Digital Modulation

1
Digital Modulation
The discontinuity between analog and digital
modulation is that in analog modulation, there
are theoretically an infinite number of possible
states for the carrier/channel in digital
modulation, there is a finite number of states
(symbols) for the carrier/channel, and we can
design the set of states to optimize decision
accuracy at the receiver.
2
Phase Shift Keying (PSK)
Bit 1
Bit 0
0
Oscillator
BPSK
0
QPSK
180o
1
1
90o
180o
01
0
1
00
10
11
3
Phase Shift Keying (PSK)
0
Oscillator
Bit 1

180o
1
QPSK
90o
0

Bit 0
180o
1
10
00
01
11
4
Differential Binary Phase Shift Keying(DBPSK)
e.g., A zero causes a 180 degree phase change, a
one causes no phase change.
p
0
1
1
1
0
0
0
1
0
0
Mixer
Rx
LPF
Data
Data
To Phase Modulator
delay
delay
5
Carrier Recovery
BP _at_ fIF
BPF _at_ 2 fIF
Loop Filter
BPSK _at_ fIF
X 2
PD
VCO
X 2
PD
LPF
Data
6
Quiz

• A 3 kHz BW analog signal of 5v p-p is quantized
  a resolution of DQ 0.020 volts.
• What is the transmitted bit rate (/- 5)?

7
Differential QPSK
Phase Reference Previous Carrier Phase
10
00
01
11
8
Quadrature Demodulation
Q-bit
Carrier Recovery (x4)
00
10
I-bit
QPSK
01
11
90o
LPF
Q-data
LPF
I-data
9
Bit Clock Recovery
T
RZ Signal
I or Q Data
Narrowband PLL fc 1/T
Recovered Bit Clock
T/2 Delay
10
Other Quadrature Modulations
8-PSK
8-QAM (8 Phase)
16-QAM
8-QAM (4 Phase)