L27 - changes from x(t)=-1 for t<0 to x(t)=1 for t>0...

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Usually given by the government (FCC). Need to purchase "licence" to use certain frequency-band. Different frequency bands will have different price, depending on the transmission range, bandwidth, channel quality, etc. Choose it to be equal to or sightly larger than
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The receiver has to have a carrier signal that is "synchronous" (or equivalently "phase coherent" to the cosine signal used in the transmitter.
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If the carrier signal oscillates much faster than x(t), then x(t) is the "envelope" of the wave form, which can be obtained by "half-wave rectification" (see p. 590-594). (1) Quality loss due to half-wave rectification (2) Cannot take negative signals. Namely, However, since the "phase" at "t>0" changes, a shnchronous detector knows that the signal
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