  Molecular communication (MC) can enable the transfer of information between
nanomachines using molecules as the information carrier. In MC systems,
multiple receiver nanomachines often co-exist in the same communication channel
to serve common or different purposes. However, the analytical channel model
for a system with multiple fully absorbing receivers (FARs) does not exist in
the literature, which is significantly different from the single FAR system due
to the mutual influence of FARs. The analytical channel model is essential in
analyzing systems with multiple FARs, including MIMO, SIMO, and cognitive
molecular communication systems. In this work, we derive an approximate
analytical expression for the hitting probability of a molecule emitted from a
point source on each FAR on a diffusion-based MC system with three or more
FARs. Using these expressions, we derive the channel model for a SIMO system
with a single transmitter and multiple FARs arranged in a uniform circular
array (UCA). We then analyze the communication performance of this SIMO system
under different cooperative receiver schemes and develop several interesting
insights.
