Evaluation of a Multi-Molecule Molecular Communication Testbed Based on Spectral Sensing

TL;DR

This work addresses the need for practical, real-time multi-molecule MC testbeds to boost throughput and enable advanced coding and networking. A flow-based testbed with non-invasive spectral sensing differentiates inks, an absorbance-based ink-intensity estimator recovers color concentrations, a simple CIR model is used for validation, and a basic difference detector demonstrates usable data rates. Findings show throughput up to about 3 bps with BER near 1 in 100, and MUMO preserves the CIR shape but increases variability and slightly reduces peak. The platform is low-cost, modular, and suited for real-time multi-node MC experiments, with potential to evaluate coding, modulation, and resource management schemes including non-orthogonal access in future work.

Abstract

This work presents a novel flow-based molecular communication (MC) testbed using spectral sensing and ink intensity estimation to enable real-time multi-molecule (MUMO) transmission. MUMO communication opens up crucial opportunities for increased throughput as well as implementing more complex coding, modulation, and resource allocation strategies for MC testbeds. An estimator using non-invasive spectral sensing at the receiver is proposed based on a simple absorption model. We conduct in-depth channel impulse response (CIR) measurements and a preliminary communication performance evaluation. Additionally, a simple analytical model is used to check the consistency of the CIRs. The results indicate that by utilizing MUMO transmission, on-off-keying, and a simple difference detector, the testbed can achieve up to 3 bits per second for near-error-free communication, which is on par with comparable testbeds that utilize more sophisticated coding or detection methods. Our platform lays the ground for implementing MUMO communication and evaluating various physical layer and networking techniques based on multiple molecule types in future MC testbeds in real time.

Figures (5)

• Figure 1: Annotated image of the entire testbed structure. From a water reservoir, the background flow is generated by a peristaltic pump through tubes of diameter $d_\mathrm{c}$. The liquid then flows past the TX which consists of three micropumps capable of injecting cyan, magenta, and yellow ink into the channel. After a channel of length $l_\mathrm{c}$ the mixture flows past the non-invasive RX with a spectral sensor of length $l_\mathrm{RX}$, and lastly arrives in a waste reservoir.
• Figure 2: Average CIR for SIMO and MUMO transmission of cyan, magenta, and yellow molecules as the estimated color intensity over time. The plot shows the SIMO CIR average, with the dark-shaded area representing the maximum deviation from the average. The best fit analytical CIR defined in Eq. (\ref{['eq:analytical_cir']}) is shown in red. Additionally, the average MUMO transmission bit-1 and bit-0 response are shown with the lighter shaded area depicting the maximum deviation from the average.
• Figure 3: Best fit initial Beta distributions $f(s)$ as defined in Eq. \ref{['eq:init_distr']} for the SIMO CIR shown in Figure \ref{['fig:avg_cir']}.
• Figure 4: System BER for a sample sequence of 312 bits sent via MUMO transmission using a range of detection thresholds $\tau$, see Eq. (\ref{['eq:slope_detection']}). Results for three different values of the symbol period $T_\mathrm{sym} = \{0.5, 1, 2\} \unit{\second}$ are depicted, representing a bit rate of 6, 3, and 1.5 bps, respectively.
• Figure 5: Selected time frame from the received signal during the random bit sequence transmission with $T_\mathrm{sym} = \qty{2}{\second}$. The plot on top depicts raw measured light intensities $I^j(t)$ for all channels, and the plot below depicts the corresponding estimated color intensities $c_i(t)$. Received bits in each symbol period are shown for the three inks at the top of the lower plot, with the single error of the entire sequence (yellow bit-1 transmitted, bit-0 detected) highlighted in red. For 4 selected symbol periods, the threshold $\tau$ and detected difference for the yellow signal are shown.