(0 t Tb) g(t) 2 # sin (2pf2t)
f2 fo Rb/4 f1 fo Rb/4 fo
The receiver for this approach is discussed next (see Fig. 2.65). This receiver??™s operating principles
are easily described by the received complex envelope waveforms that are matched filtered with
the pulse shaping used in the transmitter. The outputs are then sampled at the symbol rate, with one
symbol stream offset by Tb from the other. This was done to align the two symbol streams so that the
decision device can produce the information signal m(t).
88 CHAPTER TWO
FIGURE 2.65 Receiver block diagram for the MSK waveform generated by the Offset QPSK
approach.
t
t
t
m(t)
I(t)
Q(t)
+1
??“1
+1
??“1
+1
??“1
Tb
Tb
Tb
t a(t)
+1
??“1
Ts
t b(t ??“ Tb)
+1
??“1
Ts
FIGURE 2.64 Baseband waveforms for MSK generated from the Offset QPSK approach.
m(k) Decision
Device
(2k)Tb
(2k + 1)Tb
BPF
R(t)
Quad
Demod
~ cos(wct)
I(t)
p(t)
p(t)
Q(t)
In the above receiver block diagram, the matched filter has the following impulse response:
(2.41)
We started this section with a block diagram that contained a premodulation filter. A filter commonly
used with MSK is the Gaussian filter GMSK. The Gaussian filter has excellent group delay
characteristics.
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