563489578934

Sec. 5–10 Multilevel Modulated Bandpass Signaling 369
Baseband processing
x(t)
Binary
input
R bits/sec
Digital-to-analog
converter
bits
Multilevel
digital signal
M = 2 level
D = ––
R
symbols/sec
Signal
processing
y(t)
cos( c t)
–
sin( c t)
QAM
signal out
s(t)
(a) Modulator for Generalized Signal Constellation
Binary
input
d(t)
R-bits/sec
2-bit
serial-to-parallel
converter
Baseband processing
d 1 (t)
R/2 bits/sec
d 2 (t)
R/2 bits/sec
(b) Modulator for Rectangular Signal Constellation
–90°
Oscillator
phase
f=f c
shift
g(t)=x(t)+jy(t)
M=2 point constellation
D=R/ symbols/sec
x(t)
cos( c t)
s(t)
–
y(t)
sin( c t)
90°
Oscillator
phase
f=f c shift
/2-bit
digital-to-analog
converter
/2-bit
digital-to-analog
converter
QAM
signal out
Figure 5–31
Generation of QAM signals.
rectangular-shaped data produce a (sin xx) 2 -type power spectrum for the QPSK signal that
has large undesirable spectral sidelobes. (See Fig. 5–33.) These undesirable sidelobes can be
eliminated if the data pulses are filtered to a pulse shape corresponding to a raised cosinerolloff
filter. Unfortunately, this produces AM on the resulting QPSK signal, because the
filtered data waveform cannot change instantaneously from one peak to another when 180
phase transitions occur. Although filtering solves the problem of poor spectral sidelobes, it
creates another one: AM on the QPSK signal. Due to this AM, low-efficiency linear (Class A
or Class B) amplifiers, instead of high-efficiency nonlinear (Class C) amplifiers, are required
for amplifying the QPSK signal without distortion. In portable communication applications,
these amplifiers increase the battery capacity requirements by as much as 50%. A possible