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Unformatted text preview: ECE 562 Fall 2011 September 7, 2011 Digital Modulation • After possible source and error control encoding, we have a sequence { m n } of message symbols to be transmitted on the channel. The message symbols are assumed to come from a finite alphabet, say { , 1 , . . . , M − 1 } . In the simplest case of binary signaling, M = 2. Each symbol in the sequence is assigned to one of M waveforms { s ( t ) , . . . , s M 1 ( t ) } . • Memoryless modulation versus modulation with memory. If the symbol to waveform mapping is fixed from one interval to the next, i.e., m mapsto→ s m ( t ), then the modulation is memoryless. If the mapping from symbol to waveform in the nth symbol interval depends on previously transmitted symbols (or waveforms) then the modulation is said to have memory. • For memoryless modulation, to send the sequence { m n } of symbols at the rate of 1 /T s symbols per second, we transmit the signal s ( t ) = summationdisplay n s m n ( t − nT s ) . (1) • Linear versus nonlinear modulation. A digital modulation scheme is said to be linear if we can write the mapping from the sequence of symbols { m n } to the transmitted signal s ( t ) as concatenation of a mapping from the sequence { m n } to a complex sequence...
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 Fall '09

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