# A Joint Positioning Algorithm in Industrial IoT Environments with mm-Wave Communications
## 3 Overview of NR PRS and SRS-Pos Design
### UL Positioning Reference Signal, UL SRS-Pos  
UL SRS-Pos is used to provide the positioning measurement for UL-TDOA, UL-AoA and Multi-RTT. These measurements include UL RTOA, UL-AoA (including A-AoA and Z-AoA), reference signal received power (RSRP), and TRP transmission time differences.  
Considering the UL link restriction, SRS-Pos is a zadoff-chu sequence [14], due to the good autocorrelation, low cross-correlation and low peak to average power ratio (PAPR) characteristics. It is expressed as  
\[\left\{\begin{array}{l}\mathtt{r}_{\text{SRS-Pos}}=\mathtt{r}_{u,v}^{(a_{i}, \delta)}\\ \alpha_{i}=2\pi n_{SRS}^{c,\delta}/n_{\text{SRS}}^{\text{cs,max}}\\ \delta=\log_{2}(K_{\text{TC}}^{\text{SRS}})\end{array}\right., \tag{1}\]  
where \(\alpha_{i}\) represents a cyclic shift of port; \(n_{\text{SRS}}^{\text{cs,max}}\) is the maximum number of cyclic shifts; \(n_{\text{SRS}}^{\text{cs,i}}\in\{0,1,\ldots,n_{\text{SRS}}^{\text{cs,max}}-1\}\); \(K_{\text{TC}}^{\text{SRS}}\in\{2,4,8\}\) represents SRS-Pos comb size, which means that SRS-Pos is transmitted in a comb structure to support multi-user multiplexing within the same set of time-frequency resources, i.e., the number of multiplexed UE is \(K_{\text{TC}}^{\text{SRS}}\).  
Regarding the time-frequency resource allocation for SRS-Pos, one SRS-Pos resource occupies multiple consecutive OFDM symbols in the time domain to guarantee the performance, and the symbol number is \(N_{\text{symb}}^{\text{SRS}}\in\{1,2,4,8,12\}\). In the frequency domain, SRS-Pos can be configured up to \(N_{\text{PRS}}^{\text{SRS}}=272\) consecutive PRBs with a granularity of four PRBs [14]. Figure 2b shows an example of SRS-Pos comb-based RE mapping with \(K_{\text{TC}}^{\text{SRS}}=4\) and \(N_{\text{symb}}^{\text{SRS}}=8\), and it can be founded that SRS-Pos resource is a permuted staggered comb, i.e., the starting positions of a single SRS-pos in different OFDM symbols are different to provide a possibility of coherent combination at the receiver side. Such a design is similar to PRS.  
Further, SRS-Pos transmission can be semi-continuous, periodic, and aperiodic, according to demand and UE capabilities. In case of semi-continuous transmission, TRP can dynamically activate or deactivate SRS-Pos. In case of periodical SRS-Pos, UE starts SRS-Pos transmission once the corresponding configuration is available. In case of aperiodic SRS-Pos, TRP triggers each SRS-Pos transmission by DCI (downlink control information). Whether a UE supports aperiodic SRS-Pos transfers depends on the UE capabilities.  
Different from DL positioning, to support flexible configuration, a device can be configured with multiple SRS-Pos resource sets comprising multiple SRS-Pos resources, as  
Figure 3: Illustration of PRS and SRS-Pos comb-based time-frequency RE mapping. (**a**) no muting. (**b**) muting opt1. (**c**) muting opt2. (**d**) muting opt1+opt2.  
shown in Figure 1b. For each SRS-Pos resource set, parameters comprising the transmission type (i.e., periodic, aperiodic and semi-continuous), transmission power are unified for multiple SRS-Pos resources. Correspondingly, to support flexible resource allocation of UL positioning according to the channel status of each UE, flexible configurations for comb size, time domain duration, PRB position in the frequency domain, and so on are allowed for each SRS-Pos resource.