frequency of
  carrier or combination according to the message

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Digitization

• How do we represent this

• In analog data?

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OOK or ASK

• On-Off Keying or Amplitude Shift Keying

• shifting (switching) a carrier sinusoid on and
  off identical to unipolar binary modulation on
  DSB-SC

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FSK

• Frequency Shift Keying

• shifting the frequency of a sinusoidal carrier
  from a high frequency to a low frequency
  equivalent to FM signaling with
  a binary digital signal

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PSK

• Phase Shift Keying

• shifting a sinusoidal carrier 0o or 180o
  
  equivalent to PM signaling with a digital message
  and also to modulating a DSB-SC signal with a
  polar digital waveform

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QPSK QAM M-PSK

• Quadrature Phase Shift Keying
• Instead of shifting phase by 0o or 180o
  shift by 45o, 135o, 225o, 315o digital cell
  phones
• Quadrature Amplitude Modulation
• Shift by phase and amplitude digital microwave
  links
• M- Phase Shift Keying
• Shift by 4, 16 different phases

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Phase Plot
M
?
I Axis
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Q Axis
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Why

• One phase shift represents two bits

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11
00
01

• Others can represent 4, 16 bits
• 0000 1111 1001 1010 1100 0101 0011 0110
• 0001 0010 0100 1000 0111 1011 1101 1110
• Used in blue-tooth, low power WiFi

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Others

• MSK Minimum Shift Keying
• Minimum spacing that allows two frequencies
  states to be orthogonal
• spectrally efficient, easily generated
• GMSK Gaussian Minimum Shift Keying
• MSK premodulation Gaussian low pass filter
• increases spectral efficiency
• excellent power efficiency due to constant
  envelope
• No sudden change in phase/frequency
• Used extensively in second generation digital
  cellular and cordless telephone applications

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Real Signals

• Real Data

• Easier to recover signal ? Noise resistance

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Digitization

• How do we represent this

• In digital data?

• Sampling error
• Quantization error

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Nyquist frequency sampling error

• In order to recover all frequency components of a
  waveform, it is necessary to sample it at twice
  as fast as the highest frequency

See nothing
Related to aliasing
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Quantization Noise
A
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error
2n
10
01
Reduced by increasing n
00
Voice n 8 or 16 Commercial n
32 Telephone n 128 or 256
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Questions?