Performance of Ratio-Shift Keying Modulation for Diffusive Molecular Communication Scenarios

Molecular communication (MC) is a promising communication paradigm that utilizes molecules as information carriers, mimicking natural biological processes. Out of various modulations to encode information onto signaling molecules, on-off keying (OOK) modulation has gained popularity due to its simplicity and efficiency. However, when the channel is unknown, for example, in cases where nodes are mobile, the OOK modulation-based M C systems suffer from a high bit error probability. In this paper, we investigate dynamic MC system with mobile transmit and receiver devices. We consider a variant of ratio-shift-keying (RSK) where the information is encoded in the ratio of the concentration of two different types of signaling molecules with the same propagation characteristics. We derive the optimal maximum likelihood (ML) decoder for OOK and RSK-based MC system. We also present a simpler representation of the optimal decoder under the Poisson assumption and show that the ML decoder for RSK modulation doesn't require channel information, in contrast with the ML decoder for OOK-based MC systems. We further investigate the bit error rate for the considered system. We show that RSK performs better than OOK in a dynamic MC system where the channel varies with time. This is because the ratio of the concentrations of the two types of information molecules is preserved at the receiver, offering improved robustness to variations in channel conditions. Therefore, RSK comes to the rescue in case of unknown or difficult to estimate channel environments. Via numerical investigations, we present various design insights, including optimal symbol design, and the gain of RSK over OOK as a function of node mobility.