Design and Analysis of Molecular Communication Systems

Abstract

Molecular communication (MC) is a new communication engineering paradigm. The main characteristic of MC systems is that information is embedded in the properties of signaling molecules, e.g., their number, type, and time of release by the transmitter, which is in contrast to the conventional communication systems that embed information in the properties of electromagnetic waves, e.g., their amplitude, frequency, and phase. MC systems are envisioned to enable revolutionary applications, e.g., sensing of a target substance in biotechnology, targeted drug delivery in medicine, and monitoring of oil pipelines or chemical reactors in industrial applications. The main focus of this dissertation is on the communication theoretical design and analysis of MC systems. In particular, we develop efficient channel estimation, symbol synchronization, and detection schemes for MC systems as outlined in the following. Channel Estimation: Knowledge of the channel impulse response (CIR) is typically needed for communication system design. In the MC literature, it is typically assumed that either the CIR is known or a specific, usually oversimplified, MC channel model is adopted and the model parameters are estimated. In this dissertation, a general training-based CIR estimation framework is developed for MC systems which is not limited to a particular MC system model and does not require knowledge of channel parameters. Symbol Synchronization: Symbol synchronization refers to the estimation of the start of a symbol interval and is needed for reliable detection. Therefore, in this dissertation, several symbol synchronization schemes are developed that take into account the practical challenges of MC systems, namely that the transmitter may not be equipped with an internal clock and may not be able to emit molecules with a fixed release frequency. Coherent Detection: Coherent detectors use knowledge of the channel state information (CSI) for data recovery. MC systems are subject to signal-dependent diffusion noise due to the molecule counting process, inter-symbol interference, and external interfering molecules. Taking these impairments into account, the optimal signal-to-noise-plus-interference ratio maximizing filter is derived as a function of the instantaneous CSI of the MC system. It is shown that unlike for the additive white Gaussian noise channel in conventional wireless communications, the simple channel correlator is not optimal for molecule counting receivers. Non-Coherent Detection: Non-coherent detection schemes are desirable for application in MC systems since they do not require knowledge of the instantaneous CSI which can be difficult to obtain in practical MC scenarios. Therefore, several non-coherent multiple-symbol detectors are proposed in this dissertation. It is shown that if the number of symbols exploited for detection is sufficiently large, the performance of the proposed detectors approaches that of the optimal coherent detector which assumes perfect CSI knowledge. CSI-Free Detection: Although the proposed non-coherent detectors do not need instantaneous CSI, statistical CSI is still needed. To relax this requirement, it is shown in this dissertation that a class of codes, namely constant-composition (CC) codes, facilitate maximum likelihood (ML) sequence detection at the receiver without requiring instantaneous or statistical CSI. The corresponding CSI-free detector is derived and shown to have a simple structure. It is noted that for all of the above estimation/detection scenarios, the corresponding optimal ML (or maximum a posteriori) estimator/detector is derived and serves as a performance upper bound. In addition, several suboptimal low-complexity estimators/detectors are proposed that may be more suitable for applications in practical MC systems with limited computational capability. Performance analysis and comprehensive simulation results are provided to assess the effectiveness of all proposed estimators/detectors. Show more