# Low-Complexity Channel Codes for Reliable Molecular Communication via Diffusion
## 1 Introduction  
The telecommunications domain is constantly developing, facing all types of new demands, and therefore, while fifth-generation (5G) wireless networks are currently being deployed around the world, work is already underway for future evolutions such as sixth-generation (6G) systems [1] and even beyond. Extending the concept of Internet of Things [2] and emerging as a new network paradigm is the \"Internet of Bio-Nano Things\" (IoBNT), defined as the interconnection of nanoscale devices [3, 4]. This is a revolutionizing concept promising many applications in the biomedical field, medicine, industry, environment, agriculture, and military [5]. Communications in IoBNT need to be biocompatible, energy-efficient, and robust in physiological conditions, making electromagnetic signals a difficult choice due to issues related to biocompatibility, power, and possible health hazards. An alternative to this problem is Molecular Communications (MC), using molecules for encoding, transmitting, and receiving information. MC are feasible, being considered easier to implement than other approaches in the near term. They are scalable having an appropriate size for nanomachines, and they are bio-compatible allowing the integration with living systems [6]. MC have been captivating a lot of research interest with several novel transmitter/receiver architectures being proposed in recent years. Due to the growing interest in nanoscale communications and MC, IEEE has already started standardization efforts through the IEEE P1906.1 working group [7, 8]. Nevertheless, MC for in-body networks is still in its infancy, and it will take some time before practical systems become areality, probably not before a seventh or higher generation (7G) of wireless networks [8]. However, MC are also relevant for several industry and environment applications such as smart infrastructure monitoring, quality control, hazardous environment monitoring, and plant and animal monitoring [9]. Many of these scenarios rely on macroscale MC which is expected to evolve much faster than in-body MC, and may even allow it to be integrated into the next generation of wireless networks, namely, into 6G [8]. Regarding this type of communications, diffusion-based MC is one of the most studied channels because of its flexibility and suitability for a wide number of scenarios. The diffusion-based architecture is characterized by the random movement of molecules (Brownian motion) from higher concentrations to less [10; 11]. The system structure adopted for MC is similar to traditional radio communication systems, characterized by an end-to-end chain [12; 13]: a transmitter, a channel, and a receiver. A few MC systems based on diffusion have been experimentally tested over the last few years. The purpose is to bridge the gap between theory and practice. Due to current technology limitations, most experiments are still carried out at the macro level instead of nano. For instance, an air-driven experiment was described in [14], which was based on spraying and detecting alcohol in open space. Some difficulties identified in this experiment were the fluctuations in humidity and temperature that change the channel features, which in the molecular context may have an impact on the results obtained. In a liquid-driven experiment presented in [15], the authors used small tubes as the communication channel where information is carried out via the pH. The authors refer that the system was shown to achieve better data rates than any previous chemical communication platform tested. MC have some limitations: the stochasticity where molecules have random propagation called environmental noise, the large delays due to the much longer propagation times of molecules when compared to the speed of light, the range since known MC techniques have very short practical ranges, and the fragility because biological components can be environmentally sensitive [5; 16]. These conditions pose many challenges for achieving a reliable communication.