MODULATION THEORY 91
I-Channel
Q-Channel
Gaussian
LPF
(BTb) 2E
2E d
I-Channel
Q-Channel
dmin
dmin = K E d = 2 E
FIGURE 2.69 GMSK filtering impairment.
MSK Eye Diagram
??“1.5
??“1
??“0.5
0
0.5
1
1.5
0 4 8 12 16
Normalized Symbol Time (Ts/16)
Eye Value
FIGURE 2.70 MSK eye diagram (BTb infinity).
The corresponding MSK signal constellation is drawn in Fig. 2.71. Notice this modulation scheme
is a constant envelope waveform.
When the BTb product is reduced to 0.5, notice some ISI is present in the eye diagram when a
Gaussian filter GMSK is used (see Fig. 2.72).
A point worth mentioning is as the BTb product decreases, more filtering is applied to the information
signal. This additional filtering essentially slows down the phase trajectories. This in turn
causes an increase in ISI and symbol timing jitter. The GMSK eye diagram in Fig. 2.73 is for a BTb
product of 0.3.
92 CHAPTER TWO
MSK Constellation Diagram
??“1.5
??“1
??“0.5
0
0.5
1
1.5
??“1.5 ??“1 ??“0.5 0 0.5 1 1.5
I-Channel
Q-Channel
FIGURE 2.71 MSK signal constellation.
GMSK BT = 0.5 Eye Diagram
??“1.5
??“1
??“0.5
0
0.5
1
1.5
0 4 8 12 16
Normalized Symbol (Ts/16)
Eye Value
FIGURE 2.72 GMSK eye diagram with BTb 0.
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