THE INTERSYMBOL INTERFERENCE CHANNEL - LOWER BOUNDS ON CAPACITY AND CHANNEL PRECEDING LOSS

The discrete-time additive Gaussian intersymbol interference (ISI) cha nnel with i.i.d. (not necessarily Gaussian) input signals is considere d. Several new and old lower bounds on the capacity are derived in a u nified manner by assuming different front-end receiver filters, in par ticular the sampled whitened matched filter (SWMF) and the minimum mea n-squared error-decision feedback equalizer (MMSE-DFE) tilter. The fea tures of the bounds are demonstrated and compared in several examples with binary and quaternary input signals. It is also shown that the ef fect of an ideal post-cursor or tail cancellation, in an information-p reserving context, depends primarily on the front-end filter. While. a s is well known, ideal post-cursor cancellation at the output of the S WMF decreases information, the opposite trend is seen when an MMSE-DFE front filter is considered. This observation reflects the basic theor etical obstacles in precoding, i.e., ideal post-cursor cancellation in the presence of a pre-cursor. It is used to assess the inherent loss (in terms of information rates as compared to the rates achievable wit h the hypothetical ideal post-cursor cancellation) associated with any post-cursor cancellation technique such as preceding, DFE, or other v ariants, when operating in synergy with the MMSE-DFE front-end filter. The effect of the front-end filter on an ideally interleaved, precede d coded-modulation system is also addressed.