  The design and engineering of molecular communication (MC) components capable
of processing chemical concentration signals is the key to unleashing the
potential of MC for interdisciplinary applications. By controlling the
signaling pathway and molecule exchange between cell devices, synthetic biology
provides the MC community with tools and techniques to achieve various signal
processing functions. In this paper, we propose a design framework to realize
any order concentration shift keying (CSK) systems based on simple and reusable
single-input single-output cells. The design framework also exploits the
distributed multicellular consortia with spatial segregation, which has
advantages in system scalability, low genetic manipulation, and signal
orthogonality. We also create a small library of reusable engineered cells and
apply them to implement binary CSK (BCSK) and quadruple CSK (QCSK) systems to
demonstrate the feasibility of our proposed design framework. Importantly, we
establish a mathematical framework to theoretically characterize our proposed
distributed multicellular systems. Specially, we divide a system into
fundamental building blocks, from which we derive the impulse response of each
block and the cascade of the impulse responses leads to the end-to-end response
of the system. Simulation results obtained from the agent-based simulator BSim
not only validate our CSK design framework but also demonstrate the accuracy of
the proposed mathematical analysis.
