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发表于 2009-1-1 11:13:29
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Digital Modulation Overview
Albert H. Higashi
(602) 413-5309
Revised on June 23, 1998
Notes
Tb = Bit Time
Ts = Symbol Time = Tb*(Bits per symbol)
Bit Rate = 1/ Tb = Rb
Symbol Rate = 1/ Ts = Rs
With respect to the following examples which use a Raised-Cosine response as
the bandlimiting transmit filter, the actual implementation would employ a
square-Root Raised-Cosine filter on the transmitter and an identical filter for the
receiver which would have a composite response of a Raised-Cosine filter. The
reason for doing so is that Raised-Cosine filters belong to the Nyquist class of
filters (Zero-ISI) and the objective is to receive a zero-ISI signal after the matched
filter (noise rejection), so the optimal solution is to split the response between the
transmitter and the receiver.
Eye Patterns are diagram examples of an oscilloscope output if a signal is
triggered with a clock at the symbol rate. This diagram is useful in determining
optimal sample times for data slicing/estimation.
cosn t [e e ] o
w= jnwot+- jnwot 1
2
In exponential Fourier series, the basis functions are exponentials not
sinusoids. Therefore, spectrums of real signals exist over negative as well
as positive frequencies because the exponential index is referred to as the
frequency of that component.
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