Capacity bounds for LTI channels

Linear time invariant (LTI) channels are channels with memory and therefore introduce inter symbol interference (ISI), in general. The problem reduces to a single user AWGN channel, if the spectrum after the channel satisfies the Nyquist theorem and allows for ISI free communication. In the frequency domain, the relationship between source signal spectrum and received spectrum is straightforward, and it is but difficult to design a pulse shape with it. This approach, unlike other ones to bound the capacity of channels with memory, neither neglects the transients nor depends on data preambles (eg. OFDM).