1. Digital modulation techniques

2. Modulations systems

3. Digital modulation techniques

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

5. FSK (Frequency shift keyring)

6. PSK (Phase shift keyring)

7. Modulation Techniques
Amplitude Shift Keying (ASK)
Binary Frequency Shift Keying (BFSK)
Binary Phase Shift Keying (BPSK)

8. Amplitude Shift Keying (ASK)
In ASK, the two binary values are represented by to different amplitudes of the carrier frequency
The resulting modulated signal for one bit time is
Susceptible to noise
Inefficient modulation technique
used for
up to 1200bps on voice grade lines
very high speeds over optical fiber

9. Binary Frequency Shift Keying (BFSK)
The most common form of FSK is Binary FSK (BFSK)
Two binary values represented by two different frequencies ( f1 and f2 )
less susceptible to noise than ASK
used for
up to 1200bps on voice grade lines
high frequency radio (3 to 30MHz)
even higher frequency on LANs using coaxial cable 1 f2 f1

10. Full-Duplex BFSK Transmission on a Voice-Grade line
Voice grade lines will pass voice frequencies in the range 300 to 3400Hz
Full duplex means that signals are transmitted in both directions at the same time f1
f 3 f4 f2

11. Multiple FSK (MFSK) More than two frequencies (M frequencies) are used
More bandwidth efficient compared to BFSK
More susceptible to noise compared to BFSK
MFSK signal:

12. Multiple FSK (MFSK) MFSK signal: Period of signal element
Minimum frequency separation
MFSK signal bandwidth:

13. Phase Shift Keying (PSK)
Phase of carrier signal is shifted to represent data
Binary PSK (BPSK): two phases represent two binary digits 1 π

14. Differential PSK (DPSK)
In DPSK, the phase shift is with reference to the previous bit transmitted rather than to some constant reference signal
Binary 0:signal burst with the same phase as the previous one
Binary 1:signal burst of opposite phase to the preceding one

15. Four-level PSK: Quadrature PSK (QPSK)
More efficient use of bandwidth if each signal element represents more than one bit
eg. shifts of /2 (90o)
each signal element represents two bits
split input data stream in two & modulate onto the phase of the carrier
can use 8 phase angles & more than one amplitude
9600bps modem uses 12 phase angles, four of which have two amplitudes: this gives a total of 16 different signal elements

16. QPSK and Offset QPSK (OQPSK) Modulators

17. Example of QPSK and OQPSK Waveforms

18. Performance of ASK, FSK, MFSK, PSK and MPSK
Bandwidth Efficiency
ASK/PSK:
MPSK:
MFSK:
Bit Error Rate (BER)
bit error rate of PSK and QPSK are about 3dB superior to ASK and FSK (see Fig. 5.4)
for MFSK & MPSK have tradeoff between bandwidth efficiency and error performance

19. Performance of MFSK and MPSK
MFSK: increasing M decreases BER and decreases bandwidth Efficiency
MPSK: Increasing M increases BER and increases bandwidth efficiency

20. Quadrature Amplitude Modulation (QAM)
QAM used on asymmetric digital subscriber line (ADSL) and some wireless standards
combination of ASK and PSK
logical extension of QPSK
send two different signals simultaneously on same carrier frequency
use two copies of carrier, one shifted by 90°
each carrier is ASK modulated

21. QAM modulator

22. Two level ASK (two different amplitude levels)
QAM Variants
Two level ASK (two different amplitude levels)
each of two streams in one of two states
four state system
essentially QPSK
Four level ASK (four different amplitude levels)
combined stream in one of 16 states
Have 64 and 256 state systems
Improved data rate for given bandwidth
but increased potential error rate