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Sec. 5–10 Multilevel Modulated Bandpass Signaling 367
Low-pass
filter
FSK in
FM
detector
Binary
output
FSK in
cos( 1 t)
Binary
output
Low-pass
filter
cos( 2 t)
(a) Noncoherent Detection
(b) Coherent (Synchronous) Detection
Figure 5–28
Detection of FSK.
Binary
input
R bits/sec
Digital-toanalog
converter
bits
M=2 -level
digital signal
symbols R
D ––––––=––
sec
Transmitter
Modulated
output
Figure 5–29
Multilevel digital transmission system.
For example, suppose that an = 2-bit DAC is used. Then the number of levels in the
multilevel signal is M = 2 = 2 2 = 4, as illustrated in Fig. 3–14a for rectangular pulses.
The symbol rate (baud) of the multilevel signal is D = R>/ = 1 2 R, where the bit rate is
R = 1T b bits/s.
Quadrature Phase-Shift Keying and M-ary
Phase-Shift Keying
If the transmitter is a PM transmitter with an M = 4-level digital modulation signal, M-ary
phase-shift keying (MPSK) is generated at the transmitter output. Assuming rectangularshaped
data pulses, a plot of the permitted values of the complex envelope, g(t) = A c e ju(t) ,
would contain four points, one value of g (a complex number in general) for each of the four
multilevel values, corresponding to the four phases that u is permitted to have. A plot of two
possible sets of g(t) is shown in Fig. 5–30. For instance, suppose that the permitted multilevel
values at the DAC are -3, -1, +1, and +3 V; then, in Fig. 5–30a, these multilevel values
might correspond to PSK phases of 0, 90, 180, and 270, respectively. In Fig. 5–30b,
those levels would correspond to carrier phases of 45, 135, 225, and 315, respectively.