  A diffusion-based molecular communication system has two major components:
the diffusion in the medium, and the ligand-reception. Information bits,
encoded in the time variations of the concentration of molecules, are conveyed
to the receiver front through the molecular diffusion in the medium. The
receiver, in turn, measures the concentration of the molecules in its vicinity
in order to retrieve the information. This is done via ligand-reception
process. In this paper, we develop models to study the constraints imposed by
the concentration sensing at the receiver side and derive the maximum rate by
which a ligand-receiver can receive information. Therefore, the overall
capacity of the diffusion channel with the ligand receptors can be obtained by
combining the results presented in this paper with our previous work on the
achievable information rate of molecular communication over the diffusion
channel.
