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
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
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.
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
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