7 Coherent open loop receiver block diagram for GMSK.
BPF
BPF
Local
( )4
LNA
r(t)
Limiter ?· 4
Delay
I(t)
Q(t)
Quad
Demod
Half-Symbol
Clock
FIGURE 4.8 Coherent receiver block diagram based on quadrupling principles.
180 CHAPTER FOUR
4fc 4fc ??“
So(f)
f
8
Rs
2Ts
3
4fc +
2Ts
3
4fc ??“
2Ts
1
4fc +
2Ts
1
fd1 = ?±
FIGURE 4.9 Ideal 4-level FSK spectrum at the quadrupler output.
The spectrum of this signal can be approximated as follows, when continuing with the above assumption
of the spectral tones (see Fig. 4.9).
Next we generate a local half symbol clock signal to be used with the quadrupler output signal in
the following fashion
(4.28)
(4.29)
Using the above equation for the quadrupler output we arrive with the following:
(4.30)
We BPF this signal to only allow the spectral line at 4x??™s the carrier frequency to pass through and this
signal can be further hard limited to remove the amplitude variations (possibly due to fading). Now in
order for us to use this signal for demodulation, we require a divide by 4 operation to generate a spectral
line at the carrier frequency. This resulting signal is then used in the quadrature demodulation operation.
We have presented three techniques that use the received signal to essentially generate a spectral
component that can be used as a locally generated reference.
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