  This paper analyzes the channel impulse response of an absorbing receiver
(RX) covered by multiple non-overlapping heterogeneous receptors with different
sizes and arbitrary locations in a molecular communication system. In this
system, a point transmitter (TX) is assumed to be uniformly located on a
virtual sphere at a fixed distance from the RX. Considering molecule
degradation during the propagation from the TX to the RX, the expected molecule
hitting rate at the RX over varying locations of the TX is analyzed as a
function of the size and location of each receptor. Notably, this analytical
result is applicable for different numbers, sizes, and locations of receptors,
and its accuracy is demonstrated via particle-based simulations. Numerical
results show that (i) the expected number of absorbed molecules at the RX
increases with an increasing number of receptors, when the total area of
receptors on the RX surface is fixed, and (ii) evenly distributed receptors
lead to the largest expected number of absorbed molecules.
