Digital modulation BPSK QPSK QAM FSK

1. Digital Modulation Schemes
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2. The idea of modulation
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• Modulation changing a carrier (sine) wave to encode
information.
• Carrier signal is usually of high frequency.
• The information signal modulates the carrier signal.
• It is possible to change either amplitude, frequency or
phase.
• This will result in different digital modulation schemes
Carrier signal:

3. The idea of modulation
Info
signal
Carrier
signal
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4. Digital Modulation Schemes
• Amplitude shift keying (ASK) – change
amplitude of carrier signal
• Frequency shift keying (FSK) - change
frequency of carrier signal
• Phase shift keying (PSK) - change phase of
carrier signal
• Quadrature amplitude modulation (QAM) -
change amplitude of carrier signal
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5. Amplitude Shift Keying (ASK)
• Values represented by different amplitudes of carrier
• Usually, one amplitude is zero
– i.e. presence and absence of carrier is used
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6. Binary amplitude shift keying
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7. Implementation of binary ASK
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8. ASK - Sending Multiple Bits
Simultaneously
• Each of the three modulation techniques can be
refined to send more than one bit at a time.
• It is possible to send two bits on one wave by
defining four different amplitudes.
• This technique could be further refined to send three
bits at the same time by defining 8 different
amplitude levels or four bits by defining 16, etc.
• The same approach can be used for frequency and
phase modulation.
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9. ASK - Sending Multiple Bits
Simultaneously
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10. Phase Shift Keyeing
• Phase of the carrier is varied signal to
represent digital data.
• PSK is much more robust than ASK as it is
not that vulnerable to noise, which changes
amplitude of the signal.
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11. Figure 5.9 Binary phase shift keying
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12. Phase Modulation and PSK
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13. Quadrature PSK (QPSK)
 QPSK uses two separate BPSK modulations: one is in-phase, the
other quadrature (out-of-phase).
 The incoming bits are first passed through a serial-to-parallel
conversion that sends one bit to one modulator and the next bit
to the other modulator.
 The two composite signal created by each multiplier are sine
waves with the same frequency, but different phase.
 When they are added, the result is another sine wave, with one
of four possible phases: 450, -450, 1350 and -1350.
 There are four kinds of signal elements in the output signal
(L=4), so we can send 2 bits per signal (r=2).
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14. Figure 5.11 QPSK and its implementation
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15. Frequency Shift Keying
• Values represented by different frequencies
• Less susceptible to error than ASK
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16. FSK
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17. Multi level FSK
• Similarly to ASK, FSK can use multiple bits
per signal element.
• That means we need to provision for
multiple frequencies, each one to represent
a group of data bits.
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18. Constellation Diagrams
• A constellation diagram helps us to define
the amplitude and phase of a signal.
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19. Three constellation diagrams: ASK, BPSK, QPSK
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20. Constellation Diagrams
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21. Quadrature Amplitude Modulation -
QAM
• In practice, the maximum number of bits that can be
sent with any one of these techniques is about five
bits.
• The solution is to combine modulation techniques.
• One popular technique is quadrature amplitude
modulation (QAM) involves splitting the signal into
different phases and different amplitude.
• Quadrature amplitude modulation, therefore, is a
combination of ASK and PSK.
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22. Constellation Diagrams
(a) QPSK (b) 16-QAM (c) 64-QAM
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23. High order modulation schemes: Data
rate, bandwidth and error rate
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24. High order modulation schemes: Data
rate, bandwidth and error rate
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25. High order modulation schemes: Data
rate, bandwidth and error rate
• The advantage of using QAM is that it is a higher
order form of modulation and as a result it is able
to carry more bits of information per symbol.
• By selecting a higher order format of QAM, the
data rate of a link can be increased.
• High order modulation schemes make more
efficient use of bandwidth.
• The higher the M value, the higher the spectral
efficiency.
• LTE uses 64-QAM
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26. High order modulation schemes: Data
rate, bandwidth and error rate
• While higher order modulation rates are able to
offer much faster data rates and higher levels of
spectral efficiency for the radio communications
system, this comes at a price.
• The higher order modulation schemes are
considerably less resilient to noise and
interference.
• They require higher Eb/No
• As a result of this, many radio communications
systems now use dynamic adaptive modulation
techniques.
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27. High order modulation schemes: Data
rate, bandwidth and error rate
• They sense the channel conditions and adapt
the modulation scheme to obtain the highest
data rate for the given conditions.
• As signal to noise ratios decrease errors will
increase along with re-sends of the data,
thereby slowing throughput.
• By reverting to a lower order modulation
scheme the link can be made more reliable
with fewer data errors and re-sends.
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