Capacity of Frequency-based Channels: Encoding Information in Molecular Concentrations

We consider a molecular channel, in which messages are encoded to the frequency of objects (or concentration of molecules) in a pool, and whose output during reading time is a noisy version of the input frequencies, as obtained by sampling with replacement from the pool. We tightly characterize the capacity of this channel using upper and lower bounds, when the number of objects in the pool of objects is constrained. We apply this result to the DNA storage channel in the short-molecule regime, and show that even though the capacity of this channel is technically zero, it can still achieve a large information density.