EE 551451, Fall, 2006 Communication Systems

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EE 551/451, Fall, 2006Communication Systems

• Zhu Han
• Department of Electrical and Computer Engineering
• Class 23
• Nov. 9th, 2006

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Outline

• Digital Carrier Systems
• Carrier band vs. baseband
• Baud rate, bit rate, bandwidth efficiency
• Spectrum
• Gray coding
• Coherent, noncoherent receiver
• BER
• Comparison
• Practical implementation example

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Digital Carrier System Baseband analysis
Signal in baseband mean symbol energy signal
in carrier band mean symbol
energy Conclusion analysis of carrier band
base band. Fc0 in project
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Baud Rate, Bit Rate, Bandwidth Efficiency

• Remember channel capacity CWlog2 (1 SNR)gt fb

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Power Spectrum, ASK

• Baseband
• Sy(W)Sx(W) P(W)
• ASK Sy(t)b Acoswct, Square wave convolute with
  sinusoid.

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FSK Spectrum

• FSK two sinc added together

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BPSK Spectrum

• BPSK Sx(W) NRZ. P(t) raised cosine function.
  Sy(W) P(W)
• Rb
• baud rate

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QPSK Spectrum

• Same Rb
• Narrow BW

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Pulse Shaped M-PSK

• Different ?

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Bandwidth vs. Power Efficiency

• Bandwidth efficiency high, required SNR is high
  and low power efficiency

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QAM efficiencies

• For l 1 ? PSD for BPSK
• For l 2 ? PSD for QPSK, OQPSK
• PSD for complex envelope of the bandpass
  multilevel signal is same as the PSD of baseband
  multilevel signals
• Same baud rate, higher bit rate.
• Same bit rate, less bandwidth. But higher power

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Minimum Shift Keying spectra

• Continuous phase and constant envelop. So narrow
  spectrum

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GMSK spectral shaping
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Gray coding

• It is very unlikely that switches will change
  states exactly in synchrony. So there might be
  misunderstanding. E.g. 011-gt100
• In a digital modulation scheme such as QAM where
  data is typically transmitted in symbols of 4
  bits or more, the signal's constellation diagram
  is arranged so that the bit patterns conveyed by
  adjacent constellation points differ by only one
  bit. By combining this with forward error
  correction capable of correcting single-bit
  errors,
• it is possible for a receiver to correct
• any transmission errors that cause a
• constellation point to deviate into the
• area of an adjacent point. This makes
• the transmission system less susceptible
• to noise.
• Graduate student for 16-QAM

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Coherent Reception

• An estimate of the channel phase and attenuation
  is recovered. It is then possible to reproduce
  the transmitted signal, and demodulate. It is
  necessary to have an accurate version of the
  carrier, otherwise errors are introduced. Carrier
  recovery methods include

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Coherent BER

• PSK
• BPSK QPSK
• MPSK

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Coherent BER performance

• ASK
• FSK
• MSK less bandwidth but the same BER
• MQAM

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Non-coherent detection

• Non-coherent detection
• does not require carrier phase recovery (uses
  differentially encoded mod. or energy detectors)
  and hence, has less complexity at the price of
  higher error rate.
• No need in a reference in phase with the received
  carrier
• Differentially coherent detection
• Differential PSK (DPSK)
• The information bits and previous symbol,
  determine the phase of the current symbol.
• Energy detection
• Non-coherent detection for orthogonal signals
  (e.g. M-FSK)
• Carrier-phase offset causes partial correlation
  between I and Q braches for each candidate
  signal.
• The received energy corresponding to each
  candidate signal is used for detection.

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Differential Reception
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Differential Coherent

• DBPSK
• 3dB loss

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Non-coherent detection of BFSK
Decision stage

-
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Non-coherent detection BER

• Non-coherent detection of BFSK
• Similarly, non-coherent detection of DBPSK

Rayleigh pdf
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BER Example
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Example of samples of matched filter output for
some bandpass modulation schemes
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Comparison of Digital Modulation
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Comparison of Digital Modulation
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Spectral Efficiencies in practical radios

• GSM- Digital Cellular
• Data Rate 270kb/s, bandwidth 200kHz
• Bandwidth Efficiency 270/200 1.35bits/sec/Hz
• Modulation Gaussian Minimum Shift Keying (FSK
  with orthogonal frequencies).
• Gaussian refers to filter response.
• IS-54 North American Digital Cellular
• Data Rate 48kb/s, bandwidth 30kHz
• Bandwidth Efficiency 48/30 1.6bits/sec/Hz
• Modulation pi/4 DPSK

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Modulation Summary

• Phase Shift Keying is often used, as it provides
  a highly bandwidth efficient modulation scheme.
• QPSK, modulation is very robust, but requires
  some form of linear amplification. OQPSK and
  p/4-QPSK can be implemented, and reduce the
  envelope variations of the signal.
• High level M-ary schemes (such as 64-QAM) are
  very bandwidth efficient, but more susceptible to
  noise and require linear amplification.
• Constant envelope schemes (such as GMSK) can be
  employed since an efficient, non-linear amplifier
  can be used.
• Coherent reception provides better performance
  than differential, but requires a more complex
  receiver.

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Homework, Project

• Due 11/16/06
• 7.7.5
• 7.8.2
• 13.2.1
• 13.5.1

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Questions?