Molecular communications: Novel relaying schemes and molecular beamforming

The communication in the small scale has received a growing interest in the recent years due to two main reasons. First of all, there is a wide variety of applications for which the traditional communication technologies are not suitable, especially those related to medical treatment and healthcare. Secondly, the communication in the nano/micro scale is feasible due to the advancements in the production of the electronic devices, both in terms of cost and scale constraints. As a prominent approach towards enabling nanonetworking, molecular communications (MC) has been proposed. First and foremost, compared to the conventional EM communication, MC solves the impediment of the size of antennas. Moreover, MC is a bio-compatible solution, which is a key aspect for its proposed usage areas. There exist numerous MC systems introduced in the literature, among which MC via diffusion (MCvD) is one of the most examined ones, since it is an efficient and practical solution that utilizes the diffusive characteristics of molecules for conveying the information. The signal arriving at the intended node features a heavy-tail shape due to the diffusion dynamics of the molecules, which leads to the MCvD systems being prone to inter-symbol interference (ISI). Alleviating ISI is one of the most investigated issues when it comes to MC. Apart from that, the communication range is another limitation that characterizes MCvD, due to the signal's amplitude rapidly decreasing as the communicating nanomachines get further from each other. In order to address these challenges, novel designs and algorithms in the communication perspective are proposed in the literature. Chapter 2 of this dissertation summarizes the single-input single-output (SISO) topology of MCvD, providing the foundations in the literature that are very useful in understanding the aforementioned challenges. Moreover, a brief literature review on relaying in MC is provided, laying the groundwork for the motivation behind two of the contributions of this dissertation. The first proposed scheme is introduced in Chapter 3. This work is inspired from the fact that most of the relay-based schemes in the literature of MC consider the relay to be positioned in the middle of the two nodes whose communication it facilitates. As this my not always be achievable in practice, an asymmetrical relaying case is considered. If the parameters of the two resulting communication links have the same values, the overall performance of an asymmetrical relaying scheme is prone to error propagation due to unequal error protection for these links. This results from the fact that the quality of the channel belonging to the node closer to the relay is higher compared to the channel of the further one. In order to overcome this, utilization of molecules with different diffusion coefficients is proposed for the uplink (UL) of the relaying system. The best performance in terms of bit error rate (BER) is obtained when the emitting points release different numbers of different types of molecules. Two parameter optimization methods are proposed and the analytical results are verified by computer simulations. The scheme proposed in Chapter 4 focuses on utilizing a relay for boosting the received signal's strength, which results in an improvement in the BER performance of the communication system. This scheme is referred to as optimal relaying. The proposed system consists of a transmitting point, a receiving node, and a relay somewhere in between them. The relay absorbs a fraction of the released molecules, until some time, and then re-directs it towards the receiver (Rx), which also absorbs from the molecules emitted from the transmitter (Tx) in the meantime. In other words, the overall absorbed molecules at the Rx come from two sources: the Tx and the relay. Overlapping these two components, such that the overall amplitude of the received signal is increased, is the main motivation of this work. In order to achieve this, optimizing the time until when the relay will be absorbing is proposed by means of maximizing a function called the signal to interference difference (SID). The system is modelled analytically as well. The BER results after optimization are compared with the BER performance of the traditional SISO system, and an improvement is shown to be attained with the incorporation of the optimal relay. The last contribution of the thesis focuses on cooperative sensing in MCvD networks. Particularly, a novel paradigm is presented, which is motivated by specific scenarios where MC can find application, such as sugar level stabilization of patients with diabetes. Inspired by the concept of beamforming in radio frequency (RF) communications, molecular beamforming is presented in Chapter 5. The proposed technique can find application in systems where the actuation of nanomachines is targeted. The typical proposed network consists of several sensors (Txs) that emit molecules with the purpose of activating one actuator (Rx) at a time, whose on or off state depends on the overall received signal. However, the sensors are prone to errors, thus they may wrongly emit/not emit. For this reason, the main motivation behind this idea is that when the number of sensors increases, the actuation error rate decreases. In order to achieve this, several design techniques are proposed, such that the overall received signal is composed of delayed versions of the components arriving from different sensors. The verification of the proposed method is achieved by the analytical derivation of the actuation error probability for several scenarios. Firstly, the system where sensors are forming a uniform linear array (ULA) is considered for presenting the scheme with less complexity. Afterwards, a more realistic scenario is considered, where the locations of the sensors are random. As for the computation of the actuation error rate, three different scenarios are presented, depending on the error source and how the actuation decision is made. The analytical results are validated with the ones obtained from computer simulations. The results reveal very interesting insights about the potential of molecular beamforming for actuation accuracy in MC networks. Finally, Chapter 6 concludes the dissertation by summarizing the main outcomes obtained from the aforementioned works. Additionally, several future research directions are provided.