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Two different techniques can be employed to use LTE signals for PNT: network‐based and UE‐based. The network‐based technique was enabled in LTE Release 9 by introducing a broadcast positioning reference signal (PRS). The expected positioning accuracy with PRS is on the order of 50 m [59]. Network‐based positioning suffers from a number of drawbacks:

Figure 38.24 Experimental hardware setup, navigator trajectory, and mapper and BTS locations for ground experiment. Map data: Google Earth (Khalife et al. [18]; Khalife and Kassas [25]).

Source: Reproduced with permission of IEEE.

 The user’s privacy is compromised, since the user’s location is revealed to the network [60].Figure 38.25 BTS environment and experimental hardware setup with a mobile mapper. Map data: Google Earth (Khalife and Kassas [25]).Source: Reproduced with permission of IEEE.Figure 38.26 Navigating UAV’s true and estimated trajectory.Map data: Google Earth.

 Localization services are limited to paying subscribers and from a particular cellular provider.

 Ambient LTE signals transmitted by other cellular providers are not exploited.

 Additional bandwidth is required to accommodate the PRS, which caused the majority of cellular providers to choose not to transmit the PRS in favor of dedicating more bandwidth for traffic channels.

To circumvent these drawbacks, UE‐based PNT techniques, which exploit the existing reference signals in the transmitted LTE signals, have been developed. This section focuses on UE‐based PNT techniques. When a UE enters an unknown LTE environment, the first step it performs to establish communication with the network is synchronizing with the surrounding LTE BTSs, also referred to as Evolved Node Bs (eNodeBs). This is achieved by acquiring the PSS and the SSS transmitted by the eNodeB. The PSS and SSS can be directly exploited for navigation. Another LTE signal that can be exploited for navigation is the CRS; however, exploiting CRS is not as straightforward due to its scattered nature in time and frequency. Table 38.1 compares the salient navigation properties of PSS, SSS, and CRS.

This section is organized as follows. Section 38.6.1 discusses the LTE frame structure and reference signals that could be exploited for navigation. Section 38.6.2 presents a receiver architecture for producing navigation observables from received LTE signals. Section 38.6.3 analyzes the code phase error of SSS signals with coherent and non‐coherent DLL tracking. Section 38.6.4 shows experimental results for ground and aerial vehicles navigating with cellular LTE signals.