What is the best way to avoid or remove ISI (intersymbol interference)? What is the role of Sinc and raised cosane wave in it? In addition, what role does the equalizer play in ISI?
There is nyquist criteria that a pulse has to satisfy in order to avoid ISI (e.g. when you sample, you get contribution from only one sample and that from others are zero). The sinc pulse can be made to satify these conditions. You can also see partial repsonse signaling scheme. Equalizer is deisgn to revert the effects of channel, including ISI. This is a very long topic that needs better insight (difficult to write here in detail). You can see the book by B P Lathi.
In the condition with perfect timing, the regular sinc pulse works without ISI if the received signal is not distorted. However, when the timing is not perfect, i.e., when the signal is sampled with timing error, ISI is generated even the received signal has no distortion. Under such condition, the raised cosine pulse generates less ISI compared to the sinc pulse, since the side-lobe magnitude of the raised cosine pulse is much lower than the sinc pulse.
The major source of ISI is distortion of the received signal due to various reasons. If the received signal has been distorted, ISI is inevitable even the raised cosine pulse is used in the transmitter. Distortion could be because of non-flat frequency response of any device and the channel, the non-linear group delay, among others. Multi-path propagation in wireless communications is also a possible reason.
ISI can be removed or reduced by using various kinds of equalizers, with training or without training (i.e. blind), e.g. the adaptive decision feed-back equalizer. Equalization can also be done in frequency domain, such as what is done in the OFDM systems.
Again, it is a big (and interesting) topic, and much more words can be added further.
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