In a molecular communication network, transmitters and receivers communicate
by using signalling molecules. At the receivers, the signalling molecules
react, via a chain of chemical reactions, to produce output molecules. The
counts of output molecules over time is considered to be the output signal of
the receiver. This output signal is used to detect the presence of signalling
molecules at the receiver. The output signal is noisy due to the stochastic
nature of diffusion and chemical reactions. The aim of this paper is to
characterise the properties of the output signals for two types of receivers,
which are based on two different types of reaction mechanisms. We derive
analytical expressions for the mean, variance and frequency properties of these
two types of receivers. These expressions allow us to study the properties of
these two types of receivers. In addition, our model allows us to study the
effect of the diffusibility of the receiver membrane on the performance of the
receivers