  In Molecular Communications via Diffusion (MCvD), messenger molecules are
emitted by a transmitter and propagate randomly through the fluidic
environment. In biological systems, the environment can be considered a bounded
space, surrounded by various structures such as tissues and organs. The
propagation of molecules is affected by these structures, which reflect the
molecules upon collision. Deriving the channel response of MCvD systems with an
absorbing spherical receiver requires solving the 3-D diffusion equation in the
presence of reflecting and absorbing boundary conditions, which is extremely
challenging. In this paper, the method of images is brought to molecular
communication (MC) realm to find a closed-form solution to the channel response
of a single-input single-output (SISO) system near an infinite reflecting
surface. We showed that a molecular SISO system in a 3-D half-space with an
infinite reflecting surface could be approximated as a molecular single-input
multiple-output (SIMO) system in a 3-D space, which consists of two
symmetrically located, with respect to the reflecting surface, identical
absorbing spherical receivers.
