OFDM

V. Multi-Path

1. What is Multi-Pass

there are many kind of interference in real wireless communication. One big problem is multi-path. In analog type of TV broadcasting, ghost phenomena is caused.

Figure 25 : Multi-Path

2. The Problem of Multi-Path

The delayed signal is generated by the multi-path. This causes the overlapping between the current symbol and the previous symbol. This overlap causes the inter-symbol interference which destroys the OFDM sub-carrier orthogonality.

Figure 26 : Inter-symbol interference caused by multi-path

3. Guard Interval

In order to avoid the inter-symbol interference caused by the multi-path, guard interval is attached to the head of each symbol. The guard signal is the copy of the last part of the symbol. The delay less than the guard interval period does NOT cause the inter-symbol interference.

Figure 27 : Addition of the guard interval

Figure 28 : Orthogonality is kept by the guard interval

4. Other Effect on the Symbol

Since the delay signal is added to the original signal, the constellation is still disturbed as figure 29. This disturbance has to be corrected by the equalizing.

Figure 29 : Other effect on the symbol

5. SFN(Single Frequency Network)

Once multi-path problem is solved, the single frequency network (SFN) can be realized.

Figure 30 : Single Frequency Network

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