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Fall 2011 6 7 8 9 10 October 22, 2012 11
14 / 31 Lecture Notes 13 OFDM with Multipath x1(t) 1
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0 1 2 3 4 5 x3(t)=x1(t)+x2(t) t (s)
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0 1 2 3 4 5 t (s) EECS 455 (Univ. of Michigan) Fall 2011 October 22, 2012 15 / 31 Lecture Notes 13 OFDM with Multipath
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Total Received Signal
Second Path
(τ=0.16) Direct Path
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1.8 EECS 455 (Univ. of Michigan) 2 2.2 2.4
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time (s) Fall 2011 2.8 3 3.2 October 22, 2012 16 / 31 Lecture Notes 13 Orthogonal Frequency Division Multiplexing (OFDM)
Multipath propagation causes intersymbol interference because
the delay spread is signiﬁcant relative to the bit duration.
By making the bit duration longer the effect of intersymbol
interference can be decreased.
Longer bit duration reduces the bandwidth and the data rate.
Adding modulated carriers at other frequencies allows the data
rate to be increased.
Different carriers are chosen so that one carrier does not interfere
with other carriers.
Another way of looking at OFDM is that the bandwidth of each
carrier is narrow enough so that the channel frequency response
does not change signiﬁcantly.
IFFT produces signals that are orthogonal. Need extra time to let
multipath die out.
OFDM has worse peaktoaverage power ratio compared to single
carrier (unﬁltered system).
EECS 455 (Univ. of Michigan) Fall 2011 October 22, 2012 17 / 31 Lecture Notes 13 OFDM Signals
Consider using IQ modulation (two dimensional) on each carrier.
Let (Ik + jQk ) represent the complex data symbol transmitted on
each carrier for the data interval [0, T ].
If on each carrier we use both cosine and sine, then for each
carrier the generated signal for the interval [0, T ] is
sk (t ) = [Ik cos(2π kf0 t ) − Qk sin(2π kf0 t )]
= ℜ [(Ik + jQk ) exp(j 2π kf0 t )] . The total transmitted signal is then
N −1 sk (t ) s(t ) =
k =0 N −1 =ℜ
EECS 455 (Univ. of Michigan) (Ik +...
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This note was uploaded on 02/12/2014 for the course EECS 455 taught by Professor Stark during the Fall '08 term at University of Michigan.
 Fall '08
 Stark
 Frequency

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