In fact, as can be
seen from Fig 2.59, choosing 0.715 produces a correlation coefficient of r0.217. This point was
made so the reader has a complete picture of the effects of the chosen on system performance.
The modulation indices of interest, and ones commonly used, are related to satisfying the orthogonality
constraint. In particular, we wish to discuss 0.5 case, sometimes called Fast Frequency Shift
f fc fd (fc fd) 2fd
3
Tb
0
S1(t) # S2(t) dt 0
S2(t) 22P # cos (2pfct 2pfdt)
S1(t) 22P # cos (2pfct 2pfdt u)
MODULATION THEORY 85
BFSK Cross-Correlation Function
??“1.2
??“1
0
0.8
??“0.8
0.6
??“0.6
0.4
??“0.4
0.2
??“0.2
1
1.2
0 0.5 1 1.5 2 2.5 3
Modulation Index, b
Correlation Value
BFSK
BPSK
FIGURE 2.59 FSK correlation function.
Keying (FFSK), because this is the closest the two frequency deviations can be placed while still maintaining
the principle of orthogonality. This is more often called Minimum Shift Keying (MSK) [27].
We will shortly see that MSK has a number of wonderful properties. It can be generated and
detected in a number of ways. Let us first start with the complex envelope baseband approach.
2.2.7 Minimum Shift Keying (MSK)
We have specifically chosen to call attention to the orthogonal BFSK modulation scheme described
above.
Pages:
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158