# UWB-Based Real-Time Indoor Positioning Systems: A Comprehensive Review
## 5 Real-Time UWB Indoor Positioning Systems (RTIPSs)
### Real-Time Implementation Ranging and Topologies
#### 5.2.2 Time Difference of Arrival (TDoA)-Based Indoor Positioning Systems
The test results show that the system can predict the TDoA accurately in the presence of measurement errors or missing data. [PERSON] et al. [101,102] proposed a 3D TDoA-based IPS for tracking the workers in hazardous indoor areas of construction sites. The system hardware consists mainly of the commercial Samsung SmartTag+ (Samsung Electronics Co., Ltd., Suwon, Republic of Korea) module. The UWB chip in this tag is the NXP Trimension SR040 (NXP Semiconductors, Eindhoven, The Netherlands), which is powered by a small coin-cell battery and used for tracking objects using new Samsung  
Figure 11: TDoA IPS topologies: (**a**) TDoA; (**b**) RTDoA. smartphones with UWB technology. Four SmartTag+ modules were used as fixed anchors for indoor environments, and a Samsung smartphone was used as a mobile tag. The system's testing showed that the spatial configuration of the anchors and tags with increasing height differences has a significant impact on the accuracy of the 3D indoor localization. The system tests demonstrated an accuracy of 35 cm. [PERSON] et al. [103] proposed a scalable TDoA-based IPS \"FlexTDOA\". The system topology is based on passive tags and active fixed anchors, where the anchor clocks' synchronization is not required; instead, the flexible time-division multiple access (TDMA) scheme is used for the anchor communications. The system hardware was built using the Qrovo DW3000 UWB transceiver with the ARM cortex M4 STM32F429 ZIT6 (STMicroelectronics, Geneva, Switzerland) microcontroller for control and data processing. The ranging data were transmitted using a USB 2.0 port, and each unit had the main communication buses, such as UART, SPI, and I2C. The system units used a rechargeable Li-ion battery as a power source and the MCP73830 IC for power management. The UWB unit was configured to operate on channel 5, with 6.5 GHz and a data rate of 6.8 Mb/s. The system was tested using ten anchors and one tag. The test results show that the proposed method has better performance than classical TDoA, with an accuracy of \(\approx\)17 cm in LoS and \(\approx\)22 cm in NLoS environments. [PERSON] et al. [104] proposed a low-cost TDoA RTLS that does not require time synchronization to reduce the system complexity by using a one-way communication scheme. The tags blink a sequence of 2-nanosecond pulses on a 7.25 GHz carrier frequency, which is received by the anchors with a two-step correlation analysis to calculate the ToA of the tag at each anchor. The hardware of the system has a custom design for the anchors and the tag. The system tests used three anchors and one tag, and the achieved accuracy was \(\approx\)10 cm. [PERSON] et al. [105] proposed another approach for unsynchronized TDoA IPSs using one-way ranging for time-of-arrival calculations. The system consists of hardware and software parts; the system units include the Qrovo (DecaWave) DWM1000 UWB modules with a 2-millisecond packet transmission time and the Raspberry Pi platform (Raspberry Pi Ltd., Cambridge, UK) for control and processing. The system was tested in two different deployments: the first in a single room, with an accuracy of \(\approx\)15 cm, and the second in a multi-room environment, with 56 test points and an accuracy of \(\approx\)50 cm. Similar TDoA-based IPSs have been proposed in [106; 107; 108; 109].