Sensitivity to timing errors
 ICI and ISI occur only when the FFT interval extends over a
symbol boundary or extends over the rolloff region of a
symbol. Hence, OFDM demodulation is quite insensitive to
timing offsets.
 An relationship exists between symbol timing and the
demodulated subcarrier phase.
 Looking the figure in the above page, we can see that as the
timing changes, the phases of the subcarriers change.
 The relation between the phase  i of subcarrier i and the
timing offset  is given by
i  2f i
 In a coherent OFDM receiver, channel estimation is performed
to estimate these phase shifts for all subcarriers.
Wireless Communication Technologies 2.5.3
1
Sensitivity to timing errors
2
2
1.5
1.5
1
1
0.5
0.5
Quadrature
Quadrature
 Figure (a) shows an example of the QPSK constellation of a
received OFDM signal with a timing offset equal to 1/20 of
the FFT interval.
0
0
-0.5
-0.5
-1
-1
-1.5
-1.5
-2
-2
-1.5
-1
-0.5
0
In-phase
(a)
0.5
1
1.5
2
-2
-2
-1.5
-1
-0.5
0
In-phase
0.5
1
1.5
2
(b)
Constellation diagram with a timing error of T/20 before (a) and after (b)
phase correction.
Wireless Communication Technologies 2.5.3
2
Sensitivity to timing errors
 The timing error translates into a phase offset of a multiple of
2 / 20 between the subcarriers.
 Because of this phase offset, the QPSK constellation points
are rotated to 20 possible points on a circle.
 After estimation and correction of the phase rotations, the
constellation diagram of Fig. (b) is obtained.
Wireless Communication Technologies 2.5.3
3