Digital Communication DSP Algorithms: An Introduction

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Digital Communication DSP Algorithms An
Introduction

• Yu Hen Hu

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Outline

• Digital Modulation
• Linear Modulation QPSK
• Constant Envelope Modulation GMSK
• Combined LM and CEM QAM
• Direct Sequence Spread Spectrum (DS-SS)
• Frequency Hopped Spread Spectrum (FH-SS)

• Channel Equalization
• Linear equalizer
• Decision feed-back equalizer
• Maximum Likelihood Sequence Estimation (MLSE)
  equalizer
• Channel Coding
• Block code Reed-Solomon code
• Convolutional Code Viterbi decoding

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Digital Modulation

• Modulation is the process to translate a
  base-band message signal into a high frequency
  band-pass modulated signal to facilitate
  efficient transmission.
• Analog modulation examples include AM (amplitude
  modulation), FM (frequency modulation).
• In digital communication, the message is a
  sequence of symbols, each representing n-bit
  information.

• Each symbol will be mapped into a modulation
  signal selected from finite set of them (M2n)
• S s1(t), s2(t), , sM(t)
• Types of digital modulation
• Linear modulation
• Constant envelope modulation
• Combined LM and CEM
• Spread spectrum modulation
• Direct sequence
• Frequency hopping

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Linear Modulation QPSK

• Linear Modulation
• s(t) ReA m(t) exp(j?ct)
• For Quadrature Phase Shift Key (QPSK)
• sk(t) A cos?ct k?/2
• K 0, 1, 2, 3
• Source sequence broken into odd and even entries,
  low-pass filtered and then modulated with two
  carriers with 90o phase shift s0, s2 and s1,
  s3

LPF
X
Oscillator
odd
QPSK signal
Serial to Parallel
BPF
?
m(t)
90o
even
LPF
X
Carrier frequency Base-band
QPSK Transmitter
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QPSK (Receiver)

• AMPS, USDC (IS-54) wireless systems use ?/4 DQPSK
  modulation
• Decision circuits
• Channel equalization
• Channel decoding

LPF
X
Decision Circuit
Carrier Recovery Circuit
Symbol Timing Recovery
m(t)
Multiplex
BPF
QPSK signal
90o
Decision Circuit
LPF
X
Carrier frequency Base-band
QPSK Receiver
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Constant Envelope Modulation

• The amplitude of modulated waveform is constant.
• A Gaussian Minimum Shift Key (GMSK) modulation is
  used in GSM (global system for mobile) digital
  cellular system.

• GMSK transmitter consists of a Gaussian LPF and
  an analog FM transmitter!
• GMSK receiver can be realized with simple digital
  circuits and a VCO (voltage controlled
  oscillator).

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LM CEM QAM/OFDM

• QAM Quadrature amplitude modulation
• 0 ? t ? T
• ai,bi are grid points of a square in a 2D
  signal constellation. E.g 16QAM

• OFDM Orthogonal frequency division multiplexing
  
• 0 ? t ? T
• M transmitted signals are equal energy, equal
  duration, off-set by frequency of 1/2T Hz.
• DFT may be applied for demodulation.

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Spread Spectrum Technology

• In conventional modulation methods, the overall
  band-width of modulated signal is minimized so
  that more channels can transmit simultaneously.
• Spread spectrum, on the other hand, spread the
  modulated signal to a wide-band signal.

• Spread spectrum was developed for military
  application for its anti-jamming capability.
• In a multiple access system, spread spectrum
  techniques, in theory, allows more users to share
  the same band-width in the same region due to its
  tolerance to jamming characteristics.

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DS-SS Direct Sequence Spread Spectrum
s(t)
Baseband BPF
m(t)
X
X

• m(t) ?1 over symbol time Ts
• p(t) ?1 over chip time Tc is the pseudo-random
  sequence. p(t) can be generated using a linear
  feedback shift register clocked by the chip
  clock.
• Ts n Tc
• To demodulate, multiply sss(t) by p(t) at
  receiver. Since p2(t) 1, the signal is
  recovered. Note that receiver must know p(t) and
  correct timing information.

PN Code Generator
Oscillator fc
DS-SS Transmitter
r(t)
m(t)
X
X
cos(?ct?)
p(t)
DS-SS Receiver
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FH-SS Frequency Hopping Spread Spectrum
m(t)
s(t)
Modulator
X
FH-SS Transmitter
Oscillator
Frequency Synthesizer
p(t)
m(t)
s(t)
Wideband Filter
Bandpass Filter
X
Demodulator
Frequency Synthesizer
Synchronization System
FH-SS Receiver
p(t)