Modulation Basics
• Modulation means that, instead of transmitting the
message signal directly, we alter it in such a way that
it is more suitable to be transmitted over our channel.
• This needs to be done in such a way that the original
signal can be recovered at the receiver.
• The modulated signal will have a different frequency
than the original signal.
04-1
Amplitude Modulation
• Amplitude modulation (AM)
– the amplitude, but not the frequency, of the carrier signal
is varied according to the message signal
• Message signal can be recovered at the receiver
from the amplitude information of the received signal
04-2
AM signal example
Message signal
0
20
Modulated signal
-1.5
04-3
Binary Digital Modulation
• When the message signal is a zero, the transmitted
signal is s0(t) for 0≤t ≤Tb
• When the message signal is a one, the transmitted
signal is s1(t) for 0≤t ≤Tb
– where Tb is the bit period.
• Different modulation schemes differ in the choice of
the signals s0(t) and s1(t)
04-4
Binary On-Off Keying
• Signals are either 0 Volts or +A Volts
• Logical “0” is transmitted as 0 Volts
– signal received as < A/2 volts is interpreted as “0”
• Logical “1” is transmitted as +A Volts
– signal received as > A/2 volts is interpreted as “1”
A
Bit
Period
Tb
04-5
Binary Frequency-shift keying FSK
• Signals are either at frequency f0 or f1
– nominal carrier frequency is mid-way between these two
values
• Logical “0” is transmitted as signal at frequency f0
• Logical “1” is transmitted as signal at frequency f1
04-6
FSK Example
04-7
Minimum Shift keying MSK
• FSK with the minimum frequency separation between
signals (i.e. 1/(2Tb))
04-8
Binary Phase Shift Keying BPSK
• In BPSK, the signals are transmitted at the same
frequency, but different phase:
04-9
BPSK Example
04-10
Quadrature Phase Shift Keying QPSK
• In QPSK, pairs of bits are coded jointly – each pair
00, 01, 11, 10 forms a symbol, which is transmitted
during a symbol period .
04-11