This document discusses various digital modulation techniques including: - Amplitude Shift Keying (ASK) which represents data as changes in signal amplitude. - Frequency Shift Keying (FSK) which represents data as changes in carrier frequency. - Phase Shift Keying (PSK) which represents data as changes in the phase of the carrier signal. - Minimum Shift Keying (MSK) and Gaussian Minimum Shift Keying (GMSK) which are continuous phase modulation schemes used in wireless communications for their spectral efficiency. - Quadrature Amplitude Modulation (QAM) which combines ASK and PSK to send multiple bits per symbol.

1. *

2. We use the term modulation to refer to
changes made in a carrier
-according to the information being sent
Modulation takes two inputs
-a carrier
-and a signal

3. * MSK FSK ASK

5. * Shift Keying
To distinguish between analog and digital
modulation
-We use the term shift keying rather
than modulation

7. *
 ASK is the abbreviation of Amplitude
Shift Keying
 Ask is called On-Off Keying.
 The simplest and most common form
of operate as a switch.

8. *
 It is Abbreviated as Frequency shift keying
 Values represented by different frequencies (near
carrier)
 Less susceptible to error than ASK
 Typically used up to 1200bps on voice grade lines
 High frequency radio
 Even higher frequency on LANs using co-ax

9. *
Amplitude and frequency modulations require at least one cycle
of a carrier wave to send a single bit
unless a special encoding scheme is used
The number of bits sent per unit time can be increased
if the encoding scheme permits multiple bits to be encoded in a
single cycle of the carrier
Data communications systems often use techniques that can
send more bits
PSK changes the phase of the carrier wave abruptly
each such change is called a phase shift
* Phase of carrier signal is shifted to represent data

10. Quadrature amplitude modulation is a
combination of ASK and PSK.
Note

11. *
To send more than one bit at a time
It is possible to send two bits on
one wave by defining four different
amplitudes.
The same approach can be used for
frequency and phase modulation.

13. 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 eight different phases, and two different
amplitude for a total of 16 different possible
values, giving us lg(16) or 4 bits per value.

14. *

15. *

16. *Minimum Shift Keying
MSK is a variant of OQPSK
The rectangular symbol pulse is replaced by a half cycle
sinusoidal symbol pulse.
MSK provides a constant envelope signal which has it’s phase
continuous at all times including the interbit switching
times.
 In MSK, also the phase shifts can be detected with an I or Q
modulator.
 At even numbered symbols, the polarity of I channel conveys
the transmitted data and at odd numbered symbols the polarity
of Q channel conveys the data.
 A phase shift of + 90 degrees represents a data bit equal to 1 and a
phase shift of –90 degrees represents a data bit equal to zero

17. It possesses properties
such as:
constant envelope
spectral efficiency
good BER
performance
self-synchronizing
capability.
MSK PROPERTIES

18. *

19. *
• Gaussian Minimum Shift Keying
• Modulation scheme used in GSM
• Bandwidth-time product
• Describes the amount the symbols overlap
• BT = 0.3 for GSM networks
• Good spectral efficiency
• At the expense of some inter-symbol interference (ISI
• Data rate: 270.8 kbps

20. *
• GMSK is based on MSK
• Minimum Shift Keying
• Linear phase changes
• Spectrally efficient
• At baseband, bit transitions are represented by
½ cycle sinusoid

21. *
 Improved spectral efficiency
 Power Spectral Density
 Reduced main lobe over MSK
 Requires more power to transmit data than
many comparable modulation schemes

22. *
Binary sequence
GMSK modulated Signal
F ( )t
( )z t tI ( ) cos ( )= F
( )z t tQ ( ) sin ( )= F
5
1
0 5 10 15 20
-1
0
1
0 5 10 15 20
-1
0
1
t
0 5 10 15 20
1
-1
0 t
0 5 10 15 20
-5
0 t
0 5 10 15 20
-1
0 t
t
in rd