# UWB-Based Real-Time Indoor Positioning Systems: A Comprehensive Review
## 5 Real-Time UWB Indoor Positioning Systems (RTIPSs)
### Real-Time Implementation Ranging and Topologies
#### 5.2.1 Two-Way Ranging-Based Indoor Positioning Systems  
This method is one of the common methods for implementing real-time tracking and positioning systems. It can be implemented using two topologies: The first uses a reference anchor to calculate the distances between the ultra-wideband units and send them to the monitoring and display server via a cable or wireless connection. The second approach calculates the distances between the moving tag and the active anchors in the moving tag itself and sends the data via the cloud to the monitoring and display server. Figure 10 shows these two topologies [8]. Many tracking and positioning systems have been implemented using this method. [PERSON] et al. [79] proposed a single-sided two-way ranging (SS-TWR) positioning system based on filtering and clock compensation. The system uses the Qorvo DW1000 (Qorvo Inc., Greensboro, NC, USA) UWB modules for both tags and anchors. The proposed system uses the anchor-based topology and consisted of three anchors and one tag. The reference anchor is connected by wire to the server laptop using a serial port. The random forest algorithm estimates new distances based on previous SS-TWR algorithm values. The results of both random forest and SS-TWR algorithms have been evaluated using the root-mean-square error (RMSE), and the results showed better performance for the random forest algorithm, with an improvement of approximately 4.85 cm compared to SS-TWR. [PERSON] et al. [80] proposed a low-cost multi-tag IPS for object tracking in indoor environments. The system's UWB hardware uses the MaUWB_DW3000 (Makerfabs Corporation, Shenzhen, China) module, which includes the new Qorvo DW3000 UWB transceiver. The system consists of three fixed anchors and can track up to 16 movable tags. The trilateration technique with the least squares method has been used for system implementation and testing in both LoS and NLoS situations. All UWB measurements from the system's UWB units are accumulated by the reference anchor and forwarded to the control and monitoring server laptop via a COM port. A Python-3.7 program is used for monitoring the movable tag based on the map of the testing area. The practical tests indicate that the system can achieve an accuracy of 50 cm. [PERSON] et al. [81] proposed the design of an IPS based on TWR and trilateration algorithms. The system consists of four anchors and one movable tag. All of the units have similar hardware designs, including the Qorvo DW1000 UWB modules for distance measurements and the STM32F1032 ET6 (STMicroelectronics, Geneva, Switzerland) microcontroller for data preprocessing and UWB control tasks via an SPI communication bus and powered by a lithium battery. The experimental tests proved a low error rate of approximately 14 cm, and the authors suggested this system for underground coal mine personnel. [PERSON] et al. [82] proposed a double-sided two-way ranging (DS-TWR)-based positioning system with a Kalman filter for improving the system accuracy. Figure 10: TWR IPS topologies: (**a**) anchor-based; (**b**) tag-based. The system used the Qorvo DW1000 UWB modules for system implementation, with four fixed anchors and one movable tag responsible for the distance calculation with all ultra-wideband units and communication with the monitoring station (tag-based topology). The use of the Kalman filter enhances the system's performance and decreases the distance measurement errors in both LoS and NLoS environments. [PERSON] et al. (2020) used a derivative unscented Kalman filter (DUKF) and root-mean-square error (RMSE) to estimate and reduce the nonlinear error of anchor positions in a TWR-based IPS. The system used the Qorvo DW1000 UWB modules and micro-electromechanical system (MEMS) inertial sensor for real-time implementation with the anchor positions' error calibration, which improved the localization precision and robustness of the system. [PERSON] et al.