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An ISI channel is, as the name implies, a channel that causes Inter Symbol Interference (ISI). I am not sure if people use the term ISI channel to much but instead usually refer to a frequency selective channel that causes ISI.
A frequency selective channel is when the bandwidth of the channel is less than the bandwidth of the signal. In this case the bandwidth of the signal would be reduced which corresponds to a spreading in the time domain of the signal which is what causes ISI.
ISI occurs when one symbol overlaps with another symbol. eg, you transmit a signal from point X to Y. There'll be a direct signal reaching Y after time t1. There'll be reflections and some parts of the original signal will be reflected and due to this reflection, travel a longer path to the same destination and hence will be delayed. If they reach Y when the next symbol sent from X is there, then this interferes with that and causes ISI.
In OFDM, multiple sub carriers are used. Consider the following case. Lets say you transmit some info at a rate of 1 M symbols per sec and this causes ISI. To avoid this, What you do is, split the data among different carriers, something like sending water through many small pipes instead of one big pipe now, you send the data through each of these channels at a much smaller rate, say you have 1000 sub carriers and if you send one symbol through a channel, you send the next through another channel, so, there won't be ISI and the long delay will satisfy the delay spread condition. The basic purpose of going in for ofdm is to sustain high data rates without the use of an equalizer.
In ofdm, as the name suggests, the sub carriers are orthogonal. Orthogonality means, a cross correlation of two orthogonal signals will be an extremely low value (ideally 0). So, as long as they are orthogonal,there won't be ICI. Orthogonality is defined for the same time interval for the carriers.
off topic:Though , in msk, we say two orthogonal carriers, one is evaluated from -Tb to Tb and the other is evaluated from 0 to 2Tb.
Yes thanks for your very kind explanation..
but if I was to investigate the functioning of OFDM to help in counteracting the effects of delay ICI or ISI? what do i nneed to do?
I have already added CP... but i dont think thats enough...
Would i need to create Equalizer (i.e. digital filter)?
Please have a look at the attached pdf..
it says: The lack of guard interval makes OFDM/OQAM more
spectrally efficient, but multipath effects must be eliminated
by an equalizer. Hirosaki  has proved that a single branch
fractionally spaced equalizer is sufficient to eliminate ISI and
ICI simultaneously. Tu  independently explored the MMSE
equalization problem for single carrier OQAM transmission
systems, which can be regarded as a special case.
I have seen these sort of sentences in number of sources. thats why i was wondering weather i should use the equalizer..