  Molecular Communications (MC) is a bio-inspired communication technique that
uses molecules to encode and transfer information. Many efforts have been
focused on developing new modulation techniques for MC by exploiting
distinguishable properties of molecules. In this paper, we investigate a
particular modulation scheme where the information is encoded into the
concentration ratio of two different types of molecules. To evaluate the
performance of this so-called Ratio Shift Keying (RSK) modulation, we carry out
an information theoretical analysis and derive the capacity of the end-to-end
MC channel where the receiver performs ratio estimation based on
ligand-receptor binding statistics in an optimal or suboptimal manner. The
numerical results, obtained for varying similarity between the ligand types
employed for ratio-encoding, and number of receptors, indicate that the RSK can
outperform the concentration shift keying (CSK) modulation, the most common
technique considered in literature, when the transmitter is power-limited. The
results also indicate the potential advantages of RSK over other modulation
methods under time-varying channel conditions, when the effects of the dynamic
conditions are invariant to the type of the molecules.
