Symbiotic Molecular Communications: A New Paradigm for Simultaneous Molecule Sharing and Harvesting

Molecules serve as both information carriers and potential energy sources in molecular communications via diffusion (MCvD). However, the stochastic and non-directional nature of diffusion often prevents molecules from reaching the receiver within the desired time window, leading to degraded reliability and inefficient utilization. To address this challenge, we propose a symbiotic MCvD system that enables both molecule sharing and harvesting. A switch-controllable secondary transmitter is introduced into the conventional point-to-point MCvD model, forming primary and secondary transmission links. Information for these two links is conveyed through molecular characteristics and the operating state of the secondary transmitter, embedding the secondary link within the primary link to facilitate sharing and harvesting. We derive a closed-form, state-transition-dependent channel impulse response (CIR) to characterize state transitions, and propose a block-based absorption shift keying (B-AbSK) scheme to mitigate unintended absorption and further enhance link cooperation. Theoretical analysis of error performance and energy consumption is provided, along with gradient projection and low-complexity exhaustive search algorithms to optimize transmitter placement. Simulations confirm that both links can match or surpass conventional MCvD systems in terms of transmission reliability and molecule efficiency. Finally, a liquid-based experimental platform validates the feasibility of the proposed approach.