# Overcoming Challenges in Large-Core SI-POF-Based System-Level Modeling and Simulation
## 2 POF Modeling Challenges  
There are several challenges to overcome in order to accomplish a realistic system-level simulation of POF-based networks. They are either associated with the optical properties of the fiber, whose size and manufacturing materials are very different to those of multimode glass fibers, or related to the environmental characteristics and deployment requirements of their applications. In this sense, the implementation of short-area POF-based networks entails the use of many components such as connectors or splitters that have an impact on transmission properties [34]. Also, fiber installation in compact environments such as cars, planes and houses implies pressuring and bending the fiber [32]. Moreover, in some of these settings, the fiber can be subjected to harsh environmental conditions [35]. Therefore, the fiber model should describe its behavior, precisely encompassing all sources of variability while simultaneously optimizing computation time. In addition, all necessary component models, including all relevant parameters, should be developed and incorporated into the simulation package. First, due to the large diameter of SI-POF and the relatively large numerical aperture (NA), a huge number of propagating modes are supported and consequently, it is impractical to use modes or even mode groups to model signal flow in these POFs. To address this fundamental issue, a computationally efficient method that describes power propagation in large-core SI-POFs using the generalized power flow differential equation was developed [29]. This approach is based on approximating the very large number of modes by a continuum, where each degenerate mode group is characterized by its propagation angle. Also, in PMMA SI-POFs, mode coupling is much higher than in other multimode fibers. This fiber behavior is incorporated into the model by using a function of the propagating angle to describe mode coupling: the angular diffusion function which is estimated for each particular fiber type from experimental measurements [23]. This strong mode coupling does not, however, guarantee rapid achievement of stationary behavior, and to reach the steady-state power distribution (SSD) usually requires transmission lengths of up to 100 m [23]. Since POF links are mainly implemented for short-range applications, the usual fiber lengths are shorter than the length where the SSD has been reached (coupling length) and thus, they are usually operating in non-stationary conditions. Modeling the behavior of POFs well below their coupling length is very demanding as the fiber behavior is determined by the launching and detecting conditions, and also altered by localized disturbances caused by passive devices (such as couplers, connectors, etc.) or by installation (such as bends, stress, etc.) [36]. Another distinct feature of large-core POFs is the substantial influence of the launching and detection conditions over the transmission parameters, which explains the wide range of published values of power loss and bandwidth measurements [10; 29]. In a typical system-level simulation framework, there are transmitter models with a variety of sources whose wavelength/frequency characteristics are precisely described, but that simplify or ignore their spatial characteristics, such as its aperture and beam divergence or symmetry. Also, in simulation packages, the receiver represents a variety of photodetector models where the area of the photo-diode is either not included or, if so, only as a parameter to set the device electrical bandwidth. However, experimental measurements demonstrate that photodetectors with smaller areas filter out power at high angles, thereby enhancing the fiber transmission capacity [37]. Moreover, the transmitter and receiver connections to the fibers in POF commercial devices implement a variety of designs whose parameters determine the launching distribution and the amount of power reaching the photodetector.