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In a lecture slide I noticed that OFDM(Orthogonal frequency-division multiplexing) mitigates the problem of Intersymbol Intereference over wireless systems because the symbol time is increased, in the case when we multiplex a channel into two subchannels, each with bandwidth half of the bandwidth of the original channel.

However, I can not grasp how the length of the Symbol time is related to Intersymbol Interference?

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Intersymbol interference is a property of the communications channel; it causes information to be spread out in the time domain.

To reduce ISI, you can either decrease the time spread in the channel or increase the symbol time of the encoding method — either way, the smearing expressed as a fraction of the total symbol time is reduced.

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    \$\begingroup\$ Dave, thanks, but what you say is just definition, does not explain the underlying effect. Lets say, my symbol is 1ms, now I make it 2ms, why the Intersymbol Interference is less now? Because the symbols get more distant from each other? \$\endgroup\$ – Kristof Tak Jul 17 '14 at 15:03
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Lets consider the case where we have only two paths of a signal.

Delay Spread quantifies the multipath in the sense that it tells the difference in time between the original path and the next path of the same signal.

We have to consider the symbol length with respect to delay spread. Basically, how many symbols are encountered between the path arrivals. The goal is to have as little as possible signals affected by the arrival of the next path.

If the symbol time is smaller than the delay, the other path will interfere with the multiple symbol resulted by its own original path; Otherwise, if the symbol time is longer than the delay, the next path will interfere with less symbols (only one maybe).

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