# IoT: A Long-Reach Fully Passive Low-Rate Upstream PHY for IoT over Fiber
## 2 Materials and Methods
### Requirements for IoT over Fiber (IoToF)  
We are aiming to develop a niche solution to fill gaps from an IoT arena dominated by extremely low-cost and widespread wireless solutions. Therefore, one of the main challenges in IoTFo systems are CAPEX and OPEX. But, it should be noted that optical solutions are costly because they involves electrical-to-optical (E/O) and optical-to-electrical (O/E) interfaces--and not for the optical fiber itself. In addition, such interfaces are usually power hungry circuits, whereas optical fibers are electrically passive elements. Energy is certainly a major constraint for IoT solutions and, therefore, E/O and O/E conversions need to be addressed in IoTFoPHY architectures before benefiting from long-reach with reliability, privacy and other appealing features from optical fibers compared to wireless ones. Table 1 highlights some matching factors between conventional optical communication connectivity and desired IoTFoPHY architectures. Data throughput required by individual IoT  
devices is usually several orders of magnitude below the figures that optical interfaces can offer. The long reach and reliability from optical fibers is what IoToF seeks to bring to the IoT ecosystem. Installation and maintenance in conventional optical systems is costly, time-consuming, and expensive at E/O and O/E interfaces involving specialized splicing equipment and connectors, whereas the wireless counterpart operation is virtually effortless and costless. Therefore, IoToF should aim at extremely simple operations. In addition, conventional optical communication E/O and O/E are power consuming due to the need of biasing and cooling circuitry whereas IoToF would aim, if possible, at optical passive solutions at IoT devices. Spectral efficiency is not a major concern in optical systems as seen in the widespread use of on-off keying and direct detection to reduce cost [22]. In contrast, the extremely low-cost requirement from the IoT systems will push for IoToF solutions with limited end-to-end bandwidth, where spectral efficiency becomes an issue to be addressed. In order to put IoToF into the IoT wireless connectivity context, we focus on two relevant parameters in Figure 1: throughput and reach. Higher-frequency bands offer more channels and bandwidth, allowing more data throughput, nevertheless lower transmission distance is achieved. Lower-frequency radio waves are less perturbed during the propagation than higher-frequency waves, reaching a higher operation range, with a reduced data throughput [14]. IoToF requirements should aim at ranges beyond conventional wireless solutions, without compromising minimal throughputs achieved. It is expected for IoToF, in Figure 1, to exceed a fundamental wireless limit: Radio horizon [27]. It is the furthest reach imposed by the earth's curvature light-of-sight propagation, including atmosphere refraction effects [28]. The Radio horizon for a 30 m height gateway antenna and an end station at 1 m is around 20 km [27], which is within LoRa's and SigFox's experimentally reported ranges. Coincidently, this is also Gigabit PON (G-PON)'s maximum non extended reach [25; 29]. 20 km makes up an important optical and wireless landmark for IoToF to surpass and find its own niche. This paper puts forward the IoToF concept, suggesting a simple architecture and implements a prototype to demonstrate its feasibility. In Section 2.4, we argue that FB \begin{table}
\begin{tabular}{l l l} \hline \hline \multirow{2}{*}{**IoT Requirements**} & **Conventional Optical** & **IoToF Optical** \\  & **PHY @ ONU** & **PHY** \\  & **(per Subscriber [26])** & \\ \hline Throughput & Up to 2.5 Gbps & At least 100 bps \\ Range & Up to 20 km & At least 20 km \\ Power Consumption & Around 1 W & 0 W (_Passive_) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The internet of things (IoT) requirements and conventional optical physical connectivity (PHY) at Optical network unit (ONU) and IoT over fiber (IoToF) optical PHY.