ISI-Avoiding Modulation for Diffusion-Based Molecular Communication

Abstract

A molecular diffusion channel is a channel with memory, as molecules released into the medium hit the receptors after a random delay. Modulating over the diffusion channel is performed by choosing the type, intensity, or the released time of molecules diffused in the environment over time. Motivated by the desire to keep the encoder and decoder simple and the fact that channel state information is difficult to obtain in diffusion channels, we consider modulation schemes that avoid intersymbol interference (ISI), wherein molecules of the same type are released at time instances that are sufficiently far apart. This ensures that molecules of a previous transmission are faded in the environment, before molecules of the same type are reused for signaling. Avoiding ISI puts a constraint on the input sequence to the channel. In this paper, we study the fundamental limits on reliable communication rate, due to this constraint on input sequences. The maximum reliable transmission rate of ISI-avoiding modulations is given by the constrained coding capacity of the graph that represents the permissible transmission sequences. However, achieving the constrained coding capacity requires long blocklengths and delays at the decoder, making it impractical for simple nano-machines. The main contribution of this paper is to consider modulations with small delay (short blocklength) and show that they get very close to constrained coding capacity.