# Analysis of the Scalability of UWB Indoor Localization Solutions for High User Densities
## 3 UWB Regulations and PHY Considerations  
A typical RF system aim to serve as many users as possible. Although many techniques exist, they are not always easy to apply and not suited for all kind of technologies. UWB follows strict rules that may limit the applicability of certain mechanisms. The next two paragraphs analyze the PHY layer related aspects that influence the scalability of the system. Similar analysis for MAC and localization schemes is done in Section 4. UWB uses 500 MHz (or 900 MHz) bandwidth and as such could interfere with others operating in the same frequency bands. This is the reason the Federal Communications Commission (FCC) introduced limitations for the maximum UWB radiated power. The harmonized European standard \"ETSI EN 302 065-1 Part 2\" describes the requirements for UWB location tracking devices. UWB equipment is categorized based on the operational frequency range and the type of tracking it performs. Three different systems are defined:  
* LT1 Systems  
They operate in the 6 GHz to 9 GHz region of the spectrum and are intended for general localization of people and objects. Devices in this category can be mobile (indoors and outdoors) or fixed (indoors only).  
* LT2 Systems  
In this category the frequency range is 3.1 GHz to 4.8 GHz and devices are used for person and object tracking and industrial applications. The devices can transmit indoor and outdoor and may be fixed or mobile.  
* LAES Systems  
Similarly to the previous category, the frequency range is the same 3.1 GHz to 4.8 GHz but in LAES systems, UWB devices are used for tracking staff of emergency services such as firefighters. This implies that these systems are typically temporary and may required licenses prior usage.  
An UWB device can have more than one operating bandwidth provided that the maximum mean power spectral density and the maximum peak power are not above the limits. The maximum mean power spectral density and maximum peak power depend on the type of the UWB system, the frequency and sometimes whether the device is operating outdoor or indoor. All the different values are reported in Tables 2 and 3 (if not specified, outdoor and indoor are the same). The maximum peak power is specified as the effective isotropic radiated power (e.i.r.p.) contained within a 50 MHz bandwidth at the frequency at which the highest mean radiated power occurs. As noted in the tables, mitigation techniques exist. These allow devices to modulate their transmission power to avoid collisions and do not cause interference with other devices. In general, there are two main categories, namely active and passive mitigation techniques. Each of these includes several mechanisms but only two are applicable to our scenarios, i.e., LT1, LT2 and LEAS. The mitigation techniques are called Low Duty Cycle (LDC) and Detect And Avoid (DAA). Although they are used to improve the robustness of the communications, they may impact the overall scalability of the system.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Solution** & **Approach** & **User Density** & **Other Notes** \\ \hline [PERSON] and [PERSON], 2005 [10] & RSSI distributed localization & 200 tags & Results of simulation \\ [PERSON] et al., 2011 [11] & TDoA & 30 tags & Real life experiments with TDMA protocol \\ [PERSON] and [PERSON], 2016 [12] & Hybrid TDoA-TWR & Not evaluated & Improved system accuracy \\ [PERSON] and [PERSON], 2016 [20] & TWR & 22 tags & Industrial warehouse \\ OpenTRIS [16] & TDoA, TWR & 7500 updates/s & Communication range 20 m \\ Sewio [17] & TDoA & 1000 updates/s & Communication range 30 m \\ Be Spoon [18] & TDoA, TWR, AoA & 750 updates/s & Office like environment \\ Time Domain [19] & TWR & Unknown & Proprietary MAC protocols \\ \hline \hline \end{tabular}
\end{table}
Table 1: Related work overview.