Wireless Communication Engineering Fall 2004

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Wireless Communication Engineering(Fall 2004)

• Lecture 5
• Professor Mingbo Xiao
• Oct. 28, 2004

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Diagram of Digital Comm. System
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Basic Mixing Process

• In most communications systems the frequency
  content of the baseband data stream does not
  match the frequency transmission property of the
  transmission channel.
• For example a radio channel will have a bandpass
  response, only passing frequency components many
  times higher than those making up the input data
  stream

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Basic Mixing

• To translate our baseband data stream to the
  frequency of our transmission channel we multiply
  or mix the baseband with a carrier frequency
  (wct) ?Carrier Modulation.
• Spectrum of the modulated signal

wc
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Modulation

• The process involves modulating the amplitude,
  frequency and/or phase of a carrier sinewave.
• Carrier is commonly written cos(wct)
• Choice of modulation affects
• ease of implementation
• noise tolerance
• occupied channel bandwidth
• Two well-known analog modulation, AM and FM

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Analog Modulation AM Radio
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Analog Modulation FM Radio

• Noise has a greater effect on amplitude than
  frequency
• Sufficient to detect zero crossings to
  reconstruct the signal
• Easy to eliminate amplitude distortion
• Constant envelope, i.e., envelope of carrier wave
  does not change with changes in modulated signal
• This means that more efficient amplifiers can be
  used, reducing power demands

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Detection of FM Signal

• Noise translates into amplitude changes, and
  sometimes frequency changes
• Detection based on zero crossings the limiter
• Alternative schemes to translate limited signal
  into bit streams

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Digital Modulation

• Serve as interface to the channel.
• Map the binary information sequence into signal
  waveforms.
• Alternate certain property of the carrier.
• Carrier wave has the frequency of the wireless
  channel used for communication.
• Two well-known analog modulation, AM and FM FM
  is more resistant to noise.

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Digital Modulation Techniques
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Digital Demodulation

• Demodulation
• Process of removing the carrier signal
• Detection
• Process of symbol decision
• Coherent detection
• Receiver uses the carrier phase to detect
  signal
• Cross correlate with replica signals at
  receiver
• Match within threshold to make decision
• Noncoherent detection
• Does not exploit phase reference information
• Less complex receiver, but worse performance

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Coherent Versus Noncoherent

• Coherent (aka synchronous) detection process
  received signal with a local carrier of same
  frequency and phase
• Noncoherent (aka envelope) detection requires no
  reference wave
• Coherent examples Phase shift keying (PSK),
  Frequency shift keying (FSK), Amplitude shift
  keying (ASK), Continuous phase modulation (CPM),
  Hybrids
• Noncoherent examples FSK, ASK, Differential PSK
  (DPSK), CPM, Hybrids

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Metrics for Digital Modulation

• Power Efficiency
• Ability of a modulation technique to preserve
  the fidelity of the digital message at low power
  levels
• Designer can increase noise immunity by
  increasing signal power
• Power efficiency is a measure of how much
  signal power should be increased to achieve a
  particular BER for a given modulation scheme
• Signal energy per bit / noise power spectral
  density
• Eb / N0
• Bandwidth Efficiency
• Ability to accomodate data within a limited
  bandwidth
• Tradeoff between data rate and pulse width
• Throuput data rate per hertz R/B bps per Hz

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Other Considerations

• Robust to multipath effects
• Low cost and ease of implementation
• Low carrier-to-cochannel interference ratio
• Low out-of-band radiation
• Constant or near constant envelope
• Constant only phase is modulated
• Non-constant phase and amplitude modulated

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Illustration of Modulation
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Amplitude Shift Keying (ASK)

• Simplest form of bandpass data modulation,
  symbols are represented as discrete amplitudes of
  a fixed frequency carrier oscillator.
• Binary ASK, two symbol states, carrier is simply
  turned on or off. Also known as ONOFF Keying
  (OOK).

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Symmetry in ASK

• Spectrum of an ASK signal can be determined from
  its baseband data stream if the ASK modulation
  process if seen as a multiplication or mixing of
  the baseband symbol stream with the carrier wave.
• Consider a single frequency cos(wmt) from within
  the baseband spectrum and perform the
  mathematical multiplication with the carrier
  cos(wct) ...

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ASK Modulation

• The modulated signal becomes
• cos(wct) . cos(wmt) 0.5 cos (wc - wm)t 0.5
  cos (wc wm)t
• two identical components symmetric about the
  carrier frequency.

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ASK Modulation 2

• If we include all the components in the baseband
  stream, the resulting spectrum is again
  symmetrical about the carrier, a positive and
  reversed image of the sinc spectrum for
  unfiltered binary data.

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Generation of ASK Signals

• Simplest method for binary ASK is to use a switch
  to gate the carrier on and off, driven by the
  data signal.
• But pulse shaping at RF is difficult / expensive

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Baseband Filtering

• Using a mixer based approach, the baseband data
  stream can be prefiltered using a low pass (RRC)
  filter and this pulse shaping will be imposed as
  the envelope variation of the carrier.

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NonCoherent ASK Detection

• With ASK information is conveyed in the amplitude
  or envelope of the modulated carrier.
• Simplest form of envelope detector is a diode
  rectifier / smoothing filter, known as
  non-coherent detection.

