  This paper presents an analytical comparison of active and passive receiver
models in diffusive molecular communication. In the active model, molecules are
absorbed when they collide with the receiver surface. In the passive model, the
receiver is a virtual boundary that does not affect molecule behavior. Two
approaches are presented to derive transforms between the receiver signals. As
an example, two models for an unbounded diffusion-only molecular communication
system with a spherical receiver are unified. As time increases in the
three-dimensional system, the transform functions have constant scaling
factors, such that the receiver models are effectively equivalent. Methods are
presented to enable the transformation of stochastic simulations, which are
used to verify the transforms and demonstrate that transforming the simulation
of a passive receiver can be more efficient and more accurate than the direct
simulation of an absorbing receiver.
