# A Review of Wireless Positioning Techniques and Technologies: From Smart Sensors to 6G
## 2 Localization Techniques
### Round Trip Time (RTT)
#### 2.1.3 Channel-Based Measurements
Although the rain rate was assumed to remain unchanged for a short period of time, the RSS-based distance estimator achieved significant error reduction over the conventional approach. Channel State Information (CSI)  
The CSI is an advanced ranging technique used to provide accurate received signal parameters over the entire signal bandwidth [27]. It represents the channel properties that cause the attenuation of the signal during its transmission between the transmitting and receiving nodes, including multipath. It is only used in the orthogonal frequency division multiplexing (OFDM) systems, in which data is modulated on multiple subcarriers with different frequencies and transmitted simultaneously. The OFDM divides the data stream into multiple sub-streams and transmits them in parallel through multiple sub-channels with orthogonal frequencies. CSI can avoid the multipath effects and noise [40]. In the frequency domain, the CSI is referred as the channel impulse response (CIR), which is a fine-grained physical layer information used to describe the amplitude and phase of each subcarrier. This frequency diversity provides a unique fingerprint, which can be used for constructing a radio map. However, special hardware is required for capturing the state information, such as multiple antennas and advanced network interface cards (NICs), which increase the system's cost and complexity. The CSI information can be extracted in the frequency domain in the form of a channel frequency response (CFR), which describes the multipath propagation of the signal from the amplitude and phase frequency characteristics. The CSI of a single subcarrier can be expressed as follows:  
\[H_{i}=|H_{i}|e^{j\sin(\angle H_{i})}, \tag{8}\]  
where \(|H_{i}|\) is the amplitude response and \(\angle H_{i}\) is the phase response of the \(i\)-th subcarrier. In [40], both RSS and CSI measurements were collected at fixed locations and compared. The CSI measurements benefit from multipath effects and, therefore, provide richer and finer information. The signal attenuation ranging model established utilized the attenuation factor propagation model for converting the RSS and CSI measurements into distance values, as follows [40]:  
\[d=\alpha*10^{\frac{A-RSS}{100}}+(1-\alpha)\frac{1}{4\pi}\left[\left(\frac{c} {\frac{\gamma_{0}}{K}\sum\frac{f_{k}}{f_{c}}*||A||_{k}}\right)^{2}\sigma^{ \left]\frac{1}{2}},\right. \tag{9}\]where \(\alpha\in[0,1]\), \(A\) is the signal strength at 1m from the source, \(n\) is the path loss attenuation factor, \(\sigma\) is the environmental factor, \(c\) the radio velocity, \(f_{0}\) is the central frequency of CSI, \(K\) is the number of subcarriers, \(f_{c}\) is the calculated center frequency, and \(||A||_{k}\) is the amplitude of the \(k\)-th carrier of the filtered CSI. In [40], it was shown that the combination of RSS and CSI can provide more accurate distance information, as compared to only using an existing RSS attenuation model. Additionally, the CSI was found to be more stable in the indoor environment, while employing the ranging algorithm can further improve the ranging stability.