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Coherent ASK Detection

• A coherent detector operates by mixing the
  incoming data signal with a locally generated
  carrier reference and selecting the difference
  component from the mixer output.

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Coherent ASK Detection (Contd)

• If the modulated data signal is a(t).cos(wct) and
  the reference carrier cos(wct q) where q is the
  phase error between the source and reference
  carriers, the mixer output becomes
• a(t).cos(wct).cos(wct q)
• 0.5a(t).cos(q) 0.5a(t).cos (2wct q)
• If q 0 (reference carrier phase coherent)
  output is proportional to a(t)
• Coherent detection has better noise performance

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Frequency Shift Keying (FSK)

• Frequency shift keying has been the most widely
  used form of digital modulation until recently.
• simple to generate and detect
• insensitive to amplitude fluctuations on the
  channel
• distinct carrier frequencies represent symbol
  states

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Frequency Shift Keying (Contd)

• Can be viewed as two, separate ASK symbol streams
  summed prior to transmission

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FSK Generation

• switch gt phase jumps
• VCO (Bateman p118)
• Continuous Phase Frequency Shift Keying (CPFSK)

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Filtered FSK

• we can control the spectral shape of CPFSK by a
  pulse shaping filter prior to the modulator as in
  ASK although the mapping is not precise since
  the VCO is nonlinear.
• Commonly use a Gaussian filter as in GSM which
  uses Gaussian Minimum Shift Keying (GMSK)

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Non-coherent FSK Detection

• Simplest approach is to pass the signal through
  two bandpass filters tuned to the two signalling
  frequencies and detect which has the larger
  output averaged over the symbol period. Equates
  to two ASK detectors plus a comparator.
• coherent detection also used, more complex but
  better performance.

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Comparison of Modulation Types

• assuming equal average symbol energy
• coherent gt
• noncoherent.
• phase gt
• frequency gt
• amplitude.

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Phase Shift Keying (PSK)

• With PSK, the information is contained in the
  instantaneous phase of the modulated carrier.
• It is usually imposed and measured with respect
  to a fixed carrier of known phase coherent PSK.
  
• For binary PSK (BPSK), phase states of 0o and
  180o are used.

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Differentially Coherent PSK

• It is also possible to transmit data encoded as
  the phase change between consecutive symbols
  this is known as Differentially Coherent PSK.

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Binary Phase Shift Keying (BPSK)

• Use alternative sine wave phase to encode bits
• Simple to implement, inefficient use of bandwidth
• Very robust, used extensively in satellite
  communications

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Quadrature Phase Shift Keying

• Multilevel modulation technique 2 bits per
  symbol
• More spectrally efficient, more complex
  receiver
• Output waveform is sum of modulated Cosine and
  Sine wave
• 2x bandwidth efficiency of BPSK

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QPSK Symbols
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Minimum Shift Keying

• Special form of (continuous phase) frequency
  shift keying
• Minimum spacing that allows two frequencies
  states to be orthogonal
• Spectrally efficient, easily generated

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Generating Minimum Shift Keying
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Gaussian Minimum ShiftKeying(GMSK)

• MSK premodulation Gaussian low pass filter
• Increases spectral efficiency with sharper
  cutoff, excellent power efficiency due to
  constant envelope
• Used extensively in second generation digital
  cellular and cordless telephone applications
• GSM digital cellular 1.35 bps/Hz
• DECT cordless telephone 0.67 bps/Hz
• RAM Mobile Data

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p/4-Shifted QPSK

• Variation on QPSK
• Restricted carrier phase transition to /-p/4
  and /- 3p/4
• Signaling elements selected in turn from two QPSK
  constellations, each shifted by p/4
• Maximum phase change is 135 vs. 180 for QPSK,
  thus maintaining constant envelope (i.e.,
  amplitude of QPSK signal not constant for short
  interval during 180 phase changes)
• Popular in Second Generation Systems
• North American Digital Cellular (IS-54) 1.62
  bps/Hz
• Japanese Digital Cellular System 1.68 bps/Hz
• European TETRA System 1.44 bps/Hz
• Japanese Personal Handy Phone (PHP)

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p/4-Shifted QPSK (Contd)

• Advantages
• Two bits per symbol, twice as efficient as GMSK
• Phase transitions avoid center of diagram, remove
  some design constraints on amplifier
• Always a phase change between symbols, leading to
  self clocking

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Quadrature AmplitudeModulation (QAM)

• Quadrature Amplitude Modulation (QAM)
• Amplitude modulation on both quadrature carriers
• 2n discrete levels, n 2 same as QPSK
• Extensive use in digital microwave radio links

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Differential PSK

• simple receiver no carrier recovery mechanism
  and still good performance.
• logic 1 gt change of logic state from previous
  coded bit
• logic 0 gt no change of state from the previous
  coded bit

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QUIZzzzzzzzzzzzzzzzzzzzzz

• Why do we also need retransmission mechanism in
  the transport layer, while it is provided in the
  data link layer?
• What are the expected data rates of 3G systems in
  vehicular, pedestrian, and indoor environment?
• Give two examples and explain what is Companding?
• What are the effects of packet size in a packet
  switching network?
• If you cut a watermelon N times in horizon and M
  times in vertical, how many chunks are with skin?