# Analysis of 5G and LTE Signals for Opportunistic Navigation and Time Holdover
## 3 Results
### Visible Cell Stability  
While it is known that cellular infrastructure must meet certain minimum time and frequency stability specifications, which have generally become more stringent as data rates and network densities have increased, the implementation details can leave room for interpretation on matters such as which time scale a network may be synchronized \begin{table}
\begin{tabular}{c c} \hline
**Property** & **Value** \\ \hline PLL Loop Bandwidth & 5 Hz \\ DLL Loop Bandwidth & 2 Hz \\ FFT Size & 1024 \\ Subcarrier Spacing & 15 kHz \\ Complex Sample Rate & 15.36 MS/s \\ USRP Frontend Gain & 40 dB \\ \hline \end{tabular}
\end{table} 
Table 1: Common settings for all the recordings discussed in this paper. \begin{table}
\begin{tabular}{c c c c} \hline
**Operator 1** & **DL Frequency (MHz)** & **Band Number** & **Bandwidth (MHz)** & **Cell PhyID 2** \\ \hline Telenor & 816.0 & 20 & 10 & 94 \\ Telenor & 955.0 & 8 & 10 & 411 \\ Telenor & 1815.6 & 3 & 10 & 466 \\ Telenor & 1830.0 & 3 & 20 & 214 \\ Telenor & 2140.0 & 1 & 20 & 275 \\ Telenor & 2660.0 & 7 & 20 & 214 \\ Telenor & 2679.8 & 7 & 20 & 429 \\ Telia & 935.1 & 8 & 10 & 3 \\ Telia & 935.1 & 8 & 10 & 169 \\ Telia & 1850.0 & 3 & 20 & 169 \\ Telia & 2120.0 & 1 & 20 & 385 \\ Telia & 2630.0 & 7 & 20 & 272 \\ ICE & 465.0 & 31 & 5 & 101 \\ ICE & 796.0 & 20 & 10 & 19 \\ ICE & 1870.0 & 3 & 20 & 19 \\ \hline \end{tabular}  
* Based on the publicly available spectrum allocation at the time of publication [19]. 2 Physical ID.  
\end{table} 
Table 2: Adjacent to the NTNU campus infrastructure from all three network operators, a variety of signals spanning approximately 2 GHz of a spectrum are visible. to [20; 21]. Based on a survey of the existing literature, the stability of the time source within the base stations could be as poor as a normal quartz oscillator [22], but it is likely steered to a common time scale that is shared among the adjacent stations that are operated by the same network operator to prevent intercell interference and to meet the synchronization requirements of 5G [23]. To test this assumption directly, the data from the observed stations were gathered over periods of multiple hours using the STARE SDR. Internally, the STARE SDR tracks the epoch of arrival of signal features, including the primary and secondary synchronization sequence (PSS and SSS, respectively), versus the sampling epochs of the radio receiver, as well as converts these measurements to range deltas by multiplying the time observation with the speed of light. By smoothing these with the signal-carrier phase, a low noise estimate of the change in apparent range between the base station antenna and the user antenna is produced. Since both the transmit and receive antennas were static in these tests, the remaining measurements were, due to noise, clock deviations at the source and receiver; in addition, they also produced multipath effects. While multipath effects for static users of GNSS are not constant over long time periods due to the changing relative user-reflector-satellite geometry [24], the multipath between a static user and a static transmitter should be mostly constant (ref. Section 2.2 for further discussion), thus leaving primarily noise and clock changes in the resulting delta range measurements. The measured range deltas for Cell ID 214, 2660.0 MHz (Telenor) and Cell ID 19, 1870.0 MHz (ICE) are presented below in Figure 2a,b, respectively. While the results shown in Figure 2 seem to indicate that the two stations observed implement timing solutions with very different levels of stability, it is not yet possible to determine if this is true generally for a given network operator and service type or if this is simply anecdotal to this realization of the transmitter. During a very long segment of about ten hours of tracking the former tower, it appeared as if the timing solution was very stable even over longer periods (see Appendix B).