On Hybrid MoSK-CSK Modulation based Molecular Communication: Error Rate Performance Analysis using Stochastic Geometry

This paper studies the impact of a transmitter's molecule generation process on the capacity of a concentration-based molecular communication (MC) system. Constraints caused by the molecule generation process affect the availability of the molecules at the transmitter. The transmitter has a storage of molecules, and should decide whether to release or save the currently produced molecules. As a result, the MC system has conceptual connections with energy harvesting systems. In this paper, we consider two scenarios on the propagation channel. The first scenario assumes a channel with no inter-symbol interference (ISI), i.e., a memoryless channel. We derive bounds on the capacity of the MC system in this scenario. The second scenario assumes an MC channel with ISI, in which the output of the channel depends on the history of released molecules in the previous time-slots. Based on the assumptions that either the transmitter or the receiver knows the channel statistics, we compute a lower bound on the channel capacity.

In recent years, conveying information using molecules via diffusion has attracted significant interest in micro and nanoscale applications. Yet, very little research have been conducted on the performance analysis of large-scale molecular communication (MC) systems with swarm of interfering receivers. In this type of MC system, the collective signal strength at a desired receiver can be impaired by the interference caused by other MC transmitters. The paper presents the first tractable analytical framework for the collective signal strength at a partially absorbing receiver due to a desired transmitter under the impact of a swarm of interfering transmitters in a three-dimensional (3D) large-scale MC system using stochastic geometry. We propose Reed Solomon (RS) coding as an error correction technique to combat the effects of interference caused by the multiple transmitters and the delayed molecules from the previous bits. We derive analytical expressions for the bit error probability (BEP) to evaluate the level of reliability that can be achieved by the proposed scheme. The results obtained using Monte Carlo simulations, matched extactly with the anlytical results, justifying the accuracy of the derivations. Results reveal that the RS codes are capable of providing an acceptable level of improvement in terms of BEP. Furthermore, by carefully selecting the code length, n, and the message length, k, of the RS(n, k) code, we can fine tune the BEP improvement to match the system requirements.

The biocompatibility and nanoscale features of Molecular Communication (MC) make this paradigm, based on molecules and chemical reactions, an enabler for communication theory applications in the healthcare at its biological level ( e.g. , bimolecular disease detection/monitoring and intelligent drug delivery). However, the adoption of MC-based innovative solutions into privacy and security-sensitive areas is opening new challenges for this research field. Despite fundamentals of information theory applied to MC have been established in the last decade, research work on security in MC systems is still limited. In contrast to previous literature focused on challenges, and potential roadmaps to secure MC, this paper presents the preliminary elements of a systematic approach to quantifying information security as it propagates through an MC link. In particular, a closed-form mathematical expression for the secrecy capacity of an MC system based on free molecule diffusion is provided. Numerical results highlight the dependence of the secrecy capacity on the average thermodynamic transmit power, the eavesdropper’s distance, the transmitted signal bandwidth, and the receiver radius. In addition, the concept of secure distance in an MC system is introduced and investigated for two different techniques of signal detection, i.e. , based on energy and amplitude. The secrecy capacity can be used to determine how much secure information (bit/sec/Hz) can be exchanged and within which operative range, while the secure distance can be used to set the transmit power to obtain a secure channel at a given distance. We envision these metrics will be of utmost importance for a future design framework tailored to MC systems and their practical applications.

Molecular communication (MC) is an interdisciplinary research area with vast potential in biomedical applications, including delivery of drugs or other therapeutic molecules in cardiovascular and other pathological conditions. The bio-molecules release modeling in MC systems is an open problem in designing the physical and chemical signal interaction level in the MC hierarchy. This also includes the release of extracellular vesicles (EVs), which are shed by all cell types and serve as nano-vehicles carrying various molecular cargo. In this paper, we propose to use engineered human induced pluripotent stem cells differentiated into atrial cardiomyocytes (ACMs) in a cardiac bio-nanonetwork and mathematically model the release of therapeutic EVs under voltage clamp. We study the stochastic nature of the proposed targeted drug delivery system by modeling the release process from engineered ACMs as a Poisson process. We model the engineered ACMs as an EV emission model, where the input signal is the voltage clamp to modulate the EV release, and the output signal is the rate of released EVs. We also study the non-linearity of the proposed EV emission system by a non-linear autoregressive exogenous model. Future drug delivery applications for cardiovascular diseases can utilize the proposed EV release modeling for engineered ACMs.

