# Wireless Positioning in IoT: A Look at Current and Future Trends
## 3 Designing an IoT Positioning System
### IoT System Parameters  
This subsection discusses the relevant parameters for a positioning fit in an IoT system. **Topology** relates to a message passing from one node to another and the possibility to discover new nodes in the network. The network topology, illustrated in Figure 5, has a significant impact on how nodes with known locations are discovered by others. On a mesh topology, any node can be set as a reference node, whereas, on a star topology, only the access nodes can be defined as such. The density of the fixed nodes also plays an important role in the location accuracy. For example, a denser network with a well-spread distribution of nodes is likely to provide a better location accuracy than a network with few reference nodes all placed in the same direction from the device to be located. An IoT network typically has a star or mesh topology. In a star topology, devices can only talk to their parent device, while, in a mesh topology, nodes can exchange messages between each other. Star topologies are susceptible to single points of failure, since losing the connection to the parent means that the node will be outside the network. In a mesh topology, if a link fault occurs, the device can look for any other neighbor to connect to. Thus, mesh networks provide better coverage and, implicitly, they are likely to offer better positioning accuracy than star networks;  
Figure 4: Classification of IoT networks. **Range** of an IoT system is important in the sense that it defines an upper bound of the positioning error, which cannot be larger than the communication range. In this aspect, mesh-capable networks have a better footing for positioning purposes as any device can extend service without the need to have specific and dedicated infrastructure;  
**Channel bandwidth** is directly related to the achievable accuracy in positioning when a TOA-based estimation is used. The Cramer-Rao lower bound for any unbiased estimator [12] of a time delay \(\tau_{0}\) of a signal \(S\) is given as  
\[\mathrm{var}(\tau_{0})\geq\frac{1}{\frac{\varepsilon}{N_{0}/2} \frac{\int_{-\infty}^{+\infty}{(2\beta f)^{2}|S(f)|^{2}df}}{\int_{-\infty}^{+ \infty}{|S(f)|^{2}dt}}}, \tag{6}\]  
where \(\varepsilon\) is the signal energy, \(N_{0}\) the noise spectral density and \(\int_{-\infty}^{+\infty}{(2\beta f)^{2}|S(f)|^{2}df}\) is the mean square bandwidth of the signal. However, since we have do not have all the necessary information to accurately determine each IoT signal's spectrum density, we provide instead the multipath resolution or time-frequency resolution defined as follows:  
\[\Delta t\geq\frac{2\pi}{\Delta w}. \tag{7}\]  
Equation (7) determines how the time duration \(\Delta t\) and the spectral bandwidth \(\Delta\omega\) relate to each other. The spectral bandwidth is defined as the bandwidth that includes most of the signal's energy. In this study, we assume it to be equal to the channel bandwidth. Overall, what both Equations (6) and (7) show is that, for time-based approaches, it is favorable to have signals with high SNR and short time duration (i.e., higher bandwidth);  
**Carrier frequency** is inversely proportional to the signal wavelength and to the path losses exhibited by the signal. As we move from sub-GHz carriers towards mmWave carriers, the path losses are stronger and stronger, which results in smaller communication ranges. The differences in  
Figure 5: Comparison of a simplified IoT mesh and star network topology. path losses are due to a multitude of phenomena, but, as frequency increases, they are especially due to the smaller effective area of the devices' antennae. Overall, combining lower carrier frequencies and mesh topologies results in an enhanced service coverage;  
**Modulation types** in IoT systems rely on various digital modulation types, from Ultra Narrow Band (UNB), defined as systems with bandwidths below 1 kHz, to Ultra Wide Band (UWB) modulations, i.e., bandwidths above 500 MHz.