  We consider zero-forcing equalization of frequency selective MIMO channels by
causal and linear time-invariant precoders in the presence of intersymbol
interference. Our motivation is twofold. First, we are concerned with the
optimal performance of causal precoders from a worst case point of view.
Therefore we construct an optimal causal precoder, whereas contrary to other
works our construction is not limited to finite or rational impulse responses.
Moreover we derive a novel numerical approach to computation of the optimal
perfomance index achievable by causal precoders for given channels. This
quantity is important in the numerical determination of optimal precoders.