In this paper, we consider diffusive molecular communication (MC) systems affected by signal-dependent diffusive noise, inter-symbol interference, and external noise. We design linear and nonlinear fractionally-spaced equalization schemes and a detection scheme which combines decision feedback and sequence detection (DFSD). In contrast to the symbol-rate equalization schemes in the MC literature, the proposed equalization and detection schemes exploit multiple samples of the received signal per symbol interval to achieve lower bit error rates (BERs) than existing schemes. The proposed DFSD scheme achieves a BER which is very close to that achieved by maximum likelihood sequence detection, but with lower computational complexity.

Molecular communication is a promising nanoscale communication paradigm that enables nanomachines to exchange information by using molecules as communication carrier. Up to now, the molecular communication channel between a transmitter nanomachine (TN) and a receiver nanomachine (RN) has been modeled as either concentration channel or timing channel. However, these channel models necessitate exact time synchronization of the nanomachines and provide a relatively low communication bandwidth. In this paper, the Molecular ARray-based COmmunication (MARCO) scheme is proposed, in which the transmission order of different molecules is used to convey molecular information without any need for time synchronization. The MARCO channel model is first theoretically derived, and the intersymbol interference and error probabilities are obtained. Based on the error probability, achievable communication rates are analytically obtained. Numerical results and performance comparisons reveal that MARCO provides significantly higher communication rate, i.e., on the scale of 100 Kbps, than the previously proposed molecular communication models without any need for synchronization. More specifically, MARCO can provide more than 250 Kbps of molecular communication rate if intersymbol time and internode distance are set to 2 μs and 2 nm, respectively.

Engineering of molecular and nano-scale communication networks may benefit future applications which interact with biological systems. Molecular communication networks are composed of bionanomachines which can interact with biomolecules or cells through transmission of molecules. One promising approach for generic communications is directional transport of molecules, such as by bacteria, to transmit molecules to another bionanomachine at a specific spatial location in an environment. In this paper, bacteria move to spatial locations relative to molecular beacons. Each molecular beacon emits a different type of molecule to produce a concentration gradient. A bacterium has a chemotactic pathway tuned for estimating its direction relative to the concentration gradients, and the specific concentrations at which the chemotactic pathway saturate represents a specific spatial address. Through simulation modeling, receptor parameters of bacteria were tuned to target various spatial addresses, and the latency and success rate of communication were characterized for receivers positioned at each spatial address.

Molecular communication is a bioinspired communication that enables macro-scale, micro-scale and nano-scale devices to communicate with each other. The molecular communication system is prone to severe signal attenuation, dispersion and delay, which leads to performance degradation as the distance between two communicating devices increases. To mitigate these challenges, relays are used to establish reliable communication in microfluidic channels. Relay assisted molecular communication systems can also enable interconnection among various entities of the lab-on-chip for sharing information. Various relaying schemes have been proposed for reliable molecular communication systems, most of which lack practical feasibility. Thus, it is essential to design and develop relays that can be practically incorporated into the microfluidic channel. This paper presents a novel design of passive in-line relay for molecular communication system that can be easily embedded in the microfluidic channel and operate without external energy. Results show that geometric modification in the microfluidic channel can act as a relay and restore the degraded signal up-to 28%.

