  The objective in stochastic filtering is to reconstruct information about an
unobserved (random) process, called the signal process, given the current
available observations of a certain noisy transformation of that process.
  Usually X and Y are modeled by stochastic differential equations driven by a
Brownian motion or a jump (or Levy) process.
  We are interested in the situation where both the state process X and the
observation process Y are perturbed by coupled Levy processes. More precisely,
L=(L_1,L_2) is a 2--dimensional Levy process in which the structure of
dependence is described by a Levy copula. We derive the associated Zakai
equation for the density process and establish sufficient conditions depending
on the copula and $L$ for the solvability of the corresponding solution to the
Zakai equation. In particular, we give conditions of existence and uniqueness
of the density process, if one is interested to estimate quantities like P(
X(t)>a), where a is a threshold.
