  In the emerging field of molecular communication (MC), testbeds are needed to
validate theoretical concepts, motivate applications, and guide further
modeling efforts. To this end, this paper presents a flexible and extendable
in-vessel MC testbed based on superparamagnetic iron oxide nanoparticles
(SPIONs) dispersed in an aqueous suspension as they are also used for drug
targeting in biotechnology. The transmitter is realized by an electronic pump
for injection of the SPIONs stored in a syringe via a Y-tubing-connector. A
second pump generates a background flow for signal propagation in the main
tube, e.g., modeling a part of a chemical reactor or a blood vessel. For signal
reception, we employ a commercial susceptometer, an electronic device including
a coil, through which the magnetic particles move and non-intrusively generate
an electrical signal. We identify the physical mechanisms governing
transmission, propagation, and reception of SPIONs as signal carriers and
propose a simple two-parameter mathematical model for the system's channel
impulse response (CIR). Reliable communication is demonstrated for
model-agnostic and model-based detection methods for experiments with 400
random symbols transmitted via on-off keying modulation with a 1 s symbol
interval. Moreover, the proposed CIR model is shown to consistently capture the
experimentally observed distance-dependent impulse response peak heights and
peak decays for transmission distances from 5 to 40 cm.