This paper considers the impact of external noise sources, including interfering transmitters, on a diffusive molecular communication system, where the impact is measured as the number of noise molecules expected to be observed at a passive receiver. A unifying model for noise, multiuser interference, and intersymbol interference is presented, where, under certain circumstances, interference can be approximated as a noise source that is emitting continuously. The model includes the presence of advection and molecule degradation. The time-varying and asymptotic impact is derived for a series of special cases, some of which facilitate closed-form solutions. Simulation results show the accuracy of the expressions derived for the impact of a continuously-emitting noise source, and show how approximating old intersymbol interference as a noise source can simplify the calculation of the expected bit error probability of a weighted sum detector.

Inter-symbol interference (ISI) may result in substantial performance degradation in molecular communication systems. In this paper, we propose a more accurate and realistic ISI model compared to the literature, by quantifying the ISI in a three-dimensional fluid environment with a spherical receiver. Moreover, due to the propagation delay in a fluid environment, a large number of the molecules absorbed earlier in a transmission interval are due to the previous transmissions. Therefore, we find the optimal delay time, that the receiver should wait at the beginning of each time interval before counting the absorbed molecules, to reduce the effect of the ISI. Further, and to enhance the performance of the system, we adopt a detection approach based on multiple molecular observations at the receiver, and introduce a weighted sum detector, in which the transmission interval is divided into a number of sub-intervals. We analytically derive the weights, assigned to different sub-intervals, that minimize the bit error rate (BER). Simulations, based on the presented approaches, show the impact of the transmitter-receiver distance, the reaction rate, and the diffusion constant of the environment on the BER performance. We also show that using a weighted sum detector significantly improves the BER performance.

In the Molecular Communication (MC), molecules are utilized to encode, transmit, and receive information. Transmission of the information is achieved by means of diffusion of molecules and the information is recovered based on the molecule concentration variations at the receiver location. The MC is very prone to intersymbol interference (ISI) due to residual molecules emitted previously. Furthermore, the stochastic nature of the molecule movements adds noise to the MC. For the first time, we propose four methods for a receiver in the MC to recover the transmitted information distorted by both ISI and noise. We introduce sequence detection methods based on maximum a posteriori (MAP) and maximum likelihood (ML) criterions, a linear equalizer based on minimum mean-square error (MMSE) criterion, and a decision-feedback equalizer (DFE) which is a nonlinear equalizer. We present a channel estimator to estimate time varying MC channel at the receiver. The performances of the proposed methods based on bit error rates are evaluated. The sequence detection methods reveal the best performance at the expense of computational complexity. However, the MMSE equalizer has the lowest performance with the lowest computational complexity. The results show that using these methods significantly increases the information transmission rate in the MC.

Molecular communication provides a mechanism for the coordination and control of micro-scale and nano-scale devices. In this paper, we describe simulation models to evaluate a molecular communication system that uses molecular motors to transport information molecules over a filament network. The molecular communication system includes an abstracted sender device that releases the information molecules into the environment and a receiver device that detects those molecules as information. In biological systems, molecules move through the environment using methods such as molecular motors that walk on filaments (for certain intracellular transport) or passive diffusion through Brownian motion (for both inter and intracellular transport). We perform simulations to compare the molecular motor system and a Brownian diffusion system and measure the success rate and delay of communicating to receivers. Recent research in engineering biological systems focuses on communication on a two-dimensional surface, and thus we focus on characterizing communications between a sender and receiver that are randomly located on a surface.

Low-earth orbit (LEO) satellite communication (SatCom) has emerged as a promising technology to improve wireless connectivity in global areas. Cell-free massive multiple-input multiple-output (CF-mMIMO), an architecture proposed for next-generation networks, has yet to be fully explored for LEO satellites. In this paper, we investigate the downlink performance of a CF-mMIMO LEO SatCom network, where multiple satellite access points (SAPs) simultaneously serve the corresponding ground user terminals (UTs). Using tools from stochastic geometry, we model the locations of SAPs and UTs on surfaces of concentric spheres using Poisson point processes (PPPs) and present expressions on transmit and received signals, signal-to-interference-plus-noise ratio (SINR). Then, we derive the coverage probabilities in fading scenarios, considering significant system parameters such as the Nakagami fading parameter, the number of UTs, the number of SAPs, the orbital altitude, and the service range affected by the dome angle. Finally, the analytical model is verified by extensive Monte Carlo simulations. Simulation results indicate that stronger line-of-sight (LoS) effects and a more comprehensive service range of the UT result in a higher coverage probability, despite the presence of multi-user interference (MUI). Moreover, we found that there exist optimal numbers of UTs that maximize system capacity for different orbital altitudes and dome angles, providing valuable insights for system design.

