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AN APPLICATION IN DIGITAL COMMUNICATION
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In FSK, PSK or QAM transmissions we
have to evaluate the energy of a signal and also we have to verify orthogonality of two signals.
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Consider 
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Typically 
is called the carrier frequency (in Hz) and is such that
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Energy of 
is defined as
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.................(1)
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Now, what is 
? In a practical case we do not need to know at all! Why? Because,
. Hence, maximum value of
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which is much less than T, because
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That is, the second term within the parenthesis in (1) can be
neglected compared to the first term, and there is no need
to know the actual value of the second term. Hence
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.............................................................. (2)
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(How good is this approximation?)
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COMMUNICATIONS