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Making Molecular MIMO Work
A group of researchers, including Prof. Chan-Byoung Chae, develops a novel MIMO design for molecular communication
For communication systems operating on the microscale or nanoscale level, various technical problems occur: Is the antenna small enough? Can the wavelength of your communication signal be transmitted effectively? Are your data rates being hampered by interference?
Researchers are looking at other ways to send signals from point A to B, and they have found an innovative approach: molecular communication, where data travels as chemical instead of electromagnetic signals.
Koo, Lee, Yilmaz, Farsad, Eckford, and Chae (2016) present a novel approach in their paper, “Molecular MIMO: From Theory to Prototype” published in the IEEE Journal on Selected Areas in Communication. In their approach, they build on molecular communication via diffusion (MCvD), and add a multiple-input multiple-output (MIMO) technique to deal specifically with signal interference.
An MCvD system consists of three main components: transmitter, receiver, and fluid environment between the transmitter and the receiver. Transmitter nodes send molecules of data (symbols) into the system, and those molecules travel through the medium; the receiver nodes capture these molecules and convert them to data. Although several molecular systems have been proposed, the capabilities of the available molecular communications are rather primitive; their data rate is too low for commercial applications.
In their study, the authors added a MIMO technique to improve the efficiency of the MCvD systems. Transmitting molecules via multiple nodes may provide higher data rate communication than the single-in single-out (SISO) approach. Some of the key points of the current study are described below.
Considering ISI and ILI for a molecular communication system
MIMO techniques need to deal with inter-symbol interference (ISI) and inter-link interference (ILI). ISI occurs when a signal introduces a propagation delay that interferes with other symbol taps, whereas ILI occurs when a signal interferes with other communication links that are in physically proximity.
The molecular communication system used in this research comprises a 3D environment with two point sources and two spherical receive antennas. The communication medium is a fluid. After the transmitter releases “messenger molecules” into the fluid, those molecules travel via diffusion, and the receive antennas count the number of molecules that reach the spherical surface.
To investigate the effectiveness of their MIMO technique, the scientists first implemented a software simulation to ascertain their claim, and then constructed a macro-scale physical testbed to examine their concept in a lab .
The model function and the proposed algorithms
The team used a model function to fit the simulation data. This formula is analogous to the one in a molecular SISO system in a 3D environment, with variable control coefficients. A similar model function was used in previous studies to model noise effects in an SISO testbed and to model a molecular MIMO channel.
Next, after deriving formulas for ISI and ILI, the researchers proposed a series of detection algorithms specific to molecular MIMO systems. The authors calculated and analyzed the performance of their MIMO system in terms of the bit error rate.
In addition, the authors examined the link-level performance of the proposed detection algorithms while varying the number of emitted molecules and symbol duration.
Using SIR in the performance analysis
In their performance analysis, the authors considered signal-to-interference ratio (SIR). The SIR is the ratio of the expected number of molecules transmitted from the intended transmitter in the intended time slot to the mean ILI plus ISI for a one-shot signal.
The authors investigated the effects of various topological conditions on the SIR. The results showed that, to reduce interference, decreasing the transmitter–receiver distance and increasing the size of the receive antennas is more effective than increasing the separation of the antennas.
The world’s first molecular MIMO testbed
To verify their concept physically, the authors designed the world’s first molecular MIMO testbed, which comprises a molecular MIMO transmitter and receiver. The transmitter and receiver were equipped with multiple transmit nozzles and receive sensors and they are exploited to increase the data rate.
The propagation distance between the transmitter and the receiver was approximately 1 m. This low-cost platform is modifiable and programmable.
Experimental results show that the MIMO system could achieve transmission rates that were 1.7 times higher than those obtained from SISO systems.
The current study sheds light on achieving higher data rate communication using MIMO. Since data rates in molecular communications are affected by interference, the authors suggested using a MIMO system for MCvD that considers ISI and ILI. Four symbol detection algorithms were proposed, and the effect of varying topological conditions on the SIR was investigated. The results showed that decreasing the transmitter–receiver distance and increasing the size of the receive antennas are more effective at reducing interference than increasing the separation of the antennas. The world’s first molecular MIMO testbed to verify the proposed concept was developed, and it achieved transmission rates 1.7 times higher than those obtained from the molecular SISO system. For this testbed, the authors received the prestigious IEEE INFOCOM Best Demo Award in 2015.