Intersymbol Interference (ISI) is one of the major bottlenecks in Molecular Communication via Diffusion (MCvD) systems resulting in degraded system performance. This paper first introduces two new families of linear channel codes to minimize the effect of ISI: linear Zero Pad Zero Start (ZPZS) and linear Zero Pad (ZP) codes, ensuring that every codeword is devoid of consecutive bit-1s. Subsequently, the ZPZS linear and ZP linear codes are combined to form a binary ZP code, aiming for a higher code rate compared to the linear ZP codes, which can be decoded with a simple Majority Location Rule (MLR) algorithm. Additionally, a linear Leading One Zero Pad (LOZP) code is proposed, which relaxes the zero padding constraints considering the placement of bit-1s in the codeword as an important metric to have an improved code rate than the ZP code. Finally, a closed-form expression is deduced to compute the expected ISI for the proposed codes and to demonstrate that the expected ISI is a function of the average bit-1 density of the codewords in a code. To compare the ISI and BER performance with average bit-1 density of the proposed codes, two types of MCvD channel are considered: (i) channel without refresh, where the previously transmitted bits persist for a longer duration and (ii) channel with refresh, where the channel is cleared after each successful reception of the message. The ISI comparison, across different sequence distributions for a given length and weight, shows that the linear LOZP code exhibits superior resilience against ISI in a channel with refresh due to the placement of bit-1s at the initial positions, whereas the ZP code performs better in channel without refresh by reducing the average bit-1 density of the code. The asymptotic upper bound of the code rate is derived for the proposed codes, which depicts that a trade-off exists between the ISI and code rate. The simulation results show that the proposed family of ZP and linear LOZP codes can improve the Bit Error Rate (BER) performance by controlling the bit-1 locations and the average bit-1 density of the code, specifically where the ISI is more pronounced over the channel noise, thus providing a better reliability compared to the conventional error correcting codes at different data rate regimes.

Currently, most of the researches in molecular communication (MC) domain focus on the static MC scenarios. However, some envisioned important MC applications require mobile MC system. The investigation on mobile MC, especially the signal detection of mobile MC is limited. This work considers the problem of signal detection for mobile MC scenarios where the receiver nano-machine performs random movement. Due to the random movement of the receiver, the channel impulse response (CIR) changes over time which makes the received signal stochastic and complicated. This further complicates the signal detection in mobile MC and leads to that the state-of-the-art signal detection schemes for static MC scenarios fail for the mobile MC scenarios. To solve this issue, an adaptive detection scheme has been proposed by our group previously, based on dynamic estimation of the stochastically varying distance between the transmitter and receiver and the reconstruction of CIR in each interval. However, its computational complexity is high. Limited capability of current nano-machines desire low-complexity detection algorithm. In this work, we further propose an adaptive detection scheme for mobile MC with a low computational complexity by utilizing the local convex property of the CIR. With on-off keying (OOK) modulation, the signal of symbol "1" presents local convex property while that of symbol "0" presents local concave property. The convexity extent varies with the stochastic distance. A simple indicator, local maximum convexity is proposed which adapts to the stochastic distance. By comparing the adaptive indicator with an adaptive threshold within each symbol interval, the signal is detected without the need to estimate the stochastically changing distance or to reconstruct the CIR. Therefore, the computational load is effectively reduced. Numerical simulations are performed to evaluate the proposed scheme. The results show that the proposed scheme achieves good detection accuracy with low computational complexity and it could be a promising detection scheme for mobile MC scenarios.