# State of the Art of Telecommunication Systems in Isolated and Constrained Areas
## 2 Telecommunication Systems in an Isolated and Constrained Environment
### Optical Fiber  
The other solution to consider when talking about setting up a telecommunication station to create a network of sensors [22] with the possibility of remote data access by an internet gateway is to use optical fiber [23]. For Quebec's specific case, the North Plan was put in place by the government to reduce the digital divide by providing internet access in remote locations. Nunavik [24] is an example of this penetration of optical fiber (Figure 3) in the territory. The Cree indigenous communities have benefited from this development of optical fiber since 2011 for an amount of 29 million CAD. The second phase, which is about to be completed, will have cost 25 million CAD to deploy 800 km of optical fiber. Other communities will be connected to the Internet via optical fiber passing through undersea cables, even if the costs of deploying this solution are very high. From a physical point of view, the optical fiber can be likened to a glass or plastic wire with the property of propagating light. Since light speed is 300,000 km/s, optical fiber makes it possible to obtain information rates significantly higher than those of coaxial cables that propagate the electrical signal at only 200,000 km/s with additional losses (weakening along the cable and coupling) [25]. Optical fiber has the following advantages:  
* Lower attenuation than conventional cables propagating an electrical signal,
* Higher information rate,
* High propagation speed,
* High immunity to parasites, and
* Almost zero crosstalk. One of the significant drawbacks of this type of technology is its fragility, which must be added to a higher cost. There are two main types of optical fiber, the multimode, for which there are different modes of light propagation within the fiber core, and the  
Figure 3: Link budget. single-mode, in which there is only one mode of propagation of the light, the straight line way. In multimode optical fibers [26], there are two types of fiber, the step-index and the gradient index. The step-index optical fiber has a very large core with the disadvantage of a strong attenuation in the order of 10 dB/km. Its range is around 2 km and its speed around 100 Mbits/s. It is mainly used in local area networks (LAN). Multimode optical fiber with gradient index is also used in local area networks. Its core is of intermediate size (50 to 100 \(\upmu\)m). It has better mitigation than that of step-index fiber. Its range is around 2 km and its speed around 1 Gbit/s. Monomodal fiber is the most efficient currently. This type of fiber is used in the core of global networks because its attenuation is almost zero (around 0.5 dB/km), its range in the order of 100 km, and its high speed of 100 Gbits/s [27]. It is evident that with the advent and development of this type of support, operators have embarked on a race to transmit more and more information through an optical fiber. To channel multiple sources of information over a medium such as optical fiber, the technique consists in using multiplexing. There are two types of multiplexing:  
* Time division multiplexing (TDM)
* Wavelength division multiplexing (WDM)  
TDM consists of cutting the optical fiber bandwidth in time intervals, which shares the different sources of communication. This allows multiple low-speed digital channels to be transmitted over a high-speed medium (optical fiber). WDM uses the mixing of several optical signals of different wavelengths on the same optical fiber. The wavelengths of the various sources are relatively distant to limit interference between them. We distinguish among the WDM, Coarse WDM, Dense WDM, and Ultra-Dense WDM [28]. Their respective performances are summarized in Table 1.  
It should be noted that the structural diagram method would need a multiplexer at the input and a demultiplexer at the output, as illustrated in Figure 4. The deployment of optical fibers over hundreds of kilometers involves relays (optical amplifiers) to amplify light [29], as shown in Figure 4.