# 5G Positioning: An Analysis of Early Datasets
## 2 The GINT05G Experiment
### Data Recording  
This section describes the necessary steps needed to be performed for processing the raw data and, therefore, obtaining _ToA_ measurements [27; 32; 33]. The receiving unit recorded IQ samples generated using the Matlab 5G toolbox and transmitted by the transmitting points. The computation of 5G NR _ToA_ is performed based on the SRSs sent on the downlink channels. This represents an intentional deviation from the 3 GPP specifications, which define the use of the SRS on the uplink channel. The choice to adapt the transmission of the SRS to the downlink stems from the fact that existing tools and signal post-processing algorithms were developed for the SRS signal. The adaptation to the SRS signal consisted in prefixing each SRS symbol with a unique secondary synchronization signal (SSS) sequence; this SSS sequence helps to match a specific SRS burst to a transmitting antenna. The SRS signal is transmitted as 10 ms frames, as described in the 3 GPP specifications, and its  
Figure 4: Indoor trajectories: Takes 06, 07, and 08. Figure 3: Driveway trajectories: Takes 04 and 05.  
configuration is characterized by a 1 OFDM (orthogonal frequency-division multiplexing) symbol and comb2 transmission pattern with the following bandwidth configuration parameters (Table 1):  
This received signal was processed offline as follows: first, the SSS correlation step looked for the maximum SSS peak to allow for different transmitting antennas to be distinguished; followed by SRS correlation based on a fast Fourier transform approach. The maximum SRS correlation peak represents the _ToA_ for the signal with respect to the sampling rate. For each 10 ms frame, one _ToA_ value was generated. Although 5G NR transmitters are in general synchronized by a common 10 MHz clock source, the synchronization is not perfect. There is a delay caused by a number of factors, including different cable lengths and the connection between the antenna and ADC (analog to digital converter), and it needs to be compensated for. Furthermore, additional, non-constant latency can be introduced by each USRP on start-up, necessitating the performance of latency compensation at least once each time the setup is turned on. The following equation gives the effect of latency on the _ToA_ measurement:  
\[ToA_{R}^{BS}=\tau_{R}^{BS}+dt_{R}-dt^{BS} \tag{1}\]  
where \(\tau\) is the time of flight of the signal from the base station, and \(dt_{R}\) and \(dt_{BS}\) are, respectively, the clock offset of the receiver and of the base station with respect to a given reference time scale. In the GINTO5G experiment, all the base stations were electronically synchronized and the residual synchronization error resulted in post-set-up calibration with a magnitude of a few meters, which was used to correct the measured _ToA_. This instrumental calibration is not discussed here because it was preliminary to this work [31]. As a result, the _ToA_ contains just the geometry and the clock offset of the receiver. As further detailed in Section 3, the receiver clock offset was removed by time differencing pairs of _ToA_ measurements, leading, therefore, to _TDoA_ measurements. The calibrated observations contain errors that can be ascribed to three different sources:  
* clock jitter;
* radio channel effects (e.g., multipath);
* the accuracy of the reference measurements of the distances between antennas. Although clock jitter has a general impact on the _ToA_ quality, it cannot be directly compensated for, and in general, low-jitter clocks (picosecond jitter) have to be taken into account during system design. The equipment and conditions used in this work were experimental and well controlled, a situation difficult to reproduce on a standard mass-market device: picosecond-jitter clocks indeed suggest that such results cannot currently be reproduced with mass-market devices/smartphones. This clearly will be an object of future investigations.