# A Review of Wireless Positioning Techniques and Technologies: From Smart Sensors to 6G
## 2 Localization Techniques
### Range-Based Localization Techniques  
Range-based localization techniques first utilize the signal measurements (time, angle, or channel-based) for estimating the distance/range from various reference nodes, as described in Section 2.1. Next, the various distance measurements are combined using  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Distance Estimation** & **Features** & **Limitations** \\ \hline Time of Arrival (ToA) & - More accurate results compared to the RSSI & Precise time synchronization between nodes is required \\  & - Highly biased from the environment & - Additional hardware may be required \\ \hline Time Difference of Arrival & - Contrary to ToA, TDoA only requires precise time synchronization between the transmitting nodes & - Time synchronization between transmitting nodes is required \\ (TDoA) & - It can also be utilized with nodes equipped with speakers and microphones & - Performance is degraded in the NLoS signal propagation \\  & - Provides higher accuracy than ToA & - Microphones and speakers require LoS and free of echoes conditions for higher accuracy \\  & - It can be used with different communication technologies, i.e., ultrasound and RF & - It requires bidirectional communication capabilities \\ \hline Round Trip Time (RTT) & - It eliminates the need of utilizing different clocks and time synchronization & - It’s accuracy is biased from the different changes in the environment \\  & - It can be combined with other measurements & - The phase cannot be distinguished, so it can only determine in which fraction of the wavelength is again is received \\  & - It requires LoS \\ \hline Angle of Arrival (AoA) & - At least two transmitting nodes are required & - Signals can easily change direction, so accuracy degradation occurs \\  & - No time synchronization between the nodes is required & - Its accuracy is proportional to the nodes’ density \\  & - & Additional hardware is required (e.g., antenna arrays) \\  & - & Expensive computational cost \\ \hline Received Signal Strength & - One of the simplest and most widely used metrics & - Its accuracy is affected by the existence of obstacles and multipath propagation \\ (RSS) & - Supported by most transceivers & - Affected by NLoS \\  & - Easily scaled to larger areas and multiple users & - Affected by weather conditions, such as rain [42] \\  & - Minimum infrastructure required & - Less accurate than time-based estimations \\  & - Cost-effective computation & - Only available for ZigBee technology \\ \hline Link Quality Indicator & - Contrary to RSS, LQI is included in all received packets & - It can only be used with technologies supporting OFDM \\ (LQI) & - It can be used as a unique fingerprint & - Special hardware is required (increased cost and complexity) \\  & - It is affected by the dynamic changes of the transmitting nodes’ power \\ \hline \hline \end{tabular}
\end{table}
Table 2: Distance Estimation Techniques Comparison.  
appropriate algorithms for estimating the nodes' unknown positions. The methods described in this section include lateration, hyperbolic, angulation, and multidimensional scaling (MDS).