nyquist ISI criterion
For some filter with impulse response h(t), when applied (via linear convolution) to a signal, is ISI free if
h(nTs)={1n=00n≠0
In other words, for at every point right after every symbol (given by symbol duration Ts), the impulse response is 0, except at the first one.
In the frequency domain, this property is written as
1Ts∞∑k=−∞H(f−kfs)=1∀f
In other words, at every frequency an integer multiple of the symbol bandwidth, the frequency should be the same.
Satisfied by the raised cosine filter and sinc filter.