# IoT: A Long-Reach Fully Passive Low-Rate Upstream PHY for IoT over Fiber
## 1 Introduction  
The concept of the internet of things (IoT) is rapidly expanding its reach involving applications in different fields such as domotics, industry, healthcare, and smart cities [1; 2; 3; 4]. The original IoT concept appeared more than three decades ago, being used to combine people, processes and devices with communication technology to remotely exchange data for processing and management [5]. The term IoT was originally attributed to [PERSON] in 1999. In his definition, \"internet\" refers tothe global system of interconnected computer networks using the standard internet protocol suite (TCP/IP), which are linked by a broad array of electronic, wireless and optical networking technologies. The \"things\" term refers to real objects and living entities in the physical or material world. IoT can be considered as a global network, which allows the communication between human-to-human, human-to-things and things-to-things, each element with a unique identity [6]. Therefore, ubiquitous connectivity is an important challenge posed by IoT to communication infrastructures.  
An unexplored IoT connectivity scenario involves wired networks based on optical fiber directly attached to IoT devices. This may be necessary to go around intrinsic wireless networks limitations. Passive optical networks (PONs) could be a candidate solution, offering both extended reach and high bit rates. But for some IoT devices, the power hungry [7] and costly electrical-to-optical conversions could be a severe limitation, and such physical connectivity (PHY) may be an overkill solution for most of IoT typical data rate requirements. Therefore, there is a gap to be bridged for low data-rate transported over long distances using fully passive optical solutions.  
To the best of our knowledge, this is the first time that a fully passive optical architecture is proposed and demonstrated, where digital data modulation passively modulated over an optical signal, which is transmitted over ranges tens of kilometers, outperforming low data-rate long-reach wireless technologies such as LoRa and SigFox. In the current work, we proposed a cost-effective modulation scheme composed by a fiber Bragg grating (FBG) mechanically driven by an acoustic signal containing the IoT information. FBGs have been broadly used in the sensing field such as overhead transmission line wire sag monitoring [8], liquid level monitoring [9], temperature [10], strain [11], and acceleration [12]. In this approach, we focused on the use of the FBG not as a sensor but rather as a modulator, which allows encoding the IoT device's information directly into wavelength shifting. The Internet of things over fiber (IoToF) proposal does not intend to replace the current wireless technologies nor conventional solutions for optical communications. The main idea of IoToF is to overcome the different limitations of current wireless and optical technologies for serving IoT. IoT reinforces the concept of ubiquitous IoT by enabling such concept to reach, for instance, regions with very unfavorable wireless propagation environments and extremely limited energy availability.  
The remainder of the paper is divided as follows. Section 2 discusses IoT connectivity, requirements for IoToF, concept and operation principle of IoToF architecture, and the implementation and characterization of a FBG-based IoT prototype. In Section 3 the experimental results are reported. Finally, conclusions are drawn in Section 4.