  The derivation of suitable analytical models is an important step for the
design and analysis of molecular communication systems. However, many existing
models have limited applicability in practical scenarios due to various
simplifications (e.g., assumption of an unbounded environment). In this paper,
we develop a realistic model for particle diffusion in a bounded sphere and
particle transport through a semi-permeable boundary. This model can be used
for various applications, such as modeling of inter-/intra-cell communication
or the release process of drug carriers. The proposed analytical model is based
on a transfer function approach, which allows for fast numerical evaluation and
provides insights into the impact of the relevant molecular communication
system parameters. The proposed solution of the bounded spherical diffusion
problem is formulated in terms of a state-space description and the
semi-permeable boundary is accounted for by a feedback loop. Particle-based
simulations verify the proposed modeling approach.
