  In this paper, we present an analytical model for a diffusive molecular
communication (MC) system with a reversible adsorption receiver in a fluid
environment. The time-varying spatial distribution of the information molecules
under the reversible adsorption and desorption reaction at the surface of a
bio-receiver is analytically characterized. Based on the spatial distribution,
we derive the number of newly-adsorbed information molecules expected in any
time duration. Importantly, we present a simulation framework for the proposed
model that accounts for the diffusion and reversible reaction. Simulation
results show the accuracy of our derived expressions, and demonstrate the
positive effect of the adsorption rate and the negative effect of the
desorption rate on the net number of newly-adsorbed information molecules
expected. Moreover, our analytical results simplify to the special case of an
absorbing receiver.
