Page 424 - Fiber Optic Communications Fund

Channel Multiplexing Techniques 405

At Tx At Rx
DFT window
1 0.5 1
Subcar 1 *0.5 0 *0.5 0
0.5
Disp.
*1 channel *1
0 1 2 3 4 5 6 7 8 910 0 1 2 3 4 5 6 7 8 910
1 1
Subcar N 0 0

*1 *1
0 2 4 6 8 10 0 2 4 6 8 10
Time Time
Delay due to fiber dispersion = DΔL

Figure 9.14 Subcarriers of the OFDM system with no cyclic prefix.

N has moved out of the DFT window. This leads to two issues. First, it would interfere with the data in
the neighboring OFDM symbol if there is no guard interval between the two. This is known as intercarrier
interference (ICI) and could lead to performance degradation. Second, as can be seen from Fig. 9.14, the first
few cycles of the subcarrier N at the receiver are empty within the DFT window and, therefore, subcarriers
1 and N are no longer orthogonal over the symbol interval. The breakdown of orthogonality conditions also
leads to performance degradations.
To preserve the orthogonality, a cyclic prefix is used [27, 28]. Instead of leaving the guard interval empty,
the last few cycles of subcarrier N within a block are copied to the guard interval, as shown in Fig. 9.15.

At Tx At Rx
DFT window
Cyclic prefix

1
1
0.5
Subcar 1 *0.5 0 0.5 0
*1 *0.5
*1
0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14
1 1
Subcar N 0 0

*1 *1
0 5 10 15 0 5 10 15
Time
Time
Figure 9.15 Subcarriers of the OFDM system with cyclic prefix.