Transmission Techniques

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Transmission Techniques

• Geometric interpretation of modulated signals
• Baseband transmission
• Ultrawideband pulse transmission
• Carrier modulated systems (BPSK, QPSK, Offset
  QPSK and MSK)
• Transmission in bandlimited channels
• Fading channel performance
• Diversity

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General Criteria for Modulation Technique
Selection

• Detection efficiency
• Bandwidth efficiency
• Sensitivity to nonlinearities
• Filtering and ISI
• CCI and ACI performance
• Sensitivity to frequency and phase uncertainties
• Complexity

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Transmission System Classification

• Baseband systems signal transmitted without
  modulating with a carrier.
• Systems with carriers RF bandwidth usually much
  smaller than carrier frequency
• Ultrawideband systems either no carrier but very
  large bandwidth, or with carrier but bandwidth a
  large percentage of carrier freq.

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Baseband systems

• Used in wired systems, or in infra-red (IR)
  systems.
• Employs line coding and pulse coding.

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ULTRA-WIDEBAND SYSTEMS

• Although FCC defined UWB systems as those which
  have bandwidths exceeding 25 of their center
  frequency or 1.5 GHz, whichever is less.
• In industry, an UWB system is, which uses
  impulses that have extremely fast rise and fall
  times in sub-nanosecond range. As a result their
  bandwidths are from near-DC to several GHz. There
  is no carrier frequency in this system.

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UWB PULSE EXAMPLE
1 T. S. Rappaport et al, , Wireless
communications past events and a future
perspective. IEEE Commun. Mag., Vol. 40 Issue 5
Part Anniversary May 2002.
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Modulation Techniques of Interest

• M-ary PSK (for M2,4,8 and perhaps 16)
• M-ary FSK
• Continuous Phase FSK (MSK,GMSK)
• M-QAM
• TCM

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Geometric Representation of Signals
Suppose each waveforms represents 2 bits of
information.
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4 waveforms can be represented as points in 3D
using the following basis functions
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Amplitude Shift Keying (ASK)
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Baseband filtered ASK
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FSK Waveforms
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Frequency Shift Keying (FSK)
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Phase Shift Keying (PSK)
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Baseband filtered PSK
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Amplitude Phase Shift Keying (APK)
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Performance of BPSK
Transmitted signal
Received signal
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BER performance of BPSK
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2,4 and 8-PSK constellations
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SER of coherent M-PSK
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Why M-PSK ? (Mgt4)

• The last figure clearly demonstrates that as M
  becomes larger than 4, there is a power
  efficiency penalty. The question is why do we pay
  this penalty. The answer is in the next figure.

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Bandwidth of M-PSK
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What about QAM in wireless?

• We now know that PSK is the most popular
  modulation for many wireless systems. But M-PSK
  for Mgt8 is not used in practice. Clearly as M
  becomes large, putting the points on a single
  circuit reduces the distance for a given average
  power (or energy) as shown next.

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4 different 8-QAM constellations
a
b
c
d
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Is FSK used in cellular systems?

• We know that FSK is a basic digital modulation
  format.
• Is it frequently used in cellular systems?
• If not, why not?

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Signal separation in FSK
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Spectrum definitions
(a) 3-dB, (b) noise equivalent, (c) null-to-null,
(d) 99 power
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Bandlimiting and ISI

• When a signal is bandlimited in the frequency
  domain, it is usually smeared in the time domain.
  This smearing results in intersymbol interference
  (ISI).
• The only way to avoid ISI is to satisfy the 1st
  Nyquist criterion.
• For an impulse response this means at sampling
  instants having only one nonzero sample.

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Bandwidth requirements

• For PSK or QAM for FSK

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State diagram of QPSK
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Serial to parallel conversion in QPSK
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Phase changes in QPSK
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Envelope variations in QPSK
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State diagram of filtered QPSK(square-root
raised cosine with roll-off 0.5)
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Offset QPSK
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Serial to parallel conversion in OQPSK
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Spectral regrowth in QPSK and OQPSK
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P/4 QPSK
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GMSK Generation
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Gaussian Filter
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Bandwidth as a function of BT
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BER in AWGN and Rayleigh Fading Channels
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BPSK in Rayleigh Fading
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Objective of Diversity

• If diversity is not employed, the resulting
  efficiency would be very low, as it can be
  deduced from the comparison of AWGN vs. Rayleigh
  channel BER.
• Diversity refers to transmitting and/or receiving
  the same information via different (preferably
  independent) ways.
• Diversity combats fading and improves the BER
  performance which
• directly translates to power savings,
• increased system capacity.

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Diversity Techniques

• Space Diversity
• Receive
• Transmit
• Polarization Diversity
• Angle Diversity
• Frequency Diversity
• Path Diversity
• Time Diversity

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Some relevant concepts

• Explicit diversity (redundant transmission)
• Implicit diversity
• Coherence distance
• Coherence time
• Coherence bandwidth

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Diversity Combining Techniques

• Selection Combining
• Equal Gain Combining
• Maximal Ratio Combining

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Selection Combining
Logic
Select
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Equal Gain Combining
Estimate Phase

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Maximal Ratio Combining
Estimate Weights Phase
Phase
Weights
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Performance with Selection Diversity