# 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  
This is a flexible and scalable method for determining the location of moving tags by recording the difference in the arrival time of the signal between active anchors that must be synchronized. The advantage of this method is its low power consumption, because the identification wave needs to be sent only once from the tag to the anchors, without the need for a response or confirmation wave. The timestamps recorded at all anchors are collected by the reference anchor or a gateway and forwarded to the tracking monitoring server, where the location coordinates are calculated and the desired movement of the tags is tracked. It is also possible to implement this method in reverse, called reverse time difference of arrival (RTDoA). The anchors send synchronized blinks at specific time intervals in the active tracking area. When the moving tag enters the specified area, it receives the blinks and calculates the distances between it and the anchors using TDoA. All of the calculated distances are then forwarded to the monitoring server for implementing real-time tag monitoring using multilateration. Figure 11 shows the two TDoA topologies ([PERSON] et al., 2020). This method has been widely used in the implementation and development of indoor tracking and positioning systems due to its unique advantage of saving energy consumption, especially since the targeted systems use batteries extensively in their operation. Many studies, system developments, system tests, and evaluations for TDoA have been proposed. [PERSON] et al. [98] proposed a TDoA-based RTTS for patient tracking in healthcare environments. The system consists of four anchors and two wearable tags embedded in a wristband. The anchors are synchronized, fixed in a wall, and connected through Ethernet via a 24-port Ethernet hub. The nearest anchor for the tags is considered to be the reference anchor, and it is responsible for collecting the TDoA information. All of the data are forwarded to the system server, which is a Raspberry Pi platform (Raspberry Pi Ltd., Cambridge, UK). The server is responsible for calculating the coordinates of the movable tags. The Qorvo DW1000 UWB module has been used in both anchors and tag design, where the STM32F407 VET6 and the STM32L041K6U6D (STMicroelectronics, Geneva, Switzerland) microcontroller have been used for anchors and tags, respectively. A mobile-phone- and web-based application can be used for monitoring. [PERSON] et al. [99] proposed a high-precision wireless IPS based on TDoA combined with the Taylor algorithm. The system's UWB units have the same hardware design and can be programmed as anchors or tags. The unit's hardware consists of the Qorvo DW1000 and the STM32F103 RCT6 (STMicroelectronics, Geneva, Switzerland) microcontroller, which communicates via an SPI bus. The system testing was performed using four fixed synchronized anchors and one movable tag. The test results revealed that the system's positioning measurement errors do not exceed 10 cm. The main challenge in implementing TDoA-based IPSs is the need for time synchronization of the used anchors. One possible solution to overcome this challenge is to use a synchronization reference anchor that broadcasts periodic synchronization signals to the other anchors. Several researchers have developed the TDoA method in a way that does not require time synchronization between anchors, reducing the complexity and cost. [PERSON] et al. [100] proposed a deep learning algorithm for TDoA IPS security with missing data and measurement errors called \"DeepTAL\". First, the algorithm is trained with the system without the anchors' synchronization condition, and then it is applied to the TDoA synchronized system in real time. The system was implemented using the Qorvo DW1000 UWB radio transceiver with an ARM cortex M43 STM32f105 (STMicroelectronics, Geneva, Switzerland) microcontroller. The practical tests of the system were implemented using four anchors and one tag.