B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

5.5 Effects of Nonlinear Distortion and Interference 235
Immunity from nonlinearity is the primary reason for the use of angle modulation in
microwave radio relay systems. where power levels are high. This requires highly efficient
nonlinear class C amplifiers. In addition, the constant amplitude of FM gives it a kind of
immunity to rapid fading. The effect of amplitude variations caused by rapid fading can be
eliminated by using automatic gain control and bandpass limiting. These advantages made FM
attractive as the technology behind the first-generation (lG) cellular phone system.
The same advantages of FM also make it attractive for microwave radio relay systems. In
the legacy analog long-haul telephone systems, several channels are multiplexed by means of
SSB signals to form L-carrier signals. The multiplexed signals are frequency-modulated and
transmitted over a microwave radio relay system with many links in tandem. In this application,
however, FM is used not to reduce noise effects but to realize other advantages of constant
amplitude, and, hence, NBFM rather than WBFM is used.
Interference Effect
Angle modulation is also less vulnerable than AM to small-signal interference from adjacent
channels.
Let us consider the simple case of the interference of an unmodulated carrier A cos W e t
with another sinusoid / cos (w e + w )t. The received signal r(t) is
where
r(t) = A cos W e t + I cos (w, + w)t
= (A + I cos wt) cos W e t - I sin wt sin W e t
= E r (t) cos [w e t + 1/rd (t)]
1 / sin wt
1/r d (t) = tan - A +I cos wt
When the interfering signal is small in comparison to the carrier (/ « A),
I
1/rd (t) ::::: - sm wt
A
(5.29)
The phase of E r (t) cos [w e t + 1/r d (t)] is 1/r d (t), and its instantaneous frequency is W e + 'if! d (t).
If the signal E r (t) cos [w e t + 1/r c1 (t)] is applied to an ideal phase demodulator, the output Yd (t)
would be i/J c1 (t). Similarly, the output Yd (t) of an ideal frequency demodulator would be 'if! d (t).
Hence,
I
Yd (t) = A
sm wt
lw
Yd (t) = A
cos wt
for PM (5.30)
for FM (5.31)
Observe that in either case, the interference output is inversely proportional to the carrier
amplitude A. Thus, the larger the carrier amplitude A, the smaller the interference effect. This
behavior is very different from that in AM signals, where the interference output is independent