Of michigan 3 4 5 fall 2011 6 7 8 9 10 october 22

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Unformatted text preview: (Univ. of Michigan) 3 4 5 Fall 2011 6 7 8 9 10 October 22, 2012 11 14 / 31 Lecture Notes 13 OFDM with Multipath x1(t) 1 0.5 0 −0.5 −1 0 1 2 3 4 5 6 7 8 9 10 6 7 8 9 10 6 7 8 9 10 t (s) x2(t) 1 0.5 0 −0.5 −1 0 1 2 3 4 5 x3(t)=x1(t)+x2(t) t (s) 1 0.5 0 −0.5 −1 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 1.5 Total Received Signal Second Path (τ=0.16) Direct Path 1 x(t) 0.5 0 −0.5 −1 Constant Phase −1.5 1.8 EECS 455 (Univ. of Michigan) 2 2.2 2.4 2.6 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 peak-to-average 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 I-Q 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.

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