In digital modulation, an analog carrier signal is modulated by a discrete signal.

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Digital Modulation
Technique
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2. What is digital Modulation Technique?
• In digital modulation, an analog carrier sign
al is modulated by a discrete signal.
• Digital modulation methods can be consider
ed as digital-to-analog conversion, and
• the corresponding demodulation or detection
as analog-to-digital conversion.

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II. Digital Modulation
• Better performance and more cost effective than analog modu
lation methods (AM, FM, etc.)
• Used in modern cellular systems
• Advancements in VLSI, DSP, etc. have made digital
solutions practical and affordable

4. 4
Digital Modulation
• Performance advantages:
1) Resistant to noise, fading, & interference
2) Can combine multiple information types (voice, data, & vide
o) in a single transmission channel
3) Improved security (e.g., encryption) → deters phone cloning
+ eavesdropping
4) Error coding is used to detect/correct transmission errors
5) Signal conditioning can be used to combat hostile MRC envi
ronment
6) Can implement mod/dem functions using DSP software (inst
ead of hardware circuits).

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Digital Modulation
• Choice of digital modulation scheme
– Many types of digital modulation methods →
subtle differences
– Performance factors to consider
1) low Bit Error Rate (BER) at low S/N
2) resistance to interference (ACI & CCI) & multipa
th fading
3) occupying a minimum amount of BW
4) easy and cheap to implement in mobile unit
5) efficient use of battery power in mobile unit

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Digital Modulation
– No existing modulation scheme simultane
ously satisfies all of these requirements
well.
– Each one is better in some areas with
tradeoffs of being worse in others.

7. Digital modulation techniques

8. ASK
• ASK On-off keying (Amplitude Shift Keying) – frequenc
y is kept constant, amplitude has 2 levels (for bit 1 and
for bit 0)
The binary sequence 0010110010

10. FSK (Frequency shift keyring)

12. PSK (Phase shift keyring)

13. PSK (Phase shift keying)

16. What is Quadrature Phase Shift Keying (QPSK)?
• Quadrature Phase Shift Keying (QPSK) is a form of
Phase Shift Keying in which
two bits are modulated at once, selecting one of four possible ca
rrier phase shifts (0, 90, 180, or 270 degrees).
QPSK allows the signal to carry twice as much information as or
dinary PSK using the same bandwidth.

17. 1999 BG Mobasseri 17
QPSK
Quadrature phase shift keying
• This is a 4 level modulation.
• Every two bits is combined and mapped to
one of 4 phases of an RF signal
• These phases are 45o,135o,225o,315o
Symbol energy

18. QPSK -Quadrature Phase Shift Keying
• In Quadrature Phase Shift Keying (QPSK) two sinusoids (sin and cos) are taken as
basis functions for modulation.
• Modulation is achieved by varying the phase of the basis functions
depending on the message symbols.
• n QPSK, modulation is symbol based,where one symbol contains 2 bits. The follow
ing equation outlines QPSK modulation technique.
si(t)=2EsT√cos(2πfct+(2n−1)π4),n=1,2,3,4
where n=1, the phase shift is 45 degrees.
This is called pi/4 QPSK