# UWB-Based Real-Time Indoor Positioning Systems: A Comprehensive Review
## 5 Real-Time UWB Indoor Positioning Systems (RTIPSs)
### Obstacles and Non-Line-of-Sight Obstructions  
Calculating the wave propagation time with high accuracy leads to determining the location with high accuracy, but the time calculation may be subject to errors that cause major defects in the system's performance in general; this is especially the case inside buildings, where many sources of interference and obstacles cause defects in the calculation of the wave propagation time. An error in calculating the real distance leads to an error in determining the real location of the object to be found or tracked. Fading is one of the common phenomena that affect the performance of ultra-wideband communication systems. It occurs when the signal propagates in multiple paths, which causes signal fluctuation that affects the strength and quality of the UWB signal, leading to errors in time-of-flight calculation and increasing the system error rate. Shadowing is another common and more popular phenomenon that affects the performance of IPSs. It occurs when the UWB signals are attenuated or blocked by obstacles in the indoor environment [67]. The fading and shadowing effects are critical parameters to be considered during the system performance and error analysis as part of the UWB-based indoor positioning wireless communication system [68]. One of the most important problems affecting the accuracy of measuring propagation time is the presence of obstacles in the path of direct wave propagation between two points, which is called the non-line-of-sight (NLoS) problem. In this case, there are two possibilities: The first is for the wave to penetrate the obstacle and complete its path to the target point. In this case, the error in measuring the distance occurs due to the delay resulting from adding the penetration time. This increases the time taken to reach the target, which leads to an increase in the calculated distance and, thus, an error in determining the location. In the second possibility, the direct wave is unable to penetrate the obstacle; thus, it is reflected or deflected. In this case, the reflected or deflected indirect waves may reach the target, traveling a greater distance by bouncing \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Vendor**} & \multirow{2}{*}{**Algorithm/Topology**} & **Accuracy** & **Operation** & **Frequency** & \multirow{2}{*}{**Application**} & \multirow{2}{*}{**Reference**} \\  & & **(\(\pm\) cm)** & **Range (m)** & **Range (GHz)** & & \\ \hline Inpixon & TWR/AoA & 40 & 50 & 6.4 & Asset, employer & [50] \\ Eliko & TWR/TDoA & 50 & 70 & NA & RTLS & [51] \\ Litum & TWR/TDoA & NA & 70 & 3.25–6.75 & Asset, employer, forklift & [52] \\ WOXU & ToF/TDoA & 10 & NA & 3.24–6.74 & RTLS & [53] \\ Sewio & TDoA & 30 & 50 & 3–7 & Asset, employer, forklift & [54] \\ KKM & TDoA & 30 & NA & NA & Asset & [55] \\ Kinexon & TDoA/AoA & 20 & NA & 6.25–6.75 & RTLS & [56] \\ Pozyx & ToF & 30 & 50 & 3.5–6.5 & People, assets, vehicles & [57] \\ Zebra & ToF & 30 & 200 & 6.35–6.75 & Transportation, warehouse & [58] \\ Tsingoal & ToF/TDoA & 30 & NA & NA & RTLS & [59] \\ Ubisense & TDoA/AoA & 30 & 65 & 6–7 & Asset & [60] \\ Kathrein & ToF/TDoA & 10 & 40 & 3–10 & Vehicle, forklift & [61] \\ Ubudu & TDoA/TWR & 20 & NA & NA & Portable tracking device & [62] \\ Tracktio & TDoA & 30 & 150 & 3–7 & Worker, warehouse & [63] \\ Leisure & ToF/TDoA & 1 & 100 & 3.1–10.6 & Vehicles & [64] \\ MK & TDoA/AoA & 7 & NA & 6.5–8 & RTLS & [65] \\ BeSpoon & TWR & 10 & 880 & 3.99 & RTLS & [66] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Commercial UWB-based solutions for IPSs. off walls or obstacles, which adds a delay time that may be greater than in the first case, resulting in a greater error in determining the location of the target to be tracked [69,70]. Figure 9 explains the NLoS problem in UWB-based IPSs. Many researchers have suggested different solutions and methods to address errors caused by the non-line-of-sight problem. [PERSON] et al.