  In this paper, we consider a type of continuous time random walk model where
the jump length is correlated with the waiting time. The asymptotic behaviors
of the coupled jump probability density function in the Fourier-Laplace domain
are discussed. The corresponding fractional diffusion equations are derived
from the given asymptotic behaviors. Corresponding to the asymptotic behaviors
of the joint probability density function in the Fourier-Laplace space, the
asymptotic behaviors of the waiting time probability density and the
conditional probability density for jump length are also discussed